Effect of water activity on catotenoid degradation in dehydrated carrots

The effect of water activity on the stability of carotenoids in dehydrated carrots was studied. Freeze-dried carrots from blanched and unblanched batches were placed in air-tight glass jars containing saturated salt solutions with water activity ranging from 0.052 to 0.75, at 40°C. The equilibrium moisture, water activity, alfa- and beta-carotene, and lutein contents were analysed at different storage times. The Guggenheim-Anderson-de Boer (GAB) equation was applied to model experimental data for moisture content as a function of water activity, and to calculate the monolayer water activity, where the oxidative reactions are expected to be at minimum. Estimated monolayer water activity was 0.33 (confidential limits at 95%: 0.26 and 0.38). alfa- and beta-carotene, and lutein degradation followed pseudo-first-order kinetics in all dehydrated carrots, with rate constants ranging from 0.031 to 0.374 days-l. Similar rate constants were found between alfa- and beta-carotene, whereas lutein degraded faster. In both blanched and unblanched batches the rate of carotenoid degradation was at a minimum over the water activity range 0.31-0.54. Blanching resulted in a higher initial carotenoid content, but it accelerated carotenoid decrease during storage of dehydrated carrots.