During the manufacture of Indian milk products like rasogolla, sandesh, peda, and gulabjaman, kneading of the chhana or khoa and subsequent mixing withsugar are the important unit operations. Mostly, these operations are labour-intensive, manual, and unhygienic and lacks quality control for the finished product. In spite of the rapidly growing demand for Indian milk products in national and international markets, process automation and mechanised production of these milk products is the need of the hour in order to meet the consumer demand, food safety and quality requirements.

The use of jacketed vessels is a common practice in food industries. Batch-wise kneading, heating, cooking, and concentration are some of the major operations carried out in these vessels. Uniform mixing of highly viscous products is one of the major concerns for the food industry. In these products, mixing and kneading do not generate flow currents inside the mixing vessel. Sometimes it is necessary to take special steps to eliminate any stagnant zones inside the mixer in order to avoid the product riding around on the mixing blades.

In most vessels, elaborate designs are made for the discharge of the mixed, cooked, or concentrated products from the vessel. In all the existing designs, the vessel is tilted to pour the product out of the vessel, but the highly viscous paste-like products cannot be completely emptied out, as some amount would remain stuck to the mixing or kneading blades. The mixing blades with these designs cannot be easily removed from the vessel for manual cleaning.

Scaling on hot surface is a major problem while heating and concentrating non-foaming liquids and pastes in a jacketed vessel. In most cases, the vessel wall offers negligible thermal resistance to the heating or cooling medium, but a thin film of paste adhering to the vessel wall offers a considerable amount of resistance. This reduces heat transfer from the heating medium to the liquids or paste. In order to minimise the thermal resistance, it is necessary to fix a scraper blade very close to the vessel wall. The shape of the blade should be such that a flow current of the liquid or paste is established around the blade.



Indian Patent No 165440 describes a vat, tapered downward with a cone angle of 60o. The vat has a steam jacket, which is partitioned into four segments to provide control over the heat transfer surface area. The mixing elements, consisting of scraper blades, are placed in a manner to facilitate scraping from the entire heating surface. Discharge of the product is through a propeller located at the bottom end of the vat.

US Patent No 4123174 describes a mixer with a downwardly tapered conical wall. The mixer doesn’t bear any heating or cooling jacket for thermal treatment of product. The mixer uses a ring-shaped screw as the mixing element, and the mixing element rotates about its own axis and at the same time it rotates about a conical path along the inside wall of the vessel. Discharge of product is carried out through a product outlet tube extending vertically downward.

Objectives
The first objective of the present invention is to provide a jacketed conical vessel for mixing, heating, concentration, evaporation and cooking (both at atmospheric and subatmospheric pressures) operations involved in the manufacture of food products. Another objective is to provide a simple and straight  mixing and kneading mechanism to increase the convenience of the blades of the mixing vessel. The third objective is to provide for easy discharge of the end product from the vessel.

All About the Vessel
The invention is a single-jacketed scraped surface conical vessel. The bottom surface of the vessel is conical. The conical bottom allows the vessel to remain dry when not in use. The angle between the bottom of the vessel and the horizontal is 15o. The vessel is jacketed and there is a provision for the heating/cooling medium to be introduced into the jacket. There is also a provision to connect a steam trap and vent off valve at the jacketed portion.

Two Teflon-coated kneading and mixing blades are placed symmetrically inside the vessel. The blades are rotated by a central vertical stainless steel shaft. The blades are placed at an angle with respect to the conical and vertical surface of the vessel to avoid the possibility of the viscous products riding around on the kneading and mixing blades without reincorporating into the mixture. Of the two blades, one fits close to the inner walls of the vessel whereas the other maintains a certain amount of clearance from the walls. The blade that maintains clearance allows the product to pass through, while the other blade scrapes the product that sticks to the wall surface.

As the product passes through the clearance it gets stretched and sheared, forming a thin layer on the inner wall of the vessel—as that obtained in a thin film evaporator. This reduces the particle size of the product and increases the rate of heat transfer in the vessel.

Slots are provided on the blades to adjust the desired clearance between the vessel surface and the edge of the scraper blades. The shaft that transmits power to the kneadingcum- mixing elements is introduced from the bottom of the vessel and is connected to a speed reduction unit.

