Food Microbiology Learning outcomes

What is Food Microbiology

Food microbiology is a study of the micro-organisms that cause loss of food quality and safety, as well as those organisms that help to preserve the food and keep other organisms away.

Groups of microorganisms involved in food spoilage and food safety

1. Viruses

2. Parasites

3. Fungi

4. Bacteria

Viruses

• Reproduces in living cells

  • They need a host (e.g. human body)

  • A food is not a host → hence viruses don’t multiply in foods

  • And hence viruses are not involved in spoilage.

• 1-10 particles can cause disease

• 70% of the incidents caused by infected employees

Parasites

• Also don’t grow in food: because they need a living host

• There are two types of parasites

  • Protozoa

    • Are unicellular

    • Sources: water, meat, pets

  • Worms

    • Are multicellular

    • Sources: meat, fish, tape worm, herring worm

• Example: Herring worm (a parasite) is killed by being frozen → no herring you eat is fresh, it has always been defrosted.

Fungi types

1. Unicellular: Yeast

2. Multicellular: Mold

What are mycelium

the vegetative part of a fungus, consisting of a network of fine white filaments (hyphae).

They are branch like structures

What are hypha

each of the branching filaments that make up the mycelium of a fungus.

Bacteria

• >> 1000 species

• Classification

  • Morphology (what do they look like)

  • Cell membrane

  • Enzymes (differs per bacteria)

  • Growth conditions (oxygen, temperature)

  • Biochemical

Classification of bacteria

• Morphology (shape)

• Gram staining

• Enzyme activity

Sources of contamination (primary)

• Primary: Raw materials

  • Soil

  • Manure

  • Water

  • Air

  • Animals

  • Slaughter

Sources of contamination (Secondary)

• Secondary: Processing and food preparation

  • Water: process, cleaning

  • Air: Aerosols, dust

  • People

  • Vermin: Rodents, birds, insects

  • Package

  • Routing

What is routing?

• Routing describes how for example raw materials which are usually contaminated will be located very far from the packaging machine.

• Hence there are special routes throughout the factory to prevent contamination.

Factors affecting microbial growth

• Product characteristics (intrinsic)

• External factors (extrinsic)

• Interactions between microorganisms (implicit)

• Process factors (heating, cooling, etc.)

Spoilage level

• Spoilage level: 10⁷ /ml

  • Log N/ml = 7

Maximum bacteria concentration

• Maximum: 10⁹ / ml

  • Log N/ml=9

factors affecting microbial growth: Product characteristics (intrinsic)

• Structure & nutrients

• pH

• A_w : Water activity

factors affecting microbial growth: Product characteristics (structure & nutrients)

• Cutting a vegetable exposes nutrients. These also become availabel to microorganisms.

• Hence microbial growth occurs.

factors affecting microbial growth: Product characteristics (pH)

pH	Range	pHopt
Bacteria	4-9	6-8
Yeast	1.5-8	4.5-6
Molds	1-11	3.5-4

factors affecting microbial growth: water activity

• Measure of the amount of free (or non-chemically bound) water in a product

• Where ERH = equilibrium relative humidity

• The a_w of a food can be measured by putting the food in a container and measuring its ERH.

Know terms not numberss	Minimum aw
Halophiles (mainly bacteria)	0.75
Xerophiles (mainly bacteria)	0.60
Osmophiles (mainly yeasts)	0.60

Factors affecting microbial growth: External factors (extrinsic)

• Temperature

• MAP

• Relative humidity

Extrinsic factors to microbial growth - Temperature

• <0^oC, no microbial growth

• 0-15^oC, refrigerator, start growing

• 15-35^oC, room temp, increases microbial growth rate a lot more than in fridge

• 35-65^oC, microbes start to die

• >65^oC, inactivation most microorganisms

Extrinsic factors to microbial growth - MAP

• Vacuum packaging

• Gas packaging

  • Air replaced with a mixture -> CO₂, N₂, O₂

  • Composition changes because of product activity

• For example normally meat will turn slightly brown, customers don't like this so producers add extra oxygen because it prevents the browning.

Gasses used in MAP

• CO₂

  • Very efficient at inhibiting bacterial growth (mainly gram-, molds)

  • Dissolution in water - carbonic acid

  • CO₂ + H₂O -> H₂CO₃ -> H⁺ + HCO₃^-

    • The acid released also helps to prevent microbial growth

• N₂

  • Non inhibitory

• O₂

  • Maintains the bright red appearance of meat

  • Fat oxidation

  • 'Respiring' products: during storage: O₂ goes down, CO₂ goes up

Packaging without oxygen advantages and disadvantages and hazards

• Packaging without oxygen

  • Advantages

    • Growth inhibition of aerobes (often spoilage organisms; psychrotrophic Gram-, moulds)

    • Increase of shelf life

  • Disadvantages

    • NO growth inhibition of (facultative) anaerobes (pathogens)

