PHENOLIC COMPOUNDS, TANNINS, AND ANTIOXIDANT CAPACITY IN MICROGREENS AND MATURE PLANTS OF FOUR HORTICULTURAL SPECIES

Abstract: Microgreens are recognized for their high density of bioactive compounds, which is often associated with the metabolic changes occurring during germination. However, their superiority over mature plants is not universal and may vary depending on the species and growing conditions. This study aimed to compare total phenolic compounds, condensed tannins, and antioxidant capacity in microgreens and mature plants of red lettuce (Lactuca sativa), Swiss chard (Beta vulgaris), kale (Brassica oleracea), and arugula (Eruca sativa). The microgreens were grown in trays and harvested at seven days, whereas the mature plants were grown in a commercial greenhouse and harvested between 30 and 60 days. The samples were pre-dried at 50 °C, ground, and analyzed on a dry-weight basis. Total phenolic compounds were determined using the Folin–Ciocalteu assay, and condensed tannins were quantified using the vanillin assay with a SpectraMax^® i3x microplate reader. Antioxidant capacity was assessed using the ORAC assay by monitoring fluorescence decay with the same instrument. The experiment followed a completely randomized design with three replicates. Data were subjected to analysis of variance (ANOVA), and when the F-test indicated a significant effect (p < 0.05), means were compared using Tukey’s test at the 5% significance level. The results showed that differences between developmental stages were species-dependent. Mature red lettuce had higher total phenolic content (2,850.1 mg gallic acid equivalents [GAE]/100 g) and antioxidant capacity (6,960.2 ± 31.8 µmol Trolox equivalents [TE]/100 g) than its microgreen counterpart, which contained 508.0 mg GAE/100 g and exhibited an antioxidant capacity of 892.5 ± 8.6 µmol TE/100 g. Antioxidant capacity was also higher in mature Swiss chard (4,494.8 ± 27.5 µmol TE/100 g) and arugula (2,792.4 ± 8.1 µmol TE/100 g). Conversely, kale microgreens had higher total phenolic content (1,498.9 vs. 991.5 mg GAE/100 g) and antioxidant capacity (1,945.3 ± 17.5 vs. 1,665.2 ± 5.9 µmol TE/100 g) than mature kale. Condensed tannin contents were higher in the mature plants of all species, particularly in red lettuce, which contained 149,541.7 µg catechin equivalents (CE)/100 g, compared with 1,066.6 µg CE/100 g in the microgreens. These findings demonstrate that the mature stage does not necessarily result in lower concentrations of bioactive compounds and suggest an association between phenolic compounds and the antioxidant capacity of the samples. In conclusion, the effect of developmental stage on bioactive composition is species-dependent: mature red lettuce, Swiss chard, and arugula exhibited greater antioxidant capacity, whereas kale microgreens showed higher total phenolic content and antioxidant capacity.

Acknowledgments: The authors thank FAPERGS for the research scholarship and IFFar and ICTA–UFRGS for providing infrastructure and materials.