Microbial Growth Factors Chapter 2

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Questions and Answers

What is the optimal pH range for most microorganisms to grow?

Below 4.0

Above 9.0

Around 7.0 (correct)

Around 5.5

Which of the following is considered an intrinsic factor affecting microbial growth?

Nutrient content (correct)

Relative humidity

Gaseous environment

Temperature of storage

What role do organic acids play in microbial growth?

They are more inhibitory than inorganic acids. (correct)

They enhance energy production.

They solely promote cell division.

They have negligible effects on enzyme activity.

Which parameter is NOT an intrinsic factor influencing microbial growth?

Temperature of storage Signup and view all the answers

What is an example of an extrinsic factor that can affect microbial growth in foods?

Gas mixture Signup and view all the answers

What is the pH range for fruits that generally undergo mold and yeast spoilage?

Below 3.5 Signup and view all the answers

How does the pH of meat from fatigued animals compare to rested animals?

It spoils faster Signup and view all the answers

How does the buffering capacity of food relate to microbial growth?

It reflects resistance to pH change. Signup and view all the answers

Which factor is crucial for the favorable environment for microbial growth?

Time Signup and view all the answers

What typically happens to glycogen in well-rested meat animals following death?

It produces lactic acid that reduces pH Signup and view all the answers

How does the addition of 0.2 M Sodium chloride affect the pH growth range of Alcaligenes?

It broadens the pH growth range. Signup and view all the answers

Which pH value is considered to provide better keeping qualities for fish?

pH of 5.6 Signup and view all the answers

Which of the following statements about acids in relation to microbial growth is true?

Lower concentrations of acids than required can hinder growth. Signup and view all the answers

What is the relationship between moisture content and microbial growth?

Lack of moisture prevents microorganisms from growing. Signup and view all the answers

What is termed as biological acidity in foods?

Acidity due to microorganisms’ activity Signup and view all the answers

What is the water activity (Aw) of pure water?

1.00 Signup and view all the answers

What is the typical pH range for beef post-rigor mortis?

5.1-6.2 Signup and view all the answers

How does inherent acidity serve plants and animals?

It protects tissues from microorganisms Signup and view all the answers

Which type of bacteria require higher values of Aw for growth?

Gram-negative bacteria Signup and view all the answers

What is the purpose of using solutes in food preservation?

To tie up water and inhibit microbial growth. Signup and view all the answers

What generally favors the growth of spoilage organisms in living animals?

Other intrinsic parameters at play Signup and view all the answers

Which organism is categorized as halotolerant?

Staphylococcus Signup and view all the answers

What is the vapor pressure of a solution (P) proportional to?

The ratio of solute to solvent. Signup and view all the answers

Which of the following compounds has the highest degree of inhibition against microbial growth?

Salt Signup and view all the answers

Which of the following describes buffered foods?

They resist changes in pH. Signup and view all the answers

What is the effect of pH on enzymatic activity?

Enzymes have specific pH ranges for optimal activity. Signup and view all the answers

Which bacteria is known to raise pH by reducing butyric acid?

Clostridium acetobutylicum. Signup and view all the answers

What happens to nutrient transport in bacterial cells at acidic pH?

Nonionic compounds can enter cells more effectively. Signup and view all the answers

How can pH affect the morphology of microorganisms?

It can alter the length of hyphae in certain fungi. Signup and view all the answers

What role do hydrogen ions play in the fermentation process?

They favor fermentation. Signup and view all the answers

What happens to the pH of a substrate as temperature increases?

It becomes more acidic. Signup and view all the answers

What effect does potassium ion concentration have on yeast metabolism in acidic medium?

It stimulates glucose metabolism. Signup and view all the answers

Which of the following is not a natural inhibitor found in milk?

Lysozyme Signup and view all the answers

What is the temperature range for optimal growth of most organisms?

15 - 45 C Signup and view all the answers

Which group of microorganisms can grow at temperatures below 0 C?

Psychrophiles Signup and view all the answers

Which of the following statements about relative humidity (RH) is true?

Too high RH can lead to condensation and spoilage. Signup and view all the answers

What is the primary function of protective structures such as the skin of fruits?

Providing tough protection for seeds Signup and view all the answers

In which temperature range do thermophiles grow optimally?

55 - 75 C Signup and view all the answers

What can be a consequence of storing fresh fruits and vegetables in an environment with low relative humidity?

Loss of water and flaccidity Signup and view all the answers

Which group of microorganisms is likely to thrive at temperatures above 45 C?

Thermophiles Signup and view all the answers

What defines the oxidation-reduction potential (Eh) in a biological context?

The balance between oxidation and reduction reactions Signup and view all the answers

Which type of microorganism is primarily anaerobic?

