Ohmic Heating of Milk & Milk Products

Milk & dairy products has always been depended on thermal techniques for sterilization and for various processing. Providing product with heat is very convenient method for destruction of microbial growth in milk. Natural contamination of milk has always been microbial contamination due to high nutritious composition of milk. It provides perfect media for multiplication of microorganism. Heating of dairy and dairy products have been subjected to various categories depending on exposure to specific temperature for particular time period to heat source, like High temperature short time pasteurization, low temperature high time pasteurization, ultra-high temperature processing, etc. They have always been effective way of thermal treatment but due to exposure of product to such high temperature or to long time, nutritive composition tend to change with time even though they effectively curb the microbial load on product. The unrequired cooked flavour, burnt flavour, caramelization of natural sugar present in dairy product are few issues manufacturer face from conventional thermal processing techniques.

Looking for novel thermal processing methods, Ohmic heating also known as Joule heating or electrical resistance heating is one such technique that has been widely used for products other than dairy but now is used in dairy segments as well due to its advantage over heating time and uniformity. It is evident that Ohmic heating is a rapid method for heating of product and even distribution provided upper hand in maintaining the quality, nutritional aspects, texture and organoleptic characteristics of the milk & dairy products.

Principles

Ohmic heating involves passage of alternating electrical current through a food product. Food products tends to show certain electrical conductivity, depending on their composition, but also shows resistance to various components of food that have capability of causing  resistance that generates heat. This process involves food coming in direct contact with electrode. This process allows short time and high temperature processing which prevents browning reaction caused during thermal process and can be said that microbial inactivation of enzyme involves thermal processing.

Factors affecting Ohmic Heating

1. Electrical Conductivity of Milk:

Milk composition comprises of majorly of water, ions, minerals, salts such as sodium, potassium and chlorides. When subjected to electrical current, some conduct electricity through them while others resist, causing increase in temperature. Any change in the composition of milk shall cause changes in the electrical conductivity of milk. It is noted that cow’s milk has a conductivity of 4-6 mS/cm at 18 Deg. C. It is evident that increase in temperature increases the electrical conductivity of milk.

2. Orientation of particles:

Heating is more efficient if particles in milk are in parallel orientation in respect to electrical field provided. Heating can also be facilitated if milk composition includes smaller particles in them.

3. Field Strength:

 Field strength is proportional to the strength provided. Hence, increase in strength can easily reduce the processing time of milk. However solid concentration of milk plays an important role, as increase in solid composition doesn’t provide similar result when field strength is increases.

 4. Heater Arrangement

For better uniformity of heating in milk, configuration & geometry of heaters are important. Distance and dimension of electrodes also influence heating of milk. Investigation suggest better results are to be achieved when electrical field applied in right angle to the flow of milk. Stainless steel and tin electrode are susceptible to fouling and graphite is preferable more suitable.

5. Product Characteristics

Characteristics such as specific heat capacity, viscosity, density, composition, water, iconic components, proteins, fat, carbohydrate etc., are influencing characteristics that contribute to rise in temperature. 

 

Application in Dairy Industry

#	Processing Application	Advantages
1	Pasteurization of milk	Improved quality
2	Heating of whey solutions	Improved temperature sensitivity of whey proteins and handling of shear sensitive products
3	Heat treatment of milk	Inactivation of microorganisms and reduced protein denaturation
4	Alternative pasteurization	Inactivation of milk enzymes, alkaline phosphatase
5	Heat inactivation of bacteria and yeast	Improved quality
6	Heat treatment of whole, skimmed milk and low fat milk	Avoid fouling and heat induced damage to milk
7	Pasteurization of goat milk	Avoiding rancidity due to development of free fatty acids

 

Influence on Dairy Product Quality

Ohmic heating have higher heat conversion efficiency compared to conventional heating. It is quite offen used in processing of non-dairy foods such as tomato sauce, liquid egg, fish paste etc. but are now used for various dairy based food product.

1. Kefir: Mainville, et al., in 2001 used ohmic heating in kefir for reducing viable microbial cell counts without altering its product composition.Treatment was effective at 72 deg. C for inactivation of both yeasts and lactobacilli. However, both heat treatments caused significant microstructural changes in proteins and lipids.
2. Whey: Ayadi et al. in 2004 observed fouling in the whey protein when subjected to continuous Ohmic heating system, due to increase in temperature, as it resulted in denaturation of milk.
3. β-galactosidase: Pereira et al. in 2008 tested goat milk and observed no hydrolysis in lipids with no difference in the free fatty acid contents upon pasteurization at 72 C for 15 seconds by either conventional heat or OH.

Major disadvantage of conventional heating in fouling, which is caused due to usage of heating surface. As Ohmic heating does not have any heating surface and helps in rapid increase in temperature, it is competitively more advantageous. But this advantage cannot be sustainable for a long period of time, due to deposition on the electrode, due to higher power supply and temperature and hence it’s recommended to use higher frequency for lesser time.

Ohmic heating have high potential and wide scope in future for various unit operations such as dehydration, evaporation, blanching and extraction. These are subjected to continuous investigation as per consumer demand without compromising safety. There is need for in depth research about temperature distribution within dairy product.