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Growth Kinetics

- Know the bacterial growth curve
- Know the growth kinetics equation

What are intrinsic factors?

*Factors inherent to the food
- pH
- Available Water
- Oxidation-Reduction Potential
- Nutrients
- Antimicrobial Constituents
- Biological Structures

(1) pH

- Low pH will select for fungi
- Neutral pH will promote bacterial growth
- pH Homeostasis

(2) Water Activity

- partial pressure of water above the material / partial pressure of pure water at the same temperature
- equal to the equilibrium relative humidity (ERH), expressed as a fraction
- the higher the water activity, the more likely there will be microbial growth
- the lower the water activity, the less likely there will be microbial growth
- by lowering the water activity, we are increasing the length of the lag phase of growth and decreasing the final population

(3) Oxidation-Reduction Potential (Eh)

- substrate is OXIDIZED when it GAINS an electron
- substrate is REDUCED when it LOSES an electron
- substance that gives up electrons is the reducing agent
- substance that takes up electrons is the oxidizing agent
- when electrons are transferred from one compound to another, potential difference is created
- what affects this? poising capacity, oxygen tension, access to the atmosphere

What are the main types of microorganism?

(1) Aerobes
(2) Anaerobes
(3) Microaerophiles
(4) Facultative Anaerobes

Oxygen-loving microbes; can't live without O2; high positive Eh needed

- Bacillus, mold, yeast

Oxygen-hating microbes; can't live with O2; low negative Eh needed

- Clostridium

Requires oxygen to survive, but requires environments containing lower levels of oxygen

- Lactobacilli, Campylobacter

Able to grow in presence/absence of O2; Capable of producing energy through aerobic respiration and then switching back to anaerobic respiration

Facultative Anaerobe
- E. coli

(4) Nutrient Content

In odrer to grow and function, microbes require:
- Water
- Energy Source (AA, sugars, alcohols)
- Nitrogen Source (AA)
- Vitamins/Growth Factors
- Minerals

(5) Antimicrobial Constituents

Examples of these include:
- Plants have essential oils (cloves, allicin in garlic, eugenol in cinnamon, thymol in oregano)
- Cow's milk contains lactoferrin and lactoperoxidase
- Eggs contain lysozyme

(6) Biological Structures

Some foods provide protection against the entry of microbes, such as:
- Shell of nuts
- Ring of melons
- Hide of animals
- Shell of eggs
- Skin of fish and meats

What are extrinsic factors?

*Environmental factors
- Time
- Temperature
- Relative Humidity
- Presence/Conc. of Gases
- Presence/Conc. of Other Microbes

(1) Temperature - What are the types of microbes?

(1) Psychrophiles
(2) Psychrotrophs
(3) Mesophiles
(4) Thermophiles

Are capable of growth and reproduction in cold temperatures (Min, Optimum, Max?)

Psychrophiles (0, 15, 25)
- Flavobacterium

Rre capable of surviving or even thriving in a cold environment but have a optimum near a value of the mesophiles (Min, Optimum, Max?)

Psychrotrophs (0, 25, 40)
- Listeria

Grows best in moderate temperature, neither too hot nor too cold (Min, Optimum, Max?)

Mesophiles (10, 35, 45)
- Salmonella

Thrives at relatively high temperatures (Min, Optimum, Max?)

Thermophiles (45, 55, 70)
- Thermus

(2) Relative Humidity

- Affects the aw within food and growth of microorganism at surfaces
- When low aw valued foods are placed in a high humidity environment, the food picks up moisture until equilibrium (vice versa)
- The higher the T, the lower the RH

(3) Presence/Conc. of Gases

- CO2 and O2 most important with control of microbes; used with atmospheric packaging
- O3 ozone has been used for antimircobial activity

(4) Presence/Conc. of Other Microbes

Some organisms can produce substances that inhibit/are lethal to other microbes; these include:
- Antiobiotics
- Bacteriocins
- Hydrogen Peroxide
- Organic Acids

What is the Hurdle Concept?

- Aims to eliminate/inhibit microbial growth
- Idea is to to use multiple factors or normal level/attack to maintain healthy food and control the growth of microbes
- Using two factors is better than one extreme factor

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