The only treatment for coeliac disease, a common autoimmune disorder, is life-long adherence to a gluten-free diet. However, the replacement of wheat gluten, a key structural and functional component in bread, poses a major technological challenge for food scientists. The use of non-wheat cereal proteins, as alternatives to gluten, shows much promise in gluten-free bread making. Literature has shown that when zein, the maize prolamin protein, is subjected to wet heat above its glass transition temperature (Tg), the protein becomes viscoelastic, rubbery and dough-like. Gluten-like fibrils are visible, which form complex protein networks similar to those found in wheat dough. The resulting zein dough has viscoelastic characteristics and can be successfully used with hydrocolloids to produce gluten-free bread.
This project examined the influence of wet heat treatment and dilute organic acids (lactic acid and acetic acid) on the dough-making quality of non-wheat cereal proteins, such as kafirin and zein. Zein was the only non-wheat cereal protein to show any physical change when it was subjected to wet heat treatments, forming a dough-like substance. Acidification of the zein dough prepared at 40°C with concentrations of 0.7, 1.3 and 5.4% (v/v) organic acid in distilled water solutions, showed that the higher the concentration of acid used, the greater its effect on the dough's rheological properties. Tensile tests using a Keifer rig on zein dough showed that as the concentration of organic acid was increased from 0.7 to 1.3 and to 5.4% (v/v) the dough become softer and increasingly more extensible. The dough also exhibited less resistance to extension and reduced elasticity. CLSM revealed that the zein doughs contained a protein network, made up of fine protein fibrils, which became smoother and more homogenous as the concentration of acid was increased. Although SDS-PAGE revealed that no oligomerization took place with acid addition,
FTIR showed that zein dough prepared with distilled water at 40°C had elevated levels of β-sheets. When organic acids were added in increasing levels, corresponding increases in the quantities of α-helices in the protein were observed. Alveography showed that zein-based doughs prepared with dilute organic acids retained gases well and that the concentration of dilute organic acids influenced dough distensibility (biaxial extensibility) and stability (the ability of the dough to retain gas). Low concentration of acids (0.7 and 1.3%) increased dough stability to levels similar to that of strong wheat flour, 103 mm H2O, but higher concentrations of acids (5.4%) led to a marked reduction in dough stability. Thus, by increasing zein dough functionality to such an extent, the apparent usefulness of the doughs and their ability to retain gases produced during fermentation is reduced. Simple distensibility tests on zein doughs showed that added organic acids promoted ‘clumping’ of the fine protein fibrils in the dough network into pronounced fibres. This would account for the decreased dough stability when high levels (5.4%) of organic acids were used. Baking trials with zein doughs were not successful as adequate leavening was impossible without an acid-tolerant leavening agent.
It is believed that dilute organic acids influence the rheological properties of zein dough by creating a positively charged environment, in which the protein is partially solubilized. The higher the level of organic acid used, the greater the positive net charge and the more pronounced the effect on the protein network structure. Organic acids could also improve fluidity of the zein dough by acting as plasticizers.
From this work it can be seen that although a protein network is present in all zein-based doughs, the ability of this network to retain gases is dependant on the level of organic acids present. The functional properties of zein-doughs made with low levels of organic acids (0.7 and 1.3%) shows potential in the production of gluten-free bread for individuals suffering from coeliac disease.