The water, chemical formula H2O, is the major constituent of most foods. Although it brings any energy to food, its existence plays a very important role. It influences the structure, the appearance, taste of food and their susceptibility to degradation.
The water content of foods is very variable: 10 to 20% in cereals, 60 to 75% in meat and animal flesh, 80 to 90% in fruits and vegetables.
|Food||Water content (%)|
|Beef||50 to 70|
|Fish||65 to 81|
|Pears||80 to 85|
|Apples, peaches, oranges||85 to 90|
|Tomatoes, strawberries||90 to 95|
|Avocado, banana||74 to 80|
|Carrot, potato||80 to 90|
|Lettuce, lentils||90 to 95|
Knowing the water content of food products is often necessary for this:
- Technological need: Knowledge of the water content of food is necessary for the efficient conduct of the operations of harvesting, drying, storage or processing. It is a key parameter for evaluating and controlling the risks of deterioration during storage of foodstuffs.
- Regulatory need: If the regulations lay down the limits of water content of certain foods for reasons of hygiene or for guaranteeing the fairness of commercial transactions.
- Need Contract: In case of commercial contracts require limited water content in food.
- Need analysis: The results of analysis of food products are often expressed in relation to a fixed base (dry matter or water content standard).
Functional properties of water in food
Water has three main functions in food. These functions are:
- Function solubilization (or dispersion): Water in foods is the solvent of the hydrophilic constituents.
- Function structure: Water plays a key role in the pattern of food macromolecules, including proteins and carbohydrates. The water also determines the structure of certain constituents in the micelle. This applies, for example, casein in milk.
- Mobilization function: water, compared to other fluids, is the mobility factor of the response in food products.
Expression of the amount of water in food
The moisture content or humidity of a food is the amount of water lost by the substance when brought into balance with a true vapor pressure of zero (relative humidity of 0%). The amount of water lost is made up of water fixed by hydrogen bonds (water sorption, water held by capillary or osmotic, water solutions, water in occluse mesh and crystal water of crystallization); water chemically linked by covalent bonds is excluded.
The water content of a sample of food is expressed as% of water is reported to the mass of dry matter contained in the sample, ie the total mass of the moist of the sample.
Water activity (aw) indicates the availability of water from a medium for chemical reactions, biochemical, a change of state or transfer through a semi permeable membrane.
Water activity (aw) is the ratio of the vapor pressure of the feed water (vapor pressure of water at the surface of the product) and the pressure of water vapor pure at the same temperature q °.
The value of water activity varies between 0 (dry to the point where all the water is linked to the food, and thus quality-reactive) and 1 (pure water and solute, which is difficult to achieve especially to maintain).
The awof a solution can be calculated by the formula RAOULT:
aw= n1/ (n1+ n2)
n1= number of moles of solvent (water).
n2= number of moles of solute.
The table below shows the value of the awof solutions of different concentrations of NaCl and sucrose, measured at 25 ° C.
The water activity of a food depends on the temperature. A change of 10 ° C can cause a change in awfrom 0.03 to 0.2 depending on the type of product. Thus, the change in temperature can affect the stability of a product and plays an important role in preserving the product in a sealed package.
Relation between water content and water activity
At equilibrium, the relationship between water content and water activity (aw) of a food product at a constant temperature can be represented by a curve called sorption isotherm. For each value of aw, the isotherm gives the water content (Xeq) of the product at a given temperature.
The sorption isotherms are divided into three zones:
- Zone 1 (aw<0.3): water is strongly linked, "also called" constitution ". Water is intimately linked to the biochemical components by covalent bonds and can not be separated by techniques very severe. This water is virtually unavailable as a solvent or reagent and corresponds to the first layer (monolayer) around the dry food.
- Zone 2 (0.3 <aw<0.7): water is "weakly bound" in the form of polymoléculaires layers (multilayer) partially covering the surface of the substrate dry. Although also available as a solvent as a reagent, it is moderately reactive.
- Zone 3 (aw>0.7): water is "free" or "liquid water" that is retained on the surface of the substrate by dry hydrogen bonds. This water is available both as a reactive solvent. Only in this way that water is used by microorganisms and may allow the enzymatic reactions.
There are two types of sorption isotherms:
- Isotherme adsorption if it was determined experimentally from a dry product.
- Desorption isotherm if it was determined experimentally from a product-saturated water.
The two curves are generally different because the drying of a product leads to structural changes and irreversible porosity.
Water activity and food preservation
The importance of water activity for the stability of foods during processing and storage is illustrated in a very obvious below. Broadly speaking, optimum stability is obtained when aw is between 0.2 and 0.3.
Water activity and the oxidation reactions
Rancidity is one of the main reactions of food spoilage at low and medium water content, it is observed even for water activities ranging from 0 to about 0.2 (red curve).
Oxidation of lipids is often the limiting factor for the conservation of certain foods or dehydrated average water content. The addition of antioxidants or an increase in water content can change the data and lead to make the stability of other reactions alterations in particular non-enzymatic browning.
Water activity and non-enzymatic browning (Maillard reaction)
The rate of non-enzymatic browning increased rapidly with the water activity and reached a maximum of between 0.5 and 0.7 (gray curve). Beyond these values, the speed of this reaction decreases.
Like the oxidation of lipids, the NEB is often the limiting factor for preserving food at average water content. It is also a disturbing deterioration of reaction during dehydration operations where you have to try to cross the critical area as soon as possible and at a minimum temperature.
Water activity and enzymatic browning
The enzyme activity (orange curve) and the ultimate amount of hydrolysis considerably when the water activity exceeds 0.7.
To avoid the undesirable effect of the enzyme activity that may occur during storage of food whether or not dehydrated or frozen, there are usually a bleaching, before dehydration or freezing, which is the main goal destruction of enzymes.
Water activity and microbial activities
The growth of bacteria (black curve) is usually impossible when aw<0.90. Molds and yeasts (curves in light green and dark green) are inhibited, respectively, to an aw of 0.7 and 0.8 except some osmophiles yeasts and molds that can grow to aw of 0.6. In most cases, aw limits for growth of a microorganism different from aw limit required for the production of its toxin.
In a food, a water activity of 0.7 is considered a lower limit with all the guarantees of microbial stability. But 0.91 is a limit below which the growth of microorganisms is highly constrained. This limit was adopted by the EU directive of 1977 for food at room temperature and is even raised to 0.95 provided that it comes with a pH less than or equal to 5.2.