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Dangyang Ke and Michael Boersig

Consumer acceptance of sensory quality was significantly correlated (P = 0.05) with overall eating quality of tomatoes (Lycopersicon esculentum Mill.) determined by a trained taste panel. The overall eating quality was partitioned into three flavor components (sweetness, sourness, and aroma) and internal texture. Relationships between the sensory flavor components and concentrations of several chemical compounds were analyzed in tomatoes of eleven varieties within a range of acceptable texture. Sweetness was significantly correlated with total sugar concentration and overall flavor at P = 0.01 and 0.001. High concentrations of two ketone volatiles (6-methyl-5-hepten-2-one and 1-penten-3-one) were significantly correlated with aroma and overall flavor at P = 0.05 to 0.001. This study indicates that positive tomato flavor may be characterized by high sweetness, moderate acidity, and high concentrations of certain ketone volatiles.

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Dangyang Ke and Adel A. Kader

Fruits of `Bing' cherry (Prunus avium L.), `Red Jim' nectarine (Prunuspersica L.), `Angeleno' plum (Prunus salicina, L.), `Yellow Newtown' and `Granny Smith' apples (Malus domestica Borkh.), and `20th Century' pear (Pyrus serotina L.) were treated with 0.25% or 0.02% O2 (balance N2) at 0, 5, or 10C to study the effects of these insecticidal low-O2 atmospheres on their postharvest physiology and quality attributes. Development of alcoholic off-flavor was associated with ethanol accumulation, which was the most common and important detrimental effect that limited fruit tolerance to low O2. Relatively higher storage temperature (T), higher respiration rate (R), and greater resistance to gas diffusion (r) enhanced while relatively higher O2 concentration (C) and higher soluble solids concentration (SSC) reduced off-flavor development. Using a SAS computer program to do multiple regression analysis with T, C, R, r, and SSC as variables, models were developed for prediction of fruit tolerance to insecticidal low-O, atmospheres. Comparison of fruit tolerances and published information on the times required to completely kill specific insects by O2 levels at or below 1% suggests that low-O2 atmospheres have a good potential for use as postharvest quarantine treatments for some fruits.

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Dangyang Ke and Adel A. Kader

`Valencia' oranges [Citrus sinensis (L.) Osbeck] tolerated up to 20 days of exposure to 0.5%, 0.25%, or 0.02% O2, at 5 or 10C followed by holding in air at 5C for 7 days without any detrimental effects on external and internal appearance. Oranges stored in 0.5%, 0.25%, or 0.02% O2 had lower respiration rates, but higher resistance to CO, diffusion and higher ethanol evolution rates than those stored in air at 10C. Similar, but less pronounced, effects of the low O2 atmospheres were observed at O and SC. Respiration rates, internal CO2 concentrations, and ethanol evolution rates were generally higher at 10C than at 0C, while resistance to CO2 diffusion was lower at the higher temperature. `Valencia' oranges kept in 60% CO2 at 5C for 5 to 14 days followed by holding in air at 5C for 7 days developed slight to severe injury that was characterized by skin browning and lowered external appearance scores. Juice color, soluble solids content, pH, titratable acidity, and ascorbic acid content were not significantly influenced by either the low O2 or the high CO2 treatments. However, these treatments increased ethanol and acetaldehyde contents, which correlated with the decrease in flavor score of the fruits. Ethanol content of the oranges transferred to air following low 02 treatment correlated with CO2 production rate of the fruits at the transfer temperature and was related to ethanol evolution and probably production rates after the transfer.

