Effects of Pretreatment Solutions on Drying Yield and Bioactive Compounds of Local Grape Raisin

Abstract

The research was conducted to determine the effects of pretreatment solutions on drying yield, raisin weight, drying time, soluble solids content (SSC), titratable acidity, pH, total phenolics, total flavonoids, and antioxidant activity (2,2-diphenyl-1-picrylhydrazyl [DPPH] and ferric reducing-antioxidant power [FRAP] assays) of the local grape varieties (‘Bilbizeki’, ‘Ra¸se kewnar’, ‘Kerkü¸s’) grown in ¸Sırnak province in Türkiye. The study determined three different applications for grape drying: drying in sun (control), dipping in water with wood ash + 2.5% olive oil (AWI), and dipping in 5% potassium carbonate + 1.5% olive oil (PSI). It was determined that the raisin belonging to the control application had higher raisin weight than the raisin undergoing AWI application. Significantly, the control application had higher drying yield and drying rate in all grape varieties compared to PSI application. SSC and pH values of drying applications were found to be similar in all grape varieties. However, significantly higher acidity was obtained from the raisin of AWI only in ‘Bilbizeki’ and from the raisin of AWI and PSI in ‘Ra¸se kewnar’ compared to the raisin of the control application. In ‘Ra¸se kewnar’ and ‘Kerkü¸s’ grapes, raisin undergoing PSI application showed significantly higher total phenolics, flavonoids, and antioxidant activity (according to FRAP) than raisin undergoing both control and AWI applications. It was determined that the total flavonoids and DPPH antioxidant activity of ‘Bilbizeki’ raisin obtained from the PSI and AWI applications were significantly higher than those dried by the control application. As a result, the effects of pretreatment solutions on drying time, drying yield, and bioactive compounds have been revealed by research. In conclusion, PSI can be recommended as a pretreatment solution to preserve bioactive compounds that are important for human health and accelerate drying time.