Serial batch extraction of zein from milled maize

Abstract

Prolamine-rich, water insoluble proteins (zeins) can be extracted from milled maize by vigorous mixing in heated ethanol solutions. Whenever solvent extraction is used process cost considerations require that all the solvent be recovered. Because of the low zein content of maize rinsing extract liquid from the extracted maize particles must be done in a way that minimizes dilution. The solid mass fraction of milled grain slurry that can be pumped is 0.25 or less. Because the mass fraction of zein in maize is only approximately 0.05, the zein mass fraction of a batch extract will be less than 0.015. To increase the zein concentration, a batch extract (liquid and fines) can be repeatedly separated from the extracted solid particles and used to extract fresh milled grain. A series of batch extractions with extract reuse can approach the performance of a continuous counter-current solids/liquid extraction, which would be preferable at a commercial scale. Extract reuse is constrained by losses of liquid with the extracted grain and by reductions of the extracting capacity of the extract due to the increasing solute or fines content. By examining the extract composition and yield of a series of batches, it is possible to estimate the zein concentration that could be achieved in a continuous, countercurrent process and to examine effects of higher zein concentrations on extraction that would be inaccessible with a single batch. The centrifugate concentrations for a series of maize extractions in which the extracted maize and extract solution were cooled prior to centrifugation were analyzed. The data were fit with a model based on a maximum zein concentration in the extract. The fit indicates that the protein content of the liquid centrifugate will not exceed 2% for any series of similar batch extractions, by using centrifugation to separate the maize from the extraction slurry after cooling it to ambient temperature. This contrasts with concentrations of zein of 10% or more achieved by extracting corn gluten using similar conditions. Although the concentration of zein in gluten is higher we believe the concentration difference is mainly due to chemical changes to the zein that take place in the gluten production and the methods used to extract zein from gluten.