Cis‐trans isomerisation of lycopene and colour stability of foam—mat dried tomato powder during storage

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Abstract
Tomato powder with ∼3% moisture was obtained by foam‐mat drying tomato paste (29% solids). Samples of powder were stored at various temperatures (−10° +2° +20° and +37°c), atmospheres (air, nitrogen) and humidities (with and without in‐package desiccation), and colour changes were observed for periods of up to 1 year.The main cause of colour fading was the oxidation of lycopene. However, another phenomenon, trans—cis isomerisation of all‐trans lycopene, can occur, with apparent loss of coloration, which is later restored by re‐isomerisation. The existence of this mechanism was demonstrated by simultaneous measurements of total liposoluble colour at 420 nm and separations of carotenoid pigments by column chromatography on Al2O3.Neo‐lycopene A (6‐mono‐cis‐lycopene) and at least one other cis‐lycopene isomer were found in fresh powder and in samples stored up to 5 months. cis‐Isomers were more susceptible to oxidation.Colour changes depend upon the interplay of isomerisation and oxidation; in some cases, tomato powder was more intensively coloured after storage than when fresh. Storage at +20°c was more favourable for colour retention than storage at +2°c, or — 10°c. Excessive desiccation strongly promoted oxidation losses. Storage in an inert atmosphere improved colour retention.At +37°c non‐enzymic browning, accompanied by formation of water, caused rapid darkening although carotenoid pigments were not lost to a great extent.

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