Heating is invariably used in order to preserve milk by killing pathogenic or spoilage organisms, inactivating enzymes or by altering its physico-chemical state. In milk powder manufacture, heat is applied to produce a dried product with improved storage and desired functional properties. In general the thermal energy in the evaporation and drying stages is independent of the type of powder, as the overriding consideration in these steps is efficient energy usage in the removal of water. The amount of heat applied in the preheat is dependent on the desired end use of the powder. Though all of the stages contribute to the overall thermal effect, the functional properties of the milk powder are largely determined by the preheat treatment. Table 2.7 shows the changes that occur at each step of milk powder manufacture.
Table 2.7 Effect of individual processing steps on milk.
Process Effect
Preheat treatment Destruction of bacteria; inactivation of enzymes; denaturation of whey proteins; aggregation of whey proteins; formation of a complex between K-casein and �-lactoglobulin; shift of the soluble salts to the colloidal phase; Changes in micelle structure; Maillard reaction between protein and lactose; pH
decrease.
Evaporation Concentration of milk solids; increase in colloidal
salts; increase in micelle size; decrease in pH; limited denaturation of whey proteins.
Concentrate heating Reduction in concentrate viscosity; increase in
colloidal salts; protein interactions.
Homogenization Increase in number of fat globules; adsorption of
casein onto fat surface; decrease in protein stability. Spray drying Further removal of water; relatively minor changes
in protein and salts.
2.5.1 Preheating
A major consequence of preheating is the denaturation of the heat labile whey proteins and their interactions with the casein micelle. These heat-induced changes have been discussed in Sections 2.2 and 2.3. Preheating by direct steam injection at l 20°C for 2 min denatures ::::85% of the P-Ig and ==35% of the a-la. For low preheat treatment (72°C for 15 s) denaturation is slight (==6%) (Singh & Creamer, 199 1 a). Many of the properties of milk powders stem from the behaviour of the whey proteins.
The whey protein nitrogen index (WPNI) is a commonly used measure for classifying powders on the basis of undenatured whey protein nitrogen. Casein protein and denatured whey protein are precipitated from reconstituted milk powder using sodium chloride. The undenatured whey protein is then precipitated using amido black dye, and the excess dye determined spectrophotometrically (Sanderson, 1970a). The WPNI is an absolute measure and does not take into consideration the original amount of whey protein in the milk. A powder that is substantially denatured may be classified as being low heat because of a high protein content in the original milk. This may lead to powders with the same WPNI, but different levels of denaturation. Also the method only measures native whey proteins, and there is no indication of what has happened to the protein after denaturation. Formation of aggregates and interactions with the micelle are important factors in determining the functional properties of the powder.
Changes also occur in the mineral distribution in milk. Calcium and phosphate are transferred from soluble to the colloidal phase as heating proceeds (Walstra & Jenness, 1984). Nieuwenhuijse et al. (1988) reported a pH drop of 0.04 and 0.09 units for preheats of 72°C for 2 s and l20°C for 1 80 s, respectively. The casein proteins are relatively heat insensitive, but changes in the micelle structure occur during heating.
2.5.1.1 Changes in micelle structure
In skim milk heated at 100°C for 30 min (pH 6.65) thread like particles at the micelle surface are observed by electron microscopy. Micelle appearance in heated milk is dependent on pH. At pH 6.8 the micelles look similar to those in unheated milk. At pH 6.5 the micelles acquire amorphous appendages (Creamer et al. , 1978). The
appearance perhaps relates to the dissociation of K-casein/�-lactoglobulin complex at high pH >6.9, and the association of this complex with the casein micelles at pH <6.9
(Singh & Fox, 1987a).
During preheating a number of other changes occur to the casein micelle structure besides association with whey proteins at the micelle surface. Below 90°C changes in micelle size are minimal, but at higher temperatures micelle size increases. Dalgleish et al. ( 1987) using photon correlation spectroscopy observed the changes in average micelle diameter during heating at 130°C. Initially the size increased slowly, but just prior to visible coagulation the diameter increased rapidly. The pH at heating affects micelle size. Micelle diameter increases as the pH is increased from 6.7 to 6.9, but decreases with a further increase in the pH to 7.2 (Dalgleish et al. , 1 987). The increase in micellar size and an increase in the number of protein particles smaller than micelles after heating has been observed by electron microscopy. Disintegration of the micelle is thought to be responsible for some of the small particles (Morr, 1969; Creamer & Matheson, 1980; McMahon & Yousif, 1993).
