International Journal of Food Microbiology, 10 (1990) 59-64 Elsevier
59
FOOD 00296
In-home pasteurization of raw goat's milk by microwave treatment J. Stephen Thompson and Annemarie Thompson Regional Public Health Laboratory, Laboratory Services branch, Ontario Ministry of Health, Peterborough, Ontario, Canada (Received 30 May 1989; accepted 14 August 1989)
Raw milk has been implicated as an important source of infection with the common bacterial agents associated with gastroenteritis. While established methods of home pasteurization can be cumbersome and tedious, we have demonstrated that a domestic microwave oven can be used effectively to reduce aerobic plate counts in raw goat's milk by up to 6 log cycles without impairing the organoleptic quahty. Good keeping quality of the irradiated product was demonstrated by the 7-day holding standard plate count. Petrifilm SM and Petrifilm VRB were used successfully under field conditions. Further investigation must be undertaken to demonstrate that this method is equivalent to the legally accepted pasteurization procedures for the inactivation of such pathogenic bacteria as Salmonella spp., Listeria monocytogenes and the pasteurization standard Coxiella burnetii. Key words: Microwave pasteurization; Raw milk; Goat's milk; Pasteurization
Introduction Raw cow's milk has b e e n implicated as a n i m p o r t a n t source of i n f e c t i o n with the c o m m o n bacterial agents associated with gastroenteritis i n c l u d i n g Salmonella spp. (Jessop et al., 1985; Aldis et al., 1986; T a m b l y n , 1986), Campylobacterjejuni ( P o t t e r et al., 1984; Klein et al., 1986), a n d Yersinia enterocolitica ( S c h i e m a n n a n d T o m a , 1978). These organisms have also b e e n associated with the c o n s u m p t i o n of goat's milk in reports from C a n a d a (Jessop et al., 1985; T h o m p s o n et al., 1986), E n g l a n d ( H u t c h i n s o n et al., 1985), N o r t h e r n I r e l a n d ( W a l k e r a n d G i l m o u r , 1986) a n d A u s t r a l i a (Jensen a n d Hughes, 1980). R a w milk has b e e n i m p l i c a t e d as a vehicle for tran=mission of such other i m p o r t a n t p a t h o g e n s as Listeria monocytogenes (Hayes et al., 1986) a n d Escherichia coli 0157 : H7 ( D u n c a n et al., 1986). I n u r b a n areas raw milk is often c o n s u m e d as a ' h e a l t h food' (Potter et al., 1984; Roberts, 1985) b u t we believe that o n the farm c o n s u m p t i o n of raw milk has c o n t i n u e d because of tradition and, p e r h a p s m o r e i m p o r t a n t l y , because established m e t h o d s of home p a s t e u r i z a t i o n are c u m b e r s o m e . W e p r e s e n t in this report a simple
Correspondence address." J.S. Thompson, P.O. Box 265, Peterborough, Ontario, Canada, K9J 6Y8. 0168-1605/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
60 method of home pasteurization using household microwave ovens. All the work presented here was performed in a kitchen and not under laboratory conditions.
Materials and Methods
Four Saanen milking does were hand-milked into seamless plastic pails. Twentyfive samples of blended milk from these goats were used in this study. Each sample was between 2 and 3 1 and representative of the daily volume of household milk that could be pasteurized and consumed. After use each pail was washed and sanitized using household dishwashing detergent followed by soaking in a solution of approximately 0.5% sodium hypochlorite for approximately 10 min. A series of milkings was performed into pails that were washed but not sanitized. A 3-1 glass beaker (Pyrex No. 1000, Corning) was used as the vessel for pasteurizing each milk sample. Two models of microwave oven were used, Quasar model YM06675YW (Matsushita Electric Co., Yamatokoriyama, Japan) and Panasonic model NE9900-C (Matsushita). The pasteurization procedure was as follows: a temperature probe supplied with each microwave oven was suspended into the milk in the beaker in such a way that the probe end was in the milk to a depth of approximately 8 - 1 0 cm; medium heat (450 W) was selected, the temperature was set to rise to 65 ° C and the " T e m p e r a t u r e hold" feature was used to maintain the selected temperature for 30 min; the milk was then cooled in a household refrigerator. Portions of both raw and heat-treated milk of approximately 60 ml were poured off into sterile plastic vials (Starplex Mississauga, Ontario, Canada) immediately after processing. Methods used for analysis of raw milk samples were standard plate count (SPC) using Petrifilm SM (3M Company, Medical Products Division, St. Paul, Minnesota) (Ginn et al., 1984) and coliform count using Petrifilm VRB (3M) (Nelson et al., 1984). Appropriate dilutions were made in Butterfield's phosphate buffer to anticipate counts up to 1.5 x 106 per ml. One pasteurized milk sample was used for SPC after the sample had cooled to 25-28 ° C. The second pasteurized milk sample was stored at 4 ° C for 7 days and was then plated for SPC. SPCs were determined by directly plating 1 ml of undiluted pasteurized milk. Individual teat samples were milked into sterile vials (Starplex) and SPC performed to assess the quality of milk directly from each doe. For home storage of milk, 1 1 Mason jars (Dominion Glass) were sanitized by adding 5 cm of water to each clean bottle and microwaving for 20 min at high (650 W) power. Lids were sanitized by boiling 15-20 min on a kitchen stove.
