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’sChemical 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’sOxygen:

• 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 bad-

smelling 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