Download Cheese Lab Part 2

1c
Cheese Production - The Evolution of Cheese Making
This activity is based on labs developed by Louann Carlmagno, formerly of Genencor International and Pam DiFilippo,
Biology Teacher, San Juan High School, Citrus Heights, CA.
Background; Cheese-making is a good example of how
biotechnology has improved an industrial process. In the past,
people made cheese simply by letting the naturally occurring
bacteria in milk turn the milk sour. In that process, the bacteria use
a type of milk sugar called lactose to make lactic acid and create
energy for the cell. These milk bacteria produce special enzymes
that convert the lactose to lactic acid. Lactic acid has a low pH and
causes the milk protein, casein, to break down and fall out of
solution. These lumps of casein are called curds. Lactic acid also
causes the mixture to have a mild to slightly bitter taste. Along with other flavorful compounds, the
lactic acid gives the cheese the characteristic flavor it contains. If one batch of curdled milk produced
a good cheese, a small amount of cheese (with the curdling enzymes) was saved to start the next batch.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Today, most cheese made
commercially is produced in one
the four ways that follow. In each
method sterilized milk is used as a
starting reagent.
1. The milk may simply be left to
exposed to air and naturally
occurring bacteria. The milk
curdles because of bacterial
enzyme activity as described above.
is a form of lactic acid fermentation.
2. New curdled cheese cultures are
started with specific cultures of
selected bacteria. These “known”
bacteria also make enzymes that
milk. Buttermilk has a good culture
Lactobacillus bacteria to use as a
"starter". That's what we'll use.
of
age,
This
curdle
of
3. New cultures may be started by
the addition of purified
enzymes, like rennin, retrieved
from the cells lining the stomachs of calves (YUK!). Due to the presence of these enzymes,
calves curdle milk in their stomachs when they nurse. To retrieve the enzymes for commercial
use, companies must take calf stomachs, grind them up, and purify the rennin enzyme away
from all of the other compounds the cells make. For this reason, some vegetarians do not eat
rennin cheeses. Rennin cheeses include Asiago, most Bries, most Cheddars, and Roquefort to
name a few.
4. New cultures may be started by the addition of purified enzymes produced by genetic
engineering. Scientists found the DNA code for the cheese-making enzymes produced by
calves in regular cow cells. They cut out the cow’s rennin cheese-making code (gene) and
stuck it into fungus cells. Fungus cells then read the cow DNA and synthesize the rennin
enzyme, which scientists called “chymosin”. Then, cheese makers use the genetically
engineered enzymes to speed curdling. Fungal cultures make the curdling enzymes faster,
cheaper, and in larger amounts than inside big multicellular organisms, like cows. Chymosin
cheeses include Jack, Mozzarella, and most Swiss cheese plus many others.
For obvious reasons, modern-day cheese makers are interested in producing lots of the fastest, tastiest,
and cheapest cheese.
Purpose:
Which curdling agent produces cheese the fastest?
Which curdling agent produces the most cheese?
To examine numerical data for support of predictions.
To examine variables that can lead to invalid experiments.
Hypothesis: Chymosin is expected to produce the most volume of cheese in the shortest amount of
time since it has been engineered by Genencor scientists for that result.
Materials:
Chymosin (from Kelley Dairy Supply 1-800-782-8573)
Buttermilk
Whole Milk
P-1000 micropipet tips
10 mL pipet
10 mL pipet pump
Tube racks
11 cm Whatman Filter Paper
10 or 25 mL graduated cylinder
Animal Rennin
P-1000 micropipets
15 ml conical tubes
Plastic funnels
Procedures:
1. Using a 10 mL pipet and pipet pump, transfer exactly 7 mL of whole milk into a labeled, 15 mL
conical tube.
2. Using a pre-set micropipet, add 0.25 mL (250 uL) of one of the four curdling agents to the 7 mL of
milk.
Use either: buttermilk, rennin, chymosin, or more whole milk (control) as assigned by your
supervisor.
3. Cap the tube and gently mix by inverting 3 times. Record this “initial time”. _____
4. Place the milk-containing portion of the tube deep in your armpit, like a thermometer,
and incubate it there for at least 15 min.
5. Check for curdling every 5 minutes, recording the time to curdling in minutes. To check for
curdling, gently tilt the tube, careful to not breakup any curds. Curds are large lumps of solidified
milk. After 15 min, place the tube upright, at room temperature and check for curdling every 15
min for 2 hours. If curdling has not occurred within 2 hours, continue checking once every four
hours. With the greatest accuracy possible, record the number of minutes passed until the milk had
curdled to the greatest extent.
6. If curdling has not occurred by the end of the class period,
bring the tube home (keep at room temperature) and back to
class in 24 hours. Keep tube upright so any curds fall to the
bottom of the tube.
7. On return to the lab, measure the amount of curds (solids) and
whey (liquid) in the tube. You may be able to read the volume
each directly off the tube although it may be difficult. Better
yet, filter it as described below, using a “whey-o-meter”.
of
8. If necessary, pour the whey and curds mixture through a filter
paper funnel into a 10 mL graduated cylinder (a “whey-ometer”). Determine the volume of whey collected in the
graduated cylinder, using a pipet if necessary to measure small
amounts. By subtraction, determine the volume of curds. Can
you suggest other methods of determining the amount of curds
produced in each treatment?
9. Record the data for your sample plus one each of the other
variable groups on the data table below. Record the name of
person from whom you have gotten data.
Data Table 1: The Characteristics of Cheese made by Different Curdling Agents
Curdling Agent
Time to Curdling
Volume of
Volume of
Technician/
(min)
Whey (mL)
Curds (mL)
Comments
buttermilk
rennin
chymosin
milk (control)
How well do these single trials of the experiment support the original hypothesis? Explain.
the
Using Excel®, the lab supervisor will collect each individual’s data to make a class data table showing
multiple replications of the experiment. Averages for each variable group should also be recorded
on this data table. Averaged data is the best answer to an experimental question. Can you explain
why?
11. Using Excel®, produce two graphs; one which shows the average time to curdling for each
enzyme treatment and one which shows the average volume of curds produced by each enzyme
treatment. Use the Excel® tutorial if necessary.
12. Imagine you are an employee at a cheese curdling company and you must summarize the
results of your experiments and give your supervisor the best answer to the scientific questions
asked. Write a conclusion that reports the results of the experiment (answer to the purpose
question) including evidence and explanations for your findings. Discuss how well the results
support the hypothesis. Identify sources of errors in the technique that may lead to variations in
results. Make a recommendation to the cheese company supervisor about which curdling agent
should continue to be the focus of production. Include any variations in the procedures that
you think may improve the cheese production.
13. In the biotechnology industry, the work of others is reviewed and “OK’d” by peers. This is called
“witnessing”. When you witness data and analysis, check for:
 Accuracy of statements (and that they make sense)
 Completeness (R/E/E, PE, PA)
 Evidence (numerical with units of measurement)
 Grammatical and spelling errors
Make correction and suggestions right on the page,
 For corrections, strike a single line through the error, correct it, and initial it.
 For suggestions, write your suggestion in the margin, draw an arrow where it should be placed and
initial it.
Witness a colleague’s conclusion. After you are done witnessing, at the end of the conclusion, write
“Witnessed by” and then write your full name and date it.