ANTIOXIDANT CAPACITY OF OKARA BEVERAGES OBTAINED BY AQUEOUS AND ALKALINE EXTRACTION COMPARED WITH SOYBEAN BEVERAGE

Okara, the insoluble residue of soy beverage (SB) production, is generated in large quantities and remains underutilized in human food. This study aimed to characterize beverages obtained from dried okara by aqueous (OK1) and alkaline (OK2) extraction and to compare them with SB regarding physicochemical properties and antioxidant capacity. Soybeans were soaked for 17 h and processed into SB, which was pasteurized and stored refrigerated (4 °C) or frozen for 3 days. Okara was dried (moisture from 77.8% to 22.1%) and extracted in distilled water (~4 g/100 mL; 60 °C, 45 min; Ultra-Turrax homogenization, 500 rpm), with or without initial adjustment of the water pH to 9.1, followed by pasteurization (95 °C, 15 min), centrifugation, and filtration. Water activity (aw), pH, soluble solids (°Brix), and antioxidant capacity by the DPPH microplate assay (515 nm), expressed as Trolox equivalents (TE; R² = 0.993), were determined. Turbidity interference was eliminated by centrifugation (10,000 ×g, 10 min), dilution, and sample-specific blanks. Drying reduced okara aw to 0.68, close to that of dry soybeans (0.66), indicating a storage-stable ingredient. All beverages showed high aw (0.985–0.987), requiring refrigerated storage. In OK2, pH dropped from 9.1 to 6.4 due to the buffering capacity of okara; thus, OK2 did not differ from OK1 in pH (6.3–6.4) or soluble solids (1.0 °Brix), although it yielded a larger volume and recovered more soluble solids (22% vs. 13% of okara dry mass). SB showed 5.1 °Brix and an antioxidant capacity of 3.5–5.1 µmol TE/g dry matter, with no detectable difference between refrigerated and frozen storage. Both okara beverages were below the method's limit of quantification (< 0.6 µmol TE/g dry matter). The lower soluble solids content of okara beverages only partly explains this difference: even at a dry matter concentration equivalent to that of SB (~6.3 g dried okara/100 mL), the estimated antioxidant capacity of the okara beverage would not exceed ~25% of that of SB, assuming linear extraction (< 0.9 µmol TE/g dry matter). The actual gain would likely be smaller, since okara fiber retains more water as its proportion increases, reducing beverage volume and yield. These results suggest that soybean antioxidant compounds are mostly transferred to SB or remain bound to the okara fiber matrix, not being released by water extraction alone. In conclusion, dried okara is a stable ingredient, but aqueous or mildly alkaline extraction recovers few soluble solids and antioxidants. Strategies such as enzymatic hydrolysis of the fiber should be investigated to add value to this by-product in plant-based beverages. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES), Finance Code 88887.330127/2026-00.