The document discusses moisture sorption isotherms, which describe the relationship between water activity and moisture content in foods. It covers how temperature, hysteresis, and food composition affect isotherms. Measurement methods include gravimetric, manometric, and hygrometric techniques. Sorption isotherms have important applications in food processing and storage, such as designing drying equipment, predicting shelf-life and quality changes during storage.

1. Basics of Sorption Isotherm & Its Applications in
Food Processing
Subject : Dairy and Food Engineering-2 (DE-521)
Submitted by : Dhruv Beladiya
M.Tech ( 2nd Semester )
Department of Dairy Engineering

2. CONTENTS
Introduction
Moisture Sorption Isotherm
Effect of Temperature on Sorption Isotherm
Measurement of Sorption Isotherm
Moisture absorber
Moisture Sorption Hysteresis
Theories of Sorption Hysteresis
Application of Sorption Isotherm in Food Industry
Conclusion
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3. Introduction
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• Controlling the moisture content during the processing of foods is an ancient
method of preservation.
• This is achieved by either removing water, or binding it such that the food
becomes stable to both microbial and chemical deterioration.
• Sorption characteristics have, and are currently being examined in light of
their influence on the storage stability of dehydrated products, as well as their
effect on the diffusion of water vapour. This is a term indicating the ‘quality’
of the water content of food.
• It describes the degree of ‘boundness’ of water and hence, its availability to
participate in physical, chemical, and microbiological reactions.

4. Cont.
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• Some of the characteristics of bound water are lower vapour pressure, high
binding energy as measured during dehydration, reduced mobility as seen by
nuclear magnetic resonance (NMR), unfreezability at low temperature, and
unavailability as a solvent.
• Although each of these characteristics has been used to define bound water,
each gives a different value for the amount of water which is bound.
• aw = P/ P0 = Relative humidity / 100
• where P is the partial pressure of water in the food (atm), and P0 the vapour
pressure of pure water at the same temperature (atm).

5. Cont.
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• The relationship between water content and water activity (aw) is complex.
An increase in aw is usually accompanied by an increase in water content, but
in a non-linear fashion.
• This relationship between water activity and moisture content at a given
temperature is called the moisture sorption isotherm. These curves are
determined experimentally and constitute the fingerprint of a food system.

6. Moisture Sorption Isotherm
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• The relationship between
total moisture content and the
water activity of the food,
over a range of values, and at
a constant temperature, yields
a moisture sorption isotherm
when expressed graphically.

7. Cont.
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• (i) an adsorption isotherm is obtained by placing a completely dry material
into various atmospheres of increasing relative humidity and measuring the
weight gain due to water uptake;
• (ii) a desorption isotherm is found by placing an initially wet material under
the same relative humidity, and measuring the loss in weight.
• The adsorption and desorption processes are not fully reversible, therefore a
distinction can be made between the adsorption and desorption isotherms by
determining whether the moisture levels within the product are increasing
indicating wetting, or whether the moisture is gradually lowering to reach
equilibrium with its surroundings, implying that the product is being dried.

8. Cont.
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Classification of adsorption isotherms into five general types .
• Type I is the Langmuir, and Type II the sigmoid shaped adsorption isotherm;
however, no special names have been attached to the other three types.
• Types II and III are closely related to Types IV and V, except that the maximum
adsorption occurs at a pressure lower than the vapour pressure of the gas.

9. Cont.
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• Moisture sorption isotherms of most foods are nonlinear, generally sigmoidal
in shape, and have been classified as Type II isotherms.
• Most of the water in fresh food exerts a vapour pressure very close to that of
pure water, i.e. unity.
• This vapour pressure level is maintained until the moisture content of the
food decreases to about 22%. (Caurie)
• Foods rich in soluble components, such as sugars, however, have been found
to show Type III behaviour, this is due to the solubility of sugars in water.
• If sucrose added to starch gels sharply increased the sorption of water at
water activities higher than 0.85. (Chinachoti and Steinberg ).

