Preliminary Study On The Mechanism Of Ripening And Softening Of Postharvest Pineapple Fruit By Induced Degradation Of Cell Wall Polysaccharides

Pineapple is a typical non-respiration climacteric fruit. The fruit softens rapidly onceripening is initiated. Disassembly of cell wall polysaccharides, particularly solubilisationand depolymerization of pectin and hemicellulose, resulting in the change in the structureof the cell wall, is the main reason of fruit softening. Cell wall degrading-enzymes are themain factors of cell wall polysaccharides degradation, and recent studies show that thenon-enzymatic factors are also found to cause the degradation of cell wall polysaccharides.In this work, the experiments were carried to study the physiological changes of ‘Bali’pineapple fruits at different ripening color stages(Mature green, Green mature, Yellow ripe,Ripe, Over ripe), and the cell wall polysaccharides, including cellulose, two kinds ofpectins: water soluble pectin(WSP) and acid soluble pectin(ASP), and two kinds ofhemicelluloses:1mol/L KOH soluble hemicellulose(HC1) and4mol/L KOH solublehemicellulose(HC2), were isolated from pulp tissues of pineapple fruit. Changes in content,molecular weight and monosaccharide compositions of these polysaccharides duringstorage at ambient temperature were analyzed to evaluate effects on fruit softening andphysiological changes of pineapple fruit during softening. Moreover, cell wall degradationenzymes including polygalacturonase(PG), pectin methyl esterase (PME), β-galactosidase(β-Gal), wood glucan glycosyl transferase (XET) and cellulase (Cx) were determined tounderstand the effects of enzymatic and non-enzymatic reactions on degradation of cellwall polysaccharides in pineapple fruit tissue during storage.The main results were asfollows:(1) With pineapple fruit softening, rigidity of fruit continually declined throughstorage time but obviously at front stage of storage. Rate of respiration of pineapple fruitincreased with storage time extension but steadily at late period of storage, showing aproperty of non-respiration climacteric fruit. Contents of total soluble solids (TSS),titratable acidity (TA) of flesh tissues of pineapple fruit gradually decreased with fruitsoftening, and Vitamin C (Vc) content first increased and then declined.(2) With progress of postharvest pineapple fruit ripening and softening, the ASPcontent decreased significantly and the WSP content increased markedly, suggesting thatthe insoluble pectin degraded to become water soluble pectin and pectin acid. The contentof HC1basically remained unchanged from stageⅠtoⅡ, increased significantly at stage Ⅲ, and decreased significantly at last. The HC2content decreased at first and thenincreased, but remained unchanged on the whole. The cellulose content also remainedunchanged on the whole. These results indicated that ASP, WSP and HC1weresignificantly related to the fall of ‘Bali’ pineapple fruit hardness in the process of fruitsoftening, However, HC2and cellulose had no correlation with decline of fruit hardness.(3) There were two different molecular mass polymers in WSP, ASP, HC1fractions,including a high molecular mass polymers and a low molecular mass polymers. Withsoftening of pineapple fruits, WSP, ASP and HC1were depolymerized into low molecularmass polymer from high molecular mass polymer, the content of low molecular masspolymers increased and the content of high molecular mass polymers decreased, and themolecular mass of ASP dropped significantly. Because HC2mainly contained lowmolecular mass polymers, the molecular mass of HC2did not show obvious change in thesoftening process of pineapple fruit.(4) Pectin was rich in arabinose and galactose. With pineapple fruit softening, themolar percentage of galactose in WSP decreased obviously and the molar percentage ofarabinose slightly declined in the late period of softening, but galacturonic acid basicallyremain unchanged, and some vice chains of WSP depolymerized. The molar percentage ofgalactose in ASP also decreased significantly, galacturonic acid decreased gradually, andthe molar percentage of arabinose have slightly increased in the late period of storage. Inthe process of ASP transforming to WSP, the main chain of ASP fractured and degradedand side chain also broke at the same time. The main monosaccharide of hemicellulose isXylose and a certain amount of pectin were embedded into the hemicellulose. In theprocess of pineapple fruit softening, the main chain of HC1did not degraded, but the molarpercentage of galactose gradually declined. Degradation of HC1was subject to side chainand the structure change of pectin side chain existed in KOH soluble polysaccharides, andthe degradation of galactose in hemicellulose had close correlation with pineapplesoftening. The monosaccharide compositions of HC2did not almost degrade and had noobvious change.(5) PG, PME and β-Gal were three very important cellular wall hydrolase enzymesthat degraded pectin. The correlation analysis showed that these enzymes had a significantcorrelation with pectin content and reduce of fruit rigidity, involving in the progress ofpineapple fruit softening. With extension of storage time, PG activities in pineapple fruitfirst increased, then decreased and remained a high level of activity from the middle periodto the late period of storage duration, indicating that PG played a main role on degradationof pectin, fruit hardness decline and texture change of pineapple fruit at the late of storage. PME activities also first increased, then decreased, but earlier reached peak of activity thanPG, demethylating polygalacturonic. Change in β-Gal activity showed a similar trend withPG, resulting in the loss of pectin galactose and the damage of the integrity of the pectin.PME, β-Gal and PG had synergistic effects on hydrolysis of pineapple pectin. The recentresearch showed that XET was a enzyme hydrolyzing hemicellulose of fruit cellular cell.In present study, the correlation analysis showed that there was no significant correlationwith the decrease of pineapple fruit hardness. Cx is a group of enzymes that can hydrolyzecellulose. The correlation analysis of this study showed that there was also no significantcorrelation with the decrease of pineapple fruit hardness.(6) The experiment were conducted to study effects of non-enzymatic factor ondegradation of cellular wall polysaccharides. Hydroxyl radicals(·OH) acceleratedobviously the degradation of cell wall polysaccharides and had dosage effect. Hydroxylradicals significantly increased the release amount of total sugar and uronic acid ofsuspension of Cell wall materials (CWM) from pineapple fruit at various stage of ripening.With increment of·OH concentration, the content of the total sugars and uronic acid inpineapple fruit CWM suspension at mature green stage significantly enhanced and·OHscavenging agent can inhibit this effect.