The kinetics of acetaldehyde and ethanol accumulation in 'Hass' avocado fruit during induction and recovery from low oxygen and high carbon dioxide conditions

The kinetics of acetaldehyde (AA) and ethanol (EtOH) accumulation and pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH) activities were studied in pre-climacteric 'Hass' avocado fruit flesh during induction and recovery from hypoxic conditions at 6 degrees C. Oxygen levels <0.5% resulted in a rapid accumulation of AA and EtOH. The pattern of AA and EtOH accumulation could be described by a hyperbolic model, although the initial 96 h of EtOH accumulation was linear. The accumulation of EtOH and AA was coincident with a doubling of the extractable ADH and PDC activities after 120 h exposure. Exposure of the fruit to up to 20% CO(2) concentrations resulted in an increase in tissue levels of AA, but not EtOH. The pattern of AA accumulation under high CO(2) was similar to that under low 02, with the level of AA being higher at higher CO(2) concentrations. The AA and EtOH induced by low O(2) declined to basal levels in an exponential manner when O(2) was increased from <= 0.5 to >= 2%. The longer the duration of hypoxic induction, the longer the time required for AA and EtOH to decline to basal levels. When low O(2) induction was 48 h or less, the time required for AA and EtOH to decline to basal levels was not affected by O(2) concentrations >2%. However, after 96 h induction, the initial rate of decline in AA or EtOH was slower at lower O(2) concentrations. Including 20% CO(2) in the recovery atmosphere decreased the initial rapid rate of AA and EtOH decline, affecting EtOH levels more than AA, although both compounds reached pre-induction levels at approximately the same time. The rate of decline of ADH and PDC activity following low O(2) induction was accelerated by the presence of CO(2) in the atmosphere. Based on the rapid induction of AA and EtOH in response to low O(2) stress, and the comparable rapid recovery to basal levels after removal of the stress atmosphere, together with a seemingly high tolerance to O(2) atmospheres <2% and the similar but relatively smaller effect of CO(2) compared with O(2), it is concluded that preclimacteric 'Hass' avocados are physiologically well suited to dynamic CA storage. (C) 2006 Elsevier B.V. All rights reserved.