Effect of intrinsic factors in liquid food on the performance of ohmic heating

Abstract

Ohmic heating is a novel heating technology, which has immense potential for achieving rapid and uniform heating in foods. Even though intrinsic factors in foods such as pH and fat content are important factors influencing the survival of microorganisms, research-investigations about the effect of intrinsic factors on the ohmic heating have been limited. Therefore, effect of pH and fat content on the performance of ohmic heating was investigated in the present study. The influence of pH and fat content was assessed on the heating rate, electrical conductivity, inactivation of foodborne pathogens, and quality aspects of food. Samples with varying pH or fat content were subjected to conventional and ohmic heating after inoculated with Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes. At first, the effect of pH on the efficacy of ohmic heating was identified. For conventional heating, the heating rate was not significantly different (p > 0.05) regardless of pH, and pathogens were inactivated more effectively at lower pH. However, different patterns were observed for ohmic heating. Although heating rate and electrical conductivity were not significantly affected (p > 0.05) by lowering pH, heating rate increased with increasing pH due to higher electrical conductivity. Also, the inactivation patterns were different from conventional heating. While S. Typhimurium was inactivated most rapidly at pH 2.5, E. coli O157:H7 and L. monocytogenes were inactivated most rapidly at pH 4.5. Color, pH and oBrix values of orange juice subjected to ohmic heating was not severely affected while non-thermal effects of ohmic heating were not observed. Additionally, the effect of fat content on the efficacy of ohmic heating was examined. For conventional heating, heating rate of samples and inactivation of pathogens were not significantly different (p > 0.05) regardless of fat content. Also, a protective effect of fat on pathogens was not observed for conventional heating. In contrast to conventional heating, ohmic heating was significantly affected by fat content. Heating rate decreased with higher fat content for ohmic heating due to lower electrical conductivity. Also, the protective effect of fat on E. coli O157:H7 and L. monocytogenes was observed in samples subjected to ohmic heating. Therefore, pH and fat content should be considered as important factors which have a significant effect on the performance of ohmic heating for inactivation of foodborne pathogens.