Top all food Sucrose

point, the incidence of lactose-intolerant individuals beings to rise and increases throughout the life span, with the greatest incidence in the elderly. Both the incidence and the degree of lactose intolerance very by ethnic group, indicating that the presence or absence of lactase is under genetic control. There are two ways to overcome the effects of lactase deficiency. One is to remove the lactose by fermentation; that produces yogurt and buttermilk products. Another is to produce reduced-lactose milk by adding lactase to it. However, both products of hydrolysis, D-glucose and D-galactose, are sweeter than lactose, and at about 80% hydrolysis, the taste change becomes too evident. Therefore, most reduced-lactose, milk has the lactose reduced as close as possible to the 70% government-mandated limit for a claim. In a technology under development, live yogurt cultures are added to refrigerated milk. The bacteria remain dormant in the cold and do not change the flavor of the milk, but upon reaching the small intestine, release lactase. Other carbohydrates that are not completely broken down into monosaccharides by intestinal enzymes and are not absorbed also pass into the colon. There they also are metabolized by microorganisms, producing lactate and gas. Again diarrhea and bloating result. This problem can occur from eating beans, because beans contain a trisaccharide (raffinose) and a tetrasaccharide (stachyose) (see Sec. 4.2.3) that are not hydrolyzed to monosaccharides by intestinal enzymes and thus pass into the colon, where they are fermented. 4.2.3 Sucrose [35,39] The per person daily utilization of sucrose, usually called simply sugar or table sugar, in the United States averages about 160 g, but sucrose is also used extensively in fermentations, in bakery products where it is also largely used up in fermentation, and in pet food, so the actual average daily amount consumed by individuals in foods and beverages in about 55 g (20 kg or 43 Ib/year). Sucrose is composed of an aD-glucopyranosyl unit and a b-D-fructofursanosyl unit linked head to head (reducing end to reducing end) rather than by the usual head-to-tail linkage (Fig. 28). Since it has no reducing end, it is a nonreducing sugar. There are two principal sources of commercial source—sugar cane and sugar beets. Also present in sugar beet extract are a trisaccharide, raffinose, which has a D-galactopyranosyl unit attached to sucrose, and a tetrasaccharide, stachyose, which contains another D-galactosyl unit (Fig. 29). These oligosaccharides are also found in beans, are nondigestible, and are the source of the flatulence associated with eating beans. Commercial brown sugar is made by treating white sugar crystals with molasses to leave a coating of desired thickness. Grades range from light yellow to dark brown. Confection, or Pag e 177 FIGURE 28 Sucrose. powdered, sugar is pulverized sucrose. It usually contains 3% corn starch as an anticaking agent. To make fondant sugar, which is used in icings and confections, very fine sucrose crystals are surrounded with a saturated solution of invert sugar, corn syrup, or maltodextrin.* For many food product applications, sucrose is not crystallized; rather, it is shipped as a refined aqueous solution known as liquid sugar. Sucrose and most other low-molecular-weight carbohydrates (for example, monosaccharides, alditols, disaccharides, and other low-molecular-weight oligosaccharides), because of their great hydrophilicity and solubility, can form highly concentrated solutions of high osmolality. Such solutions, as exemplified by pancake and waffle syrups and honey, need no preservatives themselves and can be used not only as sweeteners (although not all such carbohydrate syrups need have much sweetness) but also as preservatives and humectants. A portion of the water in any carbohydrate solution is nonfreezable. When the freezable water crystallizes, that is, forms ice, the concentrations of solute in the remaining liquid phase increases, and the freezing point decreases. There is a consequential increase in viscosity of the remaining solution. Eventually, the liquid phase solidifies as a glass in which the mobility of all molecules becomes greatly restricted and diffusion-dependent reactions become very slow FIGURE 29 Sucrose, raffinose, and stachyose. (For explanation of the shorthand desig nations of structures, see Sec. 4.3.) *See Section 4.4.8 for a description of the latter two products. Invert sug ar is the equimolar mixture of D-g lucose and D-fructose formed by hydrolysis of sucrose. Pag e 178 (see Chap. 2); because of the restricted motion, these glass-state water molecules cannot crystallize. In this way, carbohydrates function as cryoprotectants and protect against the dehydration that destroys structure and texture caused by freezing. The sucrase of the human intestinal tract catalyzes hydrolysis of sucrose into D-glucose and D-fructose, making sucrose one of the three carbohydrates (other than monosaccharides) humans can utilize for energy, the other two being lactose and starch. Monosaccharides (D-glucose and D-fructose being the only significant ones in our diets) do not need to undergo digestion before absorption. 4.3 Polysaccharides [42,47,55] Polysaccharides are polymers of monosaccharides. Like the oligosaccharides, they are composed of glycosyl units in linear or branched arrangements, but most are much larger than the 20-unit limit of oligosaccharides. The number of monosaccharide units in a polysaccharide, termed its degree of polymerization (DP), varies. Only a few polysaccharides have a DP less than 100; most have DPs in the range 200–3000. The larger ones, like cellulose, have a DP of 7000–15,000. It is estimated that more than 90% of the considerable carbohydrate mass in nature is in the form of polysaccharides. Polysaccharides can be either linear or branched. The general scientific term for polysaccharides is glycans. If all the glycosyl units are of the same sugar type, they are homogeneous as to monomer units and are called homoglycans. Examples of homoglycans are cellulose (see Sec. 4.5) and starch amylose (see Sec. 4.4.1), which are linear, and starch amylopectin (see Sec. 4.4.2), which is branched. All three are composed only of D-glucopyranosyl units. When a polysaccharide is composed of two or more different monosaccharide units, it is a heteroglycan. A polysaccharide that contains two different monosaccharide units is diheteroglycan, a polysaccharide that contains three different monosaccharide units is a triheteroglycan, and so on. Diheteroglycans generally are either linear polymers of blocks of similar units alternating along the chain, or consist of a linear chain of one type of glycosyl unit with a second present as single-unit branches. Examples of the former type are algins (see Section 4.9) and of the latter guar and locust bean gums (see Sec. 4.6). In the shorthand notations of oligo-and polysaccharides, the glycosyl units are designated by the first three letters of their names with the first letter being capitalized, except for glucose, which is Glc. If the monosaccharide unit is that of a D-sugar, the D is omitted; only L sugars are so designated, such as LAra. The size of the ring is designated by an italicized p for pyranosyl or ƒ for furanosyl. The anomeric configuration is designated with a or b as appropriate; for example, an a-D-glucopyranosyl unit is indicated as aGlcp. Uronic acids are designated with a capital A; for example, an L-gulopyranosyluronic acid unit (see Sec. 4.9) is indicated as LGulpA. The position of linkages are designated either as, for example, 1 3 or 1,3, with the latter being more commonly used by biochemists and the former more commonly used by carbohydrate chemists. Using the shorthand notation, the structure of lactose is represented as bGalp(1 4)Glc or bGalp1,4Glc and maltose as aGlcp)1 4)Glc or aGlcp1,4Glc. (Note that the reducing end cannot be designated as a or b or as pyranose or furanose because the ring can open and close; that is, in solutions of lactose and maltose and other oligo-and polysaccharides, the reducing end unit will occur as a mixture of a-and b-pyranose ring forms and the acyclic form, with rapid interconversion between them; see Fig. 12.) Pag e 179 4.3.1 Polysaccharide Solubility Most polysaccharides contain glycosyl units that, on average, have three hydroxyl groups. Polysaccharides are thus polyols in which each hydroxyl group has the possibility of hydrogen bonding to one or more water molecules. Also, the ring oxygen atom and the glycosidic oxygen atom connecting one sugar ring to another can form hydrogen bonds with water. With every sugar unit in the chain having the capacity to hold water molecules avidly, glycans possess a strong affinity for water and readily hydrate when water is available. In aqueous systems, polysaccharide particles can take up water, swell, and usually undergo partial or complete dissolution. Polysaccharides modify and control the mobility of water in food systems, and water plays an important role in influencing the physical and functional properties of polysaccharides. Together polysaccharides and water control many functional properties of foods, including texture. The water of hydration that is naturally hydrogen-bonded to and thus solvates polysaccharide molecules is often described as water whose structure has been sufficiently modified by the presence of the polymer molecule so that it will not freeze. The water has also been referred to as plasticizing water. The molecules that make up this water are not energetically bound in a chemical sense. While their motions are retarded, they are able to exchange freely and rapidly with other water molecules. This water of hydration makes up only a small part of the total water in gels and fresh tissue foods. Water in excess of the hydration water is held in capillaries and