Lecture 4- Factors that Influence Microbial Growth Flashcards
Temperature
The most important environmental factor affecting growth and survival of microorganisms
- when foods are exposed to temperatures beyond the max or min temperatures of growth, cells die
Redox potential
measures the potential energy difference in a system generated by a coupled reaction in which one substance is oxidized and a second substance is reduced
- the electron donor because it reduces the oxidized substance is called the reducing agent
What are the primary means of generating energy in microbes
oxidation and reduction of substances
What is redox potential in food influenced by?
chemical composition, processing treatments, and storage conditions
Aerobes
need free oxygen
anaerobes
can only transfer electrons through fermentation, may die in even small amounts of oxygen
- they lack superoxide dismutase necessary to scavenge toxic oxygen free radicals
Facultative anaerobes
can use free oxygen, bound oxygen (CO2, NO3, SO4)
Microaerophiles
Grow better in the presence of less oxygen
What is the most important anaerobe?
Clostiridium
When oxygen is present, other organisms outcompete Clostridium or they make organic acids that limit clostridium’s growth
- when oxygen is not present Clostridium can compete for growth and result in several effects (meat spoilage, fatal foodborne illness)
Controlled Atmosphere Packaging (CAP)
atmosphere in a storage facility is altered and the levels of the gases are continually monitored and adjusted as required. This is expensive and is generally only used for the long-term storage for fruits and vegetables to maintain their freshness
Modified Atmosphere Packaging (MAP)
this method, unlike CAP, does not require a high degree of control during the entire storage period. In this method a food is enclosed in a high gas barrier packaging material; the air is removed from the package, which is then flushed with a particular gas or combination of gases and the package is hermetically sealed.
- gas flushing MAP is used to extend the life of other products such as fresh pasta, bakery products, cooked egg products, and fresh seafood
Vacuum Packaging (V)
This method involves the removal of air from the package and then sealing the package hermetically
- used in the preparation of ready-to-eat meat
pH of bacteria
yeast and mold can grow at lower pH than bacteria, gram positive bacteria grow at lower pH than gram negative
- most bacteria have a pH range of 2-3 units
Correlation between pka and %dissociation
the more positive the pKa, the less dissociation the acid is
Acetic acid
used as vinegar; very effective against bacteria, not so effective against yeast
- inhibitory action is through neutralizing the electrochemical gradient of the cell membrane as as well as denaturing PRO inside the cell walls
- recommended as a carcass wash to reduce bacterial loads
Propionic acid
used as salts of Ca in bread, cheeses, jams, jellies, and tomato purees
- effective against molds nad bacteria but not against yeast
- antimicrobial action is through acidification of the cytoplasm as well as destabilization of the membrane proton gradient
Lactic acid
used as acid or Na or K salts
- can be found in carbonated drinks, salad dressing, pickled veggies
- effective against bacteria but not yeast or mold
- neutralizes the proton gradient
Citric Acid
- used as an acid in drinks, jams, jellies, baking products, cheese, canned veggies, and sauces
- it’s antibacterial activity is unclear
Sorbic acid
- used as an acid, salts of Na, K, or Ca
- used in alcoholic and non-alcoholic drinks, processed fruits and veggies, dairy desserts, confectioneries, mayonnaise, salad dressings, spreads, and mustard
- more effective against molds and yeast than bacteria
- inhibits the function of enzymes, the synthesis of the cell wall, PRO, RNA and DNA
Benzoic Acid
- acid or as a Na salt
- inhibits the action of enzymes needed for oxidative phosphorylation
Water activity
water availability is generally expressed in terms of water activity (aw) where 0 is completely dry and 1 is pure water
Osmophiles
organisms able to live in environments which are high in sugar
Xerophiles
organisms able to live in very dry environments
What are the 3 methods for removing water from food?
- Dehydration
- Crystallization
- Adding solutes
Hurdle Concept
-Most of the factors that cause detriment to microbial growth are enough to preserve food, however, consumers want less harshly processed foods–> applying two or more methods of controlling microbial growth at lower levels can reduce growth, prevent growth, or kill microorganisms