10. Effect of Temperature on Sorption Isotherms
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• The effect of temperature on the sorption isotherm is of great importance
given that foods are exposed to a range of temperatures during storage and
processing and water activity changes with temperature.
• Temperature affects the mobility of water molecules and the dynamic
equilibrium between the vapour and adsorbed phases.
• Researchers have found that if the water activity is maintained constant, an
increase in temperature causes a decrease in the amount of sorbed water.
• aw is raised beyond the intermediate region, water begins to be sorbed by the
sugars and other low molecular weight constituents.
• The result is an increasing of the moisture content, i.e. intersection of the
isotherms.
• The intersection point depends on the composition of the food and the
solubility of sugars.

11. Measurement of Sorption Isotherms
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• Many methods are available for determining water
sorption isotherms.
These methods can be classified into three categories:
1. Gravimetric
2. Manometric
3. Hygrometric

12. 1. Gravimetric Method
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• Gravimetric Method Involves the measurement of weight changes.
• Weight changes can be determined both continuously and discontinuously in
dynamic or static systems (i.e. air may be circulated or stagnant).
• Continuous methods employ the use of electro-balances or quartz spring
balances.
• In the discontinuous systems, salt or sulphuric acid solutions are placed in
vacuum or atmospheric systems with the food material, to give a measure of
the equilibrium relative humidity.

13. 2. Manometric Method
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• Manometric Method Measures the vapour pressure of water in the vapour
space surrounding the food.
• To improve accuracy the fluid selected for the manometer is often oil instead
of mercury.
• The whole system is maintained at constant temperature and the food sample
will lose water to equilibrate with the vapour space.
• This will be indicated by the difference in height on the manometer.

14. 3.Hygrometric Method
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• Hygrometric Method measures the equilibrium relative humidity of air in
contact with a food material, at a given moisture content.
• Dew-point hygrometers detect the condensation of cooling water vapour.
• Electric hygrometers measure the change in conductance or capacitance of
hygrosensors. Most hygrosensors are coated with a hygroscopic salt, such as
LiCl, which absorbs moisture from the food sample.

15. Advantages of Static Gravimetric Technique
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• Determining the exact dry weight of the sample.
• Minimizing temperature fluctuation between samples and their surroundings
or the source of water vapour.
• Registering the weight change of the sample in equilibrium with the
respective water vapour pressures.
• Achieving hygroscopic and thermal equilibrium between samples and water
vapour source.

16. Moisture Sorption Hysteresis
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• Moisture sorption hysteresis is the phenomena by which two different paths
exist between the adsorption and desorption isotherms.
• The extent of hysteresis is related to the nature and state of the components in
a food.
• It may reflect their structural and conformational rearrangement, which alters
the accessibility of energetically favourable polar sites, and thus, may hinder
the movement of moisture.
• Several theories have been formulated to explain the phenomenon of
hysteresis, and to date, no theory has given a complete insight into the several
mechanisms and no quantitative prediction of hysteresis is available in the
literature.

17. Theories of Sorption Hysteresis
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Hysteresis on Porous Solids:
• This is observed in materials such as fruits, where the theory is based
on capillary condensation.
Hysteresis on Non-Porous Solids:
• This is observed in materials such as protein, where the theory is
based on partial chemisorption, surface impurities, or phase changes.
Hysteresis on Non-Rigid Solids:
• This is observed in materials such as starchy food, where the theory is
based on changes in structure, as these changes hinder penetration of
the adsorbate.

18. Application of Sorption Isotherm in Food Industry
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• It is highly important in food science and technology
• It can be used in Design and optimization of drying equipment
• Design of packages
• Predictions of quality of Food
• Stability of Food Product
• Shelf-life of Food
• Calculating moisture changes that may occur during storage

19. Conclusion
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• Moisture content control is an inherent feature of many food-processing
operations.
• Moisture sorption isotherms have an important role to play in the quantitative
approach to the prediction of the shelf life of dried foods due to their
sensitivity to moisture changes.
• The existence of hysteresis loops in the moisture sorption isotherms of food is
indicative of a non-equilibrium state, no matter how reproducible the data.
• The thermodynamic properties of foods including enthalpy and entropy of
sorption are essential for the design and optimization of unit operations, and
further help the understanding and interpretation of sorption mechanisms and
food-water interactions.

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