Layers of WaterLayers of Water

Horizontal Scale: Microns

Vertical Scale: AngstromsSurface of solid biopolymers

Layers of Water

• Monolayer: Also called Brunauer-Emmett-Teller Layer (BET), Extremely low moisture content

• Monolayer: Water is bound to hydroxyl groups, polysaccharides, carbonyl and amino groups of proteins

Bound Water

2-3 Layers of Water: Moderate Moisture Content

Several Layers of Water: High Moisture Content

Free Water

Radius of a water molecule:

2.73 Angstroms

Examples of Monolayer Examples of Monolayer Moisture Contents (%)Moisture Contents (%)

Gelatin: 11%Gelatin: 11% Amorphous Lactose: 6%Amorphous Lactose: 6% Spray Dried Milk: 3%Spray Dried Milk: 3%

At moisture contents below monolayer value, At moisture contents below monolayer value, there is a higher rate of lipid oxidationthere is a higher rate of lipid oxidation

At higher moisture contents there is increased At higher moisture contents there is increased Maillard browning, enzymatic and microbial Maillard browning, enzymatic and microbial activityactivity

Food is most stable at Monolayer Moisture Content

Equilibrium Moisture Content Equilibrium Moisture Content (EMC)(EMC)

If food is exposed to a constant relative If food is exposed to a constant relative humidity (Equilibrium Relative Humidity), humidity (Equilibrium Relative Humidity), after sufficiently long time, its moisture after sufficiently long time, its moisture content becomes constant. This is called content becomes constant. This is called equilibrium moisture content.equilibrium moisture content.

ERH (Equilibrium RH)

EMC (Equilibrium MC)

EMC Versus ERH CurveEMC Versus ERH Curve

Equilibrium Relative Humidity (%)

Equi

libriu

m M

oist

ure

Con

tent

Bound Water

Free Water

Monolayer

Sorptio

n

Desorpt

ion

Hysteresis

Reason For HysteresisReason For Hysteresis

Ink-Bottle EffectInk-Bottle EffectLarge Pore

Small Pore is Slow To Drain

Small Pores

Large Pores• Hysteresis results because small pores adsorb moisture quickly but they release moisture at last

• This occurs due to surface tension forces between pore walls and moisture

• Smaller the pore diameter, greater the surface tension forces

Two Types of Surface Boundary Two Types of Surface Boundary Conditions Can Be Used to Predict Conditions Can Be Used to Predict

DryingDrying

1:

EMC2:

M = Km A (Pwfood - Pwair)

Moisture ContentMoisture Content

Dry Basis (d.b.) = Mass of MoistureDry Basis (d.b.) = Mass of Moisture Mass of Solids Mass of Solids

Wet Basis (w.b.) = Mass of MoistureWet Basis (w.b.) = Mass of Moisture Mass of SolidsMass of Solids +Mass of Moisture

Resistance to Moisture TransportResistance to Moisture Transport External Resistance (Inverse of mass transfer External Resistance (Inverse of mass transfer

coefficient)coefficient)

Internal ResistanceInternal Resistance

Internal

External

InitiallyInitially Loss of moisture from the surface is compensated by the Loss of moisture from the surface is compensated by the

moisture movement from insidemoisture movement from inside The surface is saturated with waterThe surface is saturated with water Drying rate is controlled by external resistance to moisture Drying rate is controlled by external resistance to moisture

transfertransfer Drying rate is constantDrying rate is constant

After Some TimeAfter Some Time There is not sufficient supply of moisture from insideThere is not sufficient supply of moisture from inside Surface is not saturated with waterSurface is not saturated with water Drying rate is controlled by internal resistance to moisture Drying rate is controlled by internal resistance to moisture

transfertransfer Drying rate starts decreasingDrying rate starts decreasing

Rate of DryingRate of Drying

Constant Rate Period

Falling Rate Period

Settling Period

Moi

stur

e C

onte

nt

Drying Time

Critical Moisture Content

Controlled by external resistance

Controlled by both external and internal resistance

Constant Rate DryingConstant Rate Drying

Maybe very short, because surface vapor Maybe very short, because surface vapor pressure is rarely equal to the vapor pressure pressure is rarely equal to the vapor pressure of free pan of waterof free pan of water

Two Falling Rate PeriodsTwo Falling Rate Periods

Constant Rate Period

Settling Period

Moi

stur

e C

onte

nt

Drying Time

Critical Moisture Content

1st Falling Rate Period

2nd Falling Rate Period

1st Falling Rate is due to both external and internal resistance

2nd Falling Rate is caused by internal resistance only. Drying is very slow. This may be due to solid-water interaction and glass-transition effects