The characteristic "cardboard", tallowy or oxidized flavour of milk powder is the chief difficulty in the development of the market for domestic use. The flavour is almost impossible to conceal or disguise. Of the anti-oxidants used, Abbot and Waite454 found that dodecyl gállate was the best, and propyl gállate, nordihydroguaiaretic acid and 6-dodecyl-3,7,2',5'-tetrahydroxy-flavone the next best, and butylhydroxyanisole of little use for spray-dried
whole milk powder. As with oxidative changes in all milk products, copper content is an outstanding factor. Methods are available of estimating copper in milk and powder,478
2, Changes During Storage
During storage milk powder undergoes characteristic flavour and colour changes. Oxidation of the fat leads to the peculiar tallowy odour and taste, and condensation between reducing sugar aldehyde groups and certain amino groups leads to browning and "flat" and "unclean" flavours. Oxygen content controls the former and moisture content the latter. Both changes occur faster the higher the temperature, but higher moisture levels may preserve flavour better in low temperature storage (0°C) especially with higher oxygen tensions. Very low temperatures ( —18°C) appear to be un-desirable unless all oxygen is excluded with foam dried whole milk powder.479 Verhoog480 has described a rapid method for the estimation of the air entrapped in milk powders.
The shelf life of spray dried whole milk powder in the presence of oxygen is at an optimum (from the flavour view point) when the moisture content is about 3%.481»482 With batch vacuum foam-dried whole milk the time in storage before the first significant change in flavour occurred, was in-creased about 5 times when the moisture content exceeded 2-6 %.483
The major cause of browning in milk powder is the Maillard reaction between the protein and lactose,484»485 in which the e-amino group of lysine is mainly involved.486-488 During storage lactose may break down to other sugars etc., which then also combine with casein to form similar brown compounds.456»457 Mook and Williams489 have reviewed recent advances in the improvement of whole milk powder.
5. Pre-heat-treatment of Milk for Drying
When milk is heated sulphydryl groups associated with the /Mactoglobulin are liberated. They can then exert an antioxidant effect, which extends the shelf life of milk powder,49** affect flavour491 and also heat stability.492 They may even play a role in the rennet clotting of milk.493 Lyster494 has adapted the /7-chloromercuric benzoate method for the determination of the free and "masked" -SH groups in heated milk and its products.
4. Microbiological Control
Milk for powder making now usually receives a preliminary heat-treatment which kills virtually all micro-organisms except spores. Roller powders receive a second severe heat-treatment on the rollers, but for certain purposes a low-temperature spray dried product may be required (p. 116). Strict microbiological control is essential for this. The counts reported495»496 are
118
very informative on this aspect but their work was done before the more modern heat-treatment of the milk came into use.
Higginbottom497 found that 90% of roller dried milk powder had counts below 10,000/g and that aerobic spore-bearers accounted for about 40%
of the flora. Counts varied from less than 100 to over 100,000/g. Spray-dried powders contained a smaller proportion of aerobic spore-bearers but the counts varied from less than 10,000 to over 100,000,000/g.
TABLE 36. Standard for milk powders
Roller powder
As with cheese, age is of importance in considering counts and grades, because viable micro-organisms steadily decrease in milk powders unless the moisture exceeds the generally accepted limit of 5 %. There is thus an analogy between the use of temperature as a bacteriological criterion for bulk milk (p. 60) and the use of moisture content in respect of milk powders. It has been found that counts decreased on storage but at an unpredictable rate.49«*, 498 Higginbottom suggested that counts should be made within two weeks of manufacture. For technique aspects see Ref. 499. Suggested standards are given in Table 36.
In some countries (e.g. Sweden) the colony count is a property considered for quality payment. The ordinary thermoduric count, based on pasteur-ization, is not necessarily satisfactory for milk powder because of the multiple
heat-treatment the product receives, and Wallgrensoo has described a test aligned to manufacturing conditions.
Considerable variation can occur in the quality of milk and hygienic control in the manufacture of spray-dried milk. In general good plant practice with a low grade raw milk will give a better bacteriological product than poor plant practice and a high grade milk. An initial heat treatment of 90°C for 30 sec will kill all organisms except spores. Organisms such as Streptococcus thermophilus and Staphylococcus aureus can grow rapidly in milk at 42-45°C so that holding milk in a "hot well" can lead to trouble.
18. BUTTER A. General
Butter is probably the oldest manufactured milk product as in its simplest form it involves only holding the milk in a warm place and beating or shaking the cream which has risen to the top. Pre-historic cave drawings and carvings illustrate primitive types of butter churns. There are broadly two main types of butter—flavoured butter made with ripened or soured cream (see Starters, p. 143), and sweet cream butter made without starter. The former is more attractive but the keeping quality is low because the acid conditions favour the oxidation of the fat which is catalysed by trace amounts of copper and iron.
The salt content may vary from about 0*5 to 1-5 % or even higher in those countries where a salted butter is popular. Miners and others whose mode of life involves considerable loss of sweat commonly replace the lost salt by eating highly salted cheese (e.g. Caerphilly) and butter.
Salt also has the advantage that it improves keeping quality in the micro-biological sense. For example 2% salt in butter corresponds to about 12-5%
in the serum as butter usually contains just under 16% moisture, and both salt and micro-organisms occur mainly in the serum droplets.
B. Standards
In 1966 new standards were introduced in the U.K.501 requiring a maximum of 16% water, 2% milk solids other than fat, and not less than 80% milk fat, or not less than 78 % under certain conditions for salted butter.
The definition of butter is extended to allow neutralization of cream and addition of salt, lactic cultures, annatto, carotene and turmeric.