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Impact of Storage of Freshly Harvested Paddy Rice on Milled White Rice Flavor

¹USDA-ARS Southern Regional Research Center, New Orleans, LA. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. ²Corresponding author. E-mail: etchamp@srrc.ars.usda.gov ³Biological & Agricultural Engineering, University of California, Davis, CA. ⁴University of California Cooperative Extension, Oroville, CA.

Between harvest and the start of drying, paddy rice may be held for more than 24 hr at moisture contents ranging from 16% to >26%. Microbes found on the freshly harvested rice grow under these conditions and produce a wide variety of volatile compounds that impact the flavor/aroma of the white rice obtained after drying and milling of the paddy rice. The contents of 10 volatile microbial metabolites were compared in white rice obtained from paddy rice harvested at differing moisture contents and immediately dried (0 hr) or held for 48 hr before drying. No increases in volatile microbial metabolite levels were observed in white rice obtained from paddy rice that was stored at 17–21% moisture contents for 48 hr. In white rice from paddy rice stored at ≥24% moisture content, 3-methyl-butanol, 2-methyl-butanol, acetic acid, 2,3- butandiol, and ethyl hexadecanoate increased markedly with time. Also in these samples, as determined by a descriptive panel, sour/silage and alfalfa/grassy/green bean flavors significantly increased (P < 0.1) in intensity. Sour/silage, the predominant off-flavor note in the stored samples, correlated highly (r = 0.98) with 2,3-butandiol. Ethanol concentration measurements on the paddy rice correlated highly with sour/silage (r = 0.99) and 2,3-butandiol (r = 0.97), and correlated well with several other volatile microbial metabolites. Carbon dioxide measurements taken on the paddy rice did not correlate as highly (r =.78) with sour/silage. Measurements of ethanol produced in paddy rice may serve as an indicator of off-flavor/aroma development in the resultant white rice.