Agricultural/Bioengineering Agricultural/Bioengineering: Dairy Engineering 3

Question 1

What is the critical difference between UHT (Ultra-High Temperature) and HTST (High Temperature Short Time) pasteurization in terms of shelf life?

Accepted Answer

UHT achieves commercial sterility for ambient shelf life of 6–9 months while HTST requires refrigeration

Answer

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?

Accepted Answer

Vacuum lowers the boiling point of water, enabling concentration at temperatures below 70°C to minimize heat damage

Answer

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?

Accepted Answer

It separates milk into cream and skim milk by exploiting the density difference between fat and plasma

Answer

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?

Accepted Answer

It is the optimal growth temperature for Streptococcus thermophilus and Lactobacillus bulgaricus cultures

Answer

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?

Accepted Answer

Fat content of milk by acid digestion and centrifugation in a butyrometer

Answer

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.

Agricultural/Bioengineering Practice Test

Agricultural/Bioengineering Practice Test

Agricultural/Bioengineering Agricultural/Bioengineering: Dairy Engineering 3