| HPU is a novel non-thermal processing technology.Recently,it has become a research hotspot in fruit and vegetable processing area because it can effectively inactivate microorganisms while maintain the sensory and nutritional quality of fruit and vegetable products.However,the applications of HPU in fruit pulp is limited in previous studies.Therefore,the effect of HPU on strawberry pulp quality was investigated.We found that HPU could significantly change the rheological properties of strawberry pulp.To better understand this phenomenon,the changes of rheological properties,polygalacturonase?PG?,pectin methylesterase?PME?and pectin under different HPU conditions were analyzed.Finally,the possible mechanism of HPU affecting the rheological properties of strawberry pulp was proposed.The main findings and conclusions were as follows:?1?There was no significant change in pH and total soluble solids of strawberry pulp after HPU treatment in an ice bath.The L*,a*and b*values were significantly increased?p<0.05?,and the?E values of all treatment were greater than 2.The content of anthocyanin was increased while the total phenolic content was slightly decreased.The results showed that HPU can protect the color and maintain the nutrient content of strawberry pulp.?2?The apparent viscosity of strawberry pulp were significantly increased by more than4 times after HPU at different temperatures?ice bath,25°C,35°C,45°C?and 605 W/cm2 for16 min,but the difference among different treatment temperatures was not significant;The apparent viacosity was increased gradually with time and power density increasing,but the difference was small between 605 W/cm2 and 968 W/cm2 power density.The calcium content in strawberry pulp was decreased significantly after HPU treatment.Results indicated that HPU could promote gel formation,improve the viscosity of strawberry pulp and enhance its elasticity and viscous properties.?3?PG was not obviously inactivated by heat??45°C?or HPU treatment alone.Its residual activity was still above 80%after either treatment.But the combination of them could achieve better inactivation effect.The activity of PG was decreased to 13.72%by HPU treatment at 20 kHz,35°C and 605 W/cm2 power density for 16 min.The experimental results showed that HPU and heat treatment could inactivate synergistically PG only when they were simultaneously carried out.The results showed that the inactivation of PG by HPU might inhibit the degradation of pectin.?4?The residual activity of PME was above 70%after HPU treatment at different temperature?ice bath,25°C,35°C,45°C?and different power density?242 W/cm2,605W/cm2,968 W/cm2?at 0-16 min,indicating that HPU could not significantly inactivate PME.The residual PME might cause the low esterified of pectin.The?-helix content of PME was decreased about 3.3%at 35°C and 605 W/cm2 for 16 min,while the content of?-sheet and?-turn was increased by 4.2%and 6.4%respectively,which indicated that?-helix simultaneously conversion to?-sheet and?-turn.The?max of pure PME was 345 nm.Its fluorescence intensity was decreased by 42.84%at 35°C and 605 W/cm2 for 16 min.The decrease in fluorescence intensity was greater with longer treatment time and higher power density.These results indicated that the inactivation of PME by HPU was related to the change of the secondary and tertiary structure.?5?Strawberry water-soluble pectin?WSP?solution was a pseudoplastic fluid.With the prolonging HPU treatment time,its viscosity was decreased and gradually tend to newtonian fluid.HPU?35°C,605 W/cm2,2,6,16 min?could reduce the molecular weight and particle size of WSP.After 16 min treatment at 35°C and 605 W/cm2,the molecular weight and average particle size of WSP was decreased by 47.75%and 69.12%,respectively.The results indicated that HPU could change the stucture of WSP,thereby decreasing its viscosity.Based on the above results,the possible mechanism of HPU to improve the apparent viscosity of strawberry pulp might be as follow:on the one hand,the PG of strawberry pulp was significantly inactivated but the PME activity was still high after HPU treatment.Therefore,the degradation of pectin was effectively inhibited,but the low esterification of pectin was promoted.And the low esterified pectin may form a gel with Ca2+in the pulp.On the other hand,the rate at which the HPU geled the low esterified pectin was higher than the degradation of the pectin in the strawberry pulp,thereby increasing the viscosity. |
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