Synergy of combining pressure-electric field-heat for preserving shelf-stable low-acid foods was investigated. Our hypothesis was that synergy of pressure (rapid temperature increase with heat of compression as well as instant adiabatic cooling) can be simultaneously combined with ohmic heating (rapid internal heat generation) to produce microbiologically safe and better quality shelf stable low acid food products.
First, studies were conducted to characterize in situ electrical conductivity of selected vegetable products (carrot, potato, red radish) using a semi-custom made high pressure experimental setup. In situ electrical conductivities of vegetable samples under going pressure treatment at 200, 400, 600 MPa at 25ºC (holding times up to 10 min) were estimated. The hardness and stiffness of the samples were evaluated using the instrumental texture analyzer. This information was related to the crunchiness index (CI). Pressure treatment increased electrical conductivity values of all the samples as a function of pressure and holding time. Beyond a certain threshold of pressure holding time, the electrical conductivity did not change further. Knowledge of electrical conductivity values of raw, treated and frozen-thawed products was utilized to calculate tissue disintegration index (Z). Z values were found to be inversely correlated to CI.
Quality studies of pressure ohmic thermal sterilization (POTS) were carried out using a laboratory scale prototype POTS cell that loaded into a pressure chamber. It essentially consisted of a sample holder with a stationary electrode at one end and a movable electrode at the other end. Such design enabled the sample to receive simultaneous or sequential delivery of pressure-electric field and thermal treatment to products. POTS enabled the rapid temperature come-up time within 1.45 min to 105ºC under 600 MPa through synergy between pressure and electric field. POTS had the rapidest temperature come-up time to 105ºC within 1.45 min followed by ohmic heating (3.58 min) and pressure assisted thermal processing (PATP, 6.84 min). POTS treated carrots showed better cruchiness index (CI) of 0.76 as compare to PATP (CI=0.57) and ohmic heated (CI=0.62) carrots.
Finally, combined effects of pressure (600 MPa), electric field (50 V/cm), and heat (105ºC) and their selected combinations were investigated for the inactivation of two pressure-thermal resistant bacterial spores of Bacillus amyloliquefaciens and Bacillus stearothermophilus. The influence of food matrices on microbial susceptibility to the treatments was tested using green pea puree (pH 6.1), carrot puree (pH 5.0), and tomato juice (pH 4.0) while sterile 0.1% NaCl solutions (pH 5.0 & 7.0) served as controls. POTS treatment inactivated B. amyloliquefaciens and B. stearothermophilus spores suspended in 0.1 % NaCl at pH 7.0 by 4.6 and 5.6 log during 30 min treatment. B. stearothermophilus spores were more susceptible to the POTS treatment than B. amyloliquefaciens spores. Increasing acidity of the food matrices accelerated the inactivation of both spores. The spores investigated followed a non-linear inactivation kinetics and Weibull model could explain the POTS spore inactivation. In conclusion, this study demonstrated POTS is a promising technology for producing microbiologically safe and better quality shelf stable low acid vegetable products.