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Food Safety
Note: These online slides are a
supplement to in-person lecture, but not
a replacement; some details are omitted
Types of Food Contaminants
Bacteria are single-celled organisms without nuclei (simple cells) and can
reproduce in food. They harm the body with dangerous toxins.
Viruses are not, by most definitions, alive. They reproduce inside of other
cells.
Parasites include worms (animals) and single-celled protozoa. They are
generally in a dormant/encysted/egg form in food.
Fungi include molds, filamentous (string-shaped) organisms that reproduce
in food. Can tolerate more dryness than bacteria, but grow more slowly.
Toxins are poisons, such as mercury compounds and bacterial toxins.
Prions are proteins that can be “copied” by your cells, and cause Mad Cow
Disease and CJD.
Bacteria: Pathogenic E. coli
Escherichia coli is a bacterium
normally found in the intestines of
healthy people. However, there are
disease-causing strains of E. coli
that can be harmful, leading to
diarrhea, or even deadly.
Contaminated meat is a potential
source of pathogenic E. coli, as is
fecal contamination of produce. To
reduce the risk, cook meat,
especially hamburgers, thoroughly,
and wash your produce.
Bacteria: Salmonella
Salmonella is one of many members
of the Enterobacteria that produces
dangerous toxins that can cause
severe intestinal distress. Often found
on the surface of eggs. Unpasteurized
raw eggs are not safe.
Refrigerating food can slow the
growth of bacteria like Salmonella and
E. coli, but does not kill them. Keeping
food warm for a long time, rather than
cold or hot) can be especially
dangerous.
Temperature and Bacterial
Growth
Refrigerator
spoilage bacteria
include Listeria
monocytogenes,
a facultative
anaerobe that is
arguably deadlier
than botulism.
“Danger zone”
bacteria include
Campylobacter
jejuni.
For smaller portions in the home setting, using shallow
containers to refrigerate hot foods will reduce the “warm zone”
at the center of the food and slow bacterial growth.
Bacteria: Clostridium botulinum
C. botulinum causes botulism, a paralytic disease that can halt breathing. It produces
tiny quantities of the deadliest poison on earth.
The spore form of this bacterium is very resistant to heat and cold, and cannot be killed
easily. It’s common in soil, so home-canned or home-pickled foods are at risk. Since the
bacterium uses anaerobic respiration, it does not need air and can survive in a can.
Cooking does not destroy the bacterial toxin; affected cans must be thrown out.
Swallowing the bacterial spores in honey is usually harmless to adults, as our own gut
flora outcompete them. However, infants should not be given honey.
Viruses: Noroviruses (Norwalk)
Although Norwalk virus is most infamous as a “cruise ship” virus that can quickly give
gastrointestinal distress to a ship full of people eating from a shared cafeteria, it is not
limited to such close quarters.
Norwalk virus is very contagious by the fecal-oral route. Very slight traces of the virus
under the fingernails can contaminate food and give vomiting and diarrhea to many
people!
This is one of many reasons why food preparers must wash their hands, and also wear
gloves. As few as 5 viral particles may be enough to give someone the disease.
Keeping cold foods cold somewhat against bacterial
growth, but is not effective against viruses like
Norwalk, as they do not “grow” in the food anyway;
they sit in the food until it is eaten.
However, sufficient heating can make Norwalk virus
harmless.
Fungi: Bread molds
There are many, many species of mold that can affect bread, grains, and cheese.
Sometimes we cultivate these molds intentionally (edible molds like Penicillium
roqueforti). However, others are toxic and even carcinogenic.
Molds on nuts can be quite dangerous, as some produce aflatoxin, a potent carcinogen
(cancer-causing poison) that also damages the liver. Never eat moldy peanuts. Similarly,
moldy corn killed over 100 people in an outbreak in Kenya.
More common bread molds are less harmful,
but can still worsen allergies and lead to
respiratory problems. It is not safe to cut the
“moldy patch” off a piece of bread and eat
the rest, as the “fuzz” you see on the surface
is just the reproductive spores. The rest of
the fungus is invisible and underneath the
surface of the bread!
Parasites: Beef Tapeworm (Taenia)
Tapeworms are animal parasites in the flatworm
family. They attach to the gut lining with a hooked
head, or scolex, and absorb nutrients directly through
their bodies.
Unlike the worms that cause trichinosis, beef
tapeworms are relatively harmless, though very high
levels of infection can reduce nutrient intake and
appetite. In addition, it’s possible for tapeworms to
cause an obstruction of the intestines, which can be
dangerous and require surgery.
Both thorough cooking and thorough freezing kill beef
tapeworms. Not as dangerous as parasites of pork.
Not to be confused with parasitic Ascaris roundworms
that can be found on improperly washed produce, and
also, to some extent, paper money. Wash hands after
handling money.
Parasites: Trichinella
The roundworm Trichinella forms cysts in pork and
can survive unless the meat is thoroughly cooked.
Unlike the beef tapeworm, this parasite can migrate
out of the human host’s intestines, causing trichinosis.
Often, no symptoms appear, or there is some diarrhea
for a week. In other cases, migration of the larvae
through other tissues, such as muscle, leads to fever.
In rare cases, the larvae can reach the central nervous
system and even cause brain damage.
Prevention: cook pork until it is light to the bone.
However, recent guidelines have suggested that lower
temperatures are sufficient, especially after freezing,
and trichinosis is currently rare in the U.S. due to
improved sanitation.
Parasites: Cyclospora
Unwashed produce and contaminated water can contain single-celled parasites
such as Cyclospora, cause of the gastrointestinal infection cyclosporiasis.
Diarrhea from cyclosporiasis sufferers then contaminates water used to wash
produce, spreading the infection further. It is important to wash produce to remove
bacterial, parasitic, and pesticide contamination, even if the produce is supposedly
pre-washed, such as store-bought raspberries and bagged salads.
Toxins: Mercury Compounds
Toxic metals can bioaccumulate in animals.
They are absorbed by species lower on the
food chain, which are then eaten. Their
consumers are then eaten in turn by
predators. In this way, poisons such as
mercury compounds can rise up the food
chain and be biomagnified, and contaminate
the fatty tissues of predators.
Pure mercury is not as extremely toxic as
some fear, but more dangerous mercury
compounds have an organic (carbon)
component that allows them to affect the
nervous system more readily. Example:
Methylmercury.
In moderation, fish remains a healthy food
as long as you avoid apex predators such as
sharks. Pregnant women and young children
should limit intake. In addition, eating these
species can be environmentally harmful!