The influence of atmospheric CO₂ concentration on the protein, starch and mixing properties of wheat flour

Abstract

Wheat (Triticum aestivum L.) cultivars Hartog and Rosella were grown at CO₂ concentrations of 280 µL L^-1 (representing the pre-industrial CO₂ concentration), 350 µL L^-1 (ambient) and 900 µL L^-1 (an extreme projection of atmospheric CO₂ concentration). The plants were grown in naturally lit glasshouses in 7 L pots containing soil to which basal nutrients had been added and the pH adjusted to 6.5. Hartog yielded 2.4 g of grain per plant when grown at 280 µL CO₂ L^-1. This yield was increased by 38% and 75% at CO₂ concentrations of 350 µL L^-1 and 900 µL L^-1 respectively. These changes were due to increases in both grain number and individual grain weight as the level of CO₂ was raised. The yield of Rosella was unaffected by altering the CO₂ concentration. Increasing the CO₂ concentration reduced grain protein concentration of cv. Hartog from 17.4% at 280 µL CO₂ L^-1 to 16.5% and 16% at CO₂ concentrations of 350 µL L^-1 and 900 µL L^-1 respectively. The grain protein concentration of cv. Rosella was reduced from 10.7% to 10.2% by increasing the CO₂ concentration from 280 µL L^-1 to 350 µL L^-1; however, an additional increase in the CO₂ concentration to 900 µL L^-1 had no effect on grain protein concentration. In Hartog flour, the highest proportion of polymeric protein in the flour (7.7%) occurred at 280 µL CO₂ L^-1. This was reduced to 6.3% at 350 µL CO₂ L^-1 but then increased again to 7.0% at 900 µL CO₂ L^-1. These changes in concentration of polymeric protein were correlated (r²=0.58) with changes in mixing properties. The mixing time required to produce optimum dough strength was greatest at 900 µL CO₂ L^-1 (181 s), then 141 s and 151 s at 350 µL CO₂ L^-1 and 280 µL CO₂ L^-1 respectively. These changes in mixing time could not be explained by changes in grain protein concentration. The proportion of ‘B’ starch granules (<10 m diameter) increased from 25% of total weight of starch at 280 L CO₂ L^-1 to 30% at CO₂ concentrations 350 and 900 µL L^-1. There were generally no effects of CO₂ concentration on dough mixing properties or starch granule size distribution for Rosella.