Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/15/2003
Publication Date: 6/1/2004
Citation: Bergman, C.J., Goffman, F.D. 2004. A gas chromatographic procedure for determining rice degree of milling. In: Rice Technical Working Group Meeting Proceedings. February 29-March 4, 2004, New Orleans, LA. 2004 CD-ROM.
Technical Abstract: Rice degree of milling is a quantification of the amount of bran that has been removed from kernels during the milling process. All things being equal, undermilled rice will weigh more than well milled rice and thus influence its value. Kernels that have been milled to a different degree reportedly have varying functional and sensory properties. Also, degree of milling has an affect on the levels of phytochemicals in rice bran. Consequently, degree of milling is an important end-use quality characteristic to the rice industry. It is also important to the research community because of its affect on rice quality, that is, only samples that have been milled to the same degree should be compared in end-use quality related studies. This, however, is seldom documented in published research. The numerous methods that have been developed to determine degree of milling can be categorized into two groups; those that assess the amount of bran remaining on milled rice and those that measure (or predict) chemical components in the outer layers of rice kernels. These methods all have shortcomings such as a large quantity of sample is required, lengthy analysis time, and the predictive methods have not been validated with cultivars of differing genetic backgrounds. In the last few decades, however, extracting the amount of lipid in the outer layers of 5 -10 g of milled rice using a Goldfisch apparatus (i.e., surface lipids) has become the most commonly used degree of milling research tool. The objective of this research was to develop and test a rapid GC procedure for milled rice surface lipid content. Long, medium and short grain rice milling standards were obtained from the USDA GIPSA Federal Grain Inspection Service. Another group of samples included, conventional U.S. long grain cultivars, conventional U.S. medium grain cultivars, a Japanese premium quality-type cultivar, a waxy cultivar and other quality types. The samples were divided into two groups for milling, the long grains and the medium grains. Broken, diseased and immature kernels were removed from the samples and then milled using standard protocols designed to mill rice of various length wide ratios to a similar degree of milling. The milled rice surface lipid content of 6 g of each sample was determined with petroleum ether in a Goldfish extraction apparatus. The fatty acid content of rice surface lipids was also determined by gas chromatography of fatty acid methyl esters (FAMEs). FAMEs were prepared using extracts obtained from 50 mg of each milled rice sample. An internal standard, tricaprylin, was added to each sample along with isooctane. Two µL of the isooctane phase was then injected into a gas chromatograph equipped with a flame ionization detector. The oven temperature was programmed as follows: the initial temperature (160'C) was linearly increased to 167'C at a 2'C/min rate, then to 210' C at a 15' C/min rate, then to 250' C at a 10' C/min rate and the final temperature was held for 14 min. The samples were injected at a split rate of 1:8.8. GC fatty acid peaks were identified by comparing retention times to those found for a set of reference standards. Response factors were determined for all of the fatty acids found in milled rice using tricaprylin. These factors were used to determine individual fatty acid levels which were summed and reported as total surface lipids. The Goldfish surface lipid content of the samples ranged from 0.16 to 0.58%. Approximately 0.3% surface lipid content is considered well milled by the U.S. rice industry. These results indicate that some of the samples studied were what is considered to be over-milled and some were under-milled. This is in spite of the fact that standard procedures were used to mill these samples. These results are not surprising, since slight variations in length wide kernel ratios between cultivars and within s