The effects of high power ultrasonic energy on milk plasmin activity

The shelf life of pasteurized milk is limited by heat-stable proteases that undergo depolymerization, causing age gelation and bitterness. Proteolysis of milk during storage can be attributed to both native proteases and the proteases produced by bacteria, such as plasmin, which is the major native protease in milk. Ultra-high temperature (UHT) processing (>135°C for 2 s) inactivates proteases and extends shelf life up to 9 months but adversely affects sensory properties. Ultrasound is an emerging non-thermal food processing technology that is being explored as an alternative to pasteurization, as it minimize flavor loss, facilitates homogenization, and saves energy compared to thermal processing. A preliminary study by Vijayakumar et al. has shown that thermosonication (72°C, 152 μmpeakto-peak (p-p) for 3 min) inactivated proteases without producing off-flavor. With the aim of inactivating plasmin, the present study evaluated the feasibility of extending milk shelf life by combining short durations (≤ 60 s) sonication as an adjunct to pasteurization. We refer to this approach as thermosonication to reflect its use of both highpower ultrasound and thermal treatment. Batch thermosonication was conducted using a bench scale ultrasonic unit operating at a frequency of 20 kHz and a maximum output of 2.2 kW. Skim milk was sonicated at short duration with constant amplitude of 170 μm(p-p), 72°C for 10, 30 and 60 s. The enzyme activity of raw, pasteurized, and thermosonicated milk was analyzed by total plasmin assay. A plasmin activity reduction of 83 and 96% up to day 49 was observed for both 30 and 60 s sonication times. Because another preliminary study by Benner et al. reported lower total aerobic bacterial count (TAC) via psychrosonication with subsequent pasteurization, the latter process was tested for scale-up potential in a continuous flow system.