7 ater in milk and dairy products 7.1 Introduction he water content of dairy products ranges from around 2.5 to 94%(w/w) Table 7. 1) and is the principal component by weight in most dairy roducts, including milk, cream, ice-cream, yogurt and most cheeses. the moisture content of foods(or more correctly their water activity, section 7.3), together with temperature and ph, are of great importance to food technology. As described in section 7.8, water plays an extremely important role even in relatively low-moisture products such as butter(c. 16% mois- ture)or dehydrated milk powders(c 2.5-4% moisture). Water is the most important diluent in foodstuffs and has an important influence on the physical, chemical and microbiological changes which occur in dairy prod ucts. Water is an important plasticizer of non-fat milk solids 7.2 General properties of water Some physical properties of water are shown in Table 7. 2. Water has higher melting and boiling temperatures, surface tension, dielectric constant, heat capacity, thermal conductivity and heats of phase transition than similar molecules (Table 7.3). Water has a lower density than would be expected from comparison with the above molecules and has the unusual property of expansion on solidification. The thermal conductivity of ice is approxi mately four times greater than that of water at the same temperature and is high compared with other non-metallic solids. Likewise, the thermal dif- fusivity of ice is about nine times greater than that of water The water molecule(HOH) is formed by covalent(o)bonds between two of the four sp bonding orbitals of oxygen(formed by the hybridization of the 2s, 2px, 2p, and 2p, orbitals) and two hydrogen atoms( Figure 7. 1a). The remaining two sp' orbitals of oxygen contain non-bonding electrons. The verall arrangement of the orbitals around the central oxygen atom is trahedral and this shape is almost perfectly retained in the water molecule. Due to electronegativity differences between oxygen and hydrogen, the o-H bond in water is polar(a vapour state dipole moment of 1.84 D). This results in a partial negative charge on the oxygen and a partial positive charge on each hydrogen( Figure 7. 1b). Hydrogen bonding can occur between the two lone electron pairs in the oxygen atom and the hydrogen atoms of other
7 Water in milk and dairy products 7.1 Introduction The water content of dairy products ranges from around 2.5 to 94% (w/w) (Table 7.1) and is the principal component by weight in most dairy products, including milk, cream, ice-cream, yogurt and most cheeses. The moisture content of foods (or more correctly their water activity, section 7.3), together with temperature and pH, are of great importance to food technology. As described in section 7.8, water plays an extremely important role even in relatively low-moisture products such as butter (c. 16% moisture) or dehydrated milk powders (c. 2.54% moisture). Water is the most important diluent in foodstuffs and has an important influence on the physical, chemical and microbiological changes which occur in dairy products. Water is an important plasticizer of non-fat milk solids. 7.2 General properties of water Some physical properties of water are shown in Table 7.2. Water has higher melting and boiling temperatures, surface tension, dielectric constant, heat capacity, thermal conductivity and heats of phase transition than similar molecules (Table 7.3). Water has a lower density than would be expected from comparison with the above molecules and has the unusual property of expansion on solidification. The thermal conductivity of ice is approximately four times greater than that of water at the same temperature and is high compared with other non-metallic solids. Likewise, the thermal diffusivity of ice is about nine times greater than that of water. The water molecule (HOH) is formed by covalent (6) bonds between two of the four sp3 bonding orbitals of oxygen (formed