INFLUENCE OF GRAPE POMACE FLOUR PARTICLE SIZE ON THE CONTENT OF BIOACTIVE COMPOUNDS AND ANTIOXIDANT ACTIVITY OF YOGURTS

Grape pomace (GP) is an agro-industrial byproduct rich in dietary fiber and bioactive compounds. However, previous studies have reported limited incorporation of grape pomace flour (GPF) into yogurt due to its high insoluble fiber content. This study evaluated the effect of GPF particle size on the bioactive compound content and antioxidant activity of yogurt. Fresh GP (Vitis labrusca) obtained from an agricultural cooperative in Campo do Tenente, Paraná, Brazil, was lyophilized and sieved. Yogurts were prepared using whole pasteurized cow’s milk, whole milk powder, and starter cultures Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. Fermentation was conducted at 42 °C until pH 4.65–4.70. Three formulations were prepared: a control without GPF, yogurt supplemented with 2% whole GPF (A), and yogurt supplemented with 2% GPF from the 180–355 μm fraction (B). Before inoculation, the flour was added to the milk and subjected to ultrasonic treatment for 15 min. Following fermentation, undissolved particles were removed to improve sensory acceptability. This step may also have removed phenolic compounds associated with the insoluble fraction, potentially affecting the bioactive compound content of the final yogurt.  The formulations were then freeze-dried and extracted with methanol. Total phenolic content (TPC) was determined using the Folin–Ciocalteu assay, total anthocyanin content using the pH-differential method, and antioxidant activity using DPPH, ABTS, and FRAP assays. Formulation A showed 27% higher TPC than B, which did not differ significantly from the control (1.9, 2.7, and 2.1 mg gallic acid equivalents [GAE]/g for the control, A, and B, respectively). Anthocyanins were not detected in the control or A, whereas B contained 7.6 mg cyanidin-3-glucoside equivalents/L. This difference may reflect variations in anthocyanin retention or the sensitivity of the spectrophotometric method. DPPH radical-scavenging activity was highest in A (21 μmol Trolox equivalents [TE]/g), followed by the control (11.8 μmol TE/g) and B (9.4 μmol TE/g). Similarly, ABTS and FRAP activities were highest in formulation A (5.2 and 8.3 μmol TE/g, respectively), followed by B (2.0 and 3.9 μmol TE/g) and the control (0 and 0.6 μmol TE/g). Overall, yogurt supplemented with whole GPF showed the highest TPC and antioxidant activity in all three assays, indicating that whole GPF was more effective than the selected particle-size fraction in improving the measured phenolic content and in vitro antioxidant properties of yogurt. These findings suggest that differences between the flour fractions may reflect changes in their chemical composition, rather than particle size alone. Using whole GPF may simplify processing by avoiding sieving, although undissolved particles must still be removed to improve sensory acceptability.

Acknowledgments: Fundação Araucária