Binary mixtures of waxy wheat and conventional wheat as measured by nir reflectance

Authors: Delwiche, Stephen - Steve; Graybosch, Robert

Submitted to: Talanta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/27/2015
Publication Date: 1/1/2016
Citation: Delwiche, S.R., Graybosch, R.A. 2016. Binary mixtures of waxy wheat and conventional wheat as measured by nir reflectance. Talanta. 146:496-506.

Interpretive Summary: Starch makes up approximately three quarters by weight of the whole wheat kernel and is in even greater proportion upon the milling of wheat into flour. The unique chemical and physical properties of the starch-protein complex of flour give wheat unique properties for producing bread, noodles, crackers, and cake, as well as non-food applications. Although the wheat endosperm storage proteins have been studied extensively, less effort has been focused on the starch itself. Starch consists of two macromolecules, with these being amylose which possesses linear chains of glucose units, and amylopectin which possesses a combination of linear and branched chains of these same units. This slight difference in structure gives rise to different heating, gelling, and shelf life properties in food products. With the recent release in the United States of a commercial hard winter wheat variety that possesses very low levels of amylose, the milling and processing industries desire a rapid and workable procedure that will allow the verification of identity preserved higher valued low-amylose, (e.g., ‘waxy’) wheat lots. Our previous research has shown the potential of the technique known as near-infrared spectroscopy to fill this need. By application of several statistical regression procedures, the study reported herein substantiates this procedure on two successive harvest years of wheat and carefully quantifies the limits of measuring conventional wheat ‘contamination’ in waxy wheat. Essentially, the procedure is capable of measuring the level of mixing to within five to ten percent of the concentration by weight, which is considered to be sufficient for the wheat trade and processing industries. This technique can be immediately applied because of the longstanding acceptance, possession, and use of near-infrared spectroscopy in the cereals industries for the measurement of other quality determining properties such as protein, moisture, hardness, and ash. Wheat traders, millers, and processors stand to benefit from this research.

Technical Abstract: Waxy wheat contains very low concentration (generally <2%) of amylose in endosperm starch, in contrast to conventional wheat whose starch is typically 20% amylose, with the balance being the branched macromolecule, amylopectin. With the release of a commercial hard winter waxy wheat cultivar in the United States, the grain trade, milling, and processing industries seek to have a rapid technique to ensure the purity of identity preserved waxy wheat lots. Near infrared (NIR) reflectance spectroscopy, a technique widely used in the cereals industry for proximate analysis, is a logical candidate for measuring contamination level and thus is the subject of this study. Two sets of wheat samples, harvested, prepared and scanned one year apart, were used to evaluate the NIR concept. One year consisted of nine pairs of conventional: waxy preparations, with each preparation consisting of 29 binary mixtures ranging in conventional wheat fraction (by weight) of 0 to 100% (261 spectral samples). The second year was prepared in the same fashion, with 12 preparations, thus producing 348 spectral samples. One year’s samples were controlled for protein content and moisture level between pair components in order to avoid the basis for the conventional wheat fraction models being caused by something other than spectral differences attributed to waxy and nonwaxy endosperm. Likewise the second year was controlled by selection of conventional wheat for mixture preparation based on either protein content or cluster analysis of principal components of candidate spectra. Partial least squares regression, one and two-term linear regression, and support vector machine regression models were examined. Validation statistics arising from sets within the same year or across years were remarkably similar, as were those among the three regression types. A single wavelength on second derivative transformed spectra, namely 2,290 nm, was effective at estimating the mixture level by weight, with standard errors of performance in the 6 to 9 percent range. Thus, NIR spectroscopy may be used for measuring conventional hard wheat ‘contamination’ in waxy wheat at mixture levels above 10% w/w.