Dissociation of the casein proteins from the micelle increases with the severity of the heat treatment and follows the order K-, �-. a5-caseins (in order of decreasing extent) (Singh & Creamer, 199 1 b). The dissociation is also dependent on the pH at heating. At pH <6.9, K-casein and any associated �-lg or a-la eo-sediment ( 100,000 g for 1 h at 20°C) after heating at 140°C for 1 min (Singh & Fox, 1985). Above pH 6.9 K-casein, �-lg and a-la dissociate from the casein micelle.
2.5. 1.2 Mineral balance between soluble and colloidal phases
Heating milk causes a shift in the mineral balance from the soluble to the colloidal phase, in particular Ca and P, which are important in determining the stability of the casein micelles. Research into the area of heat induced changes in minerals is complicated by separation of the soluble/colloidal phases and reversibility of heat induced changes. The behaviour of minerals during processing and their role in determining the functional properties of the powder is not well understood. Reversibility of reactions is important in mineral changes that occur upon heating milk. The mineral
balance between the soluble and colloidal phases is in equilibrium and any shift away from this state during heating is reversible upon cooling (Pouliot et al. , 1989b). Therefore the time at which measurements are taken after cooling is critical. The use of an ultrafiltration membrane by Pouliot et al. ( 1989a, b) allowed for the almost immediate separation of soluble and colloidal phases at the temperature of heating. Advantages of ultrafiltration are that the minerals (Ca, P and Mg) are not concentrated in the permeate, and the mineral compounds bound to the protein in the retentate increases proportionally with the concentration ratio (Brule et al. , 1974). At present only low temperature conditions have been used (�90°C).
Ultrafiltration of milk at the heating temperature used by Pouliot et al. ( 1989a) allows for the measurement of reversible changes. Increasing heating temperature causes more of the soluble minerals to shift to the colloidal phase. The shift in soluble salts is mainly due to Ca and P, and to a lesser extent Mg and citrate. The decrease in soluble Ca, P and citrate was shown to involve two steps. During the first minute of heating the majority of the decrease takes place, after which a small decrease occurs over an extended period of time (up to 1 20 min). Mter the initial decrease, Ca, P and citrate show some reversibility before further heating results in the second step proceeding. The reversibility of Ca and P in milk heated at 85°C for 40 min increases as the cooling temperature decreases (60 � 4°C). During the 1st minute of cooling there is an initial increase in the soluble Ca and P and then a slow increase over the following 120 min. The reversibility ranges from 90-99% after 120 min of cooling at 4 to 60°C (Pouliot et al. , 1989b).
The effect of heating and reversibility can also be observed in the Ca2+ activity. Recovery of the Ca2+ activity on cooling follows a logarithmic relationship with time (Greets et al. , 1983). Thus Ca2+ activities can be determined at the point of sampling with reasonable accuracy, and past discrepancies caused by different measuring times can be eliminated. Augustin and Clark ( 199 1 ) observed that the effect of preheating conditions on the Ca2+ activity in reconstituted and recombined milks was small, though reversible changes while the milk was held at 4 oc overnight may have masked any differences caused by preheating.
2.5. 1.3 �ctose
The Maillard reaction is an important consequence of heating lactose with amino compounds as it leads to browning, off flavours, and loss of digestible lysine. The main reaction in Maillard browning is condensation of an amino compound with the carbonyl group of a sugar (Walstra & Jenness, 1984). Some of the products of the Maillard reaction give heated milk products a caramelized flavour.
2.5.2 Concentration related changes
After preheating the solids in milk are concentrated by evaporation and spray drying. Most of the water is removed during evaporation, typically 80%, in a multistage falling film evaporator. Evaporation temperatures are kept low (70 to 50°C) by vacuum, and the residence time in each stage is about 1 min (Singh & Newstead, 1992).
