Characterization of red and white cocoyam (Xanthosoma sagittifolium) roots, flours and starches during heating by low field NMR

Abstract

Cocoyams are important root and tuber staples in West African communities, but limited information exists on their physicochemical and processing characteristics. In this study low field proton relaxation analysis was used to characterize the water distribution and gelatinization behaviour during cooking of red and white cocoyam roots, as well as water dispersions of their corresponding processed flours and purified starches. Up to four fast-interacting water populations were observed in the roots, identifying water associated with starch (T2a≈1.5 ms), water interacting with the cell walls (T2b=5-10 ms), water in the cytoplasm (T2c=13-54 ms), and water in vacuoles/extracellular water (T2d=51-246 ms). Two populations were observed in 10% (w/w) flour and starch dispersions. All relaxation parameters were sensitive towards swelling and gelatinization in the flours and starches, but T22 was more sensitive towards water expelled from the matrix at temperatures above gelatinization (approx. 80°C in the roots and flours, and 75°C in the starch). Shorter relaxation times observed in the white variety roots, and a higher proportion of more restrained water, indicated that the white root variety was slightly more sensitive towards forming a gel, and that it held a higher proportion of water after gelatinization. A higher proportion of more restrained water was also observed after retrogradation in the starch and flour suspensions from the white variety. The observed differences are believed to mainly relate to differences in the starch characteristics of the two varieties, including a higher amylose/amylopectin ratio in the white roots. Furthermore, the study showed that the roots have much wider potential than their current utilization.