L. Assistant Rawa abdul Redha Aziz
FOOD MICROBIOLOGY LAB 2
Cheese Production
There are hundreds of different types of cheese, but each
is made using similar principles of coagulating the
proteins in milk to form curds, and then separating the
curds from the liquid whey.
The coagulation of milk proteins can be done as follows:
1- Using rennet - an enzyme extracted from calves’ stomachs that coagulates
the proteins in milk.
2- Fermenting to form lactic acids
3- Adding acid (e.g. lemon juice, vinegar)
4- Boiling
Cheeses may be broadly grouped into soft, moist, creamy cheeses and
hard cheese. They have a shelf life of around 3 days when refrigerated, or
5 days if covered with brine and refrigerated.
Hard Cheeses
(e.g. ‘Cheddar’ cheese) is produced using lactic acid bacteria and rennet.
Hard cheese requires greater investment in equipment and greater skill to
produce than soft cheeses.
Sterilization vs. Pasteurization
Sterilization means the complete destruction of all the micro-organisms
including spores, from an object or environment. It is usually achieved by
heat or filtration but chemicals or radiation can be used.
Pasteurization is the destruction, inhibition or removal of microbes that
may cause disease or other problems e.g. spoilage. It is usually achieved
by using heat.
Collage of Science/ Kufa University
Fourth Level
L. Assistant Rawa abdul Redha Aziz
FOOD MICROBIOLOGY LAB 2
((Steps of Cheese Making))
Collage of Science/ Kufa University
Fourth Level
L. Assistant Rawa abdul Redha Aziz
FOOD MICROBIOLOGY LAB 2
Yoghurt Production
Yoghurt is a sour milk product due to the presence of lactic acid yielded
by bacteria that underwent fermentation. Bacteria that are commonly used
in yoghurt production are Streptococcus thermophilus and Lactobacillus
bulgaricus which break down sugars in the milk and produce lactate or
lactic acid as by-product.
STAGES IN THE MANUFACTURE OF PROCESSED YOGHURT
Starter Culture
Two species of bacteria Lactobacillus bulgaricus and Lactococcus
thermophilus in approximately equal proportions, are used to make
yoghurt. Commercial producers pasteurize and homogenize the milk
before adding the starter. After stirring, the mixture is then incubated for
3-6 hours at 40-450C.
Collage of Science/ Kufa University
Fourth Level
L. Assistant Rawa abdul Redha Aziz
FOOD MICROBIOLOGY LAB 2
At this temperature the two bacteria have a mutually stimulating effect on
one another. Proteolytic enzymes from L. bulgaricus break down milk
proteins into peptides. These stimulate the growth of L. thermophilus
which, in turn, produce formic acid and carbon dioxide, growth
stimulants for L. bulgaricus. As the incubation proceeds, L. bulgaricus
converts the lactose to lactic acid and the pH falls to 4.2-4.4 which leads
to the coagulation of proteins by lactic acid and the thickening of the
yoghurt.
Further processing involves the addition of flavour, colour, fruit pulp and
heat treatment to kill off any bacteria.
METABOLIC PRODUCTS OF BACTERIA USED IN YOGHURT MAKING
Lactobacilli and Lactococcus Characterization
 Gram-positive facultative anaerobic
 Cultured on MRS medium
 pH adjusted to 6.2
 Incubation period 30oC for 48hr.
Collage of Science/ Kufa University
Fourth Level