Basics of sorption isotherm & its applications in food processing

Basics of sorption isotherm & its applications in food processing

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

2. CONTENTS 12-07-2020 2

3. Introduction 12-07-2020 3 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. 12-07-2020 4 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. a w = P/ P = Relative humidity / 100 where P is the partial pressure of water in the food ( atm ), and P the vapour pressure of pure water at the same temperature ( atm ).

5. Cont. 12-07-2020 5 The relationship between water content and water activity (aw) is complex. An increase in a w 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 12-07-2020 6 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. 12-07-2020 7 ( 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. 12-07-2020 8 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. 12-07-2020 9 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 12-07-2020 10 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. a w 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 12-07-2020 11 Many methods are available for determining water sorption isotherms. These methods can be classified into three categories: Gravimetric Manometric Hygrometric

12. 1. Gravimetric Method 12-07-2020 12 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 12-07-2020 13 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 12-07-2020 14 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 12-07-2020 15 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 12-07-2020 16 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 12-07-2020 17

18. Application of Sorption Isotherm in Food Industry 12-07-2020 18 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 12-07-2020 19 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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