Antioxidant Capacity in Fruit and Vegetables -Cultivar Variation - Storage Conditions - Human Antioxidant Status

The health-promoting effects of fruits and vegetables have been extensively discussed during the past decade since epidemiologic studies have shown relations between increased intake of fruit and vegetables and decreased incidences of several chronic diseases. It has been suggested that antioxidants are a main contributors to the observed effects, for example by acting as scavengers of free radicals. It is necessary to identify and quantify the antioxidants in plant food, but also to develop methods that measure the total effect of all antioxidants. The aims of the present study were to evaluate different methods of measuring the total antioxidant capacity (TAC), to apply them on plant foods and to human plasma. Variation in TAC of plants due to cultivar differences and storage was evaluated as well as the human antioxidant status after intake of plant food meals.

High correlations were obtained between TAC measured with two methods, a free radical scavenging method (ABTS) and a ferric reducing method (FRAP). Correlations were obtained both for strawberries and other plant extracts. The molar TAC response of six pure antioxidant substances were ranked in the following order by both methods: quercetin>ferulic acid>catechin>rutin>caffeic acid>Trolox=chlorogenic acid.

In the plant materials studied, the TAC was in the following order: strawberry=curly kale>>cauliflower>peas=white cabbage. TAC in peas, strawberries and white cabbage varied between varieties. In peas the variation was three-fold and in strawberries it was two-fold for TAC, ascorbic acid, chlorogenic acid and ellagic acid. TAC in water-soluble extracts of peas and strawberries correlated well with the ascorbic acid concentration, which also contributed a major part to TAC in peas. In strawberries the content of ellagic acid correlated with TAC in the water-insoluble extracts. Unripe strawberries had lower concentrations of TAC and phenolic antioxidants than riper berries. During cold storage for up to three days, relatively few changes in antioxidant concentration occurred. Regarding Brassica vegetables, TAC varied in cauliflowers cultivated in two consecutive years but there were no variations in TAC between the various parts of the plants. Glucobrassicin, sinigrin and glucoiberin were the most abundant glucosinolates, and for most glucosinolates differences were found in their contents between cultivars. TAC in the white cabbage samples was correlated to the content of several glucosinolates.

Intake of meals containing 500 g peas or cauliflower increased TAC in blood plasma by 14% and 12%, respectively, while 500 ml of rosehip or blackcurrant beverage did not affect the plasma TAC. The increase in uric acid after the intake of the pea and cauliflower meals was correlated with the plasma TAC, and uric acid was responsible for the major contribution to the measured increase in plasma TAC. The plasma levels of TAC were not related to TAC measured in the meals.

This study shows that methods of measuring TAC are valuable tools in revealing differences in antioxidant content between cultivars and cultivation periods of fruit and vegetables. Further investigations are, however, necessary to develop more selective indicators of antioxidant status in the blood plasma of the consumer.