Spoilage - Microbial growth Flashcards
What is the GT?
The time needed to double the population
What is the scale of the growth rate of a MO?
Log-linear
When k is used for growth rate, which scale is used?
Log-scale
When mu is used as growth rate, which scale is used?
ln-scale
Which mathematical equation is used to describe growth kinetics? Give log and ln scale
Log(N(t)) = Log(N0) + kt
Ln(N(t)) = Ln(N0) + mut
Explain what has the most effect on increasing shelf life?
Either decrease the N0 or decrease growth rate. Where the growth rate has the biggest effect on the shelf life.
Give a definition and four examples of an intrinsic factor.
Physico-chemical properties of the food.
- pH
- Nutrients
- Water activity
- Preservatives
Give a definition and three examples of an extrinsic factor.
Environment of the food.
- Temperature
- Relative humidity
- Gas composition
Give a definition and three examples of an implicit factor.
Characteristics of the MO
- Ability to compete with other MO present
- Ability to produce mycotoxins
- maximum specific growth rate
Give a definition and five examples of a processing factor.
These are actions to undertake in order to alter the food product (verbs!)
- Cooling
- Heating
- MAP
- Irradiation
- Slicing
MO have different enzymes in order to break down food products. On what type of food does Pectobacterium serratia act and how?
The bacterium acts on fruit, where pectolytic enzymes degrade the pectin structure and releases nutrients.
MO have different enzymes in order to break down food products. On what type of food does Aspergillus oryzae act and how?
The fungus acts on rice, where amylolytic enzymes act on starch to release nutrients.
MO have different enzymes in order to break down food products. On what type of food does Pseudomonas act and how?
The bacterium acts on milk, where the lipolytic enzymes act on lipids to release fatty acids (leads to rancidity)
MO have different enzymes in order to break down food products. On what type of food does Bacillus cereus act and how?
The bacterium acts on milk, where the proteolytic enzyme coagulates protein, releasing nutrients.
Give the three terms for pH preference, corresponding optimum, and MO
- Acidophile
- 0< optimum pH < 5.5
- Filamentous fungi, yeasts, LAB - Neutrophile
- 5.5 < optimum pH < 8.0
- Bacteria - Alkalophile
- 8.0 < optimum pH < 11.5
- Vibrio paraheamolyticus
Why is the ratio between [A-]/[HA] important?
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.
How can the weak acid principle be used in food preservation? Give two applications
- Fermentation: use LAB that produces weak acids and inhibit growth of pathogenic bacteria. (sauerkraut and salami)
- Acidic preservative: add acid to lower pH
What are the pitfalls in pH in food preservation? Give two
- 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.
What foods have a higher redox potential?
Foods that come into contact with air. As oxygen is an oxidant and accepts electrons, easily.
What is the difference between aerobic and anaerobic MO? What enzymes play a role?
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.
What are the five different terms in oxygen preference? Give an example for each
- Obligate aerobe: needs atmospheric O2 for growth –> Bacillus
- Microaerophile: requires O2 below 2-10% damaged by atmospheric O2 –> Campylobacter (can grow at atm. O2 when pyruvate is present)
- Facultative anaerobe: Do not require O2, grows better with O2 –> E. coli
- Aerotolerant anaerobe: grows equally well in presence or absence of O2 –> LAB (uses accumulation of manganese to destroy superoxide radical)
- Obligate anaerobe: Does not tolerate O2 and dies in its presence –> Clostridium
What factors influence redox potential?
- 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
What is the effect of CO2 on food spoilage?
- 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.
What is the difference between MO in CO2 sensitivity?
Most sensitive are moulds, oxidative gram-negative bacteria.
Most resistant are gram-positive bacteria, especially some LAB and yeasts.
