MODIFICATION OF ALPHA-FARNESENE LEVELS IN COOL-STORED GRANNY-SMITH-APPLES BY VENTILATION

'Granny Smith' apples (Malus domestica Borkh) were arranged sequential ly in low density polyethylene (LDPE) plastic tubes and ventilated wit h humidified air at 1 degrees C to characterise the effect of ventilat ion on the accumulation, oxidation and evaporative depletion of alpha- farnesene from the apple cuticle. Evaporation of alpha-farnesene into the air stream produced a vapour phase concentration gradient along th e tubes, with a concomitant decrease in the rate of evaporation of alp ha-farnesene from the fruit with distance down the tube. The concentra tion of alpha-farnesene in the apple wax rose 13-fold during the first 5 weeks of cool storage and decreased thereafter. In contrast, that i n the air rose to a maximum value at between 10 and 15 weeks' storage time and then decreased. Concentrations of alpha-farnesene oxidation p roducts (conjugated trienes) in the wax increased steadily with storag e time, but were not markedly affected by ventilation and were not wel l correlated with alpha-farnesene concentrations. Mass transfer coeffi cients between 1 and 19 nmol s(-1) m(-2) Pa-1 were obtained for evapor ative loss of alpha-farnesene from the apple surface. These coefficien ts are 10-200 times greater than those for the permeance of the fruit skin to metabolic gases which suggests that loss of alpha-farnesene oc curs by evaporative release from the cuticle wax, rather than diffusio n through the skin. Mass balance calculations for the depletion of alp ha-farnesene from the apples indicated a high rate of accumulation of alpha-farnesene during the first 5 weeks of storage followed by a shor t period of rapid depletion and then a longer period of gradual deplet ion. This suggested that the rate of loss of alpha-farnesene from the surface of the fruit depends more on storage time than on its concentr ation in the wax. Comparison between concentrations of alpha-farnesene in ventilated and non-ventilated fruit supported these conclusions an d suggested that reduction of superficial scald by ventilation may not be explicable merely by enhanced evaporative depletion of alpha-farne sene. (C) 1998 Elsevier Science B.V. All rights reserved.