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Quick and cost-effective water activity isotherm generation service

20 April 2020

Quick and cost-effective water activity isotherm generation service creates new opportunities for food and packaging development


Vapour Sorption Analyser


Product and packaging development teams in the food industry know that water activity is one of the most important characteristics, while water sorption isotherms provide invaluable insights for food safety and quality. Whereas traditional techniques for generating isotherms took weeks and were correspondingly expensive, newer and more cost-effective technology enables the time to be cut to a matter of days.

The food industry is subject to relentless pressure to create new food products, while also having to reduce the cost of food and packaging. Furthermore, there is a growing demand from consumers for less food packaging waste, and for more of the packaging to be recyclable. As a result of these factors, developers of food products and packaging are having to be more innovative today than ever before. Fortunately, a good understanding of water activity, coupled with state-of-the-art instrumentation, enables food scientists and packaging designers to create high-quality foods while minimising costs - and without compromising food safety or shelf life.

Water profoundly influences product safety and quality. To completely understand water in a product requires knowledge of the amount of water that can be held at a given energy state (water activity). One of the best ways to relate water activity to water content is with a water activity isotherm.

Water activity is the ratio of water pressure in a food (in equilibrium) to the saturation vapour pressure at the same temperature. In other words, the amount of water that can be bound into a food at a specific temperature. Pure distilled water has a water activity of one. If water is bound into the food, it is not available to microbes that could make the food unsafe, nor will it evaporate and cause undesirable changes in taste, texture or appearance.

A moisture sorption isotherm shows how water activity varies with moisture content at a constant temperature. In most cases the adsorption and desorption isotherms will differ from each other, so it is valuable to have both. Furthermore, water activity is often temperature-dependent, so a family of isotherms can be helpful for complete characterisation across a range of temperatures. Because of the complex nature of moisture sorption and desorption, it cannot be calculated reliably, which is why data has to be obtained experimentally for each food product. Modern benchtop instrumentation (for example, the Decagon AquaSorp) makes it quick, easy and cost-effective to generate accurate adsorption and desorption isotherms for an in-depth understanding of a food product's water activity.

The usefulness of isotherms, sometimes referred to as water activity graphs, in food and packaging engineering depends on high data resolution. Traditional methods for generating isotherms are laborious and time-consuming, often taking weeks. However, modern instrumentation enables isotherms to be generated in 24 hours or less, and these isotherms also have far better data resolution, which gives greater accuracy and makes them considerably more useful. High-resolution isotherms containing hundreds of data points can reveal phase transition points - at which products cake and clump or deliquesce (become liquid) - which might be difficult to identify with low-resolution isotherms. They can also be very helpful in making shelf life calculations, developing mixing models, investigating temperature abuse, determining the integrity of a protective coating or layer, and optimising packaging designs.

A further advantage of the automatic measuring system is that it needs no user intervention once it has been started, so there is no risk of human error influencing the results. Throughout the measurement range of 0.10 to 0.95 aw, the AquaSorp achieves an accuracy of +/-0.005 aw and a repeatability of +/-0.003 aw.

Decagon has developed the AquaSorp to utilise the patented high-resolution DDI (Dynamic Dewpoint Isotherm) method. The resultant high-resolution moisture sorption isotherms make it feasible to model and engineer food products and packaging in ways not previously feasible. Manufactures can therefore develop products that optimise safety, quality and profitability.

With isotherms being more accessible than ever before, they are becoming an increasingly valuable aid to product design and development. By comparing the isotherms of different formulations, it is possible to determine if a product can be adjusted to allow higher moisture content at a given water activity, or a lower water activity at a given moisture content. The result can be a moister product that is still shelf stable.

Likewise, two ingredients at the same moisture content may not be compatible when mixed together; if they have differing water activities, the water will adjust between the ingredients until an equilibrium water activity is obtained, leaving one too moist and the other too dry. For multi-component products, therefore, it is important to match the water activity of the components in order to prevent moisture migration.

A great example of this type of product is a snack comprising a cake with crème filling and icing covering. The cake, crème filling and icing have very different moisture contents, and hence very different textures, but ideally they will all have same water activity. This provides a product with variety but one that is still stable because the water activities remain in the safe range. In addition, the icing coating serves as a moisture barrier for the more moisture-sensitive cake interior.

Water sorption isotherms are a valuable tool for shelf life prediction. They can be used to determine the moisture content and water activity in a food's monolayer (a culture in a layer one cell thick), which represents its most stable state. An increase in water activity of 0.1 above the monolayer value will result in a decrease of shelf life by two to three times.

When developing food packaging, a product's isotherm can be used to determine the requirements by taking into account the product's sensitivity to moisture and the type of conditions it may be exposed to. It is possible to predict how long it will take a packaged product to reach a critical water activity when exposed to abuse conditions. The critical water activity represents a loss in stability and the corresponding time equates to the product's shelf life.

The adsorption and desorption isotherms will also show whether a water activity is different if the product is gaining or losing moisture. The practical impact is that a moisture content that is safe when drying a product (because it corresponds to a safe water activity of 0.6 aw or below) may not be safe when it is gaining moisture because it corresponds to a higher water activity level.

Companies and laboratories with very busy schedules can justify the purchase of an instrument such as the Decagon AquaSorp for generating moisture sorption isotherms as and when required. However, if isotherms only need to be generated on an occasional basis, it can be more cost-effective and almost as quick to use the isotherm generation service offered by Labcell. Full adsorption and desorption curves are generated for samples provided by the customer. Data is then returned in the form of a .csv file, together with the isotherms plotted on a graph. Only a small product sample is required - sufficient to enable three 14ml samples to be prepared for the AquaSorp - and the customer is asked for a moisture content figure taken from the same sample. Turnaround is usually one week or less, but the time depends on the type of sample and equipment availability.

To enquire about the isotherm generation service or to request a free datasheet for the AquaSorp Isotherm Generator, contact Labcell by telephoning +44 (0)1420 568150, email, or visit Labcell's website at



With compliments:

Taylor Alden Ltd
Suite 103, Highland House
165-167 The Broadway
London SW19 1NE
United Kingdom