Changes in ethanol fermentation metabolism and volatile metabolites in kiwifruit ‘Bruno' during storage at room and low temperature

【Objective】Acetaldehyde and ethanol that are produced by ethanol fermentation metabolism are the most important volatile aroma metabolites in harvested fruits. However, they may accumulate at high levels in the fruit stored under anaerobic conditions, at chilling temperature, or for a long time, and then result in physiological disorder and off-flavors in fruits, which affect the fruit flavor and quality. Kiwifruit is typically climacteric, and exists the accumulation of acetaldehyde and ethanol during storage at room temperature as well as under controlled or modified atmosphere with low O2 or high CO2, or at low temperature. In order to understand the ethanol fermentation metabolism in relation to quality in harvested kiwifruit, the changes in ethanol fermentation metabolism and the volatile aroma metabolites in kiwifruit(Actinidia chinensis‘Bruno') were investigated during storage at room(25 ℃) and low temperature(5 ℃).【Methods】The changes in the contents of pyrnvic acid, acetaldehyde and ethanol, the activities of pyruvate decarboxylase(PDC) and alcohol dehydrogenase(ADH), as well as PDC and ADH expression were measured, and the changes in aroma components and their absolute and relative contents in kiwifruit were analyzed by GC-MS during storage at room and low temperature.【Results】The pyrnvic acid content in kiwifruit increased and then decreased gradually during storage at 25 ℃ and at 5 ℃. The contents of ethanol and acetaldehyde maintained stably at low levels at the beginning 7 days and 21 days at 25 ℃ and5 ℃, respectively, and then sharply increased at 25 ℃, while gradually increased at 5 ℃ in the late period. The activities of PDC and ADH and their genes expression in kiwifruit also sharply increased duringthe beginning 10 days at 25 ℃ and then decreased, but their activities gradually increased during storage at 5 ℃, and PDC expression slowly increased, while ADH expression maintained stably low levels during the first 21 days and reached a peak on the 31 st day, then decreased gradually. Thus, low temperature resulted in significant decrease in activities of PDC and ADH, and their gene expression in kiwifruit, which could contribute to limiting the accumulation of ethanol and acetaldehyde. Moreover, the volatile compounds consisted of esters, alcohols, acids, and aldehyde ketone in kiwifruit, among which the ester was the primary volatile components. At 25 ℃, 9 or 28 esters in kiwifruit were detected on the first day or on the 16 th day, and the relative and absolute contents increased from 4.52% to 92.18% and from 12.35 μg·g-1to 1 640.95 μg·g-1during this period, respectively. At 5 ℃, 9 or 11 esters in kiwifruit were detected on the first day or on the 51 st day, and the relative and absolute contents increased from 3.75% to 4.30% and from 14.57μg·g-1to 15.92 μg·g-1during this period, respectively. Thus, the amount and type of esters in kiwifruits enhanced during storage at 25 ℃ and at 5 ℃. Unlike esters, the absolute contents of alcohols in kiwifruit increased from 39 μg·g-1to 51.69 μg·g-1, but their relative contents decreased from 14.27% to2.90% due to ethanol esterification during the period from the first day to the 16 th day at 25 ℃, while their relative and absolute contents increased slowly during storage at 5 ℃. In addition, the relative and absolute contents of acids and aldehyde ketones in kiwifruits decreased during storage at 25 ℃ and at 5 ℃.【Conclusion】Low temperature inhibited the accumulation of acetaldehyde and ethanol associated with the decrease in the activities of PDC and ADH and their gene expression, which might benefit the quality and flavor in harvested kiwifruit. Besides the direct effects on ethanol accumulation, the products of ethanol esterification might be involved in off-flavor in kiwifruits during storage, and the effects of low temperature on ethanol esterification in relation to the quality and flavor in harvested kiwifruit needed to be further addressed.