Development of Standard Procedures for a Simple, Rapid Test to Determine Wheat Color Class<xref ref-type="aff" rid="a1" ptype="aCCHEM2002792230" citart="citart1">¹</xref>

Cereal Chem. 79(2):230–237 Growing conditions, kernel characteristics, and genetics affect wheat kernel color. As a result, red and white wheats sometimes cannot be differentiated by visual examination. Soaking wheat kernels in a sodium hydroxide solution enhances the difference in color; red wheat turns a darker red, and white wheat turns straw-yellow. Previously, when NaOH was used for wheat determination of color class, only a visual assessment was made under arbitrary conditions, many times not suitable for field work. In the present work, visible reflectance spectroscopy and visual assessments were used to optimize NaOH (2 mL/g of wheat) soak time (10 min), concentration (5M or 20%), and temperature (60°C). The optimal procedure will provide users who are not laboratory trained with inexpensive, safe procedures to definitively assign wheat color class in the shortest time in field locations. Calibration and prediction of several wheat cultivars using partial least square regression were used to validate the optimal test procedure. The test differentiated even rain-bleached wheat and cultivars that were difficult to classify visually. No distinct correlation occurred between predicted color value and the number of red genes. Red and white wheats should be kept segregated because they have different end uses. Both hard red and hard white wheats are used for bread, but white wheat is also used for Asian noodles. Until recently, red and white wheat were grown in separate regions of the United States. Thus, field inspectors grading wheat at one location generally knew the expected color class of samples received. However, with the rapid change in production from hard red wheat to hard white wheat expected in the hard winter wheat growing areas in the United States (to take advantage of potential export markets), many elevators likely will be receiving both wheat classes. Wheat kernel color is influenced by the number of red genes present (Metzger and Silbaugh 1970; Flintham 1992); environmental conditions (Quartley and Wellington 1962; DePauw and McCaig 1988), and kernel characteristics such as endosperm texture (Coles and Wrigley 1976). Thus, identifying the color class of wheat may not be straightforward. Methods for determining color class include official visual identification; visual identification after soaking in potassium hydroxide (KOH) and bleach heated to 80°C (USDA 1997); visual identification after soaking in sodium hydroxide (NaOH) (Chmelar and Mostovoj 1938); colorimeters (Chen et al 1972; Bason et al 1995); machine vision (Neuman et al, 1989); visible and near-infrared (NIR) spectroscopy (Hawk et al 1970; McCaig et al 1992, 1993; Delwiche and Massie 1996; Dowell 1997, 1998; Wang et al 1999); and electrophoretic analysis (Lookhart and Wrigley 1995). Procedures using colorimeters, machine vision systems, spectrometers, or electrophoresis can improve classification accuracy over visual identification, but they can require calibration, trained personnel, significant initial costs, and considerable time. Visual classification of unsoaked kernels under visible light can be deceptive because some red wheats appear white and vice versa. Soaking in heated bleach improves visual classifications but poses safety risks. However, soaking kernels in sodium hydroxide is relatively safe and inexpensive, requires minimal user training, has never been reported to fail, and reduces subjectivity in visual determinations of color class. Although the NaOH test has been used for many years, the procedures have not been optimized. For example, reported soak times range from 5 min to 2 hr (Chmelar and Mostovoj 1938; Quartley and Wellington 1962; Kimber 1971; Lamkin and Miller 1980; DePauw and McCaig 1988). Sample sizes range from 25 kernels in 25 mL of NaOH (Quartley and Wellington 1962) to 1 kernel in 1.5 mL (Lamkin and Miller 1980). Some researchers used surfactants to facilitate kernel immersion (DePauw and McCaig 1988). Also, some researchers conducted tests at room temperature, whereas Lamkin and Miller (1980) heated the solution to 55°C to speed the color change. Before a standard test is recommended for widespread use to determine wheat color class, procedures should be developed that are safe, rapid, and inexpensive and give definitive results. Users of this procedure may not necessarily be laboratory trained personnel. When using the NaOH test, inexperienced users can draw erroneous conclusions if the color change is interpreted too soon or too late. Establishing standard procedures would ensure a more consistent interpretation of results. Thus, the objective of this research was to establish standard procedures for testing wheat using NaOH or other chemicals that will result in definitive classification of red and white wheats in the shortest time. MATERIALS AND METHODS Samples Table I lists the wheat cultivars used in this study. The set consisted of red and white wheats in a range of shades and colors, cultivars with a known number of red genes, and bleached samples from three different harvest dates with additional rainfall in between. Red wheat samples with known red genes and pure white wheat samples were provided by the USDA-ARS National Small Grain Collection Laboratory (Aberdeen, ID) and the USDA-ARS Hard Wheat Quality Laboratory (Wang 1997); the American White Wheat Producers Association (Atchison, KS); and the Federal Grain Inspection Service (FGIS) of USDA-Grain Inspection, Packers and Stockyard Administration. Additional samples were available from previous studies (Dowell 1997; Wang 1997). Scabby kernels of red and white wheat were hand-picked under magnification of samples that were apparently heavily scab-infested. Scabby kernels could easily be identified by a shriveled, pale, and thinner than normal appearance. There was no difference between scabby kernels from both the red and white cultivars as both appeared bleached. Optimization of NaOH Test NaOH concentration. A solution (20.0% w/v) was prepared by dissolving 50 g of NaOH (>98%, pellets) in 250 mL of distilled 1 Cooperative investigations, U. S. Department of Agriculture, Agricultural Research Service (USDA-ARS), and the Department of Grain Science and Industry, Kansas State University. Contribution number 01-173-J, Kansas State Agricultural Experiment Station, Manhattan, KS 66506. 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. 2 Dept. Grain Science and Industry, Kansas State University, Manhattan, KS 66506. 3 USDA-ARS, Grain Marketing and Production Research Center, 1515 College Ave, Manhattan, KS 66502. 4 Corresponding author. E-mail: [email protected] Publication no. C-2002-0204-04R. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists,