Technological Institute of the Philippines, Manila
363 P. Casal, Quiapo, Manila
Experiment No. 8
Fermentation of Lactose
Submitted by:
Submitted to:
ABAD, Jacquelyn
Engr. Juanita Apostol
SACOBO, Carl James
SALES, Abipiel Regina
SANGALANG, Steven John
SANTOS, Vince Nikko
SORIA, Jenny
Experiment No. 8
Fermentation of Lactose
1. Objective(s):
To study the production of various products formed using fermentation process.
2. Intended Learning Outcomes (ILOs):
The students shall be able to:
2.1. Produce Yogurt from fermentation of Cow’s milk.
2.2. Study the rate of fermentation of Cow’s milk.
3. Discussion:
Yogurt is made by the fermentation of lactose (milk sugar) by bacterial enzymes. This process
is anaerobic, meaning that it occurs in the absence of oxygen. Lactose is a compound sugar,
made up of the two simple sugars glucose and galactose. During the making of yogurt, the
lactose is broken down by the lactase enzyme provided by bacteria into these components.
Further processing of glucose and galactose results in the end products of Lactic Acid and
acetaldehyde. The production of lactic acid and acetaldehyde lowers the pH of the milk,
causing it to have a sour, tart taste. The lower pH also affects the casein (milk protein),
causing it to coagulate and precipitate, forming the solid curd that makes up yogurt. The
leftover watery liquid is the whey. The two bacteria most commonly used to make yogurt are
Lactobacillus bulgaricus and Streptococcus thermophilus.
Lactobacillus bulgaricus and Streptococcus thermophilus are not the only bacteria that can
convert lactose into lactic acid. Fresh milk begins acquiring microbes from the very moment it
leaves the cow. Milking equipment, people processing the milk, even bacteria in the air can
contaminate milk. To prevent the milk from going bad, all milk sold at the grocery store is
pasteurized. Pasteurization is the process in which milk is heated for a certain length of time
to kill most of the microorganisms that might be present. The U.S. Public Health Service
guidelines say that heating milk at 62.8 °C (145 °F) for 30 minutes or 71.7 °C (161 °F) for 15
seconds meets pasteurization standards. These standards are based on the amount of heat
necessary to kill most of the bacteria that are commonly found in milk.
4. Materials:
500 mL Beaker
Plastic wrap
Thermometer
Hot plate
Elastic bands
12 oz paper/plastic cups
Plastic spoons
5. Procedure:
Powdered milk
Plain yogurt with active cultures
Milk (% fat does not matter)
1. Heat about a liter of milk over the hot plate until it reaches 62.8 °C. Take note that the
2.
3.
4.
5.
milk should be in a hot water bath, not being directly heated.
Once the milk reaches 62.8 °C, remove it from heat and cool to 46 °C, then add 2
tablespoons of pre-made yogurt with active cultures per 1 liter of milk. The yogurt can
either come from a previous batch or from a store-bought
yogurt.
Stir well into the mixture, then cover the lid with plastic
wrap.
Incubate in a temperature no more than 46 °C for at least
10-12 hours.
Refrigerate.
6. Data and Results:
1. Describe the color and the consistency of your yogurt. Does it have an odor?
The yogurt that our group has made is white, and emits a slightly sweet and slightly
pungent odor.______________________________________________________________
2. How does your yogurt compare to the other group?
_________________________________________________________________________
_________________________________________________________________________
3. Compare your “home-made” yogurt to the “store-bought” variety. What effect do some
of the other ingredients have on the “store-bought” yogurt?
Our “home-made”yogurt appeared to be much more thinner in texture and sweeter, with
a little hint of sourness, compared to “store-bought” yogurt that usually had thicker
textures and usually tastes much more sour than sweet. In color-wise as well, our
“strawberry”-flavored yogurt appeared much more whiter than the pinkish tinge we were
aiming for, similar to those strawberry-flavored yogurts we see in stores. This could
probably due to the lack of food coloring our group has used, however._______________
7. Questions:
1. What are the 2 most common bacteria used in yogurt-making?
The 2 most common bacteria used in yogurt-making are Lactobacillus bulgaricus and
Streptococcus thermophilus._________________________________________________
2. During the fermentation process, what happens to the lactose (milk sugar)? What are
the end products of fermentation?
The lactose is broken down by the lactase enzyme provided by bacteria into these
components. Further processing of glucose and galactose results in the end products of
Lactic Acid and Acetaldehyde.________________________________________________
3. Why does the yogurt taste sour?
The production of lactic acid and acetaldehyde lowers the pH of the milk, causing it to
have a sour, tart taste._____________________________________________________
4. What causes milk to thicken and form “curds”?
To turn milk into yogurt, these bacteria ferment the milk, turning the lactose sugars in
the milk into lactic acid. The lactic acid is what causes the milk, as it ferments, to
thicken, taste tart, and form curds.___________________________________________
8. Conclusion:
Our yogurt appeared to be much more thinner in texture and sweeter, with a little hint of
sourness, compared to store-bought yogurt that usually had thicker textures and usually
tastes much more sour than sweet. In color-wise as well, our “strawberry”-flavored yogurt
appeared much more whiter than the pinkish tinge we were aiming for, similar to those
strawberry-flavored yogurts we see in stores. This could probably due to the lack of food
coloring our group has used, however, as well as other ingredients industries have added on
their commercial yogurt that the group is not able to acquire due to many reasons not limited
to being not sold for retail.
Aside from these observations, no other reactions or problems were encountered by the
group for this experiment and thus concluding that the yogurt production is a success.
9. Further Readings:
Ali, M. F., Bassam, A., El, M. and Spreight, J. G. (2005). Handbook of industrial chemistry:
organic chemicals. New York: McGraw-Hill Co.
Cavani, F., et al. (Eds). (2009). Sustainable industrial chemistry. Britain: Wiley-VCH
Kazmer, D. O. (2009). Plastics manufacturing systems engineering. Munich: Hanser Gardner
Pub
Valberg, H. S. (2010). Applied metal forming including FEM analysis. Cambridge: Cambridge
University Press