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TEriESIS ;

A BACTERIOLOGICAL STUDY OF THE E
HOIv'IOGENiZING PROCESS 1N *1
MAKING ECE CREAM
Frederick W. Fabian

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THESIS

 

 

 

 

 

A DAdl‘EAIOlD'iIGAL o‘l’JJJI CE 1:154

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Submitted to the Eucalty of the Lichigan Adrisultura
College in partial fulfillment of tha requirements

for the éograo of Luster of doienca.

 

THESlS

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A BACTERIOLOGIOAL STUDY CF THE HOMOGHHIZIBG

PROCESS IN MAKING ICE CREAM.

NTRODJGTIONo

In a previous paper (1) a bacteriolOgical study was made of
the influence of various manufacturing Operations upon the bacterial
content of ice cream. One of the Operations studied was homogeniz-
ing. In this Operation, which takes but a short time to complete,
an increase in.bacterial count was noted in the majority of the
samples studied. The increase noted in the mix was presumed to
be due to two causes; first, bacterial contamination of the mix
from the homagenizer, and second a breaking up of the clumps of
bacteria as the mix passed through the homOgenizer. It was to
determine whether there is an actual breaking up of the bacterial
clumps as the ice cream mix passes through the homOgenizer that

the present work was undertaken.
PREVIOUS WORK.

Hammer and Sanders (2) made a bacteriological study of’the
influence of homOgenizing the ice cream mix upon the bacterial
count both with and without pressure. when the mix was run thrnugh
the homagenizer without pressure, they found an increase in the
numbers of bacteria in all cases. However, all the samples

analyzed were from the first material passing through. This

material would serve to partially free the machine from bacterial
contamination so that subsequent material passing through should
not be contaminated from this source to nearly as great an extent.
When the mix was passed through the homOgenizer under pressure, an
increase in the number of bacteria was shown in all cases except
one. The increase in the number of bacteria when the mix was
homOgenized under pressure was not as great in most cases as without
pressure. They state that, "At least two factors are Operating to
change the bacterial content when the pressure is thrown on; first,
the machine has been in Operation longer and the contamination from
it is becoming less; second, the agitation in the machine tends to
break up any clumps that may be present and thus apparently increase
the c cunt. "

Peterson and Tracy (5) in a study to determine the relative
importance of each step in the manufacture of the mix say that,
"The increase after homOgenizing and freezing is probably due, for
the most part, to the breaking up of the bacterial clusters, Which
results in a higher count by the plate method."

PRESENT WORK}

The present work was designed to determine whether there was
a breaking up of the bacterial clumps during the process of
homagenizing. The materials going into the ice cream mix were
placed in a starter can and heated to 145°F for thirty minutes.
The mix was thoroughly stirred by mechanical paddles during the
entire pasteurizing process which usually took from forty-five

minutes to an hour to complete. After the mix had been pasteurized

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larger than tie nears-uses, an the arcra5es or all the 304120»;
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shows c“ -n3:-ace oi we p~‘3ent Lice: JOWQSOJlLlub. DnUJ an

increase in hasterial count is, no doubt, more ayparent than

real heir~ duo chiefly to the breakin; up of clusters of
organisms each in;ividual of hhidh may give rise
001033 on an agar plate."£rom their study 3f tLis process LLey
conclude that, JHemo5cnization of the mix usually causes an

increase in the bacterial count as determined by the agar-plate

1-“ , ‘I 5 . .1‘ C .‘ a 1 - . ’ ' + ._- >
method. bich an endrflfloJ is pioeably due to who creacing up

it was then passed through the homOgenizer at 2000 pounds pressure.
Samples were taken as the mix passed into the homOgenizer and as

it came out on the other side. The samples were always collected
after about half the mix had passed through the homogenizer or
toward the end but never at the beginning since it was desirable

to reduce the bacterial contamination from the homagenizeroto a

Minimum.

111$ THO D .

The samples were analyzed by two methods; the plate method
and the direct micrOSCOpic method. The numbers of bacteria by
the plate method were determined as follows: 1 gram of the mix
was weighed into an Erlenmeyer flask and 99 cc. of sterile
physiological saline solution added to it. suitable dilutions
were made from this and plated on milk—powder agar (4). The
dilutions used were 1:100; 1:1000 and l:l0,000. The plates were
incubated at 37°: and counted at the end of 48 hours. All plates
were made in duplicate and the counts represent an average of
the two best plates unless otherwise noted. All counts are
therefore per gram and not per cubic centimeter.

Brecd's direct microscOpic method (5), (6), (V), (8) with
slight modificaticns was used to determine the numbers of
individual and of groups of bacteria present. The method used
was as follows: 0.01 gram of the mix was weighed on each end of
a clean slide, and Spread evenly over an area of one square
centimeter. The slides were made in duplicate so that for each

sample of mix four one square centimeter areas were made. After

p
- ‘1 ‘

e slide es h. ad been dried they mere i “arse d in
minutes to remove the fat. the surplus zylol we
slides dried a3ain. fiflxay .e"3 then fired in 95
twenty minutes and dried. After this they mere

4‘13

r's alkaline

FwJ

seconds in Loaf

mo

:ylene blue and

xylol for five
8 removed snf the
were ent al ohol for

stained for thirty

decolorize

.L..b J.
a light blue in 95 per ent alcohol. IIe bacteria step ire; a
deeper blue than the rest of the material. because of the larger

f0 4"“

amount oft rresent in 133 c: e.m

1 U .u
than in the ”use of milk. however, satisfactory

fields are more pitted

rrcnarations for

 

comparison and other detailed information may be ohta ned. icen.y-
five or more fields iron each arcs \ re counted and averaged.
rhis made a total of ice fields for eac h sample. :he counting WES
done un-er a 1.9 millimete“ oil immersion lens. ihe factor used
was 500,000 and was derived as follows; hiune tor of the field was
.16 mm. 3y substituting in the following equatimu .X .X 150 = y
.. .. - ‘IA‘ ~1.
where equals one area of tJB smear in quare millimeters, d the

radius of the

field in millimeters "meme 1.1.“

A f‘ ‘0
tl“’ZI b.1—

and y equals the factor :1ec

-)
b

ssary to

 

bacteria found in one field of the microsc

per gram. withstituting a get 1 X 100 X
.3.lelG x (.08).

Since a hundred or more fields from each

it was necessary to find the average per

ore into terms of bacter

equals the area,

orm tie numbe r of

ICC

T .. 50.3, 00-0.

sample were counted,

field and multiply this

average by the ctor. rhis was done as follows;
5002000
T X m a number of he: aria p 0: gram, .1011 n z the hammer of
fields counted and m the total number of bacteria found in n
fields.

