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Composition of tomatoes and tomato products in antioxidants (WG1) page 90
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copper, and temperature of the juice. The longer the tomato juice is held at optimum
conditions for oxidation the lower will be the retention of ascorbic acid after processing.
The temperature to which tomato products such as tomato juice are heated in the
presence of air is the most important factor in the rate of ascorbic acid destruction; it has been
found that the rate of ascorbic acid destruction increases with increased temperature in the
presence of air. It is therefore important that juice be brought to the desiderated temperature
as quickly as possible and held for only a short period at high temperature. Concentrated
products present a further problem in retaining vitamin C.
Tomato paste has been reported to have ascorbic acid content of 49 mg per 100 g on a
solid percentage equal to that of tomato juice; however, concentrated products usually contain
less ascorbic acid than do whole tomatoes or tomato juice (Lamb et al. 1951; Hummel and
Okey 1950).
Bolcato (1936) reported less loss of ascorbic acid in the preparation of tomato
concentrates by heating at 60°C to 70 °C than at temperatures either higher or lower.
It is evident from literature on canned tomato products that a good deal of ascorbic
acid may be lost during the processing unless care is taken, but that most of the vitamins
potency may be preserved if suitable precautions are taken . Tomatoes lend themselves to the
production of juice high in ascorbic acid more readily than many other fruits, but the
importance of proper processing methods is obvious. It is also clear that an important factor in
improving the vitamin C content of commercial products is the utilization of fruits in the
canning process, which are initially high in ascorbic acid.
9.2.2 Determination of ascorbic acid in tomato products
9.2.2.1 Volumetric titration
Reagents: 2,6 dichlorphenolindophenol 0.05%
Ascorbic acid (pure)
Solution 3% of metaphosphoric acid in acetic acid at 8%