Effect of cold storage and harvest date on bioactive compounds in curly kale ( Brassica oleracea L. var. acephala)

Abstract

In this study, curly kale stored at 1 °C for 3 and 6 weeks was compared with plants remaining in the field regarding several properties related to human health and sensory quality. Cold storage had no effect on the antioxidant capacity (ORAC assay), total phenols (Folin-Ciocalteu assay) or flavonol content, but reduced the content of vitamin C and soluble sugars. The ratio ascorbic:dehydroascorbic acid decreased from about 3 to 0.5 upon storage. The largest changes in the content of bioactive compounds were found in plants that remained in the field for 6 additional weeks, including many frost days. In these plants, the levels of flavonols, total phenols and antioxidant capacity were reduced by 25–35% and the vitamin C content by more than 50%, whereas soluble sugars and dry matter increased by roughly 20% and 30%, respectively. Among all the curly kale samples, the antioxidant capacity was positively correlated with the level of total phenols ( r = 0.73, P < 0.001) and total flavonols ( r = 0.70, P < 0.001). Values obtained with a non-destructive method based on chlorophyll fluorescence (ChlF) were well correlated with the flavonol content in the leaves, with the highest correlation found for quercetin ( r = 0.76, P < 0.001). Using the ChlF method, the kinetics of the flavonol content during the experiment was recorded. The same method also showed gradients of flavonol content within the plant, with the highest values in the upper leaves and in the tip of the leaves. ChlF measurement, therefore, proved to be a quick and cost-effective alternative to chemical analyses of the flavonol content in curly kale.