210 DAIRY CHEMISTRY AND BIOCHEMISTRYlikely scheme appears to involve vesicular transport, followed by exocytosis
at the apical membrane.
IgA in colostrum is derived partly from intramammary synthesis and
partly by accumulation in the gland after being transported in the blood
from other sites of synthesis. In either case, IgA molecules are transported
into the secretory cells across the basal membrane by means of a large,
membrane-bound form of secretory component, which acts as a recognition
site. It is presumed that, following endocytosis, the sIgA complex (Figure
4.29) is transported to the apical membrane of the secretory cell where,
following cleavage of a portion of the complex, the mature sIgA complex is
secreted by exocytosis.
4.15 Functional milk proteins
The term ‘Functional Properties of Proteins’ in relation to foods refers to
those physicochemical properties of a protein which affect the functionality
of the food, i.e. its texture (rheology), colour, flavour, water sorption/binding
and stability. Probably the most important physicochemical properties are
solubility, hydration, rheology, surface activity and gelation, the relative
importance of which depends on the food in question; these properties are,
at least to some extent, interdependent.
The physical properties of many foods, especially those of animal origin,
are determined primarily by their constituent proteins, but those properties
are not the subject of this section. Rather, we are concerned with isolated,
more or less pure, proteins which are added to foods for specific purposes.
The importance of such proteins has increased greatly in recent years, partly
because suitable technology for the production of such proteins on a
commercial scale has been developed and partly because a market for
functional proteins has been created through the growth of fabricated foods,
i.e. foods manufactured from more or less pure ingredients (proteins,
fats/oils, sugars/polysaccharides, flavours, colours). Perhaps one should view
the subject the other way round, i.e. fabricated foods developed because
suitable functional proteins were available. Some functional proteins have
been used in food applications for a very long time, e.g. egg white in various
types of foamed products or gelatine in gelled products. The principal
functional food proteins are derived from milk (caseins and whey proteins)
or soybeans; other important sources are egg white, blood, connective tissue
(gelatine) and wheat (gluten).
Probably because of the ease with which casein can be produced from
skim milk, essentially free of lipids, lactose and salts, by rennet or isoelectric
coagulation and washing of the curd, acid and rennet caseins have been
produced commercially since the beginning of this century. However, until
relatively recently, they were used for industrial applications, e.g. in glues,