WATER IN MILK AND DAIRY PRODUCTS Figure 7.3 The"basal planeof ice(combinations of two planes of sligh viewed from above. The closed circles represent oxygen atoms of water m in the lowe lane and the open circles oxygen atoms in the upper plane. (a) seen from above and(b)from small fraction of the water in high-moisture foods ' Vicinal or monolayer water is bound to the first layer sites of the most hydrophilic groups Multilayer water occupies the remaining hydrophilic sites and forms a number of layers beyond the monolayer water. There is often no clear distinction between constitutional, monolayer and multilayer water since they differ only in the length of time a water molecule remains associated with the food The addition of dissociable solutes to water disrupts its normal tetra hedral structure. Many simple inorganic solutes do not possess hydrogen bond donors or acceptors and therefore can interact with water only by dipole interactions(e.g. Figure 7.5 for Nacl). Multilayer water exists in a structurally disrupted state while bulk-phase water has properties similar to
WATER IN MILK AND DAIRY PRODUCTS 299 (b) Figure 7.3 The ‘basal plane’ of ice (combinations of two planes of slightly different elevations) viewed from above. The closed circles represent oxygen atoms of water molecules in the lower plane and the open circles oxygen atoms in the upper plane, (a) seen from above and (b) from the side (from Fennema, 1985). small fraction of the water in high-moisture foods. ‘Vicinal’ or monolayer water is bound to the first layer sites of the most hydrophilic groups. Multilayer water occupies the remaining hydrophilic sites and forms a number of layers beyond the monolayer water. There is often no clear distinction between constitutional, monolayer and multilayer water since they differ only in the length of time a water molecule remains associated with the food. The addition of dissociable solutes to water disrupts its normal tetrahedral structure. Many simple inorganic solutes do not possess hydrogen bond donors or acceptors and therefore can interact with water only by dipole interactions (e.g. Figure 7.5 for NaCl). Multilayer water exists in a structurally disrupted state while bulk-phase water has properties similar to
DAIRY CHEMISTRY AND BIOCHEMISTRY Figure 7.4 The extended structure of ice. Open and shaded circles represent oxygen atoms of Figure 7.5 Arrangement of water molecules in the vicinity of sodium and chloride ions (modified from Fennema, 1985). those of water in a dilute aqueous salt solution. Ions in solution impose structure on the water but disrupt its normal tetrahedral structure. Concen- trated solutions probably do not contain much bulk-phase water and structures caused by the ions predominate. The ability of an ion to influence the structure of water is influenced by its electric field. Some ions(princi- pally small and or multivalent) have strong electric fields and loss of the inherent structure of the water is more than pensated for by the new structure resulting from the presence of the ions. However, large, mono alent ions have weak electric fields and thus have a net disruptive effect on the structure of water
300 DAIRY CHEMISTRY AND BIOCHEMISTRY C 4 Figure 7.4 The extended structure of ice. Open and shaded circles represent oxygen atoms of water molecules in the upper and lower layers, respectively, of a basal plane (from Fennema, 1985). Figure 7.5 Arrangement of water molecules in the vicinity of sodium and chloride ions (modified from Fennema. 1985). those of water in a dilute aqueous salt solution. Ions in solution impose structure on the water but disrupt its normal tetrahedral structure. Concentrated solutions probably do not contain much bulk-phase water and structures caused by the ions predominate. The ability of an ion to influence the structure of water is influenced by its electric field. Some ions (principally small and/or multivalent) have strong electric fields and loss of the inherent structure of the water is more than compensated for by the new structure resulting from the presence of the ions. However, large, monovalent ions have weak electric fields and thus have a net disruptive effect on the structure of water
WATER IN MILK AND DAIRY PRODUCTS Figure 7.6 Schet representation of the interaction of water molecules with carboxylic acid (a), alcohol(b),-NH and carbonyl groups(c) and amide groups(d) In addition to hydrogen bonding with itself, water may also form such bonds with suitable donor or acceptor groups on other molecules. Water solute hydrogen bonds are normally weaker than water-water interactions By interacting through hydrogen bonds with polar groups of solutes, the mobility of water is reduced and, therefore, is classified as either constitu tional or monolayer. Some solutes which are capable of hydrogen bonding with water do so in a manner that is incompatible with the normal structure of water and therefore have a disruptive effect on this structure. For this reason, solutes depress the freezing point of water(Chapter 11). Water can potentially hydrogen bond with lactose or a number of groups on proteins (e.g. hydroxyl, amino, carboxylic acid, amide or imino; Figure 7.6)in dairy product: Milk contains a considerable amount of hydrophobic material, especially lipids and hydrophobic amino acid side chains. The interaction of water with such groups is thermodynamically unfavourable due to a decrease in entropy caused by increased water-water hydrogen bonding(and thus an increase in structure)adjacent to the non-polar groups 7.3 Water activity Water activity(aw)is defined as the ratio between the water vapour pressure exerted by the water in a food system()and that of pure water(po)at the
WATER IN MILK AND DAIRY PRODUCTS 301 0 II Figure 7.6 Schematic representation of the interaction of water molecules with carboxylic acid (a), alcohol (b), -NH and carbonyl groups (c) and amide groups (d). In addition to hydrogen bonding with itself, water may also form such bonds with suitable donor or acceptor groups on other molecules. Watersolute hydrogen bonds are normally weaker than water-water interactions. By interacting through hydrogen bonds with polar groups of solutes, the mobility of water is reduced and, therefore, is classified as either constitutional or monolayer. Some solutes which are capable of hydrogen bonding with water do so in a manner that is incompatible with the normal structure of water and therefore have a disruptive effect on this structure. For this reason, solutes depress the freezing point of water (Chapter 11). Water can potentially hydrogen bond with lactose or a number of groups on proteins (e.g. hydroxyl, amino, carboxylic acid, amide or imino; Figure 7.6) in dairy products. Milk contains a considerable amount of hydrophobic material, especially lipids and hydrophobic amino acid side chains. The interaction of water with such groups is thermodynamically unfavourable due to a decrease in entropy caused by increased water-water hydrogen bonding (and thus an increase in structure) adjacent to the non-polar groups. 7.3 Water activity Water activity (a,) is defined as the ratio between the water vapour pressure exerted by the water in a food system (p) and that of pure water (p,) at the