An earl.1n tion is nececszry of tie terms ”individuals" and
"3roups" . by the term "individuals” is i~nt the total number

ia

-5-

of bacteria including isolated single bacteria and the individual
bacteria in groups (any organism in the process of division Was
counted as two individuals); thus the count under the column
labeled individuals represents a total of all the bacterial cells
in an average of'lOO fields times the factor. The column labeled
"groups" represents a somewhat different meaning than is usually,
ascribed to this term and should be thoroughly understood by
anyone wishing to interpret the data correctly. By this term is
meant those bacteria, either individuals or groups, so located

in the microscopic field that in the Opinion of the observer
would, if alive and plated on suitable medium, form one bacterial
colony. Thus a single individual if sufficiently isolated would
count as a "group" or several individuals if sufficiently close
to each other would constitute a "group". In the process of
counting many objects were encountered about which there might

be a reasonable doubt as to whether they were bacteria or not.

In all such cases, they were not counted as bacteria.

dESUle.

The results of the experiment are set forth in the tables
that fellow. In table I a comparison is made of the data secured
from the plate count and the direct micros00pic count. In the
second column under plate count are listed the number of bacteria
found in the ice cream mix before it was homOgenized,but after
it had been pasteurized at 145°? for thirty minutes. In the
third column are listed the numbers of bacteria found on the

plates after the ice cream mix had passed through the homogenizer

under 2000 pounds pressure.

-6-

In column four are listed the counts of individual bacteria
of the same mix under the same conditions as in column two above
except that the number of bacteria have been determined by the
modified direct microscOpic method. In column five are listed
the "groups" of bacteria of this same mix as determined by the
modified microscOpic method.

In columns six and seven are listed respectively the counts
of the same mix as listed in column three above except that the
"tndividuals" and "groups" have been determined by the direct
microscOpic method slightly modified. In other words the same
mix under identical conditions has been analyzed by two different
bacter10103ica1 methods both before and after the mechanical
Operation of homogenizing to determine the effect this process
had on the bacterial content of the mix. A comparison of the
plate count and direct microscOpic count before and after

homOgenizing are set forth in table I which follows:

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MicroscOpic Count Before and After homegenizing.

r1
I

TABLE 1.

comparison of Slate Count and Direct

 

 

 

 

 

 

V I 1
1 Plate Count 1 Direct hicroscOpic Count 1
Date T' ’T ' Beffire ' After 1
'Before ' After 'IndividualsT "Groups"‘i51hdividuals' "Groups" 7
I rj'"_"T‘ T l T 1
10-3-22 ' 5,800 '10,000 ' 1,980,000 ’ 1,160,000 ' 3,330,000 ' 3,110,000 ‘
10-10-22 '17,000 '18,000 ' 1,950,000 ' 945,000 ' 2,200,000 ' 2,000,000 '
10-17-22 '10,000 '25,000 ' 1,450,000 ' 1,100,000 ' 7,535,000 ‘ 7,120,000 '
10-23-22 '46,500 '71,000 ' 2,400,000 ’ 1,890,000 ' 3,000,000 ' 2,900,000 ‘
10-30-22 '12,500 ‘31,000 ' 2,865,000 ' 2,200,000 ' 3,825,000 ' 3,450,000 7
11-3-22 1 7,600 ’16,000 T 3,400,000 3 2,500,000 ' 4,100,000 ' 3,800,000 1
11—10-22 ‘18,000 ‘30,000 3 2,780,000 1 1,650,000 F 4,350,000 I 4,000,000 ‘
11-15—22 ’ 6,500 '23,000 1 2,800,000 ‘ 1,650,000 ’ 2,700,000 1 2,520,000 ‘
12-20-22 '19,000 '88,000 ' 2,500,000 ' 1,560,000 1 4,315,000 ' 4,000,000 ‘
3-16-23 ' 8,100 ' 4,600 ‘ 2,400,000 ' 1,550,000 ' 3,725,000 ' 3,230,000 '
3-21-23 ‘23,000 '67,000 ' 3,340,000 ' 1,880,000 ' 3,885,000 ' 3,600,000 '
4—10-23 ' 4,500 ' 8,000 ' 2,330,000 ' 1,700,000 ' 3,000,000 ' 2,565,000 ‘
4-18-23 ' 5,500 '15,000 ' 2,550,000 ' 1,585,000 ' 2,680,000 ' 2,195,000 ‘
4-10-23 '12,000 '18,000 ' 3,000,000 ‘ 2,600,000 ' 2,720,000 ' 2,340,000 '
10-8-23 ’16,000 ’58,000 ' 1,700,000 ' 1,560,000 ' 1,850,000 ‘ 1,700,000 '
10-12-23 ' 3,000 ’ 3,800 ' 1,600,000 ' 1,300,000 ' 1,900,000 ' 1,700,000 '
10-19-23 ‘24,000 ‘83,000 ' 490,000 ' 455,000 ' 465,000 ' 435,000 '
10-25-23 '14,000 '60,000 ' 850,000 ' 750,000 ’ 1,600,000 ' 1,500,000 '
11-9-23 ' 69,000 ‘75,000 ‘ 545,000 F 450,000 ' 720,000 ' 600,000 '
11-13-23 ' 6,500 ’19,000 ' 2,000,000 ' 1,500,000 ' 2,200,000 ' 2,000,000 1
11-15-23 ' 3,600 ' 6,500 ' 665,000 F 570,000 ' 570,000 ‘ 555,000 ‘
11-22-23 '12,500 51 '000 ' 3,965,000 F 2,000,000 ' 2,420,000 ‘ 2,160,000 '
11-28-23 '11,000 '45,000 ' 2,100,000 ‘ 1,600,000 ' 3,500,000 ' 3,200,000 '
12—3-23 ' 2,500 500,000 _' 1,850,000 j‘ 1,450;000 ' 3,500,000 ' 3,200,000 ‘
2-20-24 ' 4,400 ' 6,000 : 2,665,000 I 1,440,000 : 2,700,000 : 1,750,000 :
, .
DISCUSSION.

A comparison of‘columns two and three of table I shows that there

is an increase in the number of bacteria after homodenizing as cempared

to before homOgenizing as determined by the plate count in all cases

except one.

it is small.

