Radio-frequency heating is a mechanism in which we are heating the dielectric materials (especially food) at high frequency using the electromagnetic and radio frequency waves. Radio-frequency heating contains electromagnetic spectrum in terms of frequency and wavelength. As most of our food is dielectric in nature that means it contains polar molecules like water which having dipole moment and both negative and positive charges are centered inside it. Some of its applications are pasteurization, drying of milk, sterilization, etc. Microwave heating is used over conventional dairy processes to extend the shelf life of milk and various milk products. While in contrast to MWH, Radio Frequency heating provides better penetration of waves in food than MWH because of its low frequency and large wavelength nature and has easier system configuration than microwave heating. Radio Frequency heating provides uniform heating to the product.
An alternative to Radio frequency heating, is Microwave heating, it also has same applications in food processing like Radio frequency heating.
Radio Frequency heating follows the volumetric process to heat up any dielectric material like food products through the radio waves that are emerging from the magnetron and triode valve via electrodes. In this process, the dielectric substance is placed in the cavity between the electrodes and the high radio-frequency voltage is applied across the electrode, and the emerging radiations penetrate through the food system and interact with its component like protein, fat, salt, etc., and generate the heat within the food due to the frictional interaction between the molecules and that produces a high energy efficiency with the lower heating time factor.
Dielectric and ionic interaction is the two mechanisms that are required to achieve radio-frequency heating in food processing, in ionic interaction process, there are several ions present in food such as sodium and chloride, salts, etc that produce ionic conduction by the ionic movement towards oppositely charged electrode and rapid change in polarity causes the resistance heating, which ultimately results in multiple collisions of ionic molecules and distributed bond and produces heat. Dissolved ions are important contributors to heating compare to water dipoles. The basic component of most of the dairy food is water. Which is dipole in nature. When the high-frequency oscillation of water molecules under the influence of electromagnetic field collides with each other, then the higher rate of dielectric heat is generated.
Factors Having Impact on Radio-frequency heating (RFH)
There are some factors like frequency, temperature, the shape of the sample, etc that alter the heat generation process so we have to look over that in order to get the desired outcome for example: the non-uniform heat generated by microwaves creates the temperature gradients within the product that affect the volumetric heat generation process.
1. Frequency is one of the most important factors in radio frequency heating, the lower the frequency -the higher the depth of Frequency also influences the dielectric characteristics of the sample. But we have some restrictions over the selection of frequency in food processing. The frequency range of 915MHz is legally permitted in the USA and Australia for Industrial applications.
2. Composition refers to sample ingredient components and with the change in the processing stage, the composition positions of the dielectric material also vary as well as the different heating behavior need to be accounted for. As the composition of the material changes then it leads to change the dielectric properties which in turn varies in the rise of temperature. Different compositions having different loss factor, dielectric constant, and temperature of heating. The amount of water, fat, protein, minerals present in dielectric material contributes to the heat generation process. As free water and protein are favorable towards the heat generation, the amount of free water increases loss factor and dielectric constant. Milk heats at a faster rate than water due to the presence of protein and ionic components in it.
3. The shape, size, and volume of the dielectric material decided the required wavelength and power consumption in the heat-generating process, higher in the density leads to higher in heating ratio, and higher in the volume results in higher power consumption, compositions like viscosity, specific heat electrical conductivity also affects the heating rate, for example, milk fat having a low dielectric property but its heats up faster than milk because of its lower specific heat.
Radio-frequency heating factors also depend on several system-level configurations like how the system(oven) is built, its shape and size, a cavity of the oven for the flow of charged ions, generated radio waves in the system, position placement of the dielectric material, the volume of the sample, static and dynamic behavior, heating in batch or continuous, etc which directly influence the rate and the pattern of heating and for that, a power test is used to indicate the required power consumption in the given volume of sample.
Microbiological Safety of Dairy Products
One of the major issues in dairy products is food infection and how effectively we can inactivate microorganisms through treatment. We have pest control for these insects but the chemicals used in that lead to environmental concern, and also harmful for the human body. Nutrients of the dairy products are also affected by these chemicals. So, the thermal treatment came into the picture. In the thermal process, we go for the cold spot and make sure that the cold spots achieve the required temperature to guarantee the safety of dairy products. We have z value (i. e temperature coefficient) which states that bacteria are more sensitive to heat as compared to nutrients as the z value of bacteria and nutrients are around 10∘C and 25∘C.Radio-frequency heating offers the possibility of fat heating in solid and semi-solid foods by direct interaction between electromagnetic waves.
Application and effect on the quality of milk products
The effect of radio frequency heating is effectively destroyed pathogen such as Listeria innocua and E-coli k-12, in milk also be effective in pasteurization milk E-coli is more sensitive to Radio frequency heating than Listeria also using of radio frequency system, complete inactivation of pathogen was reported in <1 minute when temperature is around (60-65degree).
Some of the common applications of microwave are heating, precooking, tempering, blanching, pasteurization, sterilization, drying, etc. Both MWH and RFH are capable of pasteurization of milk. these technologies neutralize phosphatase enzyme in milk the technology mode has specified equivalent features compared to conventional pasteurization. Hence the quality of milk is exceeding and provides microbial protection. Differential heating effects of MW have been used in the development of wide variety of products using in dairy ingredient. Some of its other applications like. Drying and Baking, Defrosting, Sterilization and Pasteurization, Disinfestation.