by the hybridization of the 2s, 2p,, 2py and 2p, orbitals) and two hydrogen atoms (Figure 7.la). The remaining two sp3 orbitals of oxygen contain non-bonding electrons. The overall arrangement of the orbitals around the central oxygen atom is tetrahedral and this shape is almost perfectly retained in the water molecule. Due to electronegativity differences between oxygen and hydrogen, the O-H bond in water is polar (a vapour state dipole moment of 1.84 D). This results in a partial negative charge on the oxygen and a partial positive charge on each hydrogen (Figure 7.lb). Hydrogen bonding can occur between the two lone electron pairs in the oxygen atom and the hydrogen atoms of other
m 7.1 Approximate water content of some dairy products ified from Holland et al, 1991) Product Water(g/100 g) average ified plus SMP d° Channel Island milk, whole, pasteurized mi-skimmed, UHT Dried skimmed milk Evaporated milk, whole 8666800θ8 Goats milk, pasteurized milk. colostrum mature Fresh cream, whipping Cheeses Cheddar-type, reduced fat Cheese spread, plain reduced fa 97865 anish blue mage frais, fruit ery low fat Fu fat soft cheese Medium-fat soft chees Parmesan White cheese, average Drink Ice-cream, dairy, vanilla non-dairy, vanilla The value for pasteurized milk is similar to that for unpas- teurized milk
Table 7.1 Approximate water content of some dairy products (modified from Holland et a/., 1991) Product Water (g/lOO g) Skimmed milk, average pasteurized fortified plus SMP UHT, fortified Whole milk, average pasteurized” summer winter sterilized summer winter semi-skimmed, UHT Dried skimmed milk with vegetable fat Evaporated milk, whole Flavoured milk Goats’ milk, pasteurized Human milk, colostrum Sheep’s milk, raw Fresh cream, whipping Cheeses Brie Camembert Cheddar, average vegetarian Cheddar-type, reduced fat Cheese spread, plain Cottage cheese, plain Channel Island milk, whole, pasteurized mature with additions reduced fat Cream cheese Danish blue Edam Feta Fromage frais, fruit plain very low fat Full-fat soft cheese Gouda Hard cheese, average Lymeswold Medium-fat soft cheese Parmesan Processed cheese, plain Stilton, blue White cheese, average Whey Drinking yogurt Low-fat plain yogurt Whole-milk yogurt, plain Ice-cream, dairy, vanilla fruit non-dairy, vanilla 91 91 89 91 88 88 88 88 88 86 86 86 89 3.0 2.0 69 85 89 88 87 83 55 49 51 36 34 41 53 79 17 80 46 45 44 51 72 18 84 58 40 31 41 10 18 46 39 41 94 84 85 82 13 62 65 “The value for pasteurized milk is similar to that for unpasteurized milk
DAIRY CHEMISTRY AND BIOCHEMISTRY Table 7.2 Physical constants of water and ice( from Fennema, 1985) Molecular weight 801534 oVert Melting point at 101.3 kPa(1 atm) Boiling point at 101.3 k Pa(1 atm Critical pressu 224MPa(2186am) 0.0099cand6104kPa(4.579mmHg) Heat of fusion at0°C 6.012 kJ(1.436 kcal)mol Heat of vaporization at 100C 40.63 kJ (9.705 kcal)mol-1 50.91kJ(12.16 kcal)mol-1 Other properties at 20°C o'C (ice) 20°cice) 0999841 0.9168 09193 1787×10-3 Surface tension against 72. 75x 10-3 756×10-3 air(Nm essure(Pa) pecific heat(J kg K ) 4.1819 237×1036104×1026.104×102 1.034×10 Thermal conductivity 5983×102 5644×1022240×1022433×102 Thermal diffusivity(m's)1.4x 10-5 ~1.1×10-4~1,1×10 Dielectric constant, 8000 at3×10°H 80.5 (25C) (1.5C) Limiting value at low frequencies Parallel to c-axis of ice; values about 15% larger if perpendicular to c-axis. Table 7-3 Properties of water and other compounds (from Roos, 1997) Hydrofluoric Hydrogen Ammonia sulphide Methane Water Property (CH,) (H2O 17.0 33.35 10000 2.1374.15 Critical P(bar) 464221.5 olecules which, due to the above- mentioned differences in electronegatiy ity, have some of the characteristics of bare protons. Thus, each water molecule can form four hydrogen bonds arranged in a tetrahedral fashion around the oxygen( Figure 7. 1d ). The structure of water has been described as a continuous three-dimensional network of hydrogen-bonded molecules with a local preference for tetrahedral geometry but with a large number of strained or broken hydrogen bonds. This tetrahedral geometry is usually
