Just as the relative humidity in the air is expressed in%, so by analogy the internal relative humidity (ERH - equilibrium relative humidity) is expressed by water activity - aw
Water activity is expressed in the range of values 0 (completely dry substance) to 1 (the wettest substance, ie distilled water) and is expressed by the equation:
aw = p / ps
p - water vapor pressure above the sample surface
ps - water vapor pressure above surface of distilled water at the temperature of the measured sample
relative humidity is defined by the equation ERH = 100 x aw
Water activity is very often and erroneously mistaken for moisture, which is usual defined as percentage by weight water content in relation to dry sample weight.
The relationship between the percentage of water and water activity can be expressed by a sorption isothermal curve. For each value of water activity aw, the isothermal curve shows the moisture content at a given constant temperature. Each substance has its own isothermal curve.
Water activity aw always tries to balance the environment. Water will always move from places with high aw to places with low aw until equilibrium is reached.
Influence of water in food:
Its share has a huge impact on the quality of food, its presence has an impact on color, taste, smell, toxic substances or their degradation.
The value of water activity also shows how much free water is available to microorganisms. Each of their species (bacteria, yeast, mold) has a minimum limit aw, below which it is no longer able to reproduce.
Water activity is an important value for the chemical stability of food. Foods containing carbohydrates and proteins undergo a so-called Maillard reaction, which is more intense with increasing aw and has its peak after reaching aw = 0.6-0.7, after this limit with increasing aw, this reaction weakens.
Water activity and microorganisms:
As already mentioned, the water activity aw shows how much free water is available to microorganisms that have a minimum value of aw below which they are no longer able to multiply (this does not mean that they are not present). By measuring aw, we are able to determine which types of microorganisms are capable of multiplication, ie which are behind the degradation of coffee.
Food and hygiene government authorities (understand not moral) have already adopted limits on aw limits in coffee, which are still suitable for consumption. Water activity is taken as the amount of "free" water (available to microbes) and moisture as "bound" water (not available to microbes).
The quality of coffee is expressed in terms of climatic conditions during cultivation, processing and storage, as well as in the process of drying in the sun, when the beans are spread on the ground and the natural microbiological fermentation process has a major impact on its quality.
Microbiological contamination can occur in cherries during harvesting, fermentation, drying or storage. Bacteria, yeasts or fibrous fungi, were found in some areas of the flesh and grains during processing. Fungi in particular have a huge qualitative impact due to the production of mycotoxins, a small group of similarly formed, chemically similar metabolites of which are ochratoxins (OTA), produced due to some forms of the genus Aspergillus a Penicillium on a wide variety of foods. Detection of the presence of OTA and their removal is thus essential for the analysis of coffee quality after harvest, during transport but also for setting up proper storage. Early assurance of the aw level after harvest thus leads to protection against the formation of mold and OTA during storage. In the tropical and subtropical zones, the value of aw should be less than 0.8. Here is an overview of aw values and its impact on OTA production:
aw> 0.95 hOTA-producing teeth are unlikely to be growth, on the contrary, hydrophilic fungi and yeast grow first
aw <0.8 OTA-producing fungi will be present but do not produce toxins
aw <0.78 -0.76 OTA-producing fungi cannot grow
Therefore, the most important the point is to control the time in which the coffee remains in dry matter, and where the OTA-producing fungi they can grow up to andIn 0,8 - 0,95. According to the measurement results, it can up to 5 days or even less when drying is sufficient for effective prevention accumulation of OTA in green coffee.
In general, the maximum value of aw in range 0.67 - 0.70 and moisture content <12.5%, sufficient to protect coffee from damage.