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Processing and Bioavailability (WG2) page 34
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isomerisation, tomato paste dispersed both in water and olive oil was heated for more than 3 h
at about 75°C. Heating increased the percentage of cis isomers from 7.4% to 16.6% in the
aqueous dispersion and from 12.6 to 23.3% in the oily dispersion. The higher initial cis
isomer proportion in the oily sample (12.6%) was ascribed to a greater isomerisation
occurring during sample extraction in the presence of oil. It was concluded from these data
that lycopene isomerisation can occur to a significant extent during tomato-based food
preparation.
It was found that 20% to 30% of total lycopene consisted of cis- isomers when tomatoes
were heated at 100'°C for 1 h (Stahl and Sies, 1992a). Grtner et al. (1997) found that
lycopene bioavailability from paste and processed tomato juice was significantly higher than
from unprocessed fresh tomatoes.
Khachik et al. (1992a) examined the effect of various methods of cooking (microwaving,
boiling, steaming, stewing) on the carotenoid content of green vegetables, but only of stewing
in ripe tomatoes. Major carotenoids isolated from raw and processed tomatoes were lycopene,
ζ- and β-carotene, lycopene 5,6-epoxide, phytoene, phytofluene, neurosporene and lutein.
Most of these components were present both in the all-trans and cis forms. The authors’ main
conclusions were that tomato processing (8 minutes' stewing and industrial production of
tomato paste) did not modify the carotenoid composition of the products to a great extent,
since the same compounds were detected in the three samples. However, analysis of
individual data shows that percentages of minor carotenoids (such as β- and ζ-carotene,
lutein, phytoene and phytofluene) with respect to lycopene were much lower in tomato paste
than in raw and stewed tomatoes. This could be due both to the different carotenoid
composition of raw materials and to the effects of processing.
Labrador et al. (1998) found that tomato sauces prepared in an open stirred kettle (15 min
at 95 °C, in the open air) had significantly better colour attributes and lycopene content than
that obtained in a tubular pasteuriser (105 °C for a non-specified time sufficient to reach an Fo
value ensuring product stability), but showed a faster change in lycopene content during
storage, indicating lower stability. Also in this case, cooking in an open stirred kettle may
have given rise to greater cell destruction, and consequently to a more intense colour, greater
aeration of the mass, and a better possibility of extraction (and thus the highest value of
lycopene, not otherwise accountable for), although loss of stability during the subsequent
storage phase would also occur .
In a recent study, Nguyen and Schwartz (1998) investigated the effects of various
processing technologies on the isomerisation rate of tomato carotenoids. The analytical