Proteins as clean label ingredients in foods and beverages - Natural Food Additives, Ingredients and Flavourings - page 198

9.2.1 Hydrocolloid-like proteins
Gelatins are proteins that are characterized by a hydrocolloid-like behavior which
clearly distinguishes them from the other protein ingredients. Moreover, they are
used in a hydrolyzed form. Gelatin is made from the hydrolysis of collagen from
animals' skin and bones (e.g. bovine, pig, fi sh). Two types of gelatins can be
distinguished - type A and type B. Type A is obtained from acid-treated collagen
and type B is obtained from alkali-treated collagen. Depending on the type of
treatment, the isoelectric point (pI) of gelatin will differ; it is around pH 5 for type
B (caused by hydrolysis of glutamine and asparagine residues) and around pH 8-9
for type A. The molecular weight distribution of the two types correlates strongly
with their gelling characteristics. Gelatin will lose its gelling ability after too
extensive hydrolysis. Gelatin forms transparent polymer gels, in contrast to most
other proteins which form (turbid) particle gels.
The typical functionality of gelatins relates to their unique amino acid
composition and sequence. Collagens contain a Gly-X-Y repeating structure (X is
often proline and Y is sometimes hydroxyproline). Therefore, gelatin is unusually
high in the non-essential amino acids (i.e. those produced by the human body)
glycine and proline, and lacks certain essential amino acids (i.e. those not produced
by the human body), tryptophan, isoleucine, threonine and methionine. There can
be small variations in minor amino acids, depending on the source of the raw
material and processing technique. The repeating triplet of amino acids allows
collagen/gelatin chains to twist into a helical structure. The structural functionality
of gelatin originates from the triple helix structure formed by three separate chains
that are connected by hydrogen bonds. On heating, the hydrogen bonds weaken
and the gelatin chains fall apart into a random chain form. On cooling, interactions
between the gelatin chains are promoted by formation of hydrogen bonds resulting
in the formation of the triple helix structure (Johnston-Banks 1990).

9.2.2 Unstructured, random proteins
Caseins distinguish themselves from the other categories of proteins by lacking
the typical tertiary structure, characteristic of most proteins. Caseins constitute the
main protein fraction of most mammalian milks. In their natural habitat (milk),
caseins exist as large macromolecular assemblies called micelles. Their natural
functionality is, besides a nutritional role as a source of essential amino acids, to
solubilize the relatively large quantities of calcium phosphate required for the
growth of the neonate skeleton. The casein fraction in milk comprises four
different proteins denoted as α s1-, α s2-, β - and κ -casein. They differ in primary
structure and several genetic variants of all four proteins are known. Caseins are
characterized by having a molecular mass of around 22 kDa, and containing
phosphoserine and/or phosphothreonine residues. One of them, κ -casein, shows a
variable degree of glycosylation.
Casein can be isolated from milk by proteolytic coagulation or isoelectric
precipitation (pH 4.6). Proteolytic coagulation can be realized by using proteolytic
enzymes used in cheese manufacturing, for example, chymosin (rennet). In this

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