The PEF treatments were conducted at field intensities of 25-37 kV cm-1 (up to 50 kV cm-1)and temperature range of 30°C to 75ºC. Native milk enzymes selected for the study included alkaline phosphatase, lipase, xanthine oxidase and plasminand microbiological study included determining Total Plate Count (TPC) and Pseudomonas and Enterobacteriaceae counts in skim milk. At 30ºC PEF treatment at maximum field intensity inactivated AlP by 42% while at 60oC inactivation was higher (67%). Under these treatment conditions less than1 log reduction in TPC and Pseudomonas count and 2.1 logs reduction in the Enterobacteriaceae count was achieved at 30oC while at 60ºC TPC dropped by 2.4 logs and Pseudomonas and Enterobacteriaceae counts were reduced by 5.9 and 2.1 logs, respectively to below the detection limit of 1 CFU mL-1. Combining PEF treatment with heat increased the inactivation level of all enzymes which showed an increasing trend with increasing field intensity and temperature. Treatment time (4.8, 9.6, 19.2, 28.8 and 38.4 µs) was controlled by either changing the pulse frequencies (100-400 Hz) or product flow rate (30-240 mL min-1) at a constant field intensity of 31 kV cm-1 and it was found that changing the flow rate was a more effective way of enzyme inactivation than changing the frequency due to longer exposure time of enzymes to heat and field intensity. The size of casein micelles and fat globules was not affected by PEF treatment while severe heating of milk at 97oC for 10 min decreased both micelle and fat globule sizes marginally. The coagulation time of rennet-induced gels made from PEF-treated (35 to 50 kV cm-1) milks (whole and skim) increased as the treatment intensity increased, but remained shorter than gels made from pasteurised milk. The PEF treatment of milk at various field intensities and temperatures adversely affected the G′, G′′ and firmness of gels, but the effects were less pronounced than in gels made from pasteurised milks. This study concludes that for successful application in milk processing the PEF treatment needs to be combined with mild heat treatment. This approach could achieve safer milk with less damage to milk functionality. However, the quest for a suitable quality assurance indicator enzyme will need more extensive studies.
展开▼
