Microbiological Principles of Food

Preservation

Types of Microbes on Food

Bacteria – one celled, Procaryotic

Molds – Threadlike Fungi Yeasts – Usually creamy Fungi

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Other microorganisms of concern

Viruses – protein packet of genes

Protozoa –Complex cell - Eucaryotic

Do not multiply in foods Contaminate food

Humans Contaminated water

Bacterial Cells? Contain Genes that Program the Cell

and Help It Survive Reproduction

Binary Fission – One Cells Splits in Two 1 cell > 65,000 cells in ~4 hours

Cause Illness or Disease - Pathogens Cause Deterioration of Foods –

Spoilage Useful - fermentation

Some Microorganisms Some Microorganisms Cause DiseaseCause Disease

Microorganisms that cause disease referred to as pathogens

Few of the known microorganisms are harmful to humans

Many diseases can be transmitted from person to person or from animals to humans

Very few transmitted via foods

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Some Microorganisms Cause Some Microorganisms Cause DiseaseDisease

Majority of bacterial foodborne illnesses are caused few microorganisms• Salmonella• Campylobacter• Shigella• Clostridium perfringens• Staphylococcus aureus• Listeria monocytogenes• Clostridium botulinum

Pathogenic Microorganisms and toxins

http://www.fda.gov/Food/FoodSafety/FoodborneIllness/FoodborneIllnessFoodbornePathogensNaturalToxins/BadBugBook/default.htm

Food Spoilage and Food Safety

Not the Same! Reducing spoilage can actually

increase the risk of a harmful bacterium being present

Both Preservation and Safety must be ensured by the processor.

Processors use the “hurdles” concept to preserve foods and create safe foods.

Useful Functions of Useful Functions of MicroorganismsMicroorganisms

Food Fermentations Bread, cheese, wine, beer,

sauerkraut, sausages, and other fermented foods

Enzymes, antibiotics Degrade organic matter

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Bacteria The most important and The most important and

troublesome microorganisms for troublesome microorganisms for the food processorthe food processor

Bacteria are single-celled living Bacteria are single-celled living bodies so small that individually bodies so small that individually they can be seen only with the aid they can be seen only with the aid of a powerful microscopeof a powerful microscope

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Bacteria Bacteria have several shapes

or forms Spherical called “cocci” Rod-shaped called “rods”

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Reproduction of Bacterial Cells

Bacteria reproduce by division “Fission”

Called “growth” Cells in the active stage of

growth and metabolism are referred to as vegetative cells

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Reproduction of Bacterial Cells

Under favorable conditions, each Under favorable conditions, each cell divides on average about once cell divides on average about once every 15 or 30 minutesevery 15 or 30 minutes

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Growth of Microorganisms

MoldsMolds Multicellular organisms Form tubular filaments or

“mycelia” Branched

Reproduction Fruiting bodies or spores On aerial structures

Larger than bacteria Longer than yeasts

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Molds Widely distributed in nature

Soil and in the dust Will grow on almost any food

or substance Pastes, paper, plastics, paint

Tolerant to cold

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Molds Little heat resistance Require oxygen to grow

No growth in canned foods unless Air remaining in container Loss of vacuum

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Yeasts Single cell microscopic living

bodies Often oval or egg-shaped Smaller than molds but larger

than bacteria Reproduce mainly by budding

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Budding

Yeasts Widely found in nature Often associated with liquid

foods containing sugars and acids

More tolerant of cold than of heat Most yeast are destroyed by

heating to 170F

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Intrinsic and Extrinsic Food Parameters Affecting Safety & Spoilage “

Hurdles” Intrinsic (inside the food)

pH Moisture Oxidation-Reduction Potential Inside Food Natural Antimicrobial Constituents Biological Structures & Natural Microflora

Extrinsic (environment around the food) Temperature Relative Humidity Presence of Gases or Oxygen Antimicrobials or Added Microorganisms