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Dangyang Ke and Adel A. Kader

Fruits of peach (Prunus persica L., cv. `Fairtime') and plum (Prunus domestica L., cv. `Angeleno') were kept in air and in 0.25% or 0.02% O2 at 0, 5, or 10°C for 3 to 40 days to study the effects of temperatures and insecticidal low O2 atmospheres on their physiological responses and quality attributes. Exposure to low O2 atmospheres reduced respiration and ethylene production rates of the stone fruits. The low O2 treatments retarded color change and flesh softening of plums and maintained acidity of peaches. Exposure to the low O2 atmospheres also delayed incidence and reduced severity of internal breakdown (chilling injury) and decay of the peaches at 5°C and, therefore, maintained both external and internal appearance qualities of the fruits longer than those kept in air. The most important limiting factor for fruit tolerance to insecticidal low O2 atmospheres was development of alcoholic off-flavor which was associated with accumulation of ethanol and acetaldehyde in the fruits. The peaches and plums could tolerate exposures to the low O2 atmospheres for 9 to 40 days, depending on the temperature and O2 level used. These results suggest that stone fruits are quite tolerant to insecticidal low O2 atmospheres.

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Dangyang Ke and Adel A. Kader

Selected cultivars of several fruit species were exposed to 0.25% or 0.02% O2 at 0, 5, or 10C for short durations to investigate the potential of these treatments as quarantine procedures for postharvest insect control. Beneficial effects of such low O2 treatments included inhibition or delay of ripening processes as indicated by reduction in respiration and ethylene production rates, retardation of skin color changes and flesh softening, and maintenance of titratable acidity. While appearance was not adversely influenced by the short-term low O2 treatments, the development of alcoholic off-flavor was the most important detrimental effect, which limited the tolerance of fresh fruits to low-O2 atmospheres. Ethanol content and flavor score of the fruits had a logarithmic relationship. The threshold ethanol concentration associated with off-flavor detection (EO) increased with SSC of the commodity at the ripe stage, and it could be estimated using the following formula (Log EO)/SSC = 0.228. Using SSC of ripe fruits and average ethanol accumulation rate per day (V) from each low O2 treatment, the tolerance limit (Tl) of fruits to low O atmospheres could be predicted as follows: Tl = EO/E = 1 00.228 SSC2/V.E

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Dangyang Ke, Lili Zhou, and Adel A. Kader

`Chandler' strawberries (Fragaria ananassa Duck.) were kept in air, 0.25% O2, 21% O2 + 50% CO2, or 0.25 O2 + 50% CO2 (balance N2) at 5C for 1 to 7 days to study the effects of controlled atmospheres (CAs) on volatiles and fermentation enzymes. Concentrations of acetaldehyde, ethanol, ethyl acetate, and ethyl butyrate were greatly increased, while concentrations of isopropyl acetate, propyl acetate, and butyl acetate were reduced by the three CA treatments compared to those of air-control fruit. The CA treatments enhanced activities of pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) but slightly decreased activity of alcohol acetyltransferase (AAT). The results indicate that the enhanced PDC and ADH activities by CA treatments cause ethanol accumulation, which in turn drives the biosynthesis of ethyl esters. The increased ethanol concentration also competes with other alcohols for carboxyl groups for esterification reactions. The reduced AAT activity and limited availability of carboxyl groups due to ethanol competition decrease production of other acetate esters.

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Fairuz. El-Wazir, Dangyang Ke, and Adel A. Kader

The tolerances (based on time before detection of off-flavor) of nectarine and peach cultivars to an insecticidal controlled atmosphere of 0.25% O2 (balance N2) at 20C were 2.8, 4.0, 4.0, 4.4, 5.1, and 5.3 days for `John Henry' peaches, `Fantasia' nectarines, `Five Red' peaches, `O'Henry' peaches, `Royal Giant' nectarines, and `Flamekist' nectarines, respectively. The greater sensitivity of `John Henry' peaches to low O2 stress was associated with a higher respiration rate; faster accumulation rates of acetaldehyde, ethanol, and ethyl acetate; and a more mature and larger fruit. The tolerances of `Fairtime' peaches to 0.21% O2 + 99% CO2 at 20C, 0.21 O2 + 99% CO2 at 0C, and 0.21% O2 at 20C were 3.8, 5.0, and 6.0 days respectively. There was a good correlation between tolerance of nectarines and peaches to insecticidal atmospheres and the accumulation rates of acetaldehyde (r=-0.94, p<0.01) and ethanol (r=-0.88, p,0.01).

