Water Sorption Isotherm

The sorption isotherm describes the water activity as function of the water content expressed on a dry matter basis. The sorption isotherm is a curve that can be determined empirically and can be predicted by the Law of Rault (however it is not precise). It gives us an idea about interactions between water and other food constituents; it also gives us information about the availability of the water as function of the water content, therefore different zones in the isotherms can also be distinguished (zone1, zone 2, zone3). If the isotherms are constructed under different temperatures, additional information about the heat of evaporation can be found (important in the design of drying equipment)

BET (brunour emet teller )Isotherm: By fitting experimental data to the empirical equation, the monolayer (potential) water content can be derived. It is a very interesting parameter, because when the moisture content equals to the monolayer water, the food reaches the highest stability. It is an interesting parameter to know, because when drying food it is not recommended to dry further than monolayer water. BET is only useful in the lower part of sorption isotherm (below value of 0.4).

GAB isotherm: it is more complex, and cannot be linearized. It needs to be solved withnonlinear regression equation with statistical software. Can be used over the whole water activity range. It is a more powerful equation that BET.

There are 3 different kinds of sorption isotherms (linking water activity and water content):

ü  Type I: typical for anticaking agents (eg. Sodium silicate). These components have a large moisture adsorption capacity. When binding sites are occupied, no water uptake is possible due to which reaches a plateau in moisture content.

GRAFICO

ü  Type II: Has an S shape, the shape is a result of several interaction forces between water and  food (contains bound water- constitutional, vicinal & molayer). Typical of polymeric of polymeric food ingredients such as proteins and polysaccharides. Foods with amorphous low molecular weight will show such an hygroscopic behaviour. Such products are vulnerable for caking at too high relative humidity. It has 2 inclination points, the 1^st one responds to the addition of multilayer water causing as well to the filling of pores and capillaries,  and the 2^nd inclination responds to further swelling and dissolution of some of its components.

ü  Type III: typical of foods rich in cristaline low molecular weight components like sugar and salt. Calles J shaped sorption isotherm. Moisture uptake is very restricted until the low molecular weight components start to dissolve in the absorved water (deliquescent point).

GRAFICO

We can also  distinguish 3 zones (link with the kinds of water) in the sorption isotherm, this zones can be related to the kinds of water:  Zone III: significant change in moisture content, results in a restricted change in a restricted change of water activity; Zone II: small change of moisture content, results in a big change in water activity; Zone I: is determined by the bound water in the food: constitutional, vicinal and monolayer.

GRAPH OF ZONES

Temperature dependency

Increasing the temperature will increase the water activity and availability of water also increase. In a SI with different temperatures (eg. 30, 45, 50° SI f potatoes) you see that at fixed moisture content, there will be a shift of the sorption isotherm to the right. So it has a drastic impact on the water activity. Meaning that the increase in temperature (in aqueous food) will have a dual effect on the stability of the food because: 1.The solvent will become more available and 2. There is a kinetic factor, browman motion, so reactivity of the molecules increase.  SI at higher temperature will start to cross, it is because at higher temperature the solubility of the components increase. From the temperature dependency and from clausious claperon equation, at particular moisture content, we can calculate the heat of evaporation, energy required to remove water out of the food (at that particular moisture content). The lower the moisture content the steeper the line, because at the high water conteng (40g /100g of DM), the amunt of multilayer water in the food on total amount is very high and this is the weakest part of the chai. So the heat of evaporation will not be that high. So the slope will not be high. So the slope will be almost as free water. When we remove water from the matrix, the water will be more and more bound resulting in a bigger curve. The curves are important with regards to drying foods in industry. it is needed to calculate how much heat we need. The heat required is not equal to the normal heat of free water, because as the water evaporates the heat of evaporation also increases (due to strong interaction of bond water).

So in general the heat of sorption as function of the moisture content: al low water content, the heat of sorption will be similar to the normal water, and when you lower the water content you will see it increases. Thus we will have much more water.

Consequences: dual effect.

1.      The higher the temperature the faster the browman motion. The molecules walk fatser at higher temperatures and when they walk faster there can be more reactions.

2.      The water becomes more available, so the water activity increases. Thus solvent capability of the water increase. This will also favor the reactivity of the food.

GRAPH

In view of the temperature dependency of Aw, the sorption isotherms are temperature dependent:

-          Generally at higher temperatures and similar water contents, the water becomes more available, the water is more available, but it is also important to consider that at higher water activities, an increase in temperatures will cause an improvement in the soluvility of the substances, which results in a decrease I the water activity. Hence, at higher water activities, SI may cross each other.

-          Satability of moisture content: the stability of a food is highest when its mpisture content equals the monolayer water content.

Hysteresis

It is the relationship between water content and water activity depends upon the fact whether you dry the food or make the food humid, so there is a relationship between desorption and adsorption. If you dry your food, it will follow the solid line (by experimentally doing the sorption), when you put it in a wet environment again the same dried products) it will follow the dotted line. Meaning that the SI at the same moisture content will show higher water activity. It happens because when you dry the food and remove the water out of the capiaries, it is very difficult to put the water back into the capillaries.  Therefore at the same water content it will reach easier the monolayer water content, so the water will be more available. Another explanation is related to the impact of water content on protein structure, because water is important to retain the protein structure (by freezing you but bound water into a cristal grid). It means that if temperature is low enough we are able to take bound water, this means that we can change the hydratation state of a protein. Due to this proteins can denature (they change structure and unfold). So the water biding capacities after freezing change (water binding capacities become lower) because we are unfolding the protein (by denaturation) and the interior part of the molecule is more hydrophobic than the exterior part. Consequence: the water activity also depends on the history of the product.

GRAPH HISTERESIS

Practical consequences:

·         We can determine the amount of water in foods

·         Wc/Wa interaction

·         Availability of water as function of water content and as function of temperature.

·         Heat of evaporation

·         Derive monolayer water and stability of food.  

Criticism to concept of aw:

·          Thermodynamically – non equilibrium conditions. Theoretically there are problems because it is derived from thermodynamics. In thermodynamics we suppose that the foods are in equilibrium but foods are never in equilibrium. From thermodynamics, desorption and sorption should be the same. So the concept of water activity is still not solved. 

·          Hysteresis phenomenon- cannot explain it thermodynamically.

·          ‘strange’ observations. Eg. Amorphous lactose: the moisture content drops as function of time. It is not explainable. Also at higher water activity it collapse and becomes very dense, there is crystallization after 0.45 water activity. 

Selected effects on microbiological stability