The vessel is closed with a cover and is provided with a pressure or vacuum gauge, pressure release valve, and vent-off ball valve, sterile air and vacuum line inlet, and light and sight glasses. Provision is made for inserting a thermocouple and measuring the temperature of the product when the vessel is operated under vacuum or pressure. The top cover is tightened to the vessel through a foodgrade gasket.

The rotor is coupled to the power transmission shaft through gland packing. An outlet is provided at the bottom of the vessel. The product is discharged as a result of the centrifugal action created on the product due to rotational speed of the kneading-cummixing blades. The discharge valve is provided with two closers which are used for the discharge of liquid and high-viscosity dough and pastes.

Advantages over Comparable Vessels
The vessel is multipurpose and can readily be adopted in the preparation of milk-based products such as sandesh, peda, kalakand, and milk-based puddings. The conical bottom of the vessel, with the discharge valve provided at the bottom, facilitates complete drainage of liquid foods and cleaning solutions by gravity. This keeps the vessel dry when not in use, unlike most hemispherical vessels. Also, the unit requires less time and offers a high rate of heat transfer, proper utilisation of energy and better control over process parameters in comparison to traditional methods of manual kneading, mixing, and cooking.

In most food applications, kneading, mixing and heating or cooking are carried out as two independent unit operations using steam-jacketed vessels as described by Mc Cabe et al., (1993) and Uhl et al., (19765). The present invention combines the operations and offers mixing and cooking in a single unit.

Screw-type devices such as kneader extruders are commonly used for kneading, mixing, and cooking of highly viscous products. However, unlike the screw-type device, the present vessel is open to manual inspection during mixing and cooking and is convenient for manual cleaning.

The design of its kneading elements is quite simple, unlike that of extruders. Moreover, the top cover, which is provided with a pressure gauge, pressure release valve, ball valve, sterile air inlet, and light and sight glasses, allows the vessel to work under pressure in preparation of milk-based puddings. For the preparation of vacuum-dried powders like coconut milk, ginger, tomato, onions, the vessel can be used under vacuum to reduce the initial moisture content of the pulp to a certain level in order to reduce its drying time when dried in a vacuum dryer.

Cleaning and Dismantling of the Vessel
Some of the important steps to be considered while cleaning the vessel are:
• Fill the vessel up with water and allow it to stand for 10 minutes to remove the loosely adhered solid particles.
• Run the scraper drive for 5 minutes and open the discharge valve and flush out the contents.
• Run the equipment with caustic soda detergent (0.1 to 2 per cent hot solution of metasilicate trisodium phosphate).
• Continue the operation for 5 to 10 minutes and again open the discharge valve and flush out the contents.
• Flush out the vessel with water.

The jacketed multipurpose vessel can be used for kneading, mixing, heating and cooling operations during processing of food and pharma products. The vessel can be used for concentration (atmospheric and vacuum) and cooking (atmospheric and pressure) of nonfoaming products like tomato pulp, fruit juices, candy etc. The vessel offers easy removal of the highly viscous products handled during the manufacture of rasogolla, sandesh, gulabjamun, kheer, cheese spread, khoa, peda, kalakand, candy, etc. The vessel reduces manual processing of the products and increases the overall hygiene and shelf-life.

Dr Jatinder Kumar Sahu is Associate Professor, Department of Agricultural Engineering, School of Technology, Assam University Dr Hrishikesh Das is Professor Emeritus, Department of Agricultural and Food Engineering, Indian Institute of Technology Kharagpur
References
• Agrawala, S. P., Sawhney, I. K., and Bikrma Kumar (1987). Mechanised conical process vat. India Patent 165440
• Hans-Joachin. (1978). Mixer. US Patent 4123174
• Mc Cabe, W. L., Smith, J. C. and Harriot, P. (1985). Unit Operations in Chemical Engineering, 4th Ed. McGraw-Hill Book Company, New Delhi, India.
• Rauwendaal, C. (2001). Polymer Extrusion. 4th Ed. Hanser Publication, Munich.
• Uhl, U. W. and Gray, J. B. (1966). Mixing: Theory and Practice. Vol. I and II. Academic Press. New York.
• Van Der Wel and Gerardus, P. (2001). Intensive mixer. US Patent Application 200100224400.