الفهرس 
REVIEW OF LITERATUREOnly 14 pages are availabe for public view
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Abstract
Selection of bacterial cultures.
In this study, seven bacterial cultures were screened for their ability
to produce exopolysaccharides. It was found that the bacterial cultures of
Lactobacillus delbrueckii subsp. bulgaricus DSM 20080, Streptococcus
thermophilus CH-1, Thermophilic culture Yoflex Express 1.0 and
Thermophilic culture Yoflex CH-1 were the best. Therefore, these bacterial
cultures were selected for further studies as following:
1- Thermophilic culture Yoflex CH-1 as traditional starter
(Str.thermophilus+Lb.bulgaricus 1:1)
2- Thermophilic culture Yoflex Express 1.0 as commercial producingEPS starter.
3- Lactobacillus delbrueckii subsp. bulgaricus DSM 20080 +
Streptococcus thermophilus CH-1 (1:1) as laboratory producing- EPS
cultures.
Part I: Using of Eexopolysaccharides producing LAB starter in
making functional low-fat yoghurt.
In this part of study, Yoghurt was manufactured from fresh
milk standardized to 3% fat for full- fat and 1.5% fat for low- fat
treatments, Milk was heated to 85°C for 15 min, immediately cooled to
42ºC and inoculated with 2% starter cultures as following:
Standardized buffaloes milk with 3% fat was used as control (CF)
and made using (traditional starter culture) while low- fat milk (1.5% fat)
was divided into 3 equal portions. The first portion was served as control
(Cl) and made with (traditional starter culture), while EPS- producing
cultures either commercial (T1) or laboratory (T2) were added for the rest
2 portions, respectively. These mixtures were manufactured as mentioned
earlier in Materials and Methods section. The resultant yoghurts were
analyzed for the chemical, physiochemical properties, texture profile analysis, microbiological tests and sensory evaluation when fresh and after
7, 14 and 21 days at 5±1˚C. Microstructure of the resultant yoghurt was
examined after one day by SEM. Data obtained were statistically analyzed.
Three replicates were carried out for each treatment.
The results obtained can be summarized as follows:
1- The chemical composition (TS, fat, protein, ash and total
carbohydrate) of fresh yoghurt was nearly the same in different
treatment including the control. However, these contents increased
slightly during the storage periods.
2- The data indicated that functional low-fat yoghurt with laboratory
exopolysaccharides producing culture (T2) had the lowest value of
acidity, while the highest value was noticed with low-fat yoghurt
(Cl). The results revealed that a gradual increase was observed during
storage of all yoghurt treatments.
3- The data indicated that low-fat yoghurt (Cl) had lower pH value than
that of low-fat yoghurt treatments with exopolysaccharides producing
cultures either commercial or laboratory. The pH of the samples
decreased continuously throughout the storage period in a similar
way for all the samples.
4- Use of exopolysaccharides into low- fat yoghurt improved the yoghurt
viscosity. Values of the viscosity were increased with extending the
storage period in all treatments including controls.
5- Low-fat yoghurt with laboratory exopolysaccharides producing
culture (T2 ) was the lowest syneresis followed by low-fat yoghurt
with commercial exopolysaccharides producing culture (T1) then fullfat yoghurt control (CF) and low- fat yoghurt control (Cl) that was
highest syneresis. The syneresis contents of all yoghurt samples
tended to decrease gradually along the storage period.
6- Low-fat yoghurt made with laboratory exopolysaccharides producing
culture (T2) has the highest value of water holding capacity, while the lowest was noticed with low-fat yoghurt control without EPS–
producing culture (Cl). The water holding capacity took an opposite
trend to that of the syneresis.
7- Low- fat yoghurt with exopolysaccharides producing cultures either
commercial or laboratory (T1 and T2) were higher of acetaldehyde
content than low-fat yoghurt control (Cl). In all samples,
acetaldehyde content of the samples increased within the first 14 days
of storage and then decreased.
