Effect of Ohmic Heating on the Formation and Texture of Acid Milk Gels

Abstract

This study aimed to describe the effects of ohmic heat treatment (OHT) of milk on the formation and properties of acid milk gels. The influence of voltage gradient (25, 40, 55 V/cm), holding time (2, 16, 30 min), and final temperature (45, 65, 75, 85 °C) on rheological properties and particle size distribution was measured during the gelation process. Texture properties and syneresis of the samples were evaluated at the end of the gelation process. Scanning electron microscopy (SEM) micrographs of the acid milk gels were also taken at pH 4.6(i.e., pI or Isoelectric point of casein). Results indicated that oscillatory time sweep tests were more accurate than particle size measurements in detecting gelation onset. Complex modulus values of the final gel showed that OHT of the milk at 25 V/cm and holding at 85 °C for 16 min gave rise to a 21% higher structural strength compared to when using the conventional heat treatment (CHT) where temperature was 85 °C and holding time and come up time were 30 and 20 min respectively. In other words, by using the OHT milk, the same gel strength as that of CHT 85 °C sample was obtained by spending 185 min instead of 328 min incubation time (i.e., 40% less time). Higher voltage gradient lowered the final gel strength when temperature and holding time kept constant at 85 °C and 16 min respectively. Results indicated that the gel firmness enhanced and the amount of syneresis decreased when CHT was replaced by OHT in the manufacturing process. SEM revealed that the protein matrices of ohmic heating samples appeared to be more compact and denser with smaller pore size than observed in the conventional gel. Results indicated that OHT of milk successfully improves the quality of acid milk gels and obviate the need for increasing dry matter concentration or using additives.