Different nitrogen fertilizer application in the field affects the morphology and structure of protein and starch in rice during cooking.

Nitrogen fertilization is one of the most important cultivation practices that affects the eating quality of rice. During the cooking process, nitrogen fertilizer application in the field changed the structure of protein and starch during cooking, which eventually reduced the rice eating quality. However, the morphology and structure of rice during cooking under high nitrogen fertilizer application in the field have not been explored. The relationship between the morphological and structural changes of rice protein and starch during cooking and the rice eating quality has not been studied. In this study, we conducted field trials at two nitrogen fertilizer levels (0 N and 350 N), and the rice was cooked after harvest. Our results showed that the peak viscosity of rice flour was 3326 cp and 2453 cp at 0 N and 350 N, respectively, and the peak viscosity of rice starch was 3424 cp and 3378 cp, respectively. Rice proteins played an important role in the starch gelatinization properties and thermodynamic properties. High nitrogen fertilizer application increased the protein content of rice from 5.97 % to 11.32 %, and more protein bodies adhered to the surface of amyloplasts eventually inhibiting starch gelatinization. The rice proteins could bind to amylose-lipid complexes during cooking, promoting the formation of V-type diffraction peaks. What is more, under high nitrogen fertilizer, rice protein had more ß-sheets, which slowed the entry of water into the interior of starch molecules and prevented the destruction of the short-range ordered structure of starch. Our study provides the possibility to further improve the eating quality of rice under nitrogen fertilizer treatment.

Differences in grain protein content and regional distribution of 706 rice accessions.

BACKGROUND: Improving rice quality is one of the main goals of global rice breeding programs, and rice protein content is one of the most important factors affecting rice eating quality. The protein content of rice is mainly determined by genetic factors and also affected by environmental factors. However, the differences and regional distribution of protein content during the evolution of different rice varieties are still unclear. RESULTS: The purpose of this study was to understand the differences in grain protein content of 706 rice accessions in different regions and different rice subtypes. The influencing factors of grain protein content differences were analyzed from the perspectives of genetic characteristics, environment, rice subtypes, and breeding process. The results showed that the grain protein content of indica rice in most countries and regions was higher than that of japonica rice. From the perspective of the rice breeding process, the protein content of modern varieties in japonica was lower than that of landrace varieties. In most countries, modern varieties of indica rice had higher protein content than landrace varieties did. CONCLUSION: The environment and genetic characteristics caused the change in the protein content of rice. This study has improved our understanding of the differences in protein content of rice from different rice varieties. © 2022 Society of Chemical Industry.

Differences in starch multi-layer structure, pasting, and rice eating quality between fresh rice and 7 years stored rice.

With the continuous improvement of rice production capacity and the accumulation of reserves year by year, rice sometimes has to be stored for a long time. However, long-term storage of rice has poor sensory properties, which may be related to the structural changes of starch. Different from the previous studies on short-term storage of rice (often 3-12 months), the focus of this study was to understand the differences in starch multi-layer structure, pasting, and rice eating quality between 7 years stored rice and fresh rice. Our research indicated that 7 years stored rice showed higher hardness and lower stickiness compared to fresh rice, which ultimately led to poorer eating quality. These bad changes were related to differences in starch multi-layer structure. The 7 years stored rice had lower amylose content, a lower thickness of crystalline lamellae and short-range ordered structure of starch, and more large starch granules. In particular, the volume mean diameter of 7 years starch was more than 4 times that of fresh starch. 7 years stored rice had more large granular starch and unstable crystal structure, which led to the increase of pasting temperature and the decrease of gelatinization enthalpy during starch gelatinization, and ultimately reduced the eating quality of the rice.