‘worked’ to ensure further removal of moisture and an even distribution
of water and salt throughout the fat phase. In properly produced butter
the water is distributed as numerous droplets ( 41010 g^1 ) mostly less
than 10mm in diameter. Since the butter should contain at most around
103 cfu g^1 , most of these droplets will be sterile. In those that do contain
micro-organisms, the nutrient supply will be severely limited by the size
of the droplet. If the butter is salted, the salt will concentrate in the
aqueous phase along with the bacteria which will therefore experience a
higher, more inhibitory salt level. For example, bacteria in a butter
containing 1% salt and with a moisture content of 16% would experi-
ence an effective salt concentration of 6.25%.
Few micro-organisms survive pasteurization so the microbiological
quality of butter depends primarily on the hygienic conditions during
subsequent processing, particularly the quality of the water used to wash
the butter. Good microbiological quality starting materials are essential
though, as preformed lipases can survive pasteurization and rapidly spoil
the product during storage. Butter spoilage is most often due to the
development of chemical rancidity but microbiological problems do also
occur in the form of cheesy, putrid or fruity odours or the rancid flavour
of butyric acid produced by butterfat hydrolysis. Pseudomonads are the
most frequently implicated cause and are thought to be introduced
mainly in the wash water. Psychrotrophic yeasts and moulds can also
cause lipolytic spoilage and these are best controlled by maintaining low
humidity and good air quality in the production environment and by
ensuring the good hygienic quality of packaging materials. In this respect
aluminium foil wrappers are preferred to oxygen-permeable parchment
wrappers as they will help discourage surface mould growth.
Butter is a relatively safe commodity from a microbiological stand-
point, although there was an outbreak of listeriosis in Finland in 1998/9
which affected 18 people, four of whom died (see 7.9.5).
Margarine relies on a similar compartmentalization for its microbio-
logical stability, but uses vegetable fat as its continuous phase. Although
skim milk is often included in the formulation, it is possible to make the
aqueous phase in margarine even more deficient nutritionally than in
butter, thus increasing the microbiological stability further. With the
move towards low fat spreads containing 40% fat, the efficacy of this
system is more likely to breakdown. A higher moisture content means
that the preservative effect of salt or lactic acid, which is often included,
is diluted and that micro-organisms can grow to a greater extent in the
larger aqueous droplets. In these cases the use of preservatives may be
required to maintain stability. An approach developed at Unilever’s
laboratories in the Netherlands is based on a two stage approach where
the composition of the aqueous phase is analysed to determine its
Chapter 4 117