Results
Twenty-five milk samples of approximately 2600 ml (2000 ml to 3000 ml) were processed in the microwave ovens. Sample numbers 7, 8, 9, 14 and 22 (Table I) were tested from pails that were washed but not sanitized. The average coliform count of
61 TABLE I Standard plate count (SPC) and coliform count results of pre-pasteurization, post-pasteurization and 7-day post-pasteurization of 25 samples of fresh goat's milk Sample No.
Colony count per ml on indicated medium Raw milk
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Pasteurized milk
Coliform
SPC
SPC SPC Day0
SPC SPC Day7
4 0 9 15 10 34 51 68 150 15 40 2 5 5 0 52 0 2 1 0 23 2 0 52 6
21 000 26 000 159000 180000 13000 380000 1 500000 1 300000 1 400 000 180000 19000 4000 190000 1 100000 24000 49000 41000 3000 36 000 21000 43000 540000 70 000 140000 58000
2 5 7 5 3 0 1 0 0 0 0 0 1 3 0 0 0 0 1 2 0 0 2 2 0
6 3 0 0 0 0 0 2 0 1 0 2 1 0 2 0 1 0 0 0 0 0 0 3 2
raw milk from these pails was 55 per ml (range 2 to 150) while the average standard plate count was 1.1 x 10 6 organisms per ml (range 5.4 x 105 to 1.5 x 1 0 6 ) . The average coliform count from 20 milkings into sanitized pails was 15 per ml (range 0 to 52) and the average standard plate count was 8.8 x 104 per ml (range 3 x 103 to 4.8 x 105). The average post-pasteurization SPC from all 25 samples was 1.4 organisms per ml (range 0 to 7) while the average 7-day holding SPC was 0.9 organisms per ml (range 0 to 6). Samples 1 to 16 (Table I) were pasteurized in the Quasar microwave oven and samples 17 to 25 were pasteurized in the Panasonic oven. The average post-pasteurization SPC from each method was not notably different on inspection (with ranges of 0 to 7 and 0 to 2, respectively).
62 T A B L E II Standard plate count results (in duplicate) for right and left udder samples from four milking does Goat
N u m b e r of colonies per ml of milk Right half sample
Left half sample
A
58 6
72 8
B
43 65
38 92
C
128 93
650 1 200
D
10 23
13 31
Samples A to D (Table II) were raw milk samples directly from each half of the udder of four different does. The average SPC was 160 per ml (range 6 to 1200). Organoleptic analysis of milk stored in mason jars indicated that pasteurized milk soured within 4 days when stored in clean non-sanitized jars whereas milk stored in unopened sanitized jars was still palatable for at least 2 weeks.
