For more information on water activity (aw) and its effect on food conservation, see the following article:
Reduction of aw
You can reduce the water activity of a food by two different methods. The first is a direct method of extracting water from the food by dehydration. The second is an indirect method which is to bind the available water by the addition of the depressant activity of the water.
Reduction of aw by dehydration
Food dehydration is carried out by different techniques such as drying, evaporation or freeze-drying. It reduces the water activity of a food to values close to 0. However, a technique is costly in terms of equipment and investment point of view of energy consumption.
Reduction of aw by addition of depressant
The depressant activity of the water do not belong to any particular chemical family. The water activity depends on the nature and quantity of substances dissolved in the aqueous phase of the food (or solution).
Depressants for aw used in the food industry are the salts, including NaCl, and carbohydrates, including mono-and disaccharides. The table below gives some values of aw of solutions of different concentrations of NaCl and sucrose measured at 25 ° C.
The use of salts and carbohydrates is the technique as simple and less expensive to reduce awof a food and thus improve its keeping quality. However, it can not be considered in the case of preparations where salt or sugar play a role in the organoleptic characteristics of the food. Examples of salted anchovies, capers in salt, syrups and confectionery products.
The retention by salt or salt is present in a foodstuff to the action of salt or by pouring directly onto the surface of the food (dry salting) or by immersing the product in a salt water solution (brining ). By reducing the water activity of the product, the process will slow or block microbial development. This technique is mainly used for cheese, charcuterie and in the conservation of certain species of fish (herring, salmon, ...). It is sometimes associated with smoking.
The sugar or the addition of sugar, particularly sucrose, is a technique that is widely used in food industry.
Sugar is generally used as ingredients for improved taste, color and / or texture of certain food products such as beverages, sauces (ketchup) and several other preparations. In other food products, such as caramel and glucose syrups, sugar is the main raw material used in their preparation.
Adding sugar to be quite sufficient proportions improves keeping quality of food and thanks to its depressant effect of aw. The keeping of certain foods like sweets and glucose syrups is mainly due to the presence of sugars.
In addition to the salts and carbohydrates, can be used in other products such as proteins and their derivatives to lower the aw of food although the depressant effect is limited. In addition the price of these products is relatively high. Their use is therefore more often considered for improving the nutritional value of a commodity or to enhance the taste of it.