Colour of starch-iodine complex as index of retrogradability of starch pastes

Abstract

Starch retrogradation is mainly due to the amylose fraction of starch. Amylose quantification is usually done by iodine staining with which it forms a blue colour complex while starch retrogradability can be monitored by freeze-thaw stability measurements. In this work, spectroscopic determination of the blue value and visual monitoring of starch–iodine colour complex were used to study starch retrogradation. The results obtained were compared with that from freeze-thaw stability measurements. Native cassava starch, its carboxymethylated and cyanoethylated derivatives of different degrees of substitution (D.S) were used in the study. From the results, increase in starch derivatization reduced amylose ability to bind iodine, decreased the blue value with resultant decay in the blue colour of the starch-iodine complex. The blue black colour of the starch-iodine complex was lost in carboxymethyl starch at D.S of 0.145 and cyanoethyl starch at D.S of 0.141; at these degrees of substitution, the helical structure of amylose was no longer maintained by the starch molecules and retrogradation eliminated. Freeze-thaw stability study showed clear pastes with no evidence of retrogradation over 10 freeze-thaw cycles for derivatized starches above these degrees of substitutions. This showed a good agreement with that obtained from the colour of the starch-iodine complex. Hence derivatized starch products, which showed absence of blue black colour of starch-iodine complex would be freeze-thaw stable over a long period of cold storage. The disappearance of the blue colour of starch-iodine complex thus becomes an index of retrogradability and freeze-thaw stability of starch pastes on cold storage.