Physico Chemical Properties

During the manufacture of condensed milk certain physical changes such as development of brown colour, changes in viscosity are noticed besides increase in density/specific gravity. One of the important chemical changes noticed is decrease in pH. These changes occurring in the physico-chemical properties in condensed milk due to various processing conditions have profound effect on the final quality and self-life and storage stability of the product. Some of these physico-chemical properties are discussed below:

i)                  Density/Specific Gravity:

Removal of water in the manufacture of condensed milk increases the total solids content. Because of the increase in total solids concentration the density/sp. gr. Of condensed milk increases. The Sp. Gr. Of concentrated milk is product is thus used as a rough approximation of the percentage of total solids in the product. Baume’ hydrometers are widely used for sp. Gr. Measurements. Table 9.4 gives some of the figure for specific gravity and Baume’ readings

 

Specific gravity and Baume’ readings of condensed milks

ii) Freezing Point: Freezing point of milk is quite constant, however it is always lower than that of water and varies with composition of milk. Removal of water as well as addition of soluble substances, such as sugar, has direct effect on the freezing point of condensed milk. Table  gives figures for freezing point of condensed milk.

 

Freezing point of condensed milk

iii) Viscosity: Viscosity of condensed milk varies greatly according to the composition, processing conditions and storage period. Increase in concentration increases the viscosity. Homogenization and low temperature storage also contribute to the increase in viscosity. Viscosity during the manufacture and storage of concentrated milk is highly important for successful marketing of the product. Too high or low viscosity is not desirable in the product. Low viscosity tends to leave objectionable sugar sediment in condensed milk and fat separation in evaporated milk. Too high viscosity causes gelformation and mineral deposits in evaporated milk.

iv) Colour and Flavour: Objectionable brown colour is sometimes found in condensed milks. Exposure to high heat during manufacture and storage of condensed milks tends to darken their colour and develop a “cooked” flavour.Various factors contributing to the brown discolouration and cooked flavor production, are heat treatment, total solids concentration, pH condition, addition of compounds like sugar and stabilizers, storage temperature and time of storage.

v) Changes in pH: A decrease in pH as compared to milk can be expected in condensed milks during manufacture because of the processing steps such as preheating, concentration and sterilization. Decrease in pH has definiteeffect on the stability of concentrated milk towards heat sterilization.