Grape pomace flour in yogurt: influence of pre-fermentation addition and concentration on phenolic compounds and antioxidant capacity

Grape pomace (GP) is an underutilized industrial byproduct, despite being rich in bioactive compounds and holding potential for use in the development of functional foods. However, processing can affect the retention and incorporation of these compounds. Therefore, this study aimed to evaluate the influence of early removal of grape pomace flour (GPF) on the bioactive compound content and antioxidant capacity of yogurts with different concentrations of GPF. Fresh GP (Vitis labrusca) was supplied by an agricultural cooperative from Campo do Tenente, Paraná, which derived from winemaking process and pectolytic enzyme treatment. The material was lyophilized and yogurts were prepared with whole pasteurized cow’s milk, whole powder milk, and starter cultures of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. Fermentation was carried out at 42°C until reaching pH 4.65-4.70. Formulations consisted of a control (0% GPF) and pre-fermentation GPF addition of 2, 2.5, 3 and 5% (samples 2P, 2.5P, 3P and 5P, respectively). This was followed by a 15-minute ultrasonic bath prior to inoculation. The undissolved flour was removed before fermentation. Freeze-dried methanol-extracted samples of yogurts and GPF were analyzed for total phenolic compounds (TPC, Folin-Ciocalteu assay) and antioxidant capacity (FRAP and ABTS assays) in microplates (octuplicates). Statistical analyses (ANOVA, Tukey’s test, and Student’s t-test) were performed using GraphPad Prism, with a 5% significance level. An increase in total phenolic compounds was observed as the GPF concentration rose, as 5P presented a TPC approximately 13 times higher than 2P (2.24 vs. 0.17 mg GAE/g), while GPF alone presented nearly 10.8 mg GAE/g. However, significant differences were observed starting at 3P compared to 2P, whereas 2P and 2.5P showed values ​​that were statistically similar to each other and to the control. Antioxidant capacity also showed a concentration-dependent increase with the addition of GPF. In both assays FRAP and ABTS, 5P presented the highest antioxidant potential among yogurt formulations (65 and 31 μmol TE/g, respectively), while GPF had the highest capacity of all (175 and 65 μmol TE/g, respectively). Thus, 5P presented about 35% of GPF FRAP capacity and 48% of its ABTS capacity. Only in the FRAP assay 2P and 2.5P were not statistically different, as in ABTS all samples differed from one another. Overall, the results show that early addition and removal of GPF is a viable strategy for enriching yogurts with bioactive compounds from an underutilized byproduct, contributing to the valorization of winemaking residues. Furthermore, the methodology allowed the incorporation of phenolic compounds into yogurt while avoiding the presence of solid residues in the final product. Under the conditions of this study, a GPF concentration of 3% was associated with measurable enrichment, especially in total phenolic content.

Acknowledgments: Fundação Araucária