Biomedical Engineering Reference
In-Depth Information
(Mustakas, 1980; Serrato, 1981). The material obtained for further downstream processing
will vary depending on the process used and, in the case of soybean, could be a full fat
soybean meal or flour, defatted meal (untoasted, mildly toasted or fully toasted). The toasting
process refers to the heat treatment applied to remove residual solvent after hexane extraction.
Both mechanical and solvent extraction techniques have their limitations (Russin
et al
.,
2010). Mechanical extraction, or pressing, is limited in its applicability, particularly with
low oil content oilseeds such as soybean. Elevated temperatures employed during pressing
can also have a deleterious effect on the quality of the extracted oil and residual meal
(Sugarman, 1956 ; Nelson
et al
., 1987). Hexane extraction has many economic, environmental
and safety limitations. Economically, one of the main concerns is the stability of both
hexane supply and price due to fluctuation in the fossil fuel market (Friedrich and List,
1982 ; Lusas
et al
., 1990 ; Gandhi
et al
., 2003 ; Russin
et al
., 2010 ). Concerns also exist about
the environmental impacts of hexane use and its toxicity.
As an alternative, aqueous techniques for fat extraction are being explored. In this case,
after communition the full fat soy material is solubilized to perform a solid-liquid extraction/
separation. During this step, insoluble compounds are removed, leaving a liquid solution
containing both proteins and lipids. This solution is further separated by three-phase
centrifugation to yield a solid, an aqueous and an oil/emulsion phase, each of which can be
further processed downstream (Russin
et al
., 2010). Enzymes (e.g., lipases, cellulases) may
be added during this process to breakdown fat and carbohydrate components to facilitate
protein extraction (Russin
et al
., 2010 ). The main principle of enzyme-assisted extraction
is the use of enzymes which damage and/or degrade plant cell walls, so increasing the
permeability of the oil in the oilseed (Domínguez
et al
., 1994 ). The two main approaches
include the use of single and mixed enzymatic systems. The latter has increased utility,
given that the mixed systems allow for various enzymes to simultaneously act on the cellular
structures, leading to a more effective release of oil (Fullbrook, 1984; Domínguez
et al
.,
1993 ; Russin
et al
., 2010). The use of lipases or phospholipases to breakdown fats,
particularly in high fat aqueous extraction systems where emulsions are likely to occur, is of
interest; however, adequate care must be taken to minimize oxidation during processing, as
this could result in the generation of off-flavors in the finished product.
3.3.2.2
Protein extraction
Alkaline extraction
Aqueous alkaline extraction is one of the most commonly used techniques for protein extrac-
tion; it takes advantage of the solubility of proteins at alkaline pH. In this process the prepared
biomass, which may be full fat, partially defatted, or fully defatted, and in the case of plant
materials may or may not contain fibrous materials such as hulls, is dispersed in water using
flour:water ratios ranging from 1:5 to 1:20. The pH of the mixture is adjusted to alkaline
(pH 8-11) and the mixture is continuously stirred for 30 to 180 min to maximize protein
solubilization. During this time the pH is maintained at the desired value and the temperature
may be elevated (up to 55-65 °C) to further enhance protein solubilization and extraction.
The mixture is subsequently filtered to remove any insoluble material to yield a supernatant
containing the extracted proteins. Some extraction processes call for a second extraction of
the precipitate using similar pH as in the first extraction or higher in order to extract any
remaining proteins in the precipitate and increase protein recovery (Boye
et al
., 2010a ).
Lawhon and co-workers (1981a) found the extractability of protein in aqueous medium
(water) from full fat soy flour to be higher when using higher flour to water ratios (i.e. 1:30
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