food microbiological principles fst 241
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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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