What is Food Chemistry?

Concern about food exists throughout the world, but the aspects of concern differ with location. In underdeveloped regions of the world, the bulk of the population is involved in food production, yet attainment of adequate amounts and kinds of basic nutrients remains an ever-present problem. In developed regions of the world, food production is highly mechanized and only a small fraction of the population is involved in this activity. Food is available in abundance, much of it is processed, and the use of chemical additives is common. In these fortunate localities, concerns about food relate mainly to cost, quality, variety, convenience, and the effects of processing and added chemicals on wholesomeness and nutritive value. All of these concerns fall within the realm of food science—a science that deals with the physical, chemical, and biological properties of foods as they relate to stability, cost, quality, processing, safety, nutritive value, wholesomeness, and convenience. Food science is an interdisciplinary subject involving primarily bacteriology, chemistry, Pag e 2 biology, and engineering. Food chemistry, a major aspect of food science, deals with the composition and properties of food and the chemical changes it undergoes during handling, processing, and storage. Food chemistry is intimately related to chemistry, biochemistry, physiological chemistry, botany, zoology, and molecular biology. The food chemist relies heavily on knowledge of the aforementioned sciences to effectively study and control biological substances as sources of human food. Knowledge of the innate properties of biological substances and mastery of the means of manipulating them are common interests of both food chemists and biological scientists. The primary interests of biological scientists include reproduction, growth, and changes that biological substances undergo under environmental conditions that are compatible or marginally compatible with life. To the contrary, food chemists are concerned primarily with biological substances that are dead or dying (postharvest physiology of plants and postmortem physiology of muscle) and changes they undergo when exposed to a very wide range of environmental conditions. For example, conditions suitable for sustaining residual life processes are of concern to food chemists during the marketing of fresh fruits and vegetables, whereas conditions incompatible with life processes are of major interest when long-term preservation of food is attempted. In addition, food chemists are concerned with the chemical properties of disrupted food tissues (flour, fruit and vegetable juices, isolated and modified constituents, and manufactured foods), single-cell sources of food (eggs and microorganisms), and one major biological fluid, milk. In summary, food chemists have much in common with biological scientists, yet they also have interests that are distinctly different and are of the utmost importance to humankind. 1.2 History of Food Chemistry The origins of food chemistry are obscure, and details of its history have not yet been rigorously studied and recorded. This is not surprising, since food chemistry did not acquire a clear identity until the twentieth century and its history is deeply entangled with that of agricultural chemistry for which historical documentation is not considered exhaustive [5,14]. Thus, the following brief excursion into the history of food chemistry is incomplete and selective. Nonetheless, available information is sufficient to indicate when, where, and why certain key events in food chemistry occurred, and to relate some of these events to major changes in the wholesomeness of the food supply since the early 1800s. Although the origin of food chemistry, in a sense, extends to antiquity, the most significant discoveries, as we judge them today, began in the late 1700s. The best accounts of developments during this period are those of Filby [12] and Browne [5], and these sources have been relied upon for much of the information presented here. During the period of 1780–1850 a number of famous chemists made important discoveries, many of which related directly or indirectly to the chemistry of food. The works of Scheele, Lavoisier, de Saussure, Gay-Lussac, Thenard, Davy, Berzelius, Thomson, Beaumont, and Liebig contain the origins of modern food chemistry. Some may question whether these scientists, whose most famous discoveries bear little relationship to food chemistry, deserve recognition as major figures in the origins of modern food chemistry. Although it is admittedly difficult to categorize early scientists as chemists, bacteriologists, or food chemists, it is relatively easy to determine whether a given scientist made substantial contributions to a given field of science. From the following brief examples it is clearly evident that many of these scientists studied foods intensively and made discoveries of such fundamental importance to food chemistry that exclusion of their contributions from any historical account of food chemistry would be inappropriate. Carl Wilhelm Scheele (1742–1786), a Swedish pharmacist, was one of the greatest Pag e 3 chemists of all time. In addition to his more famous discoveries of chlorine, glycerol, and oxygen ( chemists of all time. In addition to his more famous discoveries of chlorine, glycerol, and oxygen (3 years before Priestly, but unpublished), he isolated and studied the properties of lactose (1780), prepared mucic acid by oxidation of lactic acid (1780), devised a means of preserving vinegar by means of heat (1782, well in advance of Appert’s “discovery”), isolated citric acid from lemon juice (1784) and gooseberries (1785), isolated malic acid from apples (1785), and tested 20 common fruits for the presence of citric, malic, and tartaric acids (1785). His isolation of various new chemical compounds from plant and animal substances is considered the beginning of accurate analytical research in agricultural and food chemistry. The French chemist Antoine Laurent Lavoisier (1743–1794) was instrumental in the final rejection of the phlogiston theory and in formulating the principles of modern chemistry. With respect to food chemistry, he established the fundamental principles of combustion organic analysis, he was the first to show that the process of fermentation could be expressed as a balanced equation, he made the first attempt to determine the elemental composition of alcohol (1784), and he presented one of the first papers (1786) on organic acids of various fruits. (Nicolas) Théodore de Saussure (1767–1845), a French chemist, did much to formalize and clarify the principles of agricultural and food chemistry provided by Lavoisier. He also studied CO2 and O2 changes during plant respiration (1840), studied the mineral contents of plants by ashing, and made the first accurate elemental analysis of alcohol (1807). Joseph Louis Gay-Lussac (1778–1850) and Louis-Jacques Thenard (1777–1857) devised in 1811 the first method to determine percentages of carbon, hydrogen, and nitrogen in dry vegetable substances. The English chemist Sir Humphrey Davy (1778–1829) in the years 1807 and 1808 isolated the elements K, Na, Ba, Sr, Ca, and Mg. His contributions to agricultural and food chemistry came largely through his books on agricultural chemistry, of which the first (1813) was Elements of Agriculture Chemistry, in a Course of Lectures for the Board of Agriculture [8]. His books served to organize and clarify knowledge existing at that time. In the first edition he stated, All the different parts of plants are capable of being decomposed into a few elements. Their uses as food, or for the purpose of the arts, depend upon compound arrang ements of these elements, which are capable of being produced either from their org anized parts, or from the juices they contain; and the examination of the nature of these substances is an essential part of ag ricultural chemistry. In the fifth edition he stated that plants are usually composed of only seven or eight elements, and that [9] “the most essential vegetable substances consist of hydrogen, carbon, and oxygen in different proportion, generally alone, but in some few cases combined with azote [nitrogen]” (p. 121). The works of the Swedish chemist Jons Jacob Berzelius (1779–1848) and the Scottish chemist Thomas Thomson (1773–1852) resulted in the beginnings of organic formulas, “without which organic analysis would be a trackless desert and food analysis an endless task” [12]. Berzelius determined the elemental components of about 2000 compounds, thereby verifying the law of definite proportions. He also devised a means of accurately determining the water content of organic substances, a deficiency in the method of Gay-Lussac and Thenard. Moreover, Thomson showed that laws governing the composition of inorganic substances apply equally well to organic substances, a point of immense importance. In a book entitled Considérations générales sur l’ analyse organique et sur ses applications [6], Michel Eugene Chevreul (1786–1889), a French chemist, listed the elements known to exist at that time in organic substances (O, Cl, I, N, S, P, C, Si, H, Al, Mg, Ca, Na, K, Mn, Fe) and cited the processes then available for organic analysis: (a) extraction with a neutral solvent, such as water, alcohol, or aqueous ether, (b) slow distillation, or fractional distillation, Pag e 4 (c) steam distillation, (d) passing the substance through a tube heated to incandescence, and (e) analysis with oxygen. Chevreul was a pioneer in the analysis of organic substances, and his classic research on the composition of animal fat led to the discovery and naming of stearic and oleic acids. Dr. William Beaumont (1785–1853), an American Army surgeon stationed at Fort Mackinac, Mich., performed classic experiments on gastric digestion that destroyed the concept existing from the time of Hippocrates that food contained a single nutritive component. His experiments were performed during the period 1825–1833 on a Canadian, Alexis St. Martin, whose musket wound afforded direct access to the stomach interior, thereby enabling food to be introduced and subsequently examined for digestive changes [4]. Among his many notable accomplishments, Justus von Liebig (1803–1873) showed in 1837 that acetaldehyde occurs as an intermediate between alcohol and acetic acid during fermentation of vinegar. In 1842 he classified foods as either nitrogenous (vegetable fibrin, albumin, casein, and animal flesh and blood) or nonnitrogenous (fats, carbohydrates, and alcoholic beverages). Although this classification is not correct in several respects, it served to distinguish important differences among various foods. He also perfected methods for the quantitative analysis of organic substances, especially by combustion, and he published in 1847 what is apparently the first book on food chemistry, Researches on the Chemistry of Food [18]. Included in this book are accounts of his research on the water-soluble constituents of muscle (creatine, creatinine, sarcosine, inosinic acid, lactic acid, etc.). It is interesting that the developments just reviewed paralleled the beginning of serious and widespread adulteration of food, and it is no exaggeration to state that the need to detect impurities in food was a major stimulus for the development of analytical chemistry in general and analytical food chemistry in particular. Unfortunately, it is also true that advances in chemistry contributed somewhat to the adulteration of food, since unscrupulous purveyors of food were able to profit from the availability of chemical literature, including formulas for adulterated food, and could replace older, less effective empirical approaches to food adulteration with more efficient approaches based on scientific principles. Thus, the history of food chemistry and the history of food adulteration are closely interwoven by the threads of several causative relationships, and it is therefore appropriate to consider the matter of food adulteration from a historical perspective [12]. The history of food adulteration in the currently more developed countries of the world falls into three distinct phases. From ancient times to about 1820 food adulteration was not a serious problem and there was little need for methods of detection. The most obvious explanation for this situation was that food was procured from small businesses or individuals, and transactions involved a large measure of interpersonal accountability. The second phase began in the early 1800s, when intentional food adulteration increased greatly in both frequency and seriousness. This development can be attributed primarily to increased centralization of food processing and distribution, with a corresponding decline in interpersonal accountability, and partly to the rise of modern chemistry, as already mentioned. Intentional adulteration of food remained a serious problem until about 1920, which marks the end of phase two and the beginning of phase three. At this point regulatory pressures and effective methods of detection reduced the frequency and seriousness of intentional food adulteration to acceptable levels, and the situation has gradually improved up to the present time. Some would argue that a fourth phase of food adulteration began about 1950, when foods containing legal chemical additives became increasingly prevalent, when the use of highly processed foods increased to a point where they represented a major part of the diet of persons in most of the industrialized countries, and when contamination of some foods with undesirable byproducts of industrialization, such as mercury, lead, and pesticides, became of public and Pag e 5 regulatory concern. The validity of this contention is hotly debated and disagreement persists to this day. Nevertheless, the course of action in the next few years seems clear. Public concern over the safety and nutritional adequacy of the food supply has already led to some recent changes, both voluntary and involuntary, in the manner in which foods are produced, handled, and processed, and more such actions are inevitable as we learn more about proper handling practices for food and as estimates of maximum tolerable intake of undesirable constituents become more accurate. The early 1800s was a period of especially intense public concern over the quality and safety of the food supply. This concern, or more properly indignation, was aroused in England by Frederick Accum’s publication A Treatise on Adulterations of Food [1] and by an anonymous publication entitled Death in the Pot [3]. Accum claimed that “Indeed, it would be difficult to mention a single article of food which is not to be met with in an adulterated state; and there are some substances which are scarcely ever to be procured genuine” (p. 14). He further remarked, “It is not less lamentable that the extensive application of chemistry to the useful purposes of life, should have been perverted into an auxiliary to this nefarious traffic [adulteration]” (p. 20). Although Filby [12] asserted that Accum’s accusations were somewhat overstated, the seriousness of intentional adulteration of food that prevailed in the early 1800s is clearly exemplified by the following not uncommon adulterants cited by both Accum and Filby: Annatto: Adulterants included turmeric, rye, barley, wheat flour, calcium sulfate and carbonate, salt, and Venetian red (ferric oxide, which in turn was sometimes adulterated with red lead and copper). Pepper, black: This important product was commonly adulterated with gravel, leaves, twigs, stalks, pepper dust, linseed meal, and ground parts of plants other than pepper. Pepper, cayenne. Substances such as vermillion (a-mercury sulfide), ocher (native earthy mixtures of metallic oxides and clay), and turmeric were commonly added to overcome bleaching that resulted from exposure to light. Essential oils: Oil of turpentine, other oils, and alcohol. Vinegar: Sulfuric acid Lemon juice: Sulfuric and other acids Coffee: Roasted grains, occasionally roasted carrots or scorched beans and peas; also, baked horse liver. Tea: Spent, redried tea leaves, and leaves of many other plants. Milk: Watering was the main form of adulteration; also, the addition of chalk, starch, turmeric (color), gums, and soda was common. Occasionally encountered were gelatin, dextrin, glucose, preservatives (borax, boric acid, salicylic acid, sodium salicylate, potassium nitrate, sodium fluoride, and benzoate), and such colors as annatto, saffron, caramel, and some sulfonated dyes. Beer: “Black extract,” obtained by boiling the poisonous berries of Cocculus indicus in water and concentrating the fluid, was apparently a common additive. This extract imparted flavor, narcotic properties, additional intoxicating qualities, and toxicity to the beverage. Wine: Colorants: alum, husks of elderberries, Brazil wood, and