Written by Dr Bienvenido O Juliano
Rice is the staple food of half the world. It has a highly digestible starch and good- quality protein among the cereals. Its oil is rich in linoleic acid and unsaponifiable matter (lipid antioxidants). Rice is hypoallergenic (gluten-free), although atopic dermatitis from rice albumin/globulin allergen has been reported in Japan and the US.
Eating and cooking quality of rice depends on starch properties: apparent amylose content (AC, linear starch), gelatinization temperature and gel consistency, the latter two being amylopectin (branched starch) properties. Manipulating amylose/amylopectin ratio does not increase the energy needed for starch synthesis or lower grain yield.
However, glutinous or waxy rice has lower grain yield due to 5% lower density of its endosperm as compared to nonglutinous rice. Factors affecting raw rice endosperm translucency and hardness are still not well-understood. Improving grain translucency may also improve grain yield from greater grain density. Improved hardness will increase head or whole-grain yield during milling.
With the shift of Filipino consumer preference from high-amylose to intermediate- to low-amylose rice due to appetite for softer rice, and with the rising rice per capita consumption, the questions of glycemic index (GI) and satiety index are being addressed. With the higher GI of low-AC rice than intermediate- and high-AC rice, the incidence of type 2 diabetes may increase. Although consumption of high-amylose rice with low GI is the best option, intermediate-amylose rice with medium GI would be more tolerable to Filipinos.
Consumption of brown rice with low AC in place of white rice would also reduce glycemic index. However, brown rice has only about 7% dietary fiber, which is lower than the 10% common to wheat and other whole-grain cereals, the reason why the US Food and Drug Administration only approved it as a whole grain in 2008. The relative satiety index of low-AC rice vs. high-AC rice, and of brown rice vs. white rice, is being verified with the UPLB and Food and Nutrition Research Institute.
Presumably, low-GI-high-AC rice has better satiety index because of its slower rate of digestion, or longer feeling of satiety (fullness) by the consumer. Labor class prefers high-amylose rice such as PSB Rc10. Increasing the content of resistant starch has been suggested, but it is associated with high amylose. High resistant starch content may also result in cooked texture too hard for most Filipino rice consumers.
There have been breeding efforts to improve the nutritional value of rice. While the efforts are laudable, consuming solely rice with all the nutrients needed by man is quite monotonous. Earlier efforts to improve the protein content of rice using indigenous high-protein accessions have not been successful due to low heritability of protein content and lower yields.
Protein is a macronutrient that requires more energy to synthesize than starch (0.604 g protein vs 0.853 g starch produced per g of glucose substrate). Besides, protein in the grain is derived from nitrogen absorbed by the rice plant before flowering, whereas grain starch comes from photosynthesis after flowering.
Increasing specific micronutrient content would have less adverse effect on grain yield than increasing protein content. High iron and zinc contents may require a major gene for high levels in the endosperm because of environmental effects. Golden Rice or provitamin A- rich endosperm through the incorporation of genes for provitamin A is an excellent example of improving the nutritional value of rice. Hopefully, its yellow color will not adversely affect consumer acceptance of the raw grain, since the color is reduced during cooking.
Consumption of brown rice in lieu of white rice improves the intake of vitamins B and E, oil, dietary fiber, minerals, and antioxidants, but may reduce percent mineral absorption due to complexing with phytic acid in the bran layer. One solution is to breed for low (50%) phytic acid mutant that will not appreciably reduce mineral absorption, without adversely affecting seedling vigor and grain yield. The bran layer is rich in oil and oil-soluble unsaponifiable matter constituents, gamma-oryzanols, phytosterols, and tocotrienols that are hypocholesterolemic. It is also rich in antioxidants, anthocyanins, phenols, and tannins, particularly the red and purple/black rices.
Brown rice is still quite expensive because of the present low demand and high cost of manually removing undehulled palay from brown rice out of small mills. Pigmented rice is expensive because it comes from low-yielding traditional varieties, but Philippine Rice Research Institute (PhilRice) has included them in its breeding for specialty rices. Pigmented rice is claimed to have medicinal properties. Yields will improve with having them in the semidwarf plant type. Combining the high content of these phytonutrients into one variety will be the practical breeding objective.
In the Philippines, intake of whole-grain foods like brown rice may provide greater benefits than intake of isolated rice nutraceuticals, which is attributed to the additive or synergistic effects of multiple constituents, the presence of still unidentified phytochemicals, changes in chemical structure of isolates, and/or physical factors affecting release and absorption in the digestive tract. Also, our Food and Drug Administration at present can only evaluate food additives and not phytonutrients.
Fragrance genes in rice are being consolidated to increase the level of free 2-acetyl-1-pyrroline (AP) and of bound AP to prolong its presence in the rice grain. Free AP is more volatile than bound AP, which requires heating to be released.
Organic rice free of pesticides is a healthy option but it tends to have lower protein content than ordinary rice. Its claimed softer cooked rice is probably due to its lower protein content, brought about by the slower release of nutrients from organic fertilizers to the soil. Claim for higher vitamin E content of organic rice has not been supported by experimental data.
It is vital to know the rice grain properties unique to various important rice-based products so that they will continue to be included in the breeding program. The Rice Chemistry and Food Science Division has weak linkage to the food industry since PhilRice is mainly involved in rice research and seed production. Actual rice product manufacturing should be left to or contracted to the private sector.
A critical mass of researchers is a prerequisite to undertake basic and applied research on rice. We should capitalize on our comparative advantage of available rice materials of well-defined grain properties.