Proteins as clean label ingredients in foods and beverages - Natural Food Additives, Ingredients and Flavourings - page 200

protein in its native state. The loss of the globular character of proteins during
heating can be primarily attributed to the increased entropy of the unfolded state
of the protein (Creighton 1978) and is in principle reversible. Forces that are
involved in folding and stabilizing the native protein structure are also involved in
aggregate formation. An increase in the effective hydrophobicity is an indication
of protein unfolding. When too many hydrophobic sites are exposed, intermolecular
interactions between these sites become inevitable and aggregation of protein
molecules occurs.
In addition to the non-covalent interactions, intermolecular covalent reactions
can also occur on heating in the case of cysteine-containing proteins. Aggregation
reactions, and in particular the formation of intermolecular disulfi de bonds, may
prevent the renaturation of the unfolded protein molecule upon cooling, so that
the denaturation process becomes irreversible. Therefore, the details of the
kinetics of both the unfolding and aggregation of protein molecules are important
parameters to consider when using globular proteins as building stones for tailor-
made protein-based food ingredients.

9.2.4 Complex globular proteins
Seed storage proteins are categorized as complex globular proteins. Legume
proteins, such as soy and pulses, belong to the globulin family of seed storage
proteins called legumins (11S globulin fraction) and vicilins (7S globulin).
Globulins (90% of protein fraction) are defi ned as protein extractable in dilute salt
solutions. Grains contain a third type of storage protein called gluten or
'prolamines' (see below). Legumes also contain biologically active or metabolic
proteins such as enzymes, trypsin inhibitors, hemagglutinins, and cysteine
proteases very similar to papain.
In comparison with the globular proteins described above, these globular
proteins exist as complexes as visualized in Fig. 9.1 for the most well-known
legume proteins, soy proteins. Glycinin (11S) consist of units of a basic and acidic
polypeptide that are connected via a single disulfi de bond. Depending on
conditions such as pH and temperature it forms hexameric complexes (360 kDa;
11S) or trimeric complexes (180 kDa). The pI is 4.9. β -Conglycinin (7S) exists as
a trimeric glycoprotein complex, consisting of three different subunits. The
subunits are not connected via disulfi de cross-linkages, but via hydrophobic and
hydrogen bonding. At low ionic strength a hexameric complex is predominantly
formed. The pI of this protein complex is 4.6.
Soybeans are processed into three kinds of protein preparations: soy fl our,
concentrates (70% protein) and isolates (90% protein). Soy protein concentrates
are generally prepared from de-fatted soy fl akes by aqueous alcohol extraction in
which the soluble carbohydrate fraction is removed. Soy protein isolates are
generally prepared by a two-step aqueous extraction of protein from de-hulled,
de-fatted soybean meal (by-product of oil production). The fi rst step involves
extraction of protein at a pH below 9 and clarifi cation by removal of insoluble
material. The second step is isoelectric precipitation of the protein fraction

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