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Processing and Bioavailability (WG2) page 31
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11 Drying (powder , flakes, halves) D
O
Survey of references
D Use air temperatures as low as possible
O Reduce the time of contact with warm air
A few studies deal with the effects of drying technologies on the antioxidant components
of tomatoes. Tomatoes are usually dried by drum- or spray-drying for the production of
tomato powder, and by sun- or air-drying for the production of tomato pieces (tomato halves,
quarters and slices). Drying is a critical technology in terms of oxidative damage because the
product is in most cases exposed to high temperatures and high oxygen levels for long
periods. In addition, solid concentration increases during drying, thus increasing the rate of
many oxidative and degradation reactions.
Lovric et al. (1970) produced foam-mat tomato powder (starting from tomato paste) to
study the cis-trans isomerisation of lycopene and colour stability during storage. Although
drying conditions were not specified, the authors observed a retention of soluble colour in
freshly dehydrated powder from nitrogen- and air-foamed puree of 94% and 82%
respectively. Data on carotenoid composition (determined by Al2O3 column separation
followed by spectrophotometric evaluation) of the fresh foam-mat-dried powder showed that
nearly 17% lycopene was represented by cis isomers.
More recently, Zanoni et al. (1999) evaluated oxidative and heat damage induced by airdrying
of tomato halves, which were dried using air at 80°C and 110'°C to approximately
10% final moisture content. Lycopene, ascorbic acid, 5-hydroxymethyl-2-furfural (HMF)
contents and colour were measured during drying. A marked loss of ascorbic acid occurred
during drying: 40% and 80% ascorbic acid losses were already observed at 80%, residual
moisture at 80°C and 110'°C respectively. No ascorbic acid remained in 50% moisture
samples at 110'°C, whereas a 10% residual ascorbic acid content was detected in final
products dried at 80'°C. In contrast, total lycopene had a high stability under the drying
conditions applied: no significant lycopene loss occurred in tomato dried at 80°C, and a
maximum 12% loss was observed in samples dried at 110'°C. Colour changes became evident
when the HMF content had reached 20 mg/kg d.w., that is, when the temperature of tomato
halves had reached 80°C.
Shi et al. (1999) compared the effects of different dehydration techniques on lycopene
degradation and isomerisation. In this study, whole mature tomatoes were perforated with a
set of fine needles to create pier holes on the surface (20 holes/cm 2 ); the fruits were then