    • Growth of lactic acid bacteria

  • Hazard

    • The reduced oxygen in these packages can allow the growth of bacteria called Clostridium botulinum, which produces a deadly toxin

    • Clostridium botulinum -> will prevent muscles from moving

D-Value (extrinsic factor)

• Time needed to reduce the concentration of microorganisms with a factor 10

  • How long should you heat it for the conc. of microbes to decrease. D-value is temp specific

  • Higher temp lowers D-value

  • Low value pH makes graph more steep (less time needed)

  • Low A_w makes graph less steep (more time needed) -> Low water activity leads to increased heat resistance in microorganisms. Microbes are less sensitive to heat when the surrounding environment has reduced water activity

How interaction between microorganisms can cause spoilage of food. (implicit)

• Physiological characteristics of microorganisms

• They can also influence each other positively/negatively

  • Negatively: Antagonism:

    • Some microorganisms produce antimicrobial substances that inhibit the growth of others.

    • E.g. Lactic acid produces lactic acid which creates an environment that prevents the growth of competing microbes leading to spoilage by the dominant species.

  • Positively: Synergy

    • Can promote spoilage by enhancing each other’s surivival

    • One microbe might produce essential nutrients that another microorganism requires or degrade antimicrobial components, allowing a wider range of spoilage organisms to thrive.

Formula for exponential growth of bacteria

N_t=N₀ × 2ⁿ

N_t=N₀ × 2^t/(GT)

N_t = Number of cells at time t

N₀= Initial number of cells (t=0)

n = number of generations

GT = generation time

t = time

Formula for concentration of microorganisms

Log (C) = Log (n x 1/d x 1/v)

n= colony count per plate

d= dilution (expressed as 10^-d)

v = volume of diluted sample

Fungi (unicellular)

1. Unicellular: Yeast

   • Much bigger than bacteria

   • Shape is similar to bacteria; has an oval shape.

   • Multiplies by budding (asexual) or by ascospores (sexual)

   • Ferment sugars (produces ethanol & CO2)

Fungi (multicellular)

2. Multicellular: molds

• They start growing before you can see them by human eye.

• Mold have frets that are branch like structures

  • Some have septa which are internal cross walls and some don’t have septa.

• If you have a lot of branching it is called mycelium.

• The scientific term for the fret is hypha

• Hypha is 3 micrometers

• Forms spores.

• The inner part of the mycelium is older and often have darker colors and have more spores.

  • When you touch the spores they can spread everywhere in the air.

Naming bacteria

• Genus, species, strain: Bacillus cereus ATCC 14579

• Genus should be written with a capital and in italic, species in italic

• ATCC stands for American Type Culture Collection

Classification of bacteria (gram staining)

• Gram-positive: will become purple, thick outer layer only has peptidoglycan (polysaccharide)

  • Survive dry environments better

• Gram-negative: will become pink, thin in-between layer of peptidoglycan and has outer membrane.

  • Are more sensitive

Classification of bacteria (enzyme activity)

• Oxidase test (reduce molecular oxygen to yield water and generate energy conservation): If it has oxidase it will become purple

• Catalase test (breaks down H2O2): If it has catalase, the solution will bubble

Four scenarios in contamination of microorganisms.

1. Growth - Occurs when water and nutrients are available, temp and pH are suitable. Very dependent on availability of oxygen.

2. Inactivation - When microorganisms are in a bad environment e.g. low pH.

3. Dormany - When they do not grow nor die.

4. Lethal stimulation - When bacterial spores germinate in a favorable environment in the presence of a poisonous matter for the dividing cells then death will occur.

Growth curve

What is generation time?

Generation time (τ or GT) is the average time between two consecutive generations. For microorganisms, the generation time is also called doubling time.

The reason we use log concentrations for microbes concentration

How to find the maximum growth rate

The slope of the exponential growth phase is the maximum growth rate k (log₁₀ h^-1)

How to calculate the concentration (log₁₀(C)) at a certain time point in the exponential growth phase (t in hours)

log₁₀(C_t) = log₁₀(C₀) + k ∙ t

where log₁₀(C₀) is the initial concentration (at t = 0).

How is the maximum growth rate commonly expressed?

on natural logarithmic scale (ln-scale) instead of log₁₀-scale

In that case, the growth rate is referred to as µ_max  (h^-1).

Relation between maximum growth in K vs in µ_max

maximum specific growth rate

τ is the doubling time (the time it takes for the population to double in size), usually in hours.

Serial dilutions

The concept of serial dilutions has been introduced to obtain plates with a countable number of colonies. The range between 30 and 300 colonies per plate can be considered as optimal for counting. If the number of colonies is below 30, the estimation of the number of microorganisms in the original food sample will become less accurate. If the number of colonies is above 300, the growth of the colonies will be restricted and the colonies may tend to merge.