Pretreatment and drying effect on the antioxidant, color, rehydration properties and microbial quality of four tomato varieties

Abstract

Tomato, whose consumption popularity is on the rise, is one of the most important vegetable grown in Kenya. However, 50% postharvest losses in this commodity are experienced. Drying, as a preservation method is essential in increasing its shelf stability. However, during drying quality of the product is affected as a result of heat induced reactions. Pretreatment before drying offers an effective control of these reactions. This study was therefore carried out with an objective of investigating the effect of pretreatment on the antioxidant properties, drying time, rehydration ratio, color and microbial quality of selected tomato varieties. The tomato fruits were divided into quarters and sprayed with 0.5% sodium metabisulphate, 0.5% calcium chloride and untreated (control). The quarters were oven-air, vacuum–oven, solar and freeze dried to 13% moisture content. Drying time, moisture content, total phenolics, lycopene, β-carotene, vitamin C, rehydration ratio and color were determined using standard methods. Total fungal and total bacterial count as well as Escherichia coli and Salmonella presence was determined at onset and once monthly for six months in the dried samples. Results showed that drying time was shortest in Riogrande variety (762-2097 minutes) and longest in Anna F1 variety (948-3055 minutes). Lycopene, β-carotene, total phenolic content and redness value in the fresh samples were significantly affected by variety (P<0.05). However, initial moisture content, ascorbic acid and lightness value did not differ significantly (P>0.05) in the fresh samples. Significantly (P<0.05) higher levels in lycopene, β-carotene, total phenolics, rehydration ratio, redness and lightness was observed in pretreated samples compared to the control after drying. However, the effect of pretreatment on the antioxidant properties during freeze drying was not significant (P>0.05). Percentage retention of ascorbic acid relative to the fresh samples ranged between 74.8-80.0% and was only present in freeze dried samples. At the end of the storage duration, the total bacterial growth ranged 30.0×101─49.0×101, 17.0×101─21.5×101 and 2.5×101─6.0×101 cfu/g in the control, 0.5% calcium chloride and 0.5% sodium metabisulphate samples, respectively. The maximum total fungal count during the storage period was 17.5 ×101, 12.5 ×101 and 5.0×101 in the control, 0.5% calcium chloride and 0.5% sodium metabisulphate respectively. Escherichia.coli and Salmonella were absent throughout the storage duration. These findings show that pretreatment in tomato drying is effective in maintaining the antioxidant quality and physical attributes of the dried product. In addition, pretreatment reduces microbial growth in dried tomatoes and may offer longer shelf stability.