After the production of milk at farm level, it is transformed into a variety of dairy products viz. flavoured milk, liquid creams, dried evaporated milk powder, yogurt, cheese, fermented dairy products, ice cream and traditional dairy products. For production of these value-added dairy products, there are various machines and equipments required for all these transformations. 

After Operation Flood, which started white revolution in India, our country never looked back in terms of milk production, which is rapidly increasing every year because of improved cattle breeds and healthy feed. Today, India is the largest producer of milk with 187.7 MT (NDDB 2019-20) yearly production which is a very remarkable achievement. Due to this, India is also known as the oyster of the global dairy industry. The credit goes to Dr Varghese Kurien, the father of white revolution, known as milkman of India.

Modern dairy plants are totally equipped to maneuvre raw materials and finished product with very little or no meddling. It is not uncommon to find dairy plants that process more than 1,000,000 milk per day. There are various processing operations to which milk undergoes and hi-tech equipments are used for processing this much amount of milk continuously with perfect standards and good hygiene. Some of the commonly used processing equipments in dairy industries are pasteurisers, homogenisers, evaporators, chillers, butter churners and so on.

Milk is a highly perishable commodity, which needs immediate cooling or heating after milking to prevent the growth of microorganisms and pathogens. The milk is stored in stainless steel silos at 4 °C temperature. The silos or milk storage tanks may be vertical or horizontal types, which are usually insulated for better cooling. The inner shell is made of stainless steel whereas the outer shell can be made of mild steel, stainless steel or any other metal but mild steel is more economical choice for outer shells in which dairy products are not in direct contact.

For dairy plants, where extremely large quantity of milk is processed, vertical silos are installed outside the processing section to avoid tall buildings for their accommodation. The milk is transported to dairy plants by road tankers made of stainless steel having two to three layers thermocol insulation. The dairy equipments should be designed wisely. In any storage shell, there should be smooth inner surface and no sharp corners to aid in proper cleaning. Agitators are used to avoid cream separation. From these storage silos milk is transferred in the production or processing section where it undergoes various processing operations. 

In production or processing section, the basic operation to convert the milk into various value-added milk products is heating of milk in which the temperature varies for different heating processes like pasteurisation, sterilisation, thermisation, UHT and evaporation. The most important milk processing operation i.e., heating operation, which is carried out with the help of various heat exchangers. A troublesome problem to the dairy industries is deposition of burnt milk solids on the hot surfaces of dairy process equipments known as fouling. It is a severe problem both economically as well as technically because of the following reasons:

It reduces the rate of heat transfer. 

Makes the equipment more expensive because of increased pressure drop, which enhances the pump horsepower requirement.

Increased time of operation resulting decrease in processing capacity because the equipment must be turned off for cleaning.

Leads to cleaning difficulty and increases cleaning costs.

Fouling can be a source of bacterial contamination.

The fouling is of two types viz. type A and type B: 

In fouling, the important role is played by proteins especially ß-Lactoglobulin. It is found that upto 90°C, denaturation of ß- Lactoglobulin protein governs the fouling on the surface of the heat exchanger (Lalande et al. 1985). Among various heat exchanger equipments viz. plate heat exchangers (PHE), spiral type heat exchangers, tube in tube or double pipe heat exchangers, shell and tube type heat exchanger, plate heat exchangers are widely used for pasteurisation and sterilisation of milk because of compact design, easy maintenance, cleaning, and better heat transfer coefficient (Shah and al.1988) however, these heat exchangers undergo deposition fouling, chemical fouling, biological fouling and corrosion fouling resulting in increased capital and operating costs (Sandu and Lund, 1983).

Proper energy audit and maintenance are must for these dairy-processing equipments to avoid heat loss and unnecessary energy consumption. The various measures, which would result in less fouling, are outlined below:

1) The acidity of the milk should be less

2) Milk should be preheated at 65 °C.

3) There should not be too high temperature difference between the milk and heating medium.

5) There should be circulation in turbulent flow regime.

6) Tube surfaces should be completely wetted in evaporators. 

7) The flow rate of milk should be high and there should be uniform distribution of milk all over the tube surface.

8) Air leaks should be prevented into the heat exchanger.

9) There should be proper filtration of milk before feeding into the heat exchanger.

10) A very small amount of phosphate salts should be used in milk where legally permitted, for example in evaporated milk. Most effective salt to reduce fouling is sodium pyrophosphate.

Apart from all these, altering the surface properties of these processing equipments can be an excellent option to get rid of fouling problems (Beuf et al. 2004). Dairy processing equipments or heat exchangers in which milk and milk products are in direct control, are made of SS 304 or SS 316. Covering the surface with another coating materials can help in fouling reduction, which reduces the time required to clean the heat exchangers resulting in minimum resource consumption and chemical use for cleaning. Materials like silica, SiOx, Ni-P-PTFE etc. can be used for coating and Ni-P-PTFE is found best in terms of cleaning efficiency (Beuf et al. 2004). Enzyme based coatings can also be done for antifouling effects.

All these preventive techniques and developed technologies are useful in reducing fouling, which is must to run the plant more economically as the fouling decreases the overall heat transfer and increases water consumption, electricity consumption and maintenance cost.