Effects of Hot Air and Microwave Drying on the Phenolic Components and Physicochemical Properties of Unripe Grapes (Qoura)

Background: Unripe grape (Qoura) in the forms of either intact or juice can be used as a flavoring agent in several foods of Iranian cuisines such as soup and salad due to its sour taste. Methods: In the current study, the effects of microwave (270, 540, and 720 W) and hot air drying (40, 50 and 60°C) on the polyphenol components content and chemical properties (including chroma, hue angle, L* value, total color difference, soluble sugars, acidity, total phenols and phenolic components including rutin, gallic acid, hydroxybenzoic acid, syringic acid, p-coumaric acid, and resveratrol) of Qoura obtained from unripe white seedless grape were investigated. Results: The obtained results showed that in both methods, by increasing the oven temperature and microwave power, the L value (The L* value was used to express the whiteness and darkness) was decreased. There was a notable reduction in the initial phenolic content of the sample (381.671 mg gallic acid/ 100 g) as a result of both of the used techniques. The initial content of resveratrol was decreased from 30.27 mg/100 g; to 23.07, 16.64, 10.73 mg/100 g sample in the samples treated with the air-drying method at 40, 50, 60°C, respectively. Also, by using the microwave in 270, 540, 720 W, it decreased to 5.77, 5.38. 5.26 mg/100 g sample at 270, 540, 720 Watt, respectively. Conclusion: The results of the present study revealed that the drying method affects the amount of polyphenolic compounds and physicochemical properties.