Factors affecting shrivelling and friction discolouration of pears (Pyrus communis L. )

[摘要] ENGLISH ABSTRACT: Shrivelling and friction discolouration (FD), as postharvest disorders, negatively influencethe marketability and potential shelf life of pears. By investigating the contributing factors ineach of the disorders, the potential involvement of handling and storage variables weredetermined. This allowed for a better understanding of the responsible factors that createsusceptible environments for these disorders to occur.From the moment that pears are harvested they lose weight by means of transpiration and, toa lesser extent, respiration. When excessive losses are experienced, the fruit will appearshrivelled and the marketability and shelf life are negatively influenced. By minimizing therates of weight loss, the occurrence of shrivelling among pears during the postharvesthandling can be lessened. The periods that proved to be most conducive to shrivelling (duringa simulated postharvest handling duration) were where temperatures above 0 °C wereexperienced. These short periods proved more perilous for shrivelling than lengthy storagedurations at low temperatures. This influenced the transpiration rate in such a way that thedriving force accelerated the rate of weight loss in all the cultivars that were studied.Removing field heat from fresh produce and maintaining the cold chain reduces the drivingforce behind the transpiration of the pears.In all the cultivars studied, 'Packham's Triumph', 'Beurrè Bosc' and 'Forelle', smaller andless mature fruit were more inclined to appear shrivelled. The surface area to volume ratio isfundamental in determining the rate of weight loss. This was most evident in 'Beurrè Bosc'.Although no reproducible results could be obtained from the morphological studies, literaturehas attributed this phenomenon to the composition and quantity of the cuticle layer.Reduction of weight loss was obtained by sealing of the fruit stem. This obstructed watermovement from the fruit through the xylem conducting tissue to the surrounding atmosphere.Not only did the stem appear greener and fresher, but less weight loss and subsequent shrivelwas noticeable in the treated fruit. This effect was most evident in 'Packham's Triumph' and'Beurrè Bosc', but not in 'Forelle'. 'Forelle' typically has a very short, thin stem incomparison to the other two cultivars.All the cultivars showed visual shrivel symptoms after 11 days at 18 °C. Rate of weight losswas the lowest in 'Packham's Triumph', but due to its prominent dimensions, it appearedshrivelled before any of the other cultivars. 'Beurrè Bosc' lost weight at the highest rate(0.42%.day-1).As fruit injury, in the presence of oxygen, is inevitable, the oxidative enzymatic browning ofpears will always be troublesome. This defensive mechanism partially prevents the infectionof the fruit where epidermal cells are injured. To minimize FD, impact and frictional forcesneed to be lower during both harvesting and handling practices.A laboratory scale method was developed through which reproduceable treatments could beperformed, thereby subjecting the fruit to industry related friction, rather than impact, injury.By assessing the discolouration in terms of both extent and intensity, the influence ofvariables could be determined on both 'Packham's Triumph' and 'Doyenne du Comice'pears. As also found in practice, 'Doyenne du Comice' proved to be far more susceptible toFD than 'Packham's Triumph', although the activity of the enzyme, polyphenol oxidase(PPO) was found to be higher in the latter.Although no significant difference was found between the FD encountered at fruittemperature of 3 °C and 15 °C, discolouration was greater at the higher temperature. Thismight be attributed to a greater degree of water loss, lower cell turgidity or higher enzymeactivity. Thus, fruit taken from storage and sorted directly thereafter will exhibit less FD. Thecontribution of condensation forming on the fruit, acting as lubrication, cannot be ignored.Such fruit, with high turgor pressure, might again be more susceptible to bruising which willonly be revealed well after the injury. Since enzymes, which include PPO, catalysebiochemical reactions, the availability of sufficient substrate most probably regulates theextent of this biochemical discolouration.Harvesting at optimum maturity and preventing any unnecessary friction will most definitelyreduce the occurrence of FD. The ultimate challenge remains to optimize sorting and packingconditions without compromising on fruit quality.

[发布机构] Stellenbosch University