8- Low fat yoghurt with exopolysaccharides producing cultures either
commercial or laboratory (T1 and T2) achieved the greatest diacetyl
content while low- fat yoghurt control (Cl) was the lowest diacetyl .
9- The total bacterial counts of yoghurt treatments with
exopolysaccharides producing cultures either commercial or
laboratory were higher than two controls (CF & Cl). Also, the results
indicated that gradual increasing was observed throughout the storage
period reaching the maximum after 14 days in all treatments and then
decreased with increasing the storage period.
10- The fresh and stored yoghurts made from low-fat milk with laboratory
exopolysaccharides producing culture (T2) showed higher counts of
Str. thermophilus followed by low-fat yoghurt with commercial
exopolysaccharides producing culture (T1) then the two controls.
11- The fresh and stored yoghurts made from low-fat milk with laboratory
exopolysaccharides producing culture (T2) showed highest counts of
Lb. delbrueckii subsp bulgaricus among all treatments including two
controls.
12- The use of exopolysaccharides into low-fat yoghurt led to decrease of
hardness compared to low-fat yoghurt control (Cl). The hardness
decreased by advancing the storage in all treatments including two
controls.13- All parameters of TPA (cohesiveness, springiness, and gumminess
and Chewiness) increased in the resultant yoghurt with prolonging
the storage period for full-fat yoghurt control (CF) and low-fat
yoghurt control (Cl) expect low-fat yoghurt with commercial EPSproducing culture (T1) and functional low-fat yoghurt with laboratory
EPS- producing culture (T2).
14- The microstructure of milk proteins matrix changes during processing
of yoghurt according to the type of starter and content of milk fat. So,
it can be recommended to say that for improving yoghurt quality,
EPS-producing culture either commercial or laboratory in low-fat
milk yoghurt.
15- The obtained results indicated that low-fat yoghurt with
exopolysaccharides producing cultures either commercial or
laboratory (T1 and T2) scored the highest of total scores while lowest
score was gained by low fat yoghurt control (Cl). All yoghurts were
sensory acceptable but the best yoghurt was low-fat yoghurt with
exopolysaccharides producing culture laboratory (T2).
Part II: Making functional low-fat Ras cheese using of
Exopolysaccharides producing LAB starter culture.
In this part of study, Ras cheese was manufactured from fresh
cow’s milk standardized to 3% fat for full-fat and 1.5% fat for low- fat.
Milk then heated to 72°C for 15 seconds, then cooled to 35ºC and
inoculated with 2% starter cultures as following:
Standardized cow’s milk 3% fat was used as control (CF ) and
made using (traditional starter culture) while low- fat milk 1.5% fat was
divided into 3 equal portions. The first portion was served as control (Cl)
and made with (traditional starter culture), while EPS- producing cultures
either commercial (T1) or laboratory (T2) were added for the rest 2
portions, respectively. The added cultures were mixed properly in the
cheese milk before renneting. The resultant Ras cheeses were tested for the chemical, physio-chemical properties, microbiological tests, texture
profile analysis and sensory evaluation. All parameters followed up when
fresh and after 30, 60 and 90 days of storage at 15±2 ºC. Microstructure
of resultant Ras cheese was examined after one day by SEM.
Organoleptic properties of Ras cheese were evaluated when fresh and
after 30, 60 and 90 days of storage at 15±2 ºC. Data obtained were
statistically analysed. Three replicates were carried out for each treatment.
The obtained results can be summarized as follows:
1- The data indicated that fresh low- fat Ras cheese made with laboratory
EPS- producing culture (T2) had highest yield among low-fat Ras
cheese treatment while low- fat control showed the lowest.
2- The moisture content of Ras cheese treatments with
exopolysaccharides producing cultures either commercial or laboratory
were higher than the two controls (CF & Cl). The moisture contents of
all Ras cheeses tended to decrease gradually along the ripening period.
