Determining Vitreous Subclasses of Hard Red Spring Wheat Using Visible/Near-Infrared Spectroscopy

Cereal Chem. 79(3):418–422 The percentage of dark hard vitreous (DHV) kernels in hard red spring wheat is an important grading factor that is associated with protein content, kernel hardness, milling properties, and baking quality. The current visual method of determining DHV and non-DHV (NDHV) wheat kernels is time-consuming, tedious, and subject to large errors. The objective of this research was to classify DHV and NDHV wheat kernels, including kernels that were checked, cracked, sprouted, or bleached using visible/ near-infrared (Vis/NIR) spectroscopy. Spectra from single DHV and NDHV kernels were collected using a diode-array NIR spectrometer. The dorsal and crease sides of the kernels were viewed. Three wavelength regions, 500–750 nm, 750–1,700 nm, and 500–1700 nm were compared. Spectra were analyzed by using partial least squares (PLS) regression. Results suggest that the major contributors to classifying DHV and NDHV kernels are light scattering, protein content, kernel hardness, starch content, and kernel color effects on the absorption spectrum. Bleached kernels were the most difficult to classify because of high lightness values. The sample set with bleached kernels yielded lower classification accuracies of 91.1–97.1% compared with 97.5–100% for the sample set without bleached kernels. More than 75% of misclassified kernels were bleached. For sample sets without bleached kernels, the classification models that included the dorsal side gave the highest classification accuracies (99.6– 100%) for the testing sample set. Wavelengths in both the Vis/NIR regions or the NIR region alone yielded better classification accuracies than those in the visible region only. The content of dark hard vitreous (DHV) kernels is a widely used quality factor in grading and marketing of hard red spring wheat (HRS). The DHV kernels usually have a natural translucent color and high hardness index value. The vitreousness of the DHV kernels is related to protein content and hardness (Dexter and Edwards 1998). In the United States, DHV kernels are considered as subclasses of HRS wheat (USDA 1997). The Grain Inspection, Packers and Stockyard Administration (GIPSA) uses specific criteria to visually determine the subclasses of HRS wheat. Kernels from an HRS sample are considered DHV if they are 1) dark, hard, and vitreous; 2) bleached but still hard and vitreous; 3) checked or cracked with cloudy or shadowy spots but are still dark, hard, and vitreous; or 4) hard red winter and soft red winter wheat kernels within the HRS sample, but still dark, hard, and vitreous. Kernels are considered non-DHV (NDHV) if they are 1) yellow or contain a mottled spot; 2) green immature kernels; 3) severely affected by scab; 4) sprouted; or 5) hard white, soft white, or durum wheat. Foreign material also is classed as NDHV. Three subclasses of HRS wheat are defined by the percentage by weight of DHV kernels present in a 15-g representative sample that is dockage-free with no shrunken or broken kernels. Wheat is assigned to the subclasses of dark northern spring wheat if it contains ≥75% DHV kernels; northern spring wheat if it contains 25–75% DHV kernels; or red spring wheat if it has <25% DHV kernels. The current visual method of determining DHV and NDHV wheat is time-consuming, tedious, and subject to large errors, and inspectors may not agree on classifications (Dexter and Edwards 1998). Therefore, an objective and reproducible means of determining DHV and NDHV kernels is needed. Near-infrared (NIR) spectroscopy has been used successfully to measure wheat characteristics such as hardness (Delwiche 1993); protein content (Delwiche 1995, 1998); wheat class (Delwiche and Massie 1996); color class (Wang et al 1999a,b); insect damage (Dowell et al 1998, 1999a); and scab (Dowell et al 1999b). Dowell (2000) studied the possibility of using NIR spectroscopy to identify vitreous and nonvitreous single kernels of durum wheat. Those results showed that NIR spectroscopy could be used to quantify the vitreousness of durum wheat, possibly because of protein, starch, or scattering effects on NIR absorption. The objective of this research was to classify HRS DHV and NDHV kernels, including kernels that are checked, cracked, sprouted, and bleached by using visible/ NIR (Vis/NIR) spectroscopy. MATERIALS AND METHODS Wheat Samples Fourteen wheat samples (100 g) were obtained from GIPSA and separated into DHV kernels and NDHV kernels by a four-member panel from the Board of Appeals and Review (BAR) section of GIPSA (Table I). Each kernel was placed into the DHV or NDHV class by a unanimous decision of the panel. Forty kernels from each sample were randomly selected for Vis/NIR analysis. Spectra Collection Spectra from DHV and NDHV single kernels were collected using a diode-array spectrometer (DA7000, Perten Instruments, Springfield, IL). This spectrometer measures extended visible (400–750 nm) and NIR (750–1,700 nm) reflectance at a rate of 30 spectra per second. Single kernels were placed in a black V-shaped trough (12 mm long, 10 mm wide, and 5 mm deep) and illuminated with a quartz tungsten halogen (QTH) light through a fiber bundle (8 mm diameter) positioned 13 mm from the top of the trough and oriented 45o from vertical. The reflectance probe (2 mm diameter) was oriented vertically 9.5 mm from the top of the trough. The reflectance probe carried the reflected energy to a spectrometer. Wheat kernels (560 from 14 1 Contribution No. 01-269-J from the Kansas Agricultural Experiment Station. 2 Corresponding author. Phone: 785-532-2919. Fax: 785-532-5580. E-mail: dwang @bae.kus.edu 3 Biological & Agricultural Engineering Dept., Kansas State University, Manhattan, KS 66506. 4 Grain Marketing & Production Research Center, USDA-ARS, Manhattan, KS 66502. 5 American Institute of Baking, Manhattan, KS 66502. Publication no. C-2002-0405-02R. © 2002 American Association of Cereal Chemists, Inc. TABLE I Wheat Samples Used for Classification of Dark Hard Vitreous (DHV) and Non-DHV Wheat Kernelsa DHV Wheat Samples NDHV Wheat Samples