In some cases the increase is large While in other cases

A comparison of columns four and six tabulating individuals before

and after homOgenizing, as determined by the direct microscOpic method,

shows, that in twenty out of twenty-five or 80 percent of the samples

there is an increase; nubile in ii e out of twenty-five or 20

, rcent of the samples, there is a decrease in the number of
individuals after as compare with before homegeiizing. Ihe sum“
relationship as just stated or i.ui*i'u*1s also holds t to for
”groups” tahulated in columns five and seven. It till thus he
Seen that, in the majority of 31000, thele is an rere in the

bacterial count, as determined by hoih the plate and direct
mi‘roqc pic method, after the mix ha” passed throujh the homegcnizcr.
'mnxibiuu“ih 2141.:ngle
IthfIgJals, "1100:0' any 13031 clad.

ih; individua s ”groupe', arr group siz' rc scarred in
able II. ”hat is eant 1y ir‘iviut ls an' "groups' has heen
defined earlier in this paper. hotsothr, it was thought advisaltle
to make further tabulation“ comet: ruin; tic rumber of 'roups and
group size since We are 0 09”“01 primaril" in tryiir to determine
the cause of the increase of the number of bacteria as determine;
by the plate count and a“ reported “a pre"ious in: stiratoro.

for this -eason a record was made or the numh>r f groups ard
the size of each croup as the count was made. In column seven
are tabulated all the groups of two and in column eight all the

1.

‘-

groups of “roe o more

are listed in

('_1.w
S;

column xhile the

- A‘ -Q t‘ ”a
J»: 83" Cl —- *4 x v
t

I“

size of ‘rouxs of two or more are lists i in column nine. 15
complete record pf this detailed information follows in table II.

- 9 _
TABLE II,

{5'" .
iaoulation of Indivii" c

1310, "eronwuu —~ ~

«he and uroup size,

 

 

 

 

T r I
I I ~. j
humber 01' Numb T_ i gj
Date : :Iguizidufil' Ofer :Aviiege :TOtal ' Number of T
:1 'IBef°r° H°m°een121n :I"Gr°uPS"'grr;ES; t3§ : °f 'Grogngiu
4, :“hr Homosenmngg : : . «3.03.108 °r
I i ' r6
10-3-09 : B ' 1,980,0001 1 1. I V 1* I in,
NH ‘ O f r
r A I 3'550’000' 3:1§0:8gg W 1.33 d 39 d 19 ' 20 : 3 a
l ' o ‘-‘ ‘l
I I . I Q 17 I .
10‘10-°Bx i I é’950.000' 945 000 r 2 ' I ' 11 a; 3'0
u , .200.000: 2,0001000 1 1°38 ; g3 ' 26 x 29 :, 4 7
n I i I O : ' 34 i . C
' I .035.000' 7,120:000 "I 1.3% ' 50 .r 11 ' 19 l 5
10 23 I B ' 2 4 t I . I 14 ' 4 I 10 ' “.0
"' -22. A I 5’ 00,000! 1,890 000 t 1 . I I ' “.9
, I $000,000: 2,900:000 l 1.8: : 4: ‘ 16 ' 28 I 5 0
r' I ' . ' 3 i .
10-00-23, i ; 3,855.000' 2,200 000 I 1 ' ' ' 2 : 2’4
l I ,825,0001 5,450:000 I 1.52 : 78 I 31 l 47 '
11-3 29 ' B l 3 4 I L' . 7'! 4O ' 27 ' 13 Y §.O
:- «o I A 1 4’ 00,000! 2,500 000 I 1 ’2. I I l Q.5
w x ’100'000' 3.800:ooo ' 1°59 : 2? ' 19 ' 45 . s e
I O 0 I n
p 3 x ' ‘1 ' 2 .
11'10-43. 1 : 2’Z§9.000' 1,650 000 I 1 ' ' ' 0 I 2'4
' ' 4,350,000: 4:000:000 , 1032 : 135 I 34 . 79 , q _
. ' I ~ I O I I 16 ' H :00
11—15-23! a 2 2,800,000: 1.650 000 ' 1 I I , 19 : 2.8
, , 2.700.000! 2,5201000 : 1'12 1 104 ' 33 ' 71 r 4 o
i O “'3 l o
t 1 5 10 ' ‘
12-20-,‘32' i : i,520’000! 1,560 000 l 1 I g ' 13 ; 3.0
I 1 ,315,000: 4,ooo:oco . 1'3: : g: r 45 u 47 l 5 1
' I C t 00 I n.
3‘15-231 i 1 3,490,000' 1 550 000 I 1 I ' Q ' 18 : “'6
I I 9785.000, 3,230:000 I 1.§(:5L : 25 I 53 ' 62 I 5 1
I O 8 I 3 o
_ ‘ I g I O t
3 21-23! a I 3,330,000! 1,880,000 ' 1 77 z I , 33 ; 2.3
r I ’ 59000: 3,600,000 1 1:08 , Z: I 40 ' 38 I 3 3
fl 1 I y 29 ' .
4-10"3 I E : 2’3509000' 1.700 000 1 1 n r I ' 13 ; 2.4
, , 3#300,000: 2,565:000 r 1.17 ; Z6 I 44 ' 32 I 3 0
I ‘ I O ‘ 0 I 39 I 0
4-18-23! 2 : 2,§m’0001 1,585 000 ' I I ' 21 : 2.4
. , 2.080.000! 2 195’000 . 1-69 ' 89 : 4o .
' l ’ ’ I 1023 i 56 I 27 l 49 ‘ 5.2

 

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I 1|. 1‘ VI 7.. 1| VI

0‘ V .f. C

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VI T VI 1) VA Y] T1
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11 T. V TA Y: Y5 13
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1- VI. Tl 1| Y; I I
u, 4 b +

TITY

- 10 -

TABLE 11 (continued)

 

 

 

 

 

