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  • Author or Editor: Guiwen W. Cheng x
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The relationship of phenolic composition and polyphenoloxidase activity (PPO, E.C. 1.14.18.1) to browning potential (BP) was studied in buffer extracts of peach [Prunus persica L. Batsch) and nectarine [P. persica var. nectarine (L.) Batsch] fruit skin. The BP varied among cultivars with `Flavorcrest' having the highest value and `Maycrest' the lowest. On average, over 83 % of the browning measured at the end of the S-hour incubation occurred during the first hour. The total soluble phenolics (TSP), the total anthocyanin (TA), and glutathione content (GLU) varied among cultivars, but were not significantly correlated to the BP. Of the phenolics determined by HPLC, only chlorogenic acid had a significant positive correlation and epicatechin a significant negative correlation with BP by the first hour of incubation. The PPO activity, ranging from 4 to 11 optical density units per gram dry weight per minute among peaches and nectarines, was not significantly correlated with BP. However, no browning was detected if the buffer extract was previously boiled. These results indicated that browning in the buffer extracts of peach and nectarine skin tissue depends on the presence of PPO activity and chlorogenic acid, which are major contributors to enzymatic browning.

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The formation of metallo-pigmentation and copigmentation as potential mechanisms of inking formation was investigated in peach and nectarine skin tissues. Cyanidin-3-glucoside, the most abundant anthocyanin in peaches and nectarines, formed very purple ferric complexes with an anthocyanin/iron molar ratio of two. Greenish metallo complexes between ferric ion and chlorogenic acid, caffeic acid, catechin, or epicatechin formed with an phenolic/iron molar ratio of one. The lack of copigmentation pointed out the importance to focus research on the metallo-phenolics reaction. High intensity of dark color formation was developed with cyanidin-3-glucoside, followed by caffeic acid, chlorogenic acid, catechin, and epicatechin on an equal molar basis. Citric acid acted as a strong iron chelator to prevent and reverse the formation of ferric cyanidin-3-glucoside complexes. The variety of dark and light colored spots observed on the surface of peaches and nectarines is explained by the formation of metallo-pigment complexes.

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Dark skin discoloration development on peach and nectarine cultivars was investigated in response to exogenous pH and metallic ions. The influence of skin abrasion and washing in combination with exogenous contaminants was studied in a factorial design experiment by using skin discs. Only abraded skin discs with and without washing developed discoloration after being exposed to high pH and different metallic ion concentrations. Among the metallic ion contaminants studied (Fe, Al, Cu, Sn, Zn, and Na), iron was the most effective in causing dark skin discoloration at physiological pH (3.5). Iron concentrations ≥10 ppm induced dark discoloration on abraded fruit skin. Dark discoloration development produced by exposing the skin tissue to pH levels >6 was reversible, whereas the dark discoloration induced by iron and aluminum remained stable.

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Fruit size, number of receptacle cells, and mean cell size were determined throughout development of secondary fruit of three day-neutral strawberry (Fragaria ×ananassa Duch.) cultivars grown in a greenhouse. Cells were counted after enzymatic separation of receptacle tissue, and mean cell volume was estimated from cell count and receptacle tissue volume. Size of mature fruit was small (3.8 g) in `Tillikum', medium (11.5 g) in `Tristar', and large (15.6 g) in `Selva'. Fruit size was correlated with the number of achenes per berry. Mature fruit of `Tillikum' had a lower fruit fresh weight per achene and lower achene population density (achenes per square centimeter) than the larger-fruited cultivars. The average number of cells per mature fruit was 0.72 × 106, 1.96 × 106, and 2.94 × 106 for `Tillikum', `Tristar', and `Selva', respectively. The relative difference among cultivars in the number of receptacle cells was established by the time of anthesis. In all cultivars, cell division was exponential for 10 days following anthesis and ceased by the 15th day. Mean cell volume increased slowly during active cell division, but rose rapidly and linearly for 10 days after cell division halted. Mean cell volume of all cultivars increased > 12-fold after anthesis and was ≈ 6 × 106 μm3 in mature fruit. The genotypic variation in the size of mature fruit was not the result of large differences in either duration of cell division after anthesis or mean cell volume, but rather was primarily due to differences in the number of receptacle cells established by anthesis.

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Studies on regulation of production of phenolics in strawberry (Fragaria X ananassa Duch,) fruit were initiated by monitoring phenylalanine ammonia-lyase (PAL) activity and levels of anthocyanins, flavonoids, tannins, and other soluble phenols throughout fruit ontogeny in `Tillikum'. PAL catalyzes the first step in the biosynthesis of phenylpropanoids, which are further modified into a wide variety of phenolic compounds. Peak in PAL activity (1 mol· s-1 = 1 kat) of 90 pkat· mg-1 protein was detected at 5 and 27 days after anthesis (DAA), when fruit was green and nearly ripe, respectively. PAL activity was only ≈10% of peak values in the white berry stage, when. fruit growth was most rapid. The second peak in PAL activity was followed by a rapid drop, to nearly zero in red-ripe fruit at 30 DAA. Total soluble phenols reached a maximum level soon after anthesis, just before the first peak in PAL activity, then declined to a low constant value well in advance of fruit ripening. Similar changes were observed in levels of tannins and flavonoids that, at anthesis, accounted for 44% and 51% of the soluble phenols, respectively. The concentration of anthocyanin was very low throughout most of fruit development, but beginning at 23 DAA it increased from <0.03 to >0.53 mg·g-1 fresh weight in 3 days. This accumulation paralleled the second rise in PAL activity. Accordingly, strawberry fruit have a developmental-dependent expression of PAL activity and accumulation of phenolic substances derived from the phenylpropanoid pathway.

