Download MICROBIOLOGY

MICROBIOLOGY - ALCAMO
CHAPTER 24: MICROBIOLOGY
OF FOODS
Case of the Patty Melt
• The 3rd largest outbreak of botulism in our country
occurred in October, 1983, at the Skewer Inn,
Peoria Illinois
• Between October 14th and 16th, 36 people began to
feel the paralyzing signs of botulism
• Blurred vision, difficulty swallowing and
chewing, and labored breathing
• All had to be hospitalized, and all but one person
recovered
Case of the Patty Melt
• Investigators from the
CDC came to investigate
• They identified the patty melt as the
common food eaten by the sick people
• But, they had to determine what part of the
sandwich was the culprit
• They were able to isolate Clostridium
botulinum spores from fresh onions
Case of the Patty Melt
• Once sautéed, the onions were left
uncovered on a warm stove for hours
• The onions had not been reheated before
serving
• Spores had germinated in
the anaerobic mounds of
warm onions and deposited
their toxins
Why Did It Happen??
• Most foods (even cooked foods) provide
excellent conditions for growth of MO’s
• There is plenty of organic matter, water
content is high, and the pH is usually
neutral
• We will be talking about types of organisms
that contaminate food, the consequences of
them, and how to prevent them
Food Spoilage
• Has been a problem since humans started
producing more food than could be eaten in
one meal
• Marco Polo explored new trade routes to get
spices to improve the smell and taste of
spoiled food
• Refrigeration and canning were not yet
invented
Food Spoilage
• Food is considered spoiled if it
is altered from its expected form
• Usually it will have an unpleasant
appearance, aroma, and taste
• Sometimes you can’t tell if there aren’t
enough MO’s or they deposit a toxin
• Consumption of toxins or MO’s can cause a
variety of food poisonings and infections
ENTRY OF MO’S TO FOOD
• Air – MO’s fall onto fruit and vegetables
and penetrate through a break in the skin or
rind
• Soil – crops carry soilborne bacteria into
processing plants
• Rodents and arthropods – transport MO’s
on their feet and body parts as they are
around food
ENTRY OF MO’S TO FOOD
• Human handling – processing and storage
– Careless butcher – can contaminate meat with
bacteria from an animal’s intestines
– Raw fruits and vegetables – salad bars
Factors Affecting Growth of MO’s
Water:
• Food must be moist for
MO’s to grow (water
content of 18-20%)
• MO’s do not grow in foods
like dried beans, rice, and flour
because of the low water content
Factors Affecting Growth of
MO’s
pH:
• Most foods are neutral
to slightly acidic
• Many bacteria grow well
under these conditions
• If food pH is below 5.0 molds like to grow
• Citrus foods are rarely spoiled by bacteria –
usually mold
Factors Affecting Growth of
MO’s
Physical Structure:
• It may be physically easier for the MO to
penetrate into the food
• MO’s have a harder time penetrating steak
than hamburger meat
• Mo’s can exist within the loosely packed
meat
Factors Affecting Growth of MO’s
Chemical Composition of Food:
• MO’s that metabolize
carbohydrates – found on
fruits
• MO’s that metabolize
protein – found on meats
• Presence of certain vitamins might
encourage MO’s to grow and absence of
vitamins may discourage growth
Factors Affecting Growth of MO’s
Oxygen:
• Vacuum-sealed cans (all oxygen removed)
do not support the growth of aerobic
bacteria
• Vegetables and bakery products do not
support the growth of anaerobes
Factors Affecting Growth of MO’s
Temperature:
• The refrigerator is too cold for the
growth of human pathogens
• A hot warehouse storeroom would
encourage the growth of these
Chemistry of Spoilage
• Spoilage often due to naturally occurring
chemistry of contaminating MO
– Yeasts – live in apple juice and convert sugar to
ethyl alcohol
– Certain bacteria – break down protein into badsmelling end products (rotten egg)
– Carbohydrates can break down into
• acid – sour food
• gas – causes sealed cans to swell
Chemistry of Spoilage
– In General:
• Bacteria - spoil fresh foods
• Fungi - spoil breads, fruits, and dried
foods
Food Industry Divides Food By
Its Properties
Highly Perishable – Spoil rapidly
Examples: Poultry, eggs, meats, most vegetables and
fruits, dairy products
Highly Perishable
• Meats and Fish – muscle tissue of animals
usually sterile
• Spoilage usually occurs during handling,
processing, packaging and storage
• Processed meats – even greater hazard
because they are handled so often
• Organ meats – less compact tissue and spoil
more quickly
Highly Perishable
• Poultry and Eggs – contamination is usually
from MO’s that have infected the bird
• Salmonella causes disease in turkeys and
chickens – passed on to consumers via
poultry and egg products
• Eggs are normally sterile, but shell can get
penetrated by bacteria after a few hours
Food Industry Divides Food By
Its Properties
Semiperishable – Spoil less quickly
Examples: nutmeats, potatoes, and some
apples
Food Industry Divides Food By
Its Properties
Nonperishable – Not subject to rapid spoiling
Cereals, rice, dried beans, macaroni products,
flour and sugar
