2009_Book_FoodChemistry

8.8 Sweeteners 437
Fig. 8.8. A stereoscopic representation
of the conformation
of the peptide chains of monellin
(top) and thaumatin
(bottom) (the location of tryptic
peptides that cross react
with heterologous antibodies
is indicated by thicker lines)
(according to Kim et al., 1991)
8.8.5 Thaumatins
Fig. 8.9. Monellin: schematic representation of the
A and B chains, showing the intra- and intermolecular
hydrogen bonds (— —). Using genetic engineering
techniques, the two chains were linked via a peptide
bond (→) between the amino acids residues E (B50)
and R (A2) (according to Kim et al., 1991)
was cloned and expressed in E. coli and yeast.
The protein (I) thus obtained was as sweet as
natural monellin (II). While the sweet taste of
II was completely quenched at pH 2 by heating
to 50 ◦ C, I exhibited its full sweetness at room
temperature even after being heated to 100 ◦ C.
The threshold value is f sac, g = 3000. Based on
its low stability, slow triggering and slow fading
away of taste perception, monellin probably will
not succeed as a commercial sweetener.
The fruit of Thaumatococcus daniellii contains
two sweet proteins: thaumatin I and II, with
f sac, g ∼ 2000. There are also low amounts of
three other sweet proteins (thaumatin a, b and c).
The complete amino acid sequence and the
conformation (Fig. 8.7 and 8.8) of thaumatin I,
a peptide chain with 207 amino acid residues, has
been established (Table 8.5). As a result of cross
reactions with an anti-serum against monellin
(cf. 8.8.4), sequence Y(57)FD in a β-turn is
regarded as the site of contact with the sweetness
receptor. It corresponds to sequence Y(A13)ASD
of monellin.
The sweet taste gets weaker with the increasing
acetylation of the 11 ε-amino groups of the
protein, being lost already with four acetyl
groups. In contrast, up to 7 ε-amino groups
could be modified by reductive methylation with
HCHO/NaBH 4 without reduction in the taste
intensity. The isoelectric point of the protein is
obviously of importance for its activity. Thaumatin
which is regarded as toxicologically safe
is used, e. g., in chewing gum and milk products.
Synergistic effects have been observed when
thaumatin is used in combination with saccharin
and acesulfame.