Application of Fourier transform infrared (FTIR) spectroscopy for the identification of wheat varieties

Introduction

Wheat is a major cereal crop and an important component of human diet particularly in developing countries. Wheat is considered as unique among the cereals largely due to the fact that its grain contains unique chemical and physical properties. Besides being a rich source of carbohydrates, wheat contains other valuable components such as protein, minerals, phytochemicals and vitamins.

Several techniques are used to check the differences in grains of different wheat varieties. These techniques include the FTIR spectroscopic analysis ,SDS-PAGE, spectroscopic analysis by using atomic absorption spectroscopy and some other standard methods . Chemical mapping using Fourier transform IR (FTIR) spectroscopy has been successfully applied to a wide range of agricultural products e.g. flax stem, wheat, maize, oat, rye, grains, soybean seed . 
Fourier transform infrared spectroscopy is a powerful tool for compositional analysis and has been widely used to analyze cell wall of cereal grains and composition of hard and soft milling wheat using FTIR imaging in conjunction with multivariate statistical analysis and identified spectral differences which were confined to specific cell types within the starchy endosperm tissue.

FTIR is a rapid, nondestructive, time saving method that can detect a range of functional groups and is sensitive to changes in molecular structure. FTIR provide information on the basis of chemical composition and physical state of the whole sample . Fourier Transform-Infrared spectroscopy (FTIR) has been used as a rapid, accurate and nondestructive technique for measuring many wheat quality parameters. Fourier-transform Raman spectrometry is a powerful tool for the investigation of primary cell wall characteristics at a molecular level and providing complementary information to that obtained by Fourier transform infrared micro-spectroscopy. In addition the sensitivity and accuracy of FTIR detectors along with wide variety of software algorithms have dramatically increased the practical use of infrared for quantitative analysis.

Materials and methods

Procurement of raw materials

Four commercially available wheat varieties were procured from Wheat Research Institute, Faisalabad Pakistan. The grains of each wheat variety was cleaned manually to remove damaged seeds, dust particles, seeds of other grains/crops and other impurities like metals and weeds. All the chemicals and reagents used in the study were purchased from sigma company USA.

Physical tests

The wheat grains of different varieties were analyzed for different physical characteristics like thousand kernel weight by counting one thousand wheat kernels of each variety randomly and weighing them separately and for test weight in kilograms per hectoliter (Kg/hl) according to procedure described in AACC.

Wheat milling

The tempered wheat was milled through Brabender Quadrumate Senior Mill (C.W. Brabender Instruments, Inc.) to get break, reduction and straight grade flour according to the method No. 26–95 described in AACC.

Dough rheological properties

The flour samples of each wheat variety was evaluated for farinographic characteristics by using Brabander Farinograph equipped with 50 g bowl according to procedure prescribed in AACC method No 54–21. The properties of dough were obtained by Farinograph. Dough characteristics such as water absorption, dough stability, dough development time, tolerance index and softening of dough were interpreted from Frinogram generated by Frinograph. Mixographic characteristic of straight grade flour was also estimated using mixograph according to the procedure prescribed in AACC.

Chemical tests by standard methods

The flour samples of each wheat variety was analyzed for the moisture, ash, fat and protein contents according to their respective method Nos. 44- 15A, 08–01, 30–10, 46–10 and 32–10, given in AACC.
 

Fourier transform infrared (FTIR) spectrometric analysis

1. Sample preparation

   Flour of each wheat variety was collected. These flour samples were converted into pellets and pellets were cut into thin sections. These thin sections were oven dried and their spectra were recorded on FTIR.

2. FTIR measurements

   FTIR spectra were recorded for moisture content, crude fat, crude protein, ash content and starch contents

• (i) Moisture content

  Moisture content was determined by FTIR in mid infrared range. Peaks for water was observed in the 1,640 cm⁻¹ to 3,300 cm⁻¹ as mentioned by Manley et al. 

• (ii) Crude fat

  Crude fat of flour samples was observed having peaks at 1,600 cm⁻¹ to 1,700 cm⁻¹ and 1,550 cm⁻¹ to 1,570 cm⁻¹ as mentioned by Che-Man and Setiowaty 

• (iii) Crude protein

  Crude protein will be observed having peaks of amide I band at 1,600 cm⁻¹ to 1,700 cm⁻¹ and amide II at 1,550 cm⁻¹ to 1,570 cm⁻¹ observed by Manley et al. 

• (iv) Ash contents

  Ash content was determined by FTIR using method mentioned by Armstrong et al. 

• (v) Starch contents

  Spectra of starch were recorded using FT-IR, key bands. Spectra were recorded in regions below 800 cm⁻¹, 800–1,500 cm⁻¹ (the fingerprint region), the region between 2,800 and 3,000 cm⁻¹ (C-H stretch region), and finally the region between 3,000 and 3,600 cm⁻¹ (O-H stretch region) by method as mentioned by Belton et al.

  Conclusion

  The present study clearly indicates that the physical properties showed significant differences in different wheat varieties. In the case of chemical parameters there are also large variation existed among different wheat varieties. As the mandate of the present study was to assess the physical, chemical and rheological properties of different wheat varieties by standard method and advance technique FTIR and their advantages over standard methods. On the basis of results it is concluded that these methods are expensive, time consuming and cause destruction of samples but Fourier transforms infrared (FTIR) spectroscopy is one of the most important tools used for analyzing wheat for different tests. FTIR provides an excellent means to visualize the chemical composition of different wheat varieties and it is very quick, reliable and cheaper analytical technique which can effectively be used for estimation of different wheat quality parameters. The values determined by this technique are very close to the values determined by the standard procedures and preferred over dispersive due to following reasons. It is non destructive technique, provides precise measurement which requires no external calibration, increases speed, collecting a scan every second has minimum sample preparation.