2009_Book_FoodChemistry

88 1 Amino Acids, Peptides, Proteins
Table 1.45. Content of dityrosine in some proteins after
their oxidation with horseradish peroxidase/H 2 O 2
(pH 9.5, 37 ◦ C, 24 h. Substrate/enzyme = 20:1)
Protein Tyrosine Tyrosine Dityrosine
content decrease content
prior to (%) (g/100 g
oxidation
protein)
(g/100 g
protein)
Casein 6.3 21.8 1.37
Soyamine a 3.8 11.5 0.44
Bovine serum
albumin 4.56 30.7 1.40
Gliadin 3.2 5.4 0.17
a Protein preparation from soybean.
Proteins are responsible for the distinct physical
structure of a number of foods, e. g. the fibrous
structure of muscle tissue (meat, fish), the porous
structure of bread and the gel structure of some
dairy and soya products.
Many plant proteins have a globular structure
and, although available in large amounts, are
used to only a limited extent in food processing.
In an attempt to broaden the use of such proteins,
a number of processes were developed in the
mid-1950’s which confer a fiber-like structure
to globular proteins. Suitable processes give
products with cooking strength and a meat-like
structure. They are marketed as meat extenders
and meat analogues and can be used whenever
a lumpy structure is desired.
1.4.7.2 Starting Material
The following protein sources are suitable for the
production of texturized products: soya; casein;
wheat gluten; oilseed meals such as from cottonseed,
groundnut, sesame, sunflower, safflower or
rapeseed; zein (corn protein); yeast; whey; blood
plasma; or packing plant offal such as lungs or
stomach tissue.
The required protein content of the starting material
varies and depends on the process used for
texturization. The starting material is often a mixture
such as soya with lactalbumin, or protein
plus acidic polysaccharide (alginate, carrageenan
or pectin).
The suitability of proteins for texturization varies,
but the molecular weight should be in the range of
10–50 kdal. Proteins of less than 10 kdal are weak
fiber builders, while those higher than 50 kdal are
disadvantageous due to their high viscosity and
tendency to gel in the alkaline pH range. The proportion
of amino acid residues with polar side
chains should be high in order to enhance intermolecular
binding of chains. Bulky side chains
obstruct such interactions, so that the amounts of
amino acids with these structures should be low.
1.4.7.3 Texturization
The globular protein is unfolded during texturization
by breaking the intramolecular binding
forces. The resultant extended protein chains are
stabilized through interaction with neighboring
chains. In practice, texturization is achieved in
one of two ways:
• The starting protein is solubilized and the resultant
viscous solution is extruded through
a spinning nozzle into a coagulating bath (spin
process).
• The starting protein is moistened slightly and
then, at high temperature and pressure, is extruded
with shear force through the orifices of
a die (extrusion process).
1.4.7.3.1 Spin Process
The starting material (protein content >90%, e. g.
a soya protein isolate) is suspended in water and
solubilized by the addition of alkali. The 20% solution
is then aged at pH 11 with constant stirring.
The viscosity rises during this time as the protein
unfolds. The solution is then pressed through the
orifices of a die (5000–15,000 orifices, each with
a diameter of 0.01–0.08 mm) into a coagulating
bath at pH 2–3. This bath contains an acid
(citric, acetic, phosphoric, lactic or hydrochloric)
and, usually, 10% NaCl. Spinning solutions of
protein and acidic polysaccharide mixtures also
contain earth alkali salts. The protein fibers
are extended further (to about 2- to 4-times
the original length) in a “winding up” step and
are bundled into thicker fibers with diameters