Dry Matter Loss for Hybrid Rough Rice Stored Under Reduced-Oxygen Conditions Griffiths G. Atungulu, † Supriya Thote, and Shantae Wilson ABSTRACT Cereal Chem. 94(3):497–501 The objectives of this research were to characterize dry matter loss of hybrid long-grain rough rice during storage under reduced-oxygen conditions and develop a new approach to predict the dry matter loss by using storage temperature and relative humidity data as input. Two long- grain hybrid rice cultivars, CL XL745 and XL760, harvested in the year 2015 were stored in rough-rice form in sealed glass jars at moisture contents of 12.5, 16, 19, and 21%, (wet basis) and temperatures of 10, 15, 20, 27, and 40°C for a total of 16 weeks, with samples taken at 2, 4, 6, 8, 10, 12, and 16 weeks of storage. Results revealed no differences in dry matter loss connected with the rough-rice moisture content levels and temperature during the storage duration. However, the dry matter loss was statistically different based on rough-rice cultivar. Experimental data were fitted to a dry matter loss equation for long-grain rice found in literature. The dry matter loss equation developed for conditions of grain storage without oxygen limitation did not accurately predict rough-rice dry matter loss under reduced-oxygen conditions. Equation constants generated for reduced-oxygen storage conditions were significantly lower than the typical constants used for long-grain rice in literature. Hence, integration of rice cultivar and storage conditions such as oxygen supply is crucial for accurate determination of kinetics of dry matter loss during storage of hybrid long-grain rough rice. The allowable duration that rice can remain at high moisture content during natural-air drying and storage in on-farm, in-bin systems is under researched and is vaguely determined, in part, based on dry matter loss guidelines of 0.5% (Lawrence et al. 2015). In a natural-air, in-bin drying method, typically a fan (often more than one) is used to mechanically push ambient air through a rice column, from the bottom to the top of the bin. As the ambient air moves vertically through the rice column inside the bin, the ambient equilibrium moisture content (EMC) determines the extent to which grain of a given moisture content gains or loses the moisture. The duration required for natural-air in-bin drying is directly affected by local weather conditions, which sometimes may not be conducive for complete drying, especially for upper layers of rice (Atungulu et al. 2014). When drying is incomplete, rice, especially that in the upper layers, remains at high moisture contents for prolonged du- rations, leading to excessive rice respiration and dry matter loss. Carbohydrates are the main substrates used by both molds and grain during respiration. The process of respiration is represented by glucose oxidation as follows (Seib et al. 1980): C 6 H 12 O 6 + 6O 2 ðgÞ→6CO 2 ðgÞ + 6H 2 O ð1Þ + 2; 833:4 J=g (1) According to equation 1, the breakdown of 1 g of dry matter (C 6 H 12 O 6 ) by aerobic respiration uses 1.07 g of oxygen and releases 1.47 g of carbon dioxide, 0.6 g of water, and 2,833.4 J of heat energy (Kaleta and G´ ornicki 2013). Because the grain is sold on weight basis, excessive respiration and associated dry matter loss could result in negative economic consequences to grain producers. It has been proposed that dry matter loss could be used as an indicator of the extent of grain quality deterioration during drying and storage (Chikubu 1970; Sukabdi 1979; Tangonan and Jindal 1980). Saul and Steele (1966) recommended that no more than 0.5% dry matter loss be allowed to maintain maize above U.S. number 2 grade. Relating dry matter loss to the U.S. grade for rice, Sukabdi (1979) reported that the dry matter loss at which rice remained in good condition varied with grain moisture content (Naewbanij 1985). Sukabdi (1979) reported that medium-grain rough rice (Brazos) at 22% moisture content lost up to 0.25% dry matter loss at 29.4°C before falling below U.S. number 2 grade, whereas long-grain rice (Lebonet) at 18% moisture content lost up to 0.5% dry matter loss at 29.4°C before falling below U.S. number 2 grade. Based on the foregoing observations, dry matter loss was rice type dependent. It is widely understood that, once rice is harvested, overall rice quality, including dry matter, is dependent on three factors: water activity, storage temperature, and storage duration (Mutters and Thompson 2009). Overarching these factors are microbial prevalence, pop- ulation, and rate of growth on the rice (Christensen and Lopez 1965). Atungulu et al. (2015) discussed microbial prevalence on long- grain hybrid, long-grain pureline, and medium-grain rice types at harvest. In addition, Atungulu et al. (2016) described the kinetics of microbial growth on hybrid rice during storage. The foregoing studies showed direct correlations among microbial prevalence and growth kinetics on different rice types. In particular, hybrid long- grain rough rice has a pubescent characteristic that makes it very morphologically distinct from conventional long-grain pureline and medium-grain rice. It is reported that the pubescence on the hybrid long-grain rough rice predisposes it to harbor and support microbial growth in a manner different from nonpubescent rough rice (Atungulu et al. 2016). It is hypothesized that these morphological distinctions, which result in different responses in terms of mold growth patterns, may influence rates of dry matter loss. Seib et al. (1980) used the evolution of carbon dioxide as an in- dicator to determine dry matter loss based on a nonlinear equation. Freer et al. (1990) modeled the effect of moisture and temperature on bunker-stored rice and hypothesized that rice stored in airtight or reduced-oxygen conditions would result in lowering the respiration rate and, thereby, decrease dry matter loss. The estimation by Freer et al. (1990) utilized the equation developed by Seib et al. (1980). These studies estimated the dry matter loss of rice in general terms without specifying whether it relates to the long-grain hybrid, long- grain pureline, or medium-grain category. The objectives of this research were to characterize dry matter loss of hybrid long-grain rough rice during storage under reduced- oxygen conditions and develop a new approach to predict the dry matter loss based on storage temperature and relative humidity data as input. MATERIALS AND METHODS Samples. The experiments used freshly harvested hybrid rice that was grown in the year 2015 at commercial rice fields located in Pocahontas, Arkansas. Hybrid long-grain cultivars CL XL745 and XL760 were harvested at approximately 22% moisture content (wet basis). All moisture contents reported in this study are expressed on a wet basis unless otherwise stated. The rice samples were then cleaned to remove chaff and foreign matter with a dockage tester (XT4, Carter-Day, Minneapolis, MN, U.S.A.). Following the † Corresponding author. Phone: +1.479.575.6843. E-mail: atungulu@uark.edu Department of Food Science, University of Arkansas Division of Agriculture, 2650 N. Young Avenue, Fayetteville, AR 72704, U.S.A. http://dx.doi.org/10.1094/CCHEM-07-16-0198-R © 2017 AACC International, Inc. Vol. 94, No. 3, 2017 497