Khoa Making: Traditional and Modern Equipment

Khoa – also called mawa or khoya – is one of India’s most important heat-desiccated dairy products. It serves as the base for dozens of beloved sweets like burfi, peda, gulab jamun, and kalakand. Producing khoa involves reducing milk to roughly one-fourth of its original volume through controlled heating, stirring, and scraping until it becomes a dense, semi-solid mass. The equipment used in this process has evolved significantly – from simple open pans heated over wood fires to sophisticated continuous machines capable of producing tonnes of khoa per day. Understanding this equipment is essential for anyone involved in dairy processing, whether at the cottage level or in a large commercial plant.

What is khoa and why does it need specialized equipment? 🔗

Khoa is prepared by thermally desiccating milk at atmospheric pressure until the total solids reach about 55-65%, giving it a semi-solid consistency. Buffalo milk is generally preferred because of its higher total solids, white colour, and naturally sweet taste. As moisture evaporates, the milk thickens progressively and eventually undergoes heat-induced coagulation of proteins. This stage demands vigorous scraping to prevent milk solids from sticking to the heated surface and burning.

The product is classified into three varieties – pindi (hard, used for burfi), danedar (granular, used for kalakand), and dhap (soft and moist, used for gulab jamun). Each variety requires slightly different processing intensity, making equipment design a critical factor in determining khoa quality.

Because of the constant need for scraping, temperature control, and gradual moisture removal, ordinary cooking vessels are often inadequate for consistent, large-batch production. This is exactly why the dairy industry has developed a range of equipment – from simple karahis to advanced scraped surface heat exchangers.

Traditional equipment: the karahi 🔗

The oldest and most widely recognized equipment for khoa making is the karahi (also spelled karhai), an open, wide-mouthed pan traditionally made of iron or brass, though stainless steel versions are now common. The karahi is placed over a direct heat source – wood, coal, or LPG – and milk is poured into it for boiling and concentration.

The process is straightforward. Fresh whole milk is heated while the operator continuously stirs and scrapes the bottom and sides of the pan with a flat ladle. In the initial stages, when the milk has a high water content, stirring can be slow and intermittent. But as concentration increases and milk solids start sticking to the heated surface, vigorous and continuous scraping becomes necessary to prevent scorching.

The entire process typically takes 45 minutes to 1.5 hours depending on the batch size and heat intensity. Eventually, the milk reduces to a thick, pasty mass that solidifies upon cooling – this is the finished khoa.

Advantages of the karahi 🔗

The karahi remains popular for good reasons. It is inexpensive, widely available, easy to maintain, and versatile – the same pan can be used for making ghee and other products. Small sweet shops and household producers across India still rely on it. Many consumers also prefer the slightly caramelized flavour that traditional open-pan desiccation imparts to the khoa.

Limitations of the karahi 🔗

However, the traditional karahi has significant drawbacks for any operation aiming at scale or consistency. The temperature varies during the process due to non-uniform flame distribution, which can lead to burnt patches, discolouration, and off-flavours. The process is heavily dependent on the operator’s skill, making product quality inconsistent from batch to batch. It is labour-intensive, energy-inefficient, and not suitable for large-volume production.

Semi-mechanized equipment: the jacketed kettle 🔗

The first major upgrade from the traditional karahi is the steam-jacketed kettle, a hemispherical vessel made of stainless steel with a double-layered wall. The space between the inner and outer walls serves as a jacket through which steam or hot water circulates, providing indirect heating to the milk inside.

This design addresses the biggest problem of direct-flame heating: uneven temperature distribution. With steam circulating uniformly around the vessel, the risk of localized overheating and burning is greatly reduced. A thermostat can be used to regulate the temperature, giving the operator much better control over the process.

How the jacketed kettle works 🔗

Milk is loaded into the inner vessel. Steam at a controlled pressure enters the jacket, heating the vessel wall uniformly. The operator – or in some designs, a motorized scraper mechanism – stirs and scrapes the milk as it concentrates. The scraping mechanism in jacketed kettles is specifically designed for the heavier scraping demands of khoa compared to, say, ghee making, where fouling is less severe.

Jacketed kettles are preferred by small to mid-scale dairies and larger sweet shops. They offer better energy efficiency, more uniform product quality, and reduced labour compared to the open karahi. However, they still operate in batch mode, which limits throughput.

The conical process vat 🔗

An important innovation in batch-type khoa equipment is the conical process vat (CPV), developed at the ICAR-National Dairy Research Institute (NDRI), Karnal. This equipment was patented in 1987 by Agrawala, Sawhney, and Bikram Kumar (Patent No. 165440) and represents a purpose-built solution for processing viscous dairy products like khoa.

Design and construction 🔗

The CPV features a stainless steel conical vessel with a cone angle of 60°. Its steam jacket is partitioned into four segments for efficient thermal energy use and reduced heat loss. The conical shape is the key design feature – it gives the vessel a straight-line heating surface profile, which is much easier to scrape effectively compared to the curved surface of a hemispherical kettle.

Operational advantages 🔗

The scrapers in a CPV are motorized with adjustable speed control, allowing operators to vary scraping intensity as the milk concentrates and becomes more viscous. The product is loaded from the top and can be discharged through an axial mechanism at the bottom, making handling more convenient than with hemispherical kettles.

The CPV provides more uniform heat distribution, better scraping efficiency, and improved product consistency compared to simple jacketed kettles. It is particularly well-suited for medium-scale dairy operations that need better quality control but do not require the throughput of a fully continuous system.

However, since the CPV still operates in batch mode, its production capacity is inherently limited.

Continuous khoa-making machines 🔗

For large commercial dairies producing khoa at scale, batch equipment simply cannot keep up with demand. This need led to the development of continuous khoa-making machines based on scraped surface heat exchanger (SSHE) technology. Two major designs have emerged from Indian dairy research institutions.