cavities of various sizes in the gel or tissue. Polysaccharides are cryostabilizers rather than cryoprotectants, because they do not increase the osmolality or depress the freezing point of water significantly, since they are large, high-molecular-weight molecules and these are colligative properties. As an example, when a starch solution is frozen, a two-phase system of crystalline water (ice) and a glass consisting of about 70% starch molecules and 30% nonfreezable water is formed. As in the case of solutions of low-molecular-weight carbohydrates, the nonfreezable water is part of a highly concentrated polysaccharide solution in which the mobility of the water molecules is restricted by the extremely high viscosity. However, while most polysaccharides provide cryostabilization by producing this freeze-concentrated matrix, which severely limits molecular mobility, there is evidence that others provide cryostabilization by restricting ice crystal growth by adsorption to nuclei or active crystal growth sites. Other polysaccharides may be ice nucleators. Thus both high-and low-molecular-weight carbohydrates are generally effective in protecting food products stored at freezer temperatures (typically -18°C) from destructive changes in texture and structure. In both cases, the improvement in product quality and storage stability is a result of controlling both the amount (particularly in the case of low-molecular-weight carbohydrates) and the structural state (particularly in the case of polymeric carbohydrates) of the freeze-concentrated, amorphous matrix surrounding ice crystals. Most, if not all, polysaccharides, except those with very bushlike, branch-on-branch structures, exist in some sort of helical shape. Certain linear homoglycans, like cellulose (see Sec. 4.5), have flat, ribbon-like structures. Such uniform linear chains undergo hydrogen bonding with each other so as to form crystallites separated by amorphous regions. Crystalline arrangements of this sort are called fringed micelles (Fig. 30). It is these crystallites of linear chains that give cellulose fibers, like wood and cotton fibers, their great strength, insolubility, and resistance to breakdown, the latter because the crystalline regions are nearly inaccessible to Pag e 180 FIGURE 30 Fring ed micelles. The crystalline reg ions are those in which the chains are parallel and ordered. enzyme penetration. These highly ordered polysaccharides with orientation and crystallinity comprise the exception, rather than the rule. Most polysaccharides are not so crystalline as to impart water insolubility, but are readily hydrated and dissolved. Most unbranched diheteroglycans containing nonuniform blocks of glycosyl units and most branched glycans cannot form micelles because chain segments are prevented from becoming closely packed over lengths necessary to form strong intermolecular bonding. Hence, these chains have a degree of solubility that increases as chains become less able to fit closely together. In general, polysaccharides become more soluble in proportion to the degree of irregularity of the molecular chains, which is another way of saying that, as the ease with which molecules fit together decreases, the solubility of the molecules increases. Water-soluble polysaccharides and modified polysaccharides used in food and other industrial applications are known as gums or hydrocolloids. Gums are sold as powers of varying particle size. Pag e 181 4.3.2 Polysaccharide Solution Viscosity and Stability [13] Polysaccharides (gums, hydrocolloids) are used primarily to thicken and/or gel aqueous solutions and otherwise to modify and/or control the flow properties and textures of liquid food and beverage products and the deformation properties of semisolid foods. They are generally used in food products at concentrations of 0.25–0.50%, indicating their great ability to produce viscosity and to form gels. The viscosity of a polymer solution is a function of the size and shape of its molecules and the conformations they adopt in the solvent. In foods and beverages, the solvent is an aqueous solution of other solutes. The shapes of polysaccharide molecules in solution are a function of oscillations around the bonds of the glycosidic linkages. The greater the internal freedom at each glycosidic linkage, the greater the number of conformations available to each individual segment. Chain flexibility thus provides a strong entropic drive, which generally overcomes energy considerations and induces the chain to approach disordered or random coil states in solution (Fig. 31). However, most polysaccharides exhibit deviations from strictly random coil states, forming stiff coils, with the specific nature of the coils being a function of the monosaccharide composition and linkages, some being compact, some expanded. Linear polymer molecules in solution gyrate and flex, sweeping out a large space. They frequently collide with each other, creating friction, consuming energy, and thereby producing viscosity. Linear polysaccharides produce highly viscous solutions, even at low concentrations. Viscosity depends both on the DP (molecular weight) and the extension and rigidity, that is, the shape and flexibility, of solvated polymer chain. A highly branched polysaccharide molecule will sweep out much less space than a linear polysaccharide of the same molecular weight (Fig. 32). As a result, highly branched molecules will collide less frequently and will produce a much lower viscosity than will linear molecules of the same DP. This also implies that highly branched polysaccharide molecules must be significantly larger than linear polysaccharide molecules to produce the same viscosity at the same concentration. Likewise, linear polysaccharide chains bearing only one type of ionic charge (always a negative charge derived from ionization of carboxyl or sulfate half-ester groups) assume an extended configuration due to repulsion of the like charges, increasing the endto-end chain length and thus the volume swept out by the polymer. Therefore, these polymers tend to produce solutions of high viscosity. FIGURE 31 Randomly coiled polysaccharide molecules. Pag e 182 FIGURE 32 Relative volumes occupied by a linear polysaccharide and a hig hly branched polysaccharide of the same molecular weig ht. Unbranched, regular glycans, which dissolve in water by heating, form unstable molecular dispersions that precipitate or gel rapidly. This occurs as segments of the long molecules collide and form intermolecular bonds over the distance of a few units. Initial short alignments then extend in a zipper-like fashion to greatly strengthen intermolecular associations. Other segments of other chains colliding with this organized nucleus bind to it, increasing the size of the ordered, crystalline phase. Linear molecules continue to bind to fashion a fringed micelle that may reach a size where gravitational forces cause precipitation. For example, starch amylose, when dissolved in water with the aid of heat followed by cooling the solution, undergoes molecular aggregation and precipitates, a process called retrogradation. During cooling of bread and other baked products, amylose molecules associate to produce a firming. Over a longer storage time, the branches of amylopectin associate to produce staling (see Sec. 4.4.6). In general, molecules of all unbranched, neutral homoglycans have an inherent tendency to associate and partially crystallize. However, if linear glycans are derivatized, or occur naturally derivatized, as does guar gum (see Sec. 4.6), which has single-unit glycosyl branches along a backbone chain, their segments are prevented from association and stable solutions result. Stable solutions are also formed if the linear chains contain charged groups where coulombic repulsions prevent segments from approaching each other. As already mentioned, charge repulsion also causes chains to extend, which provides high viscosities. Such highly viscous, stable solutions are seen with sodium alginate (see Sec. 4.9), where each glycosyl unit is a uronic acid unit having a carboxylic acid group in the salt form, and in xanthan (see Sec. 4.7), where one out of five glycosyl units is a uronic acid unit and another carboxylate group may be present. But if the pH of an alginate solution is lowered to 3, where ionization of carboxylic acid groups is somewhat repressed because the pKa values of the constituent monomers are 3.38 and 3.65, the resulting less ionic molecules can associate and precipitate or form a gel as expected for unbranched, neutral glycans. Pag e 183 FIGURE 33 The log arithm of viscosity as a function of the shear rate for a pseudoplastic shear-thinning fluid. Carrageenans are mixtures of linear chains that have a negative charge due to numerous ionized sulfate half-ester groups along the chain (see Sec. 4.8). These molecules do not precipitate at low pH because the sulfate group remains ionized at all practical pH values. Solutions of gums are dispersions of hydrated molecules and/or aggregates of hydrated molecules. Their flow behavior is determined by the size, shape, ease of deformation (flexibility), and presence and magnitude of charges on these hydrated molecules and/or aggregates. There are two general kinds of flow exhibited by polysaccharide solutions: pseudoplastic (by far the most common) and thixotropic. Pseudoplastic fluids are shear-thinning. In pseudoplastic flow, a more rapid increase in flow results from an increase in shear rate; that is, the faster a fluid flows, the less viscous it becomes (Fig. 33). The flow rate can be increased by increasing the applied force by pouring, chewing, swallowing, pumping, mixing, etc. The change in viscosity is independent of time; that is, the rate of flow changes instantaneously as the shear rate is changed. Linear polymer molecules form shear-thinning, usually pesudoplastic, solutions. In general, higher