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Changes in the gel filtration behavior (apparent mol mass) of cell wall pectic polymers have been observed in a number of ripening fruits, including some that express little or no detectable polygalacturonase (PG). Pectins from tomato (Lycopersicon esculentum, Mill. v. Solar Set) fruit locule tissue show limited depolymerization during ripening, although alkali-soluble polymers are of reduced mol mass relative to water- and chelator-soluble polymers (Plant Physiol. 111:447). This study addressed whether the lower mol mass of alkali-soluble polymers was a consequence of extraction or specific metabolism of these wall polymers. Pectins from sequential water and chelator extractions of ethanol-insoluble solids from mature green tomato locule tissue were subjected to alkaline conditions. The size distribution of both water- and CDTA-soluble pectins treated with weak alkali were downshifted and similar to those extracted directly by weak alkali, indicating structural similarities of the three pectin fractions. Spectrophotometric analysis showed no involvement of β-elimination hydrolysis in the apparent mol mass reduction. The alkali-treated polymers were of greatly enhanced susceptibility to PG-mediated degradation. The alkali-associated changes also occurred in response to pectinmethylesterase hydrolysis. The results indicate that deesterification can strongly influence gel filtration behavior of pectins and may explain the apparent mol mass decreases of pectins in fruits not containing PG.

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Softening and liquefaction of `Solar Set' locules was studied by examining cell wall polysaccharides during fruit developmental stages (FDS) of immature green, mature green and breaker. Ethanol insoluble solids (EIS) were sequentially extracted by H2O, CDTA, and Na2CO3 solutions. The chromatograms of gel filtration among the same-solution extracts of EISs from three FDS were similar. Gradient DEAE also yielded similar patterns among FDS in each extraction solvent, even though the patterns of Na2CO3 extracts differed from those of H2O and CDTA extracts. The mole ratio of total polyuronides decreased for Gal, Ara, and Xyl at later FDS in both EIS and in all extracted polymers. Gal had the highest mole percentage of total neutral sugars, followed by Ara, Xyl, and Rha. While the mole percentage of neutral sugars for Gal, Rha, Ara, and Xyl were relatively similar among FDS in H2O extracts, those in CDTA and Na2CO3 extracts either increased or decreased, depending on individual neutral sugar. SDS-PAGE showed increased density in locule-tissue proteins, especially one with a molecular weight of less than 20 kDa, during later FDS. Results indicate that pectin depolymerization was limited and major neutral sugars commonly composing side chains showed a net decrease.

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Hydrolytic activities in liquefying locule tissue of mature-green tomato (Lycopersicon esculentum Mill. `Solar Set') fruit were studied in pursuing the understanding of mechanisms involved in the rheological changes occurring in this tissue. Ethanol-insoluble solids (EIS) were prepared with and without enzyme-inactivating treatment. The release of uronic acids from enzymically active EIS incubated under autolysic conditions was 5-fold higher than recoveries from inactive EIS. Uronic acid release was partially inhibited by 1 mm Hg2+. Cell-free proteins extracts from active EIS exhibited hydrolytic activity against inactive EIS. Pectins released from active EIS showed no evidence of main-chain hydrolysis. Neutral sugars recovered as 80% ethanol-soluble products of autolytic reactions included glc, gal, rha, ara, xyl, and man. Gal was recovered at significant higher levels in autolysates of active EIS. Glycosidases present at high activities in locule tissue included α- and β-galactosidases, β-mannosidase, β-arabinosidase, and β-glucosidase. The results confirm our earlier findings that the metabolism of water, chelator, and alkali-soluble pectins in tomato locule tissue involves deglycosylation with no apparent depolymeriation. These changes alone appear to be inadequate in explaining the unique rheological characteristics in locule gel tissue.

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The development of ethylene preconditioning treatments for kiwifruit have made it possible to deliver ripe kiwifruit to consumers early in the season. We report on how maturity and length of storage time affect the ripening responses of kiwifruit [Actinidia deliciosa (A Chev) Liang et Ferguson cv Hayward] preconditioned with 100 ppm ethylene at 0°C for 24 hours and ripened for 10 days at 20°C. Kiwifruit freshly harvested at weekly intervals continued to soften faster in response to ethylene preconditioning compared to air controls for at least 5 weeks following commercial harvest. In contrast, kiwifruit commercially harvested and stored at 0°C for more than 2 weeks no longer responded to low-temperature ethylene preconditioning. However, kiwifruit stored more that 5 weeks were still responsive to exogenous ethylene and softened faster when exposed to continuous ethylene at either 0 or 20°C. Kiwifruit had relatively high respiration rates 1 days after transferring from 0 to 20°C, which quickly dropped to base levels within 1 day. Fruit stored >1 week at 0°C always had higher initial respiration than freshly harvested fruit on transfer to 20°C, and ethylene preconditioning increased initial respiration of freshly harvested fruit but had less of an effect on initial respiration of stored fruit. Plotting firmness against individual fruit's respiration and ethylene production revealed a distinct rise in respiration and ethylene production only after fruit softened to <6.5 N. Preconditioning fruit at 0°C did not significantly alter the timing of the climacteric respiration or ethylene peaks.

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