الفهرس 
THE LACTIC ACID BACTERIAOnly 14 pages are availabe for public view
 |  | from | 165 |  |  |
| | | from | 165 | | |
Abstract
In this study, cow milk samples were randomly collected from Milky Land tanks and examined for
chem ical composition and microbiological properties.
Lactic acid bacteria in these samples were isolated and
identified • The biochemical activities for the domin- ant lactic acid bacterial strains
(Lactobacillus bulg aricus strains ) were investigated.
Also, ( thermophilus, acidophilus ) as pure strains from (MIRCEN ) were examined, the use of most
active lactobacilli starter, isolated from cows milk with the
pure strains th ermophilusand L.acidophilus) in the manufacture of some fermented dairy products
was studied. The results obtained throughout this work could be summarized hereinafter:
PART 1 SELECTION OF Lactobacillus bulgaricus STARING
Three samples were taken from storage tank. Each sample was divided into two portions. One was used
for the chemical analysis and the other was placed in
a sterile jar for bacteriological examination.
Summd!ry ( l _;? .1 t
A. Chemical composition :
1- The total solids content of cow milk samples ranged from 11.20 to 11.50 with an average of 11.35
+ 0.01.
2- The maximum fat content of cow milk samples was
3.2 % , the minimum was 2.8% and the average was
3.0% + 0.1 .
3- The titratable acidity of cow milk samples ranged from 0.18% to 0.16% with an average of ).17% +
0.01 as lactic acid
4- The PH values ranged from 6.68 to 6.62 with an average of 6.65 + 0.01 .
B. Microbiological properties :
1- The total bacterial count in cow milk samples varied
from a min 9 minimum of 39 x 3
to maximum of 60
x 104 with an average of 44.5 x 103 C F U /ML.
2- The counts of lactic acid bacteria on M R.S medium
ranged from a minimum of 55 x 2
to a maximum of
35 x 10 with an average of 40 x 2
CFU/ml.
C. IDENTIFICATION OF LACTIC ACID BACTERIA :
Lactic acid bacteria isolated from cow milk samples on MRS medium were identified as lactobacillus
species
SummcJr y i 1 ;:’?)
by using morphological and physiological characterlstics
Growth temperature, Microscopic appearance, biochemical tests ) which were found to belong to
L.bulgaricus.
PART 2 ACTIVITY OF SINGLE AND MIXED LACTIC ACID STRAIN STARTERS :
Eleven pure single strains of L.bulgaricus have been isolated from cow milk samples (Part 1). These
isolated pure single strains were subjected to the activity test and according to the results
obtained from this test, the most active strains and the pure strains
( S.thermophilus, L.acidophilus) were judged organolept-
ically and examined for the biochemical activities.
1- The amount of acid produced in milk by L.bulgarcus(2) and (5)was higher than the corresponding
values obtained when the other L.bulgaricus strains are
grown in milk when fresh and after 1,2,3,4,5,6,7, and
8 hr incubation. The amount of acid produced in milk by L.bugaricus (5) + S.thermophilus (1:1) was
higher than the corresponding values obtained when the other mixed strains are grown in milk after
1,2,3,4,5,6, and
8 hr incubation.
2- L.buglaricus (S) was the most active single L.bulgar icus strains in acetaldehyde production,
while the mixed strains (S.thermophilus + L.bulgaricus (5)was
Summar”y ( 123)
the most active in acetaldehyde production.
3- L. bulgaricus strains (5) & (9)were the most active cultures in tyrosine liberation, i.e.
proteolytic activity. While the mixed strains (S.thermophilus L.bulgaricus (5) were the higher
active in tyrosine production than the mixed culture (S.thermophilus + L.bulgaricus (5) +
L.acidophilus
PART 3 THE USE OF MIXED LACTIC ACID STRAIN STARTEL CULTURES FOR MAKING SOME FERMENTED MILKS.
1- In this part, different types of starter cultures
(S.thermophilus. + L.bulgaricus) and (S.thermophilus
+ L.bulgaricus + L.acidophilus ) were used to produce plain Yoghurt and acidophilus yoghurt,
respectively.
The types of Yoghurt
A : Plain Yoghurt
1- 12 %, 14 % 16 % increasing the level of T.S. by using skim milk powder).
2- 12 % 14% 16% increasing the level of T.S by using sodium caseinate.
B: Acidophilus Yoghurt
1- 12 % , 14% 16% increasing the level of T.S by using skim milk powder).
2- 12 % , 14 % 16 % ( increasing the level of T.S using sodium casinate )
Summary i 124!
C Fruit yoghurt
By using Jam
5%, 7% and 10 % .
strawberry, orange, banana) with
D : Fruit Acidophilus yoghurt :
By using Jam (strawberry, Orange, banana) with 5%
7% and 10%
These types of yoghurt were analysed chemically, microbiologically and organoleptically during the
manuf acture time and after 1,2,3 weeks of cold storage. The results could be summarized as
follows :
A. Chemical composition :
1- During the manufacture time :
A. The acidity of all treatments gradually increased while the PH values decreased throughout the
manufacture time. While the yoghurt containing higher level of T.S.
by using skim milk powder showed higher amount of lactic acid but the yoghurt in which the level of
T.S. was inc reased by using sodium caseinate showed lower amount of lactic acid. Besides, the
fruit yoghurt (10% Orange Jam) had very low amount of lactic acid.
B. The acetaldehyde content of all treatments gradu ally increased after 2 h incubation until the
end of incubation time. The plain yoghurt (14% T.S) with skim milk powder had higher level of
acetaldehydP. content than
the plain yoghurt (14% T.S) with sodium caseinate.
’J’he f’rui t. yc·ghurt ( 10% Orangt Jarn) had Vt-’r’y lc,v. . ’-’\.’,--· 1
,,:
acetaldehyde content.
2- During storage (1,2,3 weeks at 5 C )
A. The acidity of all treatments slowly increased while the pH values slowly decreased throughout
the storage at
5 c
B. The acetaldehyde content of all treatments slowly decreased through the first week but through
the second and third weeks the acetaldehyde content of all treatments decreased so much.
B. Microbiological Properties:
1- During the manufacture time
The total viable count of all treatments increased after 2 h incubation until the end of
incubation time on
” Lees midium”. except the fruit yoghurt while 10% orange
Jam it was stoped at a low count.
2- During storage (1,2,3 weeks at 5 C)
A. The total viable count of all treatments slowly increased throughout the first week but
throughout the second and third week, the total viable count of all treatments decreased.
B. Yeasts and moulds were not detected in the first
week of all treatments except some of samples such as fruit yoghurt. While throughout the second
and third week the count of yeasts and moulds increased so much in most of treatments.
C. Sensory Evaluation :
Samples from treatments plain yoghurt and Acidophilus yoghurt with 14% T.S by using skim milk
powder, fruit yoghurt with 7 % jam gained tl1e higher scores when fresh
and at first week of the storage period.