Inclined scraped surface heat exchanger (ISSHE) 🔗

The ISSHE was developed by the National Dairy Development Board (NDDB), Anand. This machine consists of a feed balance tank, a positive displacement pump, and an inclined stainless steel cylinder with a steam jacket divided into multiple compartments. Inside the cylinder, a rotor fitted with scraper blades serves the dual purpose of scraping the heated surface and conveying the product along the length of the exchanger.

The machine is fed with milk that has been pre-concentrated to about 42-45% total solids. The inclination of the cylinder is critical – it allows a pool of boiling milk to form at the lower end, which is essential for developing the characteristic texture and flavour of khoa. As the product moves upward along the inclined surface, it progressively loses moisture and reaches the desired consistency.

Manufacturing trials have demonstrated that the ISSHE can maintain steady output of good-quality khoa over extended runs of 8 to 16 hours. The capacity and quality depend on factors such as the type and composition of the feed milk, feed flow rate, angle of inclination, rotor speed, and steam pressure. This machine can produce approximately 50-60 kg of khoa per hour, making it suitable for medium to large dairy plants.

Thin film scraped surface heat exchanger (TSSHE) 🔗

The TSSHE was developed at NDRI, Karnal, and uses a two-stage cascade arrangement. In the first stage, whole milk enters an SSHE whose rotor has variable-clearance blades rotating at about 200 rpm. This stage concentrates the milk to around 40-45% total solids. The partially concentrated milk is then pumped into a second SSHE with a different rotor design – typically featuring helical blades and variable-clearance blades rotating at 140-150 rpm – where final desiccation takes place.

A three-stage version of this system has also been developed, where an additional stage with modified scraper design handles the final, most viscous phase of concentration. Both SSHEs are steam-jacketed for precise temperature control.

How scraped surface heat exchangers work 🔗

A scraped surface heat exchanger works on a simple but effective principle. The product flows through a cylindrical chamber whose walls are heated (or cooled) by a medium – typically steam – circulating in an outer jacket. Inside the cylinder, spring-loaded or mechanically driven blades rotate continuously, scraping product buildup from the heat transfer surface. This serves two critical purposes: it prevents fouling and burning, and it constantly exposes fresh product to the heated wall, dramatically improving the rate of heat transfer.

For khoa making, this is ideal. The continuous scraping action handles the increasing viscosity of the concentrating milk without burning, while the enclosed design ensures hygienic processing and minimal spillage losses. The result is a uniform product with consistent colour, texture, and flavour – something that is difficult to achieve with manual methods.

Comparing the equipment: which one fits where? 🔗

Each type of khoa-making equipment serves a specific segment of the dairy industry. Here is a practical comparison:

The karahi is best suited for household use, very small sweet shops, or artisanal producers who value traditional flavour above output volume. Capital cost is minimal, but labour and energy costs per kilogram of khoa are high.

The steam-jacketed kettle fits small to medium dairies and larger sweet shops. It offers a good balance between investment cost and product quality. Batch sizes are typically 40-50 litres of milk per batch.

The conical process vat is ideal for medium-scale dairy plants that need consistent quality with semi-mechanized operation. Its motorized scrapers reduce labour dependence, and the segmented jacket design improves energy efficiency.

The continuous SSHE-based machines (ISSHE and TSSHE) are designed for large commercial dairies. They offer the highest throughput – 50 kg/hour or more – with minimal manual intervention, consistent product quality, and the ability to run continuously for extended periods.

Alternative and emerging approaches 🔗

Beyond these core equipment types, researchers have explored several alternative methods for khoa production. Roller drying involves spreading concentrated milk on heated rotating drums, where it dries into a thin film that is scraped off. Membrane technology – specifically reverse osmosis – has been used to pre-concentrate milk before thermal desiccation, reducing the energy and time required in the final heating step. Some dairies also use the Contherm-Convap system (developed by Alfa Laval), which combines a scraped surface heater with a vacuum concentrator for efficient moisture removal.

The NDDB has also developed a semi-mechanized process that pairs a condensing unit with a mechanized khoa pan, offering a practical middle path for cooperatives and mid-sized plants that may not need fully continuous systems.

Why equipment choice matters for khoa quality 🔗

The choice of equipment directly affects the sensory and compositional quality of khoa. Studies comparing traditional and mechanized methods have consistently found that mechanized khoa has better appearance, flavour, and texture than traditionally prepared khoa, mainly because of uniform heat distribution and controlled scraping.

However, some consumers and sweet makers argue that the slight caramelization and smoky notes from open-pan methods give traditional khoa a flavour that machines cannot replicate. This is why many premium sweet shops continue to use karahis or jacketed kettles despite having access to modern equipment.

From a food safety standpoint, enclosed systems like SSHEs are clearly superior – they reduce contamination risks, ensure hygienic handling, and make it easier to comply with FSSAI regulations for commercial dairy products.

What do you think? As India’s dairy industry modernizes, should traditional khoa-making methods be preserved for their unique flavour, or is the shift to fully mechanized production inevitable? How do you think equipment choices affect the taste of khoa-based sweets in your region?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC10020399/
2. https://doi.org/10.9734/cjast/2021/v40i931352
3. http://dairy-technology.blogspot.com/2014/11/khoa-making-equipment.html
4. https://ndri.res.in/dairy-engineering
5. https://www.ukessays.com/essays/sciences/manufacture-and-storage-of-khoa.php
6. https://www.alfalaval.us/products/heat-transfer/scraped-surface-heat-exchangers/scraped-surface-heat-exchangers/
7. https://www.nddb.coop/services/ppd/dairyproducts/khoa