I r T V T I T 1’
I 'Number of ' Number 'Average 'Total ' Number of 'Average
.'. lIndividual' of I 8120 "'Gr0@'§1‘0ups'Groupsjsiza of
Date ' ' Bacteria ' "Groups" ' of ’of two! of ' of ‘Grouns of
I: , 3"Groups“ or ' two ' three'two or
_!B a Before domOgenizing 1 Imore I 'or ”0' more
IA - After EOmOgeniziqgf I I I I 'I
I ’I T I I I I I
4_19_23I B I 8,000,000I 2,500,000 I 1.15 I 45 I 27 I 19 I 5.0
- i A ; 2,720,000: 2,340,000 I 1.11 I 51 I 37 I 14 I 2.5
I I I I I
10_8-23 I 5 I 1,700,000I 1,550,000 I 1.09 I 50 I 2 I 5 I 2.2
I A I 1,850,000I 1,700,000 I 1.08 I 25 I 20 I 5 I 2.1
I I I I I I I I
10_13_23I B I 1,500,000I 1,500,000 I 1.25 I 44 I 5 I 9 I 2.2
I A I 1,900,000I 1,700,000 I 1.11 I 2 I 24 I 5 I 2.1
I I I I I I I I
10_19_23I B I 490,000I 455,000 I 1.07 I 17 I 15 I 1 I 2.2
I A I 455,000I 455,000 I 1.05 I I I 8 I 1 I 2.1
I I I I I I I I
10-25_23' B ' 850,000' 750,000 ' 1.13 I 27 I 21 I 6 I 2.5
: A : 1,600,000: 1,500,000 : 1.06 I 21 I 21 I o I 2.0
I I I I
11.9_23 I 5 I 545,000I 450,000 I 1.21 I 18 I 17 I 1 I 2.1
I A I 720,000I 500,000 I 1.20 I 25 I 25 I 2 I 2.0
I I I I I I I I
11_15_25I 3 I 2,000,000I 1,500,000 I 1.55 I 57 I 51. I 5 I 2.4
I A I 2,800,000I 2,000,050 I 1.10 I 57 I 55 I 1 I 2.0
I I I I I I I I
11_15_23I B I 555,000I 570,000 I 1.17 I 15 I 15 I 2 I 2.5
’ A I 570,000I 555,000 I 1.02 I 20 I 15 I 5 I 2.2
I I I I I I I 'I
11_22_.3I 5 I 5,955,000I 2,000,000 I 1.98 I 175 I 127 I 45 I 4.5
H ' A ' 2,430,000' 2,160,000 7 1.12 I 55 35 I 0 I 2.0
r x I ‘1 I , ‘ r - I 1
11-28-23! B ' 2,130,000' 1,600,000 I 1.31 ' 99 ' 87 ' 12 ' 2.1
I A ' 3,200,000' 2,950,000 : 1.06 I 63 I 61 I 2 I 2.0
'I I I I I I I
12_5_23 ' B I 1,850,000I 1,450,000 I 1.27 I 72 I 66 I 6 I 2.1
I A I 3,500,000I 5,200,000 I 1.09 I 44 I 45 I 1 I 2.0
I I I I I I I I
2_20-24I B I 2,855,000I 1,440,000 I 1.85 I 112 I 75 I 55 I 5.1
I A I 2,700,000' 1,750,000 I 1.54 I 105 I 75 I 29 I 2,4
I I I I I 'I I I
B . Before mix Passed through Homagenizer
A . After mix fussed through Homogenizer

The size of groups most common above two are three, four and
five in the order mentioned. Groups of eirht are not infrequent
while the size of a few groups ran as large as sixty or more.
However, the larger groups occurred rather infrequently in the ice
cream mix.

It will be noticed that there is a considerable difference
between the average size of "groups" in column five and the average
size of groups of two or more in column nine. Ihe average size
of groups in column nine being considerably larger than in column
two. This is readily understood when the method of computing
these averages is explained.

The method of computing the average size of the "group” in
column five was by dividing the total number of individuals by the

in computing the

f.
I

total number of’"grouns"; while the method use?
average size of the groups of two or more in column nine was by
finding the total number of individuals occur ing in the groups
alone (not including separate individuals) and dividing this sun
t3 the total number of actual groups. Both methods of computation
show that there is a decrease in the size of groups after the mix
has passed through the homogenizer.

Since all these differences are of the same sign, it is safe
to conclude that homogenizing reduces the average size of the

clumps of bacteria in the mix.

A b’IUlLY CF 5.113 ILIFLULZLJJ" CE IICILOGLLNIZIIJG OH 12219

Is

SIZE 0 ”KB BACTERIAL GIEMEs In KILK.

When the work had been c0mpleted showing the influence of
homOgenizing on ice cream, it was thought that a similar study of
a few samples of milk might prove interesting and throw some
light on the problem. Accordingly five samples of“milk were studied
with this in view. At first it was thought advisable to secure
samples of fresh unpasteurized milk having large numbers of bacteria
present.

samples of milk coming into the dairy were analyzed until one
was found with a high bacterial plate count and then run but it
was not found to be satisfactory for this work as will be seen
later. A composit sample of milk as delivered at the dairy was
found most satisfactory and was usei in the other four cases.

"\

PRGJLD"hE.

A ten gallon can of fresh unpasteurized milk containing milk
from many different farms was taken from the vat at the College
dairy, This can of milk w s then dumped into the starter can
leading to the homegenizer and a sample taken. It was then passed
through the homegenizer without pressure. The homogenizer pressure
was then raised to 1000 pounds, 1500 pounds, 2000 pounds and
2500 pounds pressure per Square inch respectively and samples
taken as it was leaving'the homegenizer at these various pressures.
Thee- samples were then taken immediately to the laboratory and
analyzed according to the direct and plate methods as previously
described. Trevious to homegenization, the homogenizer was

rinsed with sterile water and a sample of this rinse water was

-13..

collected and analyzei by the plate method.

The results of this experiment are recorded in tables III to
VII. In table III are recorded the results obtained by using the
sample of milk containing a large number of bacteria. Only twenty-
five fields were counted because the bacteria were so numerous.
In this sample there were so many large clumps and the organisms
so many that the number of bacteria in the large clumps had to be
estimated. For this reason it was thought best to work with milk
containing fewer bacteria. This would enable a more accurate
counting of the bacteria in the groups.

In tables IV, 7, VI, and VII are recorded the results from the
compOsite samples of milk. The numbers of bacteria by the plate
count and the direct microscOpic count are recorded the same as
for the ice cream mix. The same information as to individuals,
"groups", average size of "groups", etc. are also contained in the
tables.

The Operations are lettered and have the following meaning:
Operation 3‘", fresh unpasteurized milk after being placed in the
starter can and just before passing through the homogenizer.
Operation "B" the same milk after passing through the hemogenizer
without pressure. Operation "c" same milk after passing through
the homOgenizer at 1000 pounds pressure. Operation "D" same milk
after passing through the homogenizer at 1500 pounds pressure.
Operation "E" same milk after passing through the homogenizer at
2000 pounds pressure. Operation "E" same milk after passing

through the homegenizer at 2500 pounds pressure.

- 14 _

A detailed record of the results are set forth in tables III

 

 

to 711 that follow.
IL. I‘LL; e3 Cl“ £01...11)311.I;Il;301. "1:3 SIZE OF 1'13
BASTLRIAL 0111111135 Ill LLIL"
TABLE III. Sample 30. 1, 2-28—24.
1 T ' r Average "T
' ’ Number of ' Kumber ' Size ‘
Opera-' Plate ' Individual ' of ' of ' Remarks
tion' Count ' Bacteria ' "Groups" ' "Groups" '
'm I I I I
T ’ I T I T
A ' 5 .500, ooo"‘ 50,000, 000' 2 ,520, 000 . 19.84 i 25 fields
B ' fl. 000,000' “166,300, 0 00' Bacteria spread evenly over entire field
0 I 14:0wzoooI I II n n n 1 n
D I 26 000’ 000! I n n n n n u
E I ‘23 :000 000! I n 2! n :I n n
F I) ‘55 z’éoo 000! I n n - n n n n
I :

 

Water from Homegenizer 1500 bacteria per 0.0.

2‘

ea 25 fields counted. only one plate counted.

TABLE IV. Sample no. 2, 5-18-24.

 

 

 

I I I T T —T I
' ' Number of' Number ' Average'Total ' Number of ' Average
Opera-' Plate ' Individuar of ‘ eize 'Groups' Groups of ' size
tion ' Count ' Bacteria' "Groups" ' of 'of two' 'three ‘ of
‘ ' ' '"Groups"'or mord two' or ' Groups
I I I I I I I mOre I
T I I I T I— I I
A ' 840,000' 2,700,000' 1,240,000' 2.17 ' 142 ' 85 ' 57 ' 2.8
B ' 1,000,000' 4,500,000' 1 ,580,000' 2.84 ' 154 ' 64 ' 90 ' 5.0
O ' 2,115,000' 2,570,000' 1,800,000' 1.43 ' 101 ' 72 ' 29 ' 2.5
D ’ 2,500,000' 2,400,000' 1,330, 000' 1.80 ' 97 ' 69 ' 28 ' 2.5
E ‘ 2,600,000' 1,830,000' 1,380,000' 1.52 ' 78 ' 64 ‘ 14 ' 2.2
F : 2,800,000' 2,500,000' 1,450,000: 1.73 ' 77 ' 54 ' 25 ' 2.4
I I I I I . I
Hater from HomOgcnizcr 6 bacteria per 0.0.

 

 

 

 

 

TABLE V. Sample H00 3, 3-20-24.
I INumber of I Number 'Average Lwotal I Number ofIAverage
I Plate' IIndividualI of Isize of IGroupsIGroups of Isize of
Opera-I count I Bacteria I "Groups"I"Groups"Iof two'two Ithree' Groups
tion I I I I Ior more I Ior more'
T T I T T T T T
A I 25,000 I *450,000 I 320,000 I 1.40 I 5 I 4 I 1 I 3.6
B I 68,000 I 550,000 I 300,000 I 1.83 I 19 I 15 I 4 I 2.5
.l I 124,000 I 360,000 I 290,000 I 1.24 I 10 I 9 I 1 I 2.3
D I 78,000 I 340,000 I 295,000 I 1.15 I 8 I 7" 1 I 2.1
E I 77,000 I 350,000 I 315,000 I 1.11 I 7 I 7 I O I 2.0
r I 86,000 I *350,000 I 300,000 I 1.16 I 3 I 3 I o I 2.0
I I I I I I I I
Water from Homegenizer 125,000 bacteria per c.c.
I 50 fields counted.
TABLE VI. Sample No. 4, 3-26-24.
I I I I I I 'Aver-
I Plate I number of I Number 'AverageITotal I number ofI age
Opera-I count I Individua1' of Ieize of'Groups'Groups of Isize
tionI I Bacteria I "Groups" I"Groups"of ton two'threeI of
I I I I '0: more I 'ormom'Groups
I 1 'I I f r I r
A I 435,000 I 1,870,000 I 675,000 I 2.77 I 38 I 83' 15 I 6.8
B I 2,300,000 I 3,500,000 I 1,500,000 I 2.33 I 182 I 119I 63 I 3.1
0 I 2,200,000 I 4,200,000 I 2,400,000 I 1.75 I 194 I 123I 71 I 2.8
D I 2,600,000 I 4,500,000 I 2,400,000 I 1.87 I 220 I 135' 85 I 2.9
E I 2,800,000 I 3,000,000 I 2,320,000 I 1.29 I 145 I 102I 43 I 2.5
F I 3,400,000 I 2,300,000 I 1,460,000 I 1.57 I 120 I 67I 43 I 2.5
I I I I I I I

A

water from Homegenizer 140,000 bacteria per c.c.

 

'I

YIYYIVI.

'In

71711.1.

 

 

TABLE VII. sample No. 5, 3-27-24.
I I I IAvetage I”ota1 I Number ofI
I Plate Inumber of ' Number I size I‘roupsI Groups of'Average
Opera-I count Ilnaividual I of I of Iof two‘two Ithree'eize 0!
tion I I Bacteria I‘ 'Groups" "I‘roups‘IIor mon' Iormwa’ groups
I l I I I I I I
T T I 7 f I I I
A I 250,000 I 1,950,000 I 780,000I 2.53 I 39 I 21 I 18 I 7.0
B I 400,000 I 2,400,000 I 800,000' 3.0 I 89 I 55 I 36 I 4.9
d I 72,000 I 1,700,000 I 875,000' 1.94 I 71 I 40 I 31 I 3.2
D I 105,000 I 1,400,000 I 830,000' 1.70 I 69 I 54 I 15 I 2.5
E I 285,000 I 1,400,000 I 950, 000' 1.47 I ‘56 I 37 I 19 I 3.6
E I 560,000 I 1,500,000 I1,000,000I 1.70 I 56 I 34 I 22 I 2.1
I I I 'I I I I

—_.

Rater £10m HOmOgenizer 1800 Bacteria p~r c.c.

‘wplanation of letters for all teales.

Operation A a Fresh unraste mi 91 milk from vat.

" B a Same milk rabsca tlroug? h0m03enizer without pressure.
II G . II II n II II at 1COO II
TI D - II II .1 II It! at 1500 II
n E . H I: II H II 8 1; 200.0 71
II I F I II II II II it 8 t ".3 FOO II

uldeeglOH.

A careful study of the flute sho 08 that t1 8 clumps 01 bacteria in
milk are broken up during the rroccss of homosenizit“ ibis is
esoecially marked in the first sample of milk run. In this sample
large clumps of bacteria together with many smaller olumrs were
scattered tLroughout the entire smear before the milk mas homosenized.
Af ter homng 13’ ing, however, the bacteria were evenly aistributei

throughout the entire smear and the size of the clumps greatly reduced

5 J
.3

in size. The same 018 true for the other samnles homosenized but

J

 

I‘L

TIT.

