Industrial applications of ohmic heating are mainly limited to continuous thermal processing of food. Main objectives of this research were to explore its applications in a batch type container, and investigate additional non thermal effects of electric field on bacterial spores, enzymes, carotenoids, flavonoids and quality parameters in food.
An ohmic packet made up of multilayered laminate material and capable of sterilizing food was developed. A pouch design optimization exercise was conducted to improve the heating profile and integrity of the hermetic seal at temperature and pressure conditions associated with a sterilization process. A simulation study in 3D was done for sterilization in the ohmic pouch. The mathematical model and sterilization were validated through an inoculated pack study using Geobacillus stearothermophilus spores.
Non thermal effects of electricity at frequencies of 10 kHz and 60 Hz during ohmic heating were studied on thermophilic bacterial spores of G. stearothermophilus and Bacillus coagulans. A precise capillary sized ohmic device was invented to eliminate potential source of experimental errors, and to obtain identical time temperature histories for both ohmically and conventionally heated samples. Ohmic heating at both frequencies were found to accelerate inactivation of bacterial spores. It was hypothesized that release of polar dipicolinic acid molecules and spore proteins at higher temperature conditions and their vibrations under the external electric field have resulted in an accelerated inactivation. A linearly increasing temperature analysis of the B. coagulans data suggested that Z values obtained at isothermal conditions may only be valid over a narrow range of temperature.
Effects of ohmic heating on pectin methylesterase (PME) and polygalacturonase (PG) enzymes were evaluated in fresh tomato juice at pH of 3.9 and 4.4. PME at pH 3.9 showed a high variability in the inactivation data possibly due to the interaction of multiple isozymes with the electric field. PG data at pH 3.9 showed comparatively less variation and hinted at a higher inactivation rate under ohmic heating. At pH 4.4, PG was found to show accelerated inactivation under ohmic heating in comparison to literature values for its resistant isozyme, PG1. Color values (l, a and b) were found to remain unaffected over the studied pH and temperature ranges.
Carotenoids and phenolic compounds of fresh tomato juice remained unaffected by ohmic heating. Inherent amounts of β-carotene, lycopene, phenolic acids and quercetin indicated no or minimal changes at a temperature range from 90 to 110°C at pH of 3.9 and 4.4. Naringenin content showed an interesting behavior with chalconaringenin converting to naringenin in greater proportions. Naringenin equivalents were found to increase at the temperature of 105°C under ohmic heating.