Discussion
Our results summarized in Table I showed that there was on average a 5-log reduction of the bacterial contamination for all 25 raw milk samples. The microwave oven has been shown to be an effective pasteurizer. We did not perform alkaline phosphatase testing since goat's milk phosphatase is more heat sensitive than cow's milk phosphatase and thus the test cannot be used to indicate adequate pasteurization of goat's milk (M.H. Brodsky and B.W. Ciebin, L a b o r a t o r y Services Branch, Ontario Ministry of Health, personal communication). G o a t ' s milk is susceptible to off-flavour or 'goatiness'; however, it has been demonstrated that properly pasteurized fresh milk can be stored for long periods without loss of flavour quality (Guy et al., 1985). We decided to use a lower temperature, longer time method of pasteurization based on our own flavour preference. In a preliminary test we pasteurized several batches by heating to 75 ° C and then cooling the milk. Although indications were that pasteurization would be adequate the milk had a 'cooked' flavour. Temperature distribution within the milk being treated was not measured. Because microwave cooking is known to produce ' h o t spots' and uneven temperature distribution, we opted for the slower low-temperature, long time method, theorizing that the longer time would allow temperatures to equalize. Petrifilm has been used as an alternative method for performing SPC (Ginn et al., 1984) and coliform counts (Nelson et al., 1984) from raw milk samples. We found that the same technique could be used under field conditions. In
63 fact, we were able to detect subclinical mastitis in one goat (Table II, goat C) before the condition was apparent to the animal handler. Roberts (1985) stated that any problems with goat's milk encountered during her survey related to poor hygiene during production rather than transmission of organisms from the goat. Our individual half sample analyses (Table II) were performed to assess the contribution to bacterial load from the goat and we believe that our data support this conclusion. Moreover, we performed a series of counts from samples milked into unsanitized pails and the results (Table I) of sample numbers 7, 8, 9, 14 and 22 indicated a considerable effect from the pail itself. Hand milking presents many opportunities for contamination of milk, including the personal cleanliness of the milker and the cleanliness of the milking parlour as has been suggested by others (Jensen and Hughes, 1980; Roberts, 1985). We have succeeded in reducing the aerobic plate count and increasing the shelf-life by heat treating goat's milk. Although we did not examine specifically for pathogens we can infer that reductions of aerobic plate counts of up to 6 logs indicate improved safety of the product. Additional studies including inoculating milk with specific pathogens must be undertaken. The basis for effective pasteurization is the amount of heat required to inactivate C o x i e l l a b u r n e t i i ; however, testing with C. b u r n e t i i could not be undertaken in this home pasteurization situation. To demonstrate that microwave treatment is equivalent to legally proscribed methods of pasteurization for the inactivation of pathogenic bacteria such as S a l m o n e l l a spp. or L i s t e r i a m o n o c y t o g e n e s further investigations must be carried out. These results suggest that a domestic model microwave oven could be used to heat-treat goat's milk for home consumption. The same methodology could be applied to treatment of cow's milk for home consumption. Availability of a simple pasteurization method may encourage more farm families to heat-treat milk thus reducing the number of infections related to this source.
Acknowledgements We wish McQuilken particularly manuscript
to acknowledge the support of Panasonic Canada, particularly Roxanne for supplying the microwave ovens used in this work and 3M Canada, R. Rumble for supplying Petrifilm products. For critical review of the we thank Michael H. Brodsky.
References Aldis, R., Bullen, M. and Thompson, R. (1986) Salmonella muenster - at home on the farm. Ontario Dis. Surveill. Rep. 7, 236-237. Duncan, L., Mai, V., Carter, A., Carlson, J.A.K., Borczyk, A. and Karmali, M.A. (1986) Outbreak of gastrointestinal disease in Sarnia, Ontario. Ontario Dis. Surveill. Rep. 7, 604-611. Ginn, R.E., Packard, V.S. and Fox, T.L. (1984) Evaluation of the 3M dry medium culture plate (Petrifilm SM) method for determining numbers of bacteria in raw milk. J. Food Protect. 47, 735-755.