molecular weight gums are more pseudoplastic. Gum solutions that are less pseudoplastic are said to give long flow;* such solutions are generally perceived as being slimy. More pseudoplastic solutions are described as having short flow and are generally perceived as being nonslimy. In food science, a slimy material is one that is thick, coats the mouth, and is difficult to swallow. Sliminess is inversely related to pseudoplasticity; that is, to be perceived as being nonslimy, there must be marked thinning at the low shear rates produced by chewing and swallowing. Thixotropic flow is a second type of shear-thinning flow. In this case, the viscosity reduction that results from an increase in the rate of flow does not occur instantaneously. The viscosity of thixotropic solutions decreases under a constant rate of shear in a time-dependent manner and regains the original viscosity after cessation of shear, but again only after a clearly * “Short flow” is exhibited by shear-thinning , primarily pseudoplastic, viscous solutions and “long flow” by viscous solutions that exhibit little or no shear thinning . These terms were applied long before there were instruments to determine and measure rheolog ical phenomena. They were arrived at in this way. W hen a g um or starch solution is allowed to drain from a pipette or a funnel, those that are not shear-thinning come out in long string s, while those that shear-thin form short drops. The latter occurs because, as more and more fluid exists the orifice, the weig ht of the string becomes g reater and g reater, which causes it to flow faster and faster, which causes it to shear-thin to the point that the string breaks into drops. Pag e 184 defined and measured time interval. This behavior is due to a gel solution gel transition. In other words, a thixotropic solution is at rest a weak (pourable) gel (see Sec. 4.3.3). For solutions of most gums, an increase in temperature results in a decrease in viscosity. (Xanthan gum is an exception between 0 and 100°C; sec Sec. 4.7.) This is often an important property, for it means that higher solids can be put into solution at a higher temperature; then the solution can be cooled for thickening. 4.3.3 Gels [6,18] A gel is a continuous, three-dimensional network of connected molecules or particles (such as crystals, emulsion droplets, or molecular aggregates/fibrils) entrapping a large volume of a continuous liquid phase, much as does a sponge. In many food products, the gel network consists of polymer (polysaccharide and/or protein) molecules or fibrils formed from polymer molecules joined in junction zones by hydrogen bonding, hydrophobic associations (van der Waals attractions), ionic cross bridges, entanglements, or covalent bonds, and the liquid phase is an aqueous solution of low-molecular-weight solutes and portions of the polymer chains. Gels have some characteristics of solids and some of liquids. When polymer molecules or fibrils formed from polymer molecules interact over portions of their lengths to form junction zones and a three-dimensional network (Fig. 34), a fluid solution is changed into a material that FIGURE 34 A diag rammatic representation of the type of three-dimensional network structure found in g els. This type of structure is known as a fring ed micelle structure. Parallel side-by-side chains indicate the ordered, crystalline structure of a junction zone. The holes between junction zones contain an aqueous solution of dissolved seg ments of polymer chains and other solutes. Pag e 185 has a sponge-like structure and can retain its shape. The three-dimensional network structure offers significant resistance to an applied stress causing it to behave in some respects as an elastic solid. However, the continuous liquid phase, in which molecules are completely mobile, makes a gel less stiff than an ordinary solid, causing it to behave in some respects as a viscous liquid. Therefore, a gel is a viscoelastic semisolid; that is, the response of a gel to stress is partly characteristic of an elastic solid and partly characteristic of a viscous liquid. Although gel-like or salve-like materials can be formed by high concentrations of particles (much like tomato paste), to form a true gel, the polymer molecules or aggregates of molecules must first be in solution, then partially come out of solution in junction zone regions to form the three-dimensional gel network structure. In general, if junction zones grow larger after formation of the gel, the network becomes more compact, the structure contracts, and syneresis results. (The appearance of fluid droplets on the gel surface is called syneresis.) Although polysaccharide gels generally contain only about 1% polymer—that is, they may contain as much as 99% water—they can be quite strong. Examples of polysaccharide gels are dessert gels, aspics, structured fruit pieces, structured onion rings, meat-analog pet foods, and icings. Choice of a specific gum for a particular application depends on the viscosity or gel strength desired, desired rheology, pH of the system, temperatures during processing, interactions with other ingredients, desired texture, and cost of the amount needed to impart the desired properties. In addition, consideration is also given to desired functional characteristics. These include a gum’s ability to function as a binder, bodying agent, bulking agent, crystallization inhibitor, clarifying agent, cloud agent, coating agent/film former, fat mimetic, flocculating agent, foam stabilizer, mold release agent, suspension stabilizer, swelling agent, syneresis inhibitor, and whipping agent and as an agent for water absorption and binding (water retention and migration control), adhesion, emulsification, emulsion stabilization, and encapsulation. Each food gum tends to have one or more outstanding unique property related to this list, and this property is often the basis for its choice (Table 2). 4.3.4 Polysaccharide Hydrolysis Polysaccharides are relatively less stable to hydrolytic cleavage than are proteins and may, at times, undergo depolymerization during food processing and/or storage of foods. Often, food gums are deliberately depolymerized so a relatively high concentration can be used to provide body without producing undesirable viscosity. Hydrolysis of glycosidic bonds joining monosaccharide (glycosyl) units in oligo- and polysaccharides can be catalyzed by either acids (H+ ) or enzymes. The extent of depolymerization, which has the effect of reducing viscosity, is determined by the acid strength, time, temperature, and structure of the polysaccharide. Generally, hydrolysis occurs most readily during thermal processing, because many foods are somewhat acidic. Defects associated with depolymerization during processing can usually be overcome by using more of the polysaccharide (gum) in the formulation to compensate for breakdown, using a higher viscosity grade of the gum, again to compensate for any depolymerization, or using a more acid-stable gum. Depolymerization can also be an important determinant of shelf life. Polysaccharides are also subject to enzyme-catalyzed hydrolysis. The rate and end products of this process are controlled by the specificity of the enzyme, pH, time, and temperature. Polysaccharides, like any and all other carbohydrates, are subject to microbial attack because of their susceptibility to enzyme-catalyzed hydrolysis. Furthermore, gum products are very seldom delivered sterile, and this fact must be considered when using them as ingredients. Pag e 186 Pag e 187 Pag e 188 Pag e 189 Pag e 190 Pag e 191 4.4 Starch [32,33,38,56,61] Starch unique chemical and physical characteristics and nutritional quality set it apart from all other carbohydrates. Strach is the predominant food reserve substance in plants and provides 70–80% of the calories consumed by humans worldwide. Starch and starch hydrolysis products constitute most of the digestible carbohydrate in the human diet. Also, the amount of starch used in the preparation of food products, without conunting that present in flours used to make bread and other bakery products, that naturally occurring in grains used to make breakfast cereals, or that naturally consumed in fruits and vegetables, greatly exceeds the combined use of all other food hydrocolloids. Commercial starches are obtained from cereal grain seeds, particulary from corn, waxy corn (waxy maize), high-amylose corn, wheat, and various rices, and from tubers and roots, particulary potato, sweet potato, and tapioca (cassava). Starches and modified starches have an enormous number of food uses, including adhesive, binding, clouding, dusting, film forming, foam strengthening, antistaling, gelling, glazing, moisture retaining stabilizing, texturizing, and thickening applications. Starch is unique among carbohydrates because it occurs naturally as directly particules (granules). Starch granules are relatively dense ad insoluble and hydrate only slightly in cold water. They can be dispersed in water, producing low-viscovity slurries that can be easily mixed and pumped, even at concentrations of greater than 35%. The viscosity-building (thickening) power of starch is realized only wen a slurry of granules is cooked. Heating a 5% slurry of unmodified starch granules to about 80°C (175°F) with stirring produces very high viscosity. A second uniqueness is that most starch granules are composed of a mixture of two polymers: an essentially linear polysaccharide called amylose, and a highly branched polysaccharide called amylopectin. 