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Homework 4 – Guidelines
Microbes & Food – Yogurt
45 points
Submission to Canvas is due Sunday, March 3 at 11:59PM.
OVERVIEW:
The goal of this assignment is two folds: 1) to conduct a hypothetical experiment in the Microbes
& Food project and 2) to discuss the outcome of the heirloom yogurt that you have made in
class.
For the first part of the assignment you will focus on the identification and characterization of
lactic acid bacteria isolated from yogurt. You will explain the state of the art on yogurt making
and will add your predictions and results of the hypothetical experiment in the form of a
worksheet.
For the second part of the assignment, you will discuss your team attempt at making heirloom
yogurt in the lab and will write a paragraph on the overall experiment and outcome.
Submit the assignment as a single Word file. Homework should be typed.
Narrative
Please use headings provided in italics below for each section; you may include sub-headings as
needed to organize your work. Write all of the section of your homework in paragraphs, do
not use bullet points; they are provided in the instructions just for clarity. The sections
should appear in the order presented here. Points will be subtracted otherwise.
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DESCRIPTION:
Each assignment should be/have:
12-pt Times New Roman font Double-spaced
1-inch margins
Page numbers
Divide the assignments in 4 parts: Cover page, state of the art, results & discussion, bibliography.
Cover page – 1 point:
Title – Should be interesting but concise! Author (UID), IA
State of the art – 10 points
– Introduce lactic acid bacteria and dairy fermentation and describe how yogurt is made and its
significance as a way to preserve milk
– Explain the difference between commercial and heirloom yogurts
– Introduce staining and physiological tests as a tool for the classification of microbes
– Include proper citations
Results & Discussion- part 1 – 18 points
Part 1: This part 1 is based on predictions: everything that you will write is based on
background information/literature, and it is not based on part 2. It is not based on the
results that you have seen in the lab regarding the Control and Heirloom yogurt
Hypothetical results will be presented in the form of a worksheet. This worksheet will help you
understand the industrial process of making yogurt and how yogurt products may differ based on
the microbes utilized. I suggest to watch the lecture where I discussed the process of yogurt
making earlier in the course, as well as microbial metabolism & physiological tests.
In this worksheet, you will hypothesize how a Control yogurt, made by using only
Streptococcus thermophilus and Lactobacillus bulgaricus, will differ from a Test yogurt, which
was inoculated with known lactic acid bacteria
Important: You do not need to use the tables provided in the worksheet below as they are, especially if it
is dif cult to copy and paste it into your homework. You can create your own tables as long as they
contain the same information provided in these guidelines. Make sure that each table ts within one page
and not within multiple pages
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.
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Assume that you used the following protocol for this experiment
– Milk was scalded at 85
– When temperature reaches 46C, you inoculated the milk with the appropriate yogurt starter
(see below) and incubated at 46C for 3 hour
a) Control yogurt starter contains Streptococcus thermophilus and Lactobacillus
bulgaricus
b) Test yogurt starter contains Streptococcus thermophilus, Lactobacillus bulgaricus,
Leuconostoc cremoris, Lactococcus lactis, and Lactobacillus brevis
– Incubate milk bottles at 30C* overnight
– Store yogurt at 4C (optional
*Note: we discussed 25C instead of 30C during lecture. Because of our lab incubators, we have
used 25C in Tata Hall. However, the optimal temperature is actually 30C.
.
:
s
)
C
.
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1. [6 points] a) Hypothesize the expected behavior of the Test yogurt and predict how these
changes will affect the pH, aroma/taste, and degree of curdling of the Test yogurt when
compared to the Control yogurt
Table 1. Change in pH in the Test yogurt when compared to Control yogurt. Circle or mark your
prediction for columns B & C. Do not mark anything for column A
Column A
Column B
Column C
Temperature
Compared to the 3-hour pH change in
Control, pH in the Test bottle would be
Compared to the overnight pH
change in Control, pH in the Test
bottle will be
46°C / 30°C
Lower than Equal to
Control
Control
Lower than
Control
Higher than
Control
Equal to
Control
Higher than
Control
Table 2. Curdling of Test Yogurt when compared to Control Yogurt. Circle or mark your prediction for
columns B & C. Do not mark anything for column A.
Column A
Column B
Column C
Compared to the 3-hour curdling change
Compared to the overnight curdling
Temperature in Control, curdling in the Test bottle
change in Control, curdling in the Test
will be
bottle will be
Lower than Equal to
Control
Control
Higher than
Control
Lower than
Control
Equal to
Control
Higher than
Control
.
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.
46°C / 30°C
Table 3. Aroma/taste of Test Yogurt when compared to Control Yogurt. Circle or mark your prediction
for columns B & C. Do not mark anything for column A.
Column A
Column B
Column C
Compared to the 3-hour aroma/taste
Temperature change in Control, aroma/taste in the
Test bottle will be
46°C / 30°C
Sweeter
than
Control
Equal to
Control
Sour than
Control
Compared to the overnight aroma/
taste change in Control, aroma/taste in
the Test bottle will be
Sweeter than Equal to
Control
Control
Sour than
Control
b) The rationale for the pH, curdling, and aroma/taste results that I predict at 3 hours and
overnight for the Test yogurt is:
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2. [6 points] a) Predict the results for each of the staining shown in the table below. You can use
the material discussed in lecture or outside (reliable) sources
Table 4. Microorganisms in the Test yogurt.
Name of organism
Shape of
organism
Temperature
Gram
preference (choose
characteristics between 30 & 46C)
Streptococcus thermophilus
Lactobacillus bulgaricus
Lactobacillus brevis
Lactococcus lactis
Leuconostoc cremoris
Escherichia coli (control)
b) Based on their temperature preference 30C, you can classify these microorganisms
_________________________________________________________________________ (list
them) as ____________________
c) Based on their temperature preference 46C, you can classify these microorganisms
_________________________________________________________________________ (list
them) as ____________________
3. [6 points] a) Predict the results for each of the physiological tests shown in the table below
(Yes/No). You can use the material discussed in lecture or outside (reliable) sources
Name of organism
Growth
on TSA
Growth on
MRS
Growth on
Streptococcus
agar
Growth on
MacConkey
Streptococcus thermophilus
Lactobacillus bulgaricus
Lactobacillus brevis
Lactococcus lactis
Leuconostoc cremoris
Escherichia coli (control)
.
.
_
_
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Note: Lactobacillus bulgaricus may be listed as as Lactobacillus delbruekii subsp. bulgaricus in
outside sources
Leuconostoc cremoris may be listed as Leuconostoc mesenteroides subsp. cremoris.
——————————————————————————————————————
Results & Discussion- part 2 – 15 points
Part 2: A is based on discussion of the results that YOUR TEAM have obtained after
performing the experiment (making heirloom yogurt in lab), thus, results may/may not
contradict predictions in part 1. In B, you will then compared those results with your
predictions.
A. Write a brief overview of how you made heirloom yogurt in lab, including the conditions and
type of heirloom starter use
B. Write a summary to discuss the outcome of the experiment. Describe in a well-organized,
well-formatted table he following characteristics for each stage at which the characteristics
(pH, texture/consistency, aroma) were observe
C. Discuss how the heirloom yogurt that you made in the lab compares to the predictions of part
1. If predictions and results do not match, briefly speculate on possible reasons.
——————————————————————————————————————-
Bibliography – 1 point
No page limit;
• Include full reference for all citations in narrative
• Add bibliography at the end of your writing assignment
You do not need to cite Dr. Pirino’s lectures nor the lab manual.
How to cite references within the homework
Reference articles will be included within the text as shown in the Raney & Travisano’s paper.
The articles will be cited at the end of the sentence where the article is mentioned, and will have
1) the last name of the first author followed by et al., and the year of publication, (Pirino et al,
2019) if there are more than 2 authors; or 2) the last name of the 2 authors and the year of
publication, if there are only 2 authors (Pirino and Gustafson-Brown, 2019).
If there are more than one article that need to be cited for one sentence, they will be listed in
chronological order of publication year (from the least recent to the most recent): Pirino et al.,
2015; Pirino et al., 2019.
Do not use web sites like Wikipedia or Microbewiki as references. These are sources that may
not have adequate verification and are not appropriate references. This is not to say that they are
—
d
d
.
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useless – quite often they give a good introduction to the topic you are researching and also
provide you with primary sources or references, listed at the end of the Wiki article. Follow up
on those primary sources of information.
A secondary source that is accepted is the Bergey’s manual.
Textbooks: Authors (publication date). Title. Publisher.
Bergey DH, Buchanan RE, Gibbons NE, American Society for Microbiology. (2004). Bergey’s
manual of determinative bacteriology. Baltimore: Williams & Wilkins.
Nomenclature – For all bacteria, use the binary name consisting of the genus name (or
abbreviation) and species name (e.g. Escherichia coli or E. coli). Names of the genus and
species name for all taxa are printed in italics; Genus is capitalized. Strain designations and
numbers, such as LMG18311, are not italicized. The binary name should be written out in full
the first time it appears in your paper. Thereafter, the abbreviated version should be used. Names
of categories above the genus level may be used alone. Points will be subtracted, if an incorrect
nomenclature is used.
Writing Clarity
Grammar, spelling, and style cannot interfere with understanding. Points will be subtracted
otherwise.
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