Effect of γ-Zein on the Rheological Behavior of Concentrated Zein Solutions

Zein, the prolamin of corn, is attractive to the food and pharmaceutical industries because of its ability to form edible films. It has also been investigated for its application in encapsulation, as a drug delivery base, and in tissue scaffolding. Zein is actually a mixture of proteins, which can be separated by SDS-PAGE into alpha-, beta-, gamma-, and delta-zein. The two major fractions are alpha-zein, which accounts for 70-85% of the total zein, and gamma-zein (10-20%). gamma-Zein has a high cysteine content relative to alpha-zein and is believed to affect zein rheological properties. The aim of this study was to investigate the effect of gamma-zein on the often observed phenomena of zein gelation. Gelation affects the structural stability of zein solutions, which affects process design for zein extraction operations and development of applications. The theological parameters, storage modulus (G') and loss modulus (G ''), were measured for zein solutions (27% w/w solids in 70% ethanol). beta-Mercaptoethanol (BME) was added to the solvent to investigate the effect of sulfhydryl groups on zein rheology. Modulus data showed that zein samples containing gamma-zein had measurable gelation times under experimental conditions, contrary to samples with no gamma-zein, where gelation was not detected. Addition of BME decreased the gelation time of samples containing gamma-zein. This was attributed to protein unfolding. SEM images of zein microstructure revealed the formation of microspheres for samples with relatively high content of alpha-zein, whereas gamma-zein promoted the formation of networks. Results of this work may be useful to improve understanding of the rheological behavior of zein.