4.4.1 Amylose While amylose is essentially a linear chain og (1 4)-linked a-D-glucopyranosyl units, many amylose molecules have a few aD-(1 6) branches, perhaps 1 in 180–320 units, or 0.3–0.5% of the linkages [49]. The branches in branced amylose molecules are either very long or very short, and the branch points are separatd by large distances so that the physical properties of amylose molecules are essentials those of linear molecules. Amylose molecules have molecular weights of about 10(sup>6(/sup>. The axial equatorial position coupling of the (1 4)-linked a-D-glucopyraosy units in amylose chains gives the molecules a right-handed sprial or helical shape (Fig. 35). The interior of the helix contains only hydrogen atoms and is liophilic, while the hydroxyl groups are positioned on the exterior of the coil. Most starches contain about 25% amylose (Table 3). The two so-called high-amylose corn starches that are commercially available have apparent amylose contents of about 52% and 70–75%. 4.4.2 Amylopectin [21,22,34] Amylopectin is a very large, very highly branched molecule, with branch-point linkages constituting 4–5% of the total linkages. Amylopectin consists of a chain containing the only reducing end-group, called a C-chain, which has numerous branches, termed B-chains, to which one to several third-layer A-chains are attached.* The branches of amylopectin mole- *A-chains are unbranched. B-chains are branched with A-chairs or other B-chainscules are clustered (Fig. 36) and occur as double helics. Molecular weights of from 107 to 5 × 108 make amylopectin molecules among the largest, if not the largest, molecules found in nature. Amylopectin is present in all starches, constituting about 75% of most common starches (Table 3.). Some starches consist entirely of amylopectin and are called waxy starches. Waxy corn (waxy maize), the first grain recognized as one in which the starch consists only of amylopectin, is so termed because when the kernel is cut the new surface appears vitreous or waxy. Other all-amylopectin starches are also called waxy although, as in corn, there is no wax content. Potato amylopectin is unique in having phosphate ester groups, attached most often (60%-70%) at an O-6 position, with the other third at O-3 positions. These phosphate ester groups occur about once in every 215-560 a-D-glucopyranosyl units, and about 88% of them are on B chains. 4.4.3 Starch Granules [2,21,24,37,44] Starch granules are made up of amylose and/or amylopectin molecules arranged radially. They contain both crystalline and noncrystalline regions in alternating layers. *The clustered branches of amylopectin occur as packed double helices. It is the packing together of these double-helical structures that forms the many small crystalline areas comprising the dense layers of starch granuels that alternate with less dense amorphous layers. Because the crystallinity is produced by ordering of the amylopectin chains, waxy starch granules, that is, granules without amylose, have about the same amount of crystallinity as do normal starches. Amylose molecules occur among the amylopectin molecules, and some diffuse from partially water-swollen granules. The radial, ordered arrangement of starch molecules in a granule is evident from the polarization cross (white cross on a black background) seen in a polarizing microscope with the polarizers set 90° to each other. The center of the cross is at the hilum, the origin of growth of the granule. Corn starch granules, even from a single source, have mixed shapes, with some being almost spherical, some angular, and some idented. (For the size, see Table 3.) Wheat starch granules are lenticular and have a bimodal or timodal size distribution (> 14 µm, 5–14 µm, 1–5 µm). *Starch g ranules are composed of layers somewhat like the layers of an onion, except that the layers cannot be peeled off. Pag e 193 TABLE 3 General Properties of Some Starch Granules and Their Pastes Common corn starch W axy maize starch Hig h-amylose corn starch Potato starch Tapioca starch W heat starch Granule size (major axis, µm) 2–30 2–30 2–24 5–100 4–35 2–55 Amylose (%) 28 <2 50–70 21 17 28 Gelatinization/pasting temperature (°C)a 62–80 63–72 66–170b 58–65 52–65 52–85 Relative viscosity Medium Medium hig h Very lowb Very hig h Hig h Low Paste rheolog yc Short Long (cohesive) Short Very long Long (cohesive) Short Paste clarity Opaque Very Slig htly cloudy Opaque Clear Clear Opaque Tendency to g el/retrog rade Hig h Very low Very hig h Medium to low Medium Hig h Lipid (%DS) 0.8 0.2 — 0.1 0.1 0.9 Protein (%DS) 0.35 0.25 0.5 0.1 0.1 0.4 Phosphorus (%DS) 0.00 0.00 0.00 0.08 0.00 0.00 Flavor Cereal (slig ht) “Clean” Slig ht Bland Cereal (slig ht) aFrom the initial temperature of g elatinization to complete pasting . bUnder ordinary cooking conditions, where the slurry is heated to 95–100°C, hig h-amylose corn starch produces essentially no viscosity. Pasting does not occur until the temperature reaches 160–170°C (320–340°F). cFor a description of long and short flow, see Section 4.3.2. Pag e 194 FIGURE 36 A diag rammatic representation of a portion of an amylopectin molecule. Pag e 195 Rice granules, on average, are the smallest of the commercial starch granules (1.5–9 µm), although the small granules of wheat starch are almost the same size. Many of the granules in tuber and root starches, such as potato and tapioca starches, tend to be larger than those of seed starches and are generally less dense and easier to cook. Potato starch granules may be as large as 100 µm along the major axis. All starches retain small amounts of ash, lipid, and protein (Table 3). The phosphorus content of potato starch (0.06–0.1%) is due to the presence of the phosphate ester groups on amylopectin molecules. The phosphate ester groups give potato starch amylopectin a slight negative charge, resulting in some coulombic repulsion that may contribute to the rapid swelling of potato starch granules in warm water and to several properties of potato starch pastes, namely, their high viscosities, good clarity (Table 3), and low rate of retrogradation (see Sec. 4.4.6). Cereal starch molecules either do not have phosphate ester groups or have very much smaller amounts than occurs in potato starch. Only the cereal starches contain endogenous lipids in the granules. These internal lipids are primarily free fatty acids (FFA) and lysophospholipid (LPL), largely lysophosphatidyl choline (89% in corn starch), with the ratio of FFA to LPL varying from one cereal starch to another. 4.4.4 Granule Gelatinization and Pasting Undamaged starch granules are insoluble in cold water, but can imbibe water reversibly; that is, they can swell slightly, and then return to their original size on drying. When heated in water, starch granules undergo a process called gelatinization. Gelatinization is the disruption of molecular order within granules. Evidence for the loss of order includes irreversible granule swelling, loss of birefringence, and loss of crystallinity. Leaching of amylose occurs during gelatinization, but some leaching of amylose can also occur prior to gelatinization. Total gelatinization usually occurs over a temperature range (Table 3), with larger granules generally gelatinizing first. The apparent temperature of initial gelatinization and the range over which gelatinization occurs depend on the method of measurement and on the starch:water ratio, granule type, and heterogeneities within the granule population under observation. Several stages of gelatinization can be determined using a polarizing microscope equipped with a hot stage. These are the initiation temperature (first observed loss in birefringence), the midpoint temperature, the completion or birefringence endpoint temperature (BEPT, the temperature at which the last granule in the field under observation loses its birefringence), and the gelatinization temperature range. Continued heating of starch granules in excess water results in further granule swelling, additional leaching of soluble components (primarily amylose), and eventually, especially with the application of shear forces, total disruption of granules. This phenomenon results in the formation of a starch paste. (In starch technology, what is called a paste is what results from heating a starch slurry.) Granule swelling and disruption produce a viscous mass (the paste) consisting of a continuous phase of solubilized amylose and/or amylopectin and a discontinuous phase of granule remnants (granule ghosts* and fragments). Complete molecular dispersion is not accomplished except, perhaps, under conditions of high temperature, high shear, and excess water, conditions seldom, if ever, encountered in the preparation of food products. Cooling of a hot corn-starch paste results in a viscoelastic, firm, rigid gel. Because gelatinization of starch is an endothermic process, differential scanning calorimetry (DSC), which measures both the temperature and the enthalpies of gelatinization, is *The g ranule g host is the remnant remaining after cooking under no to moderate shear. It consists of the outer portion of the g ranule. It appears as an insoluble, outer envelope, but is not a membrane. Pag e 196 widely used. Although there is not complete agreement on the interpretation of DSC data and the events that take place during gelatinization of starch granules, the following general picture is widely accepted. Water acts as a plasticizer. Its mobilityenhancing effect is first realized in the amorphous regions, which physically have the nature of a glass. When starch granules are heated in the presence of sufficient water (at least 60%), and a specific temperature (Tg, the glass transition temperature) is reached, the plasticized amorphous regions of the granule undergo a phase transition from a glassy state to a rubbery state.