Water Activity in the Food Industry
Water Activity in General
Water activity (aw) or equilibrium relative humidity (ERH) measures the water vapor pressure generated by the water present in a hygroscopic product.
Water activity is based on a scale from 0 to 1.
aw = p / ps
p is the water vapour pressure above the product surface and ps the water vapour
pressure above the surface of pure water at the product temperature
Water activity is often confused with moisture content. The moisture content of a product is usually defined as the percent weight of water content in relation to the dry weight of the sample.
At equilibrium, the relation between the percentage of water and the water activity of a hygroscopic material can be graphically represented by a curve: the sorption isotherm.
For each aw value, the sorption isotherm shows the corresponding moisture content at a given constant temperature. Each product has its own sorption isotherm.
The aw of a product will always try to reach equilibrium with the surrounding atmosphere.
Water will migrate from regions with a high aw to the regions of low aw. Water will migrate until equilibrium is reached!
The Effect of Water in Foodstuffs:
Water is also recognized in the food industry as being critical for the stability of most products. aw exerts a decisive influence on such phenomena as change in colour, taste and aroma, food poisoning and spoilage (shelf life), loss of vitamins…
Controlling the Water Activity of a Product:
The aw in foods can be controlled by using various additives (humectants), by using satisfactory packaging materials, by maintaining favorable maturation and storage conditions...
Water Activity and Microorganisms:
Water activity also indicates the amount of water which is available to micro organisms.
Each species of micro organism (bacteria, yeast, mold) has a minimum aw value below which, growth is no longer possible.
Water activity control is also an important factor for the chemical stability of foods.
Most foodstuffs contain carbohydrates and proteins and are therefore subject to nonenzymatic browning reactions (Maillard reaction). The Maillard reaction gets stronger at increasing aw values and reached its peak at aw =0.6...0.7 with further increase of aw this reaction gets rapidly weaker.
Why the Need to Measure Water Activity?
Late for work one morning, Paul forgets to put the lid back onto his cereals. When coming home later that evening, Paul finds that his cereals are all soft and no longer crunchy…
Most breakfast cereals lose their crunchiness with an aw>0.4. In this case the relative humidity of the air in Paul’s kitchen was above 40% rh and the water has migrated from the air (with a high level of relative humidity) to the cereals (with a low aw level).
In the food industry, there are two main cases where water migration could cause issues:
The first case would be when the finished product contains different components, each at a different aw level.
It is important that the aw is measured and not the moisture content as the moisture content will not help with water migration issues.
If we use the example of a cheese cake, the cheese has an aw = 0.95 whereas the biscuit base has a aw = 0.3. The water would migrate from the cheese to the biscuit, leaving you with a soggy base and a dried out cheese.
One solution would be to add humectants (sugar, salt, polymeric polyols…). The humectants will lower the aw level but not the moisture content!
Depending on the product it is also possible to lower the aw level by dehydrating and freezing.
The second issue would be the storage of the finished product and the atmosphere in which it is stored. Depending on the aw of the product and the relative humidity in the atmosphere, water migration could occur.
The main solution to avoid water migration in this case, is to use a non-hygroscopic packaging or favorable storage conditions.
Water Activity and Micro-Organisms
As mentioned previously, aw indicates the amount of water in the total water content which is available to micro-organisms. Each micro-organism has its own minimum aw value below which, growth is no longer possible (growth is no longer possible but this doesn’t mean that the micro-organisms are not present).
By measuring the aw of food stuffs it is possible to determine which micro-organisms will be able to develop.
The US Food and Drug Administration (FDA) has adopted the concept of aw for establishing moisture limits beyond which certain types of food are considered susceptible to invasion by mould and bacteria.
Often, aw is described as the amount of “free” water in a product and the moisture content, the amount of “bound” water in a product. Even though this is not a scientific description of aw, it is easy to understand that chemically bound water is not accessible to micro-organisms, whereas free water is.
What Solutions Can Rotronic Offer?
Water activity or equilibrium relative humidity is usually defined as the percent relative humidity generated in equilibrium with the product sample in a closed system at constant temperature.
ERH = 100 x aw
Therefore, aw can be measured with a relative humidity sensor provided that the conditions specified in the above definition are fulfilled.
Practical Conditions for Measuring Water Activity:
Leak proof measurement chamber: closed system.
Volume ratio: air/product. The volume of air must be kept to a minimum: a small air volume reaches equilibrium with the sample faster than a large air volume
Temperature homogeneity: any temperature difference between the sensor, the chamber and the sample will result in significant errors. The higher the aw value the greater the error will be (a 0.8aw at 25°C with a 1°C temperature difference can result in an error of 0.05aw). Constant temperature needs to be kept.
Equilibrium time: in order to read the correct water activity value, equilibrium needs to be reached! The higher the aw the longer it takes!
Calibration of the sensor: using traceable humidity standards.
Humidity and Temperature Probes:
· HC2-AW 5...50°C, 0…1aw, Large thermal mass, ±0.008aw and ±0.1K…
· HC2-AW-USB Same as HC2-AW but with direct connection to PC.
· HC2-P05 Insertion probe, -40...85°C, 0…1aw, Ø5mm x 200mm, ±0.015aw and ±0.3K…
· HC2-HP50 Insertion probe, 40...85°C, 0…1aw, Ø10mm x 280 or 500mm, ±0.008aw and ±0.1K…
Laboratory display units:
· HP23-AW-A Hand held device, 2 probe connections, Data logging, Display, AwE & AwQuick Buzzer…
· Sample Holders
WP-14-S, 14mm depth, WP-40, 40mm depth.
· Disposable Sample Containers
PS-14, containers for WP-14-S, PS-40, containers for WP-40
· Clamp Sealing Mechanism:
AW-KHS, sealing clamp
· SCS Humidity Standards
EAxx-SCS, unsaturated salt solutions, Different humidity values available: 0, 5, 10, 11, 20, 35, 50, 60, 65, 75, 80 and 95%...
Water Activity Sets:
Various sets are available, please contact us for more details.
How Rotronic Measures Water Activity:
With the Rotronic product range, there are two different ways to measure water activity: AwE and AwQuick.
AwE mode: the natural equilibration of the product is measured and the measurement process is automatically stopped once equilibrium is reached. With most products, natural equilibrium requires from 45 to 90 minutes.
AwQuick mode: this mode reduces the time required to measure water activity to a few minutes, usually with almost the same accuracy as the AwE mode.
Water activity measurement reports: Combined with the Rotronic HW4 software, it is possible to automatically generate a report as soon as the measurement is finished.
Rotronic water activity sets: Rotronic offers different sets, offering a complete solution for everyone needing to measure water activity. The sets often contain, a display units, an aw measurement device, a sample holder and disposable containers as well as Rotronic humidity standards for the calibration of the measurement device.
Accuracy and long term stability: Choosing Rotronic gives you the best accuracy on the market. This helps carry out fast and effective water activity measurements on all foodstuffs. With a long term stability of under 0.001aw, the measurement devices will not need much taking care of!
This being said, we would recommend frequent spot checks in between calibrations.
Calibration and adjustment:
Calibration and adjustment is very easy with the Rotronic product range. As all of the communication is digital, the whole calibration procedure can be done via a PC, or directly from the display unit (HP23-AW-A or the HygroLab C1) with the help of the Rotronic humidity standards.
Rotronic can also offer a factory calibration (certified or not).