Breads and Bakery Products
• Flour, eggs and sugar are the
sources of spoilage
• Most contaminants are killed
during baking but some spores
can survive
• Cream fillings and toppings are also
excellent for bacterial growth
• Most bakery products should be refrigerated
during warm weather
Grains
• 1. The mold Aspergillus flavus
produces aflatoxins in grains
– These toxins are associated with liver and colon
cancer in humans
• 2. The fungus Claviceps purpurea produces
toxins in grains (especially rye)
– These toxins may induce convulsions and
hallucinations
– The drug LSD is derived from the toxin
Food Preservation
• Centuries ago humans learned that they could
preserve food
– Drying, salting, smoking and fermenting
• In the mid 1700’s Lazarro Spallanzani showed
that if beef broth was heated it would remain
unspoiled
• Several generations later Pasteur proved that
MO’s contribute to spoilage
• The purpose remains the same – to reduce the MO
population and maintain it at a low level until
food can be eaten
Heat
• Kills MO’s by denaturing their proteins
• Useful in the process of canning:
– Machines wash and sort the food
– Subject food to steam heat for 3-5 minutes
(blanching)
– Food is peeled and cored and placed in cans
– Containers steam sterilized (autoclave)
• Process designed to eliminate the most
resistant bacterial spores (Bacillus and
Clostridium)
• If canning is not done properly, facultative
or anaerobic bacteria can grow
• They produce gas and cause the ends of the
cans to bulge
Low Temperatures
• Reduce the growth rate and reproduction of
MO’s
• Organisms are not killed, but their numbers
are kept low and spoilage is limited
• Modern refrigerator operates at about 5 C
– preserve food without destroying its appearance
and taste
– But, psychrotrophic MO’s survive (rotten eggs,
sour milk)
Low Temperatures
• Freezer keeps food at – 5 C and ice crystals
form that kill a large amount of MO’s
• But, some MO’s survive and when food
thaws, bacteria multiply quickly
– Staphylococci
– Streptococci
• Rapid thawing and cooking is best
• Food should not be refrozen because the
bacteria deposit toxins and could cause food
poisoning
TEMPERATURE
Drying Food
• Dried foods have no water
and can not support
microbial life
• Used to use the sun to dry
foods – now there are machines
• Spore-forming and capsule-producing
bacteria resist drying and can survive to
spoil the product once reconstituted
Osmotic Pressure
• Highly salted or sugared foods, the food
dehydrates as well as the MO’s
• Mo’s dehydrate, shrink and die
• Salted foods
– Ham, cod, bacon, beef, sauerkraut
• Sugared foods
– Jams, jellies, fruits, maple syrup
Chemical Preservatives
• Must inhibit growth of MO’s and also be
easily broken down and eliminated from the
body
• Smoking foods both dehydrates them and
adds chemicals that are the by-products of
smoke
• Major group of chemical preservatives are
the organic acids
– Sorbic acid, Benzoic acid, Propionic acid
Chemical Preservatives
• Some foods have their own natural
preservatives
• There are antimicrobial substances in:
– Garlic – allisin is substance that is antimicrobial
– Lysozyme in egg whites
– Benzoic acid in cranberries
Radiation
• Gamma rays – high frequency forms of
electromagnetic energy emitted by the
radioactive isotope – cobalt 60
– The radiations are not radioactive and can’t
cause food to become radioactive
• They kill MO’s by reacting with and
destroying microbial DNA
– Can cause the breakdown of chemical bonds
and formation of new ones
– Maybe form new and toxic chemicals??
Foodborne Disease
• Food infection – Person affected by
an actual MO
– Bacterial – typhoid fever, salmonellosis,
cholera
– Protozoal – amebiasis, balantidiasis, giardiasis
– Viral – hepatitis A
• Food poisoning or intoxication – Person
affected by the toxin an MO produces
– Botulism, staphylococcal food poisoning,
clostridial food poisoning
Preventing Foodborne Disease
• In the U.S. 2-10 million people are affected
by foodborne illness annually
• Can avoid them by taking basic precautions
–
–
–
–
–
–
Refrigerate perishable foods
Cover skin boils while working with food
Wash hands thoroughly
Thaw meat in refrigerator
Clean cutting boards thoroughly
Home canning should be done
carefully
WASH CUTTING BOARD
CAREFULLY
WASH HANDS
Preventing Foodborne Disease
• Leftovers are implicated
the most so refrigerate them
promptly and only keep for
a few days
• Picnics/barbecues:
– Don’t take dairy foods (cream pies, salads)
– One dish to the grill – a different one from the
grill
FOODS FROM MO’S
• Milk products:
– Lactose is the major carbohydrate in
milk
– Bacteria ferment lactose to lactic
acid and this gives foods a sour taste
(yogurt and cheese)
FOODS FROM MO’S
• Fermented foods:
– 1. less vulnerable to extensive spoilage
– 2. less likely to be vectors of foodborne illness
– 3. sour taste has been accepted and sometimes
considered a delicacy
– Sauerkraut – Leuconostoc and Lactobacillus
– Pickles – Enterobacter, Leuconostoc and
Lactobacillus
– Vinegar – Acetobacter aceti
FOODS FROM MO’S
• Other Foods:
–
–
–
–
Soy sauce
Fermented sausage
Coffee
Cocoa