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  • Author or Editor: Jun Song x
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Changes in the profile of aroma volatiles during ripening and after ethylene treatment in apple fruit have revealed a close relationship exists between ethylene production and the timing and magnitude of volatile synthesis. Therefore, AVG (ethylene biosynthesis inhibitor) was used to study the relationship between volatile biosynthesis and metabolic processes affected by ethylene in apple fruits. AVG-treated fruit were stored either for 1 month at 2C in air or 5 months in ULO condition. During the post-storage ripening, the fruits were exposed to 50 μl·liter–1 of ethylene at least 12 h/day. Aroma production was determined at 20C. Total volatile compound production by AVG-treated fruit was much lower than that of untreated fruit. A significant increase in the production of most aroma volatile after 1 month storage in air was induced by ethylene treatment to AVG-treated fruits. I was noted that branched-chain volatile, such as 2-methylbutylacetate, which originates from branched-chain amino acids, has increased prior to butylacetate and hexylacetate, which are derived from fatty acids. Ethylene treatment was unable to stimulate the production of straight-chain volatile compounds following the 5 months of ULO storage. These results are consistent with observations suggesting apple fruit lose their sensitivity to ethylene after long ULO storage.

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Hexanal vapor is a natural, metabolizable fungicide that inhibits fungal activity and enhances the aroma biosynthesis in sliced apple fruit. Whole apple fruit were inoculated at two points per fruit with Penicillium expansum at a concentration of 0.5 × 105 spore/ml and treated with hexanal vapors. Inoculated fruit were exposed to hexanal for 48 hr and kept for another 72 hr in hexanal-free air at 22°C. Treatments included 8.2–12.3 μmol·L–1 (200–300 ppm), 14.5-18.6 μmol·L–1 (350–450 ppm), and 24.8-28.9 μmol·L–1 (600–700 ppm), each with an air control. At a concentration of 200–300 ppm hexanal, there was no fungal growth during treatment, but lesion development was evident on 100% of the treated fruit following cessation of treatment. After 72 hr holding in air, lesion diameter was significantly smaller for treated fruit. When inoculated apple fruit were exposed to 350–450 ppm and 600–700 ppm hexanal vapors, the decay rate was 44.7% and 23.9%, respectively, while the decay rate of inoculated control apple fruit was 100% and 98%, respectively, after 72 hr holding in air. The development of aroma volatiles was investigated for both treated and untreated whole apple fruit. Hexanal was actively converted to aroma volatiles by `Golden Delicious' fruit and there was no detectable hexanal emanations. The amount of hexylacetate, hexylbutanoate, hexylhexanoate, hexylpropionate, butylhexanoate, and hexyl-2-methybutanoate were about 2- to 4-fold higher in treated apple fruit than in untreated apple fruit. `Mutsu' apple fruit were treated with 350–450 ppm hexanal for 48 hr and processed into apple sauce within 4 hr. An informal sensory evaluation for processed `Mutsu' apple revealed no apparent flavor difference between treated and control fruit sauce.

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After a brief description of the “history of research” of aroma volatiles of apple and strawberry fruit, possible reasons for the reduced production of these important quality attributes by particular pre- and postharvest procedures are given. Among the possible physiological factors in association with reduced aroma volatile production, a reduced ethylene sensitivity, a decline in the rate of respiration as well as the content of adenine nucleotides and limited free fatty acids as precursors for aroma volatiles biosynthesis are proposed. A hypothesis about how this sequence of events leads to reduced volatile production is given and finally some suggestions of how to improve volatile synthesis are discussed.

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The effect of polymers used in packaging on the aroma of the packaged product has been little explored. Using a package-in-a-jar system, we are able to simultaneously measure volatile production by plant organ (Malus domestica Borkh. cv. Golden Delicious) and the permeability of the packaging film to those volatiles. In this system, apple fruit were placed into a glass container or sealed in a low-density polyethylene(LDPE) package and subsequently placed into a glass container. Air or a modified atmosphere was slowly passed through the glass containers such that the O2 level in the package was similar to that in containers with no package. The package and jar head spaces were sampled for CO2, O2, ethylene, and aroma volatile analysis by gas chromatography/mass spectrometry. The effect of temperature, atmosphere and film presence to some major volatile compounds was determined. When storage temperature increased from 0°C to 22°C the production rate of hexylacetate and 2-methyl butylacetate increased 11.27- and 17.15-fold, respectively. At 0°C, as O2 decreased in concentration from 10% to 5% (v/v), hexylacetate and butylacetate declined significantly; however, 2-methyl butylacetate was not affected. This can be taken to indicate the production of 2-methyl butanol for 2-methyl butylacetate formation is not as O2 concentration dependent as straight-chain alcohols. At the same O2 concentration, non-packaged fruit evolved greater amounts of all volatiles than packaged fruit. The flux of α-farnesene, hexylacetate and 2-methyl butylacetate was 26.6-, 1.7-, and 1.4-fold higher, respectively, for fruit in glass container. The sorption of α-farnesene and some other volatiles into LDPE film is evidently considerable, altering the aroma profile of packaged produce relative to a flow-through system.

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Apple scald (peel browning) is hypothesized to involve a chilling disorder. Numerous studies have linked chloroplast fluorescence changes with chilling injury before symptom development. Therefore, chloroplast fluorescence was used for the prediction of scald in apples. `Red Delicious' apple fruit were harvested at three maturities and stored at 1 to 2C. They were removed from storage weekly and placed at ambient temperature (22C). Chloroplast fluorescence was measured at 0, 3, and 7 days after removal. A significant decline in quantum yield response (Fv/Fm), which indicates a reduction of chloroplast function, was recorded after 30 days in first-harvest fruit and 40 to 50 days in the second- and third-harvest fruit. The decline in Fv/Fm preceded scald development by ≈30 days in first-harvest fruit and 20 to 30 days in second- and third-harvest fruit. The data suggest that fluorescence changes and scald development may be related physiologically. Fruit firmness and other fruit ripening phenomena were also measured and their relationship to the fluorescence and scald development were investigated. The results indicated that the chloroplast fluorescence may be used as a predictive tool for scald development in stored apple fruit.

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Hexanal vapor inhibited hyphae growth of Penicillium expansum and Botrytis cinerea on potato dextrose agar (PDA) and on apple (Malus domestica Borkh.) slices. After 48 hours exposure to 4.1 μmol·L-1 (100 ppm) hexanal, the hyphae growth of both fungi was about 50% that of untreated controls. At a concentration of 10.3 μmol·L-1 (250 ppm), neither fungus grew during the treatment period, however, some growth of both fungi occurred 120 hours after treatment. At concentrations of hexanal vapor of 18.6 μmol·L-1 (450 ppm) or more, the growth of both fungi ceased and the organisms were apparently killed, neither showing regrowth when moved to air. When fungi were allowed to germinate and grow for 48 hours in hexanal-free air, a subsequent 48-hour exposure to 10.3 μmol·L-1 hexanal slowed colony growth relative to controls for several days and a 48-hour exposure to 18.6 μmol·L-1 stopped growth completely. Concentrations of hexanal that inhibited fungal growth on PDA also retarded decay lesion development on `Golden Delicious' and on `Jonagold' apple slices. Hexanal was actively converted to aroma volatiles in `Jonagold' and `Golden Delicious' apple slices, with hexanol and hexylacetate production strongly enhanced after 20 to 30 hours treatment. A small amount of butylhexanoate and hexylhexanoate production was also noted. Within 16 hours after treatment, no hexanal could be detected emanating from treated fruit. Since hexanal was metabolized to aroma-related volatiles by the fruit slices, the possibility of hexanal being an essentially residue-less antifungal agent seems likely. The possibility of developing a system for treating apple slices with hexanal in modified-atmosphere packages was also examined. The permeability of low-density polyethylene (LDPE) film to hexanal and hexylacetate was, respectively, about 500- and 1000-fold higher than LDPE permeability to O2. The permeability of both compounds increased exponentially with temperature, with hexanal permeability increased 6-fold while hexylacetate increased only 2.5-fold between 0 and 30 °C.

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The biological effect of corona discharge on onions (Allium cepa L.) in a commercial storage was investigated. Surface discoloration and mold were modestly but significantly reduced by the corona discharge when onions were stored for 2 or 4 weeks with or without an additional 2 weeks of shelf life under high humidity. Corona discharge treatment also reduced airborne mold spores in the storage room. No significant changes in internal decay, firmness, sprouting, or rooting, in treated onions were found.

