Natural food and beverage flavour enhancers - Natural Food Additives, Ingredients and Flavourings - page 89

(Rotzoll et al. 2005). The latter group found an aqueous solution of succinic acid
to impart a sour taste (threshold 0.7 mmol/L) followed by an umami-like taste
(threshold 0.9 mmol/L).
The Hofmann research group has termed the properties of compounds such as
succinic acid as umami taste modifi ers. They found succinic acid, as well as
L -theanine (a glutamic acid derivative), gallic acid and theogallin to enhance the
umami taste in a green tea beverage at subthreshold concentrations (Kaneko et al.
2006). Whereas in isolation the succinic acid has been denoted to have both sour
and umami taste qualities, L -theanine was found to have astringent properties at
6 mmol/L but umami and sweet properties at 24 mmol/L; only gallic acid and
theogallin had astringent properties in isolation. When prepared in model solutions
with glutamic acid, these compounds all raised the umami intensity of MSG,
depending on the level at which they were added (Kaneko et al. 2006).
Organic acids bound to sugar molecules are types of glycoside and one such
glycoside, ( S )-malic acid 1- O - D -glucopyranoside, was isolated from dried Morel
mushrooms in the Rotzoll study and found to have an umami taste; this compound
has been named ( S )-Morelid (Rotzoll et al. 2005).

4.3.2 The effect of other amino acids on umami taste
As noted above in the discussion of soy sauce, amino acids with other taste
characteristics can have an impact on the overall perception of umami. A study
evaluating a range of amino acid solutions with and without IMP (0.5M) found
that sweet amino acids (such as alanine, serine and glycine) enhanced the umami
taste of IMP. They also found that this synergistic effect was not blocked by a
sweetness inhibitor and therefore it was unlikely that the synergy was due to T1R3
activation. They proposed that the mechanism for the synergy was that the sweet
amino acids activate the T1R1 part of the heterodimer T1R1/T1R3 receptor, just
as glutamate does, and that IMP potentiates the response (Kawai et al. 2002).
In a study of Indonesian soy sauces, Lioe et al. (2004) found that the aromatic
amino acids, such as L -phenylalanine and L -tyrosine, at a subthreshold level, had
a role in enhancing umami taste. In isolation, these amino acids are characterised
by bitter taste. The same researchers went on to research these amino acids in
simple model systems and found that L -phenylalanine (0.5-5.0 mM) only enhanced
the umami taste of the MSG/NaCl mixtures and not the taste of MSG or NaCl
alone (Lioe et al. 2005). L -Tyrosine acted in the same way (at 0.5 and 1.5 mM).

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4.3.3 Taste enhancing peptides
In a molecular study aimed at selecting umami tasting molecules, Grigorov
et al. (2003) summarised 31 di- and tripeptides reported to have umami taste;
25 contained at least one glutamate unit (seven with an aspartate unit), fi ve
contained an aspartate unit and no glutamate, and one contained neither (Lys-Gly
dipeptide). The peptides had been identifi ed in six previous studies, by three
research groups, from 1973 to 1989, and one by a fl avour company (Firmenich).