eMichael Chiao
(group with Karsen Cheung and Hongbo Wang)
Lab Science 9
Bio investigation: Yogurt Culture
Relevance
Yogurt is world-widely consumed food. It is a diary product, and there are a lot of
processes to make yogurt. Factories use machineries to complete such process like
fermentation to make yogurt. In our experiment, we are trying to make yogurt without
using any machinery, but using only stuff that can be found in kitchen.
Goal
To make the yogurt is edible. (With reasonable pH values, smells, exterior, etc.)
General Expectations and models
We might make yogurt that have been fermented and undergo the process of syneresis,
and we hope we our product is edible.
Variables
Independent variable: different amount of the yogurt starter
Dependent variable: pH values
Variables held constant: temperature, amount of milk, prevent condensation,
sterilization
Materials (for both procedures)
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1 L of whole milk
one single serving containers of yogurt to use as starter cultures
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hotplate
pot with lid
2 flasks with stoppers, sterilized in boiling water
permanent marker
thermometer
pan
pH sensor
spoon
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gloves
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funnel
beaker
Method/ Procedure One
1. Heat the milk to 85–90oC (185–195oF) in a pot, keeping the pot covered to reduce
evaporation, stir frequently to prevent the milk from sticking to the bottom.
2. Remove from hotplate and place the covered pot in a pan of clean, cool water until
the stirred milk is very close to 55℃ (130oF).
3. While the milk is cooling, prepare yogurt starter.
4. Arrange 2 pre-sterilized flask with robber stopper.
5. Open the yogurt product you will use as a starter and stir with a clean fork to be
sure it is mixed evenly.
6. Add one tablespoon and a half tablespoon of the starter to two jars. Label the jar
with the brand name using a permanent marker.
7. After the milk cools to 55℃ (130oF), it will still be warm to the touch. Carefully
pour it into the flasks filling the flasks. Cover immediately with the sterile lids.
8. Place filled bottles in incubator and keep the temperature to 50℃ (122oF).
9. Do not disturb the yogurt and it will be finished in 3 hours, provided the
temperature does not drop below 40℃ (104oF). Refrigerate overnight.
10. Use the pH sensor to test the pH value of the yogurt.
Method/ Procedure Two
1. Sterilized the beaker.
2. Put the milk into the beaker and heat it up to 90℃.
3. While cooling the milk down to 55℃, put different amount the yogurt starter into
cups with lids.
4. Pour the milk into the cups and put them into the incubator for 3 hours.
5. Refrigerate
6. Use the pH sensor to test the pH value of the yogurt
CAUTION!!
Wear gloves when taking things that
have been heated.
HOT! HOT! HOT! HOT! HOT!
http://www.sciencebuddies.org/mentoring/project_ideas/MicroBio_p010.shtml
Sketch/figure
Expectations (experiment)
We are expecting that we might make yogurt from milk.
We hope that to keep the temperature at about 40-50℃ will be the best, because
bacteria in milk will be more active at that temperature during fermentation, and it
give the yogurt a sour taste. We also expect to see a densely yogurt, which is caused
by a process named syneresis.
Results
Yogurt (in flasks) that were made by procedure one.
Yogurt (in cup and flasks) that were made by procedure two.
Well, the first picture shows the yogurt that we made by following procedure one. It is
obvious that they look kind of like yogurt, but they look really disgusting: the white
part didn’t form a liquid-solid like yogurt, but small pieces of solid “yogurt”, and they
smell really weird. There is even something like water floating on the top of the flask.
The second photograph shows the yogurt that we made by following procedure two.
This time they look much more like yogurt, white, dense like, and they smell good which smells like the yogurt we bought from the market.
Table One (based on the product the made by following procedure one)
old
After 1 day
After 3 days
After 5 days
1/2 spoon of
yogurt starter
5.8 (pH)
5.7 (pH)
5.7 (pH)
1 spoon of yogurt
5.4 (pH)
5.5 (pH)
5.5 (pH)
starter
Table Two (based on the product the made by following procedure two)
new
After 1 day
After 3 days
1/2 spoon of
yogurt starter
5.1 (pH)
5 (pH)
1 spoon of yogurt
starter
5 (pH)
4.8 (pH)
Graph One (according to Table One)
the pH value of yogurt
pH value
6
1/2 spoon of yogurt
starte
5.8
5.6
1spoon of yogurt
starte
5.4
5.2
1
2
fater how many days
3
Graph Two (according to Table Two)
pH value
the pH value of yogurt
5.2
5.1
5
4.9
4.8
4.7
4.6
1/2 spoon of yogurt
starter
1 spoon of yogurt
starter
1
2
after how many days
Data Processing and Analysis
(What we’ve already know that regular yogurt’s pH is about 4~4.4, and milk is about
5~6.) In general, either in table one or table two, the pH values of 1 spoonful of
yogurt starter is always lower then 1 and half spoon of yogurt starter. The first graph
shows that it doesn’t really change, so we decide to do it again by following
procedure two.
What we’ve got here is that the pH value of the second graph is much closer to the
regular yogurt. So I thought that maybe more yogurt starter lower more pH values.
Well, the values didn’t change a lot after 2 to 3 days. Therefore, if we are going to
redo it, we probably will have to record the data at the time while we keep it warm.
