Formation of a viscoelastic dough from isolated total zein (α-, β- and γ-zein) using a glacial acetic acid treatment

Abstract

Abbreviations: 2D-PAGE -Two dimensional polyacrylamide gel electrophoresis, CLSMConfocal laser scanning microscopy, FTIR -Fourier transform infrared spectroscopy, T gglass transition temperature 1 AbstractOnly predominantly α-type zein (commercial zein) has been shown to form a viscoelastic dough. In maize zein comprises α-, β-, γ-and δ-zein (total zein). Because glacial acetic acid can fully solubilise zein, its effect was investigated. Dissolving total zein (comprising α-, β-, γ-and δ-zein)_ in glacial acetic acid and casting a film, enabled a viscoelastic dough to be formed with water above zein's T g . The dough was stronger and less extensible than commercial zein dough made without film formation. When residual acetic acid was removed from the total zein film, a dough still formed.. CLSM showed that the total zein dough fibrils were shorter and less well-aligned than those of commercial zein dough and appeared as particles. Disulphide bond cross-linking was probably responsible, for total zein dough stiffness. FTIR showed that total zein wet doughs and total zein slurry with water were predominantly β-sheet, indicating that β-sheet conformation was not directly responsible for dough formation. It is suggested that acetic acid brings about chemical changes in zein, enabling it to better interact with water molecules, counteracting disulphide bonding effects, allowing total zein to form a dough. This is the first report of viscoelastic dough formation from total zein. 2

Comparison of formation of visco-elastic masses and their properties between zeins and kafirins

“…All other zein and kafirin preparations had similar but slightly lower ratios, ranging from 56-58% α-helix for the zeins and 53-54% α-helix for the kafirins. These values are similar to the published value of 56.7% for zein containing all classes (King et al, 2016) and slightly higher than the value for kafirin of approximately 49% (Xiao, Li, Li, Gonzalez, Xia, Q.,& Huang2014). However, these values give no insight into changes of secondary structure that may be occurring on visco-elastic mass formation.…”

“…It is suggested that when the prolamin preparations were dissolved in glacial acetic acid, the proportion of α-helical conformation present would be increased as shown by Elhassan et al (2018) and then reduced when water was added during the coacervation process. This is similar to the effect found when total zein films were cast from glacial acetic acid and then made into visco-elastic masses with water (King et al, 2016). The proportion of β-sheet was slightly reduced when the protein was dissolved in glacial acetic acid and cast into a film and slightly increased again when the residual glacial acetic acid was removed.…”

“…It found no evidence of hydrolysis or polymerisation caused by acid but FTIR revealed small conformational changes possibly due to deamidation. A study on zein containing all the zein sub-classes (α-, β-, -, δ-) showed that visco-elastic masses (referred to as dough) could only be formed in water above zein's T g , after it had been dissolved in glacial acetic acid and cast into a film (King, Taylor, & Taylor, 2016). This visco-elastic mass was stronger and less extensible than commercial zein visco-elastic mass made without film formation, probably due to disulphide bond cross-linking.…”

Making Kafirin, the Sorghum Prolamin, into a Viable Alternative Protein Source

“…The fact that the kafirin extracted in 83% isopropanol was probably low in β‐ and γ‐kafirins and the presence of mercaptoethanol had a positive effect on dough formation suggested that extensive disulfide bonding was not desirable for kafirin dough functionality. This is supported by the finding that isolated total zein containing α‐, β‐, γ‐, and δ‐zein did not form a viscoelastic mass when mixed with water, unlike with commercial zein (predominantly α‐zein; King et al, ). The authors found that, by first completely solubilizing the total zein in glacial acetic acid and then drying the zein into a film, a viscoelastic mass could be formed when it was mixed with water.…”

“…Kafirin and zein, its maize homolog, are also very inert proteins even among the cereal prolamin proteins, with kafirin being rather more inert than zein (Schober et al, ). For example, only commercial zein (a zein preparation with a low level of γ‐zein subclass) will form a viscoelastic wheat gluten‐like mass when kneaded with water (King, Taylor, & Taylor, ). Zein preparations with all the subclasses and kafirin require pretreatments before they will form viscoelastic masses (see “Shear”).…”