Preferred Conditions for Bacterial Growth

Food: Carbs, Protein, Fat Acid Range pH 4.6 to 9 Temperature above 4C (40 F) Time Minimum 2 hours Oxygen Range varies by microbe Moisture above 0.85 Aw

What Microorganisms Need to Grow

Water Source of Energy Source of Nitrogen Vitamins/Growth Factors Minerals

Oxygen Requirements Require oxygen or air

Aerobes Free oxygen prevents growth

Anaerobes Majority of bacteria neither

strict aerobes nor strict anaerobes Facultative Anaerobes

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Water activity of some common foods

Most fresh foods > 0.95

Liverwurst0.96 Cheese Spread 0.95 Caviar 0.92 Fudge Sauce 0.83 Semi-moist Pet Food 0.83 Salami 0.82 Soy Sauce 0.80 Peanut Butter – 15% total moisture 0.70 Dry Milk – 8% total moisture 0.70

pH pH is a measure of acidity pH = -log of the Hydrogen Ion

Concentration Major Control Point for Bacteria Major Impact on Food Quality Classification of Foods: Acid,

Low-Acid, Alkaline

pH Ranges for Food

pH 7.0

Neutral

pH 0

pH 14

pH 2

pH 3.5

pH 9

pH 4.6

pH 5.5

pH 10

Limes

Fruits &

Berries

Veggies

Melons

Meats, Milk

Bleach SolutionsBacteria

Grow

Comparison of Growth Requirements

Organism

pH Range

Water Activity

Requires Oxygen?

Bacteria 4.4 to 9.8

Aw>0.85

Some- It Varies

Yeasts 1 to 11 Aw>0.60

Not Usually

Molds 1-12 Aw>0.60

Yes

Temperature of Growth Classification

psychrotroph

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Classification Optimum Temperature

Psychrotroph

Mesophiles

ThermophilesThermophiles

58ºF to 68ºF (14ºC to 20ºC)

86ºF to 98ºF (30ºC to 37ºC)

122ºF to 150ºF 122ºF to 150ºF (50ºC to 66ºC)

Sporeforming and Non-Sporeforming

Bacteria Most bacteria exist only as

vegetative cells• All cocci and some rods• NON-SPOREFORMERS

Some groups of rod-shaped bacteria produce spores• SPOREFORMERS• Exist as both vegetative cell and

spores

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Sporeforming and Non-Sporeforming

Bacteria Bacterial spores

• Dormant stage in normal growth cycle

• Can germinate and produce new vegetative cell

• Ability to survive a wide range of unfavorable conditions

Spores in molds and yeasts• Generally not resistant

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Vegetative Cell

Vegetative Cell with Spore

Spore

3 Microscopic Forms of a Sporeformer

Resistance of Spores to Environment

In general, bacterial spores are In general, bacterial spores are extremely resistant to heat, extremely resistant to heat, cold and chemical agentscold and chemical agents• Survive in boiling water > 16

hours

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Temperature Requirements

Every microorganisms has an optimum (most favorable) temperature range

Microorganisms grouped to indicate their relationships to temperature

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The Psychrotrophic Group

Psychrotrophs (“psychro” for cold and “trophs” for growing) Optimum of 58F-68F Grow slowly on food at

refrigerator temperatures 40F Primarily spoilage

microorganisms Few pathogens

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The Mesophilic Group Mesophiles (“meso” for middle

and “phile” for love) Optimum 86F-98F All microorganisms that affect

food safety can grow in this temperature range

Clostridium botulinum is a mesophile

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The Thermophilic Group Thermophiles (“thermo” for

heat, “phile” for love) Many are sporeforming bacteria Obligate thermophiles

Spores will not germinate and grow below 122F

Facultative thermophiles Growth occurs at 122F to 150F

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The Thermophilic Group Spores of these bacteria are so

heat-resistant that they can survive for more than 60 minutes at temperatures of 250F

Thermophilic bacteria are not pathogenic and do not affect food safety

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