Introduction To Dairy Microbiology

Step 1: Understanding the Concept:
The question asks for the cause of a specific off-flavor defect in butter known as "cheesy." This flavor is distinct from other defects like sour, rancid, or cooked flavors.
Step 2: Detailed Explanation:
(A) Growth of proteolytic bacteria in cream: Proteolytic bacteria produce enzymes (proteases) that break down proteins, primarily casein. This breakdown produces peptides and other compounds that are characteristic of the flavors found in aged cheese. If such bacteria grow in the cream before churning, these flavor compounds are carried over into the butter, resulting in a cheesy defect. This is the correct cause.
(B) Over heating of cream: This would cause a "cooked" or "scorched" flavor due to the denaturation of whey proteins and other heat-induced chemical reactions.
(C) Over neutralization of cream: Adding too much neutralizer can lead to a soapy or "neutralizer" flavor.
(D) Use of sour cream: Cream with high developed acidity (sour cream) will produce butter that has a sour or acidic taste, not a cheesy one.
Step 3: Final Answer:
The cheesy flavor in butter is caused by protein decomposition products resulting from the growth of proteolytic bacteria in the cream.

Step 1: Understanding the Concept:
The question asks for the desirable characteristics (selection criteria) for the starter cultures used in the manufacturing of Cheddar cheese.
Step 2: Detailed Explanation:
Let's evaluate each criterion:
A. Rapid acid production: This is critical. A predictable and fast rate of acid production is needed to control the cheese-making process, expel whey (syneresis), and achieve the correct texture and pH for cheddaring. This is a desired trait.
B. Bacteriophage resistance: Bacteriophages (or 'phages') are viruses that infect and kill bacteria. A phage infection can wipe out the starter culture, leading to slow or no acid production, which results in a failed batch of cheese. Therefore, resistance to phage attack is a vital criterion. This is a desired trait.
C. Salt senstivity: This is an undesirable trait. During Cheddar manufacturing, a significant amount of salt (around 2-3%) is added to the curd. The starter culture must be salt-tolerant to survive this step and continue its function during the initial stages of ripening. Salt sensitivity would be a reason to reject a culture.
D. Ripening activity: After the cheese is made, the enzymes released by the starter bacteria play a crucial role in breaking down proteins and fats during the aging (ripening) period. This enzymatic activity is what develops the characteristic sharp flavor and texture of mature Cheddar. This is a desired trait.
Therefore, the important criteria are rapid acid production, bacteriophage resistance, and ripening activity.
Step 3: Final Answer:
The correct selective criteria are A, B, and D.

Step 1: Understanding the Concept:
The question asks for the specific type of starter culture used in the production of Mozzarella cheese. The choice of culture is critical and depends on the processing conditions of the cheese variety.
Step 2: Detailed Explanation:
Mozzarella is a "pasta filata" or stretched-curd cheese. Its manufacturing process involves cooking the curd to a high temperature (around 40-45°C) and then heating and stretching it in hot water (around 75-85°C) once it reaches a specific pH (around 5.2).
This process requires cultures that can survive and produce acid at these high temperatures. Such cultures are called thermophilic (heat-loving).
Let's analyze the options:
(A) S. thermophilus and L. delbrueckii subsp. bulgaricus: Both are thermophilic lactic acid bacteria. This is the classic culture combination used for yogurt and is also perfectly suited for making Mozzarella due to its ability to rapidly produce acid at high temperatures.
(B) This mixture includes Propionibacterium freudenreichii subsp. shermanii, which is the culture responsible for the "eyes" (holes) and nutty flavor in Swiss-type cheeses. It's not used for Mozzarella.
(C) Leuconostoc mesenteroides subsp. cremoris: This is a mesophilic (moderate-temperature) culture known for producing gas (CO ) and aroma compounds (diacetyl). It is used for products like buttermilk and some cheeses, but not Mozzarella.
(D) Propionibacterium: As mentioned above, this is the culture for Swiss cheese.
Step 3: Final Answer:
The correct thermophilic culture for Mozzarella cheese is a combination of Streptococcus thermophilus and a thermophilic Lactobacillus, such as Lactobacillus delbrueckii subsp. bulgaricus.

Step 1: Understanding the Concept:
MBRT stands for the Methylene Blue Reduction Test. It is a rapid, indirect method to assess the microbiological quality of milk. The time it takes for the blue dye to become colorless (reduced) is an indicator of the bacterial load.
Step 2: Detailed Explanation:
The principle of the test is based on the metabolic activity of bacteria. When bacteria grow in milk, they consume oxygen and lower the oxidation-reduction potential of the milk. Methylene blue is a redox indicator that is blue in an oxidized state and colorless in a reduced state.
High bacterial count: The oxygen is used up quickly, the potential drops, and the methylene blue is reduced (decolorized) in a short time.
Low bacterial count: The metabolic activity is low, and it takes a long time for the dye to be reduced.
Properly pasteurized milk should have a very low count of viable bacteria. Therefore, its MBRT time should be very long. According to food safety standards (like FSSAI in India), the Methylene Blue Reduction Time for properly pasteurized milk must not be less than 5 hours. This long time indicates that the pasteurization was effective in killing most of the bacteria.
Step 3: Final Answer:
The MBRT time for properly pasteurized milk shall not be less than 5 hours.