How the ARC-SGI apply technology to improve bread wheat quality

BAREND WENTZEL, SMALL GRAIN INSTITUTE, AN INSTITUTE OF THE ARC-FIELD CROPS DIVISION, BETHLEHEM

Wheat breeding programmes strive to provide new cultivars that perform well agronomically and have suitable milling, rheological (dough behaviour) and baking properties. In South Africa, these quality norms are determined by the South African Grain Laboratory (SAGL), in conjunction with the millers and bakers.

Bread wheat quality is of utmost importance for the milling and baking industry, consequently, breeders need to take it into account during the selection process. Millers expect high flour yield, while bakers desire baking performance. Flour yield is influenced by the hardness of the wheat kernels, whereas baking performance is influenced by dough behaviour during mixing and baking.

Protein quantity and quality greatly influence dough behaviour and baking performance. Protein quantity is strongly influenced by the environment, whilst quality is determined by the genetic composition of the wheat cultivar and the environment. The quality of wheat flour relates to the protein composition and as a result, to the end-use suitability.

The ultimate challenge in cereal science is to improve wheat quality through a better understanding of its relationship to the chemical composition of wheat flour.

Approximately 80% of the endosperm protein is comprised of gluten and is the main determinant of the unique baking quality in leavened bread. Gluten confers water absorption capacity, viscosity and elasticity to dough, and can be separated in two main fractions: gliadins and glutenins. The balance between gliadin and glutenin is crucial for dough with acceptable strength and extensibility formation (Wieser, 2007).

Both fractions contain several sub-fractions, which interact and contribute to dough properties. The presence of these sub-fractions is determined by the genetic composition of the wheat plant. The quantity of a sub-fraction is influenced by the environment, such as availability of water, fertiliser and temperature during grain filling. Higher protein content does not necessarily mean an even distribution in the amount of protein fractions.

At the ARC-Small Grain Institute, several methods are applied to identify desirable protein fractions during the early stages of breeding. These methods include protein separation procedures that require small sample sizes.

Results indicate the genetic potential of the wheat plant as well as the amount of specific protein fractions present. No single scientific method exists to predict the expression of protein fractions under different environmental conditions. Consequently, trials need to be conducted at various localities, soil types and rainfall conditions to determine the adaptability of a wheat line.

Scientists identified protein sub-fractions that are detrimental to desirable dough properties. Electrophoresis makes it possible to detect these fractions in order to exclude such a parent from the crossing block; at least one parent should contain the fractions that are linked to good quality (Figure 1).

High-performance liquid-chromatography (HPLC) separates and quantifies protein fractions (Figure 2). HPLC enables scientists to identify wheat lines with a stable expression over different environments.

Higher nitrogen application favours more gliadin formation and drought increases the glutenin to gliadin ratio. Specific glutenin fractions buffer the negative impact of high temperatures during grain filling on dough properties. However, selection of these fractions needs to be carefully considered, because of the negative impact on dough properties during a season with average temperatures and rainfall. Scientists worldwide suggested the ideal would be to develop wheat cultivars for specific environments regarding wheat quality.