Properties of gluten from organic wheat measured by oscillation

Summary

Introduction
The bread making potential of wheat flour is largely related to the gluten protein. It has until now been stated that the genotype (cultivar) is the most important factor for the gluten properties and protein composition, whereas environmental factor like fertilisation has a great impact on the quantity of gluten proteins (Daniel and Triboi, 2000). In organic farming, there seems to be a stronger interaction between genotype and environment on the gluten properties and the baking properties, due to the restricted nutrient availability. Therefore, the influence of environmental factors, and especially the influence of fertilisation in organic farming must be investigated in order to grow organic bread wheat with good quality.
The importance o the wheat gluten is due to its unique viscoelastic properties. Rheological characterisation of the gluten protein network is essential to explain differences due to genotype and/or environmental factors. The dynamic rheological parameters (G', G'', and d) differ between cultivars with good and poor bread making quality. Differences in rheological parameters are mainly related to the ratio between the gliadins and glutenins (Khatkar et al., 1995, 2002). As some environmental factors influence this ratio, fundamental rheological testing may be valid for the testing of the influence of environmental factors on the gluten properties.
Experiments
At The Askov Long-term Experiments on Animal Manure and Mineral Fertilizer, an organic farming workshop was established in 1996 with the aim to study the influence of the nutrients. Three nutrients levels are compared with unmanured treatments. The spring wheat cultivar Vinjett was grown with application of two organic manures, slurry (GY) and liquid manure (FG), in three N-levels (0, 75, and 150 kg N/Ha). Additionally, FG was added at 225 kg N/Ha. Wheat grain was milled, and the gluten was isolated using a Glutomatic 2100 (ICC standard 137). The gluten was measured in oscillation in a frequency range of 0.05 Hz to 20 Hz. The stress amplitude was 100 Pa.
Results
Frequency sweeps from two samples are shown in Figure 1. In the gluten elastic behaviour is dominating. The modulus G' and G'' increased for increasing frequency. The slope (log G'/log w) was slightly different for the two samples, 0.20 for the sample with no manure added, and 0.22 with liquid manure (75 kg N/Ha). However, the phase angle d increased with increasing frequency up to 20 Hz.
Increasing levels of N increased the gluten content of the flour (Fig.2.). Application of liquid manure resulted in higher gluten contents at all N-levels. The viscoelastic behaviour of gluten in relation to type of manure and level of N is shown in Figure 3. The modulus seemed to be influenced by the type of manure, and by the level of N. There was a small increase in G' and G'' when N-level was increased from 0 to 75 kg N/Ha for liquid manure (FG), and similar for the slurry (GY) between 75 and 150 kg N/Ha. N-levels beyond these values decreased G' and G''. At the lowest N-level (75 kg N/Ha), there was a difference in G' and G'' between the two types of manure. The N-level had no effect on d for the slurry, whereas a slightly increase in d was observed for the liquid manure.
These results indicate, that the type and level of manure have an impact on the amount of gluten protein and the viscoelastic properties of gluten. The variability in the moduli in relation to N-management is probably explained by differences in the protein composition, and mainly the ratio gliadins/glutenins (Khatkar et. al., 1995, 2002)
Conclusions
Oscillatory measurements of gluten from one wheat cultivar with different application of manures indicated that type of manure, and the level of N influenced the viscoelastic properties of gluten. Thus, measurements of G', G'' and d may be valid in the studies of the influence of growth conditions on the baking quality of wheat.
References
Daniel, C. and Triboi, E. 2000. Effects of temperature and nitrogen nutrition on the grain composition of winter wheat: Effects on gliadin content and composition. Journal of Cereal Science 32 45-56.
Khatkar, B. S., Bell, A. E. and Schofield, J. D. 1995. The dynamic rheological properties of glutens and gluten sub-fractions from wheats of good and poor bread making quality. Journal of Cereal Science 22 29-44.
Khatkar, B. S., Fido, R. J., Tatham, A. S. and Schofield, J. D. 2002. Functional properties of wheat gliadins. II. Effects on dynamic rheological properties of wheat gluten. Journal of Cereal Science 35 307-313