04 Moisture Sorption Isotherm

The relationship between _____ and _____ in food at a constant temperature and pressure is described by the Moisture Sorption Isotherm (MSI).

MC, aw

In the context of Moisture Sorption Isotherms, how is Moisture Content (MC) typically expressed?

Mass of H2O per mass of dry matter.

The adhesion of adsorbate molecules to the surface of a material.

Adsorption

The penetration of molecules into the bulk or internal structure of a material.

Absorption

The adsorbate adheres to the surface through weak intermolecular van der Waals interactions without changing its chemical nature.

Physisorption

The molecule adheres to a surface through the formation of a chemical (covalent) bond.

Chemisorption

What are the 3 types of isotherms?

• I: Langmuir

• II: Sigmoidal

• III: J-Type

Which food additives are characteristic examples of materials exhibiting a Type I Isotherm?

Anti-caking agents

Why is a Type I curve ideal for anti-caking agents?

They can hold a high volume of water at low aw, keeping the surrounding food dry and unreactive.

Which isotherm type is most commonly encountered in normal food products rich in starches and proteins?

Type II

In a Type II isotherm, what does the first bend (typically aw 0.2–0.4) signify?

The transition from tightly bound monolayer water to multilayer water.

In a Type II isotherm, what does the second bend (typically aw 0.6–0.8) signify?

The transition into free, mobilizable capillary water.

Which specific food components typically display a Type III Isotherm?

Crystalline components like sucrose and salt, or humectants like glycerol.

Describe the moisture uptake of a Type III material at low to intermediate aw.

There is very little gain in moisture content as crystalline structures lack accessible active surface sites.

The point where the crystal lattice dissolves into a solution, exposing massive numbers of hydrogen-bonding sites for rapid water uptake.

deliquescence

What is the water activity (aw) range for Region I of a Moisture Sorption Isotherm?

0 to 0.25

What type of water is associated with Region I of an MSI?

Bound water (Monolayer water)

What is the solvent capacity of water in Region I of an MSI?

Zero; it cannot dissolve solutes.

Is Region I water freezable at sub-zero temperatures?

No, it is unfreezable.

What is the water activity (aw) range for Region II of a Moisture Sorption Isotherm?

0.25 to 0.80

Water found in Region II of an MSI is specifically categorized as _____ water.

Multilayer

Why do reactions like Maillard browning peak in Region II of an MSI?

Water gains enough mobility to act as a solvent, allowing reactants to dissolve and collide.

In Region II, water molecules form _____ with the existing monolayer rather than the food matrix itself.

Hydrogen bonds

What is the water activity (aw) range for Region III of a Moisture Sorption Isotherm?

0.80 to 1.0

Water in Region III is referred to as _____ or _____ water.

Free; Capillary

Approximately what percentage of total water in moisture-rich foods is accounted for by Region III?

95%

Combined, Regions I and II account for less than _____% of total water in moisture-rich foods.

5%

Is water in Region III of an MSI freezable?

Yes, it forms ice crystals at sub-zero temperatures.

It is prepared by exposing an initially wet material to atmospheres of successively decreasing relative humidity (RH).

desorption

It is prepared by exposing a dry material to atmospheres of successively increasing relative humidity (RH).

adsorption/resorption

The difference in the amount of water adsorbed and desorbed by a foodstuff at the same water activity (aw).

sorption hysteresis

In a hysteresis loop, which curve (adsorption or desorption) typically maintains a higher moisture content at a given aw?

Desorption curve

How do changes in polymer conformation (like proteins) contribute to sorption hysteresis?

Changes in folding and orientation can trap or expose different binding sites during water removal.

According to the Ink Bottle Theory, what physical feature governs the filling of a pore during Adsorption?

Wide Body Radius (R)

According to the Ink Bottle Theory, what physical feature governs the emptying of a pore during Desorption?

Narrow Neck Radius (r)

How does an increase in temperature generally affect the size of a hysteresis loop?

It decreases hysteresis due to increased water removal and movement.

How does increasing sugar or salt content in a food affect sorption hysteresis?

It increases the extent of hysteresis.

Who developed the BET equation?

Stephen Brunauer, Paul Emmett, and Edward Teller

For what water activity (aw) range is the BET equation typically applicable?

0 to 0.55

The BET equation is a _____-parameter equation.

Two

Who developed the GAB equation?

Guggenheim, Anderson, and de Boer

The GAB equation is adequate to describe experimental data for aw values up to _____.

0.95

The GAB equation is a _____-parameter equation.

Three

Why is the GAB equation more popular in the field of food technology than the BET equation?

It is applicable over a much wider range of water activity.

In the BET equation, what does the symbol c represent?

The surface heat constant (indicating how strongly the monolayer binds to the surface).

Which sorption phenomenon describes molecules adhering only through weak intermolecular interactions?

Physical Adsorption (Physisorption)

Describe the aw requirement to fill a pore in the Ink Bottle Theory during rehydration.

A relatively high aw (high environmental humidity) is required to fill the wide cavity (R).

During desorption in the Ink Bottle Theory, aw must drop to a very low level for what to occur?

For the water seal in the narrow neck to break, allowing the water in the body to escape.

What physical state change can cause an increase in hysteresis during water removal?

Phase changes such as crystallization (e.g., delayed crystallization).

In the BET equation, m represents the grams of H2O per 100g of solids at a specific _____ and _____.

Water activity (aw); Temperature (T)

Which IUPAC-approved equation is used because of its simplicity of application in monolayer determination?

BET Equation

True or False: Chemisorption is generally reversible.

False; it involves the formation of a chemical bond.

What happens to the internal pores of food during drying that contributes to hysteresis?

Dissolved solutes crystallize and block pores, physically trapping water inside.

Is the monolayer value mo measured in grams of water per gram of total food or per gram of solids?

Per gram of solids.

In a Sigmoidal (Type II) curve, what is the cause of the flat slope in Region II?

Primary binding sites are full, so additional moisture causes a disproportionate spike in aw rather than MC.

What does an MSI graph plot on its X-axis and Y-axis?

• X-axis: Water Activity (aw) or Relative Humidity

• Y-axis: Moisture Content (MC).

It represents the moisture level where the food is most stable chemically and physically.

monolayer value