The use of ohmic heating in processing of food industry

The aim of this work was to identify optimal pre-treatment conditions for cannellini beans in order to improve heating uniformity and, thus, the quality of product, during the subsequant sterilization process. Dry beans were blanched for different length of times (35 and 50 s) in a water bath set at 90°C after soaking (12 h) in tap water. Heating curves of solid-liquid mixtures (53g of beans/100g of mixture) of pre-treated beans and salt solution (1 g/100mL) were determined in a static ohmic heating device by applying a constant voltage of 100V. Results showed that the ohmic heating rate of beans increased with increasing the blanching time as a direct result of the increase of electrical conductivity. Optimal pre-treatment conditions, which allowed the ohmic heating of both phases of solid-liquid mixture at comparable rate, were found when beans were blanched for 50 s at 90°C. Beans pre-treated under optimal blanching conditions and sodium chloride solution (1g/100ml) (53g of beans/100g of mixture) were then subjected to both ohmic and in-container sterilization process. The solid-liquid mixture was sterilized at 121 °C for 10, 15 and 20 min. The parameters measured immediately before and after thermal treatments were: total microbial count, yeasts and molds, split degree, and proteins content. The experimental results have shown that, regardless the thermal method and treatment conditions the initial microbial spoilage was reduced up to below the detection limit of the method (<10 cfu/ml). When compared to the beans sterilized by conventional treatment, the samples treated by the ohmic method appeared of attractive appearance, with a lower split degree. The analyses of the chemical composition revealed a higher proteins concentration in sample processed by ohmic heating than those treated by conventional method.