Reading Guide
Food Microbiology, part 1 Ch 31 sections 1-5 (31.1-31.5)
This material is covered in chapter 31 and I will highlight the sections in the chapter that I want
you to focus your reading and review.
As we talk about the topic of food microbiology, keep in mind the following ideas: bacteria are
everywhere (even on our food), bacteria will grow when given the opportunity, and very specific
organisms cause food poisoning.
The first section (31.1) highlights the factors that influence microbial growth in foods. Think
about, “what do organisms need to grow?” (material from chapter 4) and you are on the way to making
a list. To begin with, bacterial growth is influenced by temperature. Microbes that grow best on
humans or other mammals are mesophiles and can still grow at room temperature or even in the
refrigerator. They will have a longer generation time and not grow as fast at these cooler temperatures.
Remember that some food, like fish, exist at much cooler temperatures. When these items are brought
home they will not keep as long in the refrigerator as the microbes on the fish continue to grow. What
can you do to prevent microbes from growing on food?
Water availability is also another feature that influences growth of bacteria in foods. This refers
to the amount of water that is available or accessible to the microorganism. Some foods have more
water than others and it is these foods that can support the growth of a wide range of bacteria. Look at
the lecture notes and determine which microbe could grow on ham. If you were to make a ham
sandwich, should I worry about the presence of bacteria on it?
The pH of food also influences the growth of bacteria in foods. Remember that while bacteria
prefer a pH of 6.5-7.5 to grow in, they can still grow or even tolerate an environment with a lower pH.
For example, E. coli can grow in food that has a pH of 4.0. Remember the outbreak of E. coli infections
that was caused by unproperly processed apple juice? The pH of the apple juice was 4.0, a level that
was thought to be too acidic for E. coli to grow. Unfortunately this pH is the minimum pH that E. coli can
tolerate and it was able to survive and grow very slowly. Some organisms are only able to grow in a very
acidic environment and are known as acidophiles. Others are only able to grow in an alkaline
environment and are called alkalophiles.
The atmosphere, whether or not oxygen is present can also influence bacterial growth in foods.
If an organism is an obligate anaerobe, it will only be able to grow when oxygen is removed. Can you
think if a situation in which oxygen is removed from a food item? Some organisms are obligate aerobes
and will only grow when oxygen is available to them.
The nutrients available in a food will influence the type of microbe that can grow. For example,
lettuce has less nutrients than meat and will support the growth of fewer organisms. There are some
foods that have naturally occurring biological barriers such as skins and shells. Bacteria will only be able
to grow in these products if the barrier is broken or damaged. Some foods have naturally occurring
antimicrobial chemicals that inhibit microbial growth. Eggs have lysozyme and cranberries have benzoic
acid.
Section 31.2 covers microbes used in food and beverage production. I would recommend that
you review the tables (Table 31.1, 31.2, 31.3) to see the various types of foods and beverages that can
be made with lactic acid bacteria, yeast, and molds. I just want you to be familiar with the different
types of foods and beverages that are made by microorganisms.
Section 31.3 covers the topic of food spoilage. Bacteria that cause food spoilage are usually not
the same kinds of bacteria that can cause food poisoning. Organisms that cause food spoilage grow in
food that is stored at temperatures below body temperature. If there are pathogens present on a food
item, the spoilage organisms tend to grow better than the pathogens.
The type of organism that causes the food spoilage is also a reflection of the type of food. Fish
are likely to have organisms present that can grow best in cooler temperatures (since the water in which
they exist is cooler that 37oC) and will spoil faster than meat in the refrigerator. Grains are often
contaminated with molds and some of these molds can produce toxins that are toxic to humans. A good
example of this is the ingestion of rye flour contaminated with the organism Claviceps purpurea. This
mold produced the toxin ergot and anyone who ingested this flour with the toxin had symptoms of
hallucinations, hearing voices, and tingling in the extremities.
Section 31.4 is a good introduction into the different types of foodborne illness. There are two
kinds of foodborne illness, foodborne infection and foodborne intoxication. Foodborne intoxication
results from ingestion of food that has had organisms grow in it and generate a toxin (exotoxin) that is
liberated into the food. The toxin can have an effect on the host, for example, with Clostridium
botulinum this organism produces a neurotoxin that causes muscle paralysis. Another organism that can
cause foodborne intoxication is Staphylococcus aureus. It can grow in food and produce a toxin that
causes vomiting and diarrhea. Foodborne infection results from eating food that is contaminated with
organisms, the organisms grow and multiply in the intestines, as they grow they may produce toxins,
which generate symptoms of abdominal cramps, vomiting, and diarrhea. The key difference in the two
methods of food poisoning is that in one case you ingest bacterial toxins which give you symptoms, in
the other case you ingest the bacterial organisms. Which type of food poisoning will have symptoms
appear more quickly? Why?
Review section 31.5 on food preservation. The methods listed should be familiar to you and you
should be able to list a few examples of these methods on an exam.
That’s all for chapter 31. The second part of this material (Food Microbiology, part 2) will focus
on organisms and cover the content in section 24.4 of the textbook. Use the lecture notes to guide you
as to which organisms I plan on covering in this section.