Alcohol Fermentation Practice Questions
20 free Alcohol Fermentation practice questions for the Microbiology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which microorganism is most commonly used in the industrial production of ethanol via fermentation?
- A Escherichia coli
- B Aspergillus niger
- C Lactobacillus acidophilus
- D Saccharomyces cerevisiae
Correct answer: Saccharomyces cerevisiae
Saccharomyces cerevisiae, also known as brewer's yeast, is the primary organism used due to its high ethanol tolerance and ability to efficiently ferment hexose sugars. It is preferred in both the beverage and biofuel industries.
In the alcohol fermentation pathway, pyruvate is first converted into which intermediate compound?
- A Lactate
- B Acetyl-CoA
- C Acetaldehyde
- D Glyceraldehyde
Correct answer: Acetaldehyde
Pyruvate undergoes decarboxylation by the enzyme pyruvate decarboxylase to form acetaldehyde. This step also releases carbon dioxide, which is a key byproduct of the fermentation process.
Which enzyme catalyzes the final step of alcohol fermentation, converting acetaldehyde to ethanol?
- A Pyruvate dehydrogenase
- B Lactate dehydrogenase
- C Hexokinase
- D Alcohol dehydrogenase
Correct answer: Alcohol dehydrogenase
Alcohol dehydrogenase reduces acetaldehyde to ethanol while oxidizing NADH back to NAD+. This regeneration of NAD+ is essential for glycolysis to continue in the absence of oxygen.
What is the theoretical maximum yield of ethanol from 1 gram of glucose during fermentation?
- A 0.51 g
- B 0.31 g
- C 0.75 g
- D 1.00 g
Correct answer: 0.51 g
Based on the Gay-Lussac equation, 1 mole of glucose yields 2 moles of ethanol and 2 moles of CO2. This translates to a theoretical mass yield of approximately 0.511 grams of ethanol per gram of glucose consumed.
Which of the following describes the Pasteur effect in yeast fermentation?
- A An increase in ethanol production when yeast cells shift to aerobic conditions
- B The progressive inhibition of the fermentation pathway by high ethanol concentrations
- C Reduced rate of sugar consumption when conditions shift from anaerobic to aerobic
- D The widespread death of yeast cells when exposed to excessively high temperatures
Correct answer: Reduced rate of sugar consumption when conditions shift from anaerobic to aerobic
The Pasteur effect occurs because aerobic respiration is much more energy-efficient than fermentation. When oxygen is present, yeast consumes less sugar to produce the same amount of ATP, thereby slowing down the fermentation rate.
Which raw material requires saccharification before it can be used for ethanol fermentation?
- A Corn starch
- B Molasses
- C Sugarcane juice
- D Grape juice
Correct answer: Corn starch
Starch is a complex polysaccharide that yeast cannot ferment directly. It must first be broken down into fermentable sugars like glucose through a process called saccharification, usually using amylase enzymes.
The 'Crabtree effect' refers to the occurrence of fermentation in the presence of oxygen when:
- A Nitrogen levels are very low
- B The temperature exceeds 40°C
- C Glucose concentrations are high
- D The pH of the medium is highly alkaline
Correct answer: Glucose concentrations are high
In Crabtree-positive yeasts like S. cerevisiae, high glucose concentrations trigger fermentation even if oxygen is available for respiration. This is an important consideration in industrial fed-batch fermentations to avoid unwanted ethanol accumulation during biomass growth.
What is the primary role of nitrogen in an industrial ethanol fermentation medium?
- A It is essential for yeast growth and protein synthesis
- B It functions as the terminal electron acceptor during fermentation
- C It prevents the growth of contaminating bacteria
- D It stabilizes the pH of the fermenter
Correct answer: It is essential for yeast growth and protein synthesis
While sugar provides carbon and energy, yeast cells require nitrogen (often supplied as urea or ammonium salts) to build amino acids and enzymes. Proper nitrogen levels ensure healthy yeast growth and sustained fermentation rates.
Which of the following is a significant byproduct of industrial alcohol fermentation used in the carbonated beverage industry?
- A Glycerol
- B Methane
- C Carbon dioxide
- D Hydrogen sulfide
Correct answer: Carbon dioxide
Carbon dioxide (CO2) is produced in equimolar amounts to ethanol. In large-scale operations, this gas is often captured, purified, and sold for use in soft drinks or as dry ice.
Which bacterial species is used in some industrial processes as an alternative to yeast for high-yield ethanol production?
- A Zymomonas mobilis
- B Bacillus subtilis
- C Pseudomonas aeruginosa
- D Streptococcus lactis
Correct answer: Zymomonas mobilis
Zymomonas mobilis is a Gram-negative bacterium that uses the Entner-Doudoroff pathway for fermentation. It offers higher sugar uptake rates and higher ethanol yields compared to traditional yeast, though it has a narrower substrate range.
