Influence of preparation methods on the levels of phenolic compounds and flavonoids in collard greens (Brassica oleracea var. acephala)

Abstract

Collard greens (Brassica oleracea var. acephala) are a nutritious and widely consumed leafy vegetable in Brazil, notable for their content of bioactive compounds such as phenolics and flavonoids, which are known for their antioxidant, anti-inflammatory, and antimicrobial properties. This study aimed to evaluate the influence of different preparation methods on the concentration of these compounds in collard greens, comparing raw (juice), moist heat (sautéed), and dry heat (oven-dried) preparations. The quantification of total phenolic compounds and flavonoids was performed using spectrophotometry, with gallic acid and quercetin as standards, respectively. The results indicated that the collard juice retained the highest levels of phenolic compounds (37.45 ± 1.05 mg GAE/g) and flavonoids (5.84 ± 0.48 mg QE/g), demonstrating that cold processing preserves these bioactives, which are sensitive to degradation by heat and oxidation. Heat-treated preparations showed a reduction in phenolic and flavonoid levels, with sautéed samples presenting intermediate values (30.11 ± 3.02 mg GAE/g and 4.02 ± 0.61 mg QE/g), and oven-dried samples showing the lowest contents (20.32 ± 3.29 mg GAE/g and 3.19 ± 0.12 mg QE/g). These losses may be attributed to prolonged thermal degradation and oxygen exposure during processing, which compromise the stability of bioactive compounds. Therefore, the preparation method significantly impacts the preservation of phenolic compounds and flavonoids in collard greens, and consumption in juice form represents an effective strategy to enhance the absorption of these natural antioxidants.