The Study of Enzymatic Browning Reactions in High Hydrostatic Pressure Processed Peach (Prunus persica)

Enzymatic browning reactions have been a cause of major loss in fresh produce. High pressure processing (HPP) is one of the advanced processes being studied extensively in fruits and vegetables because of its ability to extend shelf life while retaining products with natural attributes. However, color changes during storage are of crucial concern since elevated pressure levels may induce loss of membrane permeability and trigger loss of sub-cellular compartmentalization. The objectives of this study were to explore the factors that impact HPP peach discoloration as a result of enzymatic browning during a storage period of two weeks. The factors investigated included maturity of fruit as differentiated by firmness (M1 = 50--55 N, M2 = 35--40 N and M3 = 15--20 N), pressure levels from 100--500 MPa and peach cultivars including eight clingstone types (Carson, Andross, Ross, Evert, Dr. Davis, Late Ross, Halford, and Lilleland) and one freestone type (Summerset). A combination of 1H-NMR relaxometry and light microscopy evaluation was assessed to determine membrane integrity of peaches following high pressure processing. Other parameters involved in enzymatic browning, such as degree of browning, defined as difference in lightness, PPO activity, total phenols were also monitored.;Among three peach maturities, M1 had the least amount of viable cells, implying the highest HPP damage, which was in the same direction as the development of discoloration during storage. The effect of maturity on enzymatic browning reactions after HPP in the present study allows for selection of the appropriate maturity stage prior to processing in order to meet consumer demands for optimal quality.;Pressure levels in a range of 100--500 MPa cause different degrees of damage to peach cells, with the level at 200 MPa and above triggering loss of membrane integrity. The damage from HPP was determined by a decrease in T2 relaxation time and a reduction in number of viable cells, and these results were in agreement with development of brown color. Use of a combination of microstructural and 1H-NMR tools were an effective means of determining membrane de-compartmentalization at different pressure level and allowed for prediction of browning scenarios.;Peaches of various cultivars showed different susceptibility to high pressure damage. Degree of browning as well as PPO activity in the peach extract were found to correlate well to harvest time for each cultivar. Peaches from the mid-season had the lowest degree of browning, because they had sufficient time to fully develop, the fruit was thus stronger than the peaches from the early or late season. Moreover, a low level of PPO activity was observed in the extract of the mid-season group. The determination of microstructure, 1H-NMR and PPO content were an effective means of determining membrane de-compartmentalization and allowed for prediction of browning scenarios in different peach cultivars.;Calcium pretreatments were thought to have the potential to reduce the loss of cell integrity that results from high pressure treatment. Two calcium sources, calcium lactate and calcium chloride produced some changes in the T2 of cytoplasmic compartment but did not affect PPO activity or total phenols. The effect of calcium treatment in this study was very slight and not sufficient to inhibit the loss of cell integrity during high pressure treatment as revealed in peach discoloration during storage.