64 Guy. E.J., Hicks, K.B., Flanagan, J.F., Foglia, T.A. and Holsinger, V.H. (1985) Effect of storage of raw and pasteurized goat's milk on flavour acceptability, psychotrophic bacterial count, and content of organic acids. J. Food Protect. 48, 122-129. Hayes, P.S., Feeley, J.C., Graves, L.M., Ajello, G.W. and Fleming, D.W. (1986) Isolation of Listeria monocytogenes from raw milk. Appl. Environ. Microbiol. 51,438-440. Hutchinson, D.N., Bolton, F.J., Jelley, W.C.N., Mathews, W.G., Telford, D.R., Counter, D.E., Jessop, E.G. and Horsley, S.D. (1985) Campylobacter enteritis associated with consumption of raw goat's milk. Lancet (1985) i, 1037-1038. Jensen, N. and Hughes, D. (1980) Public health aspects of raw goat's milk produced throughout New South Wales. Food Technol. Aust. 32, 336-341. Jessop, J.S., Black, W.A., Mclnnes, W., Louie, K. and Greenfield, J. (1985) Outbreak of an unusual Salmonella serotype spread by goat's milk - British Columbia. Can. Dis. Weekly Rep. 11, 219-220. Klein, B.S., Vergeront, J.M., Blaser, M.J., Edmonds, P., Brenner, D.J., Janssen, D. and Davis, J.P. (1986) Campylobacter infection associated with raw milk. J. Am. Med. Assoc. 225, 361-364. Nelson, C.L., Fox, T.L. and Busta, F.F. (1984) Evaluation of dry medium film (Petrifilm VRB) for coliform enumeration. J. Food Protect. 47, 520-525. Potter, M.E., Kaufmann, A.F. and Blake, P.A. (1984) Unpasteurized milk: the hazards of a health fetish. J. Am. Med. Assoc. 252, 2048-2052. Roberts, D. (1985) Microbiological aspects of goat's milk. A Public Health Laboratory Service survey. J. Hyg. Camb. 94, 31-44. Schiemann, D.A. and Toma, S. (1978) Isolation of Yersinia enterocolitica from raw milk. Appl. Environ. Microbiol. 35, 54-58. Tamblyn, S. (1986) Raw milk again - Perth District Health Unit. Ontario Dis. Surveill. Rep. 7, 235. Thompson, J.S., Cahoon, F.E. and Hodge, D.S. (1986) Isolation rates of Campylobacter from three regions of Ontario, Canada, 1978 to 1985. J. Clin. Microbiol. 24, 876-878. Walker, S.J. and Gilmour, A. (1986) The incidence of Yersinia enterocolitica and Yersinia enterocolitica- like bacteria in goat's milk in Northern Ireland. Lett. Appl. Microbiol. 3, 49-52.
59
FOOD 00296
In-home pasteurization of raw goat's milk by microwave treatment J. Stephen Thompson and Annemarie Thompson Regional Public Health Laboratory, Laboratory Services branch, Ontario Ministry of Health, Peterborough, Ontario, Canada (Received 30 May 1989; accepted 14 August 1989)
Raw milk has been implicated as an important source of infection with the common bacterial agents associated with gastroenteritis. While established methods of home pasteurization can be cumbersome and tedious, we have demonstrated that a domestic microwave oven can be used effectively to reduce aerobic plate counts in raw goat's milk by up to 6 log cycles without impairing the organoleptic quahty. Good keeping quality of the irradiated product was demonstrated by the 7-day holding standard plate count. Petrifilm SM and Petrifilm VRB were used successfully under field conditions. Further investigation must be undertaken to demonstrate that this method is equivalent to the legally accepted pasteurization procedures for the inactivation of such pathogenic bacteria as Salmonella spp., Listeria monocytogenes and the pasteurization standard Coxiella burnetii. Key words: Microwave pasteurization; Raw milk; Goat's milk; Pasteurization
Introduction Raw cow's milk has b e e n implicated as a n i m p o r t a n t source of i n f e c t i o n with the c o m m o n bacterial agents associated with gastroenteritis i n c l u d i n g Salmonella spp. (Jessop et al., 1985; Aldis et al., 1986; T a m b l y n , 1986), Campylobacterjejuni ( P o t t e r et al., 1984; Klein et al., 1986), a n d Yersinia enterocolitica ( S c h i e m a n n a n d T o m a , 1978). These organisms have also b e e n associated with the c o n s u m p t i o n of goat's milk in reports from C a n a d a (Jessop et al., 1985; T h o m p s o n et al., 1986), E n g l a n d ( H u t c h i n s o n et al., 1985), N o r t h e r n I r e l a n d ( W a l k e r a n d G i l m o u r , 1986) a n d A u s t r a l i a (Jensen a n d Hughes, 1980). R a w milk has b e e n i m p l i c a t e d as a vehicle for tran=mission of such other i m p o r t a n t p a t h o g e n s as Listeria monocytogenes (Hayes et al., 1986) a n d Escherichia coli 0157 : H7 ( D u n c a n et al., 1986). I n u r b a n areas raw milk is often c o n s u m e d as a ' h e a l t h food' (Potter et al., 1984; Roberts, 1985) b u t we believe that o n the farm c o n s u m p t i o n of raw milk has c o n t i n u e d because of tradition and, p e r h a p s m o r e i m p o r t a n t l y , because established m e t h o d s of home p a s t e u r i z a t i o n are c u m b e r s o m e . W e p r e s e n t in this report a simple
Correspondence address." J.S. Thompson, P.O. Box 265, Peterborough, Ontario, Canada, K9J 6Y8. 0168-1605/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
60 method of home pasteurization using household microwave ovens. All the work presented here was performed in a kitchen and not under laboratory conditions.
