Gelation of the product on storage is a further problem, and is accom-panied by an increase in non-protein nitrogen.423»424 This is apparently not brought about by enzymes.425 The factors controlling physical stability (time to gelation) of sterile concentrated milk (26 % solids) milk are numerous and complex. Gelation is preceded by a reversible steep rise in viscosity, although the actual gelation is irreversible. The higher the sterilizing temperature and the lower the forewarming temperature, the shorter is the gelation time
although the flavour is better. Heat treatment must naturally be sufficient to ensure sterility and this is given by the equation
„ Time(sec) 10(Temp(°F)-250)
F° = 60 X Z
where Fo, the "amount of heat" required, is a constant and Z is the number of degrees Fahrenheit on the temperature scale over which the thermal death-time curve passes in traversing one logarithmic cycle on the death-time scale (e.g. 18).425a The greater the total solids content the less is the stability, and the rate of change in gelation rate is about three times the rate of change in concentration of solids. Stability may be twice as great in summer as in winter. Serum protein accelerates gelation but fat globule membrane material (the "agglutinin" fraction) is without effect. Rather surprisingly addition of calcium does not have much effect. Gelation rate rises sharply with temperature but there is a curious peak in the curve about 10°C. In general viscosity increases with homogenizing tem-perature.426 Conventional evaporated milk gels more slowly because it receives a much more drastic heat treatment, e.g. 113-116°C for 15 to
18 min compared with the treatment of 132°C for 70 sec used for this pro-d u c t .4 2^
Viscosity is the most important factor controlling foaming and foam stability.430 The viscosity of concentrated milk (44 ±1-5% solids) may vary from 70 to 800 centipoises according to batch variation (composition of milk) and temperature (35 to 10°C).
The best flavour is obtained by using as high a temperature and as short a time as possible for sterilization. Oxidation of the fat is minimized by reducing the dissolved oxygen and the head space in the can to the minimum.
The stability of the heated emulsion is enhanced by homogenization at 3500-4000 lb/in2. A very small concentration of a stabilizer (alginates, gums, etc.) may be used, subject to legal requirements.
Canned evaporated milk is remarkably stable when stored at 0°C. The major changes in four years are a slight loss in riboflavin and vitamin B12, a slight increase in non-protein N and a marked increase in free fatty acids.431
15. FROZEN CONCENTRATED MILK
Concentrated milk may be less stable in the frozen state than milk because the ratio of water to constituents is lower. On storage a change takes place resulting in the precipitation of the protein (p. 85). According to one theory this instability is linked with the crystallization of the lactose, which promotes instability.432"5 This lactose crystallization effect may not hold if less than 30 % solids are present.434
Whereas quiescent cooling of a supersaturated solution may result in a stable solution, cooling with agitation induces nucleation and crystal growth.
Both agitation during cooling and the presence of foreign nuclei decrease stability of frozen concentrated milk.436 This illustrates the advantages of a good quality clean milk and of high speed centrifugation (p. 84) as a desir-able pre-treatment of milk for this purpose. Addition of gums and other stabilizers gives variable results on account of interaction between the milk proteins and the stabilizers.
The protein changes are greatest with the /Mactoglobulin and a-casein, the other whey proteins showing little change.436
Serious attempts have been made in U.S.A. to put frozen concentrated milk on the market but there are clearly many problems to be solved before this product becomes a commercial proposition.
16. SWEETENED CONDENSED MILK A. General
Whereas evaporated milk depends on the efficiency of heat treatment for its keeping quality, sweetened condensed milk depends on its sugar (sucrose) concentration. This involves a very careful standardization or adjustment of the milk SNF and sugar ratio, and of the fat for the full cream product, and also control of the final moisture content.
The skim product is made in the greater quantity. It is in good demand by confectionery manufacturers and is a satisfactory outlet for skim milk.
B. Manufacture
The major factors controlling the quality of this product are: (i) correct adjustment of sugar—milk solids ratio; (ii) correct choice of "striking point" ending evaporation; (iii) chemical stability of the milk; (iv) pre-heat-treatment of the milk; (v) destruction of yeasts in sugar and avoidance of subsequent infection both in bulk and in cans; (vi) adequate "seeding" with powdered lactose.
The "striking point" is usually determined by specific gravity measure-ments, although refractometric values are sometimes used as an auxiliary aid.
Viscosity measurements are less reliable because of seasonal changes in viscosity, differences between sources of milk, and the complexity of factors affecting viscosity.437-8
"Sandiness" is due to the size of the lactose crystals exceeding about 15 μ, the threshold at which they can be detected on the tongue. A high quality product should have crystaL below 10 ^6 in size.
106 J. G. DAVIS
Some manufacturers may rely on the residual product in the pipelines, pumps and coolers for the "seeding" or acceleration of crystallization of the lactose. Cleaning of these units is not so frequent as with milk because of the biological stability of this product, but yeasts, moulds and cocci can grow slowly, and neglect of this aspect can easily lead to a poor quality product.
C. Defects