Accumulation and degradation of starch in carrot roots

Abstract

Carrot (Daucus carota L.) roots are traditionally considered as containing negligible amounts of starch. Depending on the type of extraction medium used to remove interfering sugars from root tissue however, the yield of starch content varies. To re-examine the role of starch in carrots, high-starch cultivars were identified and their starch and sugar content monitored during root development in the field and during storage. Starch up to 150 or even 180mgg−1 dry weight accumulated in some of these cultivars. Low-temperature storage (2°C) decreased the starch content dramatically and concomitantly increased the sucrose content. Sensory triangle tests and ranking tests of sweetness performed with juices produced from carrot roots revealed significant differences between freshly harvested and stored roots, indicating a gain in sweetness achieved by starch mobilization. Post-harvest sweetening of carrots was accompanied by a transient increase in total amylase activity and malto-oligosaccharides (MOS) content. In contrast, if roots were stored at room temperature (19°C), there was also a decrease in starch content and an increase in sucrose content, but total amylase activity and MOS content declined, suggesting different metabolic pathways at the two temperatures. The possible role of starch in sugar accumulation and in stress adaptation of carrot is discussed. It is concluded that starch is a key reserve carbohydrate in carrot.