The effect of pasteurization and quick sterilization or "UHT" (~140°C for about 2 sec) on the colour of milk is negligible, but sterilization (108—
115°C for 20-40 min) can make milk appreciably brown, especially if cooling
IS slOW.325, 326, 346-8
Addition of calcium can decrease the liberation of sulphydryl groups from the /Mactoglobulin in milk349 and also reduce the cooked flavour in milk, possibly by bonding with partially denatured protein.350
I. Microbiological Faults and their Control 1. General
The heat-treatment may not destroy all spores in a bottle, and the higher the spore content the greater is the chance of one or more spores surviving.
Bulk milk may contain from 1 up to 100 or more spores per ml. Spores are more numerous in autumn and winter. Suggested standards for raw milk are given in Table 10.
Pre-treatment at 135-140°C for about 2 sec (UHT) followed by steril-ization at 110°C for about 10 min in the bottle should ensure destruction of all spores. A badly contaminated milk and/or inadequate sterilization can lead to characteristic faults. If surviving spores are of the rennet-producing type and germinate, the casein may be rendered unstable to heat so that when used for tea, coffee, etc., or when boiled the sterilized milk curdles and the protein sinks to the bottom. Such milk may appear normal to sight, odour and taste.
2. Incubation Tests
When any material, food or otherwise, is packed in a sealed container and sterilized, or receives a treatment which is intended to sterilize it, ordinary plate count tests are useless for control purposes, except for special investiga-tions of faults. By far the best test for sterilized milk and cream, and for evaporated milk is to incubate a number of containers at 27°C and 55°C.
Only spores survive even an inefficient sterilizing treatment, and the object of the test is to ascertain if any spores capable of growth have survived.
Most spores can germinate and grow at 27° and 37°C, and some even at 55°C; the obligate thermophils may grow only at temperatures above 37°C.
Growth is readily detected by opening the container after, say, 14 days at 37°C and after 7 days at 55°C. If in bottles or jars, evidence of growth may be seen in the form of gas bubbles, digestion of the curd, etc. Cans often swell in such a test and bottles may burst. The only difficulty is in deciding what is a statistically satisfactory number to test per batch, and of course in providing the necessary incubator space.
TABLE 17. Sterilized milk, quality control methods Flavour Odour and taste
Colour Visual or Tintometer disc Identification as Turbidity144* (legal test but of no sterilized milk use commercially)
Keeping or bacteriological Incubation of unopened bottles quality at 27 and 55°C
Chemical Fat,* solids-not-fat,* added water*
as for milk.
Ordinary colony counts and coli tests are useless
* Legal standards involved.
For continuous processes two containers may be taken every hour and one incubated at 27° and the other at 45° or 55°C. Any clear evidence of growth fails the sample. If a large incubator is not available the containers may be stored in a warm place, e.g. a boiler room or over a hot tank which holds them in the range of 20-30°C. Although taking a longer time to detect surviving spores, this method is more closely related to use conditions.
From a commercial point of view the keeping quality at consumer-holding temperatures is the crucial problem. Spores may germinate at 37° or 55°C and not at 10-20°C in a reasonable time, say one month for sterilized milk and one year for evaporated milk. Nevertheless consumers often store sterilized foods in warm places and the use of 27° and 55°C for incubation tests is sound. Moreover, exported goods are often subjected to high tempera-ture. Apart from a few special instances it can be assumed that any sterilized food which passes both 27° and 55°C incubation tests will be satisfactory.
The presence of non-sporing-organisms in a sterilized product indicates a leaking container or imperfect seal. An incubation temperature of 27°C is most suitable for detecting this type of contamination, and is also suitable for detecting heat-damaged spores other than those of the obligate thermophils.
88
3. Routine Quality Control
The tests which should be applied to sterilized milk for routine control are summarized in Table 17.
9. ASEPTICALLY FILLED UHT (QUICK-STERILIZED) MILK A. General
One of the most interesting developments in the dairy industry in the last few years has been the introduction of milk sterilized by heating at 135-140°C for about 2 sec. It has been officially designated as UHT (ultra high tempera-ture) treated milk. In this process the milk is heated by tubular or plate heat exchangers, or by direct steam injection or mixing with steam. This treatment is a logical development from earlier methods as follows :
Process
This process requires special equipment and more elaborate control than the HT-ST process but the biggest problem in its commercial development is that of filling the sterile milk into sterile containers without contamination.
This is exceedingly difficult as milk, unlike soups, etc., cannot be subjected to these high temperatures for some time without loss of quality. The Dole aseptic canning system cannot therefore be used, and, moreover, glass con-tainers and cartons cannot be used with this method. No advantage in keeping quality is consistently gained when this type of milk is filled into bottles or cartons in the ordinary way, but filling hot into sterilized milk bottles gives an improved keeping quality.351 Bacillus cereus is usually responsible for early spoilage and corynebacteria for the later spoilage.
The greatest advance in this field has been made by the Tetrapak system.
The sterile milk is fed into a tube of black polythene-lined or aluminium-polythene laminated paper in a sterile atmosphere, the material having been
sterilized immediately before filling by passing through 15% hydrogen peroxide solution which is subsequently decomposed by heat. The tube con-taining milk is then formed into a tetrahedron and heat-sealed. If the process is successfully operated the product is sterile and of indefinite keeping quality bacteriologically, but oxidized flavours can develop in a week or so. Initially there is a slight cooked flavour which may be succeeded by a "cabbagy"
flavour due to volatile sulphur compounds. A steam injection process (Uperization) coupled with aseptic Tetrapak cartoning has been in commercial use in Bern, Switzerland, for 3 years and an indirect heating process by APV and Alfa-Laval plate heat-exchangers similarly coupled has now been per-fected by a British dairy company. This process became legally recognized in the United Kingdom in October, 1965,352 but only with indirect heating, i.e. steam injection methods are not permitted.