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Volatile emissions and chlorophyll fluorescence were investigated as potential signals of heat injury for apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] fruit. `McIntosh', `Cortland', `Jonagold', and `Northern Spy' apples were exposed to 46 °C for 0, 4, 8, or 12 hours (heat treatments). Following treatments, fruit were kept at 20 °C and evaluated after 1, 2, 4, or 7 days. Heat treatments induced volatile production including ethanol and ethyl acetate. The 8 and 12 hours heat treatments increased ethanol and ethyl acetate production in all four cultivars by as much as 170- and 11-fold, respectively, 1 day after treatments. Heat treatments also reduced ethylene production and chlorophyll fluorescence. Heat for 12 hours caused serious flesh browning. Among the cultivars investigated, `Northern Spy' and `McIntosh' were most susceptible to heat stress based on the degree of flesh browning. Correlation coefficients of heat stress induced ethanol emission and chlorophyll fluorescence with flesh browning were 0.82 and -0.66, respectively. The nondestructive measurements of ethanol emission and chlorophyll fluorescence have potential to identify stressed fruit with reduced quality or compromised storage life.

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Ethanol concentration and chlorophyll fluorescence (CF) were measured as signs of heat stress in apple fruit [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.]. `McIntosh', `Cortland', `Jonagold', and `Northern Spy' apples were placed in trays and exposed to 46 °C for 0, 4, 8, or 12 hours. Following treatments, fruit were stored in air at 0 °C and evaluated after 0, 1, 2, or 3 months. Ethanol and ethylene production, CF, peel and flesh browning, firmness, skin color, soluble solids, and titratable acidity were measured. Increases in ethanol were apparent immediately following 12-hour heat treatments as well as after 3 months. After 3 months, ethanol concentrations were 16-, 52-, 6-, and 60-fold higher in `McIntosh', `Cortland', `Jonagold', and `Northern Spy' apples than in controls, respectively. The concentrations of ethanol accumulated reflected the degree of heat-induced fruit injury. Heat treatments reduced ethylene production relative to control values. After 3 months of storage ethylene production of fruit exposed to 46 °C for 12 h was <0.48 μmol·kg-1·h-1 compared to >4.3 μmol·kg-1·h-1 for controls. Heat treatments also reduced CF which was expressed as Fv/Fm, where Fv is the difference between the maximal and the minimal fluorescence (Fm - Fo), and Fm is the maximal fluorescence. After 3 months storage at 0 °C, Fv/Fm was ≈0.2 in fruit held at 46 °C for 12 hours compared with 0.5-0.6 for control fruit. Exposure to 46 °C for 12 hours caused severe peel and flesh browning in all cultivars. Severity of peel and flesh browning increased with increasing duration of heat treatment and subsequent storage at 0 °C. `Northern Spy' apple fruit were most susceptible to heat stress based on the degree of flesh browning. Heat treatments of 8 and 12 hours reduced firmness of `McIntosh', `Cortland', and `Northern Spy', but not `Jonagold' apples. Hue angle of the green side of fruit was also reduced in `Cortland', Jonagold' and `Northern Spy' apples receiving the 8- and 12-hour treatments. Heat treatments caused a decrease in fruit tiratable acidity, but had no effect on soluble solids content. The increase in ethanol production and decrease in CF correlated with heat-induced injury, and were apparent before browning was visually apparent. Ethanol and CF have the potential to be used to nondestructively predict the severity of injury that develops during storage.

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Ethanol production and chlorophyll fluorescence were measured as signals of freezing and heat stress in apple fruit. `Cortland' and `Jonagold' apples were held at –8.5 °C for 0, 6, 12 or 24 h (freezing treatments), or at 46 °C for 0, 4, 8 or 12 h (heat treatments). Following treatments, fruit were stored at 0 °C and evaluated after 0, 1, 2, or 3 months. Following storage, fruit samples were kept for 12 h at 20 °C and then analyzed for ethanol production, chlorophyll fluorescence, and visible injury. Severity of flesh browning increased with increasing treatment time for both freezing and heat treatments. Freezing for 24 h and heating for 12 h caused severe flesh browning in both cultivars. Severity of heat-induced browning increased during storage. Increases in ethanol production were apparent 12 h following treatments and reflected the degree of stress-induced fruit injury. After 2 months of storage, ethanol concentrations peaked and were as much as 400-fold greater than that of controls. These stress treatments also reduced ethylene production and chlorophyll fluorescence. The degree of increase in stress-induced ethanol production and decrease in chlorophyll fluorescence correlated with stress-induced injury and could be used to predict the severity of injury that develops during storage. Other volatile production and their relationship to fruit stress will also be discussed.

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