The Science Behind Heat Desiccated Milk Products: Khoa, Rabri, and Basundi

When milk is slowly heated in an open pan to make khoa, rabri, or basundi, it undergoes a series of complex physico-chemical transformations. These aren’t just about removing water – the process fundamentally changes the proteins, fats, sugars, and minerals in milk, producing the distinctive textures, flavours, and colours that define these beloved Indian dairy products. Understanding these changes is essential for anyone studying dairy science or looking to improve the quality of heat-desiccated products.

What is heat desiccation of milk? 🔗

Heat desiccation refers to the controlled removal of water from milk through continuous heating and stirring. The milk is typically heated at temperatures between 85-100°C in a wide, open pan (karahi) while being constantly stirred and scraped to prevent burning. As water evaporates, milk solids become increasingly concentrated, and a cascade of chemical reactions begins transforming the liquid into a semi-solid or concentrated product.

The three main products that result from this process – khoa, rabri, and basundi – differ mainly in the degree of moisture removal and the specific techniques used during preparation. Khoa is concentrated to a semi-solid paste, rabri retains some liquid along with flaky cream layers, and basundi is a sweetened concentrated liquid that remains flowable.

Moisture removal and concentration of milk solids 🔗

The most obvious change during heat desiccation is the evaporation of water and concentration of milk solids. During khoa preparation, for example, the milk is concentrated to approximately 2.5 times its original solid content. This concentration is not merely a physical process – as water content decreases, the relative proportions of fat, protein, lactose, and minerals increase dramatically, and the interactions between these components intensify.

All milk constituents, including lactic acid, increase in proportion to the degree of concentration. This shift changes the entire colloidal system of milk. What was once a stable oil-in-water emulsion becomes a dense, semi-solid mass in which proteins, fats, and sugars are packed closely together, creating entirely new textural and flavour characteristics.

Protein denaturation and coagulation 🔗

Milk proteins undergo significant structural changes during heat desiccation. These changes are among the most important factors affecting the final texture of khoa, rabri, and basundi.

Whey protein denaturation 🔗

β-Lactoglobulin and α-Lactalbumin – the two primary whey proteins – begin to denature and self-aggregate as milk temperature rises above 77°C (or around 90°C for buffalo milk). During denaturation, these proteins lose their native folded structure and expose reactive groups. The denatured whey proteins then begin interacting with κ-casein, forming complexes that significantly affect product texture.

An important by-product of whey protein denaturation is the production of sulfhydryl compounds, which give khoa and other heat-desiccated products their characteristic cooked flavour. These volatile sulphur-containing compounds are released as the protein’s internal structure unfolds during heating.

Casein micelle changes 🔗

Casein is relatively heat-stable compared to whey proteins. However, the combined effect of intense heating and concentration, along with complexing of denatured whey proteins, substantially increases the size of casein micelles. This causes a loss of stability in the casein micelle system. The coagulation of casein tends to increase at a constant heating temperature as the concentration of milk solids goes up.

During this coagulation, casein aggregates trap both fat and moisture within their structure. The moisture becomes dispersed as fine droplets within the protein aggregates. This trapping mechanism is critical for the final body and texture of the product – it is what gives khoa its cohesive, dough-like consistency.

In rabri, the protein changes occur to a lesser degree than in khoa because rabri undergoes less intensive concentration. In basundi, whey protein denaturation and complexing with κ-casein does take place, but casein coagulation does not occur appreciably since basundi remains a flowable, liquid product.

Fat aggregation and free fat formation 🔗

Milk fat exists as tiny globules surrounded by a protective milk fat globule membrane (MFGM). During heat desiccation, two major changes affect the fat system in milk.

First, the vigorous stirring and scraping during the desiccation process physically disrupts the fat globule membrane. This releases a significant amount of free fat – approximately 50% of the total fat in the case of khoa. Free fat is extremely important for both flavour and texture. It gives the product a rich mouthfeel and helps carry fat-soluble flavour compounds. Free fat also contributes to the slightly oily or greasy surface characteristic of well-made khoa.

Second, as water evaporates, fat globules are pushed closer together, increasing the chance of interactions and aggregation among fat globules. The concentrated fat delivers a richer, more intense flavour to the final product.

In rabri, most of the fat is present in the form of cream flakes. These flakes form through heat coagulation of proteins at the milk-air interface, where the protein interacts with milk lipids to create the layers of malai that define rabri’s texture. In basundi, the fat largely remains in its native state as dispersed globules, though some free fat is released through stirring and scraping.

The Maillard reaction: browning and flavour development 🔗

Perhaps the most important flavour-developing transformation during heat desiccation is the Maillard reaction – a non-enzymatic browning reaction between reducing sugars and amino acids. In milk, the primary reactants are lactose (the reducing sugar) and the ε-amino group of lysine residues in milk proteins, particularly casein.

How the Maillard reaction proceeds in milk 🔗

The Maillard reaction in milk progresses through three stages. In the early stage, lactose reacts with the amino groups of protein to form an Amadori product (lactulosyllysine). In the intermediate stage, this Amadori product breaks down to produce reactive compounds such as hydroxymethylfurfural (HMF), various furans, and other flavour compounds. In the final stage, these intermediates polymerise to form melanoidins – the large brown pigments responsible for the colour of heat-desiccated dairy products.

The Maillard reaction generates a wide range of aroma compounds, including furans, pyranones, and pyrazines, which contribute roasted, cooked, and caramel notes to the product. This is what gives khoa its characteristic nutty, cooked flavour.

Differences in browning among khoa, rabri, and basundi 🔗

The extent of browning varies across the three products. Khoa shows moderate browning due to the extended heating and concentration involved. Rabri exhibits browning similar to khoa but to a somewhat lesser extent. Interestingly, basundi tends to brown more than khoa, because sugar is added to the milk during boiling. The added sugar accelerates the Maillard reaction, and some caramelisation also occurs – a separate browning reaction where sugars degrade directly under heat without needing amino acids.

