Baking quality of organic wheat from different cropping systems
By Lene Pedersen, Jørgen Eriksen and Ingrid K. Thomsen, Danish Institute of Agricultural Sciences
Achieving high quality bread wheat is important to increase the consumption of organic bread products. For wheat production, N accessibility during growth and grain development is critical in relation to protein accumulation in the wheat kernels. Moreover, the availability of N must be optimised in relation to wheat growth without enhanced loss of N to the environment. Therefore, the interactions between the crop varieties, the N management in organic cropping systems and the bread-making quality have been subject to investigation.
Bread-making quality of wheat is mainly related to the protein and gluten content of the grain and flour. However, also the protein and gluten composition has a great impact on the dough properties (water absorption, mixing stability), CO2 retention capability, and the bread volume. The amount and composition of proteins are affected by the genetic background, N-application, and environmental factors. Therefore, when studying effects of N management and cropping systems, the quality assessment of wheat must include determination of both the quantity and the quality of gluten.
The quality of spring wheat was studied in samples from field experiments at Askov (in 2001) and Foulum (in 2002). After harvest, the grain was milled with a flour yield of 60 – 70%. The gluten was extracted from the flour by washing with a dilute NaCl-solution and the gluten content was determined as % (w/w) of the flour. Gluten composition was studied by measuring the elastic and viscous properties of the gluten using rheological oscillation at small deformations of the gluten. This method gives a fast indication of the ratio of gliadins to glutenins in the flour, as gliadins (low molecular weight) contribute to the viscous properties and gliadins (high molecular weight) to the elastic properties of the gluten and the dough. The bread volume was measured from standard baking tests of the flour.
In Foulum, differences in bread-making quality were compared due to residual effect of grassland, organic or mineral fertilizer application. Spring wheat (cv. Leguan) sown after ploughing of grazed grass-clover pastures was supplied with 0, 115 or 230 kg total-N ha-1 in slurry. Reference spring wheat grown after oat and with mineral N-application (0-150 kg N ha-1) was included.
Spring wheat grown after ploughing-in of grazed grass-clover pastures produced grain with medium protein and gluten levels (Figure 1). Without N-application and grassland residual effect as the sole source of N, the grain had 12.0% protein and 23.5% gluten in the flour, which are suitable values for good bread-making properties. Application of N (115 or 230 kg total N ha-1 in slurry with approximately 55% NH4-N) increased protein, gluten and the total protein accumulation. However, the application of slurry had almost no effect on the bread volume. The quality of the spring wheat supplied with mineral fertilizer was in the lower range, having 9.5 to 12.8% protein and 12.4 to 27.4% gluten (Figure 1). Protein and gluten in grain following 125 kg mineral N ha-1 application corresponded to the values following grass-clover with no addition of N (12.0% protein and 23.5% gluten). The grain yield for 125 kg mineral N ha-1 was slightly higher compared to the grass-clover with no N-application (5520 and 4850 kg ha-1, 15% moisture). However, rheological properties of gluten from these two treatments differed considerably. Gluten in wheat following grass-clover had a higher extensibility and recovery and was more viscous, and the bread volume was slightly higher, 420 mL following grass-clover and 390 mL following 125 kg mineral N ha-1.
In the Askov experiments spring wheat (cv. Vinjett) was grown with application of cattle slurry or liquid manure in rates of (0, 75, 150, or 225 kg total N ha-1). Results from Askov illustrated the difference in baking quality for the two types of organic fertilizer (Figure 2). Protein and gluten content increased for increasing addition of N up to 150 kg N for slurry (SLU), and 225 kg N for liquid manure (LIQ). However, there was significantly higher protein and gluten in the grain fertilized with LIQ, probably explained by higher content of mineral N in this manure. The difference in protein and gluten content was not reflected in the baking results (Figure 2). The bread volume was equal for SLU and LIQ with application of 75 and 150 kg N ha-1, whereas 225 kg N ha-1 gave a higher volume for LIQ. This indicates that the baking quality of the protein and gluten produced from SLU was higher than for LIQ with low input of N. At high N-levels (225 kg N ha-1), the difference in gluten between the two fertilizers agreed with the bread volume. Therefore, protein and gluten contents are not the only quality parameters predicting baking quality of organic wheat, especially during limited supply of N. This is also illustrated when comparing two treatments with almost the same protein and gluten, SLU (150 kg N ha-1, 18.6% gluten) and LIQ (75 kg N ha-1, 17.5% gluten). The bread volume was 580 mL and 475 mL, respectively. Rheological measurements of the gluten from these two treatments showed that gluten from SLU was softer and more viscous than gluten from LIQ. See the result of the baking test.
The results from these two experiments indicate that in organic bread wheat cropping, the relationships among cropping system, the amount of protein and gluten in the grain and flour, and the baking results are complex. The protein and gluten contents are not sufficient for prediction of the baking quality, as the gluten properties and the following bread volume seem to be affected considerably by the N source.
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