182 
Journal of Agricultural Research 
Vol. II, No. 3 
BACTERIAL FLORA OF PASTEURIZED MILK CHEESE 
The influence of this varied flora upon the production of flavor is 
emphasized in a study of the flora of pasteurized-milk cheese. During 
the pasteurization approximately 99 per cent of the bacteria of the milk 
are killed (Sammis and Bruhn, 1912); then there are added, in the form 
of a starter, organisms belonging to the Bacterium lactis acidi group. 
But instead of ripening the milk until the desired acidity for cheese mak¬ 
ing is obtained—the usual procedure in the making of raw-milk cheese— 
a weak solution of hydrochloric acid is added to make up the required 
acidity. This is necessary in preparing pasteurized milk for cheese making. 
It is evident that the initial bacterial flora of the cheese made from 
pasteurized milk must differ from the flora of a raw-milk cheese of the 
same age, for in the latter all of the types of organisms making up the 
varied flora of milk have had an opportunity to develop without any 
other restraint than that of their symbiotic relations with the associ¬ 
ated types. 
In studying the flora of cheese in respect to its influence in flavor 
production many bacterial analyses have been made of pasteurized- 
milk cheese in every stage of the ripening process. The cultures isolated 
have been studied in the same manner as those from raw-milk cheese 
and have been classified accordingly. In Tables XIV and XV are pre¬ 
sented the results of the analyses of two pasteurized-milk cheeses, Nos. 
20 and 21, to which pure-culture starters were added. To cheese No. 20 
(Table XIII), there was added 0.75 per cent of a pure-milk culture of 
Bacterium lactis acidi , b. In striking contrast to the varied flora of raw- 
milk cheese, as presented in Tables VIII and IX, it will be observed 
that the bacterial flora of this cheese consisted almost entirely of the 
one variety of organism, B. lactis acidi , b, which was added to the milk 
for a starter. Only once was another variety of this group isolated. 
Table XIII .—Bacterial content of pasteurized-milk cheese No. 20 to which was added 
°-75 P er cen t °f a culture of Bacterium lactis acidi, b 
Age. 
Plate cultures. 
Dilution cultures. 
Bacterial 
content. 
Variety of 
Bacterium lao 
tis acidi. 
Vari¬ 
ety of 
strep- 
to- 
cocci, 
Variety of 
micrococci. 
Con¬ 
tamina¬ 
tion. 
Bacterial 
content. 
Vari¬ 
ety of 
Bacte¬ 
rium 
lactis 
acidi. 
Variety of 
Bacterium casei. 
a 
b 
i 
d 
a 
b 
b 
a 
b 
Days 
or 
state. 
Milk.. 
Curd . 
6 .... 
14.. .. 
22.. .. 
30.. .. 
38.. .. 
47 -... 
58.. .. 
72 - 
9j 600,000 
20, 000,000 
450,000,000 
175,000,000 
175,000.000 
400,000,000 
300,000,000 
1,650,000,000 
. 65,000,000 
170,000,000 
P.ct. 
P.ct. 
99-70 
98.00 
100.00 
96.80 
98.30 
95 - 75 
70.50 
100.00 
99-77 
200.00 
P.ct. 
P.ct. 
P.ct. 
P.ct. 
0.30 
2.00 
zoo, 000,000 
100,000,000 
i, 000.000,000 
100,000,000 
100,000,000 
10,000,000,000 
1,000,000,000 
1,000,000,000 
P.ct. 
p. a. 
P.ct. 
0.0001 
99-9999 
0.0001 
0.2 
3.00 
x.70 
4 - as 
I.30 
100.00 
100.00 
28.2 
99.00 
1.0000 
0.04 
O. IQ 
100,000,000 
90.90 
9.1000 
