According to Hamm (1962), the waterbinding capacity of meat is caused by the muscle proteins. Some 34 percent of these proteins are water-soluble. The main portion of meat proteins is structural material. Only about 3 percent of the total water-binding capacity of muscle can be attributed to water-soluble (plasma) proteins. The main water-binding capacity of muscle can be attributed to actomyosin, the main component of the myofibrils. The adsorption isotherm of freeze-dried meat has the shape shown in Figure 1-33. The curve is similar to the sorption isotherms of other foods and consists of three parts. The first part corresponds to the tightly bound water, about 4 percent, which is given off at very low vapor pressures. This quantity is only about one-fifth the total quantity required to cover the whole protein with a monomolecular layer. This water is bound under simultaneous liberation of a considerable amount of energy, 3 to 6 kcal per mole of water. The binding of this water results in a volume contraction of 0.05 mL per g of protein. The binding is localized at hydrophilic groups on proteins such as polar side chains having carboxyl, amino, hydroxyl, and sulphydryl groups and also on the nondissociable carboxyl and imino groups of the peptide bonds. The binding of water is strongly influenced by the pH of meat. The effect of pH on the swelling or unswelling (that is, water-binding capacity of proteins) is schematically represented in Figure 1-34 (Honkel 1989). The second portion of the curve corresponds to multilayer adsorption, which amounts to another 4 to 6 percent of water. Hamm (1962) considered these two quantities of water to represent the real water of hydration and found them to amount to between 50 and 60 g per 100 g of protein. Muscle binds much more than this amount of water. Meat with a protein content of 20 to 22 percent contains 74 to 76 percent water, so that 100 g of protein binds about 350 to 360 g of water. This ratio is even higher in fish muscle. Most of this water is merely immobilized—retained by the net-
Figure 1-33 Adsorption Isotherm of Freeze-Dried Meat
Figure 1-33 Adsorption Isotherm of Freeze-Dried Meat
Figure 1-34 Water Binding in Meat as Influenced by pH
Figure 1-34 Water Binding in Meat as Influenced by pH
work of membranes and filaments of the structural proteins as well as by cross-linkages and electrostatic attractions between peptide chains. It is assumed that changes in water-binding capacity of meat during aging, storage, and processing relate to the free water and not the real water of hydration. The free water is held by a three-dimensional structure of the tissue, and shrinkage in this network leads to a decrease in immobilized water; this water is lost even by application of slight pressure. The reverse is also possible. Cut-up muscle can take up as much as 700 to 800 g of water per 100 g of protein at certain pH values and in the presence of certain ions. Immediately after slaughter there is a drop in hydration and an increase in rigidity of muscle with time. The decrease in hydration was attributed at about two-thirds to decomposition of ATP and at about one-third to lowering of the pH.
Hamm (1959a, 1959b) has proposed that during the first hour after slaughter, bivalent metal ions of muscle are incorporated into the muscle proteins at pH 6, causing a contraction of the fiber network and a dehydration of the tissue. Further changes in hydration during aging for up to seven days can be explained by an increase in the number of available carboxyl and basic groups. These result from proteolysis. Hamm and Deather-age (1960a) found that freeze-drying of beef results in a decrease in water-binding capacity in the isoelectric pH range of the muscle. The proteins form a tighter network, which is stabilized by the formation of new salt and/or hydrogen bonds. Heating beef at temperatures over 40 °C leads to strong denaturation and changes in hydration (Hamm and Deatherage 1960b). Quick freezing of beef results in a significant but small increase in the water-holding capacity, whereas slow freezing results in a significant but small decrease in water binding. These effects were thought to result from the mechanical action of ice crystals (Deatherage and Hamm 1960). The influence of heating on water binding of pork was studied by Sherman (1961b), who also investigated the effect of the addition of salts on water binding (Sherman 1961a). Water binding can be greatly affected by addition of certain salts, especially phosphates (Hellendoorn 1962). Such salt additions are used to diminish cooking losses by expulsion of water in canning hams and to obtain a better structure and consistency in manufacturing sausages. Recently, the subject of water binding has been greatly extended in scope (Katz 1997). Water binding is related to the use of water as a plasti-cizer and the interaction of water with the components of mixed food systems. Retaining water in mixed food systems throughout their shelf life is becoming an important requirement in foods of low fat content. Such foods often have fat replacer ingredients based on proteins or carbohydrates, and their interaction with water is of great importance.
WATER ACTIVITY AND FOOD
Water activity is one of the criteria for establishing good manufacturing practice (GMP) regulations governing processing requirements and classification of foods (Johnston and Lin 1987). As indicated in Figure 1-35, the process requirements for foods are governed by aw and pH; aw controlled foods are those with pH greater than 4.6 and aw less than 0.85. At pH less than 4.6 and aw greater than 0.85, foods fall into the category of low-acid foods; when packaged in hermetically sealed containers, these foods must be processed to achieve commercially sterile conditions.
Intermediate moisture foods are in the aw range of 0.90 to 0.60. They can achieve stability by a combination of aw with other factors, such as pH, heat, preservatives, and Eh (equilibrium relative humidity).
Figure 1-35 The Importance of pH and aw on Processing Requirements for Foods. Source: Reprinted with permission from M.R. Johnston and R.c. Lin, FDA Views on the Importance of aw in Good Manufacturing Practice, Water Activity: Theory and Application to Food, L.B. Rockland and L.R. Beuchat, eds., p. 288, 1987, by courtesy of Marcel Dekker, Inc.
Acid Foods pH <4.6 aw > 0.85
Thermally Processed Low-Acid Foods Packaged in Hermetically Sealed Containers pH > 4.6
aw > 0.85
Controlled j aw Controlled Foods _ Foods j
..II i i i i
•Acidified Foods - 21 CFR 114 6 106.25
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