160 Modern Food Microbiology
A bacterium of continuing concern in milk is the etiologic agent of Johne’s disease of cattle, which
appears to play some role in Crohn’s disease of humans. The organisms in question are classified as
follows: Mycobacterium avium subsp. avium causes tuberculosis in birds and it is infectious for AIDS
patients. M. avium subsp. paratuberculosis is an obligate pathogen of ruminants and it is thought to
be involved in the etiology of Crohn’s disease. M. avium subsp. silvaticum is an obligate pathogen
of animals where it causes paratuberculosis in mammals and tuberculosis in birds.
73 M. avium subsp.
paratuberculosis is the strain of interest in cow’s milk because of its possible role in Crohn’s disease.
Of primary concern is whether the pasteurization methods in use are adequate to destroy this organism.
In one study, neither the HTST nor the LTLT method destroyed 10
3 –10
4 cfu/ml in all milk samples,
24
but in another study, up to 10
6 cfu/ml were destroyed by HTST carried out at 72
◦ C for 15 sec.
68
Crohn’s disease is an inflammatory bowel disease (regional ileitis), a condition wherein the terminal
ileum and sometimes the cecum and ascending colon are thickened and ulcerated. The lumen of the
affected region is much narrowed, resulting in intestinal obstruction.
In a survey of 814 bovine milk samples in the United Kingdom over a 17-month period in 1999–
2000, a mean of 7.8% of raw and 11.8% of pasteurized samples were positive for M. avium subsp.
paratuberculosis DNA.
22 Culture confirmation was achieved in 1.6% of the raw and 1.8% of the
pasteurized samples. The pasteurized milk samples in this study were phosphatase negative. In another
study of raw and pasteurized cows’ milk in the United Kingdom, M. avium subsp. paratuberculosis
was found in 4 of 40 (6.7%) raw and 10 of 144 (6.9%) pasteurized milk samples.
23 The investigators
found viable organisms in milk samples processed by four different treatments including heating at
73
◦ C for 25 sec.
The fate of this organism in the ripening of cheese has been investigated.
72 According to the FDA,
two options are available for producing safe cheese: (1) use only pasteurized milk, or (2) hold finished
cheese for at least 60 days at 2
◦ C. In the study noted above, a soft white cheese using pasteurized
milk that was spiked with ca. 10
6 M. avium subsp. paratuberculosis was prepared by varying pH and
NaCl content and tested for viable cells after storage (ripening). It was found that cheese made from
HTST-pasteurized milk, pH 6.0, 2% NaCl, and cured for 60 days resulted in ca. log-3 reduction of M.
avium subsp. paratuberculosis cells.
72 In a review of Crohn’s disease and the role of M. avium subsp.
paratuberculosis, Harris and Lammerding
30 concluded that the evidence for cause and effect is not
conclusive.
Spoilage
As the only natural source of the disaccharide lactose, milk undergoes microbial spoilage in a way
that is unique. Only a relatively small number of milk-borne bacteria can obtain energy from this sugar
(especially at refrigerator temperatures) in contrast to the disaccharides sucrose and maltose, and the
lactic acid bacteria are well suited to this task. The coliform bacteria are the most conspicuous utilizers
of lactose among Gram-negative bacteria. Thus, the bacterial spoilage of either raw or pasteurized
milk is conspicuous by the production of lactic acid by lactose users, with the normal pH of around
6.6 being reduced to 4.5 or so that leads to the precipitation of casein (curdling). The thermoduric
Streptococcus salivarius subsp. thermophilus strains preferentially use the glucose moiety of lactose
and excrete galactose, which is a ready substrate for nonlactose users.
The spoilage of UHT milk is caused by Bacillus spp. that survive the UHT process. Anaerobic
spores appear not to be a problem because of the relatively high Eh of milk. Among the Bacillus
species that have been recovered from spoiled products are B. cereus, B. licheniformis, B. badius,
and B. sporothermodurans.
