Understanding Types of Water in Foods and Water Activity

Water makes up 70% or more of most natural foods, yet not all of that water behaves the same way. Some of it can be squeezed out with your hands, some is locked so tightly to food molecules that even extreme cold cannot freeze it, and some sits trapped inside cells waiting to be released. Understanding these different forms of water – and the concept of water activity – is central to food processing, preservation, and safety. This knowledge helps food scientists predict how long a product will last, what texture it will have, and whether harmful microorganisms can grow in it.

Types of water in foods 🔗

Water in food exists in three primary forms: free water, bound water, and entrapped water. Chemically, all three are identical – they’re all H₂O. The difference lies in how each type interacts with other food components and how easily it can be removed or used by microorganisms.

Free water 🔗

Free water, also called available water, is the most abundant form of water in most fresh foods. It is not chemically attached to any food component and moves freely within the food matrix. Think of the juice that drips when you cut a watermelon or the liquid that separates on top of yogurt – that is free water.

Key characteristics of free water include:

Easy removal: It can be extracted by simple mechanical methods like pressing, squeezing, or centrifugation. It can also be removed by drying, evaporation, or freezing.

Acts as a solvent: Free water dissolves salts, sugars, vitamins, and other small molecules, enabling biochemical reactions and flavour development in foods.

Supports microbial growth: Because it is readily available, free water is the primary type that bacteria, yