Influence of postharvest water stress on lipoxygenase and alcohol dehydrogenase activities, and on the composition of some volatile compounds of Gewurztraminer grapes dehydrated under controlled and uncontrolled thermohygrometric conditions

Gewurztraminer grapes with a sugar content of around 212 g/L (21.7 degrees Brix) were dried at 17 degrees C, 40% relative humidity and 1.5 m/sec air flow in a 300 L thermo-conditioned tunnel. Control grapes were dried traditionally in a window ventilated room, under uncontrolled environmental conditions varying with outside climate. Tunnel-dried grapes reached the desired sugar concentration (305 g/L, 29.5 degrees Brix) in 17 days, loosing 36% of their weight. Control grapes lost only 22% of their weight and grey mould developed in several bunches at the last sampling. Titratable acidity decreased for tunnel-dried and control grapes from 6.5 g/L to 4 g/L and 5 g/L, respectively. Lipoxygenase (LOX) activity declined in both samples from 120 to 90 U/mg protein dw, with a subsequent significant increase after 20% weight loss in tunnel-treated grapes while the control grapes showed a small peak (150 U/mg protein dw) at 13% weight loss. Six carbon compound evolution showed a loose correlation with LOX activity. Alcohol dehydrogenase specific activity and the concentrations of ethanol and of acetaldehyde plus ethyl acetate showed fluctuating patterns of change, with the evolution of these three variables showing similarity, particularly evident in the tunnel-dried grapes. Carotenoids declined significantly, to increase slightly at the end of the experiment in both samples, with the decline more rapid in the control grapes. Traditional, uncontrolled conditions, did not permit constant dehydration, and provoked a rapid stress to the berries (10% of weight loss). Controlled conditions permitted uniform dehydration, postponed water stress, giving a higher quality product without loss of berries.