Water Activity

With the introdution of water activity, Food Scientists defined a unique and important property of food. It is also unique to Food Science as you do not meet this terminology in many other scientific disciplines. According to Chambers Dictionary of Science and Technology, water activity (Food Sci) is:

An expression of the amount of water present in a food, raw material of product which is available to support microbial growth. As it is reduced, the rate of growth of micro-organisms declines. The key food preservation principles are based on the reduction of water activity by removing water or by adding solutes such as sugar or salt. Symbol a_w.

As well as reducing the water activity, salt and sugar have other anti-microbial effects.

While water activity is very important for food microbiologists, it is also vitally important for food chemists and not just because of its influence on microbial growth. Water activity describes the amount of water available for reaction, any kind of reaction. As the graph below shows, knowing the water activity is also important for knowing which chemical reaction might take place:

Most reactions occurring in food require water as a solvent. Water allows the solutes to move about the material. For example, non-enzymatic browning could be also refer to vitamin stability, especially for the water soluble vitamins, ascorbic acid and thiamine. The movement of solutes allows reactants to “find” each other so a reaction will take place. Too little water – not enough movement; too much water – solutes are too dilute. Thus, these water-based reactions have a maximum reaction rate around an a_w of 0.6-0.8.

For lipid oxidation there is both a water-dependent and a water independent reaction. Thus, there are two maximum rates of lipid oxidation – one at high a_w and the other at low a_w.

Micro-organisms have specific water activities at which they grow. Bacteria require the highest a_w, with no bacterial growth below ~0.88. The food industry standard for microbial growth is 0.6, below which there is no micro-organism growth. Spores and viruses can survive but nothing will grow. In fact, micro-organisms are more sensitive to water activity than we are able to measure it (accuracy for the best water activity meter available is currently ± 0.003).

The moisture isotherm in the middle of the graph above is the relationship between moisture content and water activity. The shape of the isotherm shown is the most typical relationship for most food. Different shapes can be seen for different ingredients:

Where line B is the moisture sorption isotherm we discussed above, line A is the moisture-sorption isotherm for anti-caking agents and line C the moisture sorption isotherm for crystalline solids such as sucrose.

So what do I mean by water activity?

The simplist expression for water activity is a_w= p/p_o, where p = the partial vapour pressure of the water in the material being measure and p_o is the vapor pressure of pure water at the same temperature. Thus, the a_w of pure water is 1 as p = p_o.

Water activity is related to relative humidity by multiplying by 100:

%RH = p/p_o X 100 = a_w X 100

This has important implications for food as water migrates until the system is in equilibrium. Consider two food items, cheese (a_w ~ 0.9) and crackers (a_w ~ 0.3). If these two foods are put together, moisture will move from the highest to the lowest, and the crackers soften as they absorb water when left with the cheese too long. This is also why my favorite carrot and peanut salad should only prepared at the last minute – otherwise the crunchiness of the peanuts is lost as they absorb moisture. Water is also absorbed by some foods, such as peanuts and crackers, when they are left out in a room that has a higher humidity than their a_w.

The reason why the crackers and peanuts get soft after water is absorbed is fascinating to food chemists, especially those, like me, who are interested in the physicochemical properties of food and is something that I will discuss in later.

References

Any Food Chemistry textbook.

Bell & Labuza “Moisture Sorption”