Food Process Engineering & Operations

Matching membrane separation processes with their primary separation effects:

• Microfiltration ($0.1\text{--}10\ \mu\text{m}$ pore size) retains suspended particulates and bacteria, achieving cold sterilization $\to$ (II).
• Ultrafiltration ($1\text{--}100\text{ kDa}$ MWCO) retains proteins and macromolecules while permeating salts and sugars $\to$ (III).
• Nanofiltration (loose reverse osmosis, $200\text{--}1000\text{ Da}$ MWCO) retains organic solutes and divalent ions while allowing monovalent salts to pass (demineralization) $\to$ (IV).
• Reverse Osmosis (dense non-porous membranes, $\lt 100\text{ Da}$) rejects virtually all dissolved solutes, concentrating solutions by exclusively removing pure water $\to$ (I).

During the constant rate drying period, free water diffuses to the surface as rapidly as it can evaporate. Consequently, a continuous film of free liquid water covers the entire outer surface of the food material ('remains wet'), behaving exactly like a free water surface evaporating at the wet-bulb temperature until the critical moisture content is reached.

Option analysis:

• Remains dry: Occurs during the falling rate period after surface crusting.
• Remains wet: Correct, continuous saturated surface moisture film defines constant rate drying.
• Develops cracks: Occurs during over-drying or case-hardening.
• Melts: Abnormal thermal defect, not the definition of drying period.

Once drying reaches the critical moisture content, surface water is depleted and the drying rate ceases to be governed by external heat transfer. Instead, it becomes limited by the internal diffusion of liquid water or water vapour through the solid food matrix to the evaporating surface. As the dry layer thickens and capillary paths lengthen, mass transfer resistance continuously increases, causing the moisture content to decrease at an progressively decreasing rate ($d^2M/dt^2 \gt 0$, concave upward decay).

Option analysis:

• Decreases rapidly: Characteristic of the constant rate period.
• Remains constant: Characterizes equilibrium moisture content, where drying ceases.
• Decreases at a decreasing rate: Correct, standard mathematical definition of the falling rate drying curve.
• Decreases at an increasing rate: Physically impossible under diffusional resistance.