Quinoa (Chenopodium quinoa Willd.) is a pseudocereal originally grown in the Andean region of South America. This study focused on investigating the changes in phenolic profile and antioxidant capacity in white and red quinoa varieties after short-term fermentation with Lactiplantibacillus plantarum 299v^®. During fermentation, pH and lactic acid formation were monitored every three hours until pH was below 4.6. The quinoa phenolic profile was quantified via LC–UV–MS. Total polyphenol content (TPC) and total antioxidant capacity (DPPH and FRAP) were determined via spectrophotometric methods. The findings showed that fermentation resulted in a significant increase (p < 0.001) in TPC from 4.68 to 7.78 mgGAE·100 g⁻¹ for the white quinoa and from 5.04 to 8.06 mgGAE·100 g⁻¹ for the red quinoa variety. Gallic acid was the most abundant phenolic acid detected in unfermented quinoa samples (averaging 229.5 μg·g⁻¹). Fermented white quinoa showed an 18-fold increase in epicatechin, while catechin was found only in fermented red quinoa (59.19 μg·g⁻¹). Fermentation showed a significantly positive impact on the iron-reducing antioxidant capacity (FRAP) of quinoa (p < 0.05). Red quinoa had a higher FRAP antioxidant capacity than the white variety; a similar trend was observed with the DPPH assay. There was a significant correlation (r > 0.9, p < 0.05) between TPC and antioxidant capacity. In conclusion, short-time lactic fermentation effectively increased phenolic content and antioxidant capacity in both quinoa varieties. Overall, red quinoa showed higher polyphenol content and antioxidant capacity compared to the white variety.

Quinoa (Chenopodium quinoa Willd.) is a pseudocereal originally grown in the Andean region of South America. Due to its nutritional and functional properties, as well as its tolerance to harsh environments, quinoa has garnered increased attention in recent decades, with its production expanding to Europe and Asia. Polyphenols are secondary by-products of plants, exhibit positive impacts on health, including anti-mutagenic and antioxidant activities, which protect against oxidative-induced stress. This study focused on investigating changes in antioxidant capacity and phenolic compounds in white and red quinoa varieties during lactic acid fermentation with Lactobacillus plantarum 299v^®. Throughout fermentation, pH and lactic acid formation were monitored every two hours until the pH dropped below 4.6. Samples before and after fermentation were analyzed for antioxidant capacities using 2,2-diphenyl-1-pic-ylhydrazyl (DPPH) and ferric ion-reducing antioxidant power assay (FRAP). The total polyphenol content (TPC) was measured using the Folin–Ciocalteu method, and the polyphenol profile was identified and quantified by UPLC-MS-UV. The findings revealed that fermentation led to a significant increase (p < 0.001) in TPC from 4.03 to 7.68 mgGAE/100 g and from 4.35 to 7.96 mgGAE/100 g for the white and red quinoa varieties respectively. Fermentation showed a significantly positive impact on the iron-reducing antioxidant capacity of quinoa (p < 0.05). Red quinoa had higher antioxidant levels than the white variety, with a similar trend observed in the DPPH assay. After fermentation, the antioxidant capacity in white and red quinoa increased from 7.90 ± 0.03 to 10.48 ± 0.11 (mgTE/100 g) and from 8.78 ± 0.06 to 11.75 ± 0.10 (mgTE/100 g), respectively. Furthermore, fermentation significantly (p < 0.001) increased the content of polyphenols with high antioxidant power such as epi-catechin, catechin, 4-Hydroxybenzoic acid, vanillic acid, chlorogenic acid, rutin and quercetin in both red and white varieties. In conclusion, lactic fermentation proved to be effective for increasing the phenolic content and antioxidant capacities of both quinoa varieties.