Broschüre 2004 zum Download (pdf | 1994,28 KB) - H. Wilhelm ...

not only causes significant loss of the silage nutritional
value, but also results in high levels of spores. These
spores survive passage through the cow’s intestinal
tract and are excreted via the faeces. Contamination
of raw milk can take place via faeces-contaminated
udders that are insufficiently cleaned prior to milking.
The spores survive pasteurization of milk. Even low
levels of C. tyrobutyricum spores in milk cause off-flavours
and excessive gas formation, so-called late-blowing,
in semi-hard cheeses such a Gouda (Figure 2).
Since maize was considered an ›easy-to-ensile‹ crop
and a minor source of clostridia spores, research focused
on grass silage.
In The Netherlands milk supplies of farmers are
analyzed monthly for the presence of spores of lactic
acid-consuming, gas-forming clostridia (butyric acid
bacteria (BAB)). Farmers receive a penalty on the
milk price when the concentration of BAB spores
exceeds a limit value. Since the introduction of this
system in The Netherlands 22 years ago a significant
reduction of the average contamination level of BAB
spores in raw milk has occurred. The percentage of
milk supplies with a BAB penalty decreased between
1982 and 1990 from about 5 % to about 1.5 %. This
was mainly due to improvements in mechanisation
and techniques for making wilted grass silage and
increased awareness of farmers.
Figure 3: Concentration of BAB spores in grass silage produced in
The Netherlands in 2002 and 2003.
A side effect of these achievements was, however,
that silage research declined in priority. Interest in
silage research revived in The Netherlands recently.
The main drivers were, firstly, data indicating that
since a few years the average contamination level
of BAB spores in raw milk slowly increased and,
secondly, the increased focus of the food industry on
quality and safety. In a research programme carried
out by NIZO food research for the Dutch dairy industry,
a survey has been conducted to investigate the
microbiological and chemical composition of silages
produced at dairy farms in 2002 and 2003. In 4 %
grass silage samples the concentration of BAB spores
exceeded 10 5 /g, defined as ›poor‹, and in 16 – 23 %
samples it was between 10 4 and 10 5 /g, defined as
›moderate‹ (Figure 3). BAB spores had increased
more than 10 –100 fold in these silages, since the
level immediately after harvesting is less than 10 3 /g.
Interestingly, no correlation between BAB spores and
dry matter concentration was observed (Figure 4). In
fact, for the majority of ›poor‹ and ›moderate‹ silages
dry matter exceeded 350 g/kg, a concentration above
which theoretically no growth of BAB is expected.
In addition, no correlation between BAB spores and
pH and between BAB spores and the ammonia-N
fraction was observed (not shown). This is in line
Figure 4: Concentration of BAB spores as a function dry
matter concentration in grass silage produced in
The Netherlands in 2002 (n=160).
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