in that the actual maker or production manager is an experienced practical man who may have had no scientific training. Decisions are often made by subjective assessment which may or may not be based on simple analytical tests.
In the following discussions of technological aspects of manufacture (sections 13-20) a milk of good bacterial quality (p. 60) and suitable equip-ment in a satisfactory hygienic condition (p. 162) are taken for granted.
The more important laboratory tests for milk quality in manufacture are summarized in Table 25. It is assumed that a suitable test for general bacterio-logical quality, such as resazurin or méthylène blue, is applied to all the milk used for manufacture.
13. EVAPORATED MILK A. General
The legal standards for evaporated milk in the U.K. are for full cream 9%
fat and 22% SNF and for skim 20% milk solids. These figures are linked with the stability of the product and the composition of whole milk, which is assumed to be 3-6 % fat and 8-8 % solids-not-fat, giving a ratio of 1 to 2-44.
It is therefore necessary to standardize milk for the manufacture of full cream evaporated milk. The degree of concentration is about 2\ for full cream and 2\ for the skim product. Common defects and their causes are summarized in Table 26 and quality control methods in Table 27.
B. Homogenization
Milk for sterilized and evaporated milks is invariably homogenized to prevent the formation of a cream layer (p. 81) on storage, but separation of fat may take place on standing if the homogenization is not adequate. The lower the viscosity, the larger the fat globules and the greater the density difference between fat and the skim milk, the faster will the globules rise and
the cream layer form.52 The depth of layer is largely influenced by clustering of the globules. Maxcy and Sommer41? have devised a method for measuring the effectiveness of homogenization based on these properties. See also p. 81.
TABLE 26. Evaporated milk (unsweetened) Defect
Poor keeping quality
Sedimentation Instability or gelling
Taints, peptonization, gas
Cause Inadequate heat treatment
Milk high in heat-resistant spores Faulty pre-heat treatment Inadequate stabili-sation by phosphates, etc.
Faulty seaming of cans
General references Hunziker413
Rose414 Scott4^ Brunner416
Also as for sterilized milk.
TABLE 27. Evaporated milk: quality control methods Flavour
Colour Sterility
Chemical Deposits
Odour and taste Visual or Tintometer disc
Incubation of unopened cans at 37 and 55°C (for spores). Incubation at 27°C and plating (for
"leakers")
Fat* and total solids*
Visual examination of product and can
* Tests of legal significance.
C. Equipment
The four types of sterilizer commonly used are : (i) batch or autoclave.
These should preferably be of the rocking or rotating type, but static types are sometimes used; (ii) continuous hydrostatic (water column) type; (iii) continuous hot air type (not common) ; (iv) sequential pressure type. These usually consist of three chambers: heating, holding and cooling. All types
102
have their advantages and disadvantages, e.g. (i) are cheap but the equal-ization of heat-treatment is difficult, (ii) are reliable but not flexible, (iii) allow no balancing external pressure so that can size is limited, and (iv) are expensive but most efficient.
D. Technical Problems
The main problems with evaporated milk are sterility (or keeping quality) and physical stability. There is no simple or rapid test for measuring the rate at which the product will thicken on storage. The control of thickening can be improved by (i) careful selection of the raw milk, (ii) selection of the times and temperatures of both pre-heat-treatment and subsequent steril-ization, and (iii) selection of bicarbonate, polyphosphate, phosphate and/or citrate as stabilizing salts. Evaporated milk may become less viscous on storage (age-thinning), the viscosity varying inversely with the logarithm of storage time, while the slope of the curve varies directly with temperature.
Sterilization retards fat separation, and the most important factor is effective homogenization.417
Tin and iron, but not copper and lead, may increase appreciably in con-centration in evaporated milk stored in cans, and at 37°C darkening, fat separation and corrosion of the can proceed rapidly.418
E. Flavour
The general basis of flavour in evaporated milk is that of sterilized milk.
The higher the temperature and the longer the time of holding the more cooked is the flavour, other conditions being equal.419
In a study of three processes (conventional, HT-ST and aseptic), Sundara-rajan et al.420 found that the flavour by the aseptic process was initially best but deteriorated rapidly during the first few weeks of storage whereas that by the HT-ST process deteriorated very little. Evaporated milk made by the conventional process was consistently the poorest in flavour.
F. Bacteriological Aspects
The probability of a spore surviving in a can of evaporated milk depends not only on the heat-treatment but also on the heat-resistant spore content of the initial milk. The problem is thus basically the same as for sterilized milk (p. 83). The factors to be controlled are therefore (i) initial content of heat-resistant spores (these may be defined as those surviving 105°C for 10 minutes), (ii) pre-heat-treatment (p. 98), (iii) hygienic conditions of evaporator, (iv) time and temperature of sterilization process and (v) bac-terial condition of cans.
Aseptic filling (p. 88) of concentrated milk has been investigated, particu-larly in U.S.A., but has not advanced beyond the experimental stage (p. 103).
G. TheUseofNisin
The essential problem with evaporated milk is to ensure sterility without damaging structure, physical stability, colour, flavour and nutritive value.
It is now permissible to add nisin to evaporated milk (p. 159) and this gives a satisfactory keeping quality with less drastic heating. Nisin is particularly effective against spore-forming bacteria (cf. 60. Cheese, p. 142). The use of nisin permits a milder heat-treatment (e.g. 105°C for 15 min or 113°C for 3 min) than normal (e.g. 113°C for 15 min) (maximum temperatures listed) and the losses in the most labile vitamins (Bi2, BO and Bi) are then slightly reduced.421