3- Full- fat Ras cheese control (CF) had lowest total nitrogen content
while low- fat Ras cheese control (Cl) showed the highest among all
treatments. The total nitrogen content of all Ras cheese samples
tended to increase gradually along the ripening period.
4- Full- fat Ras cheese control (CF) had the highest fat content compared
to all treatments low-fat Ras cheeses. The fat contents of all Ras cheese
tended to increase gradually along the ripening period.
5- The ash contents of all Ras cheese samples tended to increase
gradually along the ripening period with no marked difference among
treatments.
6- Low-fat Ras cheeses showed slightly higher salt percent than full-fat
Ras cheese. The salt percent of all Ras cheese samples tended to
increase gradually along the ripening period.7- The data indicated that low-fat Ras cheese made with laboratory
exopolysaccharides producing culture (T2) has the lowest value of
acidity, while the highest value was noticed with low- fat Ras cheese
control (Cl). On the other hand, the results revealed that a gradual
increase was observed during ripening period in all treatments.
8- The data indicated that low- fat Ras cheese control (Cl) had lower pH
value than that of low-fat Ras cheese treatments with
exopolysaccharides producing culture either commercial or
laboratory. The pH of the samples decreased continuously throughout
the ripening period in a similar way for all samples.
9- Low-fat Ras cheese made with exopolysaccharides producing cultures
either commercial or laboratory (T1 and T2) were higher of soluble
tyrosine content than low-fat Ras cheese control (Cl). The soluble
tyrosine content of all samples increased as ripening period
progressed.
10- The data indicated that low- fat Ras cheese made with
exopolysaccharides producing cultures were higher of soluble
tryptophan content than low- fat Ras cheese control (Cl). The soluble
tryptophan contents of all Ras cheese samples including the two
controls tended to increase gradually along the ripening period.
11- Using EPS-producing cultures either commercial or laboratory in
low- fat Ras cheese enhance the TVFA of content cheese compared
to low- fat Ras cheese control (Cl). As ripening period progressed the
total volatile fatty acids content increase in all cheeses.
12- The total bacterial counts of Ras cheese treatments with
exopolysaccharides producing cultures were higher than two the
controls (CF & Cl). As ripening period progressed the total counts
gradually increased in all treatments to reach maximum after 60 days
and then decreased 13- Fresh and stored Ras cheese made from low- fat milk with laboratory
exopolysaccharides producing culture (T2) showed higher
Str.theromphilus and Lb. delbrueckii subsp bulgaricus counts
followed by low- fat Ras cheese with commercial exopolysaccharides
producing culture (T1) then the two controls.
14- Hardness of low-fat Ras cheese control (Cl) was the highest compared
to low- fat Ras cheese treatments made with exopolysaccharides
producing starters either commercial or laboratory (T1 and T2)
including full- fat Ras cheese control (CF), which led to an extremely
hard cheese. At the end of ripening period (90 days) at 15±2º C, the
hardness decreased in all treatments including the two controls.
15- All parameters (cohesiveness, springiness, gumminess and chewiness)
increased in the resultant Ras cheese with prolonging the ripening
period for full-fat Ras cheese control (CF), low-fat Ras cheese control
(Cl) and low- fat Ras cheese with laboratory EPS- producing culture
(T2) expect low-fat Ras cheese made with commercial EPSproducing culture (T1).
16- SEM indicated that fresh cheese containing exopolysaccharides
exhibited protein network with more and large opened cavities than
low- fat Ras cheese control (Cl).
17- The body and texture scores in full- fat Ras cheese control (CF) and
low-fat Ras cheeses including EPS-producing cultures (T1 and T2)
had higher points than those of 1ow- fat Ras cheese control (Cl).
18- Low- fat Ras cheese made with EPS-producing cultures (T1 and T2)
have almost similar sensory characteristics to the cheese containing
higher fat content (3%), and these samples were most preferred by
the panel compared to low- fat Ras cheese control, However the best
cheese was low-fat Ras cheese with laboratory EPS- producing
culture (T2) .