Kinetic Modelling of Polyphenol Oxidase Activity and Phenolic Compound Changes in Thermosonicated Strawberry Nectar During Storage

In this study, some quality changes in the optimally thermosonicated (59 degrees C-455 J g(-1)) cloudy strawberry nectar during storage were evaluated and modelled. The changes of pH (3.40-3.78), residual polyphenol oxidase-(PPO) activity (24%-11%), total phenolic-(TP) content (688.7-543.2 mg L-1), total monomeric anthocyanin-(TMA) content (142.5-84.4 mg L-1), anthocyanin, and colorless phenolic compounds were analyzed during three months of storage at 4 degrees C. The significant decrease (54%) which was observed in residual PPO activity can be related with the decrease in phenolic content of nectar. A high correlation was determined between the changes in pH and TP of nectar (R-2=0.983). While TMA content and anthocyanin compounds (pelargonidin-3-O-glucoside, pelargonidin-3-O-rutinoside, cyanidin-3-glucoside) were decreasing, chlorogenic acid, p-coumaric acid, ellagic acid, kaempferol, quercetin, (+/-)-catechin and (-)-epicatechin were increasing between the days 30 and 60. Pelargonidin-3-O-glucoside was decreased 36% at the end of storage (k=6.09x10(-5) L (mg day)(-1)). It was determined that the increase in PPO activity (R-2=0.986), degradations of TP (R-2=0.902), TMA (R-2=0.968), pelargonidin-3-glucoside (R-2=0.858), pelargonidin-3-rutinoside (R-2=0.965), and (-)-epicatechin gallate (R-2=0.871) were best fitted to second-order kinetic model. The half-time of TP content was 303 days, while the half of TMA content was degrading in 99 days.