Comparison of antioxidant activity and concentrations in organically and conventionally produced grapes, grape juice and wine : results from meta-analyses, farm and retail surveys

Abstract

A wide range of studies have investigated effects of organic farming practices on the composition of nutritionally relevant compounds in crop plants. Many studies focused on antioxidant concentrations, since plant polyphenols and other phytochemicals with antioxidant activity have been linked to a reduction in a range of chronic diseases. However, relatively few studies have compared the nutritional composition of grape and grape products such as grape juice and wine. Grapes are known to contain substantial amounts of secondary metabolites (e.g. phenolic compounds) with antioxidant activity. The main aim of the study reported here was to investigate whether and to what extent organic management practices and variety choice affect quality parameters in grape and grape products, using a range of approaches including (a) a systematic literature review and metaanalysis of published data, (b) a basket studies/retail survey in the UK and (c) a farm survey in central Crete. The systematic literature review and meta-analysis of published data was conducted to identify significant composition differences of organically and conventionally produced grape and its products. For example, significantly higher concentration of total flavonoids (P=0.017 and P=0.006) and anthocyanins (P=0.024 and P<0.001) were detected in organic compared to conventional grapes/grape products by both unweighted and weighted meta-analysis. The retail survey of table grapes available in UK supermarkets and farm survey of grapes and wine produced by organic and conventional grape and wine producers in Crete were both conducted over a 2 year period and aimed at identifying effects of agronomic practices used in organic versus conventional farming systems and potentially confounding factors (e.g. variety choice, climatic conditions) on the phytochemical composition in grapes/grape products. The retail survey of table grapes was based on collecting organic and conventional white, red and black grapes from 3 big UK supermarkets (Tesco, Sainsbury’s and Waitrose) at regular intervals in both the winter (when grapes are mainly from Southern Africa) and summer (when grapes are mainly from Mediterranean area) seasons in 2 years. For total antioxidant activity, total phenolic and anthocyanin contents there were no consistent effects of production system (organic vs conventional) but a range of significant interactions between production system and grape variety were identified. For some parameters there were also interactions between production systems and/or grape variety, and year. A significant interaction (P=0.043) between management system and variety choice was detected for the black grape variety Midnight Beauty, which had a higher concentration of total anthocyanins in organic samples compared to conventional. Additionally a positive association between total phenolic content and antioxidant activity was observed while analysing black grape varieties. The interaction between management system and production year indicated higher concentration of total phenolic content and antioxidant activity in organic samples compared to conventional in one of the experimental years (2015). A significant interaction between variety choice and production year was detected for total antioxidant activity mainly in white (P=0.002) and red (P=0.004) grape varieties. For total antioxidant activity there was also significant three-way interaction (between management system, variety choice and production year), but only in the red grape varieties (P=0.042). A significant 2-way and 3-way interactions between factors (e.g. management system and variety choice, and production year) was also detected for a range of individual anthocyanins (e.g cyanidin 3-O-glucoside, malvidin 3-O-glucoside, peonidin 3-O-glucoside). The farm survey in Crete focused on the most widely grown local red and white grape varieties, as well as wines made from these local varieties. A significant interaction between management system and variety choice (P=0.013) and between variety choice and production year (P<0.001) were detected only for total antioxidant activity, but there were also significant main effect of factors (management system, variety choice and production year). Similar to previous studies, a positive association between total phenolic content and antioxidant activity was also detected for analysed grape samples. Different to previous reports on differences in phenolic compounds between grape varieties, the concentration of total phenolic in one of white grape varieties (Vidiano) was slightly higher than in the red grape variety (Kotsifali). However, in wine samples it was confirming findings from previous studies with white wine samples having lower concentration of total phenolic comparing to red wine samples. For wine samples, only significant interaction (P=0.047) (between production system and variety choice) was detected for the total phenolic content, where the concentration was higher in conventional compared to organic red wine samples, but no significant difference in white wine. The results indicate that variety choice and pedo-climatic conditions in different years may be a strong confounding factor in studies comparing the nutritional composition of organic and conventional grapes and grape products. Future studies should therefore investigate the reasons why some varieties produce higher levels of specific antioxidants in organic, while other varieties have higher levels in conventional production systems. Also, future studies should investigate to what extent switching to organic table grape/wine consumption may provide additional health benefits.