—17..

to a less de3ree because there were fewer lar3e 3roaps and not nearly

as many organisms present. In all cases where V's milk assed thro ou3 h

6.1

the homorenizer under are sure there is a decrease 1L the size 013the
.) .

ihe data also shows that the home3e1 13 er is a source of contsmina-
tion for'in all cases there is an inc1'e ass in l:ot1 the rlatc ind
micros con 13 counts after the milk has passed through the homogenizer.
It is true that a rart of this increase might be attributed to the
breakin3 up of clumps and no dcuht was to a sli;ht extent but the
large increase should then be mainta inel throughout which is not the
case. Another interesting thing; is that the average 131259 of the
"groups” increases in most cases after the milk has passed throu3h
-he homegenizer, whi h would indicate cont mination from the

hom03enizer. ihe average size of grouys decreases slight y in most

0)

cases with an increase of rrc

J

urc. ile decLeas e is gradual thich
would indicate that as the groups get smaller they are harder to
break up.

Ihe acove data ShOuS that} :om03enizin3 has the same influeree on
the bacterial groups in milk as it does on the bacteria groups in the

ice cream mix.

A 6100! 03 SEE EIEVS OAIBACILRIA
EOUQL In 223 HCLOJLLIJdR AJD I; SEE Ice JRLAM
LIIX BflEC‘dL‘J ALID ALP¢L R hviquMJIaLm‘o
In making a bacter10103ical study of the hom03enizin3 process
milk-powder agar 333 used. By the use of this medium interesting
data as to the thes of bacteria present in t e homogenih er were

obtained. By using this medium it was th01ght that some data could

- 18 _

also be obtained as to the types of bacteria present in tie min before
and after homogenizi:13 ard also the ’res of bacteria present in the

hom03cnizer.

T1335 OE 55024313 Erasers.

ihe medium has made accordin3 to directions and the bacterial
colonies counted and recorded accordin3 to the system described by
Ayers and Judge (4) on p. 579 of their article, viz:

l. A tOt 31 count was made.

2. The strong acid-forming colonies were then counted. Ihe
strong acid-forming colonies are defined as, "”hose hlth a cloudy
zone ahOut them or a slight hazy edge."

3. weak acid-forming colonies were neut counted. All colonies
showing acid but without a cloudy zone or a slight h zy edge about
them were considered as weak acid-forming colonies.

4. The plate was tz:en floodel with a 5 percen t sOlIt ion of acetic

id and all colonies shouin3 the characteristic clearin3 shout the
colony were counted as neptonizers.

5. The difference between the sum of the strong acid formin3,
the weak acid-formin3, and tie peptoniz in3 colonies and t22e total
ccmnt were classified as all :ali formers and inert colonies.

By the use of milk-powder a3ar and classifying the colonies
according to the above scl:eme one is able to obtain a pretty 300d

J

bacter10103ical pthLrG of the types of colonies or escnt.

REOULIDO
Thy types of colonies found in the water from the h0m03enizer

are set forth in table VIII. The hom03enizer was thorou3hly rinsed

-19..

with sterile hot water by passinga ml gelloxls tlrou3h it and

collecting the 155 t pa rt of the water and plating it on the milk-

powder agar. She results show a predominance of alkaline and inert

colonies with weak acid colonies next in number with nearly as many

pe etozlizers. The strong acid— for ming colonies are few in numfier as

compared to the rest.

 

 

 

 

TABLE VIII.
Type of colonies of Bacteria Found in Later ”rem Homogenizing.
1 . 'T I’ T 1' 1“" -
r v ‘trong acid 1 Jeak acid :Poptonizers 15%5198505§13
Date' Total Gaunt ' ' , ' r* ‘r“ ’T
x : unamber: ;a unamber: )5 :Eumber: i3 .Numbera é
1922I j I _T I I r T 'T
10_3I 8 I - I _ I _ I _ I 6I75‘OOI 2I25.CD
10-10: 2,800 ' 700' 25.00' 600' 21.55! 600'21.43' 900'52.1;
10-17' 3,200 ’ 150' 6.82' 1,000' 45.45' 500'28.73' 550'25.00
10—23 5,000 I 500' 10.00' 2,0001 40.00' 1,500'50.00' 1.000120.00
10-50! 5,900 i - I - I 550' 9.25' 550' 8397' 5,190‘81.80
11-3 ' 8,000 ' 500' 6.25’ 5,000' 63 .50' 1,900'25.75' 600' 7.50
11'10: 450 t '- ' "' ' 100 ' 2“. L13 ' 200'44g44' 150 tuggvfi IJ
11-15 15 ' - ' - ' - ’ - ' 10'66.66' 5'35.33
13530; 7,500 ; - ; 4 1 4,500: 60.00: 2,000:25.55I 1,000'15.55
. I I
3716 'K0 water seved' ~ ' - ' - ' - 3' - ' - ' -j' -
5-21 1N0 water saved' - " — ‘ — " _ ' _ i _ I - '1 -
4-10 1 5,500 3 - I ' - ' - ' 500' 5. 88' a ,000I94.12
4-19 300 ' 18' 6.00' 6' 2.00' I 80'26.66' 196’35 .35
4-19 I 2,000 I 10' .50' - ' - ' 10' .50' 1,980 9.00
18-52: 5,503 ; - ; .~ 1 4,002: $7.05: 1 ,000'11.76'o :500'41. 17
- - - 0.00 1'30 00' 0' -
10-19' 5,000 I 500'.10.00' 5,500' 70.00' 800'16:00I 200' 4.00
10-25' 4,000 ' - ' - ' 500' 12.50I 2 ,500'62.50' 1,000'25.00
§%-§3: 5 43,000 : ,000: 6.97;18,000' 41.86'10 :000'23.25'12,000'27.90
I- ‘prea 6r '- n- i. ' - l u- -
11-12: 653 : - z - : 85:113.07' 400’61.54' 165’25.5a
- - - 2 00.00I — ' - ' - I -
11-33' 13.000 ' - ' ~ ' 3.500‘ 19.44' 5.000'27.77I 9.500'52.77
i352 : 140,000 :10,000: 7.14:70,000: 50.00:45,000'52.14”15,000'10.V1
I I I
2-20 : 315 ; — z - : 85' 26.98' 130'41.27‘ 100'31.74
I I I I I
1I?erage 3:58 32.90r 29.50 34303

 

the mix

through

is to

and mix it at Once and.the same
the h0m03enizer.

taken as

cent,

peptonizere next,

In

table

c...
d-

12

.L

put 0,

with

after

IX are

the homegcnizcr.

1e materials

20

recorded the resulte-of plating out a Sample of

for

v.1A I:
iue aata 1-

the mix

table

time

110 rrocedure folloued

. J. -
in a etartcr

after

" r

LA

' 3‘51 ‘11‘
Id -4 U41.

are collected

weak acid-formicg colonies coming next,

a ‘ . o r J‘.‘ '_‘
ent and stronb acid-lormin,

it had been raetcnrizcd and before it

t is 7‘5 565d

We find a predominance of alkali and inert colonies,

04.75 percent,

from

-a'-3

FEELS 16..

samples

in 131.6 301-633 flair"
can and restauriz a

through

the mix is ready to race on to the prOceec of homegenizing.

