Lesson 17. PHYSICO-CHEMICAL PROPERTIES OF CONDENSED MILK
Module 7. Physico-chemical properties of condensed milk
PHYSICO-CHEMICAL PROPERTIES OF CONDENSED MILK
Concentrated milks are liquid milk preserves with considerably reduced water content. Water is removed by evaporation. Preservation is achieved either by sterilization, leading to a product called evaporated milk , or by creating conditions that do not allow growth of microorganisms. The latter is generally realized by addition of a large quantity of sucrose and exclusion of oxygen. The resulting product is called sweetened condensed milk .
These products were initially meant for use in regions where milk was hardly or not available. The milks were packaged in small cans. The contents were often diluted with water before consumption to resemble plain milk. Currently, alternative products are used more often, such as whole milk powder or recombined milk. For the concentrated milks, some alternative forms of use are developed, and processing and packaging have been modified. The consumption of sweetened condensed milk has greatly declined.
17.2 Product Properties
17.2.1 Flavour & colour
● When the milk is used in coffee, the brown color is often desirable to prevent the coffee from acquiring a grayish hue. The brown color depends greatly on the Maillard reactions, although the color of the fat plays a part.
The viscosity of evaporated milk is often considered an important quality mark. Many consumers prefer the milk to be viscous. However, it should pour as like thin cream. This can be achieved by sterilization in such a way that visible heat coagulation is barely prevented. UHT evaporated milk is always less viscous and, therefore, κ -carrageenan is often added.
17.2.3 Age thinning
If the original milk contains bacterial lipases and proteinases due to growth of psychrotrophs, these enzymes may remain active in the evaporated milk and lead to strong deterioration, i.e., soapy-rancid and bitter flavors, and to age thinning. Evaporated skim milk may even become more or less transparent due to proteinase activity.
17.2.4 Nutritive value
The nutritional value of evaporated milk can be significantly decreased as compared to that of plain milk. In-container sterilization can destroy up to 10% of the available lysine, about half of the vitamins B1, B12, and C, and smaller proportions of vitamin B6 and folic acid. All of these changes are far smaller when UHT heating is applied.
The nutritive value of the sweetened and unsweetened condensed milk is very high. Both of them are rich in fat and fat soluble vitamins viz. A, D, E & K and body building proteins, bone forming minerals and energy giving lactose. While sweetened condensed milk is especially high in energy giving sucrose because of the added sugar, evaporated milk is suitable for infant feeding since it makes a soft curd and is easily digested.
17.2.5 Heat Stability
Concentrated milk is very less stable during sterilization than non-evaporated milk, and the fairly intensive homogenization applied decreases the heat stability further. Moreover, evaporated milk should increase in viscosity during sterilization. Essentially, the viscosity increases by incipient coagulation. A subtle process optimalization is needed to meet these requirements.
Therefore, the milk must be preheated before evaporation in such a way that most serum proteins are denatured. Otherwise, the evaporated milk forms a gel during sterilization due to its high concentration of serum proteins. Preheating is, for example, for 3 min at 120°C.
17.2.6 Effect of stabilization
The pH should always be adjusted. Preheating and evaporation have lowered the pH to about 6.2 or 6.1, and that is far below the optimum pH. In practice, Na2HPO4 . 12H2O is usually added, but NaOH can also be used.
17.2.7 Effect of homogenization
Homogenization of evaporated milk does not lead to formation of homogenization clusters. It is often observed that a slight homogenization increases HCT, which cannot be easily explained.
UHT heating of evaporated milk after homogenization is not possible. Even traditional sterilization is difficult if the milk is highly concentrated or if the evaporated milk is intensely homogenized. There are some other factors affecting heat stability. It can be improved by lowering the calcium content of the milk before evaporation by means of ion exchange. Addition of 0.05% H2O2 or about 15 µ mol Cu2+ (from 0.5 to 1 mg . kg − 1 ) after preheating but before evaporation tends to increase the heat stability.