Application of Ultrasound for extending the shelf life of curd

Abstract

Curd, an ethnic food formed by fermenting the milk with lactic acid bacteria is known to mankind from times immemorial. Curd has very limited shelf life under ambient condition, though the cosmetic defect of water expulsion is not considered as safety hazard. It fetches poor market value and therefore not acceptable for consumption. Ultrasonication is an emerging non-thermal processing technique used in food and dairy industry to improve the quality properties. In milk when sonication is applied it causes physical and chemical changes like sonolysis in water and disruption of quaternary structure in protein. Milk is treated in sonication bath at different power level (96 to 498 W) and pulsations (30-70) for 1- 2 h and is inoculated with 1 % starter culture and left for fermentation. Milk forms protein appendages of κ-casein which reacts with serum β-lactaglobulin under reduced pH condition by inoculum to form curd at 37oC. Whereas in control, the milk is heated to 90oC to create protein appendages and cooled down to 37oC and then added with starter culture to ferment as curd. The fermentation time for the control sample is 6 -7 h and for the treated sample it got reduced to 4 -5 h. The acidity of curd for control sample for day 1 and 2 is in the range of 0.89 ± 0.26 to 1.23 ± 0.02 % lactic acid respectively. For treated sample the acidity values are 0.90 ± 0.32 to 1.10 ± 0.39 % lactic acid respectively. Control curd perceived sour favour whereas ultrasonic treated samples irrespective of power and pulsation combination had good texture and sweet flavor. The firmness of the control sample is 95.05 ± 0.02 g which is less compared to the treated sample 125.71 ± 0.07 g. The texture and shelf life of the curd is improved by 2 days due to sonication treatment under ambient condition.