Food Preservation Through Control Of Water

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As mentioned in the general discussion of food preservation, the most serious limitation of shelf life and nutritional quality is from microbiological activity. This section covers briefly the concept of reducing the amount of water in food systems as a means of food preservation. By lowering the amount of water available for microorganisms to remain active and reducing mobility of potentially reactive chemical species in a food system, it is possible to extend quality of a product from both a microbiological and nutritional point of view. Food preservation processes falling in this category include concentration processes such as evaporation or freeze-concentration; dehydration including air drying, spray drying, freeze drying, and so on, and methods of adding salt or sugar and thereby lowering the available water by increasing osmotic pressure. Since all of these

Figure 4. Representation of dose-response curves for different types of microorganisms. Source: Ref. 14.

Dose (kGy)

Figure 4. Representation of dose-response curves for different types of microorganisms. Source: Ref. 14.

methods depend on lowering available water, instead of describing in complete detail each type of process, which is beyond the scope of this article, the overall general relationship between availability of water and chemical reactions or microbiological activity will be discussed in this section.

One of the most important concepts in understanding the relationship between moisture removal and shelf-life extension is that of free versus bound water in foods, which are terms that grew in importance particularly during the latter half of the twentieth century. It was realized that active water is more important to the stability of a food system than the total amount of water present, or water content (34). The term water activity (aw) is a thermodynamic property defined as the ratio of the vapor pressure of water in a food system (p) to the vapor pressure of pure water (p0) at a given temperature or the equilibrium relative humidity (ERH) of the air surrounding the food system at a given temperature, such that aw = p/p0 = % ERH/ 100. Water activity is currently the most common term used by researchers and industry professionals in the food processing business. A sorption isotherm is a graphical illustration of the relationship of water activity, or relative humidity of the vapor space surrounding a food system, and the equilibrium moisture content of that material. An isotherm is generally divided into three major regions (Fig. 5). Region A represents strongly bound water with an enthalpy of vaporization considerably higher than that of pure water. This region represents the first layer of water molecules sorbed at hydrophilic, charged and polar groups of food constituents (proteins, polysaccharides) and includes structural water (hydrogen-bonded water), monolayer water, and hydrophobic hydration water (35). This bound water is generally considered unfreezable and unavailable for chemical reactions or acting as a plasticizer.

Bound Water Food
Figure 5. General type of sorption isotherm for food products. Source: Ref. 50.

Region B includes water molecules that are less firmly bound than those in region A with an enthalpy of vaporization only slightly more than that of pure water. This water may or may not be unfreezable and is available for low-molecular-weight solutes and some biochemical reactions. It may be considered a sort of transition zone between bound and free water. Region C consists of water molecules that have properties resembling more closely those of free water, often referred to as capillary water or that which is loosely bound with no excess heat of binding detected in this region.

Concerning the effect of temperature on water activity, as temperature increases, the amount of water adsorbed usually decreases. In fact, the effect of temperature on isotherms follows a Claussius-Clayperon relationship as depicted in an integrated form, holding moisture content constant: In (a2/ai) = AHSIR(1IT1 - 1/T2), where AHS = heat of sorption (cal/mole); R = 1.987 cal/mol • K; a1 and a2 are different water activities at their respective temperatures Tl and T2. AHS refers to that amount of energy above and beyond that associated with the latent heat of vaporization. The average heat of sorption or the energy of binding of water (also referred to as Eh) can be determined by plotting the natural logarithm of a„ versus 1 IT at different moisture contents. The resulting slope of the straight line is - AHJR (Fig. 6). Iglesias and Chirife (36) tabulated values for Eb for different food systems at different temperature ranges. Several techniques of measurement of water activity have been sited (37-41). Probably the most common approach to estimating the amount of bound water is through the Brunauer-Emmef>-Teller (BET) isotherm or the "BET monolayer value" as determined from the following equation: a/m( 1 — a) = [1/mjc] + [(c — l)/(m1c)]a, where a = water activity, m = water content (g water/g solid), m1 = monolayer value, and c = constant.

Information on sorption behavior is important in concentration and dehydration processes, particularly toward

Figure 6. Graphical representation of determination of heat of sorption in food systems at constant moisture content.

the end of a dehydration process where the total heat of vaporization (AH) plus heat of sorption are of concern. Because the heat of sorption depends on the partial pressure of water over the food and on the energy of water binding in the food, knowledge of these values is helpful in the design of these processes in calculating the amount of energy required for proper drying. Furthermore, since water activity affects stability, it must be brought to an appropriate level at the end of drying that is suitable for long-term storage. Different quality factors and microbiological growth all have different sensitivities to different water activities. Knowledge of the relative reaction rates in combination with heats of sorption and rates of moisture dif-fusivity in different types of food materials will provide a food technologist with the tools to design the most efficient process by which to remove water from a food in order for it to maximize drying and optimize quality with extended shelf life. It can be seen from Figure 7 that, depending on the type of food system, different levels of water activity will need to be achieved to best reduce degradation reactions of certain types. In general, for most foods, microbiological activity can be curtailed providing a water activity below 0.91 for bacteria and below 0.80 for most molds, although some xerophilic fungi have been found to grow as low as 0.65. On the other hand, oxidation-type reactions are minimal around 0.3 to 0.4 aw, but nonenzymatic browning reactions are maximum around 0.4 to 0.60 aw. It is clear that processing condition designs are highly dependent on the individual system under consideration. The importance of hysteresis should also be mentioned. By this it is meant that the upward (adsorption side) of the curve has a different path than the desorption side of the isotherm (Fig. 5). This in combination with the effect of temperature on hysteresis is also important in design and operating and controlling drying processes and reverse osmosis.

Several types of deleterious changes may take place during dehydration, including nonenzymatic browning, lipid oxidation, vitamin degradation, pigment loss, reduced solubility and textural changes as well as flavor and aroma volatilization. Through an understanding of the influences of water activity on the rates of degradation of various food components, food technologists have the ability to design and regulate the processes during the different stages of drying to best maintain the integrity of the natural components of the system. For instance, it has been shown by several researchers that browning reactions follow zero-order kinetics after an induction period (42-44), and it is most severe toward the end of the drying period at low moisture where less evaporative cooling takes place (42). Franzen et al. (45) developed a model for nonenzymatic browning in skim milk within a temperature range of 35 to 130°C as a function of time, temperature, and moisture content. Ascorbic acid, on the other hand has been shown to be particularly sensitive at high moisture contents. To optimize vitamin C retention, a product should be dried at low temperatures during the constant rate period of drying, followed by increasing the temperature as the moisture content decreases (46). Rates of ascorbic acid degradation during drying have been reported in the literature (47-49). Different methods of de

Figure 7. Relative rates of typical deteriorative reactions in foods as a function of water activity. Source: Ref. 99.

Nonenzymatic browning

Nonenzymatic browning

Moisture And Moulds Food Graph

Bacteria growth

Bacteria growth

0.1 0.2 03 0.4 0.5 0.6 0.7 0.8 0.9 Water activity hydration and their major characteristics have been reviewed by Okos et al. (50).

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    How to manipulate water activity in meat preservation?
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