Technical Talk: June 2013

Exploring the different methods of determining wheat quality and evaluating wheat performance

Exploring the different methods of determining wheat quality and evaluating wheat performance

The use of wheat and flour is a complex process involving many steps. These evaluation tests are divided into three categories: physical, chemical and rheological.

The physical methods provide information on the appearance, density, hardness and plumpness, and help the buyer and miller obtain wheat with kernels that will produce a high yield of good quality flour. External appearance of the kernels indicates the extent of damage and fungal contamination. Visual examination of wheat kernels is part of the grading system that separates the wheat into different grades.

Test weight indicates the density of wheat kernel: the higher the number, the denser and sounder the wheat kernels are. The weight per 1,000 kernels in grams is another indication of soundness of wheat. Again, a higher number indicates better quality. These two measurements can be an indication of flour yields during the milling process.

Single-kernel characterization is an important single test that can provide a lot of information about its different characteristics. In this test, a sample of wheat kernels is prepared by removing broken kernels, weed seeds, and other foreign material. The sample is then introduced to the single-kernel characterization system (SKCS) instrument, which analyzes 300 kernels individually and records the results on a computer graph. The characteristics analyzed include kernel weight by load cell and kernel diameter. The SKCS test evaluates wheat kernel texture characteristics by measuring the weight, electrical current, and force needed to crush the kernels.

Wheat kernel hardness and texture has a major impact on the milling properties of the wheat. Kernels that are damaged by frost are typically harder and will require more energy during the milling process to reduce them to flour. There are various methods available to determine the kernel hardness, including the particle size of a ground sample using a specific grinder, or using the calibrated NIR system.

The particle size index, or PSI method, utilizes a grinder to grind the wheat and sifts the ground sample through a U.S. standard 200-mesh wire with a vibrating action. The amount of ground wheat that passes through the 200-mesh sieve is used to determine the PSI. Soft-texture wheat kernels will have a high amount of ground wheat pass through the sieve; hard-texture wheat, smaller amounts. It is important that the grinder used be standardized, as different mills will produce different results.

The basic chemical methods involve the determination of certain compounds that play a role in the functionality of the flour, such as protein, moisture, ash and total carbohydrates. Processors who are interested in the performance of the flour use these methods more, but they are also important in indicating the quality of the wheat. More advanced methods can determine a number of minor components that also play a role in functionality.

Moisture content is a very critical component of the wheat, as it could be a detrimental factor if it exceeds 15 per cent. Values above 15 per cent will encourage the growth of microbes, which can cause serious storage problems. Higher moisture will also encourage the onset of sprouting and the activation of various hydrolytic enzymes within the kernel. Millers can temper wheat in order to condition the wheat kernels by exposing the wheat to a moist environment for a certain length of time prior to milling.

Near infrared spectroscopy (NIR), which has to be calibrated for each crop year, is one of the various quick methods available for the measurement of moisture in wheat kernels. In addition to other quick methods, such as infrared heating, the traditional methodology involves laboratory drying ovens.

Protein content is an important indicator of the quality of the wheat. It is not a direct indicator of the performance of flour in baked goods, but wheat is often traded according to the protein content. NIR is a quick method to determine the percentage of protein. The Dumas method involves measuring the released nitrogen after combustion of the sample. A factor (most commonly 6.25 and 5.7) is used to multiply the nitrogen quantity and convert it into a percentage of protein, depending on the fraction of the wheat kernel. There is no direct method to determine the actual protein in samples. The major proteins responsible for wheat quality are gliadin and glutenin. These proteins are independently present in the flour, and once the flour is hydrated and mixed, they bind together to form gluten. The ratios of these proteins are important in determining the characteristics of the dough formation, such as the elasticity and extensibility of the dough. However, determining the two proteins is not easy based on current methodology, so this is not a standard test utilized by the industry as a quality-testing tool. Some of these quality characteristics may be predictable using newer solvent retention capacity (SRC) methods and the resulting gluten performance index (GPI).

Rheological properties of wheat flour normally involve the changes in the viscosity of slurries due to the quality of starch or the behaviour of the gluten proteins in dough systems. Alpha amylase activity is an indication of sprout-damaged wheat, and higher amounts of this activity will indicate that the kernels have begun to sprout, affecting the characteristics of the starch and its viscosity. Knowing the extent of starch damage of the wheat is very important to the millers. This allows them to blend sprouted wheat with sound wheat in order to control the levels of the enzyme and thus produce acceptable quality flour without discarding all the wheat.

For more information, or fee-for-service help with food technical and processing issues and needs, please contact Dr. John Michaelides at John Michaelides & Associates at 519-743-8956, or at Bioenterprise at 519-821-2960 ext. 246, or by e-mail at j.jmichaelides@gmail.com.