Water Sorption Isotherms: Key to Food Preservation

Every dried food product – from milk powder to biscuits to spices – has a unique relationship with moisture. How much water a food holds, and how tightly it holds that water, directly determines whether the product stays fresh or spoils. Water sorption isotherms are the graphical tools that map this relationship, giving food scientists a reliable way to predict stability, design packaging, and extend shelf life. If you work in food processing or preservation, understanding these curves is not optional – it’s essential.

What exactly is a water sorption isotherm? 🔗

A water sorption isotherm is a curve that plots the equilibrium moisture content of a food against its water activity (a_w) at a constant temperature. Water activity ranges from 0 (bone dry) to 1.0 (pure water) and measures how “available” the water in food is for microbial growth, enzymatic reactions, and chemical deterioration. Knowing the moisture content alone is not enough to predict food stability – you also need to know how that moisture interacts with the food matrix.

The term “isotherm” means these measurements are taken at a constant temperature. This matters because foods generally hold less moisture at higher temperatures and more moisture at lower temperatures. Each food product produces a slightly different isotherm shape depending on its chemical composition, physical structure, and the types of solutes it contains. In that sense, the isotherm acts as a unique fingerprint of a food’s moisture behaviour.

The sigmoid curve and its three regions 🔗

Most food sorption isotherms display a characteristic S-shaped (sigmoid) curve. This shape can be divided into three distinct zones, each representing a different state of water binding within the food. Understanding these zones is the foundation for making sound preservation decisions.

Region I: Monolayer water (aw below 0.2-0.3) 🔗

At very low water activity levels, water molecules are tightly bound to specific polar sites on food components – primarily proteins, starches, and other carbohydrates – through strong hydrogen bonds. This water forms what is called the monolayer: a single molecular layer coating accessible binding sites on the food surface.

Monolayer water is not available as a solvent. It cannot support microbial growth or participate in chemical reactions. It does not freeze even at sub-zero temperatures. Removing this water requires a significant amount of energy, which is why aggressive drying methods like freeze-drying are needed to bring moisture below this level. According to research published by the Institute of Food Technologists, the water activity range of 0.2-0.3 corresponds to the monolayer moisture region, and this represents the optimal moisture range for maximum shelf life of dehydrated foods.

Region II: Multilayer and capillary water (aw 0.3-0.7) 🔗

In this intermediate zone, additional water molecules stack on top of the initial monolayer, forming multiple layers. This water is also held through capillary condensation – the process by which water vapour condenses inside the tiny pores and capillaries present in the food’s physical structure.

The capillary condensation phenomenon is particularly significant in porous foods like crackers, cereals, and dried fruits. The pore geometry determines how water fills these spaces, and it also explains why foods gain and lose moisture at different rates. Water in this region has some molecular mobility and can participate in certain chemical reactions, including non-enzymatic browning and lipid oxidation, though at reduced rates compared to free water. Conventional drying techniques can remove most of the multilayer water.

Region III: Free water and dissolution of solutes (aw above 0.7) 🔗

At high water activity, water behaves essentially like bulk or free water. It moves freely through the food matrix, dissolves solutes such as sugars and salts, and is readily available for microbial growth and biochemical reactions. This is the zone where food is most vulnerable to spoilage.

Most bacteria require a_w above 0.91 to grow, yeasts need levels above 0.88, and moulds can survive down to about 0.65. The FDA’s Food Code defines potentially hazardous foods as those with an equilibrium water activity greater than 0.85, since pathogenic bacteria can begin growing above this point. When solutes dissolve in this free water, the food’s water activity changes, creating a direct link between composition and preservation potential.

Adsorption, desorption, and the hysteresis effect 🔗

Sorption isotherms can be determined in two directions. An adsorption isotherm is obtained by placing a dry food sample in environments of increasing humidity and measuring the weight gain as it picks up moisture. A desorption isotherm is obtained by starting with a wet sample and measuring weight loss as it dries out under decreasing humidity.

Here’s the important part: these two curves do not overlap. At the same water activity, a food undergoing desorption (losing moisture) holds more water than the same food undergoing adsorption (gaining moisture). This gap between the two curves is called sorption hysteresis.

Hysteresis has real practical consequences. It means that the drying history of a food affects its final stability. A food that has been dried from a high moisture state will have a different equilibrium moisture content than one that has absorbed moisture from a dry state – even at the same water activity and temperature. According to a review published in Vitae journal, hysteresis is linked to the nature and state of food components, and it reflects potential structural and conformational rearrangements that change how accessible polar binding sites are.

Several theories attempt to explain hysteresis. The ink bottle theory proposes that narrow pore openings trap water during desorption. The molecular shrinkage theory suggests that drying causes food structures to contract permanently, reducing their water-holding capacity. In porous foods like fruits, capillary condensation plays a key role, while in starchy or proteinaceous foods, structural changes in the polymer matrix are more important.

The BET equation and monolayer moisture content 🔗

One of the most valuable applications of sorption isotherms is calculating the monolayer moisture content – the amount of water needed to form a single complete molecular layer over all available binding sites in a food. The Brunauer-Emmett-Teller (BET) equation, first proposed in 1938, is the standard method for this calculation.

