Understanding the texture of cooked rice from the molecular, instrumental and sensory levels Hongyan Li BSc. Engineering A thesis submitted for the degree of Doctor of Philosophy at The University of Queensland in 2017 Queensland Alliance for Agriculture and Food Innovation i Abstract In recent years, consumer preferences have shifted towards better-quality rice, particularly towards varieties with good eating quality. Texture is an extremely important attribute for cooked rice and has been used as an indicator for consumer acceptance. Cooked rice texture is affected by a wide range of factors, such as the amylose content, postharvest processing, the milling ratio, the cooking method, etc., but the actual molecular reasons for the texture of cooked rice grains are still unclear. Since texture has been defined as a multidimensional characteristic that only humans can perceive, define, and measure, sensory descriptive analysis is a useful tool for characterizing texture properties of cooked rice. However, the cost associated with training and maintaining a descriptive panel has prompted many researchers to evaluate less costly and less time-consuming approaches. The overall objectives of this thesis are to explore the molecular mechanisms for the hardness and stickiness of cooked rice grains, increase understanding of the human textural perception of cooked rice, and develop an improved instrumental method to evaluate and/or predict the texture of cooked rice. The first chapter of this thesis reviews current understanding of the texture of cooked rice, which involves the factors affecting rice texture, the evaluation methods for cooked rice texture, and the scientific questions generating from the literature review and associating to the overall objectives of this thesis. In chapter 2, statistically and causally meaningful relationships are established between starch molecular structure (the molecular size distribution of whole (branched) starch and the chain length distribution of debranched starch) and texture (hardness and stickiness) of cooked rice grains. The amounts of amylose chains with degree of polymerization (DP) 100-20000, and of long amylopectin chains, positively correlate with hardness, while amylopectin chains with DP<70 and amylose molecular size both show negative correlations with hardness (p<0.05). There is also a significant negative correlation between stickiness and the amounts of long amylopectin chains (p<0.01). For rices with similar amylose content, the amount of amylose chains with DP 1000-2000 positively correlates with hardness while size negatively correlates with hardness (p<0.05). This indicates for the first time that, regardless of amylose content, rice varieties with smaller amylose molecular sizes and with higher proportions of amylose chains with DP 1000-2000 have a harder texture after cooking. This can be rationalized in terms of viscosity effects of long chains. ii Chapter 3 presents the first molecular understanding of stickiness between cooked rice grains by measuring the leaching and molecular structural characteristics during rice cooking. 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. A molecular adhesion mechanism is 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. Chapter 4 characterizes the cooked rice texture by descriptive sensory analysis and two instrumental methods (texture profile analysis (TPA) and dynamic rheological testing) using a set of 18 varieties of rice with a wide range in amylose content (0-30%). Panellists‘ results indicate that hardness and stickiness are the two most discriminating attributes among 13 tested textural attributes. Consistency coefficient (K*) and loss tangent (tan δ) from dynamic frequency sweep are used to compare with hardness and stickiness tested by TPA and sensory panellists, showing that K* representing hardness and tan δ representing stickiness are both statistically and mechanistically meaningful. The instrumental method is rationalized in terms of starch structural differences between rices: a higher proportion of both amylose and long amylopectin branches with DP 70–100 causes a more elastic and less viscous texture, which is readily understood in terms of polymer dynamics in solution. Finally, conclusions are presented in Chapter 5, summarizing the mechanisms for the hardness and stickiness of cooked rice, the main achievements corresponding to the objectives of this thesis, and the potential application of this study for rice industry and rice breeders. Furthermore, future works, e.g. exploring the specific location of amylose molecules within starch granules, optimizing the reference samples for sensory training, learning the effect of mastication and saliva on the rheological properties of cooked rice, are also recommended. iii Declaration by author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis. I have clearly stated the contribution of others to my thesis as a whole, including statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my research higher degree candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award. I acknowledge that an electronic copy of my thesis must be lodged with the University Library and, subject to the policy and procedures of The University of Queensland, the thesis be made available for research and study in accordance with the Copyright Act 1968 unless a period of embargo has been approved by the Dean of the Graduate School. I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis. iv Publications during candidature Journal Publications Relevant to Thesis 1. Li, H., Fitzgerald, M. A., Prakash, S., Nicholson, T. M., & Gilbert, R. G. (2017). The molecular structural features controlling stickiness in cooked rice, a major palatability determinant. Scientific Reports, 7, 43713. 2. Li, H., Prakash, S., Nicholson, T. M., Fitzgerald, M. A., & Gilbert, R. G. (2016a). The importance of amylose and amylopectin fine structure for textural properties of cooked rice grains. Food Chemistry, 196, 702-711. 3. Li, H., Prakash, S., Nicholson, T. M., Fitzgerald, M. A., & Gilbert, R. G. (2016b). Instrumental measurement of cooked rice texture by dynamic rheological testing and its relation to the fine structure of rice starch. Carbohydrate Polymers, 146, 253-263. Publications included in this thesis 1. Li, H., Prakash, S., Nicholson, T. M., Fitzgerald, M. A., & Gilbert, R. G. (2016a). The importance of amylose and amylopectin fine structure for textural properties of cooked rice grains. Food Chemistry, 196, 702-711. – incorporated in Chapter 2. Contributor Statement of contribution Li (Candidate) Designed and carried out experiments (100 %) Data analysis (100 %) Wrote the paper (90 %) Dr Prakash Review the paper (10 %) Dr Nicholson Review the paper (20 %) Prof. Fitzgerald Wrote the paper (5%) Review the paper (30 %) Prof. Gilbert Wrote the paper (5%) Review the paper (40 %) v 2. Li, H., Fitzgerald, M. A., Prakash, S., Nicholson, T. M., & Gilbert, R. G. (2017). The molecular structural features controlling stickiness in cooked rice, a major palatability determinant. Scientific Reports, 7, 43713. – incorporated in Chapter 3. Contributor Statement of contribution Li (Candidate) Designed and carried out experiments (100 %) Data analysis (100 %) Wrote the paper (95 %) Dr Prakash Review the paper (10 %) Dr Nicholson Review the paper (30 %) Prof. Fitzgerald Review the paper (20 %) Prof. Gilbert Wrote the paper (5%) Review the paper (40 %) 3. Li, H., Prakash, S., Nicholson, T. M., Fitzgerald, M. A., & Gilbert, R. G. (2016b). Instrumental measurement of cooked rice texture by dynamic rheological testing and its relation to the fine structure of rice starch. Carbohydrate Polymers, 146, 253-263. – incorporated in Chapter 4. Contributor Statement of contribution Li (Candidate)