i.Changes During Khoa Making
Preparation of khoa involves three actions viz. heating, concentration and vigorous stirring and scraping. These actions cause following physico-chemical changes:
• Denaturation of whey proteins, particularly the β-Lactoglobulin and α-Lactalbumin tend to undergo varying degree of self aggregation, denaturation/coagulation as the temperature of milk rises above 77oC (90oC in case of buffalo milk) and their interaction start with k-casein as the heating temperature rises.
• The denaturation of the whey proteins results into production of sulfhydryl compounds, which produces characteristic cooked flavour in khoa and other heat desiccated products.
• Though casein is quite stable at high temperature, but the combined effect of severe heat treatment and concentration and complexing of whey proteins with it, substantially increases the size of casein micelles. This results into loss of stability of casein micelles. The coagulation of casein tends to increase logarithmically at a constant temperature of heating with milk solids
concentration.
• Coagulation of casein and aggregation of whey proteins with it entrap part of the fat in this aggregate. The moisture also disperses as fine droplets in the aggregates of proteins.
• The removal of moisture from milk results into concentration of milk solids and changes in their status, particularly proteins and minerals. These eventually change the state of milk from liquid to solid or semi solid. All the constituents,including lactic acid, increase in proportion to the degree of concentration.
• Vigorous agitation of milk at elevated temperatures during desiccation of milk disrupts the fat globule membrane releasing some fat in form of free fat (about 50% of the total fat). Formation of this free fat is very important from the flavour and texture point of view.
• The heating of milk at high temperature causes conversion of soluble calcium and magnesium to insoluble form that leads to interaction (denaturation and hydrolytic cleavage) between protein components.
• Continuous heating of milk solids at high temperature (around boiling) causes browning. This is attributed to formation of Melanoidin pigment. This browning reaction is Maillard type due to interaction between addehyde group of lactose and free amino group of casein.
ii. Changes During Rabri Making
• Rabri is not a homogenous product. Part of it is in concentrated liquid form and part in form of solid and flaky material.
• Most of the fat is present in form of flakes. The flakes are obtained by heat coagulation of proteins on the liquid air interface and its interaction with milk lipids (malai).
• Denaturation and complexing of whey proteins with casein also result due to heating and concentration of milk but to lesser degree than in khoa.
• All other changes viz. browning, lactose and mineral status and vitamin loss are similar to that in khoa.
iii. Changes During Basundi Making
• Unlike khoa and rabri, basundi is in concentrated liquid form (i.e. flowable product).
• The rate of browning is more than khoa because sugar is added to milk during boiling and the rate of Maillard reaction is more. Some caramelization also takes place.
• Denaturation of whey protein and their complexing with k-casein also take place during basundi making. But heat coagulation of casein does not appear appreciably.
• Fat is present in its native state (dispersed in aqueous phase in the form fat globules), though some free fat releases due to vigorous action of stirring and scraping.
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