Chilling at 4°C in the dark induced lipid degradation in cucumber (Cucumis sativa L.) fruit upon rewarming at 14°C. Rates of ethane evolution by fruits rewarmed after 3 days of chilling were up to four-fold higher than those evolved by unchilled (140C) fruits (0.02-0.05 picomoles gram fresh weight-1 hour-'). This potentiation of lipid peroxidation occurred prior to irreversible injury (requiring 3 to 7 days of chilling) as indicated by increases in ethylene evolution and visual observations. Decreases in unsaturation of peel tissue glycolipids were observed in fruits rewarmed after 3 days of chilling, indicating the plastids to be the site of the early phases of chilling-induced peroxidation. Losses in unsaturation of tissue phospholipids were first observed only after chilling for 7 days. Phospholipase D activity appeared to be potentiated in fruits rewarmed after 7 days of chilling as indicated by a decrease in phosphatidylcholine (and secondarily phosphatidylethanolamine) with a corresponding increase in phosphatidic acid. These results indicate that lipid peroxidation may have a role in conferring chilling injury.examined as a primary response to chilling stress (in the absence of illumination), there is indirect evidence in the literature that suggests it may have a role in the development of chilling injury. The application of antioxidants to cucumber and pepper fruits delayed or reduced the severity of low temperature injury (26). Chilling also evokes a decrease in catalase activity in cucumber seedlings (18). These studies indicate that antioxidant defenses may be compromised in some CS plants exposed to low temperatures. In the special case where chilling is accompanied by illumination, photooxidation of cellular membranes is believed to play a role in the development of CI (22,28,29).We propose that lipid peroxidation may have a role in the development of CI, and undertook a study to provide an initial evaluation of this new hypothesis. Cucumber fruits were used as a model since the visual manifestations ofchilling injury in this tissue are well-defined (25, 26). There is also a lack of studies on CI in fruit tissues compared to whole plants, leaves, and other organs. Fruits were chilled in the absence of light to eliminate the contribution ofphotooxidative processes in the development of injury.Several theories have been advanced to account for the nature of CO.2 The primary lesion has been proposed to involve bulk membrane lipid phase transitions (12), unfavorable and direct low temperature effects on proteins and enzymes (7), the presence of high-melting membrane lipid species (15), and a redistribution of cellular calcium (13). Although data are available to support each of these theories, none appears to be universally accepted as the primary determinant of CI. The existence of a universal mechanism of CI is questionable and alternatively, several of the aforementioned factors may be required to confer low temperature sensitivity. There is also the possibility that some other unidentified ...

Esterification reactions were evaluated by using lipases fromRhizomucor miehei (Lipozyme IM20) andPseudomonas cepacia (PS‐30) with equimolar levels (1.77 mmol) of undecanoic acid and glycerol or 1,3‐propanediol (1,3‐PD) or 1,2‐propanediol (1,2‐PD) in organic solvents of log P (partition coefficient between 1‐octanol/water) values of (−0.33–4.5. Reaction yields (percentage of esterified undercanoate) with glycerol ranged from 1.4 to 72%, with greatest yields observed in solvents of log P 4.0–4.5 for Lipozyme, whereas the PS‐30 lipase was similarly effective (27–38% yield) over the full range of solvent polarities. For both enzymes, as solvent apolarity increased, so did the extent of acylation of glycerol in the final product mixture. Reaction yields with 1,3‐PD ranged from 8.1 to 64% for Lipozyme and from 18 to 84% for PS‐30 lipase, with greatest yields observed for both enzymes in solvents of log P values in the range 1.2–5.0. For both lipases, the shift to greater solvent apolarity was accompanied by an increased molar ratio of diacylated‐1,3‐PD/monoacylated‐1,3‐PD in the product mixture. Reaction yields with 1,2‐PD ranged from 2.5 to 45% for Lipozyme and from 12 to 52% for PS‐30 lipase, with greatest yields observed in solvents of log P values in the ranges 1.4–1.9 and 1.4–4.5, respectively. The shift to greater solvent apolarity was accompanied by an increased molar ratio of diacylated‐1,2‐PD: monoacylated‐1,2‐PD in the product mixture, except for Lipozyme in the three most apolar solvents (log P of 3.5–4.5) in which there was a general attenuation of activity. These results suggest the existence of a solvent polarity influence on reaction product selectivity in multiproduct reactions, which can be partially explained on the basis of differential solvation and extraction properties of solvents.

Substrate preferences for pancreatic lipase~mediated acylexchange reactions with butteroil were concentrationdependent for the series of acyl donors and alcohol acceptors evaluated. For acidolysis reactions, the initial reao tion rates and percent reaction yields after 18 h at 50 pmol acyl donor per gram substrate mixture were similar for nfatty acids and their methyl and glycerol esters. At 400-500 pmol g-1 (and greater), order of initial reaction rates and percent reaction yield was fatty acid glycerol esters > fatty acid methyl esters > fatty acids. At concentrations above 300-500 ~nol g-X, reaction inhibition was observed for fatty acid substrates, and inhibition took place at lower concentrations for the shorter-chainlength fatty acids of those evaluated (5-17 carbons}. Inhibition was primarily attributed to acidification of the microaqueous environment of the lipase. Desorption of water by the fatty acid substrate may be a secondary mode of inhibition. The concentration dependence of initial reaction rates and percent reaction yield was similar for the n-alcohol substrates evaluated (2-15 carbons} for alcoholysis reactions with butteroil. Optimum alcohol concentration was 375-500 pmol g-1 (except for butanol, which was 1 mmol g-l), above which reaction inhibition was observed. Inhibition was attributed to desorption of water from the enzyme by the alcohol substrate. Relative reactivity of classes of alcohols for this reaction system was primary alcohols > secondary alcohols > tertiary alcohols. Generally, alcoholysis reactions were faster than acidolysis reactions, and triacylglycerols were the best substrates for acidolys'rs reactions with butteroil at high levels (up to 2 mmol g-l) of acyl donor substrate.

The development of chilling injury in cucumbers stored at 4°C and 95% RH was evaluated. Irreversible damage due to chilling required 7 to 10 days as indicated by increases in tissue electrolyte leakage (from 30 to 70%), stress ethylene production (to 30-60 pMo1 g-l hr-r) and visual manifestations of injury. Low levels of ethane evolution (< 1 pMo1 g-l hr-') were observed for unchilled and continuously chilled fruits. Upon rewarming, ethane evolution was stimulated by an order of magnitude following a prior chilling exposure of at least 7 days. These results indicate that the potentiation of lipid peroxidation in chilled cucumbers is associated with the onset of irreversible injury.

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