2009_Book_FoodChemistry

34 1 Amino Acids, Peptides, Proteins
reduced to an aldehyde group. Reaction with
phenylhydrazine produces an indole derivative.
Lastly, hydantoin is saponified with alkali:
(1.70)
L-Tryptophan is also produced through enzymatic
synthesis from indole and serine with the help of
tryptophan synthase:
(1-aminocycloalkane-1-carboxylic acids) are
sweet and bitter.
The taste intensity of a compound is reflected
in its recognition threshold value. The recognition
threshold value is the lowest concentration
needed to recognize the compound reliably, as assessed
by a taste panel. Table 1.12 shows that the
taste intensity of amino acids is dependent on the
hydrophobicity of the side chain.
L-Tryptophan and L-tyrosine are the most
bitter amino acids with a threshold value of
c t bitter = 4−6 mmol/l. D-Tryptophan, with
c t sweet = 0.2−0.4 mmol/l, is the sweetest amino
acid. A comparison of these threshold values
with those of caffeine (c tbi = 1−1.2 mmole/l)
and sucrose (c tsw = 10−12 mmol/l) shows that
caffeine is about 5 times as bitter as L-tryptophan
and that D-tryptophan is about 37 times as sweet
as sucrose.
L-Glutamic acid has an exceptional position. In
higher concentrations it has a meat broth flavor,
while in lower concentrations it enhances the
characteristic flavor of a given food (flavor enhancer,
cf. 8.6.1). L-Methionine has a sulfur-like
flavor.
The bitter taste of the L-amino acids can interfere
with the utilization of these acids, e. g., in chemically
defined diets.
1.3 Peptides
1.2.6 Sensory Properties
(1.71)
1.3.1 General Remarks, Nomenclature
Peptides are formed by binding amino acids together
through an amide linkage.
On the other hand peptide hydrolysis results in
free amino acids:
Free amino acids can contribute to the flavor of
protein-rich foods in which hydrolytic processes
occur (e. g. meat, fish or cheese).
Table 1.12 provides data on taste quality and
taste intensity of amino acids. Taste quality
is influenced by the molecular configuration:
sweet amino acids are primarily found among
members of the D-series, whereas bitter amino
acids are generally within the L-series. Consequently
amino acids with a cyclic side chain
(1.72)
Functional groups not involved in the peptide
synthesis reaction should be blocked. The
protecting or blocking groups must be removed
after synthesis under conditions which retain
the stability of the newly formed peptide
bonds: