Dairy Fermentation Practice Questions
19 free Dairy 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 of the following bacterial genera is primarily responsible for the production of yogurt?
- A Bacillus and Clostridium
- B Lactobacillus and Streptococcus
- C Escherichia and Salmonella
- D Staphylococcus and Corynebacterium
Correct answer: Lactobacillus and Streptococcus
Yogurt is produced by the synergistic action of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. These Lactic Acid Bacteria (LAB) ferment lactose into lactic acid, which acidifies the milk and causes protein coagulation.
In dairy fermentation, what is the primary substrate converted into lactic acid?
- A Sucrose
- B Glucose
- C Lactose
- D Maltose
Correct answer: Lactose
Lactose is the principal sugar found in milk. Lactic acid bacteria utilize the enzyme beta-galactosidase to break down lactose into glucose and galactose before entering the glycolytic pathway.
Which specific biochemical change causes the thickening or 'setting' of yogurt and cheese?
- A The oxidation of milk lipids by aerobic spoilage organisms
- B The denaturation and aggregation of casein proteins at low pH
- C The alcoholic fermentation of milk sugars by contaminating yeasts
- D The thermal degradation of vitamins during heat processing
Correct answer: The denaturation and aggregation of casein proteins at low pH
As bacteria produce lactic acid, the pH of the milk drops toward the isoelectric point of casein (pH 4.6). This acidity causes the casein micelles to destabilize and aggregate into a semi-solid protein matrix.
Which enzyme is traditionally added to milk during cheese making to accelerate the formation of a firm curd?
- A Amylase
- B Lipase
- C Rennet
- D Catalase
Correct answer: Rennet
Rennet contains the enzyme chymosin, which specifically cleaves kappa-casein. This enzymatic action removes the protective 'hairy' layer of the casein micelle, allowing for rapid curd formation even at higher pH levels than simple acid fermentation.
What is the primary difference between homofermentative and heterofermentative lactic acid bacteria?
- A Homofermentative bacteria produce lactic acid only; heterofermentative also yield CO2 and ethanol
- B Homofermentative need molecular oxygen for growth; heterofermentative are obligate anaerobes
- C Homofermentative primarily metabolize lipids for energy; heterofermentative prefer protein substrates
- D Heterofermentative cause spoilage and off-flavors in dairy; homofermentative enhance preservation
Correct answer: Homofermentative bacteria produce lactic acid only; heterofermentative also yield CO2 and ethanol
Homofermentative LAB use the Embden-Meyerhof pathway to convert 1 mole of glucose into 2 moles of lactic acid. Heterofermentative LAB use the pentose phosphate pathway, resulting in additional byproducts like carbon dioxide and flavor compounds.
Which microorganism is responsible for the characteristic 'eyes' (holes) found in Swiss cheese?
- A Lactobacillus acidophilus
- B Propionibacterium freudenreichii
- C Penicillium roqueforti
- D Bifidobacterium pseudocatenulatum
Correct answer: Propionibacterium freudenreichii
Propionibacteria ferment lactic acid into propionic acid, acetic acid, and carbon dioxide. The CO2 gas collects in bubbles within the curd during the ripening process, forming the holes.
Which of the following describes 'Kefir' as a fermented dairy product?
- A A purely bacterial fermentation of cream produced without any yeast cultures
- B A product fermented by a symbiotic grain culture of bacteria and yeasts
- C A variety of hard cheese aged for several years under controlled conditions
- D A heat-sterilized milk product that undergoes no microbial fermentation
Correct answer: A product fermented by a symbiotic grain culture of bacteria and yeasts
Kefir is unique because it involves a symbiotic culture of Lactic Acid Bacteria and yeasts held together in a polysaccharide matrix known as kefir grains. This results in a slightly effervescent, mildly alcoholic fermented milk.
What is the purpose of 'backslopping' in traditional dairy fermentation?
- A Pressing out excess water and whey from the newly formed cheese curd
- B Using a portion of a previously fermented batch to inoculate fresh milk
- C Rubbing salt onto the surface of cheese wheels during the aging process
- D Filtering raw milk through layers of charcoal to remove unwanted flavors
Correct answer: Using a portion of a previously fermented batch to inoculate fresh milk
Backslopping is a traditional method of inoculation where a small amount of an active fermented product is used as the starter culture for the next batch. This ensures a consistent population of the desired microorganisms is maintained.
The sharp, blue-veined appearance of certain mold-ripened cheeses is due to the growth of:
- A Penicillium roqueforti
- B Saccharomyces cerevisiae
- C Aspergillus flavus
- D Rhizopus stolonifer
Correct answer: Penicillium roqueforti
Blue cheeses are inoculated with Penicillium roqueforti molds. These molds grow in the air channels within the curd, producing extracellular lipases and proteases that create the distinct color and pungent flavor.
Which compound is primarily responsible for the 'buttery' flavor in fermented dairy products like buttermilk?
- A Lactic acid
- B Ethanol
- C Butyrate
- D Diacetyl
Correct answer: Diacetyl
Diacetyl is produced by certain strains of LAB, such as Lactococcus lactis subsp. lactis biovar diacetylactis, through the metabolism of citrate. It provides the characteristic buttery aroma and flavor sought in buttermilk and butter.
Why is milk usually pasteurized or heat-treated before the inoculation of starter cultures?
- A To increase the total lactose content of the milk
- B To prevent lactic acid from forming during fermentation
- C To create a solid curd structure before adding any cultures
- D To eliminate competing spoilage organisms and pathogens
Correct answer: To eliminate competing spoilage organisms and pathogens
Heat treatment kills vegetative cells of wild bacteria and pathogens that might outcompete the starter culture or cause disease. It also denatures whey proteins, which helps improve the texture of the final fermented product.
Bacteriophages pose a significant threat to industrial dairy fermentation because they:
- A Cause excessive and uncontrolled acid production that degrades flavor
- B Introduce antibiotic resistance genes into the starter culture
- C Break down the milk proteins before the culture can act on them
- D Infect and destroy the starter culture bacteria, halting fermentation
Correct answer: Infect and destroy the starter culture bacteria, halting fermentation
Bacteriophages are viruses that target Lactic Acid Bacteria. An outbreak in a processing plant can destroy the starter culture, resulting in 'dead' vats where no acid is produced, causing massive economic losses.
Which fermented dairy product is made by using the mold Penicillium camemberti to create a white, fuzzy rind?
- A Cheddar
- B Feta
- C Mozzarella
- D Brie
Correct answer: Brie
Soft-ripened cheeses like Brie and Camembert are treated with P. camemberti. The mold grows on the surface, creating a 'bloom' or rind and secreting enzymes that soften the interior of the cheese over time.
The term 'mesophilic starter culture' refers to bacteria that grow best at which temperature range?
- A 0°C to 10°C
- B 40°C to 50°C
- C 60°C to 70°C
- D 20°C to 30°C
Correct answer: 20°C to 30°C
Mesophilic cultures, such as many Lactococcus species, are used for products like buttermilk and many cheeses that are fermented at moderate room temperatures. Thermophilic cultures are used for high-temperature processes like yogurt making.
In cheese production, the liquid portion that is separated from the solid curd is called:
- A Cream
- B Casein
- C Whey
- D Ghee
Correct answer: Whey
Whey is the watery byproduct of cheese making. It contains water-soluble proteins (whey proteins), lactose, vitamins, and minerals, and is often dried into powder for use in the food industry.
Which organic acid, besides lactic acid, is a major contributor to the flavor of Koumiss (fermented mare's milk)?
- A Acetic acid
- B Citric acid
- C Butyric acid
- D Ascorbic acid
Correct answer: Acetic acid
Koumiss involves both lactic acid bacteria and yeasts, similar to kefir, but it typically has a higher alcohol content and a sharper profile due to the production of small amounts of acetic acid alongside ethanol.
The 'ripening' or 'aging' phase of cheese involves the slow breakdown of which two components?
- A Lactose and water
- B Vitamins and minerals
- C Calcium and phosphorus
- D Proteins and lipids
Correct answer: Proteins and lipids
During ripening, proteolysis (protein breakdown) and lipolysis (fat breakdown) by bacterial and fungal enzymes create the complex flavors and textures of aged cheeses. The longer the ripening, the stronger the flavor.
What is the function of adding salt to cheese curds during processing?
- A To control moisture, suppress unwanted microbes, and enhance flavor
- B To supply energy for bacteria, promote acid production, and speed ripening
- C To trigger mold development and create blue veining in the cheese
- D To raise lactose levels, accelerate fermentation, and increase yield
Correct answer: To control moisture, suppress unwanted microbes, and enhance flavor
Salt (NaCl) draws out moisture through osmosis, helps suppress the growth of undesirable spoilage organisms, and slows down the activity of the starter culture, allowing for controlled ripening.
Which of the following is an example of a 'thermophilic' fermented milk product?
- A Cultured Buttermilk
- B Kefir
- C Yogurt
- D Cottage Cheese
Correct answer: Yogurt
Yogurt fermentation typically occurs at 42-45°C. This high temperature is required for the optimal growth and acid production of the thermophilic species Streptococcus thermophilus and Lactobacillus bulgaricus.