2 Combined Methods Approach to Food Stability

At this point, it is hoped that the reader will have realized that RVP and Mm are useful estimators of food stability but that neither is totally sufficient by itself. It has been mentioned several times that factors not accommodated by either of these approaches can have important influences on food stability and safety. The “combined methods approach” to controlling microbial growth in foods was developed specifically to deal with this reality. This approach is mentioned in a book on food chemistry because it demonstrates convincingly that (a) the RVP approach to controlling microbial growth is often inadequate when used alone, (b) the RVP approach, because it is based on a single parameter, is not a totally reliable predictor of chemical stability, and (c) the Mm approach, because it too is based on a single parameter, is unlikely to be a totally reliable predictor of chemical stability. The combined methods approach (originally called “hurdle approach”) was developed by Professor L. Leistner and others to determine conditions needed to limit the growth of microorganisms in nonsterile foods [57,58,59]. This approach involves manipulating various growth-controlling parameters in a manner such that growth will not occur; each parameter is a “hurdle” to microbial growth. The approach is best illustrated by several examples in Figure 41. The dashed, undulating line is meant to represent the progress of a microorganism in its attempt to overcome the inhibitory hurdles, with growth occurring only after all hurdles have been overcome. Size of the hurdle indicates relative inhibitory effectiveness. Obviously, a microorganism in real life would confront all hurdles simultaneously rather than in sequence as is shown, and some of the factors would act synergistically. In example 1, six hurdles are present and growth is satisfactorily controlled because the microorganisms are unable to overcome all hurdles. Example 2 is a more realistic one in which a typical microbial population is present and the various hurdles differ in their inhibitory effectiveness, with RVP and preservatives being the most potent ones. This example also results Pag e 84 FIGURE 41 The combined methods approach to controlling g rowth of microorg anisms in nonsterilized food. F is heating , t is chilling , RVP is relative vapor pressure, pH is acidification, Eh is redox potential, pres. is chemical preservative, and N is nutrients. (Adapted from Ref. 58.) in satisfactory control of microbial growth. Example 3 represents the same product and the same hurdles with a small starting population of microorganisms that result from good sanitary practices. In this example, fewer hurdles suffice. Example 4 represents the same product and the same hurdles with a large starting population of microorganisms that result from poor sanitary practices. Here, the hurdles are insufficient to provide satisfactory control of microbial growth. Example 5 represents the same hurdles and population of organisms as in Example 2, but in this instance the sample is rich in nutrients. Because of the nutrients, the hurdles that were adequate in Example 2 now are inadequate. In example 6, the product and hurdles remain unchanged but the microorganisms are given a substerilizing treatment before storage. The surviving but damaged organisms are less able to overcome the hurdles and fewer hurdles suffice. The lesson to be learned is this: Although RVP and Mm are powerful tools for predicting and controlling the properties and stability of food, there are many occasions when neither is Pag e 85 sufficient alone and other factors, such as chemical properties of the solute, pH, and oxidation—reduction potential, must be also considered. 2.13 Concluding Comments About Water Water is typically the most abundant constituent in food, is of critical importance to the desirable qualities of food, is the cause of food’s perishable nature, is a rate determinant of many chemical reactions, both desirable and undesirable, is a strong causative agent of undesirable side effects during freezing, is associated with nonaqueous food constituents in ways so complex that once these associations are disturbed by drying or freezing they can never again be completely restored, and, above all else, is frustratingly complex in behavior, inadequately studied, and poorly understood. Glossary: Molecular Mobility and Food Stability Amorphous This refers to a nonequilibrium, noncrystalline state of a substance. When saturation conditions prevail and a solute remains noncrystalline the supersaturated solution can be regarded as amorphous. An amorphous solid is generally called a glass and is characterized by a viscosity of greater than about 1012 Pa sec. Collapse This refers to both visibly evident collapse, such as the collapse of foods during freeze drying, and to collapse at the molecular level that consists of conversion from a nonequilibrium state to a lower state of free energy (relaxation). The kinetics of collapse is governed by molecular mobility, Mm, of the system. Eutectic temperature (TE) An invariant point on a temperature—composition phase diagram of a binary solution where solution can exist in equilibrium with both crystalline solute and crystalline solvent. Under equilibrium conditions, cooling at TE results in simultaneous crystallization of solvent and solute in constant proportion and at constant temperature until maximum solidification has occurred. The TE is, therefore, the highest temperature at which maximum crystallization can (but usually does not) occur. Free volume Free volume is space not occupied by molecules. A useful mental image can be created by imagining a container filled with the maximum number of bees that can be accommodated in hovering flight. “Filled” means no more bees can be accommodated in a hovering state, not that all space is occupied. The analogous space in liquids is called “free volume” and it accounts for the fact that liquids can be compressed if the pressure is great enough. Free volume can also be thought of as the “elbow room” molecules require to undergo vibrational, rotational and translational motions [41]. Both free volume and molecular mobility increase with increasing temperature. Temperature dependence of free volume is small below Tg, and large between Tm and Tg. Glass state (glassy) A substance existing as an amorphous (noncrystalline) solid is said to be in a glassy state. The Stokes viscosity (local viscosity, not bulk viscosity) is appropriate for characterizing glasses, and this value, at the temperature of incipient glass formation (Tg), ranges from 1010 to 1014 Pa sec, depending on the solute. This viscosity is sufficient to reduce translational and rotational mobility of most large molecules to a point of practical insignificance. In complex, polymer-dominated systems, very small molecules, most notably water, retain significant