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Thermal processing affects the nutritional value of food products. The nutritional value is not only determined by the content but also by the bioaccessibility of nutrients. The present study was performed to gain detailed insight into the influence of thermal processing on the degradation, isomerization, and bioaccessibility of lycopene isomers in tomato pulp, without adding any other ingredient. The bioaccessibility, which is defined as the fraction of the nutrient that can be released from the food matrix, was measured using an in vitro method. The results demonstrated the rather high thermal stability of lycopene. Although a treatment at 140 °C induced isomerization, the contribution of cis-lycopene to the total lycopene content remained small. Results also confirmed that thermal processing as such can improve the in vitro bioaccessibility of lycopene in tomato pulp, but the improvement was only significant upon treatments at temperatures of 130 and 140 °C. At such intense process conditions, one should be aware of the negative effect on other quality and nutrient parameters. Possibilities of thermal processing as such to improve the nutritional value of tomato pulp (without the addition of other ingredients) thus looks rather limited.

The effect of thermal processing on the stability of beta-carotene in carrot puree was investigated in a broad temperature range (80-150 degrees C). Heat induced changes in the stability of beta-carotene resulting in the conversion into its cis-isomers until an equilibrium state was reached after prolonged heating. By using nonlinear one-step regression analysis, the overall isomerization of all-trans-beta-carotene and the formation of individual cis-isomers could be modeled with a fractional conversion model. The Arrhenius equation was used to describe the temperature dependence of the reaction rate constants. As indicated by the low activation energies for all compounds (11 kJ mol(-1)), the isomerization rate constants showed little sensitivity toward the treatment temperature. The temperature dependence of the equilibrium concentration values after prolonged heating (C(f)) varied for the different compounds, but in all cases, a linear relation between the C(f) values and the treatment temperature could be noted. Although isomerization was observed as a result of thermal processing, it could be concluded that during industrially relevant heating processes, the retention of all-trans-beta-carotene in plain carrot puree was relatively high, which is most likely due to the presence of the protecting food matrix.

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