Food Microbiology & Safety
Helen Billman-Jacobe
Factors influencing microbial growth in food : Intrinsic factors contd
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Ray and Bhunia Ch 6
Intended learning outcomes
Distinguish between intrinsic, extrinsic and implicit factors
List the categories of intrinsic factors in food that affect microbial growth
Describe the intrinsic factors of samples of food
Explain how intrinsic factors affect the growth and survival of microorganisms in food
optimal growth preferences
Ability to compete with other microbes
Sensitivity to physical and chemical damage
pH and buffering Redox potential
Antimicrobial factors Nutrient content
Metabolic capacity
Implicit factors
Growth rate
Factors affecting microbial growth and survival in food
Extrinsic factors
Intrinsic factors
Gaseous atmosphere
temperature
Water activity
FOOD MICROBES
Common antimicrobial plant chemicals
Rosemary α-pinene
Garlic allicin
Mint Menthol
Thyme Thymol
. Microbiol. Rev. 1999; doi:10.1128/CMR.12.4.564
Cloves Eugenol
Intrinsic factors: pH = -log[H+]
Profound effect on microbial growth!
• pH scale is –ve log, so factors of 10 between every unit
• neutral = 7, acid is lower than 7, and alkaline is higher Preferred growth pH ranges of:
• Bacteria
• Filamentous fungi
pH 6.0 – 8.0 pH 4.5 – 6.0 pH 3.5 – 4.0
*microbial growth can alter pH, e.g. fermented foods
Food pH scale
Foods vary in pH
MOST FOODS ACIDIC-NEUTRAL
source: modified from p25, Adams & Moss 3rd Edn.
pH growth ranges of microbes
Optimum Growth ranges
• Filamentous fungi, pH 3.5 – 4.0
• Yeasts pH 4.5 – 6.0
• Bacteria pH 6.0 – 8.0
Ref: (2005). Modern Food Microbiology 7th Edition.
pH effects on food
•1.Flavour
pH can provide sour, tart or acidic tastes in food. Eg. Citric acid
•2.Texture
pH can affect the water holding capacity of some foods, impacting texture. pH can also have an impact on the hard or softness of items such as cheese.
•3.Appearance
The pH level plays a part in changes in pigmentation as well as the development of haze in some products. in some fruits change colour depending on pH.
•4.Shelf Stability
pH works to prevent spoilage by inhibiting bacterial growth. Lower pH can inhibit microbial growth. Most bacteria will not grow at levels below pH 4.6
•5.Fermentation
pH can affect bacteria used in the production of fermented food E.g. For Yoghurt the required pH for this is ≤4.5 (FSANZ).
Acidic pH levels can be used to control foodborne pathogens
Instrument choice
Intrinsic factors: Water Activity
Aw = measure of available water for biological functions i.e. water in its free (unbound) form
•Ions and polymers (carbohydrates) can bind water, so making it difficult for microbes to use it for growth Aw of foods varies from 0.1 to a maximum of 0.99
Moisture content and Water activity
Moisture content (%)
Amount of bound water + free water Quantitative measure of the amount of water in a sample
Water activity (Aw)
Amount of free water
Qualitative measurement of the status of the water in a system
Microbes can use the free water
Food component (starch, sugar, salt)
Water bound to a food component
Free water
Water activity (Aw)
•Water activity can be measured by placing a sample in a sealed measuring container.
•The vapor pressure of the water in the substance and the water in the air is allowed to equilibrium over time.
•The relative humidity of the air surrounding the sample is equal to the water activity of the sample
•The humidity can be measured using a device called a water activity meter
Intrinsic factors: Water Activity
Aw = measure of available water for biological functions i.e. water in its free (unbound) form
Most microbes can not grow below Aw ~ 0.6
Most bacteria cannot grow below Aw ~ 0.9
So this is a powerful means of preventing microbial growth
Intrinsic factors: Water Activity
Microbial groups able to grow at low Aw:
• Osmotolerant – can grow in the presence of high concentration of un-ionised organic compounds (e.g. sugary
• Xerotolerant – can grow on dry foods (e.g. grains)
• Halotolerant or Halophilic – grow in the presence of high concentration of salts (e.g. NaCl, such as soy sauce)
Limit below which microbes do not grow, aw ~ 0.6
But if Aw increases again – then can allow dormant microbes to grow
Intrinsic factors: Redox Potential (Oxidation-Reduction Potential)
REDOX POTENTIAL: the tendency of a medium to accept or donate electrons, as measured against a reference electrode
The redox potential is designated by Eh and is measured in millivolts (mV)
It is equivalent to the ability to oxidize or reduce
• A positive value, means it is oxidising (accepts e- )
• A negative value, means it is reducing (donates e- )
•Eh affected by pH and O2
Redox potential of some foods
Eh -200 +225 +225 +300 +380 +410
Water activity & pH
Sweet biscuits
Barley and split peas
honey pasta
crisp savoury biscuits
honey sultanas
toffees sugar
tomato paste
Intrinsic factors
nutrients inhibitors water activity pH
Redox potential
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