tends to grow poorly in milk with low levels of formate, forming
elongated, multinucleate cells.
Acetaldehyde (ethanal) is the most important flavour volatile of
yoghurt and should be present at 23–41 mg kg^1 (pH 4.2–4.4) to give
the correct yoghurt flavour. Its accumulation is a consequence of the fact
that both starter organisms lack an alcohol dehydrogenase which would
otherwise reduce the acetaldehyde to ethanol. Both will produce acetal-
dehyde from the glucose portion of lactoseviapyruvate and through the
action of threonine aldolase. The latter activity (Figure 9.5) is more
pronounced in the lactobacillus but in the streptococcus methionine has
been shown to increase levels of acetaldehydeviathreonine. Diacetyl, an
important flavour compound in many dairy products, is present at very
low levels (E0.5 mg kg^1 ) but is thought to make a contribution to the
typical yoghurt flavour.
When the fermentation is complete the yoghurt is cooled to 15–20 1 C
before the addition of fruits and flavours and packaging. It is then cooled
further to below 5 1 C, under which conditions it will keep for around
three weeks. Yoghurt is not usually pasteurized since chill storage will
arrest the growth of the starter organisms. The acidity will however
continue to increase slowly during storage.
Because of its high acidity and low pH (usually 3.8–4.2), yoghurt is an
inhospitable medium for pathogens which will not grow and will not
survive well. It is unusual therefore for yoghurt to be involved in
outbreaks of foodborne illness, although the hazelnut yoghurt botulism
outbreak in the UK in 1989 (see Section 7.5.5) is a notable exception.
Yoghurts are spoiled by acidoduric organisms such as yeasts and moulds.
Yeasts such as the lactose-fermentingKluyveromyces fragilisand, in fruit-
containing yoghurts,Saccharomyces cerevisiaeare particularly important
but the yeast-like fungusGeotrichumand surface growth of moulds such
asMucor,Rhizopus,Aspergillus,Penicillium,andAlternariacan also be a
problem. Advisory guidelines for microbiological quality have suggested
that satisfactory yoghurts should contain more than 10^8 cfu g^1 of the
starter organisms,o1 coliform g^1 ,o1mouldg^1 ando10 yeasts g^1
(fruit-containing yoghurts may contain up to 100 yeasts g^1 and remain of
satisfactory quality).
Figure 9.5 The threonine aldolase reaction
326 Fermented and Microbial Foods