Microbes in Food Flashcards by Sophie Barley

Requirement of Microbes

Appropriate environment

Appropriate Nutrition

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How do bacteria reproduce

Binary Fission
1 cell gives 2 
gives 4
gives 8
Exponential

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Generation time

Time it takes between each division

Clostridium perfringens is 7.4 minutes

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How to plot bacteria growth

Log paper to give straight line
LogNt = 0.201n + LogN0
Nt - number of cells over a period of time
No - starting number
n- number of generations

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Phases of bacteria growth in batch

lag
log
Stationary
Death

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Lag phase

Cells grow and adapt to new environement

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Log phase

Cells multiply rapidly and exponentially

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Stationary phase

No net increase in cell
Death = growth
Because resources at capacity

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death phase

Death of cells more frequent and larger than cell growth

Lack of nutrients

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Bacteria

Prokaryote

Binary fission

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Yeast

Eukaryote
Unicellular
Multiply by buddying

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Moulds

Eukaryote
Multicellular
Asexually reproduce

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Food production use of microbes

Dairy
Fermented beverages
Sauerkraut
Sausages
Sourdough
Soy sauce

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Non-microbial food spoilage

Physical (dehydration)
Chemical (oxidation of FA)
Infestation (insects and rodents)

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Microbial food spoilage

Direct effect on growth
Effect microbial enzymes on food
Has to gain access to food with correct conditions
Detect presents at 10^6-10^8

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Growth rate of microbes

Bacteria > Yeast > Moulds

Temperature and microbe growth

Same starting temperature but a lower log number of cells per gram then take longer to spoil

If lower the temperature to fridge temp (4 degrees) takes longer to spoil even if at same starting log number.

Conditions to consider microbial growth

pH
Aw (water activity)
Nutrient
Atmosphere

Mesophiles

Minimum temp - 5-15 degrees
Optimal - 30-40 degrees
Max temp - 40-47

Psychrophiles

Minimum temperature - -5-5 degrees
Optimal - 12-15
Max - 15-20

Psychrotrophs

Min - -5-5
Optimal - 25-30
Max - 30-35

Thermophiles

Optimal above 45

What effect microbial population

Hygiene
Contamination
Processing
Preservation

Why microbial testing

Safety to consumers
Stability of shelf life
Level of sanitation used during handling
Whether they meet acceptable standards

Quantitative microbial testing

Provide and estimate of microbial load | Aerobic plate count or standard plate count

Qualitative microbial test

Detect presence of specific foodborne pathogens using selective agar

Antigen-antigen microbial tests

ELISA testing

Nucleic acid base pair recognition

Probe featuring NA from target organism applied to NA extract from food samples PCR

Food preservation principles

Minimise food spoilage or food poisoning organisms | Reduce micro-organisms to start with, inhibit growth or kill microorganisms

High temperature

Kills microbes Thermal Pasteurisation Thermal Sterilisation

Low Temperature

Inhibition of growth Chilled storage Freezing

D value

Time for number of microorganisms to decrease by one decimal place High D value means greater heat resistance

Z value

Number of degrees to increase D value 10 folds

Sterilisation

Inactive micro-organisms and enzymes Deliver sensory and nutritional properties Time influenced by - heat resistance and pH Low acid food - anaerobic spores forming pathogens - need to remove clostridium botulinum spores 12D 121 degrees for 2.52 minute In high acid food generally less severe

Pasteurisation

Mild treatment below 100 degrees Kills pathogens, enzyme activity and spoilage enzymes Low acid food - eliminate pathogenic bacteria Liquid egg - salmonella 64.4 degree for 2.5mins Milk - mycobacterium tuberculosis High acidic food - inactivate enzymes as pathogens cant grow