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Food Microbiology MCQs Solutions – Boost Your Knowledge with Expert Answers

Welcome to our Food Microbiology MCQs solutions (Multiple Choice Questions) Solutions post! In this quiz post, we will provide you with expert answers and explanations to enhance your understanding of food microbiology concepts. WC Frazier, a renowned book in the field, has been compiled in a comprehensive set of MCQs to challenge your knowledge. So, let’s dive in and boost your understanding of food microbiology!


pH: pH is measured on a scale of 0 to 14, with 0 representing high acidity, 7 representing neutrality, and 14 representing high alkalinity. Microorganisms typically thrive within a pH range of 6.0 to 8.0, which is optimal for their growth. However, acid-tolerant bacteria like Lactobacillus acidophilus can survive in environments with pH values as low as 4.0. On the other hand, alkaliphilic bacteria can flourish in alkaline conditions with pH values above 9.0.

Moisture and Water Activity (aw): Moisture content is expressed as a percentage and indicates the amount of water present in a substance. Water activity (aw) is measured on a scale from 0 to 1, where 0 signifies the absence of available water, and 1 represents pure water. Different microorganisms have varying moisture requirements for growth. Bacteria typically require a minimum moisture content of around 15-20%, while yeasts and molds can grow at lower moisture levels. Foods with water activity below 0.6 are generally considered safe from microbial growth.

Oxidation-Reduction Potential (O-R Potential): The O-R potential is measured in millivolts (mV) and indicates the tendency of a substance to gain or lose electrons during chemical reactions. Microorganisms have distinct O-R potential requirements for growth. Aerobic bacteria require positive O-R potential values, typically above +300 mV, as they rely on oxygen for respiration. Anaerobic bacteria, on the other hand, prefer negative O-R potential values, generally below -100 mV, as they cannot tolerate oxygen. Facultative microorganisms can survive in a wide range of O-R potential conditions.

Food Quality and Safety: Maintaining appropriate pH levels, moisture content, and O-R potential is crucial for ensuring food quality and safety. For instance, to prevent the growth of spoilage bacteria, acidic foods such as pickles should have a pH level below 4.6. Controlling water activity is essential to prevent microbial growth in various food products. Dehydrated foods like jerky, for example, should have a water activity level below 0.85 to inhibit microbial growth. The management of O-R potential is important for preserving specific food characteristics. In the production of wine, for instance, maintaining a positive O-R potential helps prevent undesirable microbial fermentation.



#1. Nisin is produced by strains of

Nisin is a naturally occurring antimicrobial peptide that is primarily produced by strains of Streptococcus lactis. It is a polycyclic peptide that consists of 34 amino acid residues. Nisin has potent antibacterial properties, particularly against Gram-positive bacteria, including certain foodborne pathogens like Listeria monocytogenes and Staphylococcus aureus.

Streptococcus lactis, a lactic acid bacteria, is responsible for the production of nisin through a fermentation process. During fermentation, Streptococcus lactis synthesizes and secretes nisin into the surrounding environment. Nisin works by targeting and disrupting the bacterial cell membranes, leading to cell death.

Due to its antimicrobial properties and safety profile, nisin has found various applications in the food industry. It is commonly used as a food preservative to inhibit the growth of spoilage and pathogenic bacteria, thereby extending the shelf life of products. Nisin is also utilized in the production of certain dairy products, such as cheese and yogurt, to prevent the growth of unwanted bacteria and improve product quality.

The production of nisin is specific to strains of Streptococcus lactis, and it is not naturally produced by other bacteria such as Pseudomonas, E. coli, or Clostridium.

#2. The inner parts of whole healthy tissues of living plants and animals are

The correct answer is: d. both a & b

The inner parts of whole healthy tissues of living plants and animals can have both characteristics. They are generally low in microbial content and can also be considered sterile. The term “sterile” implies the absence of any viable microorganisms, while “low in microbial content” suggests that there may be some presence of microbes, but in very small quantities. Therefore, the correct answer is option d, which indicates that these tissues can exhibit both characteristics simultaneously.

#3. Pseudomonas grows well in food containing

Pseudomonas bacteria are capable of utilizing nitrates as a source of energy and can grow well in food that contains nitrates.

Bacteria, including Pseudomonas spp., have diverse nitrogen requirements that can be met by different compounds. Simple compounds like ammonia or nitrates can fulfill the nitrogen needs of bacteria like Pseudomonas spp. These bacteria have the ability to use ammonia or nitrates as nitrogen sources for growth and metabolism.

However, some bacteria, such as lactic acid bacteria, may have more complex nitrogen requirements. They may utilize or even require more complex compounds like amino acids, peptides, or proteins to meet their nitrogen needs. These compounds provide the necessary nitrogen for the synthesis of cellular components, enzymes, and other essential molecules in the bacterial cells.

In summary, while bacteria like Pseudomonas spp. can satisfy their nitrogen requirements with simple compounds like ammonia or nitrates, other bacteria like lactic acid bacteria may rely on more complex nitrogen sources such as amino acids, peptides, or proteins. The specific nitrogen requirements vary among different bacterial species and depend on their metabolic capabilities and ecological niches.


#4. Many kinds of molds are

The correct answer is:

a. proteolytic


Many kinds of molds are proteolytic, meaning they have the ability to break down proteins. Proteolytic molds produce enzymes called proteases, which can hydrolyze or break down proteins into smaller peptides and amino acids. These proteolytic enzymes are important for the mold’s nutrition, as they enable the mold to obtain the necessary amino acids from proteins in its environment. By breaking down proteins, proteolytic molds can derive nutrients and energy for their growth and survival.

#5. Proteolytic organisms utilize hydrolysis products of

The correct answer is:

d. all of the above


Proteolytic organisms have the ability to utilize the hydrolysis products of proteins, peptides, and amino acids. Proteolytic enzymes produced by these organisms break down proteins into peptides and further into individual amino acids through hydrolysis. These hydrolysis products, including peptides and amino acids, can then be taken up and utilized by the organism as a source of nutrients and energy. Therefore, proteolytic organisms can utilize all of the mentioned substances: proteins, peptides, and amino acids.

#6. Osmophilic orgainsms like yeasts grow best in

The correct answer is:

b. high concentrations of sugar


Osmophilic organisms, such as yeasts, are adapted to grow best in environments with high concentrations of sugar. These organisms have a high tolerance for sugar and can thrive in sugary environments where other microorganisms may not be able to survive. The high sugar concentration provides an osmotic environment that is favorable for the growth and metabolism of osmophilic organisms. Therefore, they exhibit optimal growth in high concentrations of sugar.

#7. The aw for pure water is

The water activity (aw) for pure water is generally considered to be 1.00. Water activity is a measure of the availability of water for biological and chemical reactions in a substance. It is expressed on a scale ranging from 0 to 1, where 0 represents completely dry conditions and 1 represents pure water. In the case of pure water, all the available water is in its free form, so its water activity is considered to be the highest value of 1.00.

#8. Organisms that grow over a wide range of pH are

Molds can grow over a wider range of pH values that can most yeasts and bacteria, and many
molds grow at acidities too great for yeasts and bacteria. Most fermentative yeasts are favored by
a pH of about 4.0 to 4.5, as in fruit juices, and film yeasts grow well on acid foods such as
sauerkraut and pickles. On the other hand, most yeasts do not grow well in alkaline substrates
and must be adapted to such media. Most bacteria are favored by a pH near neutrality, although
some, such as the acid formers, are favored by moderate acidity, and other, e.g., the actively
proteolytic bacteria, can grow in media with a high (alkaline) pH, as found in the white of a
stored egg.

#9. Avidin is present in

The correct answer is c. eggs. Avidin is a protein that is naturally found in egg whites. It plays a role in protecting the eggs from bacterial infection by binding to biotin, a B-vitamin, and preventing its absorption. Avidin is heat-stable, which means it remains intact even when eggs are cooked or heated.

#10. Browning of sugar syrups results in the production of

The correct answer is b. furfural. Browning of sugar syrups, also known as caramelization, involves the breakdown of sugars at high temperatures. During this process, furfural is produced as a result of the dehydration and polymerization reactions. Furfural contributes to the characteristic aroma and flavor of caramelized sugar.

More detailed:

When sugar syrups undergo browning, a complex series of chemical reactions occur, leading to the production of various compounds. One of the main products generated during this process is furfural (C5H4O2).