burnt sugar, among others. Flavors: bitter almonds, tincture of raisin seeds, sweet-brier, oris root, and others. Aging agents: bitartrate of potash, “oenathis” ether (heptyl ether), and lead salts. Preservatives: salicylic acid, benzoic acid, fluoborates, and lead salts. Antacids: lime, chalk, gypsum, and lead salts. Sugar: Sand, dust, lime, pulp, and coloring matters. Butter: Excessive salt and water, potato flour, and curds. Pag e 6 Chocolate: Starch, ground sea biscuits, tallow, brick dust, ocher, Venetian red (ferric oxide), and potato flour. Bread: Alum, and flour made from products other than wheat. Confectionery products: Colorants containing lead and arsenic. Once the seriousness of food adulteration in the early 1800s was made evident to the public, remedial forces gradually increased. These took the form of new legislation to make adulteration unlawful, and greatly expanded efforts by chemists to learn about the native properties of foods, the chemicals commonly used as adulterants, and the means of detecting them. Thus, during the period 1820–1850, chemistry and food chemistry began to assume importance in Europe. This was possible because of the work of the scientists already cited, and was stimulated largely by the establishment of chemical research laboratories for young students in various universities and by the founding of new journals for chemical research [5]. Since then, advances in food chemistry have continued at an accelerated pace, and some of these advances, along with causative factors, are mentioned below. In 1860, the first publicly supported agriculture experiment station was established in Weede, Germany, and W. Hanneberg and F. Stohmann were appointed director and chemist, respectively. Based largely on the work of earlier chemists, they developed an important procedure for the routine determination of major constituents in food. By dividing a given sample into several portions they were able to determine moisture content, “crude fat,” ash, and nitrogen. Then, by multiplying the nitrogen value by 6.25, they arrived at its protein content. Sequential digestion with dilute acid and dilute alkali yielded a residue termed “crude fiber.” The portion remaining after removal of protein, fat, ash, and crude fiber was termed “nitrogen-free extract,” and this was believed to represent utilizable carbohydrate. Unfortunately, for many years chemists and physiologists wrongfully assumed that like values obtained by this procedure represented like nutritive value, regardless of the kind of food [20]. In 1871, Jean Baptiste Duman (1800–1884) suggested that a diet consisting of only protein, carbohydrate, and fat was inadequate to support life. In 1862, the Congress of the United States passed the Land-Grant College Act, authored by Justin Smith Morrill. This act helped establish colleges of agriculture in the United States and provided considerable impetus for the training of agricultural and food chemists. Also in 1862, the United States Department of Agriculture was established and Isaac Newton was appointed the first commissioner. In 1863, Harvey Washington Wiley became chief chemist of the U.S. Department of Agriculture, from which office he led the campaign against misbranded and adulterated food, culminating in passage of the first Pure Food and Drug Act in the United States (1906). In 1887, agriculture experiment stations were established in the United States following enactment of the Hatch Act. Representative William H. Hatch of Missouri, Chairman of the House Committee on Agriculture, was author of the act. As a result, the world’s largest national system of agriculture experiment stations came into existence, and this had a great impact on food research in the United States. During the first half of the twentieth century, most of the essential dietary substances were discovered and characterized, namely, vitamins, minerals, fatty acids, and some amino acids. The development and extensive use of chemicals to aid in the growth, manufacture, and marketing of foods was an especially noteworthy and contentious event in the middle 1900s. This historical review, although brief, makes the current food supply seem almost perfect in comparison to that which existed in the 1800s. Pag e 7 1.3 Approach to the Study of Food Chemistry It is desirable to establish an analytical approach to the chemistry of food formulation, processing, and storage stability, so that facts derived from the study of one food or model system can enhance our understanding of other products. There are four components to this approach: (a) determining those properties that are important characteristics of safe, high-quality foods, (b) determining those chemical and biochemical reactions that have important influences on loss of quality and/or wholesomeness of foods, (c) integrating the first two points so that one understands how the key chemical and biochemical reactions influence quality and safety, and (d) applying this understanding to various situations encountered during formulation, processing, and storage of food. 1.3.1 Quality and Safety Attributes It is essential to reiterate that safety is the first requisite of any food. In a broad sense, this means a food must be free of any harmful chemical or microbial contaminant at the time of its consumption. For operational purposes this definition takes on a more applied form. In the canning industry, “commercial” sterility as applied to low-acid foods means the absence of viable spores of Clostridium botulinum. This in turn can be translated into a specific set of heating conditions for a specific product in a specific package. Given these heating requirements, one can then select specific time-temperature conditions that will optimize retention of quality attributes. Similarly, in a product such as peanut butter, operational safety can be regarded primarily as the absence of aflatoxins—carcinogenic substances produced by certain species of molds. Steps taken to prevent growth of the mold in question may or may not interfere with retention of some other quality attribute; nevertheless, conditions producing a safe product must be employed. A list of quality attributes of food and some alterations they can undergo during processing and storage is given in Table 1. The changes that can occur, with the exception of those involving nutritive value and safety, are readily evident to the consumer. 