Cereal Chem. 85(3):276-282Rice quality can vary inexplicably from one lot to another and from year to year. One cause could be the variable temperatures experienced during the nighttime hours of rice kernel development. During the fall of 2004, a controlled temperature study was conducted using large growth chambers, testing nighttime temperatures of 18, 22, 26, and 30°C from 12 a.m. until 5 a.m. throughout kernel development, using rice cultivars Cypress, LaGrue, XP710, XL8, M204, and Bengal. As nighttime temperature increased, head rice yields (HRY) significantly decreased for all cultivars except Cypress and Bengal, for which HRY did not vary among nighttime temperature treatments. Kernel mass did not vary among temperature treatments for any cultivar. Grain dimensions generally decreased as nighttime temperature increased. The number of chalky kernels increased with an increase in nighttime temperature for all cultivars but Cypress. The amylose content of Cypress and LaGrue was significantly lower at the nighttime temperature of 30°C, while total brown rice lipid and protein contents did not vary among temperature treatments for all cultivars.Rice is primarily consumed as an intact kernel and therefore production quality is largely measured by head rice yields (HRY), which is the mass percentage of rough rice kernels that remain as head rice (kernels that are ≥75% of a whole, milled kernel (USDA 2005). Broken rice is worth only 50-60% of the value of head rice, meaning that a reduction in HRY can have severe economic repercussions for rice producers. Therefore, maximizing HRY is a major concern. Producers can influence HRY by optimally choosing harvest dates to avoid kernel fissure formation due to rapid moisture adsorption in the field (Kunze 1977). Improper drying and storage procedures can also cause kernel fissuring that can reduce HRY (Daniels et al 1998).While HRY is determined in part by production practices, HRY can vary inexplicably from year to year and often from field to field, making it difficult for producers to predict yearly income and for processors to maintain a consistent end product. Moreover, in a given year, HRY can be uniform in one cultivar of rice and yet variable in another cultivar, leading to the suspicion that some cultivars are more resistant to quality variation. To achieve uniformity in the quality characteristics of rice, it is first necessary to have a clear understanding of the causes of these quality variations.Rice quality can be influenced by genetics and environmental conditions such as ambient temperature during rice plant development (Webb et al 1979). While rice genetics can be altered through breeding programs, environmental temperatures are difficult to predict and can only be manipulated to some extent with the choice of planting dates. Environmental temperature during kernel development may play an integral role in causing the observed, unexplained fluctuations in rice grain quality (Cooper et al 2006).Historical analyses have indicated that decreased yields were ...

Cereal Chem. 83(4):447-450Rice quality, specifically head rice yield (HRY), can vary inexplicably from one lot to another, and from year to year. In an effort to correlate air temperatures during various growth stages to HRY, growth staging data expressed in degree day units was used to predict the occurrence of sequential growth stages within a set of 17-year historical data, which included HRY and 50% heading dates for two long-grain rice cultivars, (Oryza sativa L) Newbonnet and Lemont, and area weather data. HRY was most strongly affected by the average daily low temperature (or nighttime temperature) during the R8 developmental stage. Lower HRY were associated with high nighttime air temperatures during this stage for both Newbonnet and Lemont. When used as a single variable in a regression model, the nighttime temperature during the R8 developmental stage explained over 25% of the variation in HRY.

Cereal Chem. 84(1):88-91Head rice yield (HRY) is the primary parameter used to quantify rice milling quality. However, HRY is affected by the degree of milling (DOM) and thus HRY may not be comparable between different lots if the DOM is different. The objective of this study was to develop a method by which HRY values can be adjusted for varying DOM values when measured by surface lipid content (SLC). Seventeen rough rice lots including long-grain and medium-grain cultivars and hybrids were harvested from two 2003 and five 2004 locations. Duplicate subsamples of each lot were milled in a McGill No. 2 laboratory mill for 10, 15, 20, or 40 sec after zero, one, two, three, and six months of storage. HRY and SLC were measured. The average HRY versus SLC slope across all milling duration data sets was 9.4. As such, it is suggested that, when milling with a McGill No. 2 laboratory mill, the HRY of a rice lot can be adjusted by a factor of 9.4 percentage points for every percentage point difference between the rice lot SLC and a specified SLC.

The net value (NV) of rice, as affected by drying costs and milling quality changes associated with harvesting rice at various moisture contents (MCs), was studied using a five-year data set comprising eight cultivars harvested over a range of MCs from 11 southern U.S. locations. A quadratic relationship was used to characterize the change in NV across harvest MC (HMC); this relationship was due to the progressively-increasing fee structure for commercial drying costs and the quadratic nature of head rice yield (HRY) changes with HMC. A sensitivity analysis revealed that as the price of brokens increased, there was a slight decrease in the HMC at which NV was maximized. Relative to the price of brokens, the optimum HMC was not influenced by fluctuations in head rice price. At a given HMC, the NV of a rice bulk increased with the price of brokens, and the extent of the increase was heavily influenced by the HRY versus HMC relationship. In all instances, the optimal HMC to maximize HRY (HMC opt-HRY) was greater than the HMC corresponding to the maximum NV (HMC opt-NV). When HMC, NV and HRY were plotted regardless of cultivar, location, or harvest year, the MC at which HRY was maximized was 21.7% whereas the MC at which NV was maximized was 18.5%, representing a 3.2 percentage point difference between HMC opt-HRY and HMC opt-NV .

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