Biology 20 Student _______________________ Mark________________________
YEAST FERMENTATION LAB
INTRODUCTION
Yeasts are eukaryotic, single-celled microorganisms that belong to the fungal kingdom. When
yeasts consume sugar and convert it to energy, they emit carbon dioxide, this is referred to as
fermentation. The yeast will be more active and develop faster if there is more sugar present.
While sugar and other sweets provide “food” for yeast, too much sugar can harm it by draining
moisture from it and preventing it from growing. Too much sugar also delays the development of
gluten. Increase the amount of yeast in the recipe or find a comparable recipe with less sugar.
Sweet yeast doughs will rise more slowly. Fermentation is sped up by a small amount of sugar,
up to 3%. Warm water makes yeast grow, cold water has the reverse effect, and hot water kills
yeast. Temperatures ranging from 0 to 47 degrees Celsius are suitable for yeast growth. Yeasts
flourish in acidic settings with pH levels between 4.0 and 4.5. They can grow in lower pH
environments than other bacteria, but not in alkaline environments. Yeasts are common in nature
and can be found on grapes and other fruits. All yeast need food, moisture, and a controlled
temperature environment in order to ferment. Its byproducts from food consumption include
carbon dioxide, alcohol, and other organic molecules.
PRE-EXPERIMENT QUESTIONS
1. What is cellular respiration? (1 mark)
______________________________________________________________________________
______________________________________________________________________________
2. What is alcohol fermentation? (1 mark)
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
3. What are you observing in this lab? (1 mark)
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
PURPOSE OF THE EXPERIMENT
Compare yeast growth at various sugar concentrations and temperatures by measuring C02
released in each reaction.
1
Biology 20 Student _______________________ Mark________________________
MATERIALS
Erlenmeyer flasks (5)
Weighing scale
Weighing boats (2)
Spatulas (2; one for sugar, one for yeast)
Permanent marker
Graduated cylinders (2; one for water, one for apple juice)
Water (300ml lukewarm and 100ml room temperature)
Apple juice (100ml at room temperature)
Balloons (5)
Tape
String
Ruler
METHODS
1. Find the workstation with your group number.
2. Make sure that your bin/station contains all the materials mentioned above.
3. Take out flasks and label each with numbers 1-5. To do this, take a piece of tape, tape
it to the flask and use the permanent marker to label flask 1-5. (make sure to write
numbers on the tape NOT flask).
4. Take out graduated cylinders and use to MEASURE and ADD 100ml of water to
flasks 1-4, and 100ml of apple juice to flask 5. Make sure to use one graduated
cylinder for water and the other for juice (NO cross-contamination).
5. Take out the weighing boats and spatulas. Make sure to use one weighing boat and
one spatula for sugar and one for yeast (NO cross-contamination).
6. WEIGH and ADD sugar and the yeast to each flask as per directions below. Make
sure to add the YEAST LAST.
Each flask should contain:
Flask 1: 100ml of lukewarm water, 6g of yeast “CONTROL” (NO SUGAR)
Flask 2: 100ml of room temp water, 6g of sugar, 6g of yeast “CONTROL” (TEMP)
Flask 3: 100ml of lukewarm water, 6g of sugar, 6g of yeast
Flask 4: 100ml of lukewarm water, 12g of sugar, 6g of yeast
Flask 5: 100ml of apple juice (10g of sugar), 6g of yeast
7. Mix the solutions gently.
8. Place a balloon on each flask and ensure its sealed securely.
9. Start a timer and mix the content in each flask periodically.
10. At each timepoint (5, 10, 15 and 20 min) measure a balloon circumference using the
string. Place the string around the center of the balloon and record the length of the
circumference.
11. Place the string against the ruler to obtain a balloon circumference in centimeters
(cm). Record the circumference in the results table. Bigger the circumference more
CO2 in a balloon.
2
Biology 20 Student _______________________ Mark________________________
HYPOTHESIS: Which flask (sugar concentration) will release the most CO2? (1 mark)
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
RESULTS (4 marks)
0 min 5 min 10 min 15 min 20 min
Flask 1
Flask 2
Flask 3
Flask 4
Graph your results (1 mark)
3
Biology 20 Student _______________________ Mark________________________
DISCUSSION
Warmth and moisture are necessary for yeast to function, that is why lukewarm water was added.
Sugar was converted to carbon dioxide by yeast.
4. Was your hypothesis correct? Explain. (2 mark)
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
5. Which flask produced the least and the most CO2? Explain why. (2 marks)
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
6. Did the amount of CO2 change over time? If so in which flasks and why. (2 marks)
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