1.3.2 Chemical and Biochemical Reactions Many reactions can alter food quality or safety. Some of the more important classes of these reactions are listed in Table 2. Each reaction class can involve different reactants or substrates depending on the specific food and the particular conditions for handling, processing, or storage. They are treated as reaction classes because the general nature of the substrates or reactants is similar for all foods. Thus, nonenzymic browning involves reaction of carbonyl compounds, which can arise from existing reducing sugars or from diverse reactions, such as oxidation of ascorbic acid, hydrolysis of starch, or oxidation of lipids. Oxidation may involve lipids, proteins, vitamins, or pigments, and more specifically, oxidation of lipids may involve triacylglycerols in one food or phospholipids in another. Discussion of these reactions in detail will occur in subsequent chapters of this book. 1.3.3 Effect of Reactions on the Quality and Safety of Food The reactions listed in Table 3 cause the alterations listed in Table 1. Integration of the information contained in both tables can lead to an understanding of the causes of food deterioration. Deterioration of food usually consists of a series of primary events followed by TABLE 1 Classification of Alterations That Can Occur in Food During Handling , Processing , or Storag e Attribute Alteration Texture Loss of solubility Loss of water-holding capacity Toug hening Softening Flavor Development of: Rancidity (hydrolytic or oxidative) Cooked or caramel flavors Other off-flavors Desirable flavors Color Darkening Bleaching Development of other off-colors Development of desirable colors (e.g ., browning of baked g oods) Nutritive value Loss, deg radation or altered bioavailability of proteins, lipids, vitamins, minerals Safety Generation of toxic substances Development of substances that are protective to health Inactivation of toxic substances secondary events, which, in turn, become evident as altered quality attributes (Table 1). Examples of sequences of this type are shown in Table 3. Note particularly that a given quality attribute can be altered as a result of several different primary events. The sequences in Table 3 can be applied in two directions. Operating from left to right one can consider a particular primary event, the associated secondary events, and the effect on a TABLE 2 Some Chemical and Biochemical Reactions That Can Lead to Alteration of Food Quality or Safety Types of reaction Examples Nonenzymic browning Baked g oods Enzymic browning Cut fruits Oxidation Lipids (off-flavors), vitamin deg radation, pig ment decoloration, proteins (loss of nutritive value) Hydrolysis Lipids, proteins, vitamins, carbohydrates, pig ments Metal interactions Complexation (anthocyanins), loss of Mg from chlorophyll, catalysis of oxidation Lipid isomerization Cis trans, nonconjug ated conjug ated Lipid cyclization Monocyclic fatty acids Lipid polymerization Foaming during deep fat frying Protein denaturation Eg g white coag ulation, enzyme inactivation Protein cross-linking Loss of nutritive value during alkali processing Polysaccharide synthesis In plants postharvest Glycolytic chang es Animal tissue postmortem, plant tissue postharvest Pag e 9 TABLE 3 Cause-and-Effect Relationships Pertaining to Food Alterations During Handling , Storag e, and Processing Primary causative event Secondary event Attribute influenced (see Table 1) Hydrolysis of lipids Free fatty acids react with protein Texture, flavor, nutritive value Hydrolysis of polysaccharides Sug ars react with proteins Texture, flavor, color, nutritive value Oxidation of lipids Oxidation products react with many other constituents Texture, flavor, color, nutritive value; toxic substances can be g enerated Bruising of fruit Cells break, enzymes are released, oxyg en accessible Texture, flavor, color, nutritive value Heating of g reen veg etables Cell walls and membranes lose integ rity, acids are released, enzymes become inactive Texture, flavor, color, nutritive value Heating of muscle tissue Proteins denature and ag g reg ate, enzymes become inactive Texture, flavor, color, nutritive value Cis trans conversions in lipids Enhanced rate of polymerization during deep fat frying Excessive foaming during deep fat frying ; diminished bioavailability of lipids quality attribute. Alternatively, one can determine the probable cause(s) of an observed quality change (column 3, Table 3) by considering all primary events that could be involved and then isolating, by appropriate chemical tests, the key primary event. The utility of constructing such sequences is that they encourage one to approach problems of food alteration in an analytical manner. Figure 1 is a simplistic summary of reactions and interactions of the major constituents

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