42.82 rer~

colonies

vdth
last

 

 

 

 

 

with C7 “ercent.
14515 IX.
Type of Colonies of Bacteria Before homogenizing.

'Total ’. ‘ A n '4 ' Alkaline and
Date , Count .Dtrong acid , Weak acid ‘_ ,1eptonizere !inert bacteria

5 ; :Unmher; 15 :humfier ;_15 Jhumber;_j$ Jhufiber : )3
10 5 ! 5,800! 450! 7.76! 5,800! 65.51!‘ 500! 8.62! 1,050! 18.11
10-101‘17E,000! 3,,200; 12. .94: 2,700! 15.88! 1,200: 7.05: 10, 900! 64.12

- C C 2 2 . 5 ' 5 O
18.2.2 42:2881 09" °:."0 95 000! 53.7.: .582: 121.32: 158%: 33.2%
10- 50! :12 ,500! - ! - ! 6 .500! 50.40! 4,000! 52.00! 2’200! 17.60
11-5 ! 7 ,600! - t - ! 4,400! 57.90! 500! 6.58! 2’700! 55.52
11.10! 18,000! - ! - ! 15,000! 72.22! 2,900! 16.11! 2 100! 11.66
11-15! 6,500! 50! 0.77! 2,500! 58.46! 800! 12.50! 5 150! 48.46
125:0: 19,000; - z - !' 6,000: 31.58; 4,500! 23.68: 8 500!.44.75

9 , '! ! .
5—16 ! 8,100! - ! - ! 100! 1.25! 450! 5.55! 7,550! 95.21
3-21 ' 23,000‘ 2 ,300' 10. 00' 650’ 2.82' 4,000' 17.59’ 16,050' 69.79
4-1 ! 4 5 0! ~ - ! - ! 7 ! 17 5r! 5 8 ! 84 45
4-13 ! 5,580! 1,400! 25.45! 450! 8.18! 1,588! 27:27! 2'1g8! 59:09
4—19 ! 12,000! 450! 5.75! 500! 2.50! 1,400! 11.66! 9 850! 82.08
10-8 ! 16,000! 600! 5.00! 11,000! 68.75! 1,800' 11.25! 2,400! 15.00
10-12! 5,000! 800! 26.66! 200! 6.66! 800! 26.66! 1,200! 40.00
10-19! 24,000! 9 ! - ! 11,000! 45.85! 4,000! 16.66! 9,000! 57.50
10-25! 14,000! - ! - ! 2,500! 17.85! 5,000! 55.71! 6,500! 46.44
11-9 :*69, 000; - : - ; 20,000: 29.00: 9,000! 15.00: 40,000: 58.00
41-12 32-53. - . - -. 428, 541.1483. 52;. $7388. 45
n - '- 0 U Q 0 p H '0
11-22! 12 .500! - ! - ! 91500! 74.40! 1:600' 12.80! 12600! 12.80
11-28' 11 ,000!’ 500! 4.54! 7,500! 68.18! 2,000! 18.18! 1,000! 9.09
12-5 ! 2,500! - ! - ,! 1,000! 40.00! 500! 12.00! 1,200! 48.00
2-20-24; 4,400; - :7 - ; 1,900: 45.16! 700! 15.90! 1,800! 40.90
Av rage 5.07 54.75}r ’17.41‘r 0T 42.82

* Only one plate counted.

Table X ShOIJS the type of'colonies pie!ailing in the mix a ter
it has passed through the homogenizer. It will be noticed that the
difference bet».een the number of colonies in the alkali and inert

group, 35.43 percent, and weak acid-forming group, 35.69 percent is
not so great as in table IX. t will be further noted that the

number of colonies of p!n601i70re has incre 0'd from 17.41 percent in

table IX to 21.81 percent in table X uhile the number of CWrong acid-

forming colonies shows but a slirht increase.
TABLE X.
Tyne of Colonies of Bacteria After Homogenizing.

A

 

 

 

 

'» r, 3’ *T 12 1 n nd
Date .lotal _ ,btrong acid .. beak acid .Peptonizers IA Ingrt gm a erie
' Count Wfimber' i illumb‘erT '36 1rhurzflwr ' f2 Taber ' ,3
3922 T l T T T I T W
10-5 ! 10, 000! 1,900! 19.00! 4,900! 49.00! 800! 8.00! 2,400! 24.00
10-10! 18I000! 5,900! 21.66! 9 I100! 50.55! 550! 5.05! 4,450! 24.72
10-17! “25,000! 1,000! 4.00! 15 ,000! 60.00! 4,000! 16.00! 5,000! 20.00
10-25! 71,000! 5,000! 4.22! 50I000! 42.25! 9,000! 12.66! 29,000! 40.84
10-50! 51,000! 2,000! 6.45! 14,000! 45.16! 1,500! 4.85! 15,500! 45.54
11-5 ! *16,000! 600! 5.75! 10,000! 2.50! 1I 600! 10.00! 5,800! 25.75
11-10! 50,000! ! - ! 25,000! 85.55! 1 I500! 5.00! 45,500! 11.66
11-15! 25,000! 5 ,000! 21.74! 8,000! 54.77! 5 I000! 15.04! 7,000! 50.45
12-20! 88,000! ’500! .57! 45,000! 51.15! 22 I000! 25.00! 20,500! 25.50
1923 I I l I l I I 1
5-16 ! 4,600: 150! 5.26! 700! 15.21:, 1,600! 53.78: 2,150: 46.74
5-21 ! 67 000 1 500! 1.94! 500! .44 1 500! .24 65 900 95.57
4-10 ! 8I000! '550! 7.00! - ! - ! ’450! 5.62! «7I000! 87.50
4-18 ! 15,000! 5 ,000! 20.00! 1,800! 12.00! '4 ,500! 50.00! 5,700! 58.00
4-19 ! 18,000! 2 I500! 15.88! 900! 5.00! 1I100! 6.12! 15,500! 75.00
10-8 ! 58,000! 1 I400! 5.68! 21,000! 55.26! 1 ,900! 5.00! 15,700! 56.05
10—12! 5,800! — ! - ! 1,800! 47.57! 1,500! 59.47! 500! 15.15
10-19! 85,000! 5,000! 6.02! 28,000! 55.75! 2,000! 26.50! 28,000! 55.75
10-25! *60,000! - ! - ! 10,000! 16.66! 40,000! 66.66! 10,000! 16. 66
11-9 ! *75,000! 2,000! 2.66! 16,000! 21.55! 16,000! 21.55! 41,000! 54.66
11-15! 19,000! 500! 2.65! 4,500! 25.68! 7,500! 59,47! 6,500! 54. 1
11-15! 6,500! - ! - ! 2,000! 50.77! 5,000! 46.15! 1,500! 25.07
11-22! 170 000! 2,000! 1.18! 40,000! 25.55! 45,000! 26.47! 85,000! 48.82
11-28! 45,000! 4,000! 8.88! 29,000! 64.44! 500! 1.11! 11,500! 25.55
12-5 ! 500,000! 5,000! 1.66! 85,000! 28.55! 170,000! 56.66! 40,000! 15.55
25-20-24! 6,000! - 1 - ! 2,000! 55.55! 2,400! 40100! 1,600! 26.66

 

‘ Only one plate counted.

