Protein Processing in Food and Bioproduct Manufacturing and Techniques for Analysis - Food and Industrial Bioproducts and Bioprocessing - page 94

with membrane separation processes such as MF and UF is fouling of the membranes during
processing, as this can affect membrane selectivity.
MF and UF have been used extensively to separate proteins from milk, soybeans, peas,
chickpeas, lentils amongst others (Romero-Baranzini et al ., 1995 ; Cheryan, 1998 ; Fredrikson
et al ., 2001 ; Boye et al ., 2010a ).
Cryo-precipitation
The solubility of some proteins decreases significantly as the temperature is reduced below
room temperature, a property that can be selectively used for their recovery. Cryo-
precipitation refers to the method of protein precipitation under refrigerated conditions or
from freezing and cold thawing.
Melcher and Fraij (1980) successfully used cryo-precipitation at -20 °C to separate a zein II
extract obtained from corn meal into a methionine-rich polypeptide fraction (cryo-precipitate)
and a cryo-supernatant containing predominantly polypeptides with amino acid composition
similar to that of the zein 1 proteins. The cryo-precipitate was readily dissolved in 70% ethanol
containing mercaptoethanol and precipitated again on cooling, indicating that the cryo-
precipitaion was reversible.
In medical terms, a cryo-precipitate is defined as a concentrate of high molecular weight
plasma proteins which precipitate in the cold (Pantanowitz et al ., 2003 ). Various researchers
have used the technique to recover cryoglobulins, a heterogenous group of immunoglobulins
(monoclonal and polyclonal) which precipitate or gel at reduced temperature (e.g., 4 °C) and
redissolve on heating to 37 °C (Weber and Clem, 1981; Wang, 1988). The purified proteins
exhibit crystalline, amorphous or gelatinous structure as a result of their differing primary
structure and physicochemical properties. All major immunoglobulin classes (IgM, IgG,
IgA) and their subclasses have been found in cryo-precipitates, including monoclonal
immunoglobluins, mixed immunoglobulins and polyclonal immunoglobulins. Other pro-
teins found in plasma that are insoluble at cold temperatures include cryofibriniogen,
heparin precipitable protein and aggregates of C-reactive protein and albumin (Stathakis
et al ., 1978 ; Shanbrom, 1980 ; Wang, 1988 ; Coelho and Wolf, 1991 ).
Francis and co-workers (2000) found that frozen saliva samples contained variable
amounts of precipitate on thawing. The cryo-precipitates comprised of low molecular
weight proteins (<14 KDa), which were either absent or present at reduced concentrations
in the supernatant.
Cryo-precipitation is, therefore, a promising technique for the recovery of high value
proteins and is a technique that is likely to be increasingly explored in the food, nutraceutical
and pharmaceutical sectors.
Electrodialysis using bipolar membranes
Proteins can be preferentially precipitated from solution using a technique known as
electrodialysis coupled with the use of bipolar membranes. The application is based on the
specific properties of electrolysis (redox reactions at the electrodes) coupled with membrane
action (Bazinet et al ., 1998) resulting in either electro-acidification or electro-alkalinization.
A bipolar membrane is a composite membrane consisting of a cation exchange layer and an
anion exchange layer which can split solvents (water and methanol, so far) into H + and OH - /
CH 3 O - at the interface under reverse potential bias (Xu and Huang, 2008).
For a solution circulated into an electrodialysis cell on the cationic side of the bipolar
membrane, where the H + are generated, the pH of the solution will decrease. Similarly,
a solution circulated on the anionic side of the bipolar membrane, where OH - are generated,