Hardness Measurement of Bulk Wheat by Single-Kernel Visible and Near-Infrared Reflectance Spectroscopy

Cereal Chem. 80(3):316–322 Reflectance spectra (400 to 1700 nm) of single wheat kernels collected using the Single Kernel Characterization System (SKCS) 4170 were analyzed for wheat grain hardness using partial least squares (PLS) regression. The wavelengths (650 to 700, 1100, 1200, 1380, 1450, and 1670 nm) that contributed most to the ability of the model to predict hardness were related to protein, starch, and color differences. Slightly better prediction results were observed when the 550–1690 nm region was used compared with 950–1690 nm region across all sample sizes. For the 30-kernel mass-averaged model, the hardness prediction for 550– 1690 nm spectra resulted in a coefficient of determination (R) = 0.91, standard error of cross validation (SECV) = 7.70, and relative predictive determinant (RPD) = 3.3, while the 950–1690 nm had R = 0.88, SECV = 8.67, and RPD = 2.9. Average hardness of hard and soft wheat validation samples based on mass-averaged spectra of 30 kernels was predicted and compared with the SKCS 4100 reference method (R = 0.88). Compared with the reference SKCS hardness classification, the 30-kernel (550– 1690 nm) prediction model correctly differentiated (97%) between hard and soft wheat. Monte Carlo simulation technique coupled with the SKCS 4100 hardness classification logic was used for classifying mixed wheat samples. Compared with the reference, the prediction model correctly classified mixed samples with 72–100% accuracy. Results confirmed the potential of using visible and near-infrared reflectance spectroscopy of whole single kernels of wheat as a rapid and nondestructive measurement of bulk wheat grain hardness. Wheat (Triticum aestivum L.) grain hardness is a primary quality trait relating wheat to milling properties and end-use (Pomeranz et al 1984; Slaughter et al 1992; Ohm et al 1998; Morris et al 1999). An excellent indication of its importance is the manner by which wheat has been generally classified in the United States into three major hardness classes: soft, hard hexaploid, and durum. Although extensively studied, no direct causal relationship between the genetic and physicochemical basis of endosperm texture has been established (Greenblatt et al 1995). Hong et al (1989) reported a slight correlation (r = 0.58) between the amount of water-soluble pentosans and endosperm texture. Bettge and Morris (2000) noted that among hard wheat samples, pentosans had a minimal role in modifying grain hardness; however, for soft wheat, pentosans appear to have a significant hardness-modifying effect that carries over into end-use quality. Grain hardness is affected by the degree of adhesion between starch granules and endosperm protein matrix (Barlow et al 1973; Simmonds et al 1973). Additionally, Barlow et al (1973) noted that the interaction between starch granules and amyloplast membranes is different between hard and soft wheats. Results from scanning electron microscopy and freeze-etching work showed that fractures during milling of hard wheat tend to pass along endosperm cell walls to yield clean, well-defined particles. Fracture through cell contents in these wheats, when it occurs, involves both starch granules and storage protein resulting in high proportion of damaged and broken starch granules. Soft wheats, on the other hand, had lower adhesion between starch and protein, thus tending to release starch granules more freely during milling. Glenn and Saunders (1990) demonstrated that intracellular space exists around the starch granules of soft, but not hard wheat, forming a discontinuity in the starch-protein matrix. This physical discontinuity provides a natural path for shearing forces during kernel disruption, leading to softer material that is easily reduced in particle size. Turnbull and Rahman (2002) provided a review of the structure of hard and soft wheat endosperm with particular emphasis on when differences in endosperm texture can be detected in the developing seed. Numerous techniques have been studied to quantify wheat hardness. Some of these techniques include near-infrared reflectance spectroscopy (NIRS) of ground meal (Williams 1979; Norris et al 1989), NIRS of whole wheat grains (Manley et al 1996), nearinfrared transmittance spectroscopy (Delwiche 1993), laser light scattering (Plantz 1983), acoustical properties of a kernel during grinding (Massie et al 1993), measuring the force required to crush kernels (Martin et al 1993; Psotka 1997; Morris et al 1999), particle size index (PSI) (Cutler and Brinson 1935;Yamazaki 1972), soft metal hardness testers (Katz et al 1959), pearling index (McCluggage 1943), and visual inspection of crushed endosperm (Mattern 1988). The NIRS of ground meal (Approved Method 39-70, AACC 2000), which was first reported by Norris et al (1989), is the method adopted by the National Institute of Standards and Technology (NIST) for kernel hardness determination. Two other AACC approved physical tests for grain hardness measurements are the particle size index (Approved Method 55-30) and the Single Kernel Characterization System (SKCS) 4100 (Approved Method 55-31). Of particular interest in this study is the SKCS 4100 because of its potential as a rapid and accurate measurement technique that utilizes a small quantity of sample for single kernel hardness measurement. Psotka (1997) reported that the SKCS provides a hardness index that does not significantly change with changes in moisture content. Psotka cited an example wherein the hardness index did not change when the moisture content increased from 10 to 15% moisture. The single kernel hardness measurements allow for differentiation of wheat samples that are mixed with soft and hard wheats. The SKCS 4100 provide the best phenotypic measure of the material properties of the wheat endosperm manifested by the action of the Hardness loci (Morris et al 1999). However, as with the other hardness measures, this is a destructive test because kernels are crushed during measurement. Also of interest in this study is NIRS, which can measure some attributes nondestructively. NIRS has been used to measure protein (Williams and Thompson 1978; Williams 1979), hardness using ground samples (Williams 1979; Norris et al 1989; Ohm et al 1998), moisture content (Williams and Thompson 1978), vitreousness (Dowell 2000), and color (Delwiche and Massie 1996; Dowell 1998). One of the current approved methods for wheat hardness measurement is wheat hardness as determined by near-infrared spectroscopy (Approved Method 39-70A). This method involves grinding samples before the spectra are taken, so this is also a destructive method. 1 Engineering Research Unit, Grain Marketing and Production Research Center, ARS-USDA, 1515 College Avenue, Manhattan, KS 66502. 2 Corresponding author. Phone: 785-776-2753. Fax: 785-776-2792. E-mail: [email protected]. Publication no. C-2003-0415-07R. 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, Inc., 2003.