Effect of pectinmethylesterase and calcium infusion on texture and structure of fruits and vegetables
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Processed fruits and vegetables are susceptible to textural quality changes and excessive softening. The overall objectives were to improve quality and to evaluate analytical methods to quantify the effect of pectinmethylesterase (PME) infusion. Strawberry, mango and eggplants were infused with plant or fungal PME with or without CaCl2, using vacuum. In strawberry, a print technique was developed to visualize PME activity after infusion. Although PME activity in infused fruit was about twice that of control fruit, no significant effects on firmness were found. Firmness of PME infused fruit was about twice that of water infused controls. Pectin, as water soluble pectin (WSP), was about twice that of chelator soluble pectin (CSP) or alkaline soluble pectin (ASP). In strawberry, mango and eggplant, PME activity increased, and % degree of esterification was changed, but did not correlate with the amount of infused PME. A putative inhibitor of PME was identified. WSP and alkaline ASP were the major pectic fractions in strawberry and mango, respectively. In mango, temperature gradient infusion with Valencia orange PME and/or CaCl2 increased gumminess and chewiness, but had no impact on hardness and adhesiveness. In eggplant, the firmness of fruits infused with a commercial fungal or Marsh grapefruit (MGF) PME was significantly increased compared to controls, immediately after treatment and after storage for 7 days at 4 °C. Ion-exchange chromatography revealed that water soluble pectin from fungal and MGF PME and/or CaCl2 infused eggplant had greater charge density than pectin from control eggplant. Cryo-scanning electron microscopy showed that treated samples with fungal PME/CaCl2 had greater cell to cell integrity than water-infused control. JIM5 (antibody to low-esterified pectin) and JIM7 (antibody to high-esterified pectin) labeled cell walls of non-infused control tissues. JIM5 showed more binding than JIM7 with cell walls of eggplant tissues from fungal PME/CaCl2 treatment. From the results of this study, factors that influence effectiveness of PME infusion and textural changes include source of PME, pH, endogenous inhibitors, porosity of commodity, type of vacuum process, cell wall composition of commodity, and variability in raw materials. In addition, choice of PME and process treatments must be optimized for each commodity.