The BET equation works by modelling the physical adsorption of water molecules onto a solid food surface. When experimental data is plotted according to this equation, it typically produces a linear relationship in the water activity range of 0.05 to 0.45, from which the monolayer value can be calculated. For most foods, the monolayer moisture corresponds to a_w values between 0.2 and 0.4.

Why does this number matter so much? Because foods stored at or slightly above their monolayer moisture content tend to show maximum stability. At this moisture level, chemical reactions are minimised, physical properties remain consistent, and sensory quality is preserved. Below the monolayer, certain reactions – particularly lipid oxidation – can actually accelerate, which is why extremely aggressive drying isn’t always beneficial.

The GAB model: a more practical alternative 🔗

While the BET equation is widely used, it has a significant limitation: it only fits experimental data well in a narrow water activity range (up to about 0.45). For a more complete picture, food scientists increasingly rely on the Guggenheim-Anderson-de Boer (GAB) model.

The GAB model extends the BET theory by introducing a third parameter that accounts for the properties of water in the multilayer region. This gives it the ability to describe sorption behaviour across a much wider water activity range – from 0.10 to 0.90. The European Project Group COST 90 on Physical Properties of Foods has recommended the GAB equation as the fundamental equation for characterising water sorption in food materials. Its combination of theoretical grounding, mathematical simplicity, and broad applicability makes it the preferred model in most industrial and research settings.

How temperature affects sorption isotherms 🔗

Temperature has a direct impact on water sorption behaviour. As temperature increases, the monolayer moisture content generally decreases. This happens because water molecules at higher temperatures have more kinetic energy, which weakens their binding to food surfaces and allows more of them to escape from sorption sites.

This means a food stored at 35°C will have a lower equilibrium moisture content than the same food stored at 25°C at the same water activity. For food manufacturers, this temperature dependency means that storage guidelines and shelf-life predictions must account for the conditions in which a product will actually be kept. A shelf-life estimate made at laboratory temperature (25°C) may not hold if the product is shipped through a hot warehouse in summer. Research on amaranth-sorghum complementary food demonstrated that shelf life shortened two to three fold at 35°C compared to 25°C.

Practical applications in food preservation 🔗

Water sorption isotherms are not just academic curves. They drive several critical decisions in the food industry.

Optimising drying processes 🔗

Manufacturers use isotherms to determine exactly how much moisture needs to be removed to reach a target water activity. Over-drying wastes energy and can damage product quality. Under-drying leaves the food vulnerable to spoilage. The isotherm provides the precise endpoint.

Packaging design 🔗

Selecting packaging materials with the right moisture barrier properties is essential for maintaining water activity during storage. Isotherm data tells engineers the maximum moisture gain a product can tolerate before its stability is compromised. For example, studies have shown that dried foods packaged in laminated aluminium pouches can maintain projected shelf life significantly longer than those in lower-barrier plastic films.

Shelf-life prediction 🔗

By combining isotherm data with packaging permeability information and expected storage conditions, food scientists can predict how long a product will remain safe and acceptable. Mathematical models like the Heiss-Eichner model use isotherm data as a key input to estimate shelf life under specific temperature and humidity conditions.

Product formulation 🔗

When developing new food products, ingredient selection directly affects the sorption isotherm. Ingredients that bind water more tightly – like certain proteins and hydrocolloids – can lower the overall water activity of a formulation while maintaining desirable texture and taste. Isotherms help formulators balance moisture, texture, and safety.

Measurement methods 🔗

Sorption isotherms must be determined experimentally – they cannot be reliably calculated from theory alone for complex food systems. The three main measurement approaches are the gravimetric method (weighing samples as they equilibrate at different humidity levels), the manometric method (measuring vapour pressure changes), and the hygrometric method (measuring the relative humidity of air in equilibrium with the food sample). Among these, the gravimetric method using controlled humidity chambers or dynamic vapour sorption instruments is the most widely used in both industry and research.

Factors that influence isotherm shape 🔗

Several factors determine the specific shape of a food’s sorption isotherm. Composition is the most important – foods rich in sugars behave differently from those rich in proteins or starches, because different molecules bind water through different mechanisms. Physical structure also matters: porous foods with many capillaries adsorb more moisture than dense, compact foods. Previous processing history – whether a food was air-dried, freeze-dried, or spray-dried – affects the accessibility of binding sites and therefore the isotherm shape. Even the crystalline versus amorphous state of components like sugars can shift the sorption curve significantly.

What do you think? How might knowledge of water sorption isotherms change the way you approach storage of dried foods in your home or facility? If two foods have the same moisture content but different water activities, what does that tell you about their relative stability?

References

1. https://www.sciencedirect.com/science/article/pii/S0960308502703052
2. https://www.ift.org/news-and-publications/food-technology-magazine/issues/2006/november/columns/laboratory
3. https://en.wikipedia.org/wiki/Capillary_condensation
4. https://www.fda.gov/food/guidance-regulation-food-and-dietary-supplements
5. http://www.scielo.org.co/scielo.php?script=sci_arttext&pid=S0121-40042011000300012
6. https://mts.intechopen.com/redirector/articles/moisture-sorption-isotherms-and-isotherm-model-performance-evaluation-for-food-and-agricultural-prod
7. https://www.sciencedirect.com/science/article/abs/pii/S0260877400001394
8. https://link.springer.com/chapter/10.1007/978-1-4939-2578-0_15
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC7026356/