Food Lipids: Chemistry, Nutrition, and Biotechnology

phatides extracted are greater, as is the refining loss. However, current industry practice
is to strive for the most complete extraction possible. Typically, residual oil
contents range 0.5–1.0%.
Flake thickness [22–25] and solvent temperature [25,26] have profound effects
on extraction rate, and empirical relationships to extraction time have been observed
[27]. While these factors are easy to control by adept operators, general lack of
understanding and appreciation often exist in practice. The moisture content of the
flakes is another factor affecting the rate of solvent extraction [28]. In most cases,
9–11% moisture is ideal. Hexane and water are immiscible, and higher moisture
contents interfere with the penetration of hexane. Lower moisture levels reduce the
structural strength of the flakes leading to the production of additional fines.
To reduce the amount of solvent used in extractors, countercurrent flow of the
solvent to the flakes is used (Fig. 8) [29]. That is, the freshest flakes contact the
oldest solvent and progress through the process until nearly oil-free flakes contact
fresh solvent. Flakes enter the extractor through a plug vapor seal that allows the
material to enter while keeping hexane vapors from escaping. The extractor is an
enclosed vessel designed to wash, extract, and drain flakes.
Two principal types of extractor have been employed over the years: immersion
extractors and percolation extractors. An immersion extractor immerses and soaks
the material in solvent (an industrial example is the Hildebrandt U-tube extractor,
and a laboratory example is the Soxhlet extractor). Generally, more solvent usage is
required by immersion extractors. Few immersion extractors processing oilseeds remain;
percolation extractors now dominate. In a percolation extractor, the solvent
percolates by gravity through a bed of material (a laboratory example of a percolation
extractor is the Goldfisch). The solvent flows over the surface of the particles and
diffuses through the material during its downward circuitous travel. Miscella flows
Figure 8 Flake flow relative to solvent flow. (Redrawn from Ref. 29.)
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.