Food Microbiology Exam 2

What range of pH do most microorganisms grow at?

6.6 - 7.5

What microorganisms are most acid tolerant?

Molds, yeasts, and LAB

What is the definition of low acid food based on?

Group 1 C. bot, pH 4.6 - 7.0

Is C. bot a concern in high acid foods?

No, pH < 4.6

Are fruits or vegetables more likely to have bacterial spoilage?

Vegetables

What happens to pH of beef during aging/tenderization?

Glycogen produces lactic acid, lowering the pH

Which muscle has more glycogen and lower pH? Rested or fatigued meat?

Rested

Do high protein foods have a higher or lower buffering capacity?

High buffering capacity

How does pH kill microbes?

Denature proteins important for cellular functions

Denature membrane and disturb regular nutrient transport

Change ionic strength of nutrients/organic acids and disturb proper nutrient transport

How do some microorganisms survive low pH?

Prevent H+ from entering, Pumping H+ as soon as it gets in, consume acids or produce alkaline compounds to neutralize acids

What water activity do spoilage bacteria grow at?

aw = 0.90

What water activity do spoilage molds grow at?

aw = 0.80

What water activity do spoilage yeasts grow at?

aw = 0.88

Which organism needs more water, gram (+) or gram (-)?

Gram (-)

What is the water activity required for growth of Staph. aureus?

aw = 0.86

What are the effects of low water activity of microorganisms?

Interfere w/ regular metabolic rxns, cells shrink as cytoplasm water leaks out, lose fluidity of cell membrane

What are the four determinants of O/R potential?

Characteristic O/R potential of original food, poising capacity, Oxygen tension of atmosphere, access of food to atmosphere

Do aerobes have a high or low Eh?

Aerobes require highly oxidized foods, (+) Eh

Do anaerobes have a high or low Eh?

Anaerobes require reduced condition, (-) Eh

A tomato soup that has a pH value at 4.5 is least likely to support the growth of the microorganism:

Group 1 C. bot

Low acid foods are defined as foods that have a pH value greater than 4.6. There are more food safety concerns and regulations for low acid foods as compared to high acid foods. The pH value of 4.6 was set because of the foodborne pathogen:

Group 1 C. bot

An aged cheese has a_w = 0.85. Which organism group will be able to grow?

Most spoilage molds

True or False? Microbes produce alkaline compounds to neutralize acidity in order to survive in low a_w food

True

The a_w > 0.85 is based on the minimum a_w for growth of this bacterium:

Staph. aureus

What is poising capacity?

The ability to resist the change of redox potential in a food

What are the two ways that the growth of microorganisms can lower Eh in foods?

Depletion of oxygen by aerobes, production of metabolic byproducts

What are the 5 factors affecting nutrient content?

Water, energy source, nitrogen source, vitamins, minerals

What is the order of growth for microorganisms in terms of water?

Molds, yeasts, gram (+), gram (-)

What vitamin do gram (+) bacteria require for growth?

Vitamin B

What is the antimicrobial compound in cruciferous plants?

Isothiocyanates

What are the antimicrobial compounds in tea?

Catechin in oolong and tannin in green tea

What are the three antimicrobial compounds in milk?

lactoferrin, lysozyme, lactoperoxidase

What is the antimicrobial compound in eggs?

lysozyme

What is the optimum growth temp. of psychrotrophs?

20-30 C, but grow at <7 C

Are yeasts thermophilic?

No, but they are psychrotrophic and mesophilic

True or False? The gas environment will impact Eh of foods

True

What are the 4 ways to protect food through chemicals?

Preservatives, sanitizers, indirect antimicrobials, acetic and lactic acids

What is benzoic acid?

1st FDA approved preservative, inhibits yeast, mold, and some bacteria. Used in low pH foods

What is sorbic acid?

Inhibits molds, yeasts, and many bacteria. Used in low pH food

What does propionic acid do?

Primarily inhibits molds

How do the acid food preservatives work?

weaken membrane potential

What foods are sulfites allowed and not allowed in?

Allowed in dried fruits and wine, not allowed in meats or foods recognized as a source of Thiamine

What does it mean for sulfites to be dual functional?

Antimicrobial and reducing agent

What are nitrites/nitrates used for?

Curing meat and fish products

Psychrotophs are an important food safety concern because they:

grow at both refrigeration and human body temperature

True or False? Vacuum packed foods are at high risk of spoilage by yeasts and molds

False

What intrinsic factor is the gas environment related to?

Redox potential, Eh

What intrinsic factor is relative humidity related to?

Water activity

What is the carcinogenic compound produced as a result of using sodium nitrite in food?

Nitrosamine

What is Thermal Death Time (TDT)?

time required to kill a given number of organisms at a specified temperature

What is the Decimal Reduction Time (D-Value)?

Time required to destroy 90% of the organisms at a given temperature

What is the Z-Value?

Degrees of temperature required to reduce 1 log of D-Value

What are the two high temperature control methods?

Pasteurization and sterilization

What is sterilization?

Destruction of all viable organisms

What foods require the 12-D concept?

low acid foods

What are the two low temperature control methods?

Freezing and refrigeration

How does low temperature control the growth of microorganisms?

Refrigeration slows most cellular activities, stopped by freezing

What is the most common psychrotroph?

Pseudomonas

How do psychrotrophs/psychrophiles maintain activities at low temp?

Enzymes

What is Modified Atmospheric Packaging (MAP)?

change of the gaseous environment on and around foods

What type of bacteria favor MAP?

LAB

Which organism is more resistant to irradiation, spore-forming or non-spore-forming?

Spore forming

Which organism is more resistant to irradiation, gram (+) or gram (-)?

gram (+)

Which microorganisms are most resistant to HPP? (most resistant to least resistant)

gram (+), gram (-), molds/yeasts, parasites