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Food/Dairy Powerpoint #2 - Exam #1

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