296 DAIRY CHEMISTRY AND BIOCHEMISTRY Table 7.2 Physical constants of water and ice (from Fennema, 1985) Molecular weight Phase transition properties Melting point at 101.3 kPa (1 atm) Boiling point at 101.3 kPa (1 atm) Critical temperature Critical pressure Triple point Heat of fusion at 0°C Heat of vaporization at 100°C Heat of sublimation at 0°C 18.01 534 0.ooo"c 100.00"C 374.15"C 22.14 MPa (218.6 atm) 0.0099'C and 610.4 kPa (4.579 mmHg) 6.012kJ (1.436kcal)mol-' 40.63 kJ (9.705 kcal) mol- 50.91 kJ (12.16kcal) mol-' Other properties at 20°C 0°C 0°C (ice) - 20°C (ice) Density (kg I-') 0.9998203 Surface tension against 72.75 x Vapor pressure (Pa) 2.337 x lo3 Specific heat (J kg-' K-I) 4.1819 Thermal conductivity 5.983 x 10' Thermal diffusivity (m2 s-I) Dielectric constant, Viscosity (Pa s) 1.002 x 10-3 air (N m-I) (J m-'s-' K-' 1 1.4 x static" 80.36 at 3 x lo9 Hz 76.7 (25'C) 0.999841 1.787 x 75.6 x 10-3 6.104 x 10' 4.2177 5.644 x 10' 1.3 10-5 80.00 80.5 (1 .5"C) 0.9168 - - 6.104 x 10' 2.1009 22.40 x lo2 - 1.1 x 10-4 91b - (- 12°C) 0.9193 - - 1.034 x 10' 1.9544 24.33 x 10' - 1.1 x 10-4 98b 3.2 - "Limiting value at low frequencies. bParallel to c-axis of ice; values about 15% larger if perpendicular to c-axis. Table 7.3 Properties of water and other compounds (from Roos, 1997) Hydrofluoric Hydrogen A m m o n i a acid sulphide Methane Water Property (NH,) (HF) W2.T (CHJ (HZO) Molecular weight 17.03 20.02 34.08 16.04 18.015 Melting point ('C) - 77.7 -83.1 - 85.5 - 182.6 0.00 Boiling point ("C) - 33.35 19.54 - 60.7 -161.4 100.00 Critical T ("C) 132.5 188.0 100.4 -82.1 374.15 Critical P (bar) 114.0 64.8 90.1 46.4 221.5 molecules which, due to the above-mentioned differences in electronegativity, have some of the characteristics of bare protons. Thus, each water molecule can form four hydrogen bonds arranged in a tetrahedral fashion around the oxygen (Figure 7.ld). The structure of water has been described as a continuous three-dimensional network of hydrogen-bonded molecules, with a local preference for tetrahedral geometry but with a large number of strained or broken hydrogen bonds. This tetrahedral geometry is usually
VATER IN MILK AND DAIRY PR( 297 104,5 (a) Figure 7.1 Schematic representations(a-c)of a water molecule and hydrogen bonding betwee maintained only over short distances. The structure is dynamic; molecules can rapidly exchange one hydrogen bonding partner for another and there may be some unbonded water molecules Water crystallizes to form ice. Each water molecule associates with four others in a tetrahedral fashion as is apparent from the unit cell of an ice crystal(Figure 7. 2). The combination of a number of unit cells, when viewed from the top, results in a hexagonal symmetry(Figure 7.3). Because of the tetrahedral arrangement around each molecule, the three-dimensional struc- ture of ice(Figure 7. 4)consists of two parallel planes of molecules lying close to each other (basal planes"). Basal planes of ice move as a unit under pressure. The extended structure of ice is formed by stacking of several basal planes. This is the only crystalline form of ice that is stable at a pressure of 1 atm at 0C, although ice can exist in a number of other crystalline forms, as well as in an amorphous state. The above description of ice is somewhat simplified; in practice the system is not perfect due to the presence of ionized
