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Observations on the Growth Responses of Streptococcus Lactis in Mastitis Milk1
OBSERVATIONS ON THE GROWTH RESPONSES OF STREPTOCOCCUS L A C T I S IN M A S T I T I S ]Y[ILK1 C. C. PROUTY Washington Agricultural Experiment Station, Division of Dairy Husbandry Several investigators have ascribed certain failures in the manufacturing of cheddar cheese to the presence of mastitis milk. Leitch (1) is of the opinion that mastitis infection should be suspected when recurring difficulties are experienced because of slow acid development, curd weakness and f a u l t y texture. H e noted also that the inclusion of even small amounts of mastitis milk in the cheese vat resulted in almost complete suspension of the desired lactic acid development. Whitehead and Cox (2) found that milk containing leucocytes in excess of 5,000,000 per ml. gave rise to a rennet curd in which streptococci were not able to develop normal amounts of acid. Davis and Mattick (3) concluded that visibly abnormal milk should be excluded in the m a n u f a c t u r i n g of cheese and that milk reacting positively to the strip cup or to the bromcresol-purple test should be regarded as being abnormal until proof is obtained that it may be used with safety. Davis (4, 5) states that slow starter is still the most common fault in cheese making and one of the most serious consequences of mastitis. He is of the opinion that this condition, together with other factors induced b y mastitis, causes more trouble than is realized in the making of cheese. According to Davis (6) the most common cause of slow starters in E n g l a n d appears to be abnormal milk from mastitis afflicted cows. He cites four changes in the milk that may influence the rate of growth of starter organisms, namely: (a) changes in the chemical composition of the milk, particularly a decrease in the lactose, casein, calcium and acidity; (b) changes in some enzymes and decreases in some vitamins and bacterial growth factors ; (c) increased number of bacteria in the u d d e r ; and (d) a p p a r e n t production, in rare instances, of substances strongly toxic to starter organisms. Davis and McClemont (7) studied the acid coagulating time in both normal and mastitis milk. With the majority of the mastitis milk samples slow growth of S. lactis and S. cremoris occurred, whereas most of the normal samples, but not all, supported normal growth of these organisms. They consider that the reason for the slow growth of these organisms, probably, is due to the abnormal chemical composition of such milk. In the present study frequent observations, over an extended period of time, were made on the growth responses of Streptococcus lactis in milk drawn from the individual infected and non-infected udder-quarters of the Received for publication March 11, 1940. 1 Published as Scientific Paper No. 451, College of Agriculture and Agricultural Experiment Station, State College of Washington. 899
900
c . c . PROUTY
same cows. The i n f e c t i o n of some of these u d d e r - q u a r t e r s was of such a n a t u r e t h a t the m i l k was d e f i n i t e l y a b n o r m a l a t some o b s e r v a t i o n p e r i o d s a n d a p p a r e n t l y n o r m a l a t other times. The e x p e r i m e n t a l d a t a p r e s e n t e d i n c l u d e o n l y a p o r t i o n of t h a t collected, b e i n g confined to a s t u d y of the m i l k f r o m t h r e e cows, b u t t h e y are r e p r e s e n t a t i v e of the f i n d i n g s w i t h m i l k f r o m other a n i m a l s .
PROCEDURE Samples were collected from the individual udder quarters in such a manner as to exclude gross contamination, and each consisted of approximately 200 ml. of milk drawn after the first few streams had been discarded, except in the case of several of the very abnormal milks where it was not possible to obtain this amount at a sampling period. Portions of the milk were inoculated with a culture of S. lactis, incubated at 18 ° to 20 ° C. and observations made on subsequent growth. The rate of growth was measured by the changes in pH value using the quinhydrone electrode supplemented by microscopic examinations of stained preparations. EXPERIMENTAL RESULTS Of the three cows u s e d i n this s t u d y , cow 114 gave m i l k f r o m one u d d e r q u a r t e r t h a t was a b n o r m a l at all times, w h e r e a s the m i l k f r o m the diseased u d d e r - q u a r t e r s of cows 126 a n d 118, as j u d g e d b y its p i t v a l u e s a n d p h y s i c a l state, f l u c t u a t e d b e t w e e n a n a p p a r e n t l y n o r m a l state a n d a d e f i n i t e l y a b n o r m a l c o n d i t i o n t h r o u g h o u t the p e r i o d of observation. A s u m m a r y of the results, as m e a s u r e d b y changes i n p H values, of n u m e r o u s series of m i l k f r o m each a n i m a l is shown i n t a b l e 1. TABLE 1 Range in p H values of mil]~ from diseased and normal quarters before and after inoculation with S. lactis
Range in pit Cow
114
Quarter
Condition of quarter
No. of samples
Range in p~l after 24 hrs. incubation
Minimum
Maxi-
Nini-
IllUlll
IaUlIl
~laximum
RF-RH and LII
LF
Diseased Normal
14
7.10 6.54
7.67 6.78
6.44 4.41
7.30 5.23
126
RF LF RH and LIt
Diseased Diseased Normal
20
6.54 6.55 6.49
7.30 7.47 6.78
4.54 4.53 4.52
7.05 7.35 5.14
118
RF RH LH and LF
Diseased Diseased Normal
22
6.68 6.66 6.49
7.30 7.05 6.82
4.68 4.70 4.30
7.10 7.04 5.10
C o w 114. The l e f t f r o n t u d d e r - q u a r t e r was i n f e c t e d w i t h a streptococcus h a v i n g b e t a h e m o l y t i c characteristics. No a t t e m p t was m a d e to
S T R E P T O C O C C U S L A C T I S I N :~IASTITIS I ~ I L K
901
further identify the organism. Milk from this quarter was alkaline in reaction at all times, and ranged in p i t values from 7.10 to 7.67. Milk from the remaining quarters ranged in p i t value from 6.54 to 6.78. The milk from the left front quarter, when .inoculated with S. lactis and incubated for 24 hours showed p H values ranging from 6.44 to 7.30 as compared to pH values ranging from 4.41 to 5.23 for milk samples from the normal quarters held under similar conditions of incubation. Microscopic examinations showed definitely that the milk from the left front quarter of this cow retarded the growth of S. lactis, thus confirming the findings as indicated by the study of the p i t values. Cow 126. Mastitis of a staphylococcic nature was present in the right front and left front quarters of this cow. l~Iilkfrom the right front quarter ranged in pH value from 6.54 to 7.30 and milk from the left front quarter from 6.55 to 7.47. Milk from the two apparently normal quarters ranged from 6.49 to 6.78 in p i t value. When inoculated with S. lactis and incubated for 24 hours the pt I values of the samples from the right front quarter ranged from 4.54 to 7.05, for the left front quarter from 4.53 to 7.35 and from the two remaining and apparently normal quarters from 4.52 to 5.14. The milk samples in which normal acid development did not occur had initial p H values of 6.9 and higher. Two samples, however, with p H values of 6.92 permitted S. lactis to develop in a normal manner as judged by decreased p H values. Cow 118. The right front and right hind quarters of this cow showed definite evidence of mastitis due either to a staphylococcus or to an organism having the morphological characteristics of organisms of the genus Corynebacterium or to both, since these organisms were present in considerable numbers in both diseased quarters. The milk from these quarters ranged in initial p i t values from 6.68 to 7.30 and from 6.66 to 7.05 respectively. The p i t values recorded for the milk from the two remaining quarters varied from 6.49 to 6.82. After inoculation with S. lactis and incubating for 24 hours, milk from the right front quarter ranged in p i t value from 4.68 to 7.10, from the right hind quarter from 5.70 to 7.04 and from the two apparently normal quarters from 4.30 to 5.10. Greater variation occurred with the milk from this cow in the initial pH value of the samples showing retarded acid development than occurred in the milk from cow 126. One sample with an initial p i t value as low as 6.68 showed marked delayed acid development, whereas other samples with initial pII values up to 6.95 showed little or no delayed acid development.