Materials and Methods
Four Saanen milking does were hand-milked into seamless plastic pails. Twentyfive samples of blended milk from these goats were used in this study. Each sample was between 2 and 3 1 and representative of the daily volume of household milk that could be pasteurized and consumed. After use each pail was washed and sanitized using household dishwashing detergent followed by soaking in a solution of approximately 0.5% sodium hypochlorite for approximately 10 min. A series of milkings was performed into pails that were washed but not sanitized. A 3-1 glass beaker (Pyrex No. 1000, Corning) was used as the vessel for pasteurizing each milk sample. Two models of microwave oven were used, Quasar model YM06675YW (Matsushita Electric Co., Yamatokoriyama, Japan) and Panasonic model NE9900-C (Matsushita). The pasteurization procedure was as follows: a temperature probe supplied with each microwave oven was suspended into the milk in the beaker in such a way that the probe end was in the milk to a depth of approximately 8 - 1 0 cm; medium heat (450 W) was selected, the temperature was set to rise to 65 ° C and the " T e m p e r a t u r e hold" feature was used to maintain the selected temperature for 30 min; the milk was then cooled in a household refrigerator. Portions of both raw and heat-treated milk of approximately 60 ml were poured off into sterile plastic vials (Starplex Mississauga, Ontario, Canada) immediately after processing. Methods used for analysis of raw milk samples were standard plate count (SPC) using Petrifilm SM (3M Company, Medical Products Division, St. Paul, Minnesota) (Ginn et al., 1984) and coliform count using Petrifilm VRB (3M) (Nelson et al., 1984). Appropriate dilutions were made in Butterfield's phosphate buffer to anticipate counts up to 1.5 x 106 per ml. One pasteurized milk sample was used for SPC after the sample had cooled to 25-28 ° C. The second pasteurized milk sample was stored at 4 ° C for 7 days and was then plated for SPC. SPCs were determined by directly plating 1 ml of undiluted pasteurized milk. Individual teat samples were milked into sterile vials (Starplex) and SPC performed to assess the quality of milk directly from each doe. For home storage of milk, 1 1 Mason jars (Dominion Glass) were sanitized by adding 5 cm of water to each clean bottle and microwaving for 20 min at high (650 W) power. Lids were sanitized by boiling 15-20 min on a kitchen stove.
Results
Twenty-five milk samples of approximately 2600 ml (2000 ml to 3000 ml) were processed in the microwave ovens. Sample numbers 7, 8, 9, 14 and 22 (Table I) were tested from pails that were washed but not sanitized. The average coliform count of
61 TABLE I Standard plate count (SPC) and coliform count results of pre-pasteurization, post-pasteurization and 7-day post-pasteurization of 25 samples of fresh goat's milk Sample No.