WATER IN MILK AND DAIRY PRODUCTS 297 Figure 7.1 Schematic representations (a-c) of a water molecule and hydrogen bonding between water molecules (d). maintained only over short distances. The structure is dynamic; molecules can rapidly exchange one hydrogen bonding partner for another and there may be some unbonded water molecules. Water crystallizes to form ice. Each water molecule associates with four others in a tetrahedral fashion as is apparent from the unit cell of an ice crystal (Figure 7.2). The combination of a number of unit cells, when viewed from the top, results in a hexagonal symmetry (Figure 7.3). Because of the tetrahedral arrangement around each molecule, the three-dimensional structure of ice (Figure 7.4) consists of two parallel planes of molecules lying close to each other ('basal planes'). Basal planes of ice move as a unit under pressure. The extended structure of ice is formed by stacking of several basal planes. This is the only crystalline form of ice that is stable at a pressure of 1 atm at O'C, although ice can exist in a number of other crystalline forms, as well as in an amorphous state. The above description of ice is somewhat simplified; in practice the system is not perfect due to the presence of ionized
DAIRY CHEMISTRY AND BIOCHEMISTRY 4.52A Figure 7.2 Unit cell of an ice crystal at 0 C. Circles represent the oxygen of water les.-indicates hydrogen bonding(Modified from Fennema, ater(H3 O, OH ) isotopic variants, solutes and vibrations within the ter molecules With the exceptions of water vapour and ice, water in dairy products contains numerous solutes. Thus, the interactions of water with solutes is of great importance. Hydrophilic compounds interact strongly with water by ion-dipole or dipole-dipole interactions while hydrophobic substances interact poorly with water and prefer to interact with each other (hydro- phobic interaction). Water in food products can be described as being free or bound. The definition of what consitiutes ' bound water is far from clear(see Fennema 1985)but it can be considered as that part of the water in a food which does not freeze at -40.C and exists in the vicinity of solutes and other non-aqueous constituents, has reduced molecular mobility and other signifi cantly altered properties compared with the ' bulk water'of the same system (Fennema, 1985). The actual amount of bound water varies in different products and the amount measured is often a function of the assay technique. Bound water is not permanently immobilized since interchange of bound water molecules occurs frequently. There are a number of types of bound water. Constitutional water is the most strongly bound and is an integral part of another molecule(e. g. within the structure of a globular protein). Constitutional water represents only a
298 DAIRY CHEMISTRY AND BIOCHEMISTRY 4.52 A Figure 7.2 Unit cell of an ice crystal at 0°C. Circles represent the oxygen atoms of water molecules, - indicates hydrogen bonding. (Modified from Fennema, 1985.) water (H30f, OH -), isotopic variants, solutes and vibrations within the water molecules. With the exceptions of water vapour and ice, water in dairy products contains numerous solutes. Thus, the interactions of water with solutes is of great importance. Hydrophilic compounds interact strongly with water by ion-dipole or dipole-dipole interactions while hydrophobic substances interact poorly with water and prefer to interact with each other (‘hydrophobic interaction’). Water in food products can be described as being free or bound. The definition of what consitiutes ‘bound’ water is far from clear (see Fennema, 1985) but it can be considered as that part of the water in a food which does not freeze at -40°C and exists in the vicinity of solutes and other non-aqueous constituents, has reduced molecular mobility and other significantly altered properties compared with the ‘bulk water’ of the same system (Fennema, 1985). The actual amount of bound water varies in different products and the amount measured is often a function of the assay technique. Bound water is not permanently immobilized since interchange of bound water molecules occurs frequently. There are a number of types of bound water. Constitutional water is the most strongly bound and is an integral part of another molecule (e.g. within the structure of a globular protein). Constitutional water represents only a