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Dangyang Ke, Elhadi Yahia, Mila Mateos, and Adel A. Kader

Changes in fermentation volatiles and enzymes were studied in preclimacteric and postclimacteric `Bartlett' pears (Pyrus communis L.) kept in air, 0.25% O2, 20% O2 + 80% CO2, or 0.25% O2 + 80% CO2 at 20C for 1, 2, or 3 days. All three atmospheres resulted in accumulation of acetaldehyde, ethanol, and ethyl acetate. The postclimacteric pears had higher activity of pyruvate decarboxylase (PDC) and higher concentrations of fermentation volatiles than those of the preclimacteric fruit. For the preclimacteric pears, the 0.25% O2 treatment dramatically increased alcohol dehydrogenase (ADH) activity, which was largely due to the enhancement of one ADH isozyme. Exposure to 20% O2 + 80% CO2 slightly increased ADH activity, but the combination of 0.25% O2 + 80% CO2 resulted in lower ADH activity than 0.25% O2 alone. For the postclimacteric pears, the three atmospheres resulted in higher PDC and ADH activities than those of air control fruit. Ethanolic fermentation in `Bartlett' pears could be induced by low O2 and/or high CO2 via 1) increased amounts of PDC and ADH; 2) PDC and ADH activation caused by decreased cytoplasmic pH; or 3) PDC and ADH activation or more rapid fermentation due to increased concentrations of their substrates (pyruvate, acetaldehyde, or NADH).

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Dangyang Ke, Leonor Rodriguez-Sinobas, and Adel A. Kader

Fruits of `Granny Smith' and `Yellow Newtown' apples (Malus domestica Borkh), `20th Century' pear (Pyrus serotina L.), and `Angeleno' plum (Prunus domestica L.) were kept in air and in 0.25% or 0.02% O2 at 0, 5, or 10C for 3, 7, 14, 25, or 35 days to study the effects of low-O2 atmospheres on their postharvest physiology and quality attributes. Soluble solids content (SSC), pH, and external appearance were not significantly influenced, but resistance to CO2 diffusion was increased by the low-O2 treatments. Exposures to the low-O2 atmospheres inhibited ripening, including reduction in ethylene production rate, retardation of skin color changes and flesh softening, and maintenance of titratable acidity. The most important detrimental effect of the low-O2 treatments was development of an alcoholic off-flavor that had a logarithmic relation with ethanol content of the fruits. The ethanol content causing slight off-flavor (Eo) increased with SSC of the commodity at the ripe stage, and it could be estimated using the following formula: (Log Eo)/SSC = 0.228. Using SSC of ripe fruits and average ethanol accumulation rate per day (VE) from each low-O2 treatment, the tolerance limit (Tl) of fruits to low-O2 atmospheres could be predicted as follows: Tl = Eo/VE = (100.228SSC)/VE.

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Dangyang Ke, Hendrik van Gorsel, and Adel A. Kader

`Bartlett' pears (Pyrus communis L.) tolerated up to 10 days of exposure to atmospheres containing 1.0%, 0.5%, or 0.25% O2 at 0, 5, or 10C without any detrimental effects on their quality attributes. The fruits also tolerated 4 to 6 days of exposure to air enriched with 20%, 50%, or 80% CO2 at the three temperatures. The beneficial effects of exposures to the O2-reduced or CO2-enriched atmospheres included reduction of respiration and ethylene production rates and retardation of skin yellowing and flesh softening. While 1.0% or 0.5% O2 and 20% CO2 did not increase ethanol and acetaldehyde contents, 0.25% O2 slightly increased and 50% or 80% CO2 dramatically increased the contents of these two volatiles in juice of the fruits. The effects of low O2 or high CO2 on the above attributes generally became more pronounced at the higher temperatures. The low O2 or high CO2 treatments did not significantly affect either soluble solids content or titratable acidity. Low O2 did not influence, but high CO2 slightly increased pH of the fruits.