Lesson 20. DEFECTS IN EVAPORATED MILKS
DEFECTS IN EVAPORATED MILKS
The major defects that are characteristics of evaporated milk are discussed here under
20.2 Non-Bacterial Defects in Evaporated Milk
Evaporated milk, in order to be marketable, must be free from symptoms of biological fermentation. It must be sterile. In addition, it must have certain physically important properties such as full of body that symbolizes richness and prevents objectionable fat separation, smooth and homogeneous texture, a pleasing natural milk flavour and a light creamy colour.
Much attention is to be paid to this group of non-bacterial defects such as curdiness due to heat coagulation, fat separation, mineral deposit, browning, bloats due to freezing, change in altitude and chemical action by milk acids upon base metal of container because it involves defective process itself.
20.3 Curdiness Due to Heat Coagulation
For the dependable prevention of curdiness and other physical defects that dissipate the marketable properties of the finished product, it is necessary to efficiently control the viscosity of the evaporated milk. The causes and prevention of curdiness due to heat coagulation are briefly summarized as follows:
A. Curdy evaporated milk is due to too low heat stability in the presence of heat treatment used for sterilization.
B. The standard commercial sterilizing process of holding at 115.6°C for 20 min or other temperature time ratios with equal lethal effect represents the minimum temperature time ratio that will insure a sterile finished product.
C. When the cooking time (minutes held at full sterilizing temperature) approaches the heat stability time, there is danger of permanent curdiness in the finished product.
D. If the heat stability of the evaporated milk is too low to ensure freedom from curdiness, it may be increased and curdiness prevented by use of higher forewarming temperature or by the proper use of casein stabilizer.
E. Too old or otherwise inferior raw milk as well as holding the evaporated milk too long or at too high a temperature before sterilization are not the frequent causes of curdy evaporated milk.
F. Higher homogenizing pressure such as pressure between 211 & 281 kg/cm2 or higher lowers the heat stability and increases the tendency to curdiness. Raw milk of inferior quality or condensed milk held improperly before sterilizing or homogenizing pressure in excess of 175 kg/cm2 increases the tendency to a curdy finished product.
20.4 Fat Separation in Storage
Adoption of homogenization as an integral function of manufacturing of evaporated milk eliminates the defect of fat separation as a serious problem. Yet even in the homogenized product, the distribution of fat globules is not permanently uniform, on progressive storage of homogenized product, it becomes higher and richer on top layer.
If objectionable intensity of fat separation is to be avoided, it is necessary in addition to efficient homogenization (the fat globules uniformly to a diameter of 2 µ or less) to
(1) Aim not for maximum heat stability but for heat stability sufficiently moderate to give evaporated milk a full body and a viscosity adequate to definitely retard fat separation in storage.
(2) To accomplish this, the processor aims at a concentrate that possesses heat stability a few minutes longer than about twice that of the sterilizing period.
(3) Varying the sterilizing process, yields the optimum viscosity for maximum fat emulsion stability.
20.5 Mineral Deposit
Under certain conditions, there is a tendency for a whitish gritty deposit to form near the bottom of the container. This deposit consists chiefly of tri calcium citrate. Its causes & prevention are as follows:
(1) Citric acid is believed to be present in milk combined with Ca and Mg. The citric acid content of milk has been found to be greatest when the cows are on green pasture.
(2) Higher the concentration of evaporated milk, the greater the tendency for appearance of a mineral deposit during storage.
(3) The temperature of storage is a controlling factor. The higher the temperature, the larger the amount of deposit. At 7.2°C there is no mineral deposition.
20.6 Browning of Evaporated Milk
The reasons for browning are discussed somewhere else. However, the causes and prevention are summarized as follows:
(1) The darkening of colour is due to reaction in the presence of sterilizing heat, between the sugar of milk and certain amino acid. This reaction causes discoloration of the proteins.
(2) The Browning effect is much less intense in the case of high temperature short time heat treatment then LTLT.
(3) The colour of evaporated milk darkens with age. The rate of darkening increases with storage temperature. At 5°C or below there is no change of colour during storage.
20.7 Bloats of Evaporated Milk
The bloats are caused due to
(2) Change in altitude and
(3) Chemical actions.
Bloats resulting from these causes are frequently occurring. When freezing, the content expands sufficiently to cause the cans to buldge at their ends, giving the impression of bloats. When subsequently transferred to temperatures at which the milk melts, the buldging ends almost invariably flip back to normal.
Change to high altitude may cause tins to buldge. This will result especially when cans are sealed at low altitude and are then transferred to high altitude regions. The danger in such cases is intensified, if the evaporated milk happens to be cold at times of filling and atmosphere at high altitude is warmer causing the air and milk in can to expand. The pressure difference between inside and outside of tins thus causes the can ends to buldge out.