Bacillus cereus is a common contaminant in raw milk. The spores survive
pasteurization and psychrotrophic strains of B. cereus often limit the keeping quality
of pasteurized milk stored at > 6 °C (Griffiths, 1992). High numbers of B. cereus in
pasteurized milk are most frequent when the cows are grazing (Slaghuis et al. 1997),
mainly owing to increased levels of spores in raw milk resulting from teat
contamination by soil (Christiansson et al. 1999). However, high numbers can also be
found in pasteurized milk while the cows are housed indoors, and this is probably
caused by additional contamination at the dairy plant (te Giffel et al. 1996; Larsen &
Jørgensen, 1997; Lin et al. 1998). There is little information available about the
sites of recontamination in the dairy. The use of typing techniques capable of
discrimination below the species level, such as fatty acid profiles and random
amplification of polymorphic DNA–polymerase chain reaction (RAPD–PCR), could
be helpful in demonstrating contamination routes (Lin et al. 1998; Nilsson et al.
Spores of B. cereus are very hydrophobic and readily adhere to surfaces of steel,
glass and rubber (Rönner et al. 1990), and short cleaning-in-place programmes do not
always eliminate all the spores (Rönner & Husmark, 1992). Spores adhering to
surfaces are more difficult to eliminate by disinfectants than spores in solution (te
Giffel et al. 1995). Many B. cereus spores germinate rapidly in milk upon heat
activation and, if allowed to propagate undisturbed on surfaces, may form biofilms
that are extremely difficult to eliminate (Mosteller & Bishop, 1993; Wirtanen et al.
1996; Kumar & Anand, 1998).
This paper describes how we demonstrated the involvement of a pasteurizer in
the contamination of pasteurized milk by B. cereus in a commercial dairy plant using
a combination of classic microbiological analyses and typing of strains by