CONDENSED AND DRIED MILKS

Module 7. Physico-chemical properties of condensed milk

Lesson 17
PHYSICO-CHEMICAL PROPERTIES OF CONDENSED MILK

17.1 Introduction

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

● Maillard reactions are of paramount importance for the flavor and color of evaporated milk. The temperature and duration of the heat treatment during manufacture determine the initial concentration of the reaction products, but ongoing Maillard reactions occur during storage, especially at a high temperature. The milk eventually develops a stale flavor also due to Maillard reactions. The flavor after a long storage time differs considerably from that directly after intense heating. This is because the complicated set of reactions involved leads to different reaction products at different temperatures. A sterilized milk flavor may be appreciated by some people when the milk is used in coffee. Off-flavors due to autoxidation need not occur.

● 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.

17.2.2 Viscosity

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.

17.2.8 Creaming

Creaming of evaporated milk eventually leads to formation of a solid cream plug that cannot be redispersed. This may be due to bridging of adjacent fat globules because of ‘fusion’ of the fragments of casein micelles in their surface layers. Accordingly, intensive homogenization is necessary. A higher viscosity of the evaporated milk often involves a slower creaming, but the relations are not straightforward. Generally, a high viscosity is due to approaching heat coagulation. The homogenized fat globules tend to participate in this coagulation and hence to form clusters, which will cream rapidly. κ -carrageenan is often added to decrease creaming rate. The homogenization has an adverse effect on the heat stability and, consequently, the homogenization pressure should not be high. The largest fat globules exhibit creaming, and therefore the aim should be to have the relative diameter of the globule size distribution as small as possible. The width is greatly affected by the type of homogenizer used.