Rice is the staple food for more than half of the world’s population and holds a pivotal position within the “grain family.” In China, rice cultivation is widely distributed, extending from Hainan Island in the south to the Heihe region of Heilongjiang Province in the north, from Taiwan in the east to the Xinjiang Uygur Autonomous Region in the west. Rice is grown in tidal fields along the southeastern coast at elevations below sea level, as well as on the Yunnan–Guizhou Plateau at altitudes exceeding 2,600 meters. Yield levels vary considerably across regions, and grain quality exhibits an even wider range of variation.
As living standards rise, higher demands are being placed on both the yield and quality of rice. So, what constitutes high-quality rice, and what criteria are used to evaluate it?
Generally, we can evaluate it from four levels:
1. Milling quality is primarily assessed by the brown rice yield, milled rice yield, and head rice yield.
2. Appearance quality, which pertains to indicators such as grain shape, chalkiness, and translucency;
3. For steaming and cooking, as well as eating quality, the key factors include amylose content, gel consistency, gelatinization temperature, grain elongation, and the stickiness, elasticity, and hardness of the cooked rice.
4. Nutritional quality, with particular emphasis on its protein content, essential amino acid profile, fat content, and levels of vitamins and minerals.
In everyday life, when people discuss the quality of rice, they typically refer to its cooking and eating qualities. High‑quality rice is generally characterized by whole grains, high transparency, pleasant palatability after cooking, a moderate texture—neither too hard nor overly sticky—and the ability to remain fresh even when cooled.
Since 80% to 90% of the rice endosperm consists of starch, the cooking and eating quality of rice is closely linked to the composition and structure of its starch.
What truly determines rice quality is the composition and structure of starch in the endosperm.
Rice starch is classified into two types: amylose and amylopectin. Amylose consists of linear polyglucose chains linked by α‑1,4‑glycosidic bonds, forming a highly branched macromolecule with little to no branching; in contrast, amylopectin is a highly branched glucose polymer in which side chains are introduced via α‑1,6‑glycosidic linkages along the main chain, which is itself composed of α‑1,4‑glycosidic bonds.
In rice quality breeding, researchers regard amylose content as a highly significant indicator. Indeed, it is the decisive factor determining the cooking and eating qualities of rice.
Let’s highlight the key point: rice with a high amylose content becomes dry and firm after cooking, whereas rice with a low amylose content turns soft and sticky.
There are many factors that influence rice quality, including variety, the ecological environment (soil and water quality), climate, and cultivation practices. Today, we’ll discuss how fertilization affects rice quality.
Nitrogen fertilizer has the greatest impact on rice quality, with its application rate directly influencing the content of amylopectin. Nitrogen is also a major constituent of protein, and its application can increase the protein content in rice. However, excessive nitrogen application reduces the head‑rice recovery rate, increases chalkiness, and deteriorates eating quality.
Phosphorus directly participates in rice photosynthesis and influences the accumulation of sugars and starch. Appropriate application of phosphate fertilizer can increase protein content and the milled‑rice recovery rate.
Potassium’s regulatory role in plant growth is primarily manifested through its activation of enzymes; accordingly, potassium fertilizers not only significantly boost yield but also enhance product quality, making potassium a widely recognized “quality element.” Application of potassium further promotes the synthesis of high-molecular-weight compounds such as lignin, starch, and proteins, while reducing the accumulation of low-molecular-weight substances like soluble sugars, amides, and amino acids.
Micronutrient and trace element fertilizers:
The application of micronutrient fertilizers helps reduce the rate and area of chalky grains. The concentrations of micronutrients in rice are positively correlated with protein and amylopectin content; the greater the uptake of micronutrients by rice plants, the higher the protein content in the grain.
Controlled-release fertilizer:
Controlled-release fertilizers can significantly enhance the activities of glutamine synthetase and glutamine pyruvate aminotransferase in rice grains, thereby improving the nutritional quality of the rice. They have little effect on brown rice yield and milled rice yield, but increase the percentage of chalky grains and the amylose content in the rice.
Humic acids, particularly active humic acids, can increase the organic carbon content of rice, thereby enhancing the accumulation of organic matter and boosting the levels of starch, vitamins, amino acids, and sugars.
(Contributed by Song Zhi of the Academy of Agricultural Sciences)
