Moisture Sorption Isotherms Characteristics of Food Products:

A Review

By Mustapha M Bello

University Putra Malaysia.

Introduction

• Knowledge of sorption properties of foods is important in food dehydration.

• Controlling moisture content during processing of food is an ancient method of preservation

• It is achieved by removing water or binding it to the food particles.

• Walter was the first to relate RWVP to microbial growth; the main cause of food spoilage.

• Scott and Salwin (1924) independently introduced the concept of water activity

Water activity

• Water exists as “free” or “bound” in food products

• Water activity describes the degree of “boundness” of water and hence its availability to participate in physical, chemical and microbial activities.

• Water activity is given by, a

Moisture sorption isotherms

• Relationship between total moisture content and the water activity of foods

• Adsorption or desorption isotherms• The two processes are not fully reversible and

this give rise to hysteresis• Brunauer classified adsorption isotherms in to 5

types based on their shape• Type 1 is Langmuir and type 2 the sigmoid

shape• No special names for type 3,4 & 5

• Types 1, 2 & 3 of Brunauer classification

1

2

3M

Water activity

• Types 3 & 4 of brunauer classification4

5

M

Water Activity

Sorption isotherms-cont

• Moisture sorption isotherms of most foods are generally sigmoid and classified as type two isotherms

• Foods reach in soluble components such as sugar, show type 3 isotherms

Effects of temperature on sorption isotherms

• Foods are exposed to range of temp. during storage and processing

• Water activity change with temperature• Increase in temperature caused decrease in

adsorbed water• At high temp foods become less hygroscopic• EMC decrease when T increases @ water

activity less than 0.7• EMC increases with T @ water activity greater

than 0.7

Measurement of sorption isotherms

• Gravimetric method: weight change

• Manometric method: vapor pressure

• Hygrometric method: measures ERH

Mathematical description of MSI

• Several models but none gives accurate result throughout the whole range of WA and for all types of foods1 The Brunauer-Emmett-Teller (BET) equation• Interpretation of multilayer SI, particularly type 1 & 2• Provides an estimation of monolayer value• Only linear in the range of 0.05-0.35 WA• Useful in defining optimum moisture content for drying and storage stability of foods2 Halsey equation suitable for type 1, 2 or 3 isotherms suitable @ WA between 0.1-0.83 Guggenheim-Anderson-De boer (GAB) Equation -semi-theoretical and the most versatile model available - it represent refined extension of the BET model - good description of sorption behaviour of almost all foods up to 0.9 WA4 Oswin equation - best for describing the isotherms of starchy food - reasonably good fit for meat and vegetables - purely emperical5 Handerson equation - it relates the water activity to the amount of water sorbed6 Smith equation - purely emperical