An investigation of the antioxidant properties and colour of glasshouse grown tomatoes

Abstract

The colour and the antioxidant content of five different cultivars of tomatoes (Lycopersicon esculentum L.), grown in glasshouses using commercial hydroponic techniques, were measured in freshly harvested tomatoes and after 2 and 5 days of storage in the dark at 15°C. The two cocktail cultivars of tomatoes, Aranca and Flavourine, were the smallest tomatoes but they contained higher dry matter contents compared with the three standard salad cultivars, Excell, Celero and Dutch Chaser. There were significant differences (P<0.001) in lycopene (the major carotenoid), ascorbic acid and total antioxidant activity in the whole fruit of the five different cultivars evaluated at harvest, while the total phenolic contents were very similar. The lycopene levels of all the cultivars increased significantly (P<0.001) after storage for 2 and 5 days at 15°C. Overall, the total phenolic content of all the cultivars did not change appreciably after storage at 15°C while the levels of ascorbic acid fell by 12.6% when stored for 5 days in the dark. There was a significant difference (P<0.001) in the total antioxidant activity between the five different cultivars when freshly harvested; the highest levels of total antioxidant activity were observed in the two cocktail cultivars, Aranca and Flavourine. Antioxidant activity showed different patterns of change depending on cultivar. An increase in the total antioxidant values could be seen in cultivars Dutch chaser and Excell, after storage for 5 days at 15°C. The total antioxidant activity values of Flavourine and Celero fell on storage while the values for stored Aranca were very similar to the values when freshly harvested.

The skin colour, as shown by Commission Internationale l'Eclairage (CIE) L*a*b* values of each cultivar were markedly different at harvest even though each cultivar was harvested at the same maturity. The a* value of each cultivar increased significantly (P<0.001) when they were stored in the dark, for either 2 or 5 days, at 15°C. Regression analysis showed that overall there was no significant relationship (R²=0.14) between the lycopene content and the a* value of all the tomatoes; however, when examined individually, the two cocktail tomatoes Aranca and Flavourine showed strong correlation—R²=0.96 and R²=0.97, respectively. Overall, the (a*/b*)² was the most reliable indicator of lycopene concentration (R²=0.96) in the tomatoes examined in this study.