Effect of Needle Micro-Perforation on Bioactive Compounds in Grape Drying

Grapes (Vitis sp.) are widely consumed and cultivated globally. In addition to being consumed fresh, grapes are processed into wine, juice or dried fruit. It is known for its bioactive compounds, such as flavonoids, polyphenols, anthocyanins, and tannins, which provide human health benefits, including antioxidants, anti-inflammatory, and cardioprotective effects. However, due to their high-water content, grapes are highly susceptible to microbial spoilage, which limits their shelf life. Drying is one of the most common methods used to extend grape shelf life. Pretreatments before drying facilitate moisture removal, accelerate the drying process, and preserve nutrients., this study investigates the potential of needle micro-perforation (0.5 mm, 1 mm, and 1.5 mm needle lengths) as an alternative to the widely used alkaline oil-in-water emulsion. Needle micro-perforation (NMP) has not been previously applied to grapes, and its effects on drying behavior and bioactive compound retention remain unexplored. The total phenolic content, antioxidant capacity (DPPH and CUPRAC), and anthocyanin content of Kavacık grapes (Vitis vinifera L. "Alphonse Lavallée" variety) were analyzed. The grapes were subjected to pretreatment and subsequently dried using static and convective methods at temperatures of 60°C, 70°C and 80°C. The bioactive compounds present in the grapes were then evaluated. Results showed that different pretreatments had varying effects on bioactive compounds. Needle micro-perforation was as effective as the alkaline dipping method in preserving total phenolics, antioxidants, and anthocyanins. In static drying, the total phenolic content of samples with needle micro-perforation was approximately 250 mg GAE per 100 g of dry matter higher than that of samples with the dipping treatment. In convective drying, this difference increased to about 400 mg GAE per 100 g of dry matter. Similarly, antioxidant capacity was about 10 µmol Trolox equivalent per gram of dry matter higher in needle micro-perforation samples under static drying and 20 µmol higher under convective drying. In conclusion, needle micro-perforation effectively preserves bioactive compounds and can be considered an alternative to the alkaline dipping method.