Summary Food Microbiology (FHM-20306). The summary is quite extensive but contains all information needed to successfully pass the exam. The summary contains figures and (parts of) the lecture slides for better understanding of certain topics and processes.
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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)
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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