Tomato by-products were produced by puree manufacturing from heat-stabilised fruits and raw fruits to simulate both conventional and innovative processing technologies. By-products were freeze-dried, ground and stored in five relative humidity environments in the range 11–75%, for 4 months at 30 °C. The aims were: (a) to investigate the effect of heating applied during tomato processing on by-product hygroscopicity and stability, (b) to find out the optimal water activity ( a w) range for by-product stability. Hygroscopicity was studied by applying the Guggenheim–Anderson–de Boer (GAB) model. By-product stability was studied by evaluating the kinetics of lycopene, β-carotene, rutin and chlorogenic acid degradation and the changes in Hunter’s colourimetric parameters during storage. By-products obtained from heat-stabilised fruit and raw fruits had the same hygroscopicity, with an average estimated n sm value of 0.080 ± 0.013 kg water/kg dry solids, corresponding to the mean a w of 0.44 and the confidence interval of 0.31 < a w < 0.51 (on the 95% probability level). During storage, in both by-products the rate constant for lycopene degradation was maximum at the a w level of 0.17 (half-life time was 38 d); it then decreased by more than threefold with increasing the a w level up to 0.75 (half-life time was 138 d). β-Carotene degradation rates had the same order of magnitude as those of lycopene and decreased with increasing the a w level in the heat-treated by-product. However, β-carotene degradation was accelerated at a w levels ⩾0.56 in the by-product obtained from raw fruits, suggesting the involvement of lipoxygenase. Chlorogenic acid and rutin were more stable than the carotenoids and showed an opposite dependence of their stability on the a w level, being significantly degraded only at the highest a w level. The degradation of these phenolics was higher in the by-product obtained from raw fruits, indicating the likely involvement of polyphenol oxidase. The colour difference Δ E represented the sum of different degradation processes, indicating that for maximum stability, i.e. minimal Δ E variation, a dehydration level corresponding to 0.22 ⩽ a w ⩽ 0.56, has to be achieved and then maintained by preventing moisture exchanges with both environment and other food components.