Protein Processing in Food and Bioproduct Manufacturing and Techniques for Analysis - Food and Industrial Bioproducts and Bioprocessing - page 87

For optimal separation efficiency, the cotyledons of seeds must be finely ground to
achieve complete cellular disruption and maximum protein and starch separation during
air classification (Tyler and Panchuk, 1982). The milling technique used must, therefore,
be capable of producing a very fine grind, yet selective enough to break up cells and cell
fragments without severely damaging the starch granules (Jones et al ., 1959 ). A portion of
the protein derived from the membranes and stroma of the choloroplasts in which the starch
granules developed cannot be milled free of the starch granules despite repeated milling
(Tyler, 1984). The purities of the coarse (starch) and the fine (protein) fractions obtained by
air classification are, therefore, often lower than what may be obtained by wet extraction.
Amongst others, high protein flours (up to 75% purity in some cases) have been success-
fully produced from wheat, soybean, beans, lentils, chickpeas and peas using air classification
(Wu and Strongfellow, 1979 , 1981 ; Tyler and Panchuk, 1982 ; Wolf et al ., 2002 ).

3.3.2 Wet processing
3.3.2.1 Material preparation

Animal by-products
Animal by-products (e.g., blood, skin, bones and offal) may serve as useful sources of
biomass for harvesting high-value proteins. Prior to extraction, proteins are solubilized by
disruption of the cells and tissues retaining them. Protein solubilization is a major critical
step affecting yield and quality of the extract. Methods available for tissue disruption include
grinding, homogenization and sonication. Mechanical disruption using colloidal mills, in
which the biomass is fed through a rotor-stator, high speed dispersion mills or bead mills,
which break down cell walls through their tumbling action, may also be used. In some
instances enzymes (e.g., zymolase, lysozyme, and lysostaphin) may be added to facilitate
tissue disruption followed by homogenization, sonication or vigorous vortexing.
Equipment selection will depend on the specific material being processed and the scale
(analytical, pilot or industrial). As animal materials frequently contain proteases that can
hydrolyze proteins of interest and decrease their functionality, appropriate precautions are
required to slow down or prevent these reactions (e.g., thermal inactivation or processing at
cold temperatures). After the appropriate level of communition has been attained, a filtration
step can be used to recuperate the supernatant containing the desired proteins for further
downstream processing.

Plant by-products
Plants contain high amounts of fiber and cellulosic material that needs to be removed prior
to protein extraction. Some plant materials are also high in fat, making a defatting step
necessary prior to protein extraction. Effectively disintegrating plant materials to obtain
particle sizes that allow for maximum fat and protein extraction is an important first step in
processing. This is best done by beginning the processing with a milling step. Examples of
milling equipment used in research and in industrial settings include centrifugal mills,
hammer mills, ball mills, roller mills and disc attrition mills. De-hulling and milling can be
done as a single unit operation or as two separate unit operations depending on the type of
equipment used and the material being processed (i.e., ease of de-hulling).
Materials such as soybeans seeds, which are high in fat (17-23%), may require a defatting
step. The techniques most commonly used are solvent extraction using hexane, mechanical
extraction and aqueous extraction. Details of these processes are summarized elsewhere