5. Psychometry & Convective drying

Food preservation by drying

Drying is a method of food preservation that inhibits the growth of bacteria, yeasts, and mould through the removal of water

Why not sun drying?

• Not safe: micro-organisms may grow during slow drying

• Unpredictable; big losses

• Not realistic in many temperate regions

• Solutions is the use of hot air

How much of the total industrial energy consumption goes to drying?

15-20% of total industrial energy consumption

Relative humidity formula

Properties of vapor and air

• Vapor and dry air mix well

• Hot air can contain more water vapor

What does a sorption isotherm tell you?

• It gives the moisture content/change in mass on the y axis needed to reach a certain water activity

When does hysteresis occur?

• Hysteresis is the difference between adsorption and desorption isotherm

• The effect does not occur in solutions; only in porous products, such as foods, hysteresis occurs

Absolute humidity (need to know!)

• molar fraction of moisture in air

Humidity ratio (or moisture content) (need to know!!)

• i.e. kg of water per kg dry air

Relative humidity definition (need to know!!)

• Relative humidity is the relative amount of vapor in air compared to the maximum saturation at a specific temperature

Dry and wet bulb temperature (need to know!)

• Dry bulb: Basically the normal air temperature

• Wet bulb: Temperature measured with a thermometer wrapped with a truly wet cloth exposed to the air flow

  • Wet bulb temperature is the temperature that a wet surface will take upon fast evaporation of the water

  • The wet bulb temperature is:

    • Not dependent on air flow rate

    • T_{wet bulb}«T_{dry bulb}

    • Dependent on RH

Why are at 100% RH the wet and dry bulb temperatures equal?

At 100% Relative Humidity (RH), the air is saturated, meaning it is at its maximum capacity to absorb moisture at its current temperature. Because the air is already saturated, the wet and dry bulb temperatures become identical

Dew point (need to know)

The dew point indicates the temperature at which water vapor starts to condense when cooling air

Adiabatic saturation

• This is a process where air contacts a wet product and takes up moisture without exchanging heat with the environment.

• In this state, the air's sensible heat is converted into latent heat to fuel evaporation, causing the air to cool down as its humidity increases.

• The air will cool down if you contact hot air with a wet product adiabatically

  • Adiabatic = without heat loss

• Of the air:

  • The sensible heat decreases

  • The latent heat increases

Tray dryer

• Allows for larger products

• Longer drying times

• e.g. raisins, dates, plums, figs, apricots

Continuous tray dryer

• Continuous throughput

• Carts enter through a sluice, and exit through another one

• More efficient use of drying air

Co-current and counter-current drying

• Counter current is more efficient but also heats up more and therefore can potentially damage the product.

Spray dryer

• For liquid products only

• Product is dispersed into very small droplets, in hot, dry air

• Drying time < 1-2s

• Droplet moisture evaporates, air cools down

• e.g. milk powder, whey protein isolate, cream; lactose

Fluidized bed drying

• Moist particles are bubbling in the continuous upward flow of air

• Used for drying with longer residence time (falling rate period)

• Used for agglomeration into larger particles (granulation)

• Used for cooling

• Feed = ‘moist’ granular product

• Can be batch or continuous

Three zones in continuous fluidized bed drying

• Agglomeration

• Drying

• Cooling

Agglomeration

• Done by fluidized bed drying

• Individual (~spherical) particles are glued to each other

• Agglomerates/granules are created

• Porous, powder density is low

• Excellent flowability

• Sometimes a compaction step may be added

Dissolution behavior of powders (dispersion)

• Because powder materials are typically soluble and hygroscopic, they suck up liquid into their internal pores through capillary action, a process called wicking

• Fast swelling/dissolution can lead to lumps, e.g. in hot water

• Addition of lecithin controls wettability and dispersion

Dissolution behavior of powders (disintegration)

• As water wicks into the pores, mechanical forces (like stirring) act on the powder bed to break it into many small pieces.

• Agglomerated powders excel here because they allow water to separate clusters before they become a solid mass

dissolution behavior of powders (dissolution)

• This is the final stage where primary particles actually dissolve.

• Interestingly, increasing stirring speed does not necessarily accelerate this stage once particles are suspended, as they are too small to be affected by convective turbulence (eddies).

• The rate of dissolution is primarily driven by the specific surface area; therefore, irregular, cracked, or highly porous particles dissolve the fastest.

influence of spray drying on solubility of milk powder

• Insolubility is hypothesized to be caused by protein aggregation, which occurs when milk components are exposed to high temperatures while still within a specific moisture rang

• Insoluble particles are only formed between 10 to 30 w/w% moisture and the rate depends on temperature

• If the particle is still containing significant internal moisture while its temperature is rising into this high range, the proteins (particularly caseins and whey proteins) can denature and aggregate. These aggregated proteins form a dense, hydrophobic network that water cannot easily penetrate or break apart during later reconstitution

Stages of droplet drying kinetics

• First stage: ideal shrinkage till locking point

• Second stage: collapse of droplet → powder morphology development

• Drying of moist solid products such as vegetables can be characterized in a similar way

Drying rate curve

Two periods:

• Constant rate period

  • Particle is still very wet

  • Evaporation of moisture is externally limiting

  • Temperature low

  • The ratio of heat transfer to mass transfer is constan

• Falling rate period

  • Particle is getting dry at the surface

  • Diffusion inside product is limiting

  • Temperature higher

Change in temperature during drying

• Red line = product, plateau = wet bulb temperature

When temperature changes, does relative humidity also change?

• Yes but the moisture content stays the same

What machines can or cannot be used when preventing protein aggregation?

• Spray drying: This is generally a milder process because the high surface area of small droplets allows for rapid drying at the wet bulb temperature. However, if the air is too hot or the particles reside too long in the dryer, the "critical moisture range" overlap with high heat leads to the formation of insoluble components

• Drum Drying: This method is much more aggressive because the milk slurry is in direct contact with a metal wall heated to 120–160 °C. This intense heat causes widespread protein denaturation and gives the powder a "cooked" or caramelized flavor, resulting in poorer solubility compared to spray-dried version

What is H_L?

H_L = H_v - H_c

What is P_T

Total pressure of the moist air (atmospheric pressure)