Natural food and beverage flavour enhancers - Natural Food Additives, Ingredients and Flavourings - page 87

enhancer but also as a fl avour precursor; on cooking it can be degraded to
hypoxanthine, releasing the pentose sugar ribose, which is highly reactive in
Maillard fl avour formation pathways. The drying and fermentation of meat and
fi sh increases their level of IMP. To produce the dried Bonito ( Katsuobushi ) in
Japan, fi sh is dried and smoked, and in some cases inoculated with mould post-
drying to promote fermentation and the breakdown of the protein to amino acids
(Blumenthal 2009). It is the dried fi sh such as bonito and sardine ( Niboshi ) that
are traditionally used, along with the glutamate extracted from kombu, to make
the Japanese dashi stock.

Yeast extracts
Yeast extracts are widely used as natural fl avour enhancers and the demand for
high quality extracts with unique or specifi c taste attributes continues to increase.
The extracts are formed by the autolysis of yeast cells, post propagation, either by
yeast enzymes or other additional enzymes such as proteases, peptidases,
phosphodiesterases and deaminases. A high initial protein content of the yeast
cells results in high levels of amino acids and similarly, a high RNA level will
result in increased levels of 5′-ribonucleotides (Stam et al. 1996). Careful control
of pH and temperature are needed to ensure the endogenous hydrolytic enzymes
are not destroyed. The yeast autolysis stage is followed by a heat-processing
stage, typically under vacuum and a fi nal evaporation stage (Mahadevan and
Farmer 2006).
The raw materials for yeast extracts are generally of the genus Saccharomyces ,
utilising baker's or brewer's yeast (Nakajo and Sano 2002), which leads to extracts
high in glutamic acid and 5′-ribonucleotides (see Table 4.1) but also with specifi c
yeast fl avours that tend to be characteristic of meat. A number of authors have
studied the fl avour volatiles of yeast extracts and have found key odours such as
2-methyl-3-furanthiol and 2-methyl-3-methyldithiofuran, which have typical
meat characteristics (Mahadevan and Farmer 2006). These authors found that
differences in process conditions led to differences in both the concentration of
fl avour precursors (and, therefore, tastants) as well as the balance of the odour
volatiles.
Yeast suppliers will often sell their extracts based on their characteristic meat-
like properties, for example as 'light, delicate' and 'dark, meaty' (Synergy 2009),
or as building blocks with more generic savoury taste extracts and more specifi c
meat-like taste extracts (DSM 2010). In recent years, yeast extract manufacturers
have developed the capacity to produce yeast extracts with differing levels of
ribonucleotides, up to 20% of the total product weight, allowing ribonucleotides
to be added to food without label declaration.
Other researchers have identifi ed that extracts from the yeast genus Candida
are much lower in characteristic meat fl avour volatiles but high in the umami taste
5′-nucleotides (Nakajo and Sano 2002) and lower, relative to Saccharomyces
extracts, in glutamic acid (Table 4.1). The researchers suggest that this type of
extract is more suitable for savoury fl avour enhancement of Japanese cuisine,
where the meaty fl avours of Saccharomyces extracts are less desirable.

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