Effects of Inactive Yeast Biostimulants on Mechanical and Color Attributes of Wine Grape Cultivars

Background: Biostimulants naturally improve plant growth, stress tolerance, and nutrient use efficiency and activate defenses by increasing protective metabolites (phenols, anthocyanins) in grapes. In viticulture, especially when using inactive yeasts, they modulate genetic expression and improve the skin resistance, color, and aroma profile of wine grapes in line with sustainable practices. Methods: Two wine grape cultivars, Merlot and Cabernet Sauvignon, were sprayed with the inactive yeast Saccharomyces cerevisiae in a single treatment in pre-veraison or in a double treatment in pre-veraison and veraison. Berry weight, must, total polyphenols, anthocyanins, and mechanical and colorimetric properties were measured on fresh grapes. Results: Two-way ANOVA revealed that titratable acidity (TA), pH, and total polyphenol content (TPC) were not affected, while mean berry weight and anthocyanin content varied by cultivar, treatment, and interaction; total soluble solids (TSS) differed only by cultivar. Inactive yeasts reduced weight in the single-treatment thesis but stabilized it in the double-treatment one; anthocyanins decreased in Cabernet Sauvignon but increased in Merlot. Mechanical and colorimetric analyses showed cultivar-dependent responses, with significant improvements in elasticity, skin thickness, and hue of berries, especially in Merlot when the treatment was applied twice. Conclusions: Inactive yeasts (IYs) showed an effect on the weight of the berries, the anthocyanins, the mechanics, and the color; Merlot significantly improved skin thickness, elasticity, and hue; and Cabernet remained less reactive to treatments.