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lowest solubility at its iso-electric point so if there is a charge at the protein surface,
the protein prefers to interact with water, rather than with other protein molecules,
thus this charge makes it more soluble. The surfaces of proteins are occupied by
amino acid residues that interact with water these amino acids are referred to as
hydrophilic amino acids and include arginine, lysine, aspartic acid, and glutamic
acid. At pH 7 the side chains of these amino acids carry charges positive for arginine
and lysine, negative for aspartic acid and glutamic acid. As the pH increases, lysine
and arginine begin to lose their positive charge, and at pHs greater than 12 they are
mainly neutral. On the other hand when pH decreases, aspartic acid and glutamic
acid begin to lose their negative charge and it has been found that at pH less than 4
they are mainly neutral.
Gel Forming Properties
It is an important functional attribute of proteins which is related to food processing.
Many foods are in the form of gels and the main structural building element in such
type of foods is protein. In addition to proteins, pectin, starches and gums are also
associated in the formation of strong gels. The process of gelation is a basic fundamental for various types of food systems like milk gels, comminuted meat and fish
products, other meat products, cake fillings, fruit jellies, bread dough’s and others.
It is the main criterion which is frequently used to evaluate quality of proteins.
Many quality characteristics like adhesiveness, gumminess, juiciness and other textural properties are directly related to the gelling properties of proteins. Visco-elastic
properties of many foods are also related to gelation properties of food proteins. The
gelation properties also affect other functional properties of proteins like water
binding capacity, oil binding capacity, emulsifying capacity, emulsion stability etc.
It also plays a major role in stabilizing various types of emulsions and foams.
Protein gels can be formed by employing different type of approaches like heating,
enzymatic process, heating in combination with salts etc. Whey protein gels can be
obtained by heating which proceeds through a series of transitions like denaturation
of native proteins, aggregation of unfolded molecules, strand formation from aggregates or association of strands into a network. It should be noted that aggregates are
formed in the presence of salts which results in the formation of strong gels.
Likewise for soy proteins gelation process is obtained by heating soya bean flour or
milk followed by addition of salt (e.g. Ca
++
, or Mg
++
) to from a gel (Cayot and
Lorient 1997; Jong et al. 2009). In case of milk proteins casein molecules are
strongly hydrophobic and thus micelles are hold together by hydrophobic bonds or
salt bridges. Gels can be obtained by enzymatic hydrolysis of k-casein obtained
from rennet CMP (caseinomacropeptide) and thus causes the micelles to aggregate
resulting in rennet gelation. For egg proteins both albumen and yolk of liquid eggs
have the capacity to form gels upon heating. Gel formation is a two-step process of
denaturation followed by aggregation of denatured proteins (Montejano et al. 1984;
Woodward and Cotterill 1986).
Food Biopolymers: Structural, Functional, and Nutraceutical Properties: Food Proteins…
lowest solubility at its iso-electric point so if there is a charge at the protein surface,
the protein prefers to interact with water, rather than with other protein molecules,
thus this charge makes it more soluble. The surfaces of proteins are occupied by
amino acid residues that interact with water these amino acids are referred to as
hydrophilic amino acids and include arginine, lysine, aspartic acid, and glutamic
acid. At pH 7 the side chains of these amino acids carry charges positive for arginine
and lysine, negative for aspartic acid and glutamic acid. As the pH increases, lysine
and arginine begin to lose their positive charge, and at pHs greater than 12 they are
mainly neutral. On the other hand when pH decreases, aspartic acid and glutamic
acid begin to lose their negative charge and it has been found that at pH less than 4
they are mainly neutral.
Gel Forming Properties
It is an important functional attribute of proteins which is related to food processing.
Many foods are in the form of gels and the main structural building element in such
type of foods is protein. In addition to proteins, pectin, starches and gums are also
associated in the formation of strong gels. The process of gelation is a basic fundamental for various types of food systems like milk gels, comminuted meat and fish
products, other meat products, cake fillings, fruit jellies, bread dough’s and others.
It is the main criterion which is frequently used to evaluate quality of proteins.
Many quality characteristics like adhesiveness, gumminess, juiciness and other textural properties are directly related to the gelling properties of proteins. Visco-elastic
properties of many foods are also related to gelation properties of food proteins. The
gelation properties also affect other functional properties of proteins like water
binding capacity, oil binding capacity, emulsifying capacity, emulsion stability etc.
It also plays a major role in stabilizing various types of emulsions and foams.
Protein gels can be formed by employing different type of approaches like heating,
enzymatic process, heating in combination with salts etc. Whey protein gels can be
obtained by heating which proceeds through a series of transitions like denaturation
of native proteins, aggregation of unfolded molecules, strand formation from aggregates or association of strands into a network. It should be noted that aggregates are
formed in the presence of salts which results in the formation of strong gels.
Likewise for soy proteins gelation process is obtained by heating soya bean flour or
milk followed by addition of salt (e.g. Ca
++
, or Mg
++
) to from a gel (Cayot and
Lorient 1997; Jong et al. 2009). In case of milk proteins casein molecules are
strongly hydrophobic and thus micelles are hold together by hydrophobic bonds or
salt bridges. Gels can be obtained by enzymatic hydrolysis of k-casein obtained
from rennet CMP (caseinomacropeptide) and thus causes the micelles to aggregate
resulting in rennet gelation. For egg proteins both albumen and yolk of liquid eggs
have the capacity to form gels upon heating. Gel formation is a two-step process of
denaturation followed by aggregation of denatured proteins (Montejano et al. 1984;
Woodward and Cotterill 1986).
Food Biopolymers: Structural, Functional, and Nutraceutical Properties: Food Proteins…