Colony count per ml on indicated medium Raw milk
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Pasteurized milk
Coliform
SPC
SPC SPC Day0
SPC SPC Day7
4 0 9 15 10 34 51 68 150 15 40 2 5 5 0 52 0 2 1 0 23 2 0 52 6
21 000 26 000 159000 180000 13000 380000 1 500000 1 300000 1 400 000 180000 19000 4000 190000 1 100000 24000 49000 41000 3000 36 000 21000 43000 540000 70 000 140000 58000
2 5 7 5 3 0 1 0 0 0 0 0 1 3 0 0 0 0 1 2 0 0 2 2 0
6 3 0 0 0 0 0 2 0 1 0 2 1 0 2 0 1 0 0 0 0 0 0 3 2
raw milk from these pails was 55 per ml (range 2 to 150) while the average standard plate count was 1.1 x 10 6 organisms per ml (range 5.4 x 105 to 1.5 x 1 0 6 ) . The average coliform count from 20 milkings into sanitized pails was 15 per ml (range 0 to 52) and the average standard plate count was 8.8 x 104 per ml (range 3 x 103 to 4.8 x 105). The average post-pasteurization SPC from all 25 samples was 1.4 organisms per ml (range 0 to 7) while the average 7-day holding SPC was 0.9 organisms per ml (range 0 to 6). Samples 1 to 16 (Table I) were pasteurized in the Quasar microwave oven and samples 17 to 25 were pasteurized in the Panasonic oven. The average post-pasteurization SPC from each method was not notably different on inspection (with ranges of 0 to 7 and 0 to 2, respectively).
62 T A B L E II Standard plate count results (in duplicate) for right and left udder samples from four milking does Goat
N u m b e r of colonies per ml of milk Right half sample
Left half sample
A
58 6
72 8
B
43 65
38 92
C
128 93
650 1 200
D
10 23
13 31
Samples A to D (Table II) were raw milk samples directly from each half of the udder of four different does. The average SPC was 160 per ml (range 6 to 1200). Organoleptic analysis of milk stored in mason jars indicated that pasteurized milk soured within 4 days when stored in clean non-sanitized jars whereas milk stored in unopened sanitized jars was still palatable for at least 2 weeks.
Discussion
Our results summarized in Table I showed that there was on average a 5-log reduction of the bacterial contamination for all 25 raw milk samples. The microwave oven has been shown to be an effective pasteurizer. We did not perform alkaline phosphatase testing since goat's milk phosphatase is more heat sensitive than cow's milk phosphatase and thus the test cannot be used to indicate adequate pasteurization of goat's milk (M.H. Brodsky and B.W. Ciebin, L a b o r a t o r y Services Branch, Ontario Ministry of Health, personal communication). G o a t ' s milk is susceptible to off-flavour or 'goatiness'; however, it has been demonstrated that properly pasteurized fresh milk can be stored for long periods without loss of flavour quality (Guy et al., 1985). We decided to use a lower temperature, longer time method of pasteurization based on our own flavour preference. In a preliminary test we pasteurized several batches by heating to 75 ° C and then cooling the milk. Although indications were that pasteurization would be adequate the milk had a 'cooked' flavour. Temperature distribution within the milk being treated was not measured. Because microwave cooking is known to produce ' h o t spots' and uneven temperature distribution, we opted for the slower low-temperature, long time method, theorizing that the longer time would allow temperatures to equalize. Petrifilm has been used as an alternative method for performing SPC (Ginn et al., 1984) and coliform counts (Nelson et al., 1984) from raw milk samples. We found that the same technique could be used under field conditions. In
63 fact, we were able to detect subclinical mastitis in one goat (Table II, goat C) before the condition was apparent to the animal handler. Roberts (1985) stated that any problems with goat's milk encountered during her survey related to poor hygiene during production rather than transmission of organisms from the goat. Our individual half sample analyses (Table II) were performed to assess the contribution to bacterial load from the goat and we believe that our data support this conclusion. Moreover, we performed a series of counts from samples milked into unsanitized pails and the results (Table I) of sample numbers 7, 8, 9, 14 and 22 indicated a considerable effect from the pail itself. Hand milking presents many opportunities for contamination of milk, including the personal cleanliness of the milker and the cleanliness of the milking parlour as has been suggested by others (Jensen and Hughes, 1980; Roberts, 1985). We have succeeded in reducing the aerobic plate count and increasing the shelf-life by heat treating goat's milk. Although we did not examine specifically for pathogens we can infer that reductions of aerobic plate counts of up to 6 logs indicate improved safety of the product. Additional studies including inoculating milk with specific pathogens must be undertaken. The basis for effective pasteurization is the amount of heat required to inactivate C o x i e l l a b u r n e t i i ; however, testing with C. b u r n e t i i could not be undertaken in this home pasteurization situation. To demonstrate that microwave treatment is equivalent to legally proscribed methods of pasteurization for the inactivation of pathogenic bacteria such as S a l m o n e l l a spp. or L i s t e r i a m o n o c y t o g e n e s further investigations must be carried out. These results suggest that a domestic model microwave oven could be used to heat-treat goat's milk for home consumption. The same methodology could be applied to treatment of cow's milk for home consumption. Availability of a simple pasteurization method may encourage more farm families to heat-treat milk thus reducing the number of infections related to this source.