Caramelization, the reaction responsible for browning, involves the thermal decomposition of sugars, particularly glucose and fructose, at elevated temperatures. As the sugar molecules heat up, they undergo a series of chemical transformations, including dehydration, fragmentation, and polymerization.

During the dehydration step, water molecules are eliminated from the sugar molecules, resulting in the formation of intermediate compounds. These intermediates then undergo further reactions, leading to the production of furfural. Furfural is a heterocyclic compound with a characteristic aroma and flavor, contributing to the desirable sensory properties of caramelized products.

Apart from furfural, other compounds such as phenols, alcohols, and ketones are also formed during the browning process. These compounds contribute to the complex flavor profile and color development of caramelized sugar syrups.

Overall, the browning of sugar syrups is a complex chemical process involving the formation of furfural and various other compounds, which contribute to the characteristic taste, aroma, and color of caramelized products.

#11. The fungus known as bread mold is

c. Rhizopus

The fungus known as bread mold is scientifically classified as Rhizopus. Rhizopus is a common genus of filamentous fungi that belongs to the class Zygomycetes. It is commonly found in soil, decaying plant material, and various food sources, including bread.

When bread becomes exposed to moisture and warmth, it creates an ideal environment for the growth of Rhizopus. The spores of Rhizopus are present in the air and can easily settle on the bread’s surface. Once the spores find suitable conditions, they germinate and produce hyphae, which are thread-like structures that extend and spread across the bread.

Rhizopus hyphae grow rapidly and release enzymes that break down the complex carbohydrates present in the bread into simpler molecules. These enzymes help Rhizopus obtain nutrients from the bread. As the hyphae continue to grow, they form a network called mycelium, which becomes visible as fuzzy, black patches on the bread’s surface. These patches are the characteristic signs of bread mold.

Rhizopus reproduces both sexually and asexually. Asexual reproduction occurs through the production of sporangia, which are spherical structures that contain spores. The sporangia develop at the tips of specialized hyphae called sporangiophores. When the sporangia mature, they burst open, releasing countless spores into the surrounding environment. These spores can be easily dispersed through air currents, facilitating the spread of Rhizopus to new food sources.

While Rhizopus is responsible for bread mold, it is important to note that other fungi, such as Penicillium and Aspergillus, can also cause mold growth on bread under certain conditions. However, in the specific context of the question, Rhizopus is the most commonly associated fungus with bread mold.

#12. Pigmented Propionibacter causes

The correct answer for the multiple-choice question is c. color defects in cheese.

Pigmented Propionibacterium species are known to cause color defects in cheese. These bacteria are commonly found in certain types of cheese, such as Swiss cheese. During the aging process, Propionibacterium produces carbon dioxide gas, which forms characteristic holes or “eyes” in the cheese. Additionally, some strains of Propionibacterium can produce pigments that contribute to the yellow or orange coloration often seen in certain types of cheese. This color development is an important characteristic in the maturation of cheese and contributes to its flavor profile.

#13. The fungus known as bread mold is

The correct answer for the multiple-choice question is c. Rhizopus.

The fungus commonly known as bread mold is Rhizopus. Rhizopus is a genus of fungi that includes species like Rhizopus stolonifer, also known as black bread mold. It is a common mold that can grow on bread and other food items, particularly in warm and humid conditions. The presence of Rhizopus can lead to the growth of fuzzy, black spore-bearing structures on the surface of bread, resulting in its characteristic moldy appearance.

#14. Pigmented Propionibacter causes

The correct answer for the multiple-choice question is c. color defects in cheese.

Pigmented Propionibacterium species are known to cause color defects in cheese. These bacteria are commonly found in certain types of cheese, such as Swiss cheese. During the aging process, Propionibacterium produces carbon dioxide gas, which forms characteristic holes or “eyes” in the cheese. Additionally, some strains of Propionibacterium can produce pigments that contribute to the yellow or orange coloration often seen in certain types of cheese. This color development is an important characteristic in the maturation of cheese and contributes to its flavor profile.

#15. Red pigmentation on spoilt food is caused by

The correct answer for the multiple-choice question is d. Serratia.