Discussion
Just like what I expect, the yogurt that we made has a lower pH values and it is very
dense. But in part, the acidity didn’t get lower enough to reach the regular pH value of
yogurt. I followed every single procedure, but the results didn’t turn out to be the
same. Why the pH value of the yogurt we made didn’t get lower enough? Is it because
the equipments are not well – sterilized? Or is it because we didn’t add enough
amount of the yogurt starter? If I have a chance, maybe I’ll do a new experiment that
the equipments are well – sterilized and having enough amount of yogurt starter to
make yogurt that are exactly like the one we can get from a market – well, probably
not as sweet as them.
Appendix I: Scientific Terminology
-lacto-globulin
The major whey protein of cow’s milk. From Greek word “bêta”, Latin word “lact-“,
and “GLOBULE + -IN2”
-lacto-globulin is heated above 85℃
-casein.”
Bacteria
Any of the unicellular prokaryotic microorganisms of the class Schizomycetes, which
vary in terms of morphology, oxygen and nutritional requirements, and motility, and
may be free-living, saprophytic, or pathogenic in plants or animals. From Greek word
“baktērion”
“Lactic acid bacteria is one kind of bacteria that we used to make yogurt from milk.”
Culture
The cultivation of microorganisms, as bacteria, or of tissues, for scientific study,
medicinal use, etc. From Latin word “cultūra”.
“We use Lactic acid bacteria for culture.”
Enzyme
Any of numerous proteins or conjugated proteins produced by living organisms and
functioning as biochemical catalysts, came from “Greek en-, in; see en-2 + Greek
zūmē.”
“ -lacto-globulin is one of the enzymes.”
Fermentation
Any of a group of chemical reactions induced by living or nonliving ferments that
split complex organic compounds into relatively simple substances. From Latin word
“fermentāt.”
“One of the processes that makes is fermentation.”
Lactic acid bacteria
Lactic acid bacteria are gram-positive usually non-motile, non-spore-forming rods and
cocci. The lactics can only obtain ATP by fermentation, usually of sugars. Since they
do not use oxygen in their energy production, lactic acid bacteria happily grow under
anaerobic conditions, but they can also grow in oxygen's presence. From old English
and Greek word.
“Lactic acid bacteria are essential to making yogurt from milk.”
Lactobacillus Bulgaricus
One of several bacteria used for the production of yogurt.
“Lactobacillus Bulgaricus is one of the bacteria that can use to produce yogurt.”
Lactose
Biochemistry. a disaccharide, C12H22O11, present in milk, that upon hydrolysis yields
glucose and galactose. From old english “LACT- + -OSE2.”
“Yogurt contains most milk constituents except lactose.”
Microorganism
Any organism too small to be viewed by the unaided eye, as bacteria, protozoa, and
some fungi and algae. From English word “micro- + organism”.
“Bacteria are one of the microorganisms.”
pH (acidity)
The quality or state of being acid. From German “PH”.
“Yogurt has a lower acidity then milk.”
Protein
Biochemistry. any of numerous, highly varied organic molecules constituting a large
portion of the mass of every life form and necessary in the diet of all animals and
other nonphotosynthesizing organisms, composed of 20 or more amino acids linked in
a genetically controlled linear sequence into one or more long polypeptide chains, the
final shape and other properties of each protein being determined by the side chains of
the amino acids and their chemical attachments: proteins include such specialized
forms as collagen for supportive tissue, hemoglobin for transport, antibodies for
immune defense, and enzymes for metabolism. The word protein is from German
“protein”.
“Yogurt is made out of protein.”
Sterilize
To destroy microorganisms in or on, usually by bringing to a high temperature with
steam, dry heat, or boiling liquid. From English “STERILE + -IZE STERILE”
“We should sterilize our equipment before we use them.”
Streptococcus Thermophilus
Streptococcus thermophilus is of major importance for the food industry since it is
massively used for the manufacture of dairy products.
“We use streptococcus thermophilus to produce yogurt.”
Synaeresis
The process by which a liquid is separated from a gel owing to further coagulation.
From Latin word "contraction".
In yogurt, it is undesirable and it occurs when the protein network is unable to firmly
retain water, which is synaeresis.
Whey
The watery part of curdled milk. It originally comes from Old English “hwæg”.
“The whey will be separated while making yogurt.”
Yogurt
The fermented milk product. It derives from the Turkish “yoğurt”.
“Yogurt is made from milk.”
Appendix II: Key Concepts, Models, "Big Ideas"
A picture of fermentation which shows how glucose converts into lactic acid
It converts lactose into lactic acid in yogurt, giving it its sour taste. During this
process, while ATP is hydrolysed, a hydrogen ion is released. ATP-derived hydrogen
ions are primarily responsible for the decrease in pH, that’s why yogurt has a lower
pH value then milk.
A picture of yogurt after syneresis.
Syneresis is the extraction of a liquid from a gel. The process above is the collection
of whey on the surface of yogurt, that’s why yogurt is much dense then milk.
Reference
http://www.magma.ca/~pavel/science/Yogurt.htm
http://www.wikipedia.org
Miller KR, Levine JS. Biology. Upper Saddle River, New Jersey: Prentice Hall; 2006:
225