Factors affecting the antioxidant composition of tomatoes (Lycopersicon esculentum)

Abstract

Tomatoes (Lycopersicon esculentum) are a valuable source of antioxidant compounds such as carotenoids (especially lycopene), phenolics and ascorbic acid. This research was conducted to investigate the effects of cultivars, growing conditions and postharvest treatments on the antioxidant composition of greenhouse grown tomatoes. It was found that the cultivar can play an important role in determining the level of antioxidant compounds present in tomatoes. The smallest cultivar, Campari, had significantly higher levels of total phenolics and flavonoids and, hence, significantly higher antioxidant activity (30-40%) compared with the larger cultivars because of its higher surface area/volume ratio. The hydrophilic extract (consisting mainly the ascorbic acid and phenolics) contributed 92-93% of the total antioxidant activity of tomatoes. The vitamin E content of the four tomato cultivars ranged from 8-31 mg/100 g dry matter and was significantly higher in Excell compared to the other cultivars. Flavourine, a cultivar with the brightest red colour had 50% higher lycopene content than the other three cultivars, Excell, Tradiro and Campari.

The skin fraction of tomatoes was found to contain significantly higher amounts of the major antioxidants and antioxidant activity compared to the pulp and seed fraction of tomatoes. On a fresh weight basis, the skin and seed fractions of tomato, together, contributed 53% of the total phenolics, 52% of the flavonoids, 48% of the total lycopene, 43% of the total ascorbic acid and 52% of the total antioxidant activity present in whole tomatoes. These results show that discarding tomato skin and seeds during home cooking or commercial processing can result in significant losses of antioxidants.

The mean total phenolics and antioxidant activity of the three cultivars harvested in summer (December-February) were 62% and 39% higher, respectively, than the spring (September-November) harvested tomatoes. This could be related to the higher amounts of solar radiation/temperature received inside the greenhouses during summer compared with spring. The lycopene content of the three cultivars was 31 % lower in tomatoes harvested in summer compared to the spring harvested tomatoes. The source of nutrients supplied to the plants played an important role in determining the antioxidant components of tomatoes, for example, use of nitrate dominant nutrient solution can significantly reduce the ascorbic acid content of tomatoes compared to ammonium-dominant and organic sources of nutrients. The mean lycopene content was 40% lower in tomatoes grown with high chloride levels and grass-clover mulch than other organic and inorganic treatments.

Suitable storage conditions are very important to retain the level of antioxidants in tomato. The postharvest storage temperature of light-red tomatoes had the biggest effect on their lycopene content. For example, lycopene content of tomatoes stored at 15 and 25°C on the tenth day of storage was approximately two-fold higher than the tomatoes stored at 7°C. The total phenolics and ascorbic acid accumulated in tomatoes during storage at all temperatures (7, 15 and 25°C) and the antioxidant activity increased from 17% to 27%. The effect of semi-drying at 42°C on the antioxidant composition of tomatoes indicated that the mean total phenolics, total flavonoids, lycopene and ascorbic acid content of the fresh tomatoes decreased by 26%,13%, 14%,and 21 %, respectively, as a result of semi-drying.

An in vitro digestion study that simulated physiological conditions was conducted to study the release of antioxidants from fresh tomatoes. A high amount (71-77%) of total phenolics and flavonoids were released from tomatoes during digestion; however, only 3 to 5% of the total lycopene was released. It was found that that 45-50% of the total antioxidant activity of tomato was released from the tomato matrix and this could be available for absorption under physiological conditions.

Overall, the findings of this study have contributed to an improved understanding of the various factors ranging from growing to processing that could affect the antioxidant composition of tomatoes. This could be used to produce tomatoes, and their products, with higher levels of antioxidants.