Natural food and beverage flavour enhancers - Natural Food Additives, Ingredients and Flavourings - page 75

enhancement; this is discussed in the chapter with particular attention paid to
umami taste synergies. Prediction of taste and fl avour sensory impact from
quantifi cation of the tastants remains a diffi cult task within a real food system and
studies attempting such predictive models are discussed. Perhaps psychophysical
relationships will remain elusive without a full understanding of the underlying
mechanism of enhancement.
Food ingredients responsible for fl avour enhancement are discussed from
effi cacy, source and availability perspectives within this chapter.

4.2 Savoury fl avour enhancement: umami tastants and
ingredients rich in umami compounds
Two basic tastes contribute to the enhancement of savoury fl avour perception:
umami and salt. The umami taste, often described as the savoury or glutamate
taste, is the most effective in savoury fl avour enhancement. Umami was fi rst
described in Japan in 1907 by Dr Kikunae Ikeda at Tokyo Imperial University
(Sano 2009), who recognised that the basic savoury taste in a Japanese dashi stock
was distinct from salt and that it could be extracted from kombu seaweed in the
form of glutamate.
Although umami and salt stimuli are both from non-volatile tastants perceived
by taste receptors within taste cells on the tongue, the receptors responsible are
quite different. Umami tastants are organic compounds that bind to more than one
type of G-protein coupled receptors (GPCRs). There are specifi c glutamate
receptors (mGLURs) and a non-specifi c receptor that is a hybrid between two type
1 taste receptors T1R1 and T1R3 (T1R1/T1R3). The hybrid receptor responds to
both the umami amino acids and the 5′-ribonucleotides, but not in the same way.
The 5′-nucleotides inosine monophosphate (IMP) and guanosine monophosphate
(GMP) alone have not been found to activate human T1R1/T1R3. What they do
achieve is to strongly potentiate the L -glutamate-induced T1R1/T1R3 response
(Li et al. 2002), an example of synergy at the taste receptor level. A later paper by
Zhang et al. (2008) proposed that glutamate binds to the T1R1 and that
5′-nucleotides bind to an adjacent site, helping to stabilise the conformation.
Salt is an ionic tastant that is perceived though ion channels on the tongue,
either specifi c sodium ion channels or non-specifi c ion channels (Lindeman
2001). Volatile fl avour compounds are perceived either ortho- or retro-nasally in
the olfactory epithelium, where organic fl avour compounds bind to olfactory
receptors (OR), themselves a type of GPCR. ORs are specifi c in that they bind
odour molecules which share similar molecular features. However, to date, the
processes which control the OR expression and specifi city are not fully understood
(Gaillard et al. 2004).
The mechanism of savoury fl avour enhancement by glutamate was established
to be at the cognitive level, not at the receptor level, by McCabe and Rolls (2007).
Using functional magnetic resonance imaging (fMRI), the researchers found that
a glutamate taste and savoury odour combinations produced greater activation

￿ ￿ ￿ ￿ ￿