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. 2017 Mar 6:7:43713.
doi: 10.1038/srep43713.

The molecular structural features controlling stickiness in cooked rice, a major palatability determinant

Hongyan Li  1   2 , Melissa A Fitzgerald  3 , Sangeeta Prakash  3 , Timothy M Nicholson  4 , Robert G Gilbert  1   2
Affiliations

The molecular structural features controlling stickiness in cooked rice, a major palatability determinant

Hongyan Li et al. Sci Rep. .

Abstract

The stickiness of cooked rice is important for eating quality and consumer acceptance. The first molecular understanding of stickiness is obtained from leaching and molecular structural characteristics during cooking. Starch is a highly branched glucose polymer. We find (i) the molecular size of leached amylopectin is 30 times smaller than that of native amylopectin while (ii) that of leached amylose is 5 times smaller than that of native amylose, (iii) the chain-length distribution (CLD: the number of monomer units in a chain on the branched polymer) of leached amylopectin is similar to native amylopectin while (iv) the CLD of leached amylose is much narrower than that of the native amylose, and (v) mainly amylopectin, not amylose, leaches out of the granule and rice kernel during cooking. Stickiness is found to increase with decreasing amylose content in the whole grain, and, in the leachate, with increasing total amount of amylopectin, the proportion of short amylopectin chains, and amylopectin molecular size. Molecular adhesion mechanisms are put forward to explain this result. This molecular structural mechanism provides a new tool for rice breeders to select cultivars with desirable palatability by quantifying the components and molecular structure of leached starch.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Stickiness of all rice samples measured from TPA.
Figure 2
Figure 2. SEC weight distributions of branched starch molecules, wbr(logRh), normalized to the highest peak.
(a) Weight distributions for native grain starch. (b) weight distributions for leached starch. The grey area denotes the Rh range of amylopectin in native grain and leachate, respectively.
Figure 3
Figure 3. SEC weight CLDs of debranched starches.
All distributions were normalized to the amylopectin peak. (a) Weight CLDs for native grain starch. (b) Weight CLDs for leached starch. The grey area denotes the Rh range of amylopectin in native grain and leachate, respectively.
Figure 4
Figure 4. The postulated molecular mechanism for stickiness between cooked rice grains and the TPA probe.
The surface layer of the sticky one has more amylopectin molecules with higher proportion of short chains with DP ≤ 36 and bigger molecular size, while the surface layer of the less sticky one has less amylopectin molecules (diluted by amylose molecules), fewer short chains with DP ≤ 36, and smaller molecular size.
See this image and copyright information in PMC

References

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