This two-part article describes an invention for the production of chhana, an Indian cottage cheese. In the first part of the article, the authors elaborate on the product, the drawbacks of the traditional methods of production and give a brief description of the production unit.

Field of invention
The present invention relates to a process that involves the continuous production of chhana. In particular, it explains a process for the continuous production of chhana using a heat-acid coagulation unit with a vacuum-assisted inclined strainer.

Summary of the invention
In the process, a duplex plunger pump is used for metering and dosing the milk and acid solution in the heat-acid coagulation unit. A heat exchanger is used to heat the whole milk to 90–95°C prior to acidification. After mixing the acid solution with the heated milk, the milk-acid mixture is allowed to move through a vertical column. The intermittent flow characteristic of the duplex pump facilitates the proper mixing of the milk and acid solution inside the column before the coagulation of milk. The coagulated chhana, whey mass from the discharge end of the column, is held in a holding section prior to the separation of milk solids through an inclined strainer. Vacuum is maintained under the strainer to remove the adequate amount of whey from the coagulated mass.



Background
The coagulation of milk is a common practice in all dairy sectors. Coagulation is essentially the formation of a gel by destabilising casein micelles, causing them to aggregate and form a network, which partially immobilises water and traps fat globules in the newly formed matrix. This may be accomplished by an enzyme, acid or a combination of heat and acid treatment. Accordingly, coagulation of milk is called enzymic coagulation, acid coagulation or heat-acid coagulation.

Chhana, which is the Indian counterpart of soft cottage cheese, is one of the heat-acid coagulated products of milk. The concentrated and preserved milk solids in the form of chhana provide sound nutrition, novelty of flavour and texture to the consumers. With an estimated annual milk production of 100 million tonnes at an annual growth rate of 5–6%, India today stands as the largest milk-producing country in the world, and it contributes about 15% to the world’s milk production. Chhana forms the base and filler material for the manufacture of a large variety of milk products like paneer, rasogolla, sandesh, gulab jaman, etc.

Chhana is a rich source of fat and protein. It also contains fat-soluble vitamins A and D. With high protein and a low sugar content, chhana is highly recommended for diabetic patients. According to the Bureau of Indian Standard (BIS, 1969), chhana should not contain more than 70% moisture, and the milk fat content should not be less than 50% of the dry matter. According to the Prevention of Food Adulteration Rules (1976), chhana is defined as a milk product that is obtained by precipitating a part of milk solid by boiling whole milk obtained from the cow and/or
buffalo or a combination thereof by addition of lactic acid, citric acid or any other suitable coagulating agent and subsequent drainage of whey.

Despite being referred to as the Indian counterpart of cheese, chhana is more dissimilar in its native characteristics than cheese. The differences in quality are caused by the temperature of coagulation and subsequent handling. Whereas a low temperature is employed for both cheese and casein, boiling of milk is usually required for the preparation of chhana. Unlike cheese, freshly prepared chhana is free from the acidic odour. Chhana resembles cheeses including ricotta, Queso Blanco and other related Latin American cheeses.

Traditionally, chhana is prepared by heating milk to its boiling point in a steam-jacketed kettle to denature the whey protein present in milk. The heated milk is allowed to cool to 75–80°C by constant stirring. The milk is added with a suitable coagulating agent like lactic acid, citric acid or calcium lactate solution as the milk is stirred slowly using a ladle. The process is continued until the milk gets precipitated in lumps, which settle down at the bottom of the vat.

The precipitation of milk involves destabilisation of casein micelles aided by relatively higher temperature. Acid affects the stability of casein directly by disturbing the charges carried by the particles and indirectly by releasing the calcium ion from colloidal calcium-caseinate-phosphate complex. The destabilisation results in the formation of large, firm and cohesive structural aggregates from the normal colloidal dispersion of casein micelles in which milk fat, other colloidal and soluble solids are entrained with deproteinated whey. The clear whey floating on the top is drained out using a muslin cloth. The product thus obtained is chhana.

The yield and quality of chhana depends on the type of milk, heat treatment given to milk prior to acidification, temperature at the time of coagulation, acidity of milk-acid mixture and residence time of the coagulated chhana—whey mixture before the separation of milk solids from whey.

Limitations of the traditional process of making chhana
The traditional method of heat-acid coagulation of milk for chhana preparation, which is being adopted in most of the dairy sectors is cumbersome, time consuming, labour intensive and difficult for attaining high-quality standards. Some of the limitations of the existing practices are listed below:

• Heat treatment of milk and its acid coagulation is carried out batchwise.

• The extent of heating and cooling rate of milk prior to acid coagulation is not carried out optimally. The heating, cooling and coagulation of milk are carried out as three independent unit operations.

• The quality of chhana varies from batch to batch due to non-uniform treatment of process parameters.

• The batchwise drainage of whey through a muslin cloth using manual labour is not a hygienic operation.

• Human intervention during the various unit operations leads to microbial contamination of chhana and reduces its shelf life.

Due to the above limitations, the traditional method of heat-acid coagulation of milk for preparation of chhana is too rudimentary to meet large-scale demands. There is a need to develop time, energy and/or cost-efficient processes suitable for the industrial production of chhana. The objective of the present invention is to provide a process for continuous production of chhana with aninclined straining unit.

Description of the invention
According to the present invention, the process for continuous production of chhana comprises the following steps:

• Mixing acid with heated milk

• Passing the milk-acid mixture through a heat-acid coagulation unit to give the required residence time for coagulation of milk solids to chhana

• Draining the whey through a vacuum-assisted inclined strainer from the chhana-whey mixture.



The capacity of the heat-acid coagulation unit developed is 60 l.hr-1 of milk. Figure 1 shows the process layout of the unit. The detailed descriptions of the various sections of the unit are given below.

Dosing section
Dosing of milk and acid solution to the chhanamaking unit is carried out using a duplex plunger pump. The pump is provided with a manual adjustment of flow from 0–100% of the capacity of the pump. Varying the stroke length of the piston could vary the flow rate of the milk and acid solution.

The milk from the storage tank is pumped to a heat exchanger, where it is heated and flows to a coagulation column, whereas acid solution is directly pumped to the coagulation column. The intermittent flow characteristic of the pump creates an additional turbulence for the milk-acid mixture inside the coagulation column.

Milk-heating section
Due to the low flow rate of milk in the developed unit, milk is heated to its boiling point prior to acidification using a helical tube heat exchanger. The heat exchanger consists of a 6-mm diameter and a 5-m long stainless tube placed inside a mild steel jacket.

By using saturated steam generated by an electrode boiler, milk at ambient temperature is heated to 90–95°C before acidification. Two thermocouples (T1 and T2) are provided at the milk inlet and outlet of the heat exchanger to indicate the temperature of milk before and after the heat treatment. The pressure gauge provided in the heat exchanger indicates the saturated steam pressure inside the shell of the heat exchanger.

For the medium and large-scale production of chhana, a plate heat exchanger (PHE) can be used for heating milk instead of a helical tube heat exchanger. In the case of heating using a PHE, milk can be heated to 90–95°C using hot water. The hot water can be generated in the PHE under pressure. (To be continued)

JK Sahu is an Associate Professor, Department of Agricultural Engineering, School of Technology, Assam University, MD Singh is an Associate Professor, Department of Dairy Engineering, SG Institute of Dairy Technology, Patna H Das is Professor Emeritus, Department of Agricultural and Food Engineering, Indian Institute of Technology Kharagpur