Impact on nutritional value 🔗

The Maillard reaction has a downside: it reduces the bioavailability of lysine, an essential amino acid. As lysine reacts with lactose, it becomes nutritionally unavailable. Research from the National Dairy Research Institute, Karnal, has shown that available lysine decreases progressively as milk is converted into khoa, with the loss being greater when neutralised or acidified milk is used. Buffalo milk and khoa samples showed higher concentrations of browning indicators (HMF, furosine, and lactulose) compared to cow milk samples, primarily due to buffalo milk’s higher lactose and protein content.

Changes in lactose 🔗

Lactose undergoes several transformations during heat desiccation beyond its participation in the Maillard reaction.

One notable change is the isomerisation of lactose to lactulose. Lactulose is absent in raw milk but forms during heating, particularly under alkaline conditions. It serves as a useful indicator of the intensity of heat treatment applied to milk.

Additionally, lactose crystallisation can occur in the finished product. Large crystals of lactose have been observed in ordinary khoa, which can create a sandy or gritty texture if not properly managed. The extent of crystallisation depends on factors like moisture content, cooling rate, and storage conditions.

Mineral changes 🔗

The mineral system of milk is also affected during heat desiccation. As moisture is removed, all minerals become concentrated. However, there is more to it than simple concentration.

Heating at high temperatures causes soluble calcium and magnesium to convert to insoluble forms. This shift affects how proteins interact with each other and with mineral ions, influencing the texture and stability of the final product. A portion of milk salts gets precipitated during khoa preparation. When traditional iron pans (karahi) are used, the iron content of the product also increases considerably, as some iron leaches from the pan into the product during the prolonged heating process.

Vitamin losses during heat desiccation 🔗

The intense and prolonged heating involved in making khoa, rabri, and basundi inevitably leads to losses of heat-sensitive vitamins. Vitamin C and B-group vitamins (especially thiamine and B12) are the most affected, as they degrade progressively with increased temperature and duration of heating. Vitamin C is also consumed in Maillard-type reactions, further reducing its availability. Fat-soluble vitamins like A and D are more stable but can still be partially lost during extended heating.

How processing differences affect the final product 🔗

The specific physico-chemical changes in each product depend heavily on the processing conditions used.

Khoa 🔗

Khoa involves the most intense combination of heating, concentration, and vigorous stirring. This leads to extensive whey protein denaturation, casein coagulation, high free fat release, and significant Maillard browning. The result is a dense, semi-solid product with a characteristic cooked, nutty flavour and a grainy-yet-cohesive texture. The type of milk also matters – khoa from buffalo milk has a smooth, soft body due to its high fat content, while cow milk khoa tends to be stickier because of insufficient free fat release.

Rabri 🔗

Rabri is a heterogeneous product – part concentrated liquid and part solid, flaky material. The cream layers (malai) that form on the milk surface during simmering are periodically collected on the sides of the pan. These flakes form through heat coagulation of proteins at the liquid-air interface and their interaction with milk fat. The protein denaturation and browning reactions are similar to khoa but less intense.

Basundi 🔗

Basundi remains a concentrated but still flowable liquid. Whey protein denaturation occurs but casein does not coagulate appreciably. The rate of browning in basundi is actually higher than in khoa because of the sugar added during boiling, which accelerates both the Maillard reaction and caramelisation. Fat remains mostly dispersed in its native form as globules.

Factors influencing physico-chemical changes 🔗

Several factors determine the extent and nature of these transformations during heat desiccation:

Temperature of heating plays a major role. Higher temperatures speed up moisture removal and protein denaturation but also increase the risk of excessive browning or off-flavours. Most traditional producers work at temperatures around 85-95°C for a good balance between efficiency and product quality.

Type of milk significantly affects the outcome. Buffalo milk, with its higher fat and protein content, produces khoa with better body and texture compared to cow milk. Research has shown that buffalo milk results in higher concentrations of Maillard reaction indicators like HMF and furosine due to its higher initial lactose and protein levels.

Acidity of the milk is another important factor. Developed acidity and neutralisation of milk significantly affect the extent of the Maillard reaction and protein oxidation. Neutralised milk samples consistently show the highest concentrations of browning indicators and protein-bound carbonyls, because alkaline conditions favour the Maillard reaction.

Agitation and scraping intensity determines the extent of fat globule membrane disruption and free fat release. It also affects heat distribution and prevents localised burning, which can cause off-flavours.

Summing up 🔗

The transformation of fluid milk into khoa, rabri, or basundi involves a carefully orchestrated set of physico-chemical changes – moisture removal, protein denaturation and coagulation, fat globule membrane disruption and free fat release, the Maillard browning reaction, lactose isomerisation and crystallisation, mineral precipitation, and vitamin degradation. Each of these changes contributes to the final product’s texture, flavour, colour, and nutritional profile. The degree to which these changes occur depends on the processing intensity, type of milk, and specific preparation technique used for each product.

What do you think? How might modern dairy processing technologies be designed to better control the Maillard reaction in heat-desiccated products – maximising desirable flavours while minimising lysine loss? And do you think the physico-chemical differences between cow milk and buffalo milk khoa could influence consumer preferences in ways that go beyond tradition?

References

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2. https://pmc.ncbi.nlm.nih.gov/articles/PMC5583103/
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4. https://www.intechopen.com/chapters/38829
5. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/maillard-reaction
6. http://ecoursesonline.iasri.res.in/mod/page/view.php?id=90185
7. https://courseware.cutm.ac.in/wp-content/uploads/2020/06/Khoa-and-paneer-2.pdf
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC12154226/