Effect of adjusting the p H value of mastitis milk to that of normal milk on the subsequent growth responses of S. lactis Since the most favorable reaction for the growth of S. lactis corresponds to that of normal milk, the relatively high pFi values of mastitis milk tend
902
c . c . PROUTY
to create an unfavorable environmental condition for the best development of this organism. Also, a considerable amount of physiological activity is necessary on the p a r t of the culture, in producing a~l a m o u n t of lactic acid sufficient to reduce the p H value of mastitis milk to t h a t of normal milk. Numerous experiments, therefore, were carried out in which the p H value of the mastitis milk was adjusted to t h a t of the normal milk f r o m the healthy udder-quarters by the addition of lactic acid. Both the normal milk samples and the adjusted mastitis milk samples were inoculated with S. lactis and observations made in the usual manner. On incubation the p H values of the adjusted mastitis milk, in almost every instance, failed to decrease as r a p i d l y as did the p H values of the normal milk. Usually there was little or no appreciable change in the p H values of the adjusted mastitis milk during the first fifteen to t w e n t y hours, a f t e r which time f a i r l y r a p i d acid development took place with the final p i t values d r o p p i n g to a p p r o x i m a t e l y the same levels as those of the n o r m a l milk samples.
Effect of mixing normal and mastitis milk on the subsequent growth of S. lactis Numerous experiments were carried out in which v a r y i n g proportions of normal and mastitis milk were mixed, inoculated with the test culture and f r e q u e n t observations made during the incubating period. The extent of the r e t a r d i n g effect depended on the degree of abnormality. M a n y of the samples of definitely abnormal milk in which the p H values were 7.2 and higher exerted a r e t a r d i n g effect u p o n the growth of S. lactis when mixed with normal milk in as little as 10 per cent concentrations. Other samples in which the growth of S. lactis was only slightly retarded, lost this characteristic when diluted with as little as 10 per cent of normal milk.
Effect of pasteurizing mastitis milk on the subsequent growth responses of S. lactis Since it has been shown by H a m m e r and B a k e r (8) t h a t the growth of s t a r t e r organisms is more r a p i d in milk t h a t has been subjected to high pasteurizing temperatures, numerous experiments were carried out in which portions of mastitis milk were subjected to t e m p e r a t u r e s of 62.5, 65.5 and 68.5 ° C. for periods of 30 minutes. Pasteurizing at 62.5 ° C. had little or no effect on the subsequent growth of S. lactis. Pasteurizing some samples at 65.5-68.5 ° C. f o r 30 minutes tended to p a r t i a l l y overcome the r e t a r d i n g influence on the growth of S. lactis. W i t h most of the milk samples, however, heat t r e a t m e n t at these t e m p e r a t u r e s had little or no appreciable effect.
STREPTOCOCCUS LACTIS IN ~ASTITIS
MILK
903
DISCUSSION OF RESULTS With the milks used in this study the retarding effects on the growth of S. lactis cannot be attributed to the high p H values nor to the presence of a thermolabile inhibiting substance such as that associated with excessive numbers of leucocytes. Although leucocyte counts were not recorded in this study, the samples that were definitely abnormal contained excessive numbers of these cells as indicated by the examination of stained preparations for the presence of S. lactis. In this respect the results reported herein differ from those of Whitehead and Cox (2) in which the inhibiting effect of abnormal milk containing large numbers of leucocytes was entirely removed by heating the milk for 30 minutes at 63 ° C. The results of this study tend to bear out the contention of Davis and McClemont (7) that the slow growth of S. lactis in mastitis milk probably is associated with the changed chemical composition of such milk. While the extent of the retarding effect of mastitis milk usually was associated with the degree of abnormality such, however, was not always the case. Under the prevailing conditions of milk production it is unlikely that any considerable quantities of definitely physically abnormal milk would find its way into that used for commercial purposes. Certain occasions might arise, however, where milk, that appears normal in its physical properties yet possessing definite inhibiting action on the growth of S. lactis, may be present in sufficient amounts to interfere with manufacturing proeessses dependent upon the development of this organism. The variations noted in the growth responses of S. lactis in the milks from the three cows used in this study may have been due in part, to the respective types of mastitis involved. Unfortunately, no milk for extensive study was available from a cow suffering with Streptococcus agalactiae type of mastitis, the type most common among dairy cows. However, in connection with this study, numerous observations were made using milk from S. agalactiae infected udder-quarters with the same general results. SUMMARY
The growth responses of S. lactis, as judged by changes in the p i t values and microscopic examinations have been studied in separate samples of milk drawn from the infected and non-infected udder-quarters of cows suffering with mastitis. 1V[ilk with an initial pH value greater than 6.9 usually failed to support: the growth of S. lactis in an active manner, whereas normal milk from the. other udder-quarters showed normal acid development. The growth responses of S. iactis varied somewhat in the milk drawn from the three cows under study.
904
c . c . PROUTY
A d j u s t i n g the p H v a l u e of m a s t i t i s m i l k to t h a t of n o r m a l m i l k r e s u l t e d i n o n l y p a r t i a l l y o v e r c o m i n g d e l a y e d acid d e v e l o p m e n t . M a s t i t i s m i l k t r e a t e d i n this m a n n e r c h a n g e d little i n p H v a l u e u n t i l a f t e r fifteen to t w e n t y hours, a f t e r w h i c h t i m e acid d e v e l o p m e n t was q u i t e r a p i d . The a d d i t i o n of as little as 10 p e r cent of v e r y a b n o r m a l m a s t i t i s m i l k to n o r m a l m i l k h a d a r e t a r d i n g effect on S. lactis d e v e l o p m e n t . P a s t e u r i z i n g some s a m p l e s of m a s t i t i s m i l k at 65.5 ° C.-68.5 ° C. f o r 30 m i n u t e s t e n d e d to p a r t i a l l y overcome the r e t a r d i n g i n f l u e n c e o n the g r o w t h of S. lactis; however, w i t h most of the s a m p l e s s t u d i e d h e a t t r e a t m e n t at these t e m p e r a t u r e s h a d little or n o a p p r e c i a b l e effect. REFERENCES (1) LEITCH,RENWICKH. Mastitis milk in cheese-making. Scottish J. of Agr., 19: 343349. 1936. (2) WHITEHEAD,~UGH ROBISON~AND Cox~ GEOFFREYALAN. Observations on some factors in the milk of individual cows which modify the growth of lactic streptococci. Biochem. J.~ 27: ]Part 1, 951-959. 1933. (3) DAVIS, J. G., AND ~/IATTICK,A. T . R . Mastitis in relation to cheese-making. Agr. Prog. 13: 126-133. 1936. (4) DAVIS,J.G. Mastitis and the dairy industry. Dairy Ind. Nov., 1938. (5) DAVIS, J. G. Mastititis. IV. Mastitis in relation to cheese making. Agr. Prog, 16: 9-15. 1939. (6) DAVIS,J . G . The management of starters. Dairy Ind. March, 1938. (7) DAVIS, J. G., AND MCCLEMONT,J. Studies in mastitls. V. Mastitis in relation to cheese-making. J. of Dairy Res., 10: 94-103. 1939. (8) HAMMER,B. W., AND BAKER, M. P. Studies on starters. Part III. Effect of the pasteurization exposure on the rate of coagulation of a starter. Iowa Agr. Exp. Sta. Res. Bull. 106. 1928.
900
c . c . PROUTY
same cows. The i n f e c t i o n of some of these u d d e r - q u a r t e r s was of such a n a t u r e t h a t the m i l k was d e f i n i t e l y a b n o r m a l a t some o b s e r v a t i o n p e r i o d s a n d a p p a r e n t l y n o r m a l a t other times. The e x p e r i m e n t a l d a t a p r e s e n t e d i n c l u d e o n l y a p o r t i o n of t h a t collected, b e i n g confined to a s t u d y of the m i l k f r o m t h r e e cows, b u t t h e y are r e p r e s e n t a t i v e of the f i n d i n g s w i t h m i l k f r o m other a n i m a l s .
PROCEDURE Samples were collected from the individual udder quarters in such a manner as to exclude gross contamination, and each consisted of approximately 200 ml. of milk drawn after the first few streams had been discarded, except in the case of several of the very abnormal milks where it was not possible to obtain this amount at a sampling period. Portions of the milk were inoculated with a culture of S. lactis, incubated at 18 ° to 20 ° C. and observations made on subsequent growth. The rate of growth was measured by the changes in pH value using the quinhydrone electrode supplemented by microscopic examinations of stained preparations. EXPERIMENTAL RESULTS Of the three cows u s e d i n this s t u d y , cow 114 gave m i l k f r o m one u d d e r q u a r t e r t h a t was a b n o r m a l at all times, w h e r e a s the m i l k f r o m the diseased u d d e r - q u a r t e r s of cows 126 a n d 118, as j u d g e d b y its p i t v a l u e s a n d p h y s i c a l state, f l u c t u a t e d b e t w e e n a n a p p a r e n t l y n o r m a l state a n d a d e f i n i t e l y a b n o r m a l c o n d i t i o n t h r o u g h o u t the p e r i o d of observation. A s u m m a r y of the results, as m e a s u r e d b y changes i n p H values, of n u m e r o u s series of m i l k f r o m each a n i m a l is shown i n t a b l e 1. TABLE 1 Range in p H values of mil]~ from diseased and normal quarters before and after inoculation with S. lactis
Range in pit Cow
114
Quarter
Condition of quarter
No. of samples
Range in p~l after 24 hrs. incubation
Minimum
Maxi-
Nini-
IllUlll
IaUlIl
~laximum
RF-RH and LII
LF
Diseased Normal
14
7.10 6.54
7.67 6.78
6.44 4.41
7.30 5.23
126
RF LF RH and LIt
Diseased Diseased Normal
20
6.54 6.55 6.49
7.30 7.47 6.78
4.54 4.53 4.52
7.05 7.35 5.14
118
RF RH LH and LF
Diseased Diseased Normal
22
6.68 6.66 6.49
7.30 7.05 6.82
4.68 4.70 4.30
7.10 7.04 5.10
C o w 114. The l e f t f r o n t u d d e r - q u a r t e r was i n f e c t e d w i t h a streptococcus h a v i n g b e t a h e m o l y t i c characteristics. No a t t e m p t was m a d e to
S T R E P T O C O C C U S L A C T I S I N :~IASTITIS I ~ I L K
901
further identify the organism. Milk from this quarter was alkaline in reaction at all times, and ranged in p i t values from 7.10 to 7.67. Milk from the remaining quarters ranged in p i t value from 6.54 to 6.78. The milk from the left front quarter, when .inoculated with S. lactis and incubated for 24 hours showed p H values ranging from 6.44 to 7.30 as compared to pH values ranging from 4.41 to 5.23 for milk samples from the normal quarters held under similar conditions of incubation. Microscopic examinations showed definitely that the milk from the left front quarter of this cow retarded the growth of S. lactis, thus confirming the findings as indicated by the study of the p i t values. Cow 126. Mastitis of a staphylococcic nature was present in the right front and left front quarters of this cow. l~Iilkfrom the right front quarter ranged in pH value from 6.54 to 7.30 and milk from the left front quarter from 6.55 to 7.47. Milk from the two apparently normal quarters ranged from 6.49 to 6.78 in p i t value. When inoculated with S. lactis and incubated for 24 hours the pt I values of the samples from the right front quarter ranged from 4.54 to 7.05, for the left front quarter from 4.53 to 7.35 and from the two remaining and apparently normal quarters from 4.52 to 5.14. The milk samples in which normal acid development did not occur had initial p H values of 6.9 and higher. Two samples, however, with p H values of 6.92 permitted S. lactis to develop in a normal manner as judged by decreased p H values. Cow 118. The right front and right hind quarters of this cow showed definite evidence of mastitis due either to a staphylococcus or to an organism having the morphological characteristics of organisms of the genus Corynebacterium or to both, since these organisms were present in considerable numbers in both diseased quarters. The milk from these quarters ranged in initial p i t values from 6.68 to 7.30 and from 6.66 to 7.05 respectively. The p i t values recorded for the milk from the two remaining quarters varied from 6.49 to 6.82. After inoculation with S. lactis and incubating for 24 hours, milk from the right front quarter ranged in p i t value from 4.68 to 7.10, from the right hind quarter from 5.70 to 7.04 and from the two apparently normal quarters from 4.30 to 5.10. Greater variation occurred with the milk from this cow in the initial pH value of the samples showing retarded acid development than occurred in the milk from cow 126. One sample with an initial p i t value as low as 6.68 showed marked delayed acid development, whereas other samples with initial pII values up to 6.95 showed little or no delayed acid development.
Effect of adjusting the p H value of mastitis milk to that of normal milk on the subsequent growth responses of S. lactis Since the most favorable reaction for the growth of S. lactis corresponds to that of normal milk, the relatively high pFi values of mastitis milk tend
902
c . c . PROUTY
to create an unfavorable environmental condition for the best development of this organism. Also, a considerable amount of physiological activity is necessary on the p a r t of the culture, in producing a~l a m o u n t of lactic acid sufficient to reduce the p H value of mastitis milk to t h a t of normal milk. Numerous experiments, therefore, were carried out in which the p H value of the mastitis milk was adjusted to t h a t of the normal milk f r o m the healthy udder-quarters by the addition of lactic acid. Both the normal milk samples and the adjusted mastitis milk samples were inoculated with S. lactis and observations made in the usual manner. On incubation the p H values of the adjusted mastitis milk, in almost every instance, failed to decrease as r a p i d l y as did the p H values of the normal milk. Usually there was little or no appreciable change in the p H values of the adjusted mastitis milk during the first fifteen to t w e n t y hours, a f t e r which time f a i r l y r a p i d acid development took place with the final p i t values d r o p p i n g to a p p r o x i m a t e l y the same levels as those of the n o r m a l milk samples.
Effect of mixing normal and mastitis milk on the subsequent growth of S. lactis Numerous experiments were carried out in which v a r y i n g proportions of normal and mastitis milk were mixed, inoculated with the test culture and f r e q u e n t observations made during the incubating period. The extent of the r e t a r d i n g effect depended on the degree of abnormality. M a n y of the samples of definitely abnormal milk in which the p H values were 7.2 and higher exerted a r e t a r d i n g effect u p o n the growth of S. lactis when mixed with normal milk in as little as 10 per cent concentrations. Other samples in which the growth of S. lactis was only slightly retarded, lost this characteristic when diluted with as little as 10 per cent of normal milk.
Effect of pasteurizing mastitis milk on the subsequent growth responses of S. lactis Since it has been shown by H a m m e r and B a k e r (8) t h a t the growth of s t a r t e r organisms is more r a p i d in milk t h a t has been subjected to high pasteurizing temperatures, numerous experiments were carried out in which portions of mastitis milk were subjected to t e m p e r a t u r e s of 62.5, 65.5 and 68.5 ° C. for periods of 30 minutes. Pasteurizing at 62.5 ° C. had little or no effect on the subsequent growth of S. lactis. Pasteurizing some samples at 65.5-68.5 ° C. f o r 30 minutes tended to p a r t i a l l y overcome the r e t a r d i n g influence on the growth of S. lactis. W i t h most of the milk samples, however, heat t r e a t m e n t at these t e m p e r a t u r e s had little or no appreciable effect.
STREPTOCOCCUS LACTIS IN ~ASTITIS
MILK
903
DISCUSSION OF RESULTS With the milks used in this study the retarding effects on the growth of S. lactis cannot be attributed to the high p H values nor to the presence of a thermolabile inhibiting substance such as that associated with excessive numbers of leucocytes. Although leucocyte counts were not recorded in this study, the samples that were definitely abnormal contained excessive numbers of these cells as indicated by the examination of stained preparations for the presence of S. lactis. In this respect the results reported herein differ from those of Whitehead and Cox (2) in which the inhibiting effect of abnormal milk containing large numbers of leucocytes was entirely removed by heating the milk for 30 minutes at 63 ° C. The results of this study tend to bear out the contention of Davis and McClemont (7) that the slow growth of S. lactis in mastitis milk probably is associated with the changed chemical composition of such milk. While the extent of the retarding effect of mastitis milk usually was associated with the degree of abnormality such, however, was not always the case. Under the prevailing conditions of milk production it is unlikely that any considerable quantities of definitely physically abnormal milk would find its way into that used for commercial purposes. Certain occasions might arise, however, where milk, that appears normal in its physical properties yet possessing definite inhibiting action on the growth of S. lactis, may be present in sufficient amounts to interfere with manufacturing proeessses dependent upon the development of this organism. The variations noted in the growth responses of S. lactis in the milks from the three cows used in this study may have been due in part, to the respective types of mastitis involved. Unfortunately, no milk for extensive study was available from a cow suffering with Streptococcus agalactiae type of mastitis, the type most common among dairy cows. However, in connection with this study, numerous observations were made using milk from S. agalactiae infected udder-quarters with the same general results. SUMMARY
The growth responses of S. lactis, as judged by changes in the p i t values and microscopic examinations have been studied in separate samples of milk drawn from the infected and non-infected udder-quarters of cows suffering with mastitis. 1V[ilk with an initial pH value greater than 6.9 usually failed to support: the growth of S. lactis in an active manner, whereas normal milk from the. other udder-quarters showed normal acid development. The growth responses of S. iactis varied somewhat in the milk drawn from the three cows under study.
904
c . c . PROUTY
A d j u s t i n g the p H v a l u e of m a s t i t i s m i l k to t h a t of n o r m a l m i l k r e s u l t e d i n o n l y p a r t i a l l y o v e r c o m i n g d e l a y e d acid d e v e l o p m e n t . M a s t i t i s m i l k t r e a t e d i n this m a n n e r c h a n g e d little i n p H v a l u e u n t i l a f t e r fifteen to t w e n t y hours, a f t e r w h i c h t i m e acid d e v e l o p m e n t was q u i t e r a p i d . The a d d i t i o n of as little as 10 p e r cent of v e r y a b n o r m a l m a s t i t i s m i l k to n o r m a l m i l k h a d a r e t a r d i n g effect on S. lactis d e v e l o p m e n t . P a s t e u r i z i n g some s a m p l e s of m a s t i t i s m i l k at 65.5 ° C.-68.5 ° C. f o r 30 m i n u t e s t e n d e d to p a r t i a l l y overcome the r e t a r d i n g i n f l u e n c e o n the g r o w t h of S. lactis; however, w i t h most of the s a m p l e s s t u d i e d h e a t t r e a t m e n t at these t e m p e r a t u r e s h a d little or n o a p p r e c i a b l e effect. REFERENCES (1) LEITCH,RENWICKH. Mastitis milk in cheese-making. Scottish J. of Agr., 19: 343349. 1936. (2) WHITEHEAD,~UGH ROBISON~AND Cox~ GEOFFREYALAN. Observations on some factors in the milk of individual cows which modify the growth of lactic streptococci. Biochem. J.~ 27: ]Part 1, 951-959. 1933. (3) DAVIS, J. G., AND ~/IATTICK,A. T . R . Mastitis in relation to cheese-making. Agr. Prog. 13: 126-133. 1936. (4) DAVIS,J.G. Mastitis and the dairy industry. Dairy Ind. Nov., 1938. (5) DAVIS, J. G. Mastititis. IV. Mastitis in relation to cheese making. Agr. Prog, 16: 9-15. 1939. (6) DAVIS,J . G . The management of starters. Dairy Ind. March, 1938. (7) DAVIS, J. G., AND MCCLEMONT,J. Studies in mastitls. V. Mastitis in relation to cheese-making. J. of Dairy Res., 10: 94-103. 1939. (8) HAMMER,B. W., AND BAKER, M. P. Studies on starters. Part III. Effect of the pasteurization exposure on the rate of coagulation of a starter. Iowa Agr. Exp. Sta. Res. Bull. 106. 1928.