Kafirin viscoelastic masses : factors influencing their formation and rheological behaviour

Abstract

Sorghum is the world‟s fifth most important cereal crop. It is a drought–tolerant and widely cultivated in Africa. Therefore, the use of locally produced sorghum in bread products would be advantageous to farmers and consumers in Africa. However, sorghum flour does not produce wheat flour-like doughs as kafirin, its prolamin storage protein does not exhibit viscoelastic properties like wheat gluten. Recently, kafirin viscoelastic mass formation by coacervation with water from glacial acetic acid has been reported. The objective of this study was to determine how various factors influence the functionality of kafirin viscoelastic masses with the aim of producing kafirin-based doughs with similar rheological characteristics as gluten-based doughs. Firstly, the effects of various different extraction solvents on kafirin composition and structural conformation were investigated. All the kafirin preparations formed viscoelastic masses by coacervation, regardless of the extractant used. FTIR data indicated that kafirin viscoelastic mass formation did not depend on the secondary structure of the protein. Secondly, the effects of kafirin and zein (maize prolamin) composition from various sources on their rheological properties were also investigated using stress-relaxation and dynamic rheological analyses. Kafirins and zeins from all sources studied formed viscoelastic masses. Kafirin masses were much firmer and had a much higher elastic component than zein masses, which had predominantly viscous flow properties. Both were softer than gluten. Maintenance of kafirin elasticity when stored appears to require γ-kafirin. Thirdly, the effects of final acetic acid and protein concentration during the coacervation process were investigated, with the aim of producing food-compatible functional masses. Coacervation with reduction in the final acetic acid concentration down to 0.1% still allowed formation of kafirin and zein viscoelastic masses with functionality retained when stored at 4oC for an extended period; indicating an irreversible molecular change with dissolution in glacial acetic acid. A minimum of between 5 and 10% prolamin in glacial acetic acid was required for viscoelastic mass formation at low final acetic acid concentration (5%). Kafirin displayed a similarly high elastic component to gluten, whereas zein exhibited more viscous flow properties. A model is proposed to explain these behaviours. When the masses are compressed, the force is sufficient to break hydrogen bonds but the strong covalent disulphide bonds will remain intact. During compression, zein masses will deform more than kafirin and more energy will be dissipated, whereas kafirin with its higher number of disulphide bonds will exhibit greater resistance to compression, and more energy will be stored. On removal of the force, the kafirin mass will release the stored energy and recover almost to its original shape, with hydrogen bond reformation. However, zein with its lower number of disulphide bonds, will release insufficient energy to return to its original shape. Both kafirin and zein proteins may be composited to obtain a viscoelastic mass with the desired balance of elasticity and viscous flow properties as gluten.