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?

<p>Temperature of storage (A)</p> Signup and view all the answers

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

<p>Gas mixture (A)</p> Signup and view all the answers

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

<p>Below 3.5 (A)</p> Signup and view all the answers

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

<p>It spoils faster (C)</p> Signup and view all the answers

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

<p>It reflects resistance to pH change. (D)</p> Signup and view all the answers

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

<p>Time (D)</p> Signup and view all the answers

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

<p>It produces lactic acid that reduces pH (B)</p> Signup and view all the answers

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

<p>It broadens the pH growth range. (A)</p> Signup and view all the answers

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

<p>pH of 5.6 (C)</p> Signup and view all the answers

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

<p>Lower concentrations of acids than required can hinder growth. (B)</p> Signup and view all the answers

What is the relationship between moisture content and microbial growth?

<p>Lack of moisture prevents microorganisms from growing. (B)</p> Signup and view all the answers

What is termed as biological acidity in foods?

<p>Acidity due to microorganisms’ activity (A)</p> Signup and view all the answers

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

<p>1.00 (C)</p> Signup and view all the answers

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

<p>5.1-6.2 (C)</p> Signup and view all the answers

How does inherent acidity serve plants and animals?

<p>It protects tissues from microorganisms (B)</p> Signup and view all the answers

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

<p>Gram-negative bacteria (D)</p> Signup and view all the answers

What is the purpose of using solutes in food preservation?

<p>To tie up water and inhibit microbial growth. (C)</p> Signup and view all the answers

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

<p>Other intrinsic parameters at play (A)</p> Signup and view all the answers

Which organism is categorized as halotolerant?

<p>Staphylococcus (A)</p> Signup and view all the answers

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

<p>The ratio of solute to solvent. (B)</p> Signup and view all the answers

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

<p>Salt (D)</p> Signup and view all the answers

Which of the following describes buffered foods?

<p>They resist changes in pH. (B)</p> Signup and view all the answers

What is the effect of pH on enzymatic activity?

<p>Enzymes have specific pH ranges for optimal activity. (A)</p> Signup and view all the answers

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

<p>Clostridium acetobutylicum. (D)</p> Signup and view all the answers

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

<p>Nonionic compounds can enter cells more effectively. (D)</p> Signup and view all the answers

How can pH affect the morphology of microorganisms?

<p>It can alter the length of hyphae in certain fungi. (B)</p> Signup and view all the answers

What role do hydrogen ions play in the fermentation process?

<p>They favor fermentation. (A)</p> Signup and view all the answers

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

<p>It becomes more acidic. (C)</p> Signup and view all the answers

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

<p>It stimulates glucose metabolism. (B)</p> Signup and view all the answers

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

<p>Lysozyme (A)</p> Signup and view all the answers

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

<p>15 - 45 C (D)</p> Signup and view all the answers

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

<p>Psychrophiles (D)</p> Signup and view all the answers

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

<p>Too high RH can lead to condensation and spoilage. (C)</p> Signup and view all the answers

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

<p>Providing tough protection for seeds (B)</p> Signup and view all the answers

In which temperature range do thermophiles grow optimally?

<p>55 - 75 C (D)</p> Signup and view all the answers

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

<p>Loss of water and flaccidity (B)</p> Signup and view all the answers

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

<p>Thermophiles (B)</p> Signup and view all the answers

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

<p>The balance between oxidation and reduction reactions (A)</p> Signup and view all the answers

Which type of microorganism is primarily anaerobic?

<p>Obligate anaerobes (C)</p> Signup and view all the answers

What impact does low oxygen tension have on microbial growth?

<p>It inhibits the growth of strict aerobes. (B)</p> Signup and view all the answers

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

<p>Water (C)</p> Signup and view all the answers

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

<p>Sugar is more easily metabolized than amino acids. (B)</p> Signup and view all the answers

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

<p>Gram-positive bacteria (C)</p> Signup and view all the answers

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

<p>Reducing sugars (C)</p> Signup and view all the answers

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

<p>Molds are usually strict aerobes. (C)</p> Signup and view all the answers

Flashcards

Intrinsic Factors

Factors within the food itself that affect microbial growth.

pH

A measure of acidity or alkalinity in food.

Moisture Content

The amount of water available in food for microbial growth.

Extrinsic Factors

Environmental factors outside the food that affect microbial growth.

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Temperature of Storage

The temperature at which food is stored.

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Nutrient Content

The amount of nutrients available in food for microbial growth.