Obligate anaerobes Signup and view all the answers

What impact does low oxygen tension have on microbial growth?

It inhibits the growth of strict aerobes. Signup and view all the answers

Which of the following is NOT considered a nutrient for microorganisms?

Water Signup and view all the answers

How do sugar and amino acids compare as energy sources for microorganisms?

Sugar is more easily metabolized than amino acids. Signup and view all the answers

Which group of bacteria is generally found to require B vitamins for growth?

Gram-positive bacteria Signup and view all the answers

Natural reducing agents found in foods include which of the following?

Reducing sugars Signup and view all the answers

Which statement best describes the growth of molds based on their oxygen requirements?

Molds are usually strict aerobes. Signup and view all the answers

Study Notes

Chapter Two: Factors Affecting Microbial Growth in Foods

The chapter aims to explain intrinsic and extrinsic factors influencing microbial growth in food.

Objective

Students will be able to explain intrinsic and extrinsic factors affecting microbial growth in food.

Requirements for Microbial Growth

Nutrients: Carbohydrates, proteins, lipids, minerals, and vitamins are essential. Water, while not a nutrient, is vital for biochemical reactions.
Organic Growth Factors: Needed in varying degrees by different microorganisms (Gram+ > Gram- > yeasts > molds).
Energy Sources: Sugars, alcohols, amino acids, and complex molecules are important energy sources.
Nitrogen Sources: Amino acids are the primary nitrogen source, followed by proteins.
Vitamins: Bacteria often require B vitamins for proper growth.

Intrinsic Factors

pH: A measure of hydrogen ion concentration (-log[H+]).
- Most organisms grow best at around pH 7.0.
- Bacteria have the narrowest pH range, followed by yeasts, then molds.
- Pathogens have the narrowest pH range.
- Organic acids are more inhibitory than inorganic acids.
- Foods typically fall within an acid to neutral pH range (fruits → vegetables → meats → milk).
- Buffering capacity of food reflects its resistance to pH changes.
- Acidic conditions (below the minimum required for growth) will prevent energy production vital for maintaining internal pH, and affect enzyme activity and denature proteins, DNA, and other molecules, causing a longer lag time and slower growth.
Moisture Content (Water Activity - Aw): Defined as the ratio of water vapor pressure of the food substrate to the vapor pressure of pure water.
- Most fresh foods have Aw > 0.99.
- Bacteria need higher Aw values compared to fungi.
- Negative Gram bacteria have higher Aw requirements compared to positive Gram bacteria.
- Some spoilage bacteria do not grow below Aw 0.91.
- Spoilage molds can grow at Aw values as low as 0.8.
- Food poisoning bacteria (e.g., Staphylococcus aureus) can grow at Aw as low as 0.86.
- Clostridium botulinum does not grow at Aw below 0.94.
Oxidation-reduction potential (Eh): Reflects the availability of free oxygen.
- Low Eh = anaerobic (no oxygen); High Eh = aerobic (oxygen present).
- Microorganisms are grouped as aerobes, anaerobes, facultative anaerobes, or microaerophiles.
  - Molds are typically aerobic.
  - Yeasts are aerobic or facultative.
  - Bacteria range from strict anaerobes to strict aerobes.

Intrinsic Factors (continued)

Antimicrobial Constituents: Naturally occurring chemicals that hinder microbial growth (e.g., spices, milk components, eggs, fruits).
Biological Structures: Protective structures (primary defense) that some foods have, like the skin of fruits, shells of eggs and nuts.

Extrinsic Factors

Temperature: Microorganisms have growth and survival ranges relative to temperature.
- Optimum temperature for growth for many microorganisms is between 15-45°C.
- Different groups of microorganisms prefer different temperature ranges (psychrophiles, psychrotrophs, mesophiles, thermophiles, thermoduric). Important in food preservation.
Relative Humidity (RH): Measure of water activity in the gas phase.
- Low RH can lead to water loss and spoilage in foods. High RH can encourage spoilage by condensation.
Gases: Controlled/modified atmosphere storage (e.g., CO2 to retard fungal rot).

Summary

Microorganisms need nutrients, favorable conditions, and time for growth.
Factors impacting microbial growth in food:
- Intrinsic: pH, moisture content, Eh, antimicrobial constituents, and biological structures.
- Extrinsic: Storage temperature, relative humidity, and gases in the environment.

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Description

This quiz covers Chapter Two, which explains the intrinsic and extrinsic factors affecting microbial growth in foods. Students will learn about essential nutrients, growth factors, and the impact of pH and other environmental conditions on microorganisms. Prepare to delve into the biological requirements for microbial proliferation in food contexts.