Red pigmentation on spoilt food is commonly caused by the bacterium Serratia. Certain species of Serratia, such as Serratia marcescens, can produce a red pigment called prodigiosin. When present on spoiled food, the prodigiosin pigment can result in a distinctive red or pink discoloration. This bacterium is often associated with food spoilage and is known to thrive in various environments, including refrigerated conditions. It is important to practice proper food handling and storage to prevent the growth of spoilage bacteria like Serratia.

Genus Serratia Many species produce a pink or magenta pigment and may cause red
discolorations on the surface of foods. S. marcescens is the most common species.

#16. Aeromonas hydrophila is a

The correct answer for the multiple-choice question is c. human pathogen.

Aeromonas hydrophila is a bacterium that is known to be a human pathogen. It can cause a range of infections in humans, including gastrointestinal infections, wound infections, and septicemia. This bacterium is commonly found in aquatic environments, such as freshwater and estuarine systems. While Aeromonas hydrophila primarily affects humans, it can also cause infections in certain animals. It is important to handle and cook food properly, especially seafood, to reduce the risk of contamination and infection by this pathogen.

#17. E. carotovora causes

The correct answer for the multiple-choice question is b. bacterial soft rot.

E. carotovora, also known as Erwinia carotovora, is a bacterial species that is commonly associated with causing bacterial soft rot. This bacterium can infect a variety of plant tissues, leading to the breakdown and decay of the affected tissues. It can cause soft, mushy, and discolored areas in fruits, vegetables, and other plant-based materials. Bacterial soft rot can be detrimental to agricultural crops and stored produce, causing significant economic losses.

#18. The yellow-orange pigment of which genus causes discoloration on the surface of meat?

The correct answer for the multiple-choice question is a. Flavobacterium.

The yellow-orange pigment that causes discoloration on the surface of meat is typically associated with the genus Flavobacterium. Certain species within this genus, such as Flavobacterium lutescens, produce a pigment that can result in the yellow-orange discoloration of meat. This pigmentation is often considered a sign of meat spoilage and can be an indicator of bacterial contamination. It is important to handle and store meat properly to prevent the growth of spoilage bacteria and the development of discoloration.

#19. The genus Photobacterium causes

The correct answer for the multiple-choice question is a. phosphorescence of meat.

The genus Photobacterium is known for causing phosphorescence in meat. Certain species of Photobacterium, such as Photobacterium phosphoreum, possess bioluminescent properties. When these bacteria grow on meat, they can produce light, leading to a phenomenon known as phosphorescence. This can give the meat a shiny or glowing appearance, and it is considered a spoilage indicator as it can indicate the presence of bacterial contamination. It is important to note that phosphorescent meat is not safe for consumption and should be discarded.

Phosphorescence. This rather uncommon defect is caused by phosphorescent or luminous
bacteria, e.g., Photobacterium spp., growing on the surface of the meat.

Genus Photobacterium The genus includes coccobacilli and occasional rods which can be
luminescent. They are not widespread; however, P. phosphoreum has been known to cause
phosphorescence of meats and fish.

#20. Dairy mold is

Oospora (Geotrichum) spp. Oospora (Geotrichum) lactis, called the dairy mold, grows on
soft cheeses and during ripening sometimes suppresses other molds as well as surfaceripening bacteria. The curd gradually becomes liquefied under the felt. O. rubrum and O.
crustacea produce a red coloration, and O. aurianticum forms orange to red spots. O.
caseovorans causes “cheese cancer” of Swiss and similar cheeses. Bumps of growth become
filled with a white, chalky mass.

The correct answer for the multiple-choice question is b. Geotrichum.

Dairy mold is commonly associated with the genus Geotrichum. Geotrichum species are commonly found in dairy products, such as cheese and yogurt, and they can contribute to the ripening and flavor development of these products. They are filamentous fungi that can appear as white or off-white mold on the surface of dairy items. While some species of Geotrichum are considered beneficial in dairy production, others may cause spoilage or off-flavors if they grow excessively

#21. Trichothecium is commonly called

Trichothecium The common species, T. roseum ( Figure 2.14), is a pink mold which grows on
wood, paper, fruits such as apples and peaches, and vegetables such as cucumbers and
cantaloupes. This mold is easily recognized by the clusters of two-celled conidia at the ends of
short, erect conidiophores. Conidia have a nipplelike projection at the point of attachment, and
the smaller of the two cells of each conidium is at this end.


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