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Summary food microbiology FHM20306
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SPOILAGEDecrease quality of food product
Sustainability planet
Food security (enough to eat) waste and need of food (hunger)
Microbial spoilage
Reactions occur 107 cfu/gram or ml spoilage noticeable (smell, slime, color change)
Growth is needed to get spoilage, organisms are in products but can only grow (growth = exponential) when the conditions
change
1 organism 107 organisms in 8 hours under optimal conditions
Quantification: calculation with well-chosen unit and appropriate number of significant digits
Kinetecs influenced by:
- Intrinsic factors: chemical (nutrients, pH, aw, preservation)
- Extrinsic factors: environment (temperature, relative humidity, gas)
- Implicit factors: properties/interactions mo (umax, interactions, succession in time)
- Processing factors: contamination/ change of in- or extrinsic factors
Initial contamination: animals/plants contain mo
- Animals: intestines/skin
- Plants: soil/manure/water
Secondary contamination:
- Water: process, rinsing, cooling, cleaning
- Equipment: machines, tools, surfaces
- Air: aerosols, dust
- People: hands, hair, coughing, sneezing
- Vermin: rodents, birds, insects
HISTORY OF FOOD
Mo (threaten us, bother us, help us): Antoni van leeuwenhoek
Pasteurisation: louis Pasteur
Methods to store food
- Prehistory: hunting and food gathering (storage)
- 15000 BC: domestic production of animals and crops, kept dry, away from air & light, salting, olive oil, honey, air
drying, smoking, fermentation
Development of regulation:
AD: prohibition
Romans: not allowed to sell spoiled food
Leo VI: forbid blood sausages
Swiss cities: prohibited to sell fish from the day before
Since 1960: science, industry
1900: microbial testing, pasteurization
1922: can sterilization
1930: pasteurization
1960: GMP
1971: HACCP
1980: start of quantitative microbiology
1995: QRA
SOURCES OF CONTAMINATION
Bacteria: (1-5 um)
Cocci: Rods: Other shapes:
,Fungi: (10 um)
Yeast Moulds
Viruses: (40x smaller than cocci) need a host to multiply
Parasites need a host
Mo are everywhere but they need a good environment to grow, environment selects
MICROBIAL GROWTH
GROWTH KINETICS AND FACTORS INFLUENCING GROWTH
Bacterium well adapted to environment and enough nutrients multiply
Generation time (=gt): time needed to double
t=0 min t=20 min t=40 min t=60 min t=8h
Generation 0 Gen 1 Gen 2 Gen 3
1 organism 2 organisms 4 organisms 8 organisms
20=1 21=2 22=4 23=8 224=1,7*107
Log N(t) = Log N(0) + (Log(2)/gt) * t
Worst case: exponential growth
lnN(t) = lnN(0)+u(=specific growth rate)*t
u = ln(2)/gt
Shelf life t=(lnN(t) – lnN(0))/u
Extension shelf life: decrease growth rate/decrease initial level of
contamination
NUTRIENTS AND STRUCTURES (INTRINSIC FACTORS)
Mo are organisms that have metabolism (catabolism: the metabolic routes that are involved in the degradation of a carbon-
and energy source to generate precursors for cell components and energy for cell maintenance & anabolism: the metabolic
routes involved in the biosynthesis of polymeric cell compounds (DNA, RNA, protein, lipids, cell wall constituents))
Chemo-heterotrophs: use preformed molecules from other organisms as energy and carbon source
Enzymes are important in metabolism: help to transform substrate (“food”) in products (“metabolites”)
food may contain:
- Starch, glycogen, lactose, glucose
- Protein, peptides, amino acids
- Lipids, free fatty acids
- Spore elements
- Vitamins
- Water
Protect nutrients:
- Physical barrier (for example shell of nuts)
- Macromolecules resistant to degradation (for example peel of fruit or fatty lining of meat)
- Aim: hinder growth: lack of access to water/nutrients, no protection against environment
Enzymes that break down barriers:
, Nutrients and barriers in preservation:
- Product formulation: increase or decrease nutrients (fermentation)
- Processing: introduce of remove barriers (water in oil emulsion = corporatization)
PH AND PRESERVATION BY ACID
pH: acidity of substance
pH = -log[H+]
Acid <7, neutral 7, alkaline > 7
Preferences mo:
Most: pH +/- 7
Yeast: pH 4,5
Moulds: pH 3,5
Difference between acids:
- Strong acids: totally dissociated
- Weak acids: pH=pKa+log([A-]/[HA])
Reduce growth rate using weak acids: higher pka more HA HA can enter cell reduce growth rate
Fermentation: lactic acid lower pH HA can enter cell
Acid preservative: lover pH
Pathogenic bacteria can still grow, inactivate these with for example sterilization
WATER ACTIVITY (INTRINSIC) AND RELATIVE HUMIDITY (EXTRINSIC) (AW AND RH)
Aw: free water available
Aw = P (food) / Po (water)
Reduce aw by adding solutes (salt, sugar) or by removing
water (freezing,drying)
Local changes in aw
- Bulk commodities and packaged products:
Sun: T up, water evaporates
Sun down: T down, water condensates in
package
- Effect accelerated by mo (for example: fungi
produce moisture allow other organisms to
grow faster)
Product reformulation:
- Health
- Real life example
- Effect new product formulation
- Effect of people
REDOX POTENTIAL/GASEOUS ATMOSPHERE
Redox potential (eh)= tendency of a medium to accept or donate electrons
Redox reactions: oxidant + H+ + ne reductant (en omgekeerde reactie)
Medium saturated with air: high eh (400mV)
Oxygen from atmosphere can acts as electron accepter, yielding reactive substances
O2 + 2H+ + e H2O2 (hydrogen peroxide)
O2 + e O2- (superoxide anion radical)
Aerobic organisms: most energy via oxidative phosphorylation with O 2 as terminal electron accepter
Enzymes disable toxic products of oxygen
SOD: 2O2- + 2H+ H2O2 + O2
Catalase: 2H2O2 2H2O + O2
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