2009_Book_FoodChemistry

16 Legumes
16.1 Foreword
Ripe seeds of the plant family Fabaceae, known
commonly as “legumes” or “pulses”, are an important
source of proteins for much of the world’s
population ∗ . The extent of the production of major
legumes is illustrated in Table 16.1. Legumes
contain relatively high amounts of protein (Table
16.2). Hence, they are an indispensable supply
of protein for the “third world”. Soybeans and
peanuts are oil seeds (cf. 14.3.2.2.5) and, even in
industrialized countries, are used as an important
source of raw proteins.
With regard to the biological value, legume proteins
are somewhat deficient in the S-containing
amino acids (Table 16.3 and 1.8).
Antinutritive substances, e. g., allergenic proteins,
proteinase inhibitors, lectins and cyanogenic glycosides,
are found in food raw materials. These
substances will be described in this chapter since
a large variety have been identified in legumes.
16.2 Individual Constituents
16.2.1 Proteins
About 80% of the proteins from soybean can be
extracted at pH 6.8. A large number of these proteins
can be precipitated by acidification at pH 4.5
(cf. Figure 1.54). This pH-dependent solubility is
used in large-scale preparations of soy proteins.
Fractionation of legume proteins using solubility
procedures, as applied to cereals by Osborne
(cf. 15.2.1.2), yields three fractions: albumins,
globulins, and glutelins, with globulins being predominant
(Table 16.4).
∗ Semi-ripe peas and beans are considered as vegetables
(cf. Chapter 17).
16.2.1.1 Glubulines
The high content of globulins in seeds indicates
that they function mostly as storage proteins,
which are mobilized during the course of
germination.
The globulin fraction can be separated by ultracentrifugation
or chromatography into two major
components present in all the legumes: vicilin
(∼7S) and legumin (∼11S). Legumin from
soybeans is called glycinin and from peanuts is
called arachin. Molecular weights and sedimentation
coefficients for the 7 S and 11 S globulins
isolated from various legumes are presented in
Table 16.5.
The 11 S globulins originate from a protein precursor
(M r ∼ 60,000) which is split into an acidic
α-polypeptide (pI ∼ 5) and a basic β-polypeptide
(pI ∼8.2) by cleaving the peptide bond between
Asn (417) and Gly (418) (cf. Table 16.6). These
two polypeptides are connected by a disulfide
bridge between Cys (92) and Cys (424) and
are regarded as one subunit. Six such subunits
join to give 11 S globulin, the hydrophobic
β-polypeptides evidently forming the core of
the subunits and of the entire structure. Little is
known about the tertiary and quaternary structure.
On the other hand, the amino acid sequences
of the subunits of the 11 S globulins of a number
of legumes are known. They were mainly
derived from the nucleotide sequences of the
coding nucleic acids. As an example, Table 16.6
shows the sequences of legumin J from the
pea (Pisum sativum) and glycinin A 2 B la from
the soybean (Glycine max). Homology exists
between the 11 S globulins of different legumes.
Variable regions are primarily found in the acidic
α-polypeptide, while the basic β-polypeptide
is conservative with slight variability in the
region of the C-terminal. Conserved residues are
uniformly distributed throughout the sequence.
The α/β cleavage site is conserved in all the
proteins studied until now (cf. Table 16.7). Thus,
H.-D. Belitz · W. Grosch · P. Schieberle, Food Chemistry 746
© Springer 2009