Comparative evaluation of herbal tea, syrup and kombucha prepared from purple corn (Zea mays L.) cob and stevia: physicochemical properties, antioxidant activity and consumer acceptability [pdf]

Background.Purple corn (Zea mays L.) cob is an anthocyanin-rich agricultural by-product that is usually discarded.Although a cob-stevia tea has previously been described, technologically different beverages made from the cob have rarely been compared.This study aimed to characterise dried purple corn cob and stevia (Stevia rebaudiana Bertoni) leaves and to compare a herbal tea, syrup and kombucha prepared from the cob in terms of their physicochemical properties, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging activity and consumer acceptability.Materials and methods.Moisture, water extract, ash fractions, and crude fibre contents of the dried raw materials were compared with reference limits adapted from SNI 3836:2013 and ISO 3720:2011, while classes of secondary metabolites were identified using qualitative tests.Tea bags were brewed for 3, 5, or 7 min.Sugar-free syrups sweetened with stevia leaf infusion contained 0-30% (v/v) cob infusion.A cob infusion was fermented with a symbiotic culture of bacteria and yeast (SCOBY) for 8 days, either without a sweetener or with 10% (w/v) sucrose or a commercial stevia-based sweetener.Half-maximal inhibitory concentrations (IC 50 ; lower values indicate higher activity) are reported as mean ± SD of triplicate determinations.Acceptability was assessed using five-point hedonic scales.Results.The values obtained for the cob batch used in the tea and syrup and for the stevia leaves were within all seven reference limits.The IC 50 of the tea decreased from 102.75 ±0.14 to 47.07 ±0.03 µg/mL as brewing time increased from 3 to 7 min, reaching a value similar to that of commercial herbal tea (47.22 ±0.04 µg/ mL).Syrup IC 50 decreased non-linearly from 186.10 ±0.21 to 70.29 ±0.03 µg/mL with increasing cob infusion, while the viscosity of cob-containing syrups decreased by 61.2-62.6%during a six-cycle stability test.The fermented beverages showed 44.6-63.0%lower IC 50 values than a separately brewed unfermented cob infusion, with the lowest IC 50 value obtained for the commercial stevia-based sweetener (64.66 ±0.24 µg/ mL).By comparison, a commercial kombucha showed approximately twice the antioxidant activity (32.82 ±0.36 µg/mL).Among the kombuchas, the beverage prepared with the commercial stevia-based sweetener had the highest acceptability (92.4%).