Fresh tomatoes and other processed tomato products make a significant contribution to human nutrition owing to the concentration and availability of several nutrients in these products and to their widespread consumption. Composition tables show that ripe tomato (Lycopersicon esculentum, Mill.) contains 93-95% water and low levels of solid matter.
Tomatoes contain usually from 5.5 to 9.5% total solids, of which about 1% is skins and seeds. The percentage of solids in tomatoes varies over wide limits for a number of reasons, such as variety, character of soil and especially the amount of irrigation and rainfall during the growing and harvesting season.7
In as much as tomato products, such as pulp and paste, are evaporated to a definite specific percentage of solids, their yield per t of tomatoes varies with the composition of the raw tomatoes used in their manufacture. In tomato juice, the fraction of insoluble solids (cellulose, lignin, pectic substances) varies from 15 to 20% of total solids.8
The soluble solids are in main part constituted by free sugars. The free sugars of tomatoes are predominantly reducing sugars.9 The quantity of sucrose found in tomatoes is so negligible that it may be ignored for all practical purposes.10 Sucrose rarely exceeds 0.1% on a fresh weight basis. The reducing sugars, which usually make up from 50 to 65% of tomato solids, are mainly glucose and fructose. The total sugar content of fresh tomato is found to vary from 2.19 to 3.55%.11 Leoni8 reported that, in general, more fructose than glucose was present (ratio 54/46). The polysaccharides in tomatoes make up about 0.7% of tomato juice. Pectins and arabinogalactans constitute about 50%, xylans and arabinoxylans about 28% and cellulose about 25%.9
The acid in tomatoes is generally considered to be almost entirely citric, and free acids are almost always determined as citric monohydrate. Some workers have reported the presence of malic acid in quantities often exceeding those of citric acid, while traces of tartaric, succininc, acetic and oxalic acids have also been reported. Chromatographic analyses reported by Miladi et al.9 have separated eight organic acids from tomato juice. Malic acid was found to be the second major organic acid in fresh juice whereas pyrrolidone carboxylic acid was found to be the second major organic acid in the processed juice. Processing of tomato juice results in an increase in total acid. It was found that acetic acid is increased by 32.1% apparently owing to oxidation of aldehydes and alcohols during processing and deamination of amino acids, such as the breakdown of alanine via pyruvic acid. An increase in citric and malic acids after processing was also noted. Crean12 indicated that sugars can decompose on heating in the presence of acids to give acetic, lactic, fumaric and glycolic acids.
There are 19 soluble amino acids in fresh tomato juice. Miladi et al.9 reported that glutamic acid makes up to 48.45% of the total weight of amino acids in fresh tomato juice. Second highest in concentration is aspartic acid. The amino acid with the lowest measurable concentration is proline. Processing of fresh tomato juice at 104°C for 20 min results in a substantial increase in the free amino acids as a result of denaturation and partial hydrolysis of protein. The greatest increase occurs in glutamic and aspartic acids, alanine and threonine. Asparagine and glu-tamine disappear during processing owing to the loss of amide ammonia (NH3) to form glutamic and aspartic acids, which partially account for the increase in ammonia in canned juice. It could also be due to glutamine and asparagine deamination and formation of pyrrollidone carboxylic acid.
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