Discovery of a Remarkable Oscillatory Color Change in the Iodine
Starch Reaction during the Early Stage of Acid Hydrolysis of PotatoStarchBiochemistry

Hayato Shoyama , Shintaro Yamamoto
Fukuoka Prefectural Meizen High School, Kurume, Japan; Fukuoka Prefetual Meizen High School,Kurume City, Japan

We noticed a strange oscillatory color change when observing the acid hydrolysis of potato starch using iodine.
This change involved repeating deep and pale blue colors in the reaction sample during the early stages ofhydrolysis. This oscillation is unlike the color variation described in several textbooks and is not observed with corn
and rice starch. Upon further investigation, we found that this phenomenon is due to the structure of potatoamylopectin, which consists of relatively long branched chains of -glucose and is resistant to hydrolysis. This
research represents the first study of this oscillatory change in the acid hydrolysis of starch monitored using
iodine.
We first confirmed that the absorbance of iodine coloration was linked to the average degree of polymerization((DP) ) of the decomposed polymer. However, it is difficult to visualize chain length increasing during hydrolysis.
To investigate this further, we conducted hydrolysis experiments using enzymatically debranched potato starch,
and also potato amylose and potato amylopectin. No oscillatory phenomenon was observed with debranchedstarch or amylose but was observed with potato amylopectin, and thus we presume that the phenomenon iscaused by the branched structure of amylopectin. Our measurements showed that the (DP) of branched-chain
potato amylopectin is larger than that of corn amylopectin, and the rate of acid hydrolysis of potato amylopectin isslower than that of corn.
The change is presumably due to the structure of potato amylopectin and its resistance to hydrolysis. These
results suggest that a linear polysaccharide chain, which can form an iodine-amylose-like complex, may be
produced intermittently by the cluster structure of amylopectin as hydrolysis proceeds.

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