Protein Processing in Food and Bioproduct Manufacturing and Techniques for Analysis - Food and Industrial Bioproducts and Bioprocessing - page 85

COO - COO -

| |

NH 3 +

+ H 3 N

H C

|

C
|

H

R R

D-amino acid L-amino acid

Figure 3.2 Conformation of D and L amino acids.

monomers, sometimes called “subunits”, associate to form complex structures stabilized by
non-covalent bonds.
Amino acids may be classified as acidic, basic, sulfur containing, hydroxylic, aliphatic,
hydrophobic or amidic (Table 3.1). The number and type of amino acids in a given protein
determine, respectively, its size and net charge at different pHs. Proteins vary in molecular
mass, ranging from <10 000 Da to >1 million Da, depending on the number of amino acids
contained within the protein. Environmental conditions, such as pH and the presence of
salts, can alter the charge state of amino acid residues and, consequently, the net charge of
the protein. The isoelectric point of a protein is defined as the pH at which the net charge on
the protein is zero. These two properties of proteins (i.e. charge and size) play a critical role
in the technologies and processing conditions used for their separation.
Another property of food proteins that is of interest in processing is their solubility.
Osborne (1924) classified proteins based on their solubility as follows: water soluble
proteins (albumins), salt soluble proteins (globulins), alcohol soluble proteins (prolamins)
and acid and alkali soluble proteins (glutenins). The solubility of proteins under aqueous
conditions can be further modified by heat treatment, hydrolysis and the presence of protein
structure perturbing chemical reagents. Depending on the extent of heat treatment, proteins
can unfold, exposing more hydrophilic amino acids, which can enhance solubilization.
In contrast, harsh heat treatment can result in extensive denaturation, association and
irreversible aggregation, which can reduce protein solubility. Hydrolysis of proteins using
chemical agents, such acid or alkali or enzymes, can break up proteins into smaller fragments,
which can increase their solubility.

3.3 PROTEIN SEPARATION PROCESSES IN FOOD
AND BIOPRODUCT MANUFACTURING

Proteins can be processed to obtain enriched flours, concentrates or isolates for various
food and bioproduct applications. In this chapter, the term bioproduct is used to indicate
functional foods, nutraceutical products, cosmetic products and other industrial (non-food)
products.
In general, protein flours may contain up to 65% w/w protein on a dry basis (db), whereas
concentrates and isolates contain >65% (w/w, db) and >80-90% (w/w, db) protein, respec-
tively. Microbiological and quality standards will vary for different applications.
Proteins used in the food and bioproduct sectors are derived from either animal or plant
sources, which vary significantly in their lipid, carbohydrate and protein content. Proteins of
interest may be separated using dry or wet processing techniques. Dry processing primarily
involves air classification and is frequently applied to plant materials such as cereals
(e.g., wheat) and grain legumes (e.g., peas, chickpeas, lentils) containing high amounts of
starch and protein. Wet processing has several processing steps. Some of the major unit