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What’s in your milk? |
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Informations |
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Project:
What’s in your milk? |
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Developed By:
Isabelle Côté and Isabelle Raiche |
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Type of Project:
Experiment |
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Category:
Applied science and technology |
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Class:
Senior 1 |
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Age of Participant:
16 |
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School:
Le Mistral |
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Teacher:
Johanne Belzile |
Project presented at the 1998 Eastern Quebec regional
final of the Bell Science Fair
Selected for the 1998 Quebec final (Montreal) of the Bell Super Science
Fair
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Aim |
To qualitatively compare the amount of bacteria
contained in milk that is pasteurized and milk that is unpasteurized.
To repeat the experiment with samples of cow’s, sheep’s,
goat’s and human breast milk. Finally, to compare these various
types of milk with one another.
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Hypothesis |
There are probably more bacteria in the unpasteurized samples than in
the pasteurized samples.
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Protocol,
Part A: Pasteurization |
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Materials: |
thermometer
presto (sterilizer)
beaker
samples of goat’s, cow’s, sheep’s and human breast
milk
stopwatch
test tubes with screw-top stoppers
double boiler
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pencil to
identify the test tubes
tap water
thermometer stand
test tube tongs
ice bath
universal stand
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Experiments |
Sterilize the test tubes at 121°C and at 15
lbs. of pressure for 15 minutes in the presto, being sure to loosen the
test tube stoppers to avoid having them explode under pressure.
Identify the test tubes and place 10 mL of each type of milk in the
corresponding test tube.
Submerge the test tubes in a double boiler (prepare using a metal basin,
a beaker and water) maintained at a temperature of 64°C for 34 minutes,
30 seconds.
Cool rapidly by plunging the test tubes into an ice bath.
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Part B: Preparing the culture media |
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Materials: |
magnetic
agitator
plastic containers
pouches of powdered culture media: one violet-red bile agar and
one plate count agar
burner
distilled water
presto (sterilizer)
double boiler
tap water
balance
2 Erlenmeyer flasks with screw-top stoppers |
spatula
thermometers
universal stand
thermometer stand
sterile pipettes
sterile petri dishes
70% ethanol
incubator
hot plate
insulated handle |
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Experiments |
Prepare the two culture media in their respective Erlenmeyer flasks,
following the recipe indicated on the pouch:
- Weigh the requisite amounts of powder.
- Dissolve in distilled water.
Place the loosely stoppered (to prevent pressure from accumulating) Erlenmeyer
flasks in the presto.
Sterilize the media at 121°C and at 15 lbs. of pressure for 15 minutes.
Submerge the solutions in a double boiler maintained at 49°C (prepared
using a basin, a hot plate, a thermometer, a thermometer stand and a
universal stand).
Allow the media to cool until the water temperature is stabilized and
the Erlenmeyer flask is cool enough to handle.
Disinfect the work area with 70% ethanol.
Identify the petri dishes.
Pipette 0.2 mL of milk into the corresponding sterile petri dishes using
a sterile Pipette.*
Pour approximately 20 mL of the culture media into each of the petri
dishes.*
Gently mix the components, being careful to keep the lid closed.
Let stand until congealed.
Incubate the inverted petri dishes at 32°C for 48 hours.
Observe with or without a magnifying glass.
*During these experiments, it is important
to avoid opening the petri dishes or speaking, and to be sure to flame
any instruments used in order to avoid contamination. Remember to keep
your environment sterile.
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Results |
After 48 hours, no bacteria formed in the various
pasteurized milks. As for the unpasteurized milks, we noted the presence
of a large number of coliform bacteria in the cow’s milk (violet-red
bile agar medium) and a very large number of various species of bacteria
(plate count agar medium). To our great surprise, the breast milk developed
fewer bacteria than the other types. The cow’s milk seemed to be
inhabited by countless varieties of bacteria in both media. No coliform
bacteria were found in the goat’s, sheep’s or breast milk;
only a small number were detected in the plate count agar, making it
possible to identify the presence of different species of bacteria.
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Interpretation |
Cow’s milk contains more bacteria than other
types and breast milk contains fewer bacteria. Pasteurization completely
destroys all types of bacteria.
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Conclusion |
It is important to pasteurize milk before drinking
it to avoid health problems. Goat’s and sheep’s milk contain
fewer bacteria than cow’s milk, possibly because of the ways in
which these animals eliminate waste. Sheep and goat excrement is dryer
than cow excrement, which is much more liquid and therefore more likely
to lead to contamination. Although it would have been interesting to
identify the different types of bacteria present, we were unable to do
so because of a lack of equipment.
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General
information on milk |
Proportions (in %) of the main components
of milk according to species (weight/volume ratio)
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Water |
Dry extracts |
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Proteins |
Fats |
Sugars |
Minerals |
Total |
| Breast milk |
88.1 |
1.3 |
3.6 |
6.8 |
0.2 |
11.9 |
| Cow’s milk |
87.3 |
3.4 |
3.8 |
4.7 |
0.8 |
12.7 |
| Sheep’s milk |
83.4 |
5.4 |
0.2 |
0.2 |
0.8 |
16.6 |
| Goat’s milk |
87.3 |
3.6 |
4.1 |
4.2 |
0.8 |
12.7 |
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 Water
The water found in milk comes from the blood and is controlled by
the amount of lactose secreted by the cells in the mammary glands.
Sugars
The main sugar in milk is lactose, which is synthesized by the mammary
cells from glucose and galactose. These two components are present in
milk in small quantities. Lactose is a milk solution. Although it is
a sugar, it does not taste sweet. This sugar is similar in all types
of milk since its form does not vary according to the species of animal
or its eating habits. Many people are lactose intolerant, meaning they
are unable to digest lactose. These people do not have the enzyme, known
as lactase, needed to digest the milk sugar. They can, however, eat yogurt
and certain types of cheeses that have undergone fermentation and, consequently,
do not contain lactose.
Fats
The concentration of milk fat varies tremendously according to the
breed of cow or its eating habits. Milk fat occurs in the form of triglycerides
(in emulsion in milk). The colour of milk is the result of the fats in
which carotene and xanthophyll are dissolved. The particles are surrounded
by a protective layer of phospholipids. They have a negative electric
charge, which keeps them apart from one another.
Proteins
The concentration of proteins in milk varies with the percentage
of fat: the higher the fat content, the greater the number of proteins.
Albumin and globulin make up 20% of the proteins in milk. The major nitrogenous
matter in milk is casein (80%). Milk curdles when its pH exceeds a value
of 4.6, the isoelectric point of casein. The micelles of casein form
a gel.
Vitamins
and minerals
Milk is an excellent source of minerals. It helps young people grow
and adult bones stay healthy. Calcium and phosphorous are highly digestible
in milk, due in part to their combining with casein. Milk does not contain
enough iron to meet the needs of young people, but its low concentration
is sufficient to prevent the growth of bacteria.
Milk is a complex substance and an almost complete
food that contains more than 100 substances in the form of emulsions,
suspensions or solutions. The composition of milk varies according to
species, season, lactation stage, and a number of other factors.
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© 2002, Conseil de développement du loisir scientifique (CDLS). This
document is distributed by the Conseil de développement du loisir scientifique.
For more information, visit our Web site at www.cdls.qc.ca. |
The opinions expressed
in this section are those of the authors and do not necessarily
reflect the opinions of Merck Frosst or its employees. |
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Water