* . However, the peak for absorption of energy associated with this transition is not often seen by DSC because the regions of crystallinity, that is, the ordered, packed, double-helical branches of amylopectin, are contiguous and connected by covalent bonds to the amorphous regions and melting of the crystallites immediately follows the glass transition. Because the enthalpy of initial melting (Tm) is so much larger than that of the glass transition, the latter is usually not evident. Melting of lipid-amylose complexes occurs at much higher temperatures (100–120°C in excess water) than does melting of the amylopectin double-helical branches packed in crystalline order. These complexes are made with single-helical segments of amylose molecules when a starch paste containing monoacyl lipids is cooled. A DSC peak for this event is absent in waxy starches (without amylose). Under normal food processing conditions (heat and moisture, although many food systems contain limited water as far as starch cooking is concerned), starch granules quickly swell beyond the reversible point. Water molecules enter between chains, break interchain bonds, and establish hydration layers around the separated molecules. This plasticizes (lubricates) chains so they become more fully separated and solvated. Entrance of large amounts of water causes granules to swell to several times their original size. When a 5% starch suspension is gently stirred and heated, granules imbibe water until much of the water is absorbed by them, forcing them to swell, press against each other, and fill the container with a highly viscous starch paste. Such highly swollen granules are easily broken and disintegrated by stirring, resulting in a decrease in viscosity. As starch granules swell, hydrated amylose molecules diffuse through the mass to the external phase (water), a phenomenon responsible for some aspects of paste behavior. Results of starch swelling can be recorded using a Brabender Visco/amylo/graph, which records the viscosity continuously as the temperature is increased, held constant for a time, and then decreased (Fig. 37). By the time peak viscosity is reached, some granules have been broken by stirring. With continued stirring, more granules rupture and fragment, causing a further decrease in viscosity. On cooling, some starch molecules partially reassociate to form a precipitate or gel. This process is called retrogradation (see Sec. 4.4.6). The firmness of the gel depends on the extent of junction zone formation (see Sec. 4.3.3). Junction zone formation is influenced (either facilitated or hindered) by the presence of other ingredients such as fats, proteins, sugars, and acids and the amount of water present. 4.4.5 Uses of Unmodified Starches Starches serve a variety of roles in food production. Principally they are used to take up water and to produce viscous fluids/pastes and gels and to give desired textural qualities (see also Sec. 4.4.9). The extent of starch gelatinization in baked goods strongly affects product properties, including storage behavior and rate of digestion. In some baked products, many starch granules remain ungelatinized. In certain cookies and pie crust that are high in fat and low in *A g lass is a mechanical solid capable of supporting its own weig ht ag ainst flow. A rubber is an undercooled liquid that can exhibit viscous flow. (See Chap. 2 for further details.) Pag e 197 FIGURE 37 Representative Brabender Visco/amylo/g raph curve showing viscosity chang es related to typical starch g ranule swelling and disinteg ration as a g ranule suspension is heated to 95°C and then held at that temperature. (The instrument imparts moderate shear to the system.) Tp is the pasting temperature, that is, the temperature at which a viscosity increase is recorded by the instrument. water, about 90% of the wheat starch granules remain ungelatinized (as observed microscopically and as evidenced by their slow rate of attack by amylases). In other products, such as angel food cake and white bread, which are higher in moisture, about 96% of the wheat starch granules are gelatinized and many become deformed. Food companies value the clear, cohesive pastes produced from waxy maize starch. Potato starch is used in extruded cereal and snack food products and in dry mixes for soups and cakes. Rice starch produces opaque gels useful for baby food. Waxy rice starch gels are clear and cohesive. Wheat starch gels are weak; their slight flavor may be due to residual flour components. Tuber (potato) and root (tapioca) starches have weak intermolecular bonding and swell greatly to give high-viscosity pastes. Because the highly swollen granules break easily, the viscosity quickly decreases with only moderate shear. Starches are often modified (see Sec. 4.4.9) before use in foods. Pag e 198 4.4.6 Retrogradation and Staling [36,37,44] As already pointed out, cooling a hot starch paste generally produces a viscoelastic, firm, rigid gel. The formation of the junction zones of a gel can be considered to be the first stage of an attempt by starch molecules to crystallize. As starch pastes are cooled and stored, the starch becomes progressively less soluble. In dilute solution, starch molecules will precipitate, with the insoluble material being difficult to redisolve by heating. The collective processes of dissolved starch becoming less soluble are called retrogradation. Retrogradation of cooked starch involves both the two constituent polymers, amylose and amylopectin, with amylose undergoing retrogradation at a much more rapid rate than does amylopectin. The rate of retrogradation depends on several variables, including the molecular ratio of amylose to amylopectin; structures of the amylose and amylopectin molecules, which are determined by the botanical source of the starch; temperature; starch concentration; and presence and concentration of other ingredients, such as surfactants and salts. Many quality defects in food products, such as bread staling and loss of viscosity and precipitation in soups and sauces, are due, at least in part, to starch retrogradation. Staling of baked goods is noted by an increase in crumb firmness and a loss in product freshness. Staling begins as soon as baking is complete and the product begins to cool. The rate of staling is dependent on the product formulation, the baking process, and storage conditions. Staling is due, at least in part, to the gradual transition of amorphous starch to a partially crystalline, retrograded state. In baked goods, where there is just enough moisture to gelatinize starch granules (while retaining a granule identity), amylose retrogradation (insolubilization) may be largely complete by the time the product has cooled to room temperature. Retrogradation of amylopectin is believed to involve primarily association of its outer branches and requires a much longer time than amylose retrogradation, giving it prominence in the staling process that occurs with time after the product has cooled. Most polar lipids with surfactant properties retard crumb firming. Compounds such as glyceryl monopalmitate (GMP), other monoglycerides and their derivatives, and sodium stearoyl 2-lactylate (SSL) are incorporated into doughs of bread and other baked goods, in part to increase shelf life. 4.4.7 Starch Complexes [3] Amylose chains are helical with hydrophobic (lipophilic) interiors and are able to form complexes with linear hydrophobic portions of molecules that can fit in the hydrophobic tube. Iodine (as I3-) complexes with both amylose and amylopectin molecules. Again, the complexing occurs within the hydrophobic interior of helical segments. With amylose, the long helical segments allow long chains of poly (I3-) to form and produce the blue color used as a diagnostic test for starch. The amyloseiodine complex contains 19% iodine, and determination of the amount of complexing is used to measure the amount of apparent amylose present in a starch. Amylopectin is colored a reddish-purple by iodine because the branches of amylopectin are too short for formation of long chains of poly (I3¯). Polar lipids (surfactants/emulsifiers and fatty acids) can affect starch pastes and starch-based foods in one or more of three ways as a result of complex formation: (a) by affecting the processes associated with starch gelatinization and pasting (that is, the loss of birefringence, granular swelling, leaching of amylose, melting of the crystalline regions of starch granules, and viscosity increases during cooking), (b) by modifying the rheological behavior of the resulting pastes, and (c) by inhibiting the crystallization of starch molecules associated with the retrogradation process. The specific changes observed upon addition of lipid depend on its structure, the starch employed, and the product to which it is added. Pag e 199 4.4.8 Hydrolysis of Starch [43,48,52] Starch molecules, like all other polysaccharide molecules, are depolymerized by hot acids. Hydrolysis of the glycosidic bonds occurs more or less randomly to produce, initially, very large fragments. Commercially, hydrochloric acid is sprayed onto wellmixed starch, or stirred moist starch is treated with hydrogen chloride gas; the mixture is then heated until the desired degree of depolymerization is obtained. The acid is then neutralized, and the product is recovered, washed, and dried. The products are still granular, but break up (cook out) easily. They are called acid-modified or thin-boiling starches, and the process of making them is called thinning. Even though only a few glycosidic bonds are hydrolyzed, the starch granules disintegrate much more easily during heating in water. Acid-modified starches form gels with improved clarity and increased strength, even though they provide less solution viscosity. Thin-boiling starches are used as film formers and adhesives in products such as pan-coated nuts and candies and whenever a strong gel is desired, such as in gum candies like jelly beans, jujubes, orange slices, and spearmint leaves and in processed cheese loaves. To prepare especially strong and fast-setting gels, a high-amylose corn starch is used as the base starch. More extensive modification with acid produces dextrins. Low-viscosity dextrins can be used at high concentrations in food processing. They have film-forming and adhesive properties and are used