Valorization of apple pomace by extraction of insoluble dietary fiber and production of IDF-enriched apple juice using different levels of hydrocolloids

This study examines the effect of different drying methods (hot air oven, microwave, and freeze drying) on the quality characteristics of apple pomace (AP) to enhance its value as a byproduct. Freeze-dried AP was the best sample due to the highest water and oil holding capacity (12.64 and 1.90 g/g, respectively), the highest antioxidant activity (90.64%), desirable total phenolic content (TPC), and brighter color (L* = 71.20). Accordingly, insoluble dietary fiber (IDF) of freeze-dried AP was extracted and added to apple juice (AJ) to produce a novel health-beneficial drink. The effect of added IDF (1% w/v) on AJ properties as an innovative and nutritious ingredient with the participation of different stabilizers (carboxymethyl cellulose, high methoxyl pectin, and tragacanth gum) with various concentrations (0.50, 0.33, and 0.25%w/v) and their combination as the other novel part of our study was examined. The viscosity (17.5–240.5 cp), suspension stability (17.5–100%), and cloud value (0.15–0.48) were significantly increased in IDF-enriched AJ with different stabilizers. The AJ containing 0.33% tragacanth and 0.50% tragacanth + pectin had the desirable viscosity (88.5 cp and 61 cp, respectively), suspension stability (100% and 96%, respectively), and cloud value (0.21 and 0.20, respectively) in comparison to the other samples with different applied hydrocolloids; for this reason, the effect of stabilizers (0.33% of tragacanth and 0.50% of tragacanth + pectin) on drink stability containing IDF (1%) of freeze-dried AP during 16 days of cold storage was investigated at regular intervals. Significant changes were observed in the physicochemical properties of treatments over time in pH, acidity, turbidity, color, physical stability, TPC, antioxidant activity (AA), and zeta potential. The decrease in physical stability of all samples during storage (100 to 13%) was attributed to a reduction in absolute zeta potential, likely due to the formation of brown pigments, which increased turbidity. A decrease in AA (47.02 to 0.46%) with an increase in TPC (0.08 to 0.22 mg/ml) related to the changed measurable polyphenols without AA during storage. Therefore, this study showed that valorization of AP as one of the massive byproducts and producing a functional AJ containing IDF is feasible in the food industry.