55 Paenibacillus spp. have been isolated also from UHT-treated products.
A bacterium of continuing concern in milk is the etiologic agent of Johne’s disease of cattle, which
appears to play some role in Crohn’s disease of humans. The organisms in question are classified as
follows: Mycobacterium avium subsp. avium causes tuberculosis in birds and it is infectious for AIDS
patients. M. avium subsp. paratuberculosis is an obligate pathogen of ruminants and it is thought to
be involved in the etiology of Crohn’s disease. M. avium subsp. silvaticum is an obligate pathogen
of animals where it causes paratuberculosis in mammals and tuberculosis in birds.
73 M. avium subsp.
paratuberculosis is the strain of interest in cow’s milk because of its possible role in Crohn’s disease.
Of primary concern is whether the pasteurization methods in use are adequate to destroy this organism.
In one study, neither the HTST nor the LTLT method destroyed 10
3 –10
4 cfu/ml in all milk samples,
24
but in another study, up to 10
6 cfu/ml were destroyed by HTST carried out at 72
◦ C for 15 sec.
68
Crohn’s disease is an inflammatory bowel disease (regional ileitis), a condition wherein the terminal
ileum and sometimes the cecum and ascending colon are thickened and ulcerated. The lumen of the
affected region is much narrowed, resulting in intestinal obstruction.
In a survey of 814 bovine milk samples in the United Kingdom over a 17-month period in 1999–
2000, a mean of 7.8% of raw and 11.8% of pasteurized samples were positive for M. avium subsp.
paratuberculosis DNA.
22 Culture confirmation was achieved in 1.6% of the raw and 1.8% of the
pasteurized samples. The pasteurized milk samples in this study were phosphatase negative. In another
study of raw and pasteurized cows’ milk in the United Kingdom, M. avium subsp. paratuberculosis
was found in 4 of 40 (6.7%) raw and 10 of 144 (6.9%) pasteurized milk samples.
23 The investigators
found viable organisms in milk samples processed by four different treatments including heating at
73
◦ C for 25 sec.
The fate of this organism in the ripening of cheese has been investigated.
72 According to the FDA,
two options are available for producing safe cheese: (1) use only pasteurized milk, or (2) hold finished
cheese for at least 60 days at 2
◦ C. In the study noted above, a soft white cheese using pasteurized
milk that was spiked with ca. 10
6 M. avium subsp. paratuberculosis was prepared by varying pH and
NaCl content and tested for viable cells after storage (ripening). It was found that cheese made from
HTST-pasteurized milk, pH 6.0, 2% NaCl, and cured for 60 days resulted in ca. log-3 reduction of M.
avium subsp. paratuberculosis cells.
72 In a review of Crohn’s disease and the role of M. avium subsp.
paratuberculosis, Harris and Lammerding
30 concluded that the evidence for cause and effect is not
conclusive.
Spoilage
As the only natural source of the disaccharide lactose, milk undergoes microbial spoilage in a way
that is unique. Only a relatively small number of milk-borne bacteria can obtain energy from this sugar
(especially at refrigerator temperatures) in contrast to the disaccharides sucrose and maltose, and the
lactic acid bacteria are well suited to this task. The coliform bacteria are the most conspicuous utilizers
of lactose among Gram-negative bacteria. Thus, the bacterial spoilage of either raw or pasteurized
milk is conspicuous by the production of lactic acid by lactose users, with the normal pH of around
6.6 being reduced to 4.5 or so that leads to the precipitation of casein (curdling). The thermoduric
Streptococcus salivarius subsp. thermophilus strains preferentially use the glucose moiety of lactose
and excrete galactose, which is a ready substrate for nonlactose users.
The spoilage of UHT milk is caused by Bacillus spp. that survive the UHT process. Anaerobic
spores appear not to be a problem because of the relatively high Eh of milk. Among the Bacillus
species that have been recovered from spoiled products are B. cereus, B. licheniformis, B. badius,
and B. sporothermodurans.
55 Paenibacillus spp. have been isolated also from UHT-treated products.