July, 1923 Research Bulletin No. 81
STUDIES ON STREPTOCOCCUS
PARACITROVORUS GROUP
By B. W. HAMMER AND M. P. BAKER
AGRICULTURAL EXPERIMENT STATION
IOWA STATE COLLEGE OF AGRICULTURE
AND MECHANIC ARTS
CE) CURTISS, Director
DAIRY SECTION
THE LIBRARY OF The
NIV ERSITY OF iLLINOIS:
AMES, IOWA
SUMMARY
A study of 124 cultures of 8. paracitrovorus from milk, cream
and butter confirms the characterization of the group as those
streptococci that produce a definite but variable increase in the
acidity of milk and a high volatile acidity, which is greatly in-
creased by the adding of citric acid to the milk to be fermented.
Organisms having the same general appearance in ditmus milk
as S. paracitrovorus but which fail to give a high volatile acidity
or to show an increased volatile acidity when citric acid is added
to the milk are readily isolated from dairy products.
Cultures of 8. paracitrovorus destroyed the citric acid in the
milk in which they had grown, while the organisms failing to
produce a high volatile acidity, did not.
In general S. paracitrovorus produced considerable CO, but
there was no clear cut difference in the CO, production of these
organisms and of other types so that it seems a determination of
the CO, produced will not afford a means of identifying S. para-
citrovorus cultures.
Certain of the S. paracitrovorus cultures show considerable
resistance to heat and are undoubtedly capable of withstanding
the usual pasteurization exposures.
Ropiness was observed in one culture of S. paracitrovorus out
of the 124 studied.
No satisfactory basis was found for the division of the S.
paracitrovorus cultures into types; further work may, however,
provide such a basis.
S. paracitrovorus is rather widely distributed in milk and
cream, It seems to be more common than SN. citrovorus.
Studies on the
Streptococcus Paracitrovorus Group
By B. W. HAMMER AND M. P. BAKER
For a number of years the dairy section of the Iowa Agricul-
tural Experiment Station has been interested in the organisms
that are capable of producing considerable volatile acid in milk.
These organisms have been repeatedly isolated from starters, in
which they are evidently of importance from the standpoint of
flavor and odor development, and largely on the basis of a study
of these starter-cultures they have been divided into two species
—Streptococceus citrovorus and Streptococcus paracitrovorus.
The present paper deals with S. paracitrovorus from the stand-
point of its characters when isolated from milk, cream and but-
ter, special attention being given to whether or not the charac-
ters suggest a division into types. The distribution of the or-
ganisms in these dairy products is also briefly considered.
CHARACTERIZATION OF THE 8. PARACITROVORUS GROUP
The S. paracitrovorus group is considered to include the
streptococci that produce a definite but variable increase in the
acid of milk and a high volatile acidity ; the volatile acidity pro-
duced is, moreover, greatly increased in amount if sterile citric
acid solution is added to the milk to be fermented.
The appearance of the S. paracitrovorus cultures in litmus
milk, which is of importance because it is the medium into which
colonies from dairy products are most often picked, is quite
variable, largely because of differences in the rate and extent of
acid production. Occasionally a culture is secured which shows
considerable reddening of the litmus in a day or so and when
a heated loop is plunged into it gas bubbles may form and rap-
idly come to the surface; such cultures are likely to produce
a gassy curd in a comparatively short time. With other cul-
tures the acid development is slower and when coagulation
occurs the curd formed may be without gas holes due apparently
to the fact that the gas development had largely ceased before
coagulation occurred.t Some S. paracitrovorus cultures never
“seem to show coagulation and gas development is difficult to
observe at any time by the usual methods.
1. The importance of coagulation during the time of active gas de-
velopment in the production of a gassy curd has been dealt with by the
Iowa station in a study of the formation of gas in the making of cot-
tage cheese. Studies on the Formation of Gas in Milk, Hammer, B. W,
iets <ptiasta. kes, Bulw 27, Jan. LOG,
742755
20
There seems to be a gradation from the S. paracitrovorus
organisms which show a high total acidity in milk down thru
those which show a low total acidity to a type which does not
cause a reddening of the litmus because of the very small acid
production and which has been ealled S. citrovorus. This or-
ganism, like S. paracitrovorus, produces considerable volatile
acid in milk and an increased amount of volatile acid on the
addition of sterile citric acid solution to the milk.
ISOLATION AND IDENTIFICATION OF 8S. PARACITROVORUS
The isolation of S. paracitrovorus from such materials as sour
milk, sour cream, ete., can usually be readily accomplished by
plating out on whey agar and, after incubating at room temper-
ature, picking colonies into litmus milk. The usual type of SV.
lactis cultures can be easily eliminated by the coagulation and
reduction that they cause. Cultures of S. paracitrovorus that
coagulate rather quickly are occasionally encountered but can
usually be detected by a few gas bubbles that appear along the
side of the tube and in such cultures there is often a copious
evolution of gas when a heated needle is plunged into it. The
more common behavior of the S. paracitrovorus organisms that
are picked into litmus milk is a slow acid development that is
usually evident in two or three days at room temperature by
a change in the color of the litmus. Cultures producing a slow
reddening of the litmus that are not 8S. paracitrovorus are also
commonly found however and certain of these which have been
studied are streptococci and cannot be distinguished from S.
paracitrovorus by any rapid tests such as morphological or eul-
tural. The odor of S. paracitrovorus cultures is quite charac-
teristic and after experience with the group it is possible to make
a reasonably accurate separation of the acid producing strepto-
cocci that are not typical S. lactis into 8. paracitrovorus and
those yielding only a low volatile acidity. A determination of
the volatile acid production is the final test in this differentiation
and a second determination on milk to which sterile citric acid
was added before inoculating supplies added proof.
Some of the differentiating media such as casein agar made
with peptone and lactose or whey agar to which transparent
milk? is added at the time of pouring plates seem to be helpful
in the isolation of S. paracitrovorus since by their use many
of the S. lactis colonies can be detected without picking into
litmus milk. The cultures that are picked, however, must be
studied as are those secured from whey agar plates.
2. Transparent Milk as a Bacteriological Medium. Brown, J. How-
ard, and Howe, Paul E. Jr. Bact. 7, S. 1922, p. 511.
21
METHODS USED
The S. paracitrovorus cultures studied were secured by plat-
ing the various materials on whey agar and picking a consider-
able number of colonies into litmus milk. The cultures giving
the changes expected in litmus milk were plated to insure purity
and then tried out for volatile acid production before being
definitely considered as 8. paracitrovorus.
