Protein Processing in Food and Bioproduct Manufacturing and Techniques for Analysis - Food and Industrial Bioproducts and Bioprocessing - page 89

or 1:25 soy flour to water ratio by weight) than when using lower ratios (i.e., 1:12 or using
a double extraction at 1:10 followed by 1:6). In the former case, extraction at 60 °C for
30 min at pH 6.6, 8.0 and 9.0 gave nitrogen recoveries of 91.4, 94.2 and 89.3%, respectively.
In the latter case (1:12 flour to water ratio) the extraction was done at pH 9.0 and a recovery
of 80.4% nitrogen was obtained, whereas for the double extraction (1:10 followed by 1:6)
at pH 2.5, 81.8% nitrogen recovery was obtained. The authors indicated that water ratios
influenced nitrogen extractability more significantly than pH.
pH and temperatures used during alkaline extraction must be carefully chosen to avoid
extensive denaturation as well as the development of by-products, such as lysinoalanine
(N6-(DL-2-amino-2-carboxyethyl)-L-lysine), an unusual amino acid implicated as a renal
toxic factor in rats. Lysinoalanine has been found in proteins of home-cooked and commercial
foods and ingredients and was initially thought to occur in both edible and non-food proteins
only after alkali treatment. However, some reports have shown that it can be generated in
various proteins when heated under non-alkaline conditions (Sternberg et al ., 1975 ).
Acid extraction
The solubility of some proteins increases under acidic conditions (i.e., pH <4). This low
pH range can, therefore, be used to solubilize proteins prior to their recovery. The principle
of acid extraction is similar to that of alkaline extraction, except that the initial protein
extraction is conducted under acidic conditions. The acid extraction technique is generally
used less frequently than the alkaline extraction technique and, as with alkaline extraction,
processing conditions can influence the yield and purity of the finished product. Using
this technique, Alli and co-workers (1993) extracted a bipyramidal crystalline protein
preparation from dried seeds of white kidney bean ( Phaseolus vulgaris ) by extracting
ground seeds with citric acid solution (0.4 N, pH 4.0) followed by refrigeration (4 °C, 18 h)
to precipitate the protein material. The isolate obtained contained 95.7% protein.
Salt extraction
The selective extraction of proteins in aqueous solutions having different ionic strengths can
be used for their fractionation and separation. The process is based on the salting-in and
salting-out phenomenon of food proteins.
At low molarities (0.5-1 M), ions of neutral salts promote the solubilization of proteins
(“salting in”). Interactions between the ions and charges of proteins reduce electrostatic
attractions between protein molecules enhancing their solubility. Additionally, the hydration
of the ions increases the solvation of the proteins contributing to increased solubility.
At higher salt concentrations (>1 M), competition between salts and proteins for available
water forces the proteins to precipitate (“salting out”). This phenomenon can, therefore, be
used for protein recovery as explained later.
For salt soluble globulins as per the Osborne classification, addition of salts to the extracting
medium facilitates their solubilization. After extraction, extensive dilution of the solution
can cause these globulins to precipitate out of solution especially at low temperatures
leading to their fractionation.
Foam fractionation of proteins
Foam fractionation (or separation) is an adsorptive bubble separation technique in which
soluble, surface-active substances can be removed from solution by preferential adsorption
at the gas-liquid interface (Wang and Liu, 2003). Proteins contain both hydrophilic and
hydrophobic amino acid residues that are surface active. During foam formation, bubbles