Postharvest dehydration of wine white grapes to increase genistein, daidzein and the main carotenoids

Wine white grape bunches of the Grechetto variety were dehydrated at 10, 20 and 30 °C, RH 45% and forced air ventilation of 1.5 m/s. Chemical and metabolic changes due to the effect of dehydration were studied at various stages of weight loss: 10%, 20%, 30% and 40%. Berry colour at 10 and 20 °C tended to become greener with dehydration but at 30 °C, at the final sampling, the colour darkened. Acidity decreased in all samples, while sugars increased. Total phenol content increased at 10 °C until 30% weight loss was reached and then declined, while at 20 and 30 °C the concentration decreased immediately. The contents of lutein and β-carotene (respectively 68 and 58 mg/kg d.w.), representing the 80% of total carotenoids, did not change significantly until the 30% of weight loss, when at 30 °C the value increased above all for lutein while at 10 and 20 °C, the contents decreased significantly. Daidzein, at 10 °C, rose significantly from about 150 μg/kg d.w. to 1434 μg/kg d.w. at 20% weight loss and then declined; at the same weight loss percentage, the genistein concentration began to increase. At 20 °C both isoflavones rose until the end of the experiment, reaching values similar to the sample at 10 °C. A temperature of 30 °C was deleterious to grape isoflavones. A discussion on the changes in isoflavones related to temperature and time is reported.