In studying the cultures isolated particular attention was
given to the characters that past experience suggests as being
of importance with the typical milk organisms. Certain char-
acters which are extremely valuable with some groups are be-
heved to be of little value with those that are primarily milk
types and their study has largely been omitted.
The volatile acid production in milk or milk to which citric
acid had been added was determined by the method previously
used by the Iowa Agricultural Experiment Station? which gives
the results as the ec. of N/10 alkali required to neutralize the
first liter of distillate when a 250 gram portion was distilled with
steam after the addition of 15 ec. of approximately N/1 H.SO,.
The total acidity was determined by titrating 20 grams of milk
with N/10 NaOH, using phenolphthalein as an indicator, and
calculating as lactic acid.
When citric acid was added to milk it was sterilized as an
aqueous solution and added to the sterile milk just before imocu-
lating. The method outlined by Supplee and Bellis,* for the
estimation of citric acid in milk and milk products was used for
determining the presence or absence of citric acid in milk fer-
mented by various organisms.
SOURCES OF CULTURES STUDIED
The S. paracitrovorus cultures studied were isolated from
milk, cream and butter, most of them being secured when these
products were plated and colonies picked for the purpose of
classifying the organisms contained. A number of cultures of
another type that were looked upon as possible 8. paracitrovorus
when the original cultures were first observed because of the
change produced in litmus milk were studied but failed to show
a high volatile acidity in milk. Some of the results obtained with
these organisms are given in order to bring out clearly the im-
portant characters of S. paracitrovorus.
A total of 124 cultures of S. paracitrovorus were isolated and
studied but not all of them were studied from the standpoint
of each character dealt with; when it seemed that a certain
determination was of no importance for either characterization
or classification it was investigated with only a part of the cul-
tures.
meres tulis: la.Agr. Exp. Stas 55,63, and’ 66.
4. Citric Acid Content of Milk and Milk Products. Supplee, G. C.,
and Bellis, B. Jour. Biol. Chem. 48, O. 1921. p. 453.
22
RESULTS SECURED
OBSERVATIONS ON MORPHOLOGY
The description of the morphology given by the Iowa station”
for the 8S. paracitrovorus cultures isolated from starters apples
in general to those secured from milk, cream and butter. The
cells were almost spherical, altho usually slightly longer than
broad. A paired arrangement was very common but short
chains were frequent in preparations made from agar slopes and
also from milk. Occasionally chains somewhat longer than those
usually found were seen but these were not correlated with any
other variation observed and were finally considered to be of no
special significance. The organisms giving the same general ap-
pearance in litmus milk as S. paracitrovorus but which failed to
produce a high volatile acidity showed the same general morph-
ology as the organisms classed as typical S. paracitrovorus. Both
eroups showed some variations in the size of cells but they were
quite alike and the size of each agreed with the dimensions re-
ported by the Iowa station for the S. paracitrovorus cultures
from starters.
The data obtained on the morphology of the 8S. paracitrovorus
organisms indicate that it is of no value in characterizing or
classifying the group. Organisms producing only a low volatile
acidity showed essentially the same morphology as did the high
volatile acid producers and morphology was entirely valueless
in attempting to select organisms of the latter group.
ROPINESS WITH S. PARACITROVORUS
Of the 124 8. paracitrovorus cultures studied one—no. 27 of
table I—was regularly ropy when grown in milk. The mor-
phology of this organism with the ordinary stains showed nothing
unusual and it was impossible to find any evidence of capsules.
The formation of a ropy condition in milk by organisms of
the S. paracitrovorus group is rather to be expected since ropi-
ness occasionally occurs in other groups of organisms common in
milk. It is not lhkely, however, that the production of ropiness
will be of value from the standpoint of the classification of the
S. paracitrovorus organisms since it is of no importance with
the other groups on account of its variability. Data secured at
the lowa® station showed that certain pure cultures of S. lactis
showing ropiness yielded non-ropy cultures in small numbers
on plating rather regularly and that these non-ropy cultures
showed a morphology essentially like the ropy ones.
5. Volatile Acid Production of S. lacticus and the Organisms Asso-
ciated with it in Starters, Hammer, B. W., Res. Bul. la, Agr bx ptesca:
a (Oly IEP Oe
6: Studies on Ropiness in Cultures of S. lactis, Hammer, B: WW.) Res:
Bulls la eA Sr Bip tee stein 4.0 degen
23
TOTAL AND VOLATILE ACID PRODUCTION
The total and volatile acidities in milk and the volatile acidity
in milk to which 0.4 percent citric acid had been added are given
in table I for the 124 cultures of 8. paracitrovorus; the source
of each organism is also included.
The total acidities produced in milk varied from 0.27 to 0.99
percent, calculated as lactic acid, with an average of 0.46 per-
eent. The distribution of the cultures in the different acid
ranges is Shown in table II; the minimum, maximum and aver-
age volatile acidity for each range is also shown and these values
will be referred to under the discussion of the volatile acid pro-
duction. From this table it will be seen that 33.1 percent of
the cultures studied fell in the acid range from 0.41 to 0.5 per-
cent while 24.2 percent fell in the acid range from 0.31 to 0.4
percent; accordingly over one half (actually 57.3 percent) of
the cultures studied produced acidities ranging from .31 to .5
percent. Outside of these limits the acidities produced by the
cultures showed wide variations.
The total acidity produced in milk by a certain culture of VS.
paracitrovorus would be expected to show variations in different
tests, as do other organisms producing acid in milk, In a con-
siderable number of trials repeated determinations of the total
acidity produced by a certain culture of S. paracitrovorus
showed wide differences. It was a rather common experience to
plate out a pure culture of one of these organisms and after
picking colonies into litmus milk find that some coagulated and
some did not. In one instance a culture that had quickly formed
a gassy curd at room temperature after being inoculated was
plated out and 20 eolonies picked into litmus milk; none of the
cultures developing showed coagulation after one month at room
temperature and after two months only three were coagulated
and these showed no gas holes. The variation in the acid pro-
duction may be due to variations in the composition of the milk,
in the temperature at which the milk is held and probably in
other factors; it indicates that the presence or absence of coagu-
lation cannot be considered of importance in identifying cul-
tures of S. paracitrovorus, since the usual differences in acid
production may result in one organism causing coagulation in
milk and another not.
