| Pulsed electric field (PEF) technology is one of the most studied non-thermal processing technologies recently, the need that using different PEF sterilization processes and operating points according to the characteristics of different food matrix practically should be adopted with its commercializing gradually. Pumpkin pulp is characteristic of high viscosity, high conductivity, rich in bubble and so on. Moreover, it is so easy to bacterial contamination in processing that the quantity of thermoduric bacteria in the final production (pumpkin powder) is out of limits constantly and the storage durability of production is bad. Though there are a large number of researches on the application of PEF technology to fruit and vegetable juice sterilization, studies on PEF sterilization of high viscosity fruit and vegetable pulp are not too many. So the application of PEF technology to pumpkin pulp sterilization was studied in this article. In addition, applying the vacuum freeze drying (VFD) technology to the production of pumpkin powder can better keep product’s color, incense, taste and improve the added value of product. But the cost and energy consumption of VFD are large. Therefore, the response surface methodology (RSM) was used to optimize the vacuum freeze drying process of pumpkin powder, improve drying rate and productivity, ensure the quality of pumpkin powder at the same time. The brief introduction of whole research was as follow:(1) Electric field strength, treatment time and synergism temperature had outstanding effects on the sterilization of PEF treatments. With the electric field strength, treatment time and synergism temperature increasing in the range of experimental parameters, the killing effects of PEF were improved.(2) Different microorganisms had different tolerances to the PEF treatment in pumpkin pulp, the order of the tolerance was Penicillium> Staphylococcus aureus> Salmonella> Escherichia coli> Yeast; the tolerance of Bacillus stearothermophilus was stronger than Bacillus subtilis.(3) Though the pumpkin pulp’s characteristics that high viscosity and high conductivity were adverse to the application of PEF technology, taking the use of moderate heat synergy with PEF treatment could reduce the PEF treatment parameters and difficulties, enhance the effect of PEF sterilization, maintain the quality of pumpkin pulp at the same time. The effect of moderate heat synergy with PEF sterilization was remarkable, especially in 40-60℃. With the synergism temperature increasing, the lethal effects of PEF treatment to Yeast, Penicillium, Escherichia coli, Salmonella, Staphylococcus aureus and thermophilic bacteria (Bacillus stearothermophilus and Bacillus subtilis) were remarkably improved in pumpkin pulp.(4) It had showed good synergistic effect that total bacterial count of pumpkin pulp decreased remarkably at the treatment of 60℃ mild heat combined with 20.14 kV/cm,109.25 us PEF. Moderate heat in combination with PEF process was applied to unsterilized and uninoculated pumpkin pulp. The initial total bacterial count was 4.832±0.641g cfu/g, the initial thermophilic bacteria count was 3.378±0.231g cfu/g, after treated by 60℃,20.14 kV/cm, 109.25μs PEF the total bacterial count was 0-30 cfu/g, thermophilic bacteria was not detected.(5) The effects that PEF treatment on the pumpkin pulp’s pH, conductivity, soluble solid content, total sugar content, pectin content were not outstanding. Different electric field strength and different treatment time of PEF treatment in the range of experimental parameters didn’t have significant effects on viscosity, color and carotenoid content of pumpkin pulp. The low synergism temperatures(4,20,40 and 50℃) of PEF treatment didn’t have dramatical effect on the pumpkin pulp’s viscosity, color and carotenoid content. After treated by 60℃,20.14 kV/cm, 109.25 μs PEF, the pumpkin pulp’s viscosity rose somewhat, color changed slightly(AE value was 1.17), carotenoid content decreased by 22.82%. But the harmful effects to pumpkin pulp’s quality caused by PEF treatments were much smaller compared with heat treatment (95℃, 5min).(6) The eutectic point temperature and melting point temperature of pumpkin pulp were-12℃and-6℃ respectively measured by electric resistivity method, which offered the reference for pumpkin pulp pre-freezing process. In order to freeze the material with different thickness to-22℃, the required time was different. With the thickness of pumpkin pulp increasing, the required pre-freezing time also increased gradually.(7) In the optimal experiments for vacuum freeze drying process of pumpkin powder, the response surface method was used. Meanwhile, the pressure (A), the hot plates temperature (B) and material thickness (C) were chosen as three factors. Taking pectin retention rate, carotenoid retention rate, drying rate and productivity as measurement indexes, after these four indexes were respectively graded, the synthesis scores (Y) were obtained by subjective weighting method. The regression equation was:Y= 94.8-2A+4B+0.5C+2.5AB+0.5AC-2.5BC-4.6542-10.65B2-S.6SC2The analysis of variance showed that this regression equation had preferable fitness, could be used to forecast and analyze the synthesis scores (Y). When the pressure, hot plates temperature and material thickness were 66.61 Pa,61.69℃ and 9.00mm respectively, the theoretical synthesis score was 96.3056 forecasted by this regression equation. Influence of both pressure and hot plates temperature on the synthesis score was significant, hot plates temperature’s significance was greater than the pressure’s, and the influence of material thickness was not outstanding. In the interaction of every factor, the interaction effects of the pressure with the hot plates temperature and the hot plates temperature with the material thickness were outstanding. |
PDF Full Text Request