CONDENSED AND DRIED MILKS

Module 14. Physico-chemical changes taking place during manufacture of dried milks

Lesson 39
WPN INDEX AND HEAT DENATURATION

39.1 Introduction

Heat treatment of the original product or the concentrate can cause denaturation of serum proteins; the conditions during spray drying are rarely such as to cause extensive heat denaturation. The extent of denaturation is an important quality mark in relation to the use of milk powder. For instance, if the powder is to be used in cheese making, practically no serum protein should have been denatured in view of the rennetability; in infant formulas, on the other hand, the rennetability should be poor.

39.2 WPNI

The extent of the denaturation of serum protein can be used as a measure of the heating intensity applied. This is true also where denaturation by itself may be of no importance, but other changes associated with intense heat treatment are. An example is the flavour of a powder to be used in beverage milk, which requires a mild heat treatment. An intensive heat treatment is needed for some other uses, for instance, to acquire good stability against heat coagulation in the manufacture of recombined evaporated milk, or a high viscosity of the final product when making yogurt from reconstituted milk. It is also desired if milk powder is used in milk chocolate; presumably, Maillard products contribute to its flavor.

The whey protein nitrogen index (WPN index) is generally used to classify milk powders according to the intensity of the heat treatment(s) applied during manufacture. To that end, the amount of denaturable serum protein left in the reconstituted product is estimated, usually by making acid whey and determining the quantity of protein that precipitates on heating the whey. This can be done by Kjeldahl analysis of protein nitrogen or by means of a much easier turbidity test that is calibrated on the Kjeldahl method. The result is expressed as the quantity of undenatured serum protein per gram of skim milk powder. The classification is shown in Table 39.1 as follows:

Table 39.1 Whey protein nitrogen indices for heat classification of milk powders

39.3 Damage Caused by Heating

High drying temperatures can result in undesirable changes in the dried product. Generally, it is only after the powder has been dissolved again that the changes involved are noticed. Three quite different categories of undesirable changes can be distinguished:

39.3.1 Heat denaturation and killing of microbes

a. T he reaction rate is highly dependent on temperature, but the reaction is much slower and less dependent on temperature at low water content.

b. Some results for the inactivation of phosphatase are showing that, a higher average drying temperature and the higher dry-matter content of the liquid gives more inactivation because this goes along with a higher viscosity and hence, on average, larger drops. Therefore, a longer heating time is needed at a dry-matter content in which the inactivation rate still is appreciable. However, the largest drops contain the greatest amount of material.

c. Killing of bacteria is higher as the average drying temperature is higher. The initial increase of survival of bacteria with increasing dry-matter content is presumably due to a substantial decrease in heat sensitivity of the organism.

d. Heat denaturation of globular proteins and consequently, inactivation of enzymes and killing of microorganisms greatly depend on water content. It may also occur that removal of water increases the concentration of a reactant or catalyst for heat inactivation; this is presumably the case for chymosin in whey, because at, say, 40% dry matter, a _w and diffusivity are not greatly lowered.

e. When spray-drying a starter culture, survival of bacteria is of paramount importance. Often, a relatively large proportion of an inert material, generally maltodextrin, is added to the liquid before drying, which lowers the temperature sensitivity of the bacteria. In this way, survival rates over 80% can be achieved.

39.3.2 Insolubility

a. Part of the protein may be rendered insoluble during the drying process due to heat coagulation.

b. The powder contains particles that do not dissolve in water, but the amount is a tiny fraction of the powder.

Fig. 39.1 shows that insolubility increases with increasing outlet temperature and increasing dry-matter content of the milk. It also greatly depends on drier design. Presumably, heat coagulation mainly occurs in some (large) drops or powder particles that recirculate in the drier and become wetted again. A cumulation of high temperatures and high dry matter content for a relatively long time then causes the problem. Modern driers tend to give very small insolubility figures.

39.3.3 Formation of hair cracks

a. These can form at high drying temperatures because the outer rind of a drying droplet soon reaches a glassy state; the pressure gradients developing in the particle then cause these very thin cracks to form.

b. In case of whole milk powder, part of the fat can now be extracted from the powder by solvents like chloroform or light petroleum. The extractable fat is often called free fat, but that is a misleading term.

39.3.4. Autoxidation of lipids

Autoxidation of lipids follows quite a different pattern (Fig. 39.2, curve 4). The reaction rate is high for low a _w . Possible causes are that water lowers the lifetime of free radicals, slows down the decomposition of hydroperoxides, and lowers the catalytic activity of metal ions, such as Cu²⁺. The rate of many reactions greatly depends on the water content; an example is given in Fig. 39.3

Fig. 39.2 shows Rate of Maillard reactions (—) and of protein becoming insoluble (---) in concentrated skim milk at high temperature (~80°C) as a

function of water content. (Walstra et al., 2006)

Because of an increase in the concentration of reactants, the rate of bimolecular reactions at first increases due to removal of water. On further increase of Q* , the reaction rate often decreases again; this decrease would be caused by reduced diffusivity. A good example is the Maillard reaction (Figure 39.3, curve 5). The irreversible loss of solubility of milk protein in milk powders, and the rate of gelation of concentrated milks show a trend similar to curve 5. On removal of water from milk, it thus is advisable to pass the level of approximately 10% water in the product as quickly as possible.

Fig. 39.3 shows Relative reaction rate ( K_r) of various reactions plotted against the water activity (a_w) of (concentrated) skim milk (powder). The upper abscissa scale gives the water content (% w/w).

(1) Growth of Staphylococcus aureus ;

(2) oxidative degradation of ascorbic acid;

(3) enzyme action (e.g., lipase);

(4) lipid autoxidation;

(5) Maillard reaction (non-ezymatic browning). (Walstra et al., 2006)

Thus, the influence of some process variables on product properties is inevitably complex as the relationship between two parameters may depend on other variables and also it is mostly not possible to vary only one process variable. However, it is observed that a two-stage drying provides increased possibilities to make powder with various desirable characteristics. The major process variables and properties of the dried product affected are listed hereunder in Table 39.2.

Table 39.2 Major process variable and properties of the dried products influenced