| Thermal processing is considered to be the most effective way to kill pathogenic microorganisms in milk,but it can also cause damage to the protein,lactose,vitamins and other components,resulting in a reduction in the nutritional value of milk.In order to ensure food safety and minimize the impact of processing on milk quality,highpressure-low-temperature(HPLT)treatment,a non-thermal processing technology,was applied to milk sterilization.A cooling system suitable for conventional high pressure equipment was developed to achieve rapid HPLT processing of milk,and the generation of ice crystal phase transition was confirmed by the temperature-pressure curves.The lethal and injurious effects of HPLT treatment on Escherichia coli(ATCC 25922)in milk were investigated by selective and non-selective media.The destruction of cell membrane and cell wall structure by phase transitions was observed by the scanning electron microscope and the transmission electron microscope.In addition,the changes in protein,lipid and sensory properties of milk after HPLT treatments were also studied.By comparing with two typical heat treatments,high temperature short time pasteurization(HTST)and ultra high temperature sterilization(UHT),the effects of different treatment methods on milk quality were analyzed.The main conclusions are as follows:(1)At sub-zero temperatures,a lower pressure can be used to achieve bactericidal effects that can be achieved at higher pressures at room temperature.The results of colony counting showed that the number of lethal and injured Escherichia coli in milk after HPLT treatment(-25 °C,300 MPa)was 2.84 lg cfu/m L and 1.56 lg cfu/m L,respectively.Its bactericidal effect was the same as that of two pulse high pressure treatment under 400 MPa at room temperature.HPLT treatment can induce ice Ⅰ-ice Ⅲphase transition,destroy the cell membrane and cell wall of Escherichia coli,thus enhancing bactericidal effect.By increasing the number of pressure pulses,part of the injured bacteria was transformed into lethal ones,thereby avoiding their own repair and growth.(2)HPLT treatment can reduce the loss of milk whey protein and free amino acids during processing,but too high pressure(over 300 MPa)would lead to a decrease in the casein content of milk.HPLT treatment had no significant effect on the α-lactalbumin content of milk.The β-lactoglobulin content of milk decreased by 22.3% after HPLT treatment at-25 ℃ and 400 MPa,while the β-lactoglobulin content of milk after HTST pasteurization and UHT sterilization decreased by 45.2% and 62.9%,respectively.The test results of total sulfhydryl content showed that HPLT treatment had less influence on the spatial structure of milk protein compared with thermal treatments.(3)The milk fat treated by HPLT showed better oxidation stability.The fat content of milk was not affected by HPLT treatment,while the fat content of milk decreased by13.4% and 11.6% after HTST pasteurization and UHT sterilization,respectively.After stored at 4 ℃ for 15 days,the peroxide value and malondialdehyde content of milk after HPLT treatment were 0.81 meq/kg and 0.19 μg/m L,respectively,which were significantly lower than those of heat treatment group(P < 0.05).(4)The results of sensory evaluation showed that the HPLT milk was obviously superior to the UHT milk in flavor,tissue status and color,and its sensory characteristics were almost the same as those of the HTST milk.The results of color analysis showed that the total color difference(ΔE)between HPLT(-25 °C,300 MPa)milk and the control group was 2.71.The viscosity test results showed that HPLT treatment had no significant effect on the viscosity of milk,while the milk viscosity after HTST pasteurization and UHT sterilization decreased by 19.2% and 17.6%,respectively. |
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