Development and evaluation of flow through ohmic heating assisted pulsed light treatment system for preservation of fruit juice

Abstract

Ohmic heating is a promising thermal processing technology having wide application in food processing. This causes instantaneous heat generation in the food product, creating a rapid and uniform heating that reduces thermal abuse, unlike the conventional thermal processing methods. Pulsed light (PL) treatment is one of the non- thermal processing technologies that have been proven to be effective against a number of food borne pathogens. But the practical application of the PL treatment on food products is limited due to its low penetration depth. These noval technologies could contribute to shorten processing times, energy savings, and highly balanced safe food; however, they alone still cannot guarantee food safety without damaging the food’s quality. Therefore, a new concept combining ohmic heating with PL has been extensively evaluated. This combination technology would optimize each of the individual technology’s strengths and reduce each of their individual weaknesses. The present study envisages development of an ohmic assisted PL treatment system for pineapple and cashew apple juices and evaluation of the developed system towards maintaining their quality characteristics, microbial safety, and storage stability. In this study, an ohmic heating batch system and a flow through PL system was developed and fabricated to pasteurize the pineapple and cashew apple juice. The process parameters viz. Voltage gradient (10, 12.5 and 15V/cm), Process temperature (50, 55 and 60°C) and treatment time (1, 3 and 5 min) of ohmic heating and PL dosage (10, 12.5 and 15 J/cm 2 ), sample-source distance (5, 10 and 15 cm) and flow rate (150, 200 and 300 ml/min) of the PL system were optimized separately based on the physic-chemical and microbial quality characteristics of both fruit juices on responses using Response Surface Methodology (RSM). The optimized condition for ohmic heating of pineapple juice were obtained at a voltage gradient of 14.02 V/cm, holding time of 2.31 min and treatment temperature of 55.26°C and that of cashew apple juice were a voltage gradient of 14.53 V/cm, process temperature of 55.25°C with holding time of 2.77 min. The optimum process operating conditions for pineapple juice were found at a PL dosage of 13.69J/cm 2 , sample-source distance of 10.26 cm and flow rate of 165.06 ml/min and that for cashew apple juice were a PL dosage of 12.49 J/cm 2 , sample-source distance of 8.63 cm at a flow rate of 164.01 ml/min. The optimised process operating conditions of both treatments were selected for analysis of ohmic assisted PL treatment process. The optimally treated ohmic assisted PL treated pineapple and cashew apple juice samples were found to be superior to individual treatments as well as other combined ohmic and PL treatment combinations studied in terms of physico-chemical, microbiological and organoleptical characteristics. The ohmic assisted PL treated pineapple and cashew apple juice resulted in more than 5 log reductions in Escherichia and Listeria strains from initial count thus confirms to the recommendations by FDA safety standards for fruit juices. Storage studies of ohmic assisted PL processed samples at optimised process operating conditions revealed that, the process could provide a shelf life of 25 days and 30 days respectively for pineapple and cashew apple juice under refrigerated conditions of 4±2°C retaining their physico- chemical and sensory characteristics while keeping the microbial level at safe limits.