translational and rotational mobility at temperatures well below sample Tg. Vibrational mobility, of course, does not cease until the temperature is reduced to absolute zero. Pag e 86 Glass transition temperature (Tg, ) The glass transition temperature, Tg, is the temperature at which a supersaturated solution (amorphous liquid) converts to a glass. This is a second-order transition involving a step change in specific heat at the transition temperature, allowing this transition to be measured by differential scanning calorimetry (first-order transitions involve changes in physical state among gases, liquids, and crystalline solids). Tg values are observed in substances that contain sizeable regions that are amorphous or partially amorphous (all food tissues and many other foods), regardless of whether they contain ice. For partially crystalline polymeric substances, only the amorphous regions exhibit a glass transition. The Tg is dependent on solute type and water content. is a special Tg that applies only to samples containing ice, and only when ice has been formed so maximum freeze-concentration occurs (very slow cooling). For a given solute, the observed is a quasi-invariant point on a temperature-composition state diagram (not invariant because maximum ice formation is extremely difficult to obtain during typical measurement techniques, thus causing the observed to drift slightly lower with extended storage time). Below Tg or of a complex sample, all but small molecules lose their translational mobility while retaining limited rotational and vibrational mobility. Macromolecular entanglement This refers to the interaction of large polymers in a random fashion without chemical bonding and with or without hydrogen bonding. When entanglement of macromolecules is sufficiently extensive (requires a minimum critical concentration of the macromolecule and time), a visoelastic entanglement network forms. This type of amorphous network can be dispersed by dilution, and can exist in conjunction with microcrystalline gels, which cannot be dispersed by dilution. Metastable state This refers to a state of pseudo-equilibrium or apparent equilibrium that is stable over practical times. A metastable state is not, however, the most stable equilibrium because it possesses free energy that is greater than that of the global equilibrium state under the same conditions of pressure, temperature and composition. A metastable state can exist—that is, conversion to a more stable equilibrium state of lower free energy will not occur—if the activation energy is sufficiently high to prevent conversion to an equilibrium state of lower free energy during the period of interest. Molecular mobility (Mm) This refers to either translational or rotational motion of molecules (vibrational mobility is not of concern in the context of food stability). Molecular weight, number average (MWn) where n is the number of molecules of a given molecular species and M is the molecular weight of the same species, with i kinds of molecules present. Molecular weight, weight average (MWw) Plasticizer A substance incorporated into a polymeric material to increase its deformability. A true solvent is always a plasticizer, but a plasticizer is not always a true solvent [106,117]. A plasticizer decreases the Tg of a polymer. Water is a highly effective plasticizer of hydrophilic, amorphous polymers. Its low molecular weight leads to increased free volume, decreased local viscosity, and increased Mm. Pag e 87 Relaxation Relaxation refers to passage from a nonequilibrium state to a more stable (lower free energy) state. This term is also used to indicate the decay of a stress. Rubbery state A term used to describe the viscoelastic nature of large polymers in the temperature range Tm to Tg, that is, when the substance or a part of the substance is between the glassy and liquid states. Small polymers and solutes of low molecular weight that exist in this temperature zone are highly viscous but not elastic, and are not called rubbery [27]. State diagram A phase diagram supplemented with lines depicting boundaries of various nonequilibrium and metastable states. Such a diagram is sometimes called a “supplemented phase diagram.” Temperature of solute crystallization/dissolution ( ) for simple and complex aqueous samples is the highest temperature at which a crystalline solute can exist in equilibrium with an aqueous solution of a given composition. The highest temperature at which this can occur for a given sample is the sample’s initial crystallization temperature. The also can be thought of as the incipient saturation temperature during cooling, or the temperature at which the last of a crystalline solute, present in a saturated solution, melts or dissolves upon warming. On a temperature—composition phase diagram for a binary aqueous solution, values for samples with differing temperatures constitute the curve. Temperature of ice melting/crystallization ( ) for simple and complex aqueous samples This is the temperature at which ice can exist in equilibrium with an aqueous solution of a given composition. The highest temperature at which this can occur is the initial freezing point. On a temperature—composition phase diagram for a binary aqueous solution, values for samples with differing initial ratios of solute to water constitute the curve. Vitrification Solidification of an entire sample as a glass, that is, no crystallization of solvents or solutes. Acknowledgments This chapter was reviewed totally or in part by Larry Beuchat, Nicole Brake, Kenneth Connors, Theodore Labuza, Marcus Karel, George Zografi, David Reid, Pieter Walstra, Harry Levine, and Richard Hartel. The helpful suggestions of these respected colleagues and good friends are acknowledged with deep appreciation. Abbreviations and Symbols C1C2 Constants for the WLF equation Cg Solute concentration (wt%) existing in sample at Tg Solute concentration (wt%) existing in sample at DE Dextrose equivalent DS Dominating solute ERH Percent equilibrium relative humidity Mm Molecular mobility MSI Moisture sorption isotherm MWn Number-average molecular weight (see glossary) MWw Weight-average molecular weight (see glossary) h Viscosity of sample at temperature T Pag e 88 hg Viscosity of sample at Tg (or ) p Vapor pressure of the sample p0 Vapor pressure of pure water T Sample temperature Tc Collapse temperature TE Eutectic temperature Tg Glass transition temperature of the sample Glass transition temperature of a maximally freeze-concentrated sample Tm Either or Melting or freezing temperature of water in a solution Crystallization or melting (dissolution) temperature of solute in a solution Tr Recrystallization temperature W Water content of sample (wt%) Wg Water (wt%) existing in sample at Tg Unfrozen water (wt%) existing in sample at Wm (or m1) “Monolayer” water content Bibliography Blanshard, J. 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