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Food Microbiology > Spoilage - Microbial growth > Flashcards

Spoilage - Microbial growth Flashcards

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1
Q

What is the GT?

A

The time needed to double the population

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2
Q

What is the scale of the growth rate of a MO?

A

Log-linear

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3
Q

When k is used for growth rate, which scale is used?

A

Log-scale

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4
Q

When mu is used as growth rate, which scale is used?

A

ln-scale

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5
Q

Which mathematical equation is used to describe growth kinetics? Give log and ln scale

A

Log(N(t)) = Log(N0) + kt
Ln(N(t)) = Ln(N0) + mut

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6
Q

Explain what has the most effect on increasing shelf life?

A

Either decrease the N0 or decrease growth rate. Where the growth rate has the biggest effect on the shelf life.

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7
Q

Give a definition and four examples of an intrinsic factor.

A

Physico-chemical properties of the food.
- pH
- Nutrients
- Water activity
- Preservatives

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8
Q

Give a definition and three examples of an extrinsic factor.

A

Environment of the food.
- Temperature
- Relative humidity
- Gas composition

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9
Q

Give a definition and three examples of an implicit factor.

A

Characteristics of the MO
- Ability to compete with other MO present
- Ability to produce mycotoxins
- maximum specific growth rate

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10
Q

Give a definition and five examples of a processing factor.

A

These are actions to undertake in order to alter the food product (verbs!)
- Cooling
- Heating
- MAP
- Irradiation
- Slicing

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11
Q

MO have different enzymes in order to break down food products. On what type of food does Pectobacterium serratia act and how?

A

The bacterium acts on fruit, where pectolytic enzymes degrade the pectin structure and releases nutrients.

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12
Q

MO have different enzymes in order to break down food products. On what type of food does Aspergillus oryzae act and how?

A

The fungus acts on rice, where amylolytic enzymes act on starch to release nutrients.

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13
Q

MO have different enzymes in order to break down food products. On what type of food does Pseudomonas act and how?

A

The bacterium acts on milk, where the lipolytic enzymes act on lipids to release fatty acids (leads to rancidity)

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14
Q

MO have different enzymes in order to break down food products. On what type of food does Bacillus cereus act and how?

A

The bacterium acts on milk, where the proteolytic enzyme coagulates protein, releasing nutrients.

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15
Q

Give the three terms for pH preference, corresponding optimum, and MO

A
  1. Acidophile
    - 0< optimum pH < 5.5
    - Filamentous fungi, yeasts, LAB
  2. Neutrophile
    - 5.5 < optimum pH < 8.0
    - Bacteria
  3. Alkalophile
    - 8.0 < optimum pH < 11.5
    - Vibrio paraheamolyticus
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16
Q

Why is the ratio between [A-]/[HA] important?

A

Undissociated acids [HA] pass the membrane of MO more easily. Inside the cell it dissociates, and makes the cell more acidic. More H+. The MO wants to stabilize and pump out the H+, but this costs energy. So the MO will have less energy for growth. Thus it reduces growth.

17
Q

How can the weak acid principle be used in food preservation? Give two applications

A
  1. Fermentation: use LAB that produces weak acids and inhibit growth of pathogenic bacteria. (sauerkraut and salami)
  2. Acidic preservative: add acid to lower pH
18
Q

What are the pitfalls in pH in food preservation? Give two

A
  • Pathogenic bacteria can grow in low-acid food (pH >4.5) need to receive a stringent cook to inactivate pathogens.
  • Other MO can grow and increase pH:
    1. Yeasts and moulds dissimilate acid
    2. Acid-tolerant bacteria can grow, mostly proteolytic
    3. Pathogenic bacteria can grow after the pH has increased.
19
Q

What foods have a higher redox potential?

A

Foods that come into contact with air. As oxygen is an oxidant and accepts electrons, easily.

20
Q

What is the difference between aerobic and anaerobic MO? What enzymes play a role?

A

Where aerobic MO mostly get energy via oxidative phosphorylation with O2 as terminal electron acceptor, and superoxide dismutase and catalase disable toxic products of oxygen, anaerobic organisms do not have these enzymes and therefore cannot grow in the presence of O2.

21
Q

What are the five different terms in oxygen preference? Give an example for each

A
  1. Obligate aerobe: needs atmospheric O2 for growth –> Bacillus
  2. Microaerophile: requires O2 below 2-10% damaged by atmospheric O2 –> Campylobacter (can grow at atm. O2 when pyruvate is present)
  3. Facultative anaerobe: Do not require O2, grows better with O2 –> E. coli
  4. Aerotolerant anaerobe: grows equally well in presence or absence of O2 –> LAB (uses accumulation of manganese to destroy superoxide radical)
  5. Obligate anaerobe: Does not tolerate O2 and dies in its presence –> Clostridium
22
Q

What factors influence redox potential?

