Food Microbiology summary
Spoilage
Microbial spoilage: Mostly metabolic products produced during growth (smell, slime, colour change), but
sometimes also microbes themselves (mould). Spoilage becomes notable at 107 cells per gram/mL food. Shelf
life depends on initial contamination (N0) and growth time (µ).
Initial contamination: Primary contamination of raw materials (animals and plants) and secondary
contamination (water, equipment, air, people, vermin)
Microbial growth:
! Food is spoiled before the stationary phase is reached
Micro-organisms:
1. Bacteria:
- Cocci (coc, duplococ, streptococ, staphylococ) ±1µm
- Rods (rod, rod with flagella, rod with endospore) ±0.6-0.8 x 1-5 µm
- Other shapes (vibrio, spirilli)
2. Yeasts – unicellular organisms (with buds) ±10µm
3. Filamentous fungi – multicellular organisms (with spores)
4. Virus – strictly no micro-organism, as it needs a host to multiply ±25-30nm
Microorganisms are everywhere, but the environment selects: A suitable environment has resources needed
for growth (nutrients) and physio-chemical conditions that don’t hinder the organism
Generation time (GT) – time needed for the population to double (t = 0 → 20 = 1 cell, t = 5 → 25 = 32 cells)
GT = ln(2)/µ
Exponential growth is log-linear growth: Growth rates on log10 and ln scale:
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,Growth kinetics: study of increase of cell number in time (growth rate)
Growth rate influenced by:
- Intrinsic factors (physio-chemical properties of food, such as nutrients, pH, water activity and
preservatives)
- Extrinsic factors (properties of food environment, such as temperature, relative humidity and gas
composition)
- Implicit factors (properties and interactions of micro-organisms, such as umax, interactions,
succession in time)
- Processing (changes of food/environment/micro-organisms to preserve or to process to desired
product)
Growth rates of micro-organisms:
Micro-organisms have metabolism like humans do (except viruses):
- Catabolism – metabolic routes involved in degradation of an energy and carbon source to generate
precursors for cell components and energy for cell maintenance
- Anabolism – metabolic routes involved in biosynthesis of polymeric cell compounds (DNA, RNA,
protein, lipids, cell wall constituents)
Most micro-organisms are chemo-heterotrophs: They use preformed molecules from other organisms as
energy and carbon source.
Antimicrobial barriers hinder growth by ensuring lack of access to water and nutrients:
- Physical barrier (shell of nuts)
- Macromolecules, resistant to degradation (peel of fruit or fatty lining of meat)
! Antimicrobial barriers do not protect against the environment (UV)
Microbial enzymes can degrade barriers and release nutrients: pectolytic enzymes act on pectine, amylolytic
on starch, lipolytic on lipids, proteolytic on protein.
We can change a food by product formulation (increase or decrease nutrients, such as fermenting) and
processing (introduce or remove barriers, such as compartmentalization)
pH preferences micro-organisms:
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,Types of micro-organisms – pH preference:
- Acidophile – 0 - 5.5
- Neutrophile – 5.5 – 8
- Alkalophile – 8 - 11.5
Pumping out protons of the cell requires energy and slows down
growth:
The type of acid is of relevance:
The higher the pKa, the higher
the effect of HA pumped into
the cell and there dissociating
and lowering the inside pH
! Weak acids therefore work the
best against micro-organisms
pH applications in food preservation:
- Fermentation – use of lactic acid bacteria to produce weak acids that lower the pH and inhibit
growth of pathogenic bacteria
- Acidic preservative – add acid to lower the pH
Pitfalls of pH in food preservation:
- Pathogenic bacteria can grow in low-acid foods
- Other microorganisms grow and increase the pH (yeasts and moulds dissimilate acid > acid-tolerant
bacteria can grow > pathogenic bacteria can grow)
Redox potential (Eh) – tendency of a medium to accept or donate electrons (oxidator + e- + H+ <-> reductor).
Electron donors (H2) have a negative potential. Electron acceptors (O2) have a positive potential.
! Whole grain wheat and meat have a negative Eh. Spinach, lemons and pears have a positive Eh.
Oxygen from the air can act as an electron acceptor (oxidator), yielding reactive substances:
- O2 + 2H+ + 2e- -> H2O2 (hydrogen peroxide)
- O2 + e- -> O2- (superoxide anion radical)
Aerobic organisms:
- Most energy via oxidative phosphorylation with O2 as terminal electron acceptor
- Enzymes disable toxic byproducts (Superoxide dismutase (SOD) O2- > H2O2 and catalase H2O2 > H2O)
Anaerobic organisms:
- No SOD or catalase, thus no growth at high redox potentials (in presence of O2)
Types of micro-organisms – oxygen preference:
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, The redox potential of foods may change due to:
- Higher access to O2 results in a higher Eh (e.g. after milling or grinding)
- Lowering O2 content results in a lower Eh (e.g. after canning or fermenting)
- Low pH: many H+, so a higher Eh (Nernst equation Eh = E’0 + (RT/nF)*ln((oxidator*H+)/(redactor))
- Microbial growth: less O2, resulting in a lower Eh
Growth inhibition by CO2: CO2 dissolves -> CO2 + H2O <-> H2CO3 (carbonic acid) <-> H+ + HCO3- (bicarbonate):
! Growth inhibition by CO2 has a larger effect with some O2 present than without, and has a larger effect at
low temperatures, due to a higher solubility at lower temperatures.
! Moulds and oxidative Gram- bacteria are most sensitive to CO2, where yeasts and Gram+ bacteria are most
resistant to CO2.
Vacuum packaging – exclude all air from the package. Residual respiration of the product and micro-organisms
consumes the last O2 present. High levels of CO2 develop and inhibit flora.
MAP (modified atmosphere packaging) – increasing CO2 concentration and decreasing O2 concentration
- Atmosphere changes during storage because of respiration of the product, micro-organisms and
permeability of the packaging to gases
CAP (controlled atmosphere packaging) – used for large storage rooms
- Atmosphere kept constant
Water activity (aw) – measure of free water in a food product
aw = P/P0 = 1/100*ERH
P – partial vapour pressure of the food at temperature T
P0 – vapour pressure of pure water at temperature T
ERH – equilibrium relative humidity in % at temperature T
! aw is an intrinsic factor while RH is an extrinsic factor
aw of food products:
Types of micro-organisms – water activity: Preservation pitfall 1: Non-microbial spoiling:
! Generally, Gram+ bacteria are able to grow at a lower aw than Gram- bacteria
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