in products such as pan-coated roasted nuts and candy. They are also used as fillers, encapsulating agents, and carriers of flavors, especially spray-dried flavors. They are classified by their cold water solubility and color. Dextrins that retain large amounts of linear chains or long chain fragments form strong gels. Hydrolysis of starch dispersions with either an acid or an enzyme produces first maltodextrins. Maltodextrins are usually described by their dextrose equivalency (DE). The DE is related to the degree of polymerization (DP) through the following equation: DE = 100/DP. (Both DE and DP are average values for populations of molecules.) Therefore, the DE of a product of hydrolysis is its reducing power as a percentage of the reducing power of pure dextrose (D-glucose); thus, DE is inversely related to average molecular weight. Maltodextrins are defined as products with DE values that are measurable, but less than 20. Maltodextrins of lowest DE are nonhygroscopic, while those of highest DE (that is, lowest average molecule weight) tend to absorb moisture. Maltodextrins are bland with virtually no sweetness and are excellent for contributing body or bulk to food systems. Hydrolysis to DE values of 20–60 gives mixtures of molecules that, when dried, are called corn syrup solids. Corn syrup solids dissolve rapidly and are mildly sweet. Table 4 lists functional properties of starch hydrolysis products. Continued hydrolysis of starch produces a mixture of D-glucose, maltose, and other TABLE 4 Functional Properties of Starch Hydrolysis Products Properties enhanced by g reater hydrolysis a Properties enhanced in products of less conversion b Sweetness Viscosity production Hyg roscopicity and humectancy Body formation Freezing point depression Foam stabilization Flavor enhancement Sug ar crystallization prevention Fermentability Ice crystal g rowth prevention Browning reaction aHig h-conversion (hig h-DE) syrups. bLow-DE syrups and maltodextrins. Pag e 200 malto-oligosaccharides. Syrups of this composition are produced in enormous quantities. One of the most common has a DE of 42. These syrups are stable because crystallization does not occur easily in such complex mixtures. They are sold in concentrations of high osmolality, high enough that ordinary organisms cannot grow in them. An example is waffle and pancake syrup, which is colored with caramel coloring and flavored with maple flavoring. Three to four enzymes are used for the industrial hydrolysis of starch to D-glucose. a-Amylase is an endo-enzyme that cleaves both amylose and amylopectin molecules internally, producing oligosaccharides. The larger oligosaccharides may be singly, doubly, or triply branched via (1 6) linkages, since a-amylase acts only on the (1 4) linkages of starch. a-Amylase does not attack double-helical starch polymer segments or polymer segments complexed with a polar lipid (stabilized single helical segments). Glucoamylase (amyloglucosidase) is used commercially, in combination with an a-amylase, for producing D-glucose (dextrose) syrups and crystalline D-glucose. The enzyme acts upon fully gelatinized starch as an exo-enzyme, sequentially releasing single Dglucosyl units from the nonreducing ends of amylose and amloypectin molecules, even those joined through (1 6) bonds. Consequently, the enzyme can completely hydrolyze starch to D-glucose, but is used on starch that has been previously depolymerized with a-amylase to generate small fragments and more nonreducing ends. b-Amylase releases the disaccharide maltose sequentially from the nonreducing end of amylose. It also attacks the nonreducing ends of amylopectin, sequentially releasing maltose, but it cannot cleave the (1 6) linkages at branch points, so it leaves a pruned amylopectin residue termed a limit dextrin, specifically a beta-limit dextrin. There are several debranching enzymes that specifically catalyze hydrolysis of (1 6)-linkages in amylopectin, producing numerous linear but low-molecular-weight molecules. One such enzyme is isoamylase; another is pullulanase. Cyclodextrin glucanotransferase is a unique Bacillus enzyme that forms rings of (1 4)- linked a-D-glucopyranosyl units from starch polymers. The enzyme can form six-, seven-, and eight-membered rings, which are respectively alpha-, beta-, and gamma-cyclodextrins. Since the normal helical conformation of a linear portion of a starch molecule contains six to seven glucosyl units per turn of the helix, this transfer of a glycosidic bond from one that joins adjacent segments of a spiral to one that forms a doughnut-like circular structure is easy to picture. These products, originally called Schardinger dextrins after their discoverer, are now known as cyclodextrins or cycloamyloses. They have the ability to complex with hydrophobic substances that are held in the center of the ring. Through such complexing of guest molecules, volatile essential oils can be converted into dry powders in which the flavoring or aromatic substance is protected from light and oxygen but is readily released when the complex is added to an aqueous system because of the water solubility of the cyclodextrin. Cyclodextrins are not yet approved for food use in the United States. However, chiral supports that are useful for chromatographic separations are made by converting cyclodextrins into insoluble polymeric materials. In a similar application, insoluble polymeric beads of cyclodextrins have been shown to be useful for removal of bitter components of citrus juices. Corn syrup is the major source of D-glucose and D-fructose. To make a corn syrup, a slurry of starch in water is mixed with a thermally stable a-amylase and put through a cooker where rapid gelatinization and enzyme-catalyzed hydrolysis (liquefaction) takes place. After cooling to 55–60°C (130–140°F), hydrolysis is continued with glucoamylase, whereupon the syrup is clarified, concentrated, carbon-refined, and ion-exchanged. If the syrup is properly refined and combined with seed crystals, crystalline D-glucose (dextrose) or its monohydrate can be obtained. For production of D-fructose, a solution of D-glucose is passed through a column con- Pag e 201 taining bound (immobilized) glucose isomerase. The enzyme catalyzes the isomerization of D-glucose to D-fructose (see Fig. 5) to an equilibrium mixture of approximately 58% D-glucose and 42% D-fructose. Higher concentrations of D-fructose are usually desired. [The high-fructose corn syrup (HFCS) most often used as a soft drink sweetener is approximately 55% D-fructose.] So the isomerized syrup is passed through a bed of cation-exchange resin in the calcium salt form. The resin binds D-fructose, which can be recovered to provide an enriched syrup fraction. 4.4.9 Modified Food Starch [1,56,58] Food processors generally prefer starches with better behavioral characteristics than provided by native starches. Native starches produce particularly weak-bodied, cohesive, rubbery pastes when cooked and undesirable gels when the pastes are cooled. The properties of starches can be improved by modification. Modification is done so that resultant pastes can withstand the conditions of heat, shear, and acid associated with particular processing conditions and to introduce specific functionalities. Modified food starches are functional, useful, and abundant food macroingredients and additives. Types of modifications that are most often made, sometimes singly, but often in combinations, are crosslinking of polymer chains, non-crosslinking derivatization, depolymerization (see Sec. 4.4.8), and pregelatinization (see Sec. 4.4.10). Specific property improvements that can be obtained by proper combinations of modifications are reduction in the energy required to cook (improved gelatinization and pasting), modification of cooking characteristics, increased solubility, either increased or decreased paste viscosity, increased freeze-thaw stability of pastes, enhancement of paste clarity, increased paste sheen, inhibition of gel formation, enhancement of gel formation and gel strength, reduction of gel syneresis, improvement of interaction with other substances, improvement in stabilizing properties, enhancement of film formation, improvement in water resistance of films, reduction in paste cohesiveness, and improvement of stability to acid, heat, and shear. Starch, like all carbohydrates, can undergo reactions at its various hydroxyl groups. In modified food starches, only a very few of the hydroxyl groups are modified. Normally ester or ether groups are attached at very low DS values (degrees of substitution).