BIOL 255 - Food Microbiology

What are the possible ways microorganisms can interact with our food source?

Negative: deterioration/spoilage, foodborne diseases

Positive: food production (yogurt, bread, wine), food processing, food preservation, probiotics

What is food spoilage?

The process of change in the physical and chemical properties of food, making it unfit for consumption

What is microbial spoilage?

Caused by microorganisms producing substances that change color, texture, and odor until food is unfit for consumption

Physical causes of spoilage

Physical harm to outer layers during harvesting/processing/distribution

increases contamination risk

Chemical causes of spoilage

Changes in color and flavor

begins after harvest

fat breakdown changes odor

enzymatic spoilage

What factors control microbial growth in food?

Intrinsic factors (pH, moisture, nutrients, structure)

Extrinsic factors (temperature, humidity, oxygen)

Intrinsic factors

pH, water content (aw), nutrient content, biological structure

How does pH influence microbial growth?

Increased acidity preserves food; pathogens grow poorly below pH 4.6

How does water content influence microbial growth?

Water activity (aw)

fresh foods have higher aw and support microbial growth

How does nutrient content influence microbial growth?

Microbes grow best in nutrient-rich foods

meats high in protein/lipids

plants high in carbohydrates

How does biological structure influence microbial growth?

Skins, shells, and outer coverings protect against microbial entry

maturity affects protection

pathogens survive but don’t replicate on coverings

Extrinsic factors

Temperature, relative humidity, atmosphere

How does temperature limit microbial growth?

Low temps slow enzyme activity and membrane fluidity

high temps denature cell components

How does relative humidity limit microbial growth?

Dry conditions limit growth

moisture in closed environments promotes spoilage bacteria and molds

How does atmosphere affect microbial growth?

Gases can be toxic or alter ecology

oxygen needs determine growth

aerobes don’t grow anaerobically

Methods of microbial control in food

Removal (filtration), low temps, high temps, pasteurization, water availability reduction, chemical preservation, bacteriophages, radiation

Low temperature control methods

Refrigeration reduces/slows microbial growth

contamination still possible below –10°C

High temperature control methods

Canning kills spoilage microbes but not all; retort processing at 115°C for 25–100 min; spoilage occurs if pre-spoiled, underprocessed, or contaminated during cooling

How water availability influences microbial growth

Dehydration/freeze drying slows or stops biochemical processes

How chemicals are used to slow microbial growth

Use GRAS agents: organic acids, sulfite, ethylene oxide gas, ethyl formate, sodium nitrite

effectiveness depends on pH

What foods are most likely to be irradiated? Why?

Meat, seafood, fruits, vegetables

gamma radiation extends shelf life, sterilizes

does not make food radioactive

kills microbes via peroxide formation

Two primary types of foodborne disease

Foodborne infections: ingestion of pathogen

Food intoxications: ingestion of toxins

How are foodborne diseases transmitted?

GI tract exposure, contaminated utensils, cross contamination, poor hygiene

What are probiotics?

Live microorganisms that provide health benefits

Reduce inflammation, control diarrhea, anticancer effects, modulate Crohn’s, treat enteric disease

Common probiotic organisms and benefits

Lactobacillus acidophilus & Bifidobacterium: improve lactose intolerance, intestinal health, lower cholesterol, increase anti-tumor activity

What are prebiotics?

Fibers that stimulate growth/activity of beneficial bacteria in the digestive system