![Specific water activity = actual equilibrium vapour pressure of air space above food
equilibrium vapour pressure of pure water at the same temperature
OR
aw = p
p˳
Water activity has a close relation with the composition of water in the food but it also have a
relation with temperature, where it is connected to boiling and freezing points, equilibrium
relative humidity [ERH] and also with osmotic pressure.
aw = p = ERH
p˳ 100
Where p = partial vapour pressure of food moisture at temperature, T
p˳= saturation vapour pressure of pure water at T
ERH = equilibrium relative humidity at T](https://image.slidesharecdn.com/wateractivity-210802061426/85/Water-activity-and-different-instruments-to-determine-Aw-4-320.jpg)
![ RELATIONSHIP OF aw WITH MOISTURE CONTENT
Moisture content and water activity are not the same even though they look similar. Moist food
doesn’t necessarily have higher water activity than the dry food. For example, when taking fresh
fruits and jams, both have fairly high moisture content, but the water activity in fresh fruit is
higher than that of jam, this is due to the availability of more free water in fruits rather than in
jam. In jam, water is bound structurally and chemically by sugar. Moisture content is very
important in food processing operations, where it indicates whether the drying process should
be continued or stopped. Water activity is important in term of determining the shelf life and
stability of any product.
RELATIONSHIP OF aw WITH EQUILIBRIUM RELATIVE HUMIDITY [ ERH ]
ERH = aw x 100
Equilibrium relative humidity is the percentage of vapour pressure of water in a food to vapour
pressure of pure water at ideal temperature and pressure.](https://image.slidesharecdn.com/wateractivity-210802061426/85/Water-activity-and-different-instruments-to-determine-Aw-5-320.jpg)
Water activity and different instruments to determine Aw
- 1. SUBMITTED BY, ABEL JACOB THOMAS MSc 1st YEAR
- 2. WATER IN FOOD All foods contain at least some water and they are present inside in three distinct forms ; 1. Free water : Free water is the water that can be easily extracted from foods by squeezing, cutting or pressing. They are loosely present inside the food products. Free water is the only water that can be directly used by the micro organism hence amount of free water determines many properties of food like stability, shelf life etc. 2. Adsorbed water : Water that is attached to the surface of molecules like polysaccharides and proteins. It cant be readily squeezed out of the system or by applying pressure and hence this is not readily available for micro organisms. 3. Bound water : Bound water is the water that is physically trapped within crystals, such as crystalline starch, or other substances in food. This water is tightly bound inside food components and hence have reduced mobility.
- 3. WATER ACTIVITY Water activity is the important parameter of water in terms of food safety. Water activity or aw is the ratio of vapour pressure of water in food material to vapour pressure of pure water at ideal temperature and pressure conditions. The water activity of pure water is taken as 1.0 Food usually contains free water and bound water. Bound water is bound to the surface of large molecules of cell structures. This water is not usually available to the microbial growth freely or to participate in chemical and enzymatic reaction and enhance the process of spoilage. Usually the total amount of bound water in a food has no relation to food stability. The free or available water in a food supports microbial growth and participates in chemical and enzymatic reactions, increasing the chance of spoilage. Hence it is the amount of free water which is called water activity and is more important for food stability than the total water content. Therefore we can generally define water activity as an indicator of amount of free water in food.
- 4. Specific water activity = actual equilibrium vapour pressure of air space above food equilibrium vapour pressure of pure water at the same temperature OR aw = p p˳ Water activity has a close relation with the composition of water in the food but it also have a relation with temperature, where it is connected to boiling and freezing points, equilibrium relative humidity [ERH] and also with osmotic pressure. aw = p = ERH p˳ 100 Where p = partial vapour pressure of food moisture at temperature, T p˳= saturation vapour pressure of pure water at T ERH = equilibrium relative humidity at T
- 5. RELATIONSHIP OF aw WITH MOISTURE CONTENT Moisture content and water activity are not the same even though they look similar. Moist food doesn’t necessarily have higher water activity than the dry food. For example, when taking fresh fruits and jams, both have fairly high moisture content, but the water activity in fresh fruit is higher than that of jam, this is due to the availability of more free water in fruits rather than in jam. In jam, water is bound structurally and chemically by sugar. Moisture content is very important in food processing operations, where it indicates whether the drying process should be continued or stopped. Water activity is important in term of determining the shelf life and stability of any product. RELATIONSHIP OF aw WITH EQUILIBRIUM RELATIVE HUMIDITY [ ERH ] ERH = aw x 100 Equilibrium relative humidity is the percentage of vapour pressure of water in a food to vapour pressure of pure water at ideal temperature and pressure.
- 6. FACTORS AFFECTING aw Drying – Water activity is decreased by physically removing water. eg: beef jerky Solutes - Water activity is reduced by adding solutes such as salt or sugar. eg: jams, cured meats Freezing - Water activity decreases on freezing. eg: water removed in form of ice Combination - One or more of the above can be combined for a greater influence on water activity. eg: salting and drying of fish
- 7. IMPORTANCE OF aw Water activity (aw) is one of the most critical factors in determining quality and safety of foods. Water activity affects the shelf life, safety, texture, flavour and smell of foods. While temperature, pH and several other factors influence if and how micro organisms grow in a product, water activity may be the most important factor in controlling spoilage. Most bacteria, for example, do not grow at water activities below 0.91, and most molds cease to grow at water activities below 0.80 By measuring water activity, it is possible to predict which micro organism will and will not be potential sources of spoilage. Water activity determines the lower limit of available water for microbial growth. Water activity can play a significant role in determining the activity of enzymes and vitamins in foods and can have a major impact on their colour, taste and aroma.
- 8. SORPTION ISOTHERM Food sorption isotherm describes the thermodynamic relation between water activity and moisture content in a food product at constant temperature and pressure. Food sorption isotherm are useful in : 1. Optimization of drying equipment 2. Design of packages 3. Prediction of quality 4. Shelf life 5. Calculating moisture changes that may occur during storage Sorption isotherm can be generated from adsorption or desorption process, the difference between these is known as hysteresis. Water sorption by food products is a process in which water molecules progressively and reversibly mix food solvents via chemical adsorption sorption and physical absorption Physical adsorption is the adsorption of water by food materials by weak Vander Waal's force of attraction. Hence it can be reversible. It takes place at low temperature and requires comparatively lower heat of absorption. The absorbed water molecules form monomolecular layers. In chemisorption, water molecules are adsorbed by chemical forces like covalent and ionic bond and are hence irreversible. The process take place at higher temperature and require high heat of adsorption. The water molecules are found as monomolecular layers.
- 10. The sorption isotherm of a typical food product is given in the graph. The isotherm has divided into three regions, A,B and C. A → A is the steeper portion in the curve and water in the region corresponds to strongly bound water. Its enthalpy of vaporization is higher than that of pure water. The bound water includes: i. Structural water – The water which is bound to the cell structure and membranes. ii. Monolayer water – The water adsorbed by hydrophilic and polar group of food components like polysaccharides and proteins. This water is adsorbed in monolayers and is not available for microbial growth. Hence monolayer water is also included in bound water. The concept of monolayer water was first introduced by Langmuir. So monolayer water is also called Langmuir water. B → Flat region. This region comprises water molecules which are less bound than the first region. They are usually present in small capillaries. The enthalpy of vaporization is slightly higher than bound water. Hence this class of bound water is a transition from bound water to free water. C → Shoot up stage. In this region the water are held in large voids, large capilliaries and crevices. The water in this region is loosely bound to the food material and is available for microbial growth. Since the water in this region are found freely in capilliaries, they attain maximum flowability.
- 11. MEASUREMENT OF WATER ACTIVITY 1. Vapour pressure manometer – Assuming water vapour fugacity to be approximately equal to its pressure, direct measurement of pressure has been extensively used measure water activity of foods. The method for measurement of water vapor pressure of food consists of placing 10-50 g sample in a sample flask while another flask is filled with a desiccant. Keeping the sample flask isolated, the system is evacuated to less than 200µm Hg, this is followed by evacuation of the sample. Upon equilibration for 30-50 min the pressure exerted by the sample is indicated by the difference in oil manometer height. The sample flask is subsequently excluded from the system, and the desiccant flask is opened. Water vapor is removed by sorption into the desiccant and the pressures exerted by volatiles and gases are indicated by the difference in manomeric legs. The water activity is then calculated from these pressure differences.
- 12. 2. Hair hygrometer The principle underlying this instrument is that the keratinous protein of hair absorbs moisture from the atmosphere with commensurate stretching. If the hair strands are fixed at one end, attached to an indicating level arm on the other end and allowed to come into equilibrium with the substance to be measured, the ERH can be read directly. These instruments suffers from a lack of sensitivity, however they are useful for range finding and rough estimates of aw.
- 13. 3. Wink's weight equilibrium method – The procedure involves samples of the product under test to be brought to actual weight equilibrium when exposed to different relative humidities. The apparatus consists of small desiccators in which saturated salt solutions are placed. Petri dish containing the sample is kept inside the desiccators. Equal amount of samples (5-10 g) in different Petri dishes are exposed to different relative humidities ranging from 0-98 % inside desiccators at a particular temperature. The gain or loss in weight of the sample is determined at particular intervals of time. Water activity or equilibrium relative humidity is determincd from the curve drawn with weight gain or loss against relative humidities.
- 14. 4. Freezing point depression – This method at present, has been employed by a number of with reasonably good results. In this technique the depression in freezing point determines the effective gram moles of solutes that are present. This value is then used by means of an equation to arrive at the aw. This method is most appropriate for aw ranges in excess of 0.8. Obviously it would not be suitable for aw analysis of solid material.
- 15. 5. Hygrometers – There are several type of electric hygrometers that have been developed and commercially available for indirect determination of aw. Saturated salts usually are used for these purpose. These instruments are portable, are convenient to use and needs relatively short equilibrium time. The major drawbacks include sensor fatigue and sensor poisoning by volatiles such as glycol, ammonia, acetone, and other organic substances. There are three important hygrometers used. a) Resistive electrolytic hygrometer – Resistive electrolytic hygrometer uses a sensitive element which are in form of a liquid electrolyte. Resistive electrolytic hygrometer is held between two small glass rods by capillary force. The electrolyte conducts electricity and produce resistance and if it absorbs water usually in the form of water vapour, the electrolyte senses and produce a change in resistance. The sample to be analyzed is placed in the sample holder, the electrolyte absorbs water from the sample and produces a corresponding change in the resistance which is recorded by a detector attached to the hygrometer. The resistance is directly proportional to the water activity and hence the aw can be calculated from the resistance shown by the detector. The main element present in this hygrometer is the liquid electrolyte. The volatiles present in the sample may influence the performance of the measurement. This can be avoided by using chemical protection filters that absorb volatile compound before arriving at the sensor.
- 17. b) Capacitance hygrometer – The capacitance hygrometer consist of two charged plates seperated by a polymeric membrane dielectric. A polymeric membrane dielectric is actually an insulator which has the ability to hold charge and produce polarization (charge difference). When sample is placed in a sample holder of the hygrometer, the water present in the sample is absorbed by the polymeric membrane dielectric which inturn produce a capacitance. The capacitance is measured by a capacitor or a detector attached to the instrument. Capacitance hygrometers are not affected by volatile chemicals and hence they do not require cleaning. The results are more accurate than resistive capacitance hygrometer but in this hygrometer capacitance is measured instead of resistance. Since capacitance is directly proportional to aw, the capacitance reading directly gives the water activity data.
- 19. c) Dew point hygrometer - In dew point hygrometer, a mirror is placed over a closed sample chamber. The mirror acts as a sensing element and absorb water in the form of water vapour. The mirror temperature is adjusted to slightly above 0℃. The water vapour absorbed from the sample condenses on the mirror as liquid water (dew). The point at which a dew is formed on the mirror is called dew point of the mirror. The dew point is measured and sensed by an optical sensor attached to the mirror which inturn reflect light into the water droplets and produces reflected data. Earlier, the dew point temperature was used to find the relative humidity of the chamber which was inturn related to water activity. But now, the dews of an optical sensor completely outdo the earlier method by producing an accurate result. This method is usually the most fastest method to measure water activity but the sensor and the mirror requires cleaning.