Effect Of High Pressure Treatment On Sterilization, Enzyme Inactivation And Quality Of Kiwifruit Juice

The effects of high pressure and heat treatments on sterilization, enzyme inactivation and quality of kiwifruit juice were investigated. Pressure levels ranging from 200 to 600 MPa and temperatures varying from 10 to 50℃were applied for up to 30 min.Results show that total number of bacteria declined with pressure and temperature increased and arrived commercial asepsis under 400 MPa at 30℃. After 30-day storage period at 4℃, kiwifruit juice was still safe while 50℃was used as treated temperature combined high pressure treatment.Results revealed that, at each temperature, an increase in pressure level results in a decrease of enzyme activity. Pressures higher than 400 MPa could be combined with mild heat (≤50℃) to accelerate enzyme inactivation. Prolongation of the exposure time had no great effect after the first 15 min. Regarding temperature, results showed that POD activity decreased when the temperature increased.Compared with control, POD activity at 10℃showed a slight increase (P>0.05) under treatment at 200 MPa from 10 to 20 min, whereas at 400 and 600 MPa, the activity decreased with regard to that observed at 200 MPa. POD activity at 30℃was higher after treatment at 200 MPa for 15 min and the activity decreased following longer exposure time intervals. The slope of POD in kiwifruit juice at 30℃was slightly decreased compared with that in a model system. Real food systems show a protective effect of food ingredients at pressures applied for POD inactivation. Effect of three main factors (pressure, temperature and time) in high pressure treatment was investigated, and all experiments were designed with RSM and SVM.Polyacrylamide gel electrophoresis (Native PAGE) and activity detection were carried out on the partially purified enzyme before and after treatment. At the beginning, two staining bands were observed on the untreated enzyme. This suggested that there may be two isoenzymes. After 15 min of treatment at 200 MPa at 30℃, a new band was observed. All bonds disappeared as pressure increased at 600 MPa. Purified POD isoenyme I and II changed differently after high pressure. From this observation, there may be several isoenzymes that have different resistance to high pressures.There were no significant difference on pH, Brix and conductivity after treatment, while little difference on content of Vc, amino acid and color. And the effect of pressure is less than temperature. The main aromatic compounds of kiwifruit juice was 2-Hexenal(69.35) and Hexanal(11.23%). And the main component was aldehyde compounds(85.139%). The high pressure treatment enhanced the flavors of kiwifruit juice since the aldehyde contents increased after treatment. The effect of high pressure treatment on rheological properties of kiwifruit juice was investigated. The results show that kiwifruit juice samples behaved like pesudoplastic non-Newtonian fluids and were coincident with Herschel-Bulkley model under all conditions. Kiwifruit juice displayed thixotropy when pressure up to 400 MPa. The value of G″increased while G′decreased after high pressure treatment, which indicated the enhancement of viscosity and reduce of flexibility. And the difference affected the rheological properties of kiwifruit juice after high pressure treatment. The particle size distribution of kiwifruit juice was changed after high pressure treatment.