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. Plasmin is the major native protease in milk. Ultra High temperature (UHT) inactivates proteases and extends shelf life up to 9 months but the sensory properties are adversely affected. Ultrasound is one emerging technology that is being applied for food processing. Since power ultrasound operates by means of mechanicals vibrations, this non-thermal technology is being explored as an alternative to pasteurization, as it minimize flavor loss, helps in homogenization, and saves energy significantly better than thermal processing. Preliminary study has shown that thermosonication (72°C, 152 μmpeak-to-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 integrating ultrasound at short durations (≤ 60 s) as an adjunct to pasteurization for extended milk shelf life. Current study of 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. Enzyme activity of raw, pasteurized and thermosonicated milk was analyzed by total plasmin assay. The result showed a reduction of 83 and 96% in plasmin activity up to day 49 for 30 and 60 s sonication times. Since another preliminary study 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. An ultrasonic continuous flow system was tested using Branson’s flow-through “donut” horn. The donut horn, which vibrates radially, was placed inside a 5.5 L sonication reactor. The amplitude was maintained at 12 μmp-p and the feed flow rate was varied at 4 and 6 L/min. Samples that were treated only with psychrosonication showed no difference in plasmin activity compared to raw. However, milks that were psychrosonicated and pasteurized showed lower plasmin activity than raw milk. No significant difference was found in plasmin activity between control pasteurized and psychrosonicated samples followed by pasteurization, thus confirming that the major reduction of plasmin activity is a result of the pasteurization. Based on these results, thermosonication is a promising pretreatment method for shelf life extension as an adjunct to pasteurization. However, psychrosonication, under the conditions of this study, is not a feasible technology to be used in the dairy industry to extend milk shelf life.