What is the typical alcohol concentration at which wild strains of S. cerevisiae begin to suffer from growth inhibition?
- A 8-12%
- B 2-4%
- C 20-25%
- D 40-50%
Correct answer: 8-12%
Most standard yeast strains are inhibited when ethanol reaches 8-12% by volume. Industrial 'turbo' or tolerant strains have been bred to withstand concentrations up to 18-20% to increase process efficiency.
In the production of fuel ethanol, the process of 'denaturing' involves:
- A Adding substances to render the ethanol undrinkable
- B Removing residual water through fractional distillation
- C Breaking down yeast cell walls
- D Neutralizing the acidity of the mash
Correct answer: Adding substances to render the ethanol undrinkable
Fuel ethanol is denatured by adding small amounts of gasoline or other chemicals. This is done for legal and tax purposes to ensure the alcohol is not consumed as a beverage.
The use of 'immobilized yeast' in industrial fermentation provides which advantage?
- A It enables continuous fermentation and easy yeast recovery
- B It eliminates the requirement for a sugar substrate
- C It renders the yeast cells resistant to viral contamination
- D It increases the temperature of the fermenter automatically
Correct answer: It enables continuous fermentation and easy yeast recovery
Immobilizing yeast on beads or membranes allows for a higher cell density within the reactor. It facilitates continuous processing and simplifies the downstream recovery of the yeast for reuse.
Which physiological stress is the most common cause of 'stuck' or sluggish fermentations in industry?
- A Excessive light exposure during fermentation
- B Excessive dissolved oxygen in the growth medium
- C Reduced atmospheric pressure
- D Nutrient deficiency or ethanol toxicity
Correct answer: Nutrient deficiency or ethanol toxicity
Fermentations can stall ('stuck') if yeast cells become stressed by high ethanol concentrations, lack of nitrogen, or extreme temperatures. Managing these parameters is critical for ensuring complete sugar conversion.
Which enzyme is added during the 'mashing' phase of grain-based ethanol production to liquefy starch?
- A Protease
- B Alpha-amylase
- C Glucoamylase
- D Beta-glucanase
Correct answer: Alpha-amylase
Alpha-amylase is used to break down the long-chain starch molecules into shorter dextrins. This reduces the viscosity of the mash, preparing it for further breakdown into fermentable glucose.
The term 'vinasse' refers to which part of the industrial alcohol process?
- A The high-purity ethanol collected from the top of the column
- B The nutrient-rich liquid waste remaining after distillation
- C The primary sugar source derived from grapes
- D The enzyme complex used to start fermentation
Correct answer: The nutrient-rich liquid waste remaining after distillation
Vinasse is the bottom residue or effluent left after the distillation of fermented mash. Because of its high organic load, it requires significant wastewater treatment or is repurposed as fertilizer.
Why is pH control important during the initial stages of industrial fermentation?
- A To prevent the loss of ethanol through evaporation
- B To favor yeast growth and inhibit contaminating bacteria
- C To ensure the CO2 remains dissolved in the fermentation broth
- D To prevent the metal fermenter from corroding
Correct answer: To favor yeast growth and inhibit contaminating bacteria
Yeast prefers a slightly acidic environment (pH 4.0-5.5). Maintaining this range helps prevent the proliferation of competing bacteria that could produce unwanted organic acids and reduce ethanol yield.
Which technology is used to produce 'anhydrous' (water-free) ethanol for blending with gasoline?
- A Simple batch distillation
- B High-speed centrifugation
- C Molecular sieve adsorption
- D Filtration through charcoal
Correct answer: Molecular sieve adsorption
Standard distillation cannot remove water beyond the azeotropic point (95.6% ethanol). Molecular sieves use zeolite beads to adsorb the remaining water molecules, producing 99.5%+ pure anhydrous ethanol.
What is the purpose of 'pitching' in industrial microbiology?
- A The removal of sediment from the bottom of the tank
- B Inoculating the fermenter with a yeast starter culture
- C The rapid cooling of the mash after boiling
- D The measurement of the sugar content using a hydrometer
Correct answer: Inoculating the fermenter with a yeast starter culture
Pitching refers to the inoculation of the fermentation medium with a starter culture of yeast. The pitching rate (number of cells per ml) is carefully calculated to ensure rapid onset of fermentation.
Fusel oils, which are small amounts of higher alcohols produced during fermentation, are derived primarily from the metabolism of:
- A Lipids
- B Amino acids
- C Nucleic acids
- D Complex carbohydrates
Correct answer: Amino acids
Fusel oils (such as isoamyl alcohol) are produced via the Ehrlich pathway. Yeast deaminates and decarboxylates amino acids in the medium, resulting in these higher alcohols which contribute to the flavor and aroma of the final product.