A
  • Food type (contact with air)
  • Milling, grinding –> Higher acces to air –> higher Eh
  • Low pH –> many H+ –> Higher Eh
  • Microbial growth consume O2 –> lower Eh
23
Q

What is the effect of CO2 on food spoilage?

A
  • CO2 dissolves in water, making carbonic acid which has a weak acid effect. But it also partly dissociates into bicarbonate and H+ which lowers the pH.
  • It has a larger effect when combined with O2 than without.
  • Larger effect at low temperatures: higher solubility CO2.
24
Q

What is the difference between MO in CO2 sensitivity?

A

Most sensitive are moulds, oxidative gram-negative bacteria.
Most resistant are gram-positive bacteria, especially some LAB and yeasts.

25
Q

What are three different applications in preservation with modifying air?

A
  • Vacuum packaging
  • Modified atmosphere packaging
  • Controlled atmosphere packaging
26
Q

Why is it not the best solution to vacuum pack fresh meat? What is then the best way?

A

As residual respiration occurs of the product and/or MO it consumes O2. When there is no O2 left, the residual blood will not be red, turning the product grey. –> MAP

27
Q

Why can the atmosphere within MAP still change after sealing?

A

The product and MO might use oxygen for respiration, and the packaging might be a little permeable to gasses.

28
Q

What is the defenition of water activity?

A

The amount of free water available that is not bound to food

29
Q

What are the differences between MO with regard to preference in water activity? Give four terms and examples for each one.

A
  1. Xerophillic: grow on dry foods, awmin = 0.6 –> Fungi as Aspergillus (dehydrated foods)
  2. Osmophilic: grow in high concentrations of unionized compounds (sugar), awmin = 0.62 –> Yeasts as Zygosaccharomyces baillii (jam)
  3. Halotolerant: Grow in salted products (tolerate elevated levels of NaCl) –> S. aureus (cooked meat products)
  4. Halophilic: requires high levels of NaCl usually abobe 0.2 M, to grow –> Vibrio parahaemolyticus (inshore water/ fish)
30
Q

How can you reduce water activity?

A

Adding salt, sugar, freezing, drying or a combination.

31
Q

What is a preservation pitfall when reducing water activity?

A
  • Lipid oxidation: spoilage due to lipid oxidation
  • Local changes in water activity: in bulk commodities, the temp might locally increase or drop, resulting in evaporation or condensation. The latter effect is accelerated by MO like fungi producing extra moisture, allowing other MO to grow as well.
  • Product reformulation for health reasons: reducing salt or sugar. Example: substituting sugar with aspartame, which does not reduce water activity.
32
Q

What is the difference between microbicidal and microbistatic effects?

A

Microbicidal effect will kill the MO, whereas microbistatic effect will only inhibit growth. (inhibition of growth is not equal to inactivation)

33
Q

What four preservatives are naturally present in milk?

A
  • Lactoperoxidase
  • Lysozyme (attacks peptidoglycan)
  • Lactoferrin (iron chelating)
  • Anitbodies
34
Q

What do the four natural preservatives in eggs do?

A
  • Lysozyme
  • Avidine (binds biotin)
  • Conalbumine (Iron)
  • Alkalic pH + eggshell barrier
35
Q

What are the five different terms in temperature preference between MO? Give the term the definition (Tmin, Tmax, Topt) and an example

A
  1. Psychrophile (cold loving)
    - Tmin = -5 to 5
    - Topt = 12-15
    - Tmax = 15-20
    - Moulds (frozen low water activity foods)
    - MO from cold-blooded organisms (fish)
  2. Psychrotolerant
    - Tmin = -5 to 5
    - Topt = 25-30
    - Tmax = 30-35
    - Pseudomonas (rich, wet, O2)
    - Lactobacilli (rich, wet, no O2)
  3. Mesophile
    - Tmin = 15-20
    - Topt = 30-40
    - Tmax = 40 -47
    - Microflora warm-blooded animals (man, cattle, poultry)
    - Entero’s as E. coli, Salmonella
  4. Thermophile
    - Tmin = 40-45
    - Topt = 55-75
    - Tmax = 60-90
    - Baccillus stearothermophilus
    - Desulfotomaculum nigrificans
    - Cl. thermisaccharolyticum
  5. Thermotolerant (thermoduric)
    withstand T >Tmax survive mild pasteurization
    - Vegetative: Microbacterium, Enterococcus
    - Sporeformers: Baccillus (O2), Clostridium (no O2)

Food Microbiology (7 decks)

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