Lecture 4: Factors that influence microbial growth

Question 1

what parameters can be used alone or in combination to limit bacterial growth in a product

Answer 1

1. temp
2. redox potential
3. pH
4. water activity

Question 2

how can temperature affect the growth of bacteria

Answer 2

bacterial growth and survival is dependant on temperature. microbial growth is accomplished by enzymatic reactions (catalytic rate). When temperature is at a minimum mebrane gelling and transport processes are so slow that growth cannot occur but as temp increase enzymatic rxns occur at faster rates up until they reach a temp optimum. once passed this optimum protein denaturation, collapse of the cytoplasmic membrane and thermal lysis occurs.

Question 3

___ are responsible for growth and food spoilage at refrigeration temperatures but grow much faster at room temp

Answer 3

psychrotrophs

Question 4

what is redox potential?

Answer 4

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

Question 5

what is a reducing agent?

Answer 5

The electron donor because it reduces the oxidized substance is called the reducing agent

Question 6

what is the primary means of generating energy in microbes?

Answer 6

oxidation and reduction

Question 7

___ acts as a terminal electron acceptor which is a high energy reaction

Answer 7

free oxygen

Question 8

redox potential in food is influenced by

Answer 8

1. its chemical composition
2. processing treatments
3. storage condition

Question 9

explain the redox potential of different microorganisms

Answer 9

Aerobes: Need free oxygen (O2)
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, or SO4)
Microaerophiles: grow better in the presence of less oxygen

Question 10

most important anaerobe in food is

Answer 10

clostridium

Question 11

when oxygen isnt present clostridium can compete for growth and result in what effects?

Answer 11

Clostridium laramie can cause meat putrification

Clostridium botulinum can cause a fatal foodborne illness

Question 12

what are the three terminologies used to alter the atmosphere in foods in order to preserve and increase their acceptance quality?

Answer 12

1. controlled atmosphere packaging
2. modified atmosphere packaging (map)
3. vaccuum packaging

Question 13

Controlled Atmosphere Packaging (CAP):

Answer 13

The 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.

Question 14

Modified Atmosphere Packaging (MAP):

Answer 14

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.

Question 15

Vacuum Packaging (VP):

Answer 15

This method involves the removal of air from the package and then sealing the package hermetically.

Question 16

Benefits of modified atmosphere packaging?

Answer 16

• can control or reduce the growth of undesirable microorganisms in food
• It also retards enzymatic and respiratory activities of fresh foods.
• Aerobes can no longer grow in products that are vacuum packaged or flushed with 100% CO2, 100% N2, or a combination.
• However, under these conditions anaerobes can grow, and other techniques must be used to control their growth.
• This greatly extends shelf-life

Question 17

why is VP used in the prep of ready to eat meats but not fresh meat?

Answer 17

red meat turns purple due to reduced myoglobin so this is not used on fresh red-meat

Question 18

Gas flushing (Map) is used to extend the shelf life of other products like

Answer 18

as fresh pasta, bakery products, cooked egg products, and fresh seafood

Question 19

only few bacteria can grow at pH of less than __ or greater than ___

Answer 19

2,10

Question 20

T or F: yeasts and molds can grow at lower pH than bacteria

Answer 20

True

Question 21

T or F: Gram positive bacteria grow at higher pH than gram negative

Answer 21

False (higher)

Question 22

acids commonly found in food are

Answer 22

1. acetic
2. propionic
3. lactic
4. citric
5. sorbic
6. benzoic
7. paraben

Question 23

why are acids that have lower dissociation rates ie weak acids problematic for bacteria?

Answer 23

Undissociated molecules are lipophilic, they enter into the cell and dissociate to generate H+ in the cytoplasm
This causes an increase in cellular internal pH which destroys the proton gradient, and thus the ability of the cells to generate energy

Question 24

use of acetic acid in food

Answer 24

• Is usually used as vinegar (5-10% acetic acid), or as sodium or calcium salts at >25% in pickles or salad dressings. Very effective against bacteria, not so effective against yeast. The inhibitory action is through neutralizing the electrochemical gradient of the cell membrane as well as denaturing proteins inside the cell walls. Acetic acid has been recommended as a carcass wash to reduce bacterial loads.

Question 25

use of propionic acid in food

Answer 25

– Is used as salts of calcium or sodium (0.1-0.2%) in bread, cheeses, jams, jellies, and tomato puree. It is effective against molds and bacteria but not against yeast. The antimicrobial action is through acidification of the cytoplasm as well as destabilization of the membrane proton gradients

Question 26

use of lactic acid in food

Answer 26

Is used as acid or as sodium or potassium salts up to 2%. It can be found in carbonated drinks, salad dressing, and pickled vegetables. It is effective against bacteria but not yeast or mold. It neutralizes the proton gradient. Lactic acid producing bacteria are covered heavily in FDSC442.

Question 27

use of citric acid in food

Answer 27

• Is used as an acid >1% in drinks, jams, jellies, baking products, cheeses, canned vegetables, and sauces. Its antibacterial activity is unclear.

Question 28

use of sorbic acid in food

Answer 28

Used as either an acid, or as salts of sodium, potassium, or calcium. It is used in alcoholic and non-alcoholic drinks, processed fruits and vegetables, dairy desserts, confectioneries, mayonnaise, salad dressings, spreads, and mustards. It is more effective against molds and yeasts than bacteria. It inhibits the function of enzymes, the synthesis of the cell wall, protein, RNA and DNA.

Question 29

use of benzoic acid in food

Answer 29

Used as an acid or as a sodium salt such as in alcoholic and non-alcoholic drinks, processed fruits and vegetables, dairy desserts, confectioneries, mayonnaise, salad dressings, spreads, and mustard. It inhibits the action of enzymes needed for oxidative phosphorylation.

Question 30

state the relative effectivity of natural acids

Answer 30

Acetic Acid >Propionic Acid >Sorbic Acid, and Benzoic Acid> Lactic Acid > Citric acid

Question 31

what is osmosis

Answer 31

water moves from an area of high water concentration to an area of low water concentration

Question 32

in an area of low water activity, water flows ___ of the cell

Answer 32

out

Question 33

osmophiles

Answer 33

organisms able to live in environments which are high in sugar

Question 34

xerophiles

Answer 34

organisms able to live in vert dry environments

Question 35

what are the 3 methods of removing water from food and explain them briefly

Answer 35

1) Dehydration
Dried seeds, grains, tubers and fruit resist
Roller drying such as powdered milk, instant coffee or meat jerky
Freeze drying such as camping meals, coffee
Smoked salmon the heat kills many microorganisms, the survivors are controlled by low Aw, and there are many antimicrobial substances in the smoke
2) Crystallization
Crystal candies
3) Adding solutes (salt, sugar, honey, starch)
Salami, jams, jellies, honey, maple syrup, soft granola bars

Question 36

what is the hurdle concept?

Answer 36

Applying two or more methods of controlling microbial growth at lower levels can reduce growth, prevent growth, or kill microorganisms. Instead of setting one parameter to an extreme limit, hurdle technology deoptimizes a variety of factors, so that lower concentrations of each are needed to inhibit bacterial growth
Hurdle technology assaults multiple homeostatic processes; and is most effective when it combines two stressors that act by different mechanisms

Question 37

Hurdle effects can be ___ or ___. Explain

Answer 37

1. additive or synergistic

-The MIC of hurdle A and B are plotted on the x and y axes, and a line is drawn to connect the points
Experiments are conducted to determine the MIC of A and B in various concentrations.
If the points fall on the line the effects are additive.
If the points fall below the line they are synergistic
If the points fall above the line they are antagonistic.