Agricultural/Bioengineering Agricultural/Bioengineering: Dairy Engineering 3 — Questions and Answers
Question 1: What is the critical difference between UHT (Ultra-High Temperature) and HTST (High Temperature Short Time) pasteurization in terms of shelf life?
- UHT produces a refrigerated shelf life of 30 days vs. 14 days for HTST
- UHT achieves commercial sterility for ambient shelf life of 6–9 months while HTST requires refrigeration (Correct answer)
- HTST destroys more spores than UHT because of longer hold times
- UHT uses lower temperatures to preserve more heat-sensitive vitamins
Correct answer: UHT achieves commercial sterility for ambient shelf life of 6–9 months while HTST requires refrigeration
UHT processing (135–145°C for 2–4 s) achieves commercial sterility, eliminating spore-formers and enabling ambient storage, while HTST (72°C/15 s) only achieves pasteurization requiring cold chain.
Question 2: In the context of evaporative concentration of milk, what is the significance of operating under vacuum?
- Vacuum increases the boiling point, allowing more energy to be transferred to the milk
- Vacuum lowers the boiling point of water, enabling concentration at temperatures below 70°C to minimize heat damage (Correct answer)
- Vacuum removes dissolved oxygen that causes fat oxidation during evaporation
- Vacuum increases viscosity of concentrate, improving pump efficiency
Correct answer: Vacuum lowers the boiling point of water, enabling concentration at temperatures below 70°C to minimize heat damage
Reducing pressure inside the evaporator lowers water's boiling point (e.g., to ~50°C at ~12 kPa), allowing gentle concentration that preserves heat-labile nutrients and flavor.
Question 3: What is the function of a separator (centrifuge) in a fluid dairy processing line?
- It removes heavy metal contaminants from milk using centrifugal sedimentation
- It separates milk into cream and skim milk by exploiting the density difference between fat and plasma (Correct answer)
- It clarifies milk by removing casein micelles from whey
- It standardizes fat content by blending fixed proportions of whole and skim milk
Correct answer: It separates milk into cream and skim milk by exploiting the density difference between fat and plasma
A centrifugal separator applies g-forces up to 6,000–10,000 × g to exploit the ~100 kg/m³ density difference between fat globules and skim milk, yielding cream and skim milk streams.
Question 4: In yogurt manufacturing, what is the purpose of the incubation temperature of approximately 42–45°C?
- This temperature range pasteurizes the milk during fermentation
- It is the optimal growth temperature for Streptococcus thermophilus and Lactobacillus bulgaricus cultures (Correct answer)
- It promotes whey expulsion (syneresis) to achieve the target moisture content
- It denatures alpha-lactalbumin to improve the viscosity of the final product
Correct answer: It is the optimal growth temperature for Streptococcus thermophilus and Lactobacillus bulgaricus cultures
42–45°C is the co-optimal temperature for both yogurt starter cultures, maximizing acid production rate and achieving target pH (~4.6) in 4–6 hours.
Question 5: What does the Gerber method measure in dairy quality testing?
- Total bacterial count using a rapid turbidimetric assay
- Fat content of milk by acid digestion and centrifugation in a butyrometer (Correct answer)
- Protein content by Kjeldahl nitrogen determination
- Lactose concentration using a polarimetric technique
Correct answer: Fat content of milk by acid digestion and centrifugation in a butyrometer
The Gerber method uses sulfuric acid to dissolve non-fat components and amyl alcohol to promote fat separation, then centrifuges the butyrometer to read fat percentage directly.
Question 6: In continuous butter making (e.g., Fritz process), what is the role of the working zone following churning?
- It pasteurizes the butter by heating to 72°C before packaging
- It kneads and works the butter to achieve uniform moisture distribution and expel excess buttermilk (Correct answer)
- It crystallizes the fat phase by rapid chilling to –10°C
- It adds salt and cultures to convert sweet cream to cultured butter
Correct answer: It kneads and works the butter to achieve uniform moisture distribution and expel excess buttermilk
The working zone mechanically kneads the butter mass to achieve homogeneous water-in-fat emulsion structure and bring moisture to the legal maximum of 16%.
Question 7: What is the significance of the 'critical control point' (CCP) in a dairy HACCP plan for fluid milk?
- It is a documentation checkpoint where batch records are reviewed by the quality manager
- It is a step where a control measure can be applied to prevent, eliminate, or reduce a food safety hazard to acceptable levels (Correct answer)
- It identifies the point in the process where spoilage bacteria first multiply above threshold
- It defines the maximum allowable antibiotic residue level before milk is accepted
Correct answer: It is a step where a control measure can be applied to prevent, eliminate, or reduce a food safety hazard to acceptable levels
A CCP is a process step at which a control measure (e.g., pasteurization temperature) can be applied and monitored to prevent or eliminate a food safety hazard.
Agricultural/Bioengineering Agricultural/Bioengineering: Dairy Engineering 3
What is the critical difference between UHT (Ultra-High Temperature) and HTST (High Temperature Short Time) pasteurization in terms of shelf life?