Continuously Distributed Glass Transition of Maca (Lepidium meyenii Walpers) Powder and Impact on Caking Properties

Abstract

Water sorption isotherm, glass transition temperature (Tg), and caking properties of maca (Lepidium meyenii Walpers) powder were investigated. A broad endothermic shift reflecting continuously distributed glass transition was detected by differential scanning calorimetry, which prompted the evaluation of onset Tg and offset Tg. From the onset and offset Tg curves and water sorption isotherm, critical water activity (water activity at glass transition temperature of 25 °C) was determined to be 0.258 for the onset Tg and 0.480 for the offset Tg. The critical water activity values were validated by isothermal mechanical relaxation measurement. Caking properties of maca powder were compared with maltodextrin. The degree of caking and cake hardness for maltodextrin increased at a water activity just above critical water activity for onset Tg. Maca powder, on the other hand, showed a small degree of caking in the water activity range of 0.225 to 0.576, and the degree of caking drastically increased at a higher water activity. The hardness of maca cakes increased gradually at water activity values greater than 0.432. Since maca powder showed a continuously distributed glass transition, the molecular mobility required for caking was considered to have been provided incrementally by the increase in water activity above critical water activity for the onset Tg.