FOOD MICROBIOLOGY

Role of Microorganisms in Foods

Food Spoilage

Food Poisoning

Food Preservation

Food Production

Microorganisms in Food

Bacteria

Moulds

Yeasts

Viruses

Sources of Microorganisms in Foods

Sources of Microorganisms in Foods

Plants: Lactic acid bacteria, moulds, yeasts, Pseudomonas, Bacillus

Animals: Intestinal tract – E. coli, Salmonella, Campylobacter, Yersinia, Listeria; Skin – Staphylococcus, Micrococcus, Propionibacterium

Air: moulds, Micrococcus, Bacillus, Clostridium

Soil: Pseudomonas, Bacillus, Clostridium

Sources of Microorganisms in Foods

Sewage: many enteropathogenis bacteria and viruses

Humans: intestinal and skin bacteria, human pathogenig bacteria (Staphylococcus aureus, Salmonella, Shigella, E. coli) and viruses (Norovirus, hepatitis A)

Other sources: Food ingredients, equipment, flies, wild animals, etc.

Parameters of Foods That Affect Microbial Growth

Intrinsic parameters

Extrinsic parameters

Intrinsic Parameters

pH

Moisture contents

Oxidation-reduction potential

Nutrient content

Antimicrobial contituents

Biological structures

pH of Selected Foods

Food pHBroccoli 6.5

Cabbage 5.4-6.0Corn 7,3Onion 5.3-5.8Celery 5.7-6.0Spinach 5.5-6.0Potatoes 5.3-5.6Tomatoes 4,2-4,3Apples 2.9-3.3Banana 4.5-4.7

Food pHOranges 3.6-4.3Grapes 3.4-4.5Milk 6.3-6.5Butter 6.1-6.4Cheeses 4.9-5.9Beef 5.1-6.2Chicken 6.2-6.4Fish 6.6-6.8Crabs 7.0

Egg-white 7.3-9.6

Microorganisms Lower limit

Optimum pH

Upper limit

Thiobacillus thiooxidans 0.5 3.0 6.0

Lactobacillus acidophilus

4.2 6.2 7.0

Escherichia coli 4.5 7.0 9.0

Bacillus alcalophilus 8.5 10.6 11.5

Yeasts 2.0 4.0 7.0

Moulds 1.0 7.0 11.0

Tetrahymena pyriformis

4.0 7.3 9.0

Impact of pH on the Growth of Microorganisms

Water activity

Food Bacteria Fungi

1.00 Blood, milk vegetables, fruits

Most bacteria

0.94 Ham Most G+ bacteria

0.85 Salami Staphylococcus Saccharomyces rouxii (in NaCl)

0.80 Marmalade, jam Penicillium, Aspergillus

0.60 Dried fruits, honey

Saccharomyces rouxii (in saccharose)

Water Activity (aw) Values for Growth of

Microorganisms in Foods

Extrinsic Parameters

Temperature of storage

Relative humidity of environment

Presence and concentration of gases

Presence and activity of other microorganisms

Indicators of Microbial Product Quality

Microorganisms and/or their metabolites

They should be present and detectable numbers

Their numbers should have a negative correlation with product quality (food spoilage)

They should be easily detected in short period of time

Indicators of Microbial Product Quality

Microorganisms Products

Acetobacter Fresh cider

Bacillus Bread dough

Clostridium Hard cheeses

Byssochlamys Canned fruits

Lactobacillus Beers, wines

Lactococcus Raw milk

Yeasts Fruit juice

Zygosaccharomyces bailii Mayonnaise

Indicators of Microbial Product Quality

Metabolites Applicable Food Product

Histamine Canned tuna

Ethanol Apple juice

Lactic acid Canned vegetables

Volatile fatty acids Butter, cream

Indicators of Food Safety - criteria

Should be easily and rapidly detectable

Should be easily distinguishable from other microbes

Always be present when the pathogen of concern is present

Their numbers ideally should correlate with those of the pathogen of concerns

Indicators of Food Safety

Escherichia coli

Enterococcus spp.

Staphylococcus aureus

Food Spoilage

Food Spoilage

Microbial food spoilage can be caused by: a) growing microbes; b) microbial enzymes

Bacteria, yeast and moulds multiply in food

Viruses and parasites do not multiply in food

Detection of Food Spoilage

Microorganisms mustmultiply and attain certain level of viable cell per g, ml or cm2

105 : some bacterial exotoxins can be detected

106-7: changes in odor and color

108-9: changes in texture, slime formation

Some Important Food Spoilage Microorganisms

Psychrotrophic (5oC) aerobic microorganisms:Pseudomonas fluorescens, Acinetobacter, Moraxella, moulds

Psychrotrophic facultatively anaerobic microorganisms: Brochotrix thermospacta, Lactobacillus viridescens, yeasts

Thermoduric bacteria: Bacillus, Clostridium

Thermophilic (40-90oC) bacteria: Clostridium, Bacillus, Streptococcus thermophilus

Aciduric microorganisms: Lactobacilli, yests and moulds

Food Spoilage

Utilization of food nutrients by microorganisms – general preference: 1) carbohydrates; 2) proteins; 3)lipids

Microbial growth in succession: 1) bacteria; 2) yeasts, 3) moulds

Penicillium sp.

Some End Products from Microbial Metabolism of Food Nutrients

Carbohydrates: CO2, H2O, lactate, acetate, butyrate, ethanol, diacetyl, acetoin

Proteins: CO2, H2, NH3, H2S, amines, disulfides, putrescine

Lipids: fatty acids, glycerol, aldehydes, ketones

Food Preservation

Food protection with chemicals

Food protection with modified atmospheres

Radiation protection of foods

Protection of foods with low-temperature

Food protection with high temperatures

Other food protection methods

Food protection with chemicals

Benzoic acid

Sorbic acid

Nitrites and Nitrates

NaCl and sugars

Antibiotics (monensin, tylosin, natamycin)

Bacteriocins (nisin, sakacin)

Food protection with modified atmospheres

Vacuum packaging

CO2

Modified atmosphere packaging ( a mixture of O2, CO2 and N2;

high-oxygen, low oxygen)

Radiation protection of foods

Utraviolet light

Gamma rays

G+ bacteria more resistant than G-bacteria

Protection of foods with low-temperature

Temperature (oC) Microbial activity

10 to 50 Rapid growth, enzymatic activity

0 to -10 Slow growth, enzymatic activity

0 to -15 Very slow growth of some microbes(yeast, moulds), enzymatic activity

-15 to –20 No growth, some enzymatic activity(decarboxylases, lipases)

To - 40 Only lypolytic activity

Food protection with high temperatures

Temperature (oC) Effect on microorganisms

10 to 50 Rapid growth, enzymatic activity

50 to 60 Yeast and moulds die

50 to 70 Inactivation of enzymes (wet heating)

60 to 85 Most bacteria die

80 to 90 Inactivation of enzymes (dry heating)

121 Endospores die (wet heating)

180 Endospores die (dry heating)

Other food protection methods

Drying

High hydrostatic pressure

Bakteria No. of cases

Salmonella 150 000

Campylobacter 150 000

Yersinia 10 147

Escherichia 2 664

Brucella 2 386

Listeria 860

Mycobacterium 49

Most Frequent Foodborne Diseases (EC, 2002)

Zoonoses

EFSA, 2008

oCampylobakteriosis (190 566 případů ročně)

oSalmonelosis (131 468)

oVTEC (3159)

oBrucelosis (619)

oListeriosis (1381)

oEchinocososis (891)

oTrichinelosis (670)

oQ-fever (1594)