2.5.2. 1 Evaporation
While numerous studies have been carried out on the physico-chemical behaviour of milk, research into concentrates has been hampered by difficulties in concentrate handling, in particular age thickening and separation of the colloidal and soluble phases. Whey protein denaturation during evaporation is probably minimal as the temperatures are low (:Q0°C). At low heat treatment (72°C for 15 s) a small decrease in the amount of undenatured 13-lg and a-la (1-2%) after evaporation was observed by Singh and Creamer ( 1991 a). Increasing the total solids also reduces denaturation of whey proteins in skim milk (McKenna & O'Sullivan, 197 1 ), and 13-lg in cheese whey (Nielsen et al.,
1973; Hillier et al. , 1979), though a-la denaturation increases (Hillier et al. , 1979). The immunoglobulins and BSA which are more heat sensitive than 13-lg may denature during evaporation, especially in the first effect which is at about 70°C. The heat load from the evaporator may come into consideration in the manufacture of low heat powders. Casein micelles increase in size mainly due to coalescence of the micelles. If milk is
preheated so that most of the whey proteins are associated with the micelle the increase in size is less (Walstra & Jenness, 1984).
Soluble minerals continue to move into the colloidal phase and Ca2+ activity increases
slightly during evaporation (Walstra & Jenness, 1984). Nieuwenhuijse et al. ( 1988)
suggest that changes during the evaporation process are fast enough to overcome most differences induced by preheating.
The concentrate viscosity restricts the maximum concentration that can be achieved without adversely affecting the properties of the spray dried powder (Bloore & Boag, 198 1 ). Factors which affect concentrate viscosity are preheating temperature and holding time, concentrate total solids, and temperature and holding time of the concentrate. Bloore and Boag ( 198 1) observed that preheating at a high temperature, short holding time ( 1 13°C for 10 s) produced concentrates with a lower viscosity than concentrates preheated at a low temperature, long holding time (80°C for 120 s), even though the extent of denaturation was similar. The viscosity increases with the total solids content, markedly so above 45% total solids. It becomes necessary to heat the concentrate prior to spray drying, from sooc to 70°C to reduce the viscosity (Muir, 1980; Bloore & Boag, 198 1 ). The combination of high temperature (>60°C) and holding time can lead to aggregation and gelation, producing powders with poor solubility characteristics (Muir, 1980).
2.5.2.2 Spray drying
Atomization of the concentrate gives a large surface area over which drying can take place. The droplets are brought into immediate contact with a stream of dry heated air (200°C). As the droplet passes through the dryer the moisture evaporates and the temperature of the droplet remains relatively stable (=70°C). The outlet air temperature of the dryer is important as by the time the droplet has reached that point, most of the moisture has been removed and the semi-dried droplet is susceptible to heat damage (Singh & Newstead, 1992).
Little is know about the protein and mineral changes during spray drying. The design and operation of the drier are important factors as they affect the amount of thermal treatment that the concentrate is exposed to. For the most part the properties of reconstituted milk powder preheated at low temperatures are similar to that of the raw
milk. Singh & Creamer ( 1991 a) observed that spray drying had no significant effect on the denaturation of a-la and �-lg A and B in the manufacture of low heat (72°C for 1 5 s) and high heat ( 1 10-l20°C for 2-3 min) powders. The calcium activity of milk powders immediately after being reconstituted is lower than in raw milk, but increases linearly with the logarithm of time (Greets et al. , 1983). The spray drying step is thought to have an important effect of Ca2+ activity (Greets et al. , 1983) though further work is required to determine what the exact relationship is.
OBJECTIVES
The research work was carried out using skim milk in order to achieve the following objectives.
e Obtain a better understanding of the reactions of milk proteins occurring during preheating, evaporation and spray drying steps of milk powder manufacture.
e Develop mathematical models to describe protein reactions during heat treatment of milk.
e Investigate the effects of artificial and seasonal modifications to milk composition on protein reactions.
As preheating has a large impact on the properties of the milk powder, the whey protein interactions occurring in this step were chosen as the main focus for investigation in this study.