U". "| _-r‘. q - "" Hr,- " ~.r.. . ‘ " "- :‘
0;;LAIQ‘IU-&D .'.L\£s.hJ.Uls) ‘val .bigJ

'r v ‘1': - v; , 1 «j--:v1\,; “ "- "\ "Y" 4‘. '~ ‘ “
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1n the computations persente .3es more used instead of numbers
of bacteria because it was believed that this mould give more aeouratr
results.

“he grit netieel mean of the differences in ner.emtegee of the
different typee Of bacteria before and after homOg enizirg was deter-
mined. the standard deviation of these differences wee computed and
"gtt Idente ” _e MIG: (9) of 5e tarmining the Lirnifieenee of t.”

was used. In table XII the results of theLe comrututions are set

 

 

lown.
TABLE XII.
Iatuletion of the Results of Gomputetion.
Y T I I r
' Number ' AVere ' standard ' '
Type ' Of ' bi iieiL" e "e‘J-ntion of' Z ' Odds
'Experi- ' ' Lifferenees '
' ments ' ' Y '
T I I T T
Strong acid ' 3 ' +1.93 ' 8.68 ' .2223 ' 5:1
Weak acid ' 25 ’ - .83 ' 20.50 ' .0304 ' 111
Peptonizers ' 25 f +2.46 ' 14.90 ‘ .1650 ' 2:1
Alkali and Inert ' 2 1 -6,45 I 17.20 I .5750 I z
I I I I I

 

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In statiet eel analysis odds of less than 30:1 are not coneide ed
hi3hly Li3nifiear t.. In the li3ht of this fact we can not reliably
state that the bar; entr3e of any type of bacteria is Changed by
homosenizing. This Simply indicates but even tho.13h beetLr ia may be
added to the ice cream mix from the- hom0‘er zer that the dif an
types ere added in the same relativ e ratio as they already existed in
the mix before omowjn Zing. lhis is not hard to understand since the

bacterial ecntemi ine. ion in the homegenizer is due moetly to inadequate

or imprOper cleanin3 of the mt chine after tin mix has passed through.

Another point brou3ht out by the computation is the fact that
durin3 the nrocess of hom03 Hni in3 that the groupsof the diff Hr nt
types are broken up in about the same relative preportion.

GJLERAL LIeSUdLIOh.

In carrving out this work an attempt was made to follow as nearly
as possible factory condition s so th. an actual picture would be
obtained of the bee teriolo ical changes durin3 the process of homegeniz~
ing. In this way it was hoped to throw some light on the questions
raised by the author in a previous paper and also by other workers in
th 13 field.

A close study of the data would seem to indicate that there is
no def nite relationship betheen the number of colonies on milk-powder
agar and the number of bacteria found by the direct microscOpic count
in the ice cream mix. The type of organisms predominating in the

ample mi ht be one cause for this. If a sample had a large number of
heat resistant bacteria present, a 31 renter nmi her would shot on the
plate count. A majority of the samples showed an increase in the
number of colonies by the plate method after homo; miging. The same
was true with the direct microscOpic method. This held true both
for the individuals and "3r01ps'. From a study of this data it would
seem that there was some contributin3 cause to the increases shown.

A further stuly of table II begins t throw some lijht upon the
cause for the increase after 1:0m03 Lnizing. In every single case the
"group" size is less. t is true that in some cases the differenc
is negligible. HoweVer, if one considers that the mix has been

continuously avitated from 45m minutes to an hour at a temperature of

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for at least a part of the 5.12 pereent increase of the peptonizers
found in the m 3 after homegenizirg. the only otter notable change
in the rreeentagee was in the ease of the alkali and inert groups
where there was a decrease after as compare to before hOmogenizing.
The strong aoid-‘orminx anfl the week acii-forming types also shovwi

an increase after hom03enizin3. this may here been due partly to

1

the breaking up of clumps and nertly to the contamination from the

[3

However, a st tisticel analysis of’the significance of the
slight changes in the percentages of the different types before and
after hom03enizin3 shows that they are not to be taken too
seriously since the aide are not sufficient to make them highly

significant.

GOJJLJJIOLS.

1. In a majority of the

O
L.)

sees, homo geniz in3 increased the numb r
of bacter a1 colonies in the ice cream mix deternincl by the plate
count using milk-powder agar.

2. In a majority of the ca ases, hom03enizin3 ncreased the number
of individuals and "groups' as determined by the direct micrOs conic
method.

3. the average size of the "groups" and also the avora3 size
of groups of two or more in the ice cream mix are decreased. In
VlBN of the fact tin t all these d fferences are of the same sim , it
is safe to conclude that homegcnizing reduces t1 e avera3e size of
bacterial groups in the ice .ream mix.

4. ”he bacterial clumrs in milk are decreased in size by passin3
through the hom03enizer. Increasing the pressure has the 3cnera1
tendency to decrease th e avrra3e size of the groups.

5. Lilk rass 0:1 throu3h ti. e homegcnizer without pressure show
an increase in colonies by the plate method and an increase in
individuals and "groups" by the direct microscopic method.

6. sterile rinse water from the h0m03e nir or contain e1 from 2 to
140,000 bacterial colonies per c.c.

7. The type of colonies found in the ice cream mix were stron3

acid-forming Weak acid-forminJ, pantonizers and alkali and inert.

:he reneral conclusion that may be drawn from the data as a whole

()

is, that, tie increase in the bacteria content after hom03cnizin3 is

due to tu causes; Tirst, the breaking up of bacterial clumrs and,
ccond, contamination from the hom03enizer. It would 11ercfore, seem
that a part of the increase noted in the Trevious nancrs was rartly

apparent and partly real.

 

 

 

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