Food Microbiology &
Helen Billman-Jacobe
Extrinsic factors
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Reading: Ray and Bhunia Chapter 6 and 37
Intended learning outcomes
Explain what are the extrinsic factors that affect the growth of microorganisms in food Relate the extrinsic factors to the control of microorganism 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
Extrinsic factors: humidity
The humidity of the atmosphere around a food will result form the interplay between the water activity of the food
An equilibrium is achieved in a closed vessel. E.g. biscuits in a sealed container stay crisp (low Aw) but if left in the humid air they absorb water and become soft.
There are a number of ways we can manipulate the humidity to maintain Aw (either keep foods moist or dry)
Factors influencing microbial growth in food
intrinsic factors = factors in the food
Extrinsic factors= environmental factors
Compare the extrinsic factors affecting the salamis hanging in the background and the fresh meats in the chiller cabinet
https://www.mcalpinehussmann.co.nz/specialty-deli.shtml
Deli display cabinet – example
Factors influencing microbial growth in food
intrinsic factors = factors in the food
Extrinsic factors= environmental factors
Compare the extrinsic factors affecting the salamis hanging in the background and the fresh meats in the chiller cabinet
https://www.mcalpinehussmann.co.nz/specialty-deli.shtml
Extrinsic factors:Temperature
Cell growth is dependent on chemical reactions
Temperature directly affects the rates of chemical rx. ~ 10°C rise, will double a reaction rate
~ 10°C fall, will half the reaction rate
This changes outside the growth range (no growth)
•storage of foods – usually cold, -20°C to 5°C •Some are stable at ambient temp. (10-35°C) •Ready-to-Eat (hot) – usually 50-60°C
Extrinsic factors:Temperature
Three main ‘temperature groups’ of microbes
1. Thermophiles
– high temp: opt ~ 55°C (range 45-70°C)
2. Mesophiles – opt ~ 35°C (range 10-45°C)
3. Psychrophiles – opt ~15°C (range -5 to +20°C)
Two more groups are also important in food microbiology: These properties are implicit factors of the microbes
Psychrotroph – can grow at refrigerator temperature (0-5°C) regardless of their optimum growth temperature.
Thermoduric – can survive pasteurization heat treatments.
Thermoduric bacteria (from pasteurized milk)
By microscopy these were endospore-forming, Gram positive Bacteria, probably Bacillus spp.
Extrinsic factors: Gaseous atmosphere
Remember that different microbes have different requirements for oxygen
Microaerophiles
The gaseous atmosphere around a food will influence which microbes can grow
In food packaging we can manipulate the atmosphere by adding different gas mixtures such as CO2, N2
Modification of Food Atmosphere
Modification of Food Atmosphere
https://www.boconline.co.uk/en/processes/controlling- and-modifying-atmospheres/modified-atmosphere- packaging/index.html
Control by modified atmosphere
CAP: controlled atmosphere packaging MAP: modified atmosphere packaging AP: active packaging
VP: vacuum packaging
vacuum packed fish
note the tight fitting, thick plastic, packaging
Vacuum Packing (VP)
• product placed in a bag, air evacuated and residual O2 absorbed (film must collapse on the surface entirely)
• Affects mainly fast growing aerobic microbes
• Anaerobes/Fac.Anaerobes, including any pathogens, are not greatly
Commonly used for meats – where VP + chill storage will allow 5x longer storage compared to aerobically stored (and chilled) meats.
MAP: Modified Atmosphere Packaging
Flush food / products packed with a gaseous mix of CO2, N2 and O2
CO2 has inhibitory effect on many microbes
Water and carbon dioxide combine to form carbonic acid (H2CO3), a weak acid
N2 creates an inert atmosphere, retards microbial growth, and slows rancidity O2 maintains oxy-myoglobin in packed meat (retains its bright red appearance)
Actual ratios of gases vary depending on type of food.
MAP: Modified Atmosphere Packaging
A problem with MAP is that the gas composition can change during food storage as a result of:
• Product and microbial respiration
e.g. removal of O2 by aerobes/fac. anaerobes
• Dissolution of CO2 into aqueous phase
• Differing rates of gaseous exchange across various types of packaging
film / membranes
Such changes in gas composition can reduce the inhibitory effect of the initial gas mixture
Controlled atmosphere packaging (CAP)
To overcome the problems with MAP (where the gas composition can change over time), a controlled gas environment of food/ product was developed.
In CAP, the gas is of constant composition throughout storage.
• e.g. apples/ pears are stored at sub-ambient temperature and transported under 10% CO2
• CO2 is effective in controlling mould growth and reduces ethylene
(*ethylene promotes post-harvest ripening of fruits, so its control by CO2 helps maintain/preserve fruit during transport/storage)
Controlled atmosphere packaging (CAP)
Usually this is done in large impervious containers, e.g. special shipping containers, or grain storage silos. Reducing the O2 level is also of benefit for killing any insect pests/rodents
that have managed to get into the
The gas composition is kept constant by continuous flushing or monitoring and adjustment.
AP: active packaging (‘intelligent packaging’)
• active components in packaging system interact with the contents or environment, to extend shelf life and quality
• e.g. moisture control, O2 generators, CO2 controllers, oxygen scavengers, odour removal
Extrinsic factors
Read about commercial active packaging
Read about MAP products
https://www.boconline.co.uk/en/processes/controllin g-and-modifying-atmospheres/modified-atmosphere- packaging/index.html
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