Food Microbiology and Chemistry

Summarize the chemical and microbiological factors that affect food safety.

Objectives

هدف

Food Spoilage

Six Factors That Affect Growth

Potentially Hazardous Foods

Food Additives

Overview

مرور

Food Spoilage

Microorganisms are the most important cause of food spoilage.

Spoilage microbes:

Lactic Acid bacteria - common cause

Pseudomonas species of bacteria – commonly causes meat to spoil

Fungi – cause moldy bread

Food Spoilage (cont.)

Chemical Food Spoilage

Chemical reactions can also reduce food quality

Exposure to air, light

Breakdown of fat in foods form off-flavors and off-orders

Reactions caused by enzymes that are naturally present in the food.

Six Factors that Affect Growth

There are six main factors that affect microbial

growth in food:

F- Food

A- Acid

T- Temperature

T- Time

O- Oxygen

M- Moisture

F (“Food” for the microbes to eat)

Nutrients in food affect microbial growth:

Sources of energy (e.g., sugars, proteins)

Sources of nitrogen (e.g., proteins)

Vitamins, Minerals

In order for bacteria to grow, the food has to have the right nutrients for the bacteria

A (Acid--pH of the food)

pH scale – 0-14pH range of acids – 0-7pH range of bases – 7-14pH 7 is neutral

Microorganisms grow best at pH close to neutralIf the pH of a food is lowered, pathogens may still grow, but much slower

Acid (cont.)

Most pathogens do not grow at pH < 4.6

Some yeast and molds can grow down to pH 1.5

Temperature and Time

Heating/cooking foods (to an internal temperature of 74°C), kills pathogens

Some toxins/spores are not destroyed by heat

Staph aureus

Clostridium botulinum, Bacillus cereus

Vegetative and Spore Cells

When food is heated (cooked) to 74°C

and held <57°C SPORES survive Spores can become vegetative cells again if food is not cooled properly

Vegetative cells grow to high numbers and some produce toxins

How Long do Bacteria Need to Grow?

Bacteria can double in number every 15-30 minutesExample: bacteria with a 20 min doubling time:

Start

20 Min

40 Min

1 Hour

TIME

How Long do Bacteria Need to Grow?

Exercise: 1 bacteria on a piece of potentially hazardous food sitting at 90°F, with a 20 minute doubling time:

How many bacteria after 1 hr? 4 hrs? 8 hrs?

What if there were 10,000 bacteria to start with?

Time

Lag Phase

Gro

wth

Pha

se

Resting Phase

Death Phase

Bacterial Growth Curve

Number of

Bacteria (log10)

Time

Changing the Bacterial Growth Curve

*Sub-optimal means lowered: pH, AW , Temperature, etc.

Sub-optimal growth

conditions*

Rap

id B

acte

rial G

row

th

Con

ditio

ns

Much longer Lag Phase

Number of Bacteria

(log10)

Temperature and Time (cont.)

If food is held on a hot serving line pathogens can NOT grow

Pathogens need time to grow to cause disease

Illness is caused by improper cooling of leftover foods

Foods cool faster in the refrigerator than on the counter

5°C

Danger Zone

Keep cold foods cold

Keep hot foods hot

“Food Safety Thermometer”

Temperature range of rapid pathogen

growth=

Dan

ger

Zon

eH

otC

old

57.2oC

Oxygen (Oxidation-Reduction Potential)

Generally refers to the amount of Oxygen present

Most bacterial pathogens can grow with or without Oxygen

HOWEVER, some food pathogens can only grow when no Oxygen is present

Many spoilage bacteria and molds require Oxygen to grow (obligate aerobic)

Moisture (water)

Water is important for growth of bacteria

Water in food is present in two forms:

BOUND

FREE

Bound water is unavailable for bacteria to grow

Moisture (cont.)

Water Activity (AW)

A measure of the FREE WATER--water that is available for microbial growth

AW can range from:

0.00 (no free water) to 1.00 (all free water)

Pathogens do not grow/produce toxins at

Aw < 0.85

Potentially Hazardous Foods (PHF)

Definition: A food that supports growth of microorganisms

Examples of PHF’s:

Raw or cooked foods of animal origin:

Meat, Poultry, Waterfoods, Eggs, Dairy Products

Some fruit and vegetables

Potentially Hazardous Foods (PHF)

Which F&V are PHF’s?Cut melons (like cantaloupe)

And the pH of some melons is >4.6

Heated/cooked fruits and vegetables

Rice, beans, baked potatoes, etc. after they are cooked

Seed sprouts (such as alfalfa sprouts)

Garlic in oil mixtures

Food Additives

Additive

Government approved substances added to food during production or processing

Make food taste better, look better, and have a longer shelf life (prevent microbial growth and chemical reactions)

Adulterated food:

Adulterated food means one that contains poisonous ingredients, unapproved additives, or is composed of decomposed or unsanitary ingredients to help cover up spoilage

Food Additives

SUMMARY

Food Spoilage

Six Factors That Affect Growth

Potentially Hazardous Foods

Food Additives

Review Questions

What is the most important cause of food spoilage?

What can reduce food quality?

What forms off flavors and off orders?

List the 6 factors that affect growth.

At what pH does microorganisms grow best?

What pH do most pathogens not grow?

How long do bacteria need to double in growth?

What causes illness in leftover foods?

What is the temperature range of rapid pathogen growth?

Pathogens do not grow/produce toxins at what water level (Aw)?

What is PHF?

Name at least 2 foods considered PHF.

What is an additive?

Review Questions

Questions?

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