Question

Vacuum crystallizers are preferred in sugar manufacturing because

• A. They increase boiling point
• B. They increase solubility
• C. They eliminate need for centrifugation
• D. They prevent caramelization by reducing temperature

Hint:

Operating under vacuum lowers the system pressure, which drops the boiling point of the liquid. This allows for rapid concentration and crystallization of heat-sensitive sugar syrups without heat damage.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks why vacuum crystallizers (pans) are preferred over atmospheric crystallizers during the sugar crystallization stage of cane or beet sugar processing.

Step 2: Detailed Explanation:

• Thermal Sensitivity of Sugar: Concentrated sugar syrups (sucrose solutions) are highly heat-sensitive. Under high temperatures, sucrose undergoes rapid non-enzymatic browning reactions:

• Caramelization: Direct thermal degradation of sugar at temperatures typically above $160^\circ\text{C}$ (or lower under prolonged heating and high concentrations), producing dark-colored compounds and bitter flavors.

• Inversion: Thermal breakdown of sucrose into glucose and fructose, which reduces crystallization yield.

• Vacuum Operation Principle: Operating a crystallizer under vacuum reduces the headspace pressure inside the vessel. According to thermodynamic principles, lowering the pressure reduces the boiling point of the solution.

• Low-Temperature Boiling: This boiling point depression allows the highly concentrated sugar syrup to boil and crystallize at significantly lower temperatures (typically $60^\circ\text{C} - 70^\circ\text{C}$ instead of $> 100^\circ\text{C}$). This low-temperature operation prevents caramelization, maintaining a white, high-purity sugar product.

• Why Other Options are Incorrect:

• They increase boiling point: Vacuum decreases the boiling point, not increases it.

• They increase solubility: Lower temperatures actually decrease solubility, which is desirable during crystallization to promote supersaturation and crystal growth.

• They eliminate need for centrifugation: Centrifugation is still required downstream to separate the sugar crystals from the mother liquor (molasses).

Step 3: Final Answer:
Vacuum crystallizers are used to depress the boiling point, enabling crystallization at low temperatures to prevent thermal caramelization of sucrose, corresponding to option (D).