* DS values are usually <0.1 and generally in the range 0.002–0.2. Thus, there is, on average, one substituent group on every 500-5 D-glucopyranosyl units. Small levels of derivatization change the properties of starches dramatically and greatly extend their usefulness. Starch products that are esterified or etherified with monofunctional reagents resist interchain associations. Derivatization of starches with monofunctional reagents reduces intermolecular associations, the tendency of the starch paste to gel, and the tendency for precipitation to occur. Hence this modification is often called stabilization, and the products are called stabilized starches. Use of difunctional reagents produces crosslinked starches. Modified food starches are often both cross-linked and stabilized. Chemical reactions currently both allowed and used to produce modified food starches in the United States are as follows: esterification with acetic anhydride, succinic anhydride, the mixed anhydride of acetic and adipic acids, 1-octenylsuccinic anhydride, phosphoryl chloride, sodium trimetaphosphate, sodium tripolyphosphate, and monosodium orthophosphate; etherification with propylene oxide; acid modification with hydrochloric and sulfuric acids; bleaching *The deg ree of substitution (DS) is defined as the averag e number of esterified or etherified hydroxyl g roups per monosaccharide unit. Both branched and unbranched polysaccharides composed of hexopyranosyl units have an averag e of three hydroxyl g roups per monomeric unit. Therefore, the maximum DS for a polysaccharide is 3.0. Pag e 202 FIGURE 38 Structures of starch monoester phosphate (left) and diester phosphate (rig ht). The diester joins two molecules tog ether, resulting in crosslinked starch g ranules. with hydrogen peroxide, peracetic acid, potassium permanganate, and sodium hypochlorite; oxidation with sodium hypochlorite; and various combinations of these reactions. Modified waxy maize starches are especially popular in the U.S. food industry. Pastes of unmodified common corn starch will gel, and the gels will generally be cohesive, rubbery, long textured, and prone to syneresis (that is, to weep or exude moisture). However, pastes of waxy maize starch show little tendency to gel at room temperature, which is why waxy maize starch is generally preferred as the base starch for food starches. But pastes of waxy maize starch will become cloudy and chunky and exhibit syneresis when stored under refrigerator or freezing conditions, so even waxy maize starch is usually modified to increase the stability of its pastes. The most common and useful derivatives employed for starch stabilization are the hydroxypropyl ether, the monostarch phosphate ester, and the acetate ester. Acetylation of starch to the maximum allowed in foods (DS 0.09) lowers the gelatinization temperature, improves paste clarity, and provides stability to retrogradation and freeze-thaw stability (but not as well as hydroxypropylation). Starch phosphate monoesters (Fig. 38) made by drying starch in the presence of sodium tripolyphosphate or monosodium orthophosphate can be used to make pastes that are clear and stable, have emulsifying properties, and have freeze-thaw stability. Monostarch phosphates have a long, cohesive texture. Paste viscosity is generally high and can be controlled by varying the concentration of reagent, time of reaction, temperature, and pH. Phosphate esterification lowers the gelatinization temperature. In the United States, the maximum allowable DS with phosphate groups is 0.002. Preparation of an alkenylsuccinate ester of starch attaches a hydrocarbon chain to its polymer molecules (Fig. 39). Even at very low degrees of substitution, starch 1-octenylsuccinate molecules concentrate at the interface of an oil-in-water emulsion because of the hydrophobicity of the alkenyl group. This characteristic makes them useful as emulsion stabilizers. Starch 1- octenylsuccinate products can be used in a variety of food applications where emulsion FIGURE 39 Preparation of starch 2-(1-octenyl)succinyl ester. Pag e 203 stability is needed, such as in flavored beverages. The presence of the aliphatic chain tends to give the starch derivative a sensory perception of fattiness, so it is possible to use the derivatives as a partial replacement for fat in certain foods. Hydroxypropylstarch (starch-O-CH2-CHOH-CH3), prepared by reacting starch with propylene oxide to produce a low level of etherification (DS 0.02–0.2, 0.2 being the maximum allowed) is a product with properties similar to those of starch acetate because it similarly has “bumps” along the starch polymer chains—that is, it is a stablized starch. Hydroxypropylation reduces the gelatinization temperature. Hydroxypropylstarches form clear pastes that do not retrograde and withstand freezing and thawing. They are used as thickeners and extenders. To improve viscosity, particularly under acidic conditions, acetylated and hydroxypropylated starches are often also crosslinked with phosphate groups. The majority of modified food starch is crosslinked. Crosslinking occurs when starch granules are reacted with difunctional reagents that react with hydroxyl groups on two different molecules within the granule. Crosslinking is accomplished most often by producing distarch phosphate esters. Starch is reacted with either phosphoryl chloride, PO3Cl2, or sodium trimetaphosphate in an alkaline slurry, then dried. Linking together of starch chains with phosphate diester or other crosslinks reinforces the granule and reduces both the rate and the degree of granule swelling and subsequent disintegration. Thus, granules exhibit reduced sensitivity to processing conditions (high temperature; extended cooking times; low pH; high shear during mixing, milling, homogenization, and/or pumping). Cooked pastes of crosslinked starches are more viscous, *heavier bodied, shorter textured, and less likely to break down during extended cooking or during exposure to low pH and/or severe agitation than are pastes of native starches from which they are prepared. Only a small amount of crosslinking is required to produce a noticeable effect; with lower levels of crosslinking, granules exhibit hydration swelling in inverse proportion to DS. As crosslinking is increased, the granules become more and more tolerant to physical conditions and acidity, but less and less dispersible by cooking. Energy requirements to reach maximum swelling and viscosity are also increased. For example, treatment of a starch with only 0.0025% of sodium trimetaphosphate greatly reduces both the rate and the degree of granule swelling, greatly increases paste stability, and changes dramatically the Brabender Visco/amylo/graph viscosity profile and textural characteristics of its paste. Treatment with 0.08% of trimetaphosphate produces a product in which granule swelling is restricted to the point that a peak visosity is never reached during the hot holding period in a Brabender Visco/amylo/graph. As the degree of crosslinking increases, the starch also becomes more acid stable. Though hydrolysis of glycosidic bonds occurs during heating in aqueous acid, chains tied to each other through phosphate crosslinks continue to provide large molecules and an elevated viscosity. The only other crosslink permitted in a food starch is the distrach ester of adipic acid. Most crosslinked food starches contain less than one crosslink per 1000 a-D-glucopyranosyl units. Trends toward continuous cooking require increased shear resistance and stability to hot surfaces. Storage-stable thickening is also provided by crosslinked starches. In retort sterilization of canned foods, crosslinked starches, because of their reduced rate of gelatinization and swelling, maintain a low initial viscosity long enough to facilitate the rapid heat transfer and temperature rise that is needed to provide uniform sterilization before granule swelling brings about the ultimately desired viscosity, texture, and suspending characteristics. Crosslinked *Note in Fig ure 37 that maximum viscosity is reached when the system contains hig hly swollen g ranules. Crosslinked g ranules hold tog ether. Thus, there is minimal loss of viscosity after the peak is reached. Pag e 204 starches are used in canned soups, gravies, and puddings and in batter mixes. Crosslinking of waxy maize starch gives the clear paste sufficient rigidity so that, when used in pie fillings, cut sections of pie hold their shape. Starches that are both crosslinked and stabilized are used in canned, frozen, baked, and dry foods. In baby foods and fruit and pie fillings in cans and jars, they provide long shelf life. They also allow frozen fruit pies, pot pies, and gravies to remain stable under long-term storage. Modified food starches are tailor-made for specific applications. Properties that can be controlled by combinations of crosslinking, stabilization, and thinning of corn, waxy maize, potato, and other starches include, but are not limited to, the following: adhesion, clarity of solutions/pastes, color, emulsion stabilization ability, film-forming ability, flavor release, hydration rate, moisture holding capacity, stability to acids, stability to heat and cold, stability to shear, temperature required to cook, and viscosity (hot paste and cold paste). Some characteristics imparted to the food product include, but are not limited to, the following: mouth feel, reduction of oil migration, texture, sheen, stability, and tackiness. 4.4.10 Cold-Water-Soluble (Pregelatinized) Starch Once starch has been pasted and dried without excessive retrogradation, it can be redissolved in cold water. The largest commercial quantity of such starch is made by flowing a starch-water slurry into the nip between two nearly touching and counterrotating, steam-heated rolls. The starch slurry is gelatinized and pasted almost instantaneously, and the paste coats the rolls where it dries quickly. The dry film is scraped from the roll and ground. The resulting products, known as pregelatinized starches or instant starches, are precooked starches. They are also prepared using extruders. Both chemically modified and unmodified starches can be used to make pregelatinized starches. If chemically modified starches are used, properties introduced by the modification(s) carry through to the pregelatinized products; thus, paste properties such as stability to freeze-thaw cycling can also be characteristics of pregelatinized starches. Pregelatinized, slightly crosslinked starch is useful in instant soup, pizza topping, and extruded snacks and in breakfast cereals. Pregelatinized starches can be used without cooking. Finely ground pregelatinized starch forms small gel particles like a watersoluble gum, but when properly dissolved gives solutions of high viscosity. Coarsely ground products “dissolve” much more readily and produce dispersions of lower viscosity and with graininess or pulpiness that is desirable in some products. Many pregelatinized starches are used in dry mixes such as instant pudding mixes; they disperse readily with high-shear stirring or when mixed with sugar or other dry ingredients. 