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Microbial Growth Requirements

Nutrients, favorable environment, time and absence of inhibitors are needed for microbes to grow.

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Favorable Environment

Conditions such as temperature, pH, gas and the absence of inhibitors that support microbial growth.

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Buffered foods

Foods that resist changes in pH.

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pH effect on microbial enzymatic activity

Changes in pH affect how enzymes in microorganisms work.

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pH effect on nutrient transport

pH changes the ability of nutrients to enter microbial cells.

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pH effect on microbial morphology

Changes in pH can change the shape of microorganisms.

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pH effect on microbial metabolism

Changes in pH can affect how microorganisms use nutrients.

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Acidic pH and nutrient uptake

At acidic pH, non-ionized compounds enter cells.

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pH and microbial membrane integrity

Extreme pH changes can damage the cell membranes.

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pH and temperature

Increasing Temperature, lowers the pH.

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Effect of salt concentration on microbial growth

Higher salt concentrations can affect the pH range where microbes grow and make younger cells more susceptible to toxins, while increasing the lag phase.

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Water activity (Aw)

Ratio of water vapor pressure of a food to the vapor pressure of pure water at the same temperature.

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Water activity (Aw) and microbial growth

Microbes need a certain amount of water activity to grow; different types of microbes need different levels of water activity.

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Effect of moisture content on food preservation

Removing moisture (desiccation) inhibits microbial growth, preserving food. Water activity (Aw) is crucial for microbial growth.

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Halophilic bacteria

Bacteria that tolerate or require high salt concentrations for growth

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Osmophilic yeasts

Yeasts that tolerate high sugar concentrations.

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Xerophilic molds

Molds that tolerate dry conditions.

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Preserving food using solutes

Adding solutes like salt, sugar, or glycerol to food ties up water, reducing microbial growth and extending shelf life.

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Food pH and Bacteria Growth

Foods with pH values below 3.5, like some fruits, are resistant to bacterial growth, while those with pH above 5.6, like some meats, are susceptible.

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Meat Spoilage & Rigor Mortis

Meat from fatigued animals spoils faster than meat from rested animals after death because of different pH levels at the end of rigor mortis.

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Fish Keeping Quality & pH

Fish with a lower pH (closer to 5.6) generally has better keeping qualities than fish with a higher pH (around 6.2 - 6.6).

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Biological Acidity in Foods

Some foods, like fermented milk, have acidity produced by microorganisms. This type of acidity is different from inherent acidity of plants.

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Fruits and Microorganisms

Fruits generally have a pH low enough to prevent the growth of many spoilage microorganisms, which is a way to protect the seeds.

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Animal Tissue pH and Microorganisms

The pH of living animal tissue is more favorable for spoilage organism growth than many fruits, other intrinsic factors are also at play.

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pH of Beef After Rigor Mortis

The typical range of pH levels for beef after rigor mortis is 5.1 to 6.2.

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Glycogen Conversion in Meat

Glycogen in the meat of well-rested animals converts to lactic acid after death, reducing the pH.

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Oxidation

The loss of electrons by an atom or molecule.

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Reduction

The gain of electrons by an atom or molecule.

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O-R Potential

A measure of the tendency of a substance to gain or lose electrons, reflecting the availability of free oxygen.

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Strict Anaerobe

A microbe that can only grow in the absence of oxygen.

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Strict Aerobe

A microbe that requires oxygen for growth.

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Facultative Anaerobe

A microbe that can grow in the presence or absence of oxygen.

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Microaerophile

A microbe that requires a low oxygen concentration for growth.

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Redox Potential of Food

The O-R potential of a food is influenced by factors like its chemical composition and exposure to oxygen.

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Eugenol

A natural inhibitor found in cloves that can prevent or reduce the growth of microorganisms.

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Lactoferrin

A protein found in milk that binds iron, making it unavailable for microbial growth.

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Lysozyme

An enzyme found in egg whites that breaks down bacterial cell walls.

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Organic Acids

Acids found in fruits, like citric acid, that can inhibit microbial growth.

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Hydrocinnamic Acid Derivatives

Naturally occurring compounds found in fruits that have antimicrobial properties.

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Protective Structures

Physical barriers on foods, like fruit skins and eggshells, that act as a first line of defense against microbes.

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Optimum Temperature

The temperature at which microbial growth is fastest.

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Psychrophiles

Microorganisms that thrive in cold temperatures (below 15°C).

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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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Factors Affecting Microbial Growth in Food PDF

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.

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