Science of Food - Lecture 13 & 14

Major food-associated groups include ______, _______l _______

bacteria, yeasts, and molds

Viruses and parasites can also cause _______

foodborne illness

Beneficial Microorganisms

fermentation, flavor development, preservation,

probiotics

Pathogenic Microorganisms

can cause foodborne illness

Temperature and Bacterial Growth

Refrigeration slows growth of many organisms

Freezing generally stops growth but does not reliably kill all microorganisms

Acidic foods

generally provide less favorable conditions for many bacteria

Water activity

Drying, salting, and adding sugar can reduce available water

Examples: dried fruit, jerky, jams

Beneficial Bacteria: Lactic Acid Bacteria

Convert sugars into lactic acid

Fermentation

produces acids and characteristic flavors

Why ferment foods?

To preserve vegetables and fruits

To develop characteristic sensory properties

(i.e., flavor, aroma, and texture)

To destroy naturally occurring toxins

and undesirable components in raw materials

To improve digestibility, especially of some

legumes

To enrich products with desired microbial

metabolites

To create new products for new markets

To increase dietary value

yeasts tolerate

acidic environments

Baker's Yeast or Saccharomyces cerevisiae

• Ferments sugars and produces carbon dioxide

• Carbon dioxide becomes trapped in dough

• Gas production helps bread rise

Yeast in Beer and Wine

Yeasts convert sugars into ethanol and carbon dioxide

Sourdough is made using a community of yeasts and lactic acid bacteria

(LAB)

Yeasts ferment

sugars in the flour and produce carbon dioxide (CO₂).

Lactic acid bacteria

ferment sugars and produce lactic acid and other organic acids.

Yeast Spoilage

Refrigeration can slow growth but may not eliminate it

Yeasts in Fruit Juices

Fruit juices are acidic and contain fermentable sugars

Certain yeasts can grow despite low pH

Molds in Foods

Many produce visible colonies on food

Mold in Cheese

Controlled cultures are used rather than uncontrolled mold growth

Mold on Fruits and Vegetables

Proper storage and handling slow spoilage

Mycotoxins

Some molds can produce toxic secondary metabolites called mycotoxins

Aflatoxin: A Food Example

• Aflatoxin is a mycotoxin

peanuts, corn, and tree nuts

Kimchi

Lactic acid bacteria drive fermentation

Sauerkraut

actic acid bacteria

Pickles

Can be produced by fermentation or by acidification with vinegar

Miso

Uses molds, yeasts, and bacteria during fermentation

What Is Food Spoilage?

Spoilage does not automatically mean the food is dangerous

• Pathogens may be present without obvious spoilage

Milk Spoilage

Pasteurization reduces many microorganisms but does not prevent all post-processing contamination

Pasteurization reduces many microorganisms but does not prevent all post-

processing contamination

A strong fishy odor is generally a sign of loss of freshness and spoilage, but odor alone cannot reliably determine whether fish is safe to eat, because some pathogens may be present without producing an obvious odor.

Food infection

living microorganisms are consumed and cause infection

Food intoxication

toxin is already present in the food

Toxin-mediated infection

organism is consumed and produces toxin in the

intestine

Salmonella

poultry, eggs, meat, produce

Campylobacter

poultry, raw poultry juices can spread contamination

Pathogenic E. coli

undercooked ground beef, leafy greens, and other

contaminated foods

E. coli and Ground Beef

Grinding distributes surface contamination

throughout the meat, Ground beef therefore requires thorough cooking

Listeria monocytogenes

Listeria monocytogenes, important concern in ready-to-eat refrigerated foods

Staphylococcus aureus

transferred to foods by food handlers

Clostridium botulinum

Produces botulinum neurotoxin under suitable conditions

Bacillus cereus

Spore-forming bacterium

Vibrio

Associated especially with raw or undercooked seafood

Norovirus

Virus that causes acute gastroenteritis

Hepatitis A Virus

Viruses can survive in foods; but do not grow in foods

The Danger Zone

Temperature Control Foods should be held: Below 40°F OR Above 140°F

FATTOM

Conditions That Promote Bacterial Growth

F

Food - Nutrients needed for growth

A

Acidity - Most bacteria prefer low-acid foods

T

Time - More time allows more microbial growth

T

Temperature - Many pathogens grow rapidly at moderate

temperatures

O

Oxygen - Some microbes require oxygen; others do not

M

Moisture - Microorganisms need available water