The volatile acidity produced in milk by the S. paracitrovorus
cultures studied (table I) varied from 12.5 to 40.0 and
averaged 28.6.* There seemed to be no very definite rela-
tionship between a high total acidity and a high volatile
*As pointed out under “Methods Used” these values represent the ce.
of N/10 alkali required to neutralize the first liter of distillate when
a 250 gram portion was distilled with steam after the addition of 15
ec. of approximately N/1 HeSO..
24
TABLE I—THE SOURCE, AND TOTAL AND VOLATILE ACID PRODUCTION OF
THE S. PARACITROVORUS CULTURES STUDIED
Volatile acidity in
Culture number Source Total acidity | ———————_—_ a
in milk Milk plus
Milk 0.4% citric acid
LC ee eee ees Souracreainiy sos se ee .5D 18.5 70.0
PO 8 ee oe SOUrpCrea ines ee “BH 31.0 5120
PASO t ee SOURPCICAIi =e eee .36 32.0 72.4
Para ea SOuUreGre a eae eee 39 21.0 88.5
Parayee one ee Source sins. == aes .40 23.8 76.0
NeW ATS | Apex eta Ee Sour crea ing aes 43 32.0 78.5
JET Wiel tos | Yee ee ee Sour creania.--=soeee .30 26.2 66.9
Pare eres SOULACrEa = eee .62 30.9 82.5
Pala dee ae Sour Creaiieesse eee 59 SHEA) 76.8
joecass § Se 20 SOULNCreAli =a eee ee 259 32.4 80.0
(etwas ahh eee & SOUETCredn == eee .38 29.6 Slay,
IPB AAs ee) Sa SOUrCreaIn sae see 60 Bites 81.8
Palen 3 oe Sourscresnp eee 42 33.4 82.8
OTe Pan ee eens SOurECrealnis = eee -40 24.0 With
PAT. eb 2. eee Sourvcreanies ee ape 22.8 (Glee
PAs ae lO eee Sour cream 2-2 .58 31.6 Cond
detianguly GW ep ce ees SOUr Clea ee 22a .42 29.4 72.4
PaTrasveels sees Soureerea m= .29 19.1 66.5
AL eee LO pete ees SOUT CLea ni n= === ae .45 26.8 86.9
Para eae Sour Cre nin ee ae .39 31.8 74.4
Pa hinge Lee ee SOutsCclea l= ae 52 31-0 77.8
atl see eee SOUrecre ale. === .36 Done 87.0
ieee 2a eres oa SOUteCre ain seas 639 26.0 63.2
PATO ade eee SOUT. Crean =e .48 31.0 toU
Pata eee oo Sour (cream si) 31.0 73.0
PAT ote oO aa SOULECTOAT === === ena .39 30.4 70.9
1G PAP See cee Sourecte@arnp e222. =a 259 36.6 Hleoe
Pata nu28 2. eee = Sourecreatim eee .48 25.5 76.6
Petia =e Sounpecrenniy sae eee .48 36.1 83.9
Para eo eae Sourecren eee 31 95.6 87.0
Palisa Sourscereame eon 253 28.9 74.7
(Be Ehe BPA Se a ye SO cre sin) eee 30 21.9 68.0
Para wos eee eee SS OUT Cre alae eee PA 20.3 80.4
Pata agote ee Sourscreali eee ee .46 23.0 72.3
Paraneoo eae os Soutseres ae ae £50 31.9 12.2
Pi ee oO eee SOUTECTGs Titman ae .48 31.6 81.0
Para me (eee SOUie Che aimee 45 PA ses 61.0
Para aeose ee Sourschen ae eee RET 25.1 12.8
Para aoe = Sour cream 22-222. 2 254 36.5 77.9
ieee, Ci Seurscream 2 soso. 54 33.4 73.0
Para SoOurscre 21a 43 26.4 74.5
Parad eee SO uliew Cie cri) eee 56 29.1 80.5
Para) 243 Sea SOUEECTCAI a= =e an nae 61 19.4 74.0
1etige, CR ne Sourere alia eee eee .50 82.5 44.4
Patiie4) eee Sournmencn lm aaeoaee ae ae BD, Son 85.2
Seed Oe eee Sour creams eee .40 23.6 61.4
POT Oe ee ee Sour creamy 2 oe .40 26a tose
(Pala oe see ae Butter woe ee ee .28 HDA T 50.0
Para, 0490.2. Butter eee 44 37.3 77.9
POT 0 ae ee Butter ee eee oe ee .46 20.0 75.8
Pavarsia Butter 22 ee ee .b4 o2ee R23
Ralarolge oer Butter eee eee 30 33.0 75.0
Pardee DS poe ee Bitter ee ee .74 32.0 85.0
PAT Oe 4 eee Butter ee eee Set Zou. 70.0
AEH ATT aoe Soe cres is eee .55 32.8 83.7
IDAalhes pO ee Butters eee ee TG 88.4 80.2
Pas AO (ee oe eee Butter sence .99 81.4 75.4
Paraeeoo es Buttereeeee eee pe ha Se Ped Sag 41 38.0 89.0
Para ae Ope Buttere oe ose ee eA Bien 59.5
Para Oe eee SOU Cr Co, 1) eee ea .78 34.0 85.0
IPA a6) eee SOUT sere din nee [83 28.0 76.6
PAT Rem O pee eee Sourrercanis see .86 Bie O 69.0
Lee We hee Sie, Sees oe SoOuracredin a= see ees .39 bears 66.2
NetWarahe ave ee cot as oe SOMPMecredi =e eee 48 25.9 83.5
PAT hee One eee SOUnSCrCAln oo .45 B20 96.6
PAT ae OO. eso oe SOUT CrGA Ty == eee .60 40.0 83.0
25
TABLE I—Continued
Volatile acidity in
Culture number Source Total acidity |-——-——____-_———_
in milk Milk plus
Milk 0.4% citrie acid
ay MeeeeOd eo = an SOUrs Creag o- = = eee ADT 28.6 62.5
TEE, SOUP Cre Alig ae ee -Sy/ 28.8 61.4
LEONE 0 SOUT ChCA IIs! oa ears 44 28.9 73.4
rat, ae SOUtmere plies ae 41 17.6 80.4
IEC G7 a SOUDECrea Ii g= === aaa mae 41 30.0 57.5
lesniGhy 212i SOUPECTCAIN = eee ee .39 95.3 67.7
PA ae ee fois. soak el SOU CLe hill a eee .40 30.0 v0 e4
Tei "7c eee arr SOUDeCrCAlI Maes eens .46 82.0 (ALW
Ded ih AT re SOUmCcreaniee see 52 39.0 81.0
Pie, aie SOUPECreC al === = ee 43 26.5 69.9
IEC, “Wie SOUDMCrCA lI) areas eee 525 24.3 (200
Do. 1 ——— SoOurmerealne= =e eee .36 B2eo 77.9
Tet hoi (0) ne ISOM Cre niin 30 19.3 76.9
Se i ‘Sour eran 222.02 22. .28 26.2 Wei
LES Wetley US ee eee SOUrecrea nin == eee .49 33.5 89.5
Lets ty ars SOUMMCLE A lee ees .38 84.9 78.9
1 gy ‘Soursereain 252.2262. 40 34.7 86.0
JER. et SOULS Crea Ie eee ae 50 34.0 85.0
Ue Wee SO cee = Sour cream = 22. == .36 32.0 76.8
Wea RA OG oe SOUMECCCAT eee ee eee .48 34.8 (Del
12h Beli SGOUmuCreanive eos = 44 33.5 78.7
eae Sooo eo SOUrserenins seo eee ee 28 20.0 70.1
WANA OO 2a ken. SOURPCIGAI ae 2 ee .29 SEO) 77.0
Peano O0o2 2 oo: Sour Creamy ss ee 45 26.0 75.8
Jeph to) ier SOULSCl6A Meese eee nO 35.1 78.2
IB oa Ctolibe Gee tem 2 re ae 45 30.7 65.4
IPG he Sours creainmesss oe .45 26.8 68.5
i2ariis 4) Sour crea ee 100 30.0 64.0
lenin, 955) Sourecreaiies. ==. =e .29 32.0 78.0
wanraenc96. 2. VC cere Se ane ee 47 26.5 76.9
edhe, (ore aa Nea teeSualte lee =e .36 29.0 83.5
ICR Wish eee Metallic buttermilk --_- 54 28.9 64.8
apa 09,22 Metallic buttermilk —__- .70 aoe2 80.3
earaes LOO pes ee. Metallic buttermilk —_-- 20D 834.0 83.0
Parade elles oe =: Metallic DUttermes == ons .56 31.0 68.0
1s et oa kl ee Bitters ce eee ee 4c 15.8 48.5
arame Os co) 5 2s Butterer eae .86 28.0 50.0
PAT LOs oe Sweet cream butter___- AUS 36.0 85.0
Iara \Obs2=. 22 2.- Bitterman. Ss eee .46 97.5 74.0
Para L0Gm.=-—- = Mil eee oe ee 47 29.9 68.5
Para lO, sc. 25.2 Mil kgeeee ee ee > eee ee 53 26.7 77.8
Inara use. oo VE i) ee i en eee 45 32.8 80.0
1247075 | 3) a ir Mil knee ees cee a .48 TO) 72.0
PAT vee lO Milk-geeeeresa ewes 2 ees 54 31.8 78.2
Wardell o. Milkaeeeece = tae Fe .65 31.0 87.0
ae lo ee SS VET) ee ae Se eS ee 29 Py atl 58.4
eid, Mbp CU GUW ES - 2 Ct Say oe heer Be ea .38 eo 66.0
arnt 2s 2 Mil ieee ae ee ee ee .56 26.4 75.5
ions The Butter having ‘burnt’’
TlaVOlEe es eee n= .49 30.0 73.0
ae NOs ss es Butter having “burnt”
flavOt meee eee eee .58 ilenes 73.0
Leite a Butter having “burnt”
EON. ae ee ee Boe .58 26.2 75.6
iia 10 ae Butter having ‘‘burnt”’
i aiViO tae eee 47 24.0 82.0
SE Weta a Co ee Butter having “burnt”
{laviorwee= wees eee .49 26.4 64.5
Ae Lae sees can |SYU ELT, See Sew ee .28 76: 57.3
IEW, BD SOlliss Cre At eee at ron 12.9 59.4
TEC Se. BAZ Aelia ee moter Buble es eee ee .36 125 56.6
Paine teom ean .S SoOumeecreant 2226-22 -22- ear 13:55 60.0
AT G4 2S ee Sourecrealtees. 2s. ee AF 1lS}.ah 80.3
Wihtobbaa\wags) 2 All 1220 44.4
Witp-quan\eped, oS ae .99 40.0 96.6
AVeLai Gmc mee ree 46 28.6 73.9
26
TABLE II—DISTRIBUTION OF S. PARACITROVORUS CULTURES IN DIFFERENT
ACID RANGES
No. of |% of total] Minimum | Maximum] Average
eultures cultures | vol. acid. | vol. acid. | vol. acid.
Cultures giving total acidities
UPigbO” 231 p sete nee wee ae 17 13.7 13.1 34.0 23.4
Cultures giving total acidities
from 0.31 to 0.4 inclusive 30 24D: eA 9) 34.9) 26.8
Cultures giving total acidities
from 0.41 to 0.5 inelusive A 33.5 15.8 88.0 29.0
Cultures giving total acidities
from 0.51 to 0.6 inclusive 23 18.5 18.5 40.0 31.8
Cultures giving total acidities i
from 0.61 to 0.7 inelusive 4 ee 19.4 36.2 29.4
Cultures giving total acidities
ADO Vel RT a ieee ee owe 9 hae) 28.0 38.4 33.4
acidity since some of the cultures giving a rather high total
acidity gave a comparatively low volatile acidity while other
cultures giving a low total acidity yielded a comparatively high
volatile acidity. From table If it will be seen that while there
was a tendency toward a slightly lower average volatile acidity
among the cultures in the lower acid ranges than among those
in the higher acid ranges, the differences are of very little if any
significance.
From table I it is evident that all of the S. paracitrovorus
cultures showed a pronounced increase in the volatile acidity
produced when 0.4 percent citric acid was added to the muk
before inoculation. The volatile acidities produced in this modi-
fied milk by the 124 cultures studied varied from 44.4 to 96.6
and averaged 73.9. The increase in the volatile acidity caused
by the citric acid added was quite variable either on the
actual or percentage basis as would be expected from the
variations that occur in the volatile acidity produced in normal
milk. The inerease in the volatile acid production with all the
cultures of S. paracitrovorus when citric acid was added to the
milk is further proof that the citric acid normally present in
milk is one of the important sources of the volatile acid pro-
duced. These findings with the 124 cultures studied greatly
strenethen the evidence of the increase previously reported’ for
a limited number of 8. paracitrovorus cultures isolated from
starters,
Table III gives data similar to that of table I for 27 cultures
that were tentatively selected as S. paracitrovorus but which,
when the volatile acidities were determined, proved to be another
type. These cultures gave total acidities quite like those pro-
duced by the S. paracitrovorus cultures but only low volatile
7. See ref. 5.
27
acidities and it was these latter values that first indicated the
organisms could not be classed as 8. paracitrovorus. None of
the cultures showed a significant increase in the volatile acidity
when citric acid was added to the milk fermented and this abso-
lute correlation between the low volatile acidity in milk and
the failure to get an increase in the volatile acidity when citric
acid was added before inoculating is further proof that the
organisms were not 8. paracitrovorus. The failure to get an
increase in volatile acid by the addition of citric acid with these
organisms that produce only a low volatile acid is an additional
indication that the citric acid normally present in milk must be
a source of volatile acid in the case of the organisms producing
a high volatile acidity.
CITRIC ACID FERMENTATION
The results obtained on the increase in the volatile acid pro-
duction in milk by S. paracitrovorus as a result of the addition
TABLE III—THE SOURCE, AND TOTAL AND VOLATILE ACID PRODUCTION OF
CULTURES NOT S. PARACITROVORUS BUT APPEARING LIKE
IT IN LITMUS MILK
Volatile acidity in
Culture Source Total acidity - oo
number in milk Milk plus
Milk 0.4% citric acid
‘Cini! Sie SOU CLealiesss= eee 7, Dial 6.1
Cilio ts SOUre Clea leet= ee ee .38 Eyer 6.8
Cia eee See SOULHCreAI pe ee eee 39 Gan 7.8
Sine Lt ee SOUTPCECAMIN ee =. = ee ea aE sey 5.4 4.7
Oil Sa aes VE eee ee eee. Se Eee 742, 4.6 5.4
Se Oe VET Kae eee rr ee 41 Tok 10.7
COLE” ee SOUrECrCa iinet = ee .40 6.4 5.5
(Ck) a er Bitter yee eee oe ee 42 4.0 5.0
(O10 hoe Glue (Mrs hail Se ke .43 6.6 6.7
Obs Kt ae eee BiGteTeeee sees. ee .49 4.9 5.4
SC gleelieos BUG te tgeee™ ee ee ee ree 2 a Sail 2.8
CORDINE,~ iby ti ce IBiGGerc gee a .46 5.8 5.8
‘CCUG SABE SOunwere ale eee eee 63 4.6 6.5
Chaat SOUDSCLE QTM ese eee 51 6.5 10.7
Cielo Butter from sweet. cream__ .25 3.8 2.9
GulemelG.22= Butter ee re .45 6.7 6.0
CRA fanaa Se BUC Crae eee eee ee .48 3.6 3.9
Coulee Sas UGLC eee are ee is as .49 6.6 6.0
(Oiotiy Pak eA ee PES UETRG GT. ere ee =e ee 47 8.3 8.7
C0 S OU Cha Te a eee ee 54 5.4 5.5
Citi, Die =e SOUMRECCO AI eee eee eee E “49 6.2 6.5
(Obi) Oe SOUrseCreaMimese 2esee = yi 4.5 4.3
(Cibile VS IV eae ee eae ee aval 3.4 4.1
GE, i ee Mice eet OT ee SA 47 4.5 4.3
CChbT hs yd soy oe Milica ee eS Pe ee .5T 6.7 8.0
Oul.26-__..___|Butter having “burnt’’
TLV OTe eee oe .63 38 G52
Cals 275. 5.2: SGRTigt eset 56 5.1 5.2
Woeapeoai vine, = 22. eye 21 Spill 2.38
NL AUEMELEN ee eee .63 8.3 10.7
IAVOTACE: poe oe ee tos ok 45 9.4 6.0
28
of a sterile citric acid solution to the milk before inoculation
suggested tests for citric acid* on milk in which these organ-
isms had grown. The milk was sterilized in the autoclave and
then the organisms allowed to grow for from 9 to 11 days at
room temperature before the tests for citric acid were made.
Forty-seven cultures of 8. paracitrovorus, taken without any
attempt at selection, were tried and all of them completely de-
stroyed the citric acid present in the milk as determined by the
method employed (see ‘‘ Methods Used’’) while the uninoculated
checks showed the usual amounts of citric acid. Five organisms
of the group whose original milk cultures suggested S. paracit-
rovorus but which failed to yield a high volatile acidity or a
significant increase in volatile acidity when citric acid was added
to the milk before inoculation were tried out for their effect on
the citric acid of milk and with all five organisms there was no
evidence of any decrease. These findings with the two groups
of organisms are what would be expected when their action on
citric acid added to milk in which they are grown is taken into
account.
Bosworth and Prucha*® found that the citrie acid in milk was
changed to acetic acid and carbon dioxide during the souring
and also that starter and buttermilk contained some agent eap-
able of bringing about this change. Kunz® observed that the
citric acid gradually decreased with the aging of milk, particu-
larly when soured and Supplee and Bellis'® concluded that,
‘‘the eitrie acid content of milk decreases during aging in the
presence of high developed acidity, and is more rapid in raw
milk than in pasteurized milk.’’ From the work of these inves-
tigators it is evident that there is one or possibly more agents
in milk, starter and buttermilk that are capable of destroying the
citric acid present in milk. While the increased volatile acid
production of S. paracitrovorus in milk, when citric acid is
added, suggests that this organism is capable of destroying eitric¢
acid, determinations showing the disappearance of citrie acid in
milk in which this organism has grown is undoubtedly more
convineing proof.
CARBON DIOXIDE PRODUCTION
The evolution of gas by S. paracitrovorus in milk and milk
plus peptone was reported by the Iowa station™ in the descrip-
ticn of this organism. The production of gas with S. paracitro-
*The authors are indebted to F. F. Sherwood for assistance in mak-
ing these tests.
8. The Fermentation of Citric Acid in Milk, Bosworth, Alfred W.,
and) Prucha, M. Je ech iBbuly NY. ASE xpt. state Nee Lomas
9. The Determination of Citric Acid in Milk. Abstracted in Exp. Sta.
Rec. 36. 1917. p. 415.
10. See ref. 4.
Le See rela:
29
vorus 18- however very different than the gas production with
such typical gas-formers of milk as Bact. aerogenes since many
cultures of S. paracitrovorus can be studied in a routine way
without the formation of gas bubbles being noted. In the usual
cultures, the gas production with this organism is apparently
so slow that the diffusion of the gas into the air seems to remove
it and gas bubbles ordinarily do not form. The production of
gas should however offer a suitable method for the preliminary
selection of organisms that are likely to be S. paracitrovorus, if
the gas formation could be readily determined. Attempts were
made to detect gas formation by sealing freshly inoculated milk
cultures with such materials as paraffine, bees wax, vaseline and
various mixtures of these with each other or with paraffine oil,
the sealing material being poured on in a heated condition, and
the cultures then watched for a forcing up of the seal. Some
of the results were very satisfactory and the plugs were readily
shoved up but in other trials it was evident that the seals were
not tight and that gas could escape between the seal and the
wall of the tube. It seemed to be impossible to secure a material
that uniformly gave satisfactory seals. This was apparently in
part due to the coefficient of expansion being so high that the
contraction of the material on cooling was likely to draw it away
from the wall of the tube but it seemed that getting milk along
the wall above the surface of the milk during the inoculating
and handling of the tubes so that the seals could not be gotten
tight, was also a factor contributing to the unsatisfactory results.
The tubes used by Eldredge and Rogers'* in determining the
CO, production of organisms isolated from Emmental cheese
were next tried out as a means of detecting gas formation. Ten
ee. of milk were added to one arm of each tube and after stop-
pering with cotton, sterilizing and inoculating, 10 ce. of
N/10 Ba(OH), solution were added to the other arm. The cot-
ton stoppers were then cut off level with the glass, shoved down
and rubber stoppers very firmly inserted. The usual incubation
was 10 days at room temperature, after which the excess
Ba(OH), was titrated.
The effect on the CO, production of adding citric or lactic acid
to the milk before inoculating was also determined. A sterile
lactic acid solution was prepared so that 1 ¢c., the amount to be
added to the 10 ce. of sterile milk in each tube, contained 1/30
ec. of Mercks lactic acid U. S. P. VIII. The citric acid was
prepared so that 1 ee. of solution contained 1/30 gram of crystal-
lized citric acid (1 mol. H,O of crystallization). The acid solu-
tions were added just before inoculating; the citric acid in the
amounts used caused a rapid coagulation of the milk while the
lactic acid did not.
12. The Bacteriology of Cheese of the Emmental Type. Eldredge,
ia and Rogers, L. A. Centbl.’ f. Bakt. etc. 40. 1914," p, 5.
30
Dupheate determinations of the CO, production were made on
milk alone and on milk plus lactic or citric acid and while the
results were not encouraging from the standpoint of agreement,
a series of organisms were run both with and without adding
acid to the milk.
Sixty-four S. paracitrovorus cultures were studied for CO,
production in milk, in milk plus lactic acid, and in milk plus
citric acid. In the milk alone the cc. of N/10 Ba(OH), equiva-
lent to the CO, produced varied from 2.5 to 8.8 with the values
for most of the cultures ranging from 4.0 to 6.5 ce. The effect
of adding the acids—lactie or citric—was quite variable. With
some organisms the CO, produced was increased by adding the
acids, as might be expected in the case of citric acid from its
effect on the volatile acid production, while with others no such
increase was observed. The failure to get an increase may have
been due to the acid limiting the growth of the organisms in
certain cases altho this does not seem lkely because some of
the organisms that failed to show an increase, produced quite
large amounts of acid in milk and accordingly would be assumed
to tolerate considerable acid.
A number of cultures that gave the same gross appearance in
litmus milk as S. paracitrovorus but which did not produce a
high volatile acidity were tried for CO, production. In general
the results suggest a lower value for these organisms than for
S. paracitrovorous but there was no definite difference and the
figures for one group shade over into those for the other so that
the CO, production does not seem to offer a means of separation
upon which any considerable reliance can be placed. The CO,
production of S. lactis was also studied and a great deal of varia-
tion in cultures, all of which produced the usual low volatile
acidity, was found; in some instances the values ran as high as
the typical S. paracitrovorus cultures.
ACID FORMATION FROM VARIOUS MATERIALS
In the description of S. paracitrovorus the Iowa station™ re-
ported on acid production as follows: ‘‘ Beef extract bouillon
containing fructose, galactose, glucose, lactose, maltose, or, with
certain organisms, sucrose usually showed a sleht increase in
acidity at either 37° C. or room temperature but with glycerol,
mannitol, salicin, raffinose, inulin or starch no such inerease was
observed.’’
Additional fermentation tests were run at room temperature
with a number of S. paracitrovorus cultures and these were in
entire agreement with the above statement except that some of
the cultures fermented salicin. Accordingly salicin should be
classed with sucrose as being sometimes fermented and some-
times not.
18. See ref. 5.
31
Considerable variation was observed in the acid production
with different cultures in the same bouillon. There was a gen-
eral tendency, modified, however, by a number of exceptions,
for the cultures producing the higher acidities in milk to yield
high acidities in at least some of the bouillons.
The results obtamed up ‘to the present time do not suggest
that the fermentation of various materials will afford a method
of dividing up the S. paracitrovorus group.
RESULTS WITH JANUS GREEN
Altho the S. paracitrovorus group has very little power of
reduction, sterile milk to which 1 ce. of a sterile aqueous solution
of janus green (0.5 grams per liter) had been added per 10 ce.
was tried out with some of these organisms since janus green
has been found to be of value in showing variations in the reduc-
ing power of the S. lactis group. The reducing power of the
S. paracitrovorus cultures was so low that certain of the cultures
showed only a shght pink color in the bottom of the tube even
after periods of growth such as 20 days at 21° C.; some few
showed a pink color thruout the tube and occasionally this ap-
pearance was presented in as short a time as 4 days. From work
done with S. lactis it seems that Janus green has a restraining
action on bacterial-growth and while conditions may be different
with the S. paracitrovorus group the time required with many
cultures before there was any change in the milk to which janus
ereen had been added also suggests a restraining action due to
the indicator. A few cultures of the organisms appearing lke
S. paracitrovorus in litmus milk but giving only a low volatile
acid were also tried in janus green milk and the changes pro-
duced were quite like those produced by S. paracitrovorus. It
seems evident from the small amount of work done that janus
ereen is valueless either as a means of identifying the S. paracit-
rovorus organisms or dividing the group up into types.
GOMPARKISON OF THE GROWTH AT 37° C. and 21° C.
With organisms producing visible changes in milk only slowly,
a comparison of the growth at different temperatures is rather
difficult without elaborate methods, such as counting, and these
cannot be applied to a large number of cultures without a great
deal of work. The method followed in comparing 37° C. and
21° C. for the growth of the 8S. paracitrovorus type was to inocu-
late two tubes of Ltmus milk from the same culture with the
same loop and hold one at each temperature. After a short time
the cultures were observed for the extent of the changes in the
appearance of the litmus, since this can be assumed to be due to
the amount of growth, and a record made as to whether one
temperature seemed to favor these changes more than the other.
32
TABLE IV—INFLUENCE OF 87° C. AND 21° C. ON THE GROWTH OF S. PARA-
CITROVORUS AND LOW VOLATILE ACID PRODUCERS APPEARING
LIKE IT IN LITMUS MILK
Growth best Growth best No difference
Athomeees at 2ie8@= in growth
Total |—————— ——
cultures
No. | Percent | No. | Percent | No. | Percent
S7 Daracitrovorus eae e aaa 116 20 Ip) 56 48.3 40 34.5
Low volatile acid producers
appearing like S. para-
citrovorus in litmus milk__ 23 6 26.1 9 39.1 8 34.8
Table IV gives the results obtained on S. paracitrovorus and
also on the organisms appearing like S. paracitrovorus in litmus
milk but giving only a low volatile acid production. With the
116 S. paracitrovorus cultures tried 17.2 percent seemed to grow
better at 387° C., 48.3 percent at 21° C. and with 34.5 percent
there was no observable difference, while with the 23 organisms
of the other group 26.1 percent seemed to grow better at 37°
C., 39.1 percent at 21° C. and with 34.8 percent the two tempera-
tures seemed to be equally good. These data show that with both
eroups of organisms there is a certain variation in the way
different cultures respond to different temperatures. With
neither, however, is there a clear cut dividing line with some or-
gvanisms growing well at a certain temperature and others grow-
ing poorly or not at all, but rather a gradual variation with all
degrees of response on the part of the organisms.
RESISTANCE TO HEAT
The isolation of S. paracitrovorus from pasteurized materials
such as sweet cream suggests that at least certain of the organ-
isms of this group are rather resistant to heat. Preliminary
trials were made on a number of the cultures. The method used
was to stand tubes of litmus milk in a large volume of water to
a depth considerably greater than the depth of the milk in the
tube; after the milk had taken on the temperature of the water it
was inoculated with the organism to be studied and then held the
desired time, care being taken that the water remained at a con-
stant temperature. The tubes were then placed in cold water
for rapid cooling. After holding, the tubes were examined for
gsrowth by noting the appearance of the ltmus milk, staining,
and often by culturing on a whey agar slope. While this method
may not be as accurate as the use of Sternberg bulbs or capillary
tubes, it seemed that it would supply conditions the most like
those existing in commercial pasteurization, and for that reason
the method was followed. There was no evidence of scum for-
mation on the surface of the heated milk; this would not be ex-
33
pected since the cotton stopper and column of air above the milk
would largely prevent evaporation.
The exposures used were, for the most part, 30 minutes at
60, 65, 70 and 75° C. Some cultures failed to resist 60° C. for
30 minutes but the majority did, while 65° C. for 30 minutes
destroyed a considerable number. Only a very few cultures
resisted 70° C. for 30 minutes. Certain cultures were run a
number of times at the different exposures and there was in
general a very good agreement between the results obtained on
different days; in a few instances discrepancies occurred but
these are to be expected since such factors as the age of the cul-
tures, the acidity, etc., probably influence the resistance of an
organism to heat and in addition there is a variation in the
resistance of individual cells in the same culture.
Table V gives the final results obtained on cultures which were
run by the method above outlined, all the tests on a culture being
earried out with the same inoculating material. These data con-
firm the preliminary tests in all respects.
The results given show considerable variation in the heat re-
sistance of different cultures of S. paracitrovorus. This is to
be expected from the data reported for S. lactis as well as for
other organisms and there seems to be little reason to expect that
organisms whose morphological, cultural and bio-chemical fea-
TABLE V—HEAT RESISTANCE OF S. PARACITROVORUS
+ EQUALS GROWTH — EQUALS NO GROWTH
Heated for 30 minutes at Heated for 30 minutes at
Culture eS ao ay Culture |— ee
(BOP AC Boll Ose 165 4 CO rat LO. (GOP Ok HGS TO Th (Ok 1 4GP LO;
ae eS 2S — “= — — [240 eee e. — - oe —
Gh 1 5 se ee ++ -- — — AS a oe — — — —
Ap el a — — — — 40.0 eee de — = 2s
eee. + + — — 50 were eee — — == —
(ile oo ee — — — — Sige ee ae — — —
i 233 + — — — 5 aeeeweie. = — — — —
ct ae — — — 54> ee — — — —
No ae “ft — -= ~- DO pe eee te — — — —
ik Ape _ — — — LOO Rees eee + — — —
G3, 2-year + —_ — — PST Se eR Ae rote =F — — —
1h 3 —- — os -— 26 eee + -f- — —
itl = =e -- — — ~- DAE | Bie Se apes ++ + = +
100) ee ak os — = As Raph Nae Oe ee as =f = =
ili aie ae = — SORE een aia == == =
Co a + — — — Soh ae ae aie — —— —
CCl oer — _— — — Bi heer es - _- oo —
Of ee = AO SS — — — — Sige ee — + — —
Os pees eS — — — — Bip eee ee + — — —
TSS ot ele a -— — —- — Bio) patio nka eee ae — — - —
ICON 0 pe eh -= —_ — —- 4 es bie =f. ++ --
LG are eee => te — — (Oe eee =f == i =
TRIN ie in eee 4- -+- -- -= Abyss ae — —- — —
JV 5 + =e + — 4 Opens see fe ce — —-
liigmessasse et ob -- = -—— 6 ee -- — — —
ih = Si a. a — _— wpe eS -f + — —_
Ml Gierece + + Fa —_ Ae ren EE =e = a= =s
ile == ee + ar _— —_— Shite ee sta — = —
———
34
tures are essentially alike would have exactly the same resistance
to heat. Moreover variations in this resistance, unless extraor-
dinary, cannot be considered of significance from the standpoint
of classification except where they are correlated with other dif-
ferences.
The tests on heat resistance that have been reported show that
S. paracitrovorus may be expected to frequently resist pasteuri-
zation exposures and this explains the rather common presence
of these organisms in pasteurized products.
Tests of the resistance to heat were also run on a number of
the cultures having an appearance in litmus milk suggestive of
S. paracitrovorus but which failed to yield a high volatile acid-
ity. In general these organisms responded much as did the S.
paracitrovorus group and there seemed to be no possibility of
separating the two types of organisms on a basis of heat resist-
ance.
VARIATIONS IN STARTERS PREPARED WITH DIFFERENT S.
PARACITROVORUS CULTURES
The preparation of satisfactory starters by mixing cultures of
S. citrovorus or S. paracitrovorus and S. lactis has already been
reported by the Iowa station.’* Many starters have been pre-
pared in this way using S. paracitrovorus cultures with S. lactis
and it seems certain that excellent starters can be secured from
such mixtures. Satisfactory results are not always obtained,
however, and there is undoubtedly a difference in S. paracitro-
vorus cultures from the standpoint of the character of the
starters they will yield. From certain observations it seems
that S. paracitrovorus cultures are very consistent in the quality
of starter than can be secured with them. One culture that was
studied over a considerable period, quite regularly gave the best
starter In any series in which it was used; it came originally
from a sample of off-flavored cream, and accordingly its source
did not suggest its value as an organism to be used in starters.
Altho there are variations in the quality of the starters
vielded by different cultures of S. paracitrovorus and these
represent important differences, they cannot at present be con-
sidered as a satisfactory basis on which to divide the organisms
into types because they are not known to be correlated with any-
thing else. Moreover differences in the flavor and aroma of the
products formed by the organisms are probably due to extremely
small differences in metabolism which cannot readily or accur-
ately be detected in cultures so that these differences in metabol-
ism are not likely to be recognized as related to the variations in
flavors and odors produced.
14. See ref. 5.
3)
RELATIONSHIP BETWEEN S. PARACITROVORUS AND ORGAN-
ISMS PREVIOUSLY DESCRIBED
The relationship of the organisms here referred to as S. para-
citrovorus to organisms previously described and named is a
matter of importance from the standpoint of proper nomencla-
ture. Freudenreich’ isolated an organism ‘‘ Streptococcus b’’
which Migula'® later named S. kefir. Evans" studied organisms
which she considered to be S. kefir and also reported some of
the characters of another type which was referred to as Strep-
toccus X. Evans found that both these organisms produce con-
siderable volatile acid and it seems that these two types are more
closely related to S. paracitrovorus than any of the other organ-
isms reported in the literature.
S. kefir is reported as producing gas in milk but the S. paracit-
rovorus cultures as observed in milk are usually not recognized
as gas formers, except when coagulation occurs early. As already
pointed out, the gas produced is readily lost to the air without
the formation of bubbles in the medium unless a curd which
can hold the gas is formed. Evans reported that S. kefir rapidly
acidifies peptone milk, with curdling in from three to six days,
and that the curd is rent with escaping gas. The cultures of
S. paracitrovorous studied sometimes did and sometimes did not
curdle peptone milk and escaping gas was not regularly ob-
served. Moreover the description of the lactose agar plate colo-
nies of 8. kefir given by Migula is not a description of the colony
of S. paracitrovorus. .
S. X. is reported as curdling milk in five days with partial
reduction. The organisms studied as S. paracitrovorus certainly
do not do this and while, as already pointed out, the question
of coagulation is not considered of importance because it may
depend on a very slight increase in acid development, if the
results obtained with S. Y give this character importance, then
S. X can hardly be considered to be the same as the S. paracit-
rovorus cultures studied.
One of the important characters of S. paracitrovorus is the
fermentation of citric acid. Data on the action of S. kefir and
S. X on this material are not known to be available, altho the
high volatile acid production of these organisms together with
the correlation usually found between a high volatile acid pro-
duction and the fermentation of citric acid suggests that they
ferment it. From data presented by the Iowa station in a series
of reports it seems that the fermentation of citric acid with the
production of a high volatile acidity together with the reddening
15. Bakteriologische Untersuchungen uber den Kefir. Freudenreich,
HWmaVOnlemmOentbl. Bakt. 3) 1897s p. 92.
16. System der Bakterien, Migula, W. 2. 1900. p. 44.
17. A Study of the Streptococci Concerned in Cheese Ripening,
Mvanseewlice CC. sty, Ager Res. 13. Apr. 22, 1918) p. 235.
2 042501814
3011
of litmus milk is unusual and important enough to justify §
being considered as a characterization of a group of organisr
If such a group is justified, the data presented herein show t
certain variations such as the acid produced, the resistance 4
heat, ete. exist and suggest that, if the organisms are at presege
divided up on the basis of minor variations, only confusion ci
result as so frequently occurs when descriptions of organisigg
are written from a study of only a small number of cultures. j
is entirely possible that some of the organisms such as S. keggaae
and S. X represent certain combinations of characters that mim
be found in the group that has been referred to as S. paracitig
vorus and that further studies will yield evidence justifyi#
some sort of a separation into types. It will be necessary, hog
ever, to guard against a division on the basis of such differen
as the coagulation and non-coagulation of milk which are depengg
ent on neghgible differences in acid production, instead of §
differences that are of some real significance.
DISTRIBUTION OF THE S. PARACITROVORUS TYPE
The distribution of the 8S. paracitrovorus group was not
vestigated systematically but the data secured in connectif
with a study of considerable numbers of samples of dairy prog
ucts allow of certain conelusions. Sour milk and cream veg
commonly show these organisms and frequently in consideralge
numbers. They have a been secured from sweet milk ai
eream and have been found rather commonly in sweet cream wig
a high bacterial count but it seems that sour milk or cream off
more suitable materials for isolation than do these products §
a sweet condition. This is to be expected since S. paracitrovor@m
like S. lactis, grows well in milk even after acid developme§g
milk organism. Butter samples have frequently yielded S. pay
citrovorus; this is in all probability due to the presence of the
organisms in starters but their resistance to pasteurization &
posures suggests that some may have come from the cream.
From the observations made it seems that S. paracitrovorus®
much more frequently encountered in dairy products than
S. citrovorus. This is particularly true of sour milk and crea@
from which organisms producing no observable change in litm®
milk (which inelude S. citrovorus) are only very rarely encow
tered. :
The wide distribution of the 8. paracitrovorus group in daig
products indicates that there must be some common source §
contamination that supplies these organisms. Altho no attemg
has been made to locate this, the fact that these organisms gr@
so well in combination with S. lactis suggests that the usu
sources of this organism such as utensils and the ERE GLau
cow may also be important sources {Of §.\pRadterdb
sep 24 1930
UNIVERSITY OF LLLINOIS.