The Equilibrium Relative Humidity ERH

This term is normally abbreviated to ERH. The ERH is deduced by extrapolating the weight loss values over a range of water activities greater and lesser than that of the product. Where the two lines intersect lies the water activity of the product. This extremely tedious and time consuming method has largely been superseded by instruments that measure the water activity directly. The ERH still has practical importance since it indicates the conditions under which the product can be stored without deterioration. 

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For a food system in equilibrium with a gaseous atmosphere (i.e. no net gain or loss of moisture to or from the system caused by differences in the vapour pressure of water), the equilibrium relative humidity (ERH) is related to aw by ... 

Water activity of foods is an important thermodynamic property affecting stability with respect to physical, chemical and microbiological changes. Water activity, av, is the ratio of the vapor pressure of water in a system, (Pl)sy, to the vapor pressure of pure water, (Pv)w, at the same temperature. It is equal to the equilibrium relative humidity (ERH) established in the surrounding air. Thus ... 

The extent to which a solute reduces aw is a function of the chemical nature of the solute. The equilibrium relative humidity (ERH) in percentage is ERH/100. ERH is defined as ... 

Microorganisms and enzymes need water in order to be active preservation of foods can be sought by reducing their moisture content to a point where the food-spoilage of pathogenic microorganisms are inhibited. Scott [ 1,2] called the equilibrium relative humidity (ERH), which is the availability of water in a food medium. Water Activity (a, ). a, is defined as the ratio of vapour pressure (p) of solution to the vapour pressure (Po) of the solvent, usually water ... 

In the solid-state fermentation process, the water activity is variable because of evaporation and metabolism. In order to keep the water activity constant, the humidity of the supplied air should be controlled or a humidifier should be installed in the fermentor. The relationship between the water activity within the solid substrate and the equilibrium relative humidity (ERH) of the surrounding air can be expressed as ... 

Biochemical constituents may partially immobilize water by stopping vaporization and lowering its chemical reactivity. The state and availability of water in food is just as important as the content of water. As a measurable quantity, water activity, expressed as a is used to express the availability of water in a food product. It is defined as the partial lowering in partial pressure created by the food and is the ratio of the partial pressure of water in the food to the partial pressure of pure water at the same temperature. Water activity is also equal to the equilibrium relative humidity (ERH) divided by 100. 

A food product is in equilibrium with its surroundings when its internal vapor pressnre is in equilibrium with the outside vapor pressure. The moisture content of the product at this stage is called the equilibrium moisture content (EMC). The corresponding vapor pressure in surrounding air at the same temperature is called the equilibrium vapor pressure. The ratio of the equilibrium vapor pressure to the saturation vapor pressure is known as the equilibrium relative humidity (ERH), or water activity. 

The effects of fungal fermentation on the moisture adsorption and retention properties of defatted peanut flour have been reported (28). At 8 and 21OC, little difference was noted between the moisture contents of freeze-dried ferments and untreated samples equilibrated at relative humidities ranging from 14 to 75%. However, marked changes were noted above 75% equilibrium relative humidity (ERH), where the control samples did not adsorb as much moisture as did the ferments. These differences were attributed to an increased ratio of exposed hydrophilic to hydrophobic groups resulting from fermentation. 

There are two common methods (static and dynamic) for measuring the EMC-equilibrium relative humidity (ERH) relationships of grains and oilseeds. The static method is also known as the EMC method and the dynamic method is also known as the ERH method. 

Relative humidity has been defined by Aguilera and Stanley (1990) using the equation RH (%) = F/po X 100%, where P is the partial water vapor pressure in an atmosphere and po equals the water vapor pressure of pure water at the same temperature. Thus, the water activity of a food can be determined by the prevalent relative humidity in the atmosphere surrounding the food when in an equilibrium state. It can be expressed by the equation a = F/po = RH% X 1/100 (in equilibrium state) = ERH% X 1/100, where ERH is an expression of equilibrium relative humidity (Watson and Harper, 1987). 

Thus, under ideal conditions, ERH is the % relative humidity of an atmosphere in which a foodstuff may be stored without a net loss or gain of moisture. Water activity, together with temperature and pH, is one of the most important parameters which determine the rates of chemical, biochemical and microbiological changes which occur in foods. However, since aw presupposes equilibrium conditions, its usefulness is limited to foods in which these conditions exist. 

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