Technology of Milk & Milk Products

• Classification of Expansion Devices in Refrigeration Systems:
• Regulate refrigerant mass flow and reduce pressure from condenser to evaporator:
• (I) Capillary tube: Constant-bore restrictor used in domestic refrigerators.
• (II) Thermostatic Expansion Valve (TEV/TXV): Modulating valve maintaining constant evaporator superheat.
• (III) High-side float valve: Liquid-level float valve discharging condensed liquid to flooded evaporators.
• All three are standard refrigeration expansion devices.
• Conclusion: (I), (II) and (III) (Option 2).

Correct option: (2) (I), () and (Ill)

• Glass Transition Temperature ($T_g'$): In ice cream mixes, the glass transition temperature of the maximally freeze-concentrated matrix ($T_g'$) typically lies between $-32^\circ\text{C}$ and $-35^\circ\text{C}$. At or below $T_g'$, molecular mobility ceases, and water is vitrified (converted into an amorphous solid state) without ice crystal growth or recrystallization.
• Role of Cryogenic Freezing:
• Conventional scraped surface freezers freeze ice cream down to $-5^\circ\text{C}$ to $-6^\circ\text{C}$ ($50\%$ water frozen).
• Hardening tunnels reach $-25^\circ\text{C}$ to $-30^\circ\text{C}$ ($85-90\%$ water frozen).
• To surpass and freeze below the maximally freeze-concentrated glass transition temperature ($T_g'$), ultra-rapid cryogenic freezing using liquid nitrogen ($-196^\circ\text{C}$) or liquid carbon dioxide ($-78.5^\circ\text{C}$) is required.
• Correct Option: Option 4 (Cryogenic freezing).

• Microbial Stabilizers in Ice Cream:
• Xanthan Gum: An extracellular polysaccharide (exopolysaccharide) produced by the aerobic fermentation of the bacterium Xanthomonas campestris. It is a microbial stabilizer used to impart pseudoplastic viscosity, enhance freeze-thaw stability, and prevent ice recrystallization in ice cream.
• Guar Gum and Locust Bean Gum (LBG): Galactomannans derived from the endosperm of plant seeds (Cyamopsis tetragonoloba and Ceratonia siliqua).
• $\kappa$-Carrageenan: A sulfated galactan extracted from marine red algae (Rhodophyceae, e.g., Chondrus crispus).
• Correct Option: Option 1 (Xanthan gum).