Acknowledgements We wish McQuilken particularly manuscript
to acknowledge the support of Panasonic Canada, particularly Roxanne for supplying the microwave ovens used in this work and 3M Canada, R. Rumble for supplying Petrifilm products. For critical review of the we thank Michael H. Brodsky.
References Aldis, R., Bullen, M. and Thompson, R. (1986) Salmonella muenster - at home on the farm. Ontario Dis. Surveill. Rep. 7, 236-237. Duncan, L., Mai, V., Carter, A., Carlson, J.A.K., Borczyk, A. and Karmali, M.A. (1986) Outbreak of gastrointestinal disease in Sarnia, Ontario. Ontario Dis. Surveill. Rep. 7, 604-611. Ginn, R.E., Packard, V.S. and Fox, T.L. (1984) Evaluation of the 3M dry medium culture plate (Petrifilm SM) method for determining numbers of bacteria in raw milk. J. Food Protect. 47, 735-755.
64 Guy. E.J., Hicks, K.B., Flanagan, J.F., Foglia, T.A. and Holsinger, V.H. (1985) Effect of storage of raw and pasteurized goat's milk on flavour acceptability, psychotrophic bacterial count, and content of organic acids. J. Food Protect. 48, 122-129. Hayes, P.S., Feeley, J.C., Graves, L.M., Ajello, G.W. and Fleming, D.W. (1986) Isolation of Listeria monocytogenes from raw milk. Appl. Environ. Microbiol. 51,438-440. Hutchinson, D.N., Bolton, F.J., Jelley, W.C.N., Mathews, W.G., Telford, D.R., Counter, D.E., Jessop, E.G. and Horsley, S.D. (1985) Campylobacter enteritis associated with consumption of raw goat's milk. Lancet (1985) i, 1037-1038. Jensen, N. and Hughes, D. (1980) Public health aspects of raw goat's milk produced throughout New South Wales. Food Technol. Aust. 32, 336-341. Jessop, J.S., Black, W.A., Mclnnes, W., Louie, K. and Greenfield, J. (1985) Outbreak of an unusual Salmonella serotype spread by goat's milk - British Columbia. Can. Dis. Weekly Rep. 11, 219-220. Klein, B.S., Vergeront, J.M., Blaser, M.J., Edmonds, P., Brenner, D.J., Janssen, D. and Davis, J.P. (1986) Campylobacter infection associated with raw milk. J. Am. Med. Assoc. 225, 361-364. Nelson, C.L., Fox, T.L. and Busta, F.F. (1984) Evaluation of dry medium film (Petrifilm VRB) for coliform enumeration. J. Food Protect. 47, 520-525. Potter, M.E., Kaufmann, A.F. and Blake, P.A. (1984) Unpasteurized milk: the hazards of a health fetish. J. Am. Med. Assoc. 252, 2048-2052. Roberts, D. (1985) Microbiological aspects of goat's milk. A Public Health Laboratory Service survey. J. Hyg. Camb. 94, 31-44. Schiemann, D.A. and Toma, S. (1978) Isolation of Yersinia enterocolitica from raw milk. Appl. Environ. Microbiol. 35, 54-58. Tamblyn, S. (1986) Raw milk again - Perth District Health Unit. Ontario Dis. Surveill. Rep. 7, 235. Thompson, J.S., Cahoon, F.E. and Hodge, D.S. (1986) Isolation rates of Campylobacter from three regions of Ontario, Canada, 1978 to 1985. J. Clin. Microbiol. 24, 876-878. Walker, S.J. and Gilmour, A. (1986) The incidence of Yersinia enterocolitica and Yersinia enterocolitica- like bacteria in goat's milk in Northern Ireland. Lett. Appl. Microbiol. 3, 49-52.