4.4.11 Cold-Water-Swelling Starch Granular starch that swells extensively in cold water is made by heating common corn starch in 75–90% ethanol or by a special spray-drying process. The product is dispersible in sugar solutions or corn syrups by rapid stirring; the resulting dispersion can be poured into molds, where it sets to a rigid gel that can be sliced easily. The result is a gum candy. Cold-water-swelling starch is also useful in making desserts and in muffin batters containing particles, such as blueberries, that otherwise would settle to the bottom before the batter is thickened by heating during baking. Pag e 205 4.5 Cellulose: Modifications and Derivatives [60] Cellulose is the most abundant organic compound, and therefore the most abundant carbohydrate, on earth because it is the principal cell-wall component of higher plants. (It can be argued that D-glucose is the most abundant carbohydrate and organic compound if we consider cellulose as a combined form of this monomeric building block.) Cellulose is a high-molecular-weight, linear, insoluble homopolymer of repeating b-D-glucophyranosyl units joined by (1 4) glycosidic linkages (Fig. 40). Because of their linearity and stereoregular nature, cellulose molecules associate over extended regions, forming polycrystalline, fibrous bundles. Crystalline regions are held together by large numbers of hydrogen bonds. They are separated by, and connected to, amorphous regions. Cellulose is insoluble because, in order for it to dissolve, most of these hydrogen bonds would have to be released at once. Cellulose can, however, through substitution, be converted into water-soluble gums. Cellulose and its modified forms serve as dietary fiber because they do not contribute significant nourishment or calories as they pass through the human digestive system. Dietary fiber does, however, serve important functions (see Sec. 4.12). A purified cellulose powder is available as a food ingredient. High-quality cellulose can be obtained from wood through pulping and subsequent purification. Chemical purity is not required for food use because cellulosic cell-wall materials are components of all fruits and vegetables and many of their products. The powdered cellulose used in foods has negligible flavor, color, and microbial contamination. Powdered cellulose is most often added to bread to provide noncaloric bulk. Reduced-calorie baked goods made with powdered cellulose, not only have an increased content of dietary fiber, but also stay moist and fresh longer. 4.5.1 Microcrystalline Cellulose [51] A purified, insoluble cellulose termed microcrystalline cellulose (MCC) is useful in the food industry. It is made by hydrolysis of purified wood pulp, followed by separation of the constituent microcrystals of cellulose. Cellulose molecules are fairly rigid, completely linear chains of about 3000 b-D-glucopyranosyl units and associate easily in long junction zones. However, the long and unwieldy chains do not align over their entire lengths. The end of the crystalline region is simply the divergence of cellulose chains away from order into a more random arrangement. When purified wood pulp is hydrolyzed with acid, the acid penetrates the lower density, amorphous regions, effects hydrolytic cleavage of chains in these regions, and releases individual, fringed crystallites. The released crystallites grow larger because the chains that constitute the fringes now have greater freedom of motion and can order themselves. Two types of microcrystalline cellulose are produced, both of which are stable to both heat FIGURE 40 Cellulose (repeating unit). Pag e 206 and acids. Powdered MCC is a spray-dried product. Spray-drying produces agglomerated aggregates of microcrystals that are porous and sponge-like. Powdered MCC is used primarily as a flavor carrier and as an anticaking agent for shredded cheese. The second type, colloidal MCC, is water dispersible and has functional properties similar to those of water-soluble gums. To make colloidal MCC, considerable mechanical energy is applied after hydrolysis to tear apart the weakened microfibrils and provide a major proportion of colloidal-sized aggregates (<0.2 µm in diameter). To prevent rebonding of the aggregates during drying, sodium carboxymethylcellulose (CMC) is added (see Sec. 4.5.2). CMC aids in redispersion and acts as a barrier to reassociation by giving the particles a stabilizing negative charge. The major functions of colloidal MCC are to stabilize foams and emulsions, especially during high-temperature processing; to form gels with salve-like textures (MCC does not dissolve, nor does it form intermolecular junction zones; rather it forms a network of hydrated microcrystals); to stabilize pectin and starch gels to heat; to improve adhesion; to replace fat and oil, and to control ice crystal growth. MCC stabilizes emulsions and foams by adsorbing at interfaces and strengthening interfacial films. It is a common ingredient of reduced-fat ice cream and other frozen dessert products. 4.5.2 Carboxymethylcellulose [11,26] Carboxymethylcellulose (CMC) is widely and extensively used as a food gum. Treatment of purified wood pulp with 18% sodium hydroxide solution produces alkali cellulose. When alkali cellulose is reacted with the sodium salt of chloroacetic acid, the sodium salt of the carboxymethyl ether (cellulose-O-CH2-CO2-Na+) is formed (Table 2). Most commercial sodium carboxymethylcellulose (CMC) products have a degree of substitution (DS) in the range 0.4–0.8. The most widely sold type for use as a food ingredient has a DS of 0.7. Since CMC consists of long, fairly rigid molecules that bear a negative charge due to numerous ionized carboxyl groups, electrostatic repulsion causes its molecules in solution to be extended. Also, adjacent chains repel each other. Consequently, CMC solutions tend to be both highly viscous and stable. CMC is available in a wide range of viscosity types. CMC stabilizes protein dispersions, especially near their isoelectric pH value. Thus, egg white is stabilized with CMC for codrying or freezing, and milk products are given improved stability against casein precipitation. 4.5.3 Methylcelluloses and Hydroxypropylmethylcelluloses [16,17] Alkali cellulose is treated with methyl chloride to introduce methyl ether groups (cellulose-O-CH3). Many members of this class of gums also contain hydroxypropyl ether groups (cellulose-O-CH2-CHOH-CH3). Hydroxypropylmethylcelluloses (HPMC) are made by reacting alkali cellulose with both propylene oxide and methyl chloride. The degree of substitution with methyl ether groups of commercial methylcelluloses (MC) ranges from 1.1 to 2.2. The moles of substitution (MS) *values with hydroxypropyl ether groups in commercial hydroxypropylmethylcelluloses range from 0.02 to 0.3. (Both the methylcellulose and hydroxypropylmethyl- *The moles of substitution or molar substitution (MS) is the averag e number of moles of substituent attached to a g lycosyl unit of a polysaccharide. Because reaction of a hydroxyl g roup with propylene oxide creates a new hydroxyl g roup with which propylene oxide can react further, poly(propylene oxide) chains, each terminated with a free hydroxyl g roup, can form. Thus, because more than three moles of propylene oxide can react with a sing le hexopyranosyl unit, MS rather than DS must be used. Pag e 207 cellulose members of this gum family are generally referred to simply as methylcelluloses.) Both products are cold-water soluble because the methyl and hydroxypropyl ether group protrusions along the chains prevent the intermolecular association characteristic of cellulose. While a few added ether groups spread along the chains enhance water solubility of cellulose (by decreasing internal hydrogen bonding), they also decrease chain hydration by replacing water-binding hydroxyl groups with less polar ether groups, giving members of this family unique characteristics. The ether groups restrict solvation of the chains to the point that they are on the borderline of water solubility. Hence, when an adqueous solution is heated, the water molecules of polymer solvation dissociate from the chain and hydration is decreased sufficiently that intermolecular associations increase and gelation occurs. Reducing the temperature once again brings about solubility, so the gelation is reversible. Because of the ether groups, the gum chains are somewhat surface active and absorb at interfaces. This helps stabilize emulsions and foams. Methylcelluloses also can be used to reduce the amount of fat in food products through two mechanisms: (a) they provide fat-like properties so that the fat content of a product can be reduced, and (b) they reduce adsorption of fat in products being fried. The gel structure produced by thermogelation provides a barrier to oil, holds moisture, and acts as a binder. 4.6 Guar and Locust Bean Gums [19,20,31] Guar and locust bean gums are important thickening polysaccharides for both food and nonfood uses (Table 2). Guar gum produces the highest viscosity of any natural, commercial gum. Both gums are the ground endosperm of seeds. The main component of both endosperms is a galactomannan. Galactomannans consist of a main chain of b-D-mannopyranosyl units joined by (1 4) bonds with single-unit a-D-galactopyranosyl branches attached at O-6 (Fig. 41). The specific polysaccharide component of guar gum is guaran. In guaran, about one-half of the D-mannopyranosyl main-chain units contain a D-galactopyranosyl side chain. The galactomannan of locust bean gum (LBG, also called carob gum) has fewer branch units than does guaran and its structure is more irregular, with long stretches of about 80 underivatized D-mannosyl units alternating with sections of about 50 units in which almost every main chain unit has an a-D-galactopyranosyl group glycosidically connected to its O-6 position. Because of the difference in structures, guar gum and LBG have different physical

Leave a Reply

Your email address will not be published.