Which Rice and Why? A Healthier Choice

Lu, Louise Weiwei
Rush, Elaine
Lu, Jun
Monro, John
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Doctor of Philosophy
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Auckland University of Technology

Rice has been a staple grain of the human diet for 9,000 years. Currently, more than half of the world’s population derives one-third of their total daily dietary energy intake from rice. The most popular rice product, white rice, has the oil-rich bran and germ layer removed to prolong shelf life and, compared with brown rice, requires less cooking and preparation. However, many product varieties of freshly cooked white rice are composed of rapidly digestible starch and when consumed may trigger a rapid and prolonged rise of blood glucose among people with impaired glucose tolerance or diabetes. Regular and long-term consumption of freshly cooked white rice increases the glycaemic load (GL) of the diet and may be associated with an increased risk of hyperglycaemia and the development of type 2 diabetes, obesity and other metabolic diseases. In multicultural societies such as Auckland, New Zealand, the consumption of rice per capita is increasing. This series of experiments aimed to investigate in vitro and in vivo the effect of popular rice products and method of preparation on the digestibility of starch and glycaemic responses. The rice product that had the best starch profile and lowest GL was then investigated for sensory acceptability by participants who commonly consume cooked rice. The first series of in vitro experiments investigated five popular rice products: medium-grain white rice, medium-grain brown rice, long-grain brown rice, basmati rice and parboiled rice. Samples of each cooked product had the starch digestibility profile and glucose release measured when rice was freshly prepared (Chapter 4) and then when cooked rice was stored for two to 24 hours at 4 ºC, reheated, and minced (Chapter 5). The velocity and the extent of glucose release during in vitro enzymatic starch digestion over 180 minutes were compared. No significant difference in total starch content was observed among the five uncooked rice products. After full gelatinisation (i.e., cooking), the in vitro glucose release (gram glucose / gram dry weight base) within the first 20 minutes of digestion showed that medium-grain white rice reached 78.4% (SD ± 3.9%), basmati rice 41.5% (± 6.8%), medium-grain brown rice 36.5% (± 0.2%), long-grain brown rice 26.6% (± 2.3%) and parboiled rice 27.5 (± 5.8%) of the starch as glucose. After 180 minutes, the in vitro glucose release from whole grains of medium-grain white rice reached 98.3% (± 1.1%), basmati 87.7% (± 3.1%), medium-grain brown 76.5% (± 1.8%), long-grain brown 71.5% (± 2.5%) and parboiled 81.2% (± 1.0%). Up to eight hours of cold storage at 4 ºC and mincing the cold rice to 2,400 µm did not significantly change the glucose release trajectory in any of the five rice products (within 10% reduction, P = 0.1). When stored for more than 10 hours, the trajectory was reduced significantly. For medium-grain white, medium-grain brown and long-grain brown, the reduction was around 20% (P = 0.05), for basmati around 30% (P = 0.05) and for parboiled around 40% (P = 0.01). Conversely, for all five rice products, reheating the rice to 65 ºC for at least 15 minutes increased the rate and extent of glucose release by around 20% (P = 0.01) and mincing significantly increased glucose release in various degrees. Among the rice products, minced freshly cooked medium-grain white rice had the highest overall glucose release trajectory, reaching 90% of the total available glucose after 40 minutes, whereas minced parboiled rice that had been stored at 4 ºC for 24 hours and then reheated had the lowest at 60% after 180 minutes. Clinical recommendations to help improve post prandial blood glucose concentrations could include replacement of freshly cooked medium-grain white rice with parboiled rice with the optimal treatment (cold storage at 4 ºC for 24 hours) and appropriate food safety precautions (reheating to 65 ºC for at least 15 minutes). The next step was to measure the glucose response to rice prepared this way in healthy participants. Twenty-eight apparently healthy participants had two-hour (0, 30, 60, 90 and 120 minutes) postprandial glycaemic responses measured on three separate occasions when they consumed 140 g (140 g ± 0.3 g) of either freshly cooked medium-grain white rice, freshly cooked parboiled rice or reheated parboiled rice that had been stored for 24 hours at 4 ºC. All rice was served warm at 65 ºC. Chewing time, chewed particle size distribution, and perceptions of satiety and palatability over two hours were also assessed. The 24-hours cold-stored and reheated parboiled rice resulted in a significantly lower blood glucose concentration trajectory (42%, P < 0.001) than freshly cooked medium-grain white rice and 12% lower (P = 0.001) than freshly cooked parboiled rice. Longer chewing time (6.34 seconds/10 g of rice compared with freshly cooked medium-grain white (P = 0.026) and higher palatability score (“visual appeal” was 2.0 higher (P = 0.001), “smell” was 1.0 higher (P = 0.034), “taste” was 1.5 higher (P = 0.023), and “overall palatability” was 1.9 higher (P = 0.003)) might have impacted on the slower rise of glucose response of the reheated parboiled rice. Further study is required to investigate the sensory acceptability of the rice products to investigate whether rice prepared by the optimal treatment could be accepted as part of the diet. Finally (Chapter 7), 64 participants rated their acceptance and liking of the freshly cooked and also reheated medium-grain white rice, medium-grain brown rice and parboiled rice, that is, six treatments all served warm at 65 ºC. All six rice samples (three freshly cooked and three reheated) were perceived to be similarly acceptable by participants (average 5.3 out of 10 score ± 0.2). No significant difference in overall liking was found in any of the six rice samples. Similarly, no significant differences in the acceptability of colour and sweetness were observed. Among all rice samples, the texture and the flavour of freshly cooked warm medium-grain white were less preferred (scored 4.6 ± 0.7 out of 10; 4.3 ± 0.6, respectively) compared with other samples (P < 0.05). Over 50% of all participants preferred both reheated parboiled rice (5.6 ± 0.6 out of 10) and freshly cooked parboiled rice (5.2 ± 0.6 out of 10) as a daily regular staple grain to freshly cooked medium-grain white rice (4.5 ± 0.6 out of 10, P < 0.05). This series of experiments suggests that reheated parboiled rice, which has the lowest starch digestibility and glucose impact (both in vitro glucose release and in vivo glucose response), can be accepted as a healthier alternative to freshly prepared medium-grain white rice. The strength of this study is the in vitro and then in vivo exploration of the glycaemic impact of rice products freshly prepared or stored for 24 hours and reheated. While limited by small sample sizes, this is a proof of concept and principle that needs now to be tested in real world settings with those most at risk. Reheated parboiled rice could be a staple rice that offers solutions for long-term glycaemic management and contributes to solutions for the grand challenges (e.g., type 2 diabetes and obesity) to global health because of its relatively better nutrient profile, low in vitro starch digestibility, low postprandial glycaemic response, promotion of satiety and palatability, and generally positive overall likeability.

Rice , Parboiled , Cold storage , Reheating , In vitro glucose release , Starch digestibility , Rapidly digested starch , Slowly digested starch , Resistant starch , In vivo glucose response , Chewing , Satiety , Liking preference
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