Abstract: The aim of this paper is to investigate the effects of harvest date on the overall quality of two japonica rice varieties, and to analyze the relationships among various physicochemical properties for the purpose of providing fundamental data for the optimization of paddy field management and the improvement of rice processing quality. Methods: The cultivars Nanjing 5055 and Nanjing 9108 were harvested at 6-day intervals from 36 to 66 days after heading (DAH). Thousand-grain weight, appearance quality, milling characteristics, rapid viscosity analyzer (RVA) profile and cooking quality of rice grains were measured. Moreover, One-way analysis of variance (ANOVA), correlation analysis, and principal component analysis were conducted on the data obtained. The thousand-grain weights and starch contents of two japonica rice varieties harvested at 48-54 DAH were significantly higher than those harvested at other dates, respectively (P < 0.05). The surface brightness (L*) values of two milled japonica rice varieties gradually increased and then did not change significantly (P > 0.05), and the a* and b* values decreased little by little. Both brown rice rate and head rice rate increased to peak values at 54 DAH for Nanjing 5055 and at 48 DAH for Nanjing 9108, and decreased slightly afterwards. The peak viscosity and breakdown values of rice RVA profiles versus harvest date showed V-shaped curves, and followed a highly significantly positive correlation with each other (P < 0.01) while the opposite trend was observed for setback and pasting temperature. Resilience and peak time did not significantly vary with harvest date (P > 0.05). For the two varieties, the hardness value of cooked rice decreased to the minimum values at 48-54 DAH, which was highly significantly positively correlated with breakdown (P < 0.01), and highly significantly negatively correlated with setback (P < 0.01). Gumminess gradually decreased, which was highly significantly influenced by harvest date (P < 0.01). Principal component analysis showed that the overall quality of these rice varieties could be described by processing quality and nutritional contents, rice cooking performance and cooked rice palatability. The quality of the japonica rice varieties was greatly influenced by harvest date. The suitable harvest time could not only increase the appearance quality and head rice rate, but also improve the starch gelatinization properties and cooking quality of japonica rice. Under the experimental conditions of this study, the optimal harvest time was 48-54 DAH for Nanjing 5055, and 54-60 DAH for Nanjing 9108.

Key words: harvest date, overall quality, rapid viscosity analyzer profiles, cooking properties, principal component analysis