Agricultural/Bioengineering Agricultural/Bioengineering: Dairy Engineering 4 — Questions and Answers
Question 1: When sizing a cooling system for a raw milk receiving tank, which parameter is most critical for preventing microbial growth?
- Achieving 4°C within 2 hours of milking to slow mesophilic bacterial growth (Correct answer)
- Reaching 10°C within 4 hours since psychrotrophic bacteria do not grow above 10°C
- Cooling to 0°C immediately to create a bacteriostatic environment
- Reducing temperature to 15°C within 30 minutes to prevent somatic cell increases
Correct answer: Achieving 4°C within 2 hours of milking to slow mesophilic bacterial growth
Grade A PMO standards require raw milk be cooled to ≤4.4°C (40°F) within 2 hours to inhibit mesophilic pathogens while preventing freezing.
Question 2: In lactose crystallization for the production of lactose monohydrate powder, what is the primary role of the 'seed' crystals added to the supersaturated lactose solution?
- Seeds lower the viscosity of the solution to improve pump efficiency
- Seeds provide nucleation sites to initiate controlled crystal growth and prevent amorphous solidification (Correct answer)
- Seeds adjust pH to the isoelectric point of lactose
- Seeds act as antimicrobial agents to prevent fermentation during crystallization
Correct answer: Seeds provide nucleation sites to initiate controlled crystal growth and prevent amorphous solidification
Seeding with fine lactose crystals provides nucleation sites that direct crystal growth in a controlled manner, producing uniform α-lactose monohydrate crystals rather than a glassy amorphous mass.
Question 3: A cheesemaker notices that the curd is too soft and not cutting cleanly after rennet addition. Which corrective action is most appropriate?
- Increase milk temperature above 42°C to speed coagulation
- Add more starter culture to lower pH more rapidly
- Extend the coagulation time or increase the CaCl₂ dose to achieve a firmer gel before cutting (Correct answer)
- Cut the curd immediately at a finer size to compensate for softness
Correct answer: Extend the coagulation time or increase the CaCl₂ dose to achieve a firmer gel before cutting
Soft curd indicates incomplete coagulation; extending set time or adding CaCl₂ strengthens the gel network before cutting, preventing curd shattering and fat losses in whey.
Question 4: What is the purpose of standardizing fat content in milk before processing into a product like 2% reduced-fat milk?
- To comply with legal compositional standards and deliver consistent product to consumers (Correct answer)
- To increase the protein-to-fat ratio for improved nutritional labeling claims
- To reduce microbial load since fat globules harbor surface bacteria
- To lower the freezing point of milk to prevent ice crystal formation during cold storage
Correct answer: To comply with legal compositional standards and deliver consistent product to consumers
Standardization blends cream and skim milk streams to achieve the legally defined fat content (2.0% for reduced-fat milk) required by FDA standards of identity.
Question 5: In an ice cream mix formulation, what is the primary function of an emulsifier such as mono- and diglycerides?
- They increase the sweetness intensity of the mix to reduce sugar usage
- They promote partial coalescence of fat globules during freezing, improving overrun stability and texture (Correct answer)
- They prevent lactose crystallization (sandiness) in the finished ice cream
- They increase viscosity of the mix to reduce the need for stabilizer gums
Correct answer: They promote partial coalescence of fat globules during freezing, improving overrun stability and texture
Mono- and diglycerides displace milk proteins from fat globule surfaces, promoting controlled partial coalescence that creates a fat network supporting air bubbles and giving ice cream its characteristic body.
Question 6: What does the term 'overrun' mean in the context of ice cream processing?
- The excess mix produced beyond the planned batch size due to ingredient variation
- The percentage increase in volume due to air incorporation during freezing (Correct answer)
- The amount of butterfat above the minimum legal standard in the formulation
- The temperature overshoot during continuous freezer operation
Correct answer: The percentage increase in volume due to air incorporation during freezing
Overrun = [(volume of ice cream – volume of mix) / volume of mix] × 100%; typical values are 80–100% for retail ice cream, giving a lighter, airier texture.
Question 7: In a dairy plant using CIP (Clean-in-Place) systems, what is the correct sequence of cleaning steps?
- Hot water rinse → caustic wash → hot water rinse → acid wash → final rinse → sanitize (Correct answer)
- Acid wash → caustic wash → water rinse → sanitize → dry
- Caustic wash → sanitize → water rinse → acid wash → water rinse
- Water rinse → sanitize → caustic wash → acid rinse → dry
Correct answer: Hot water rinse → caustic wash → hot water rinse → acid wash → final rinse → sanitize
Standard dairy CIP sequence: pre-rinse (remove bulk soil) → hot caustic (remove protein/fat) → intermediate rinse → acid wash (remove mineral deposits) → final rinse → sanitizer.
Agricultural/Bioengineering Agricultural/Bioengineering: Dairy Engineering 4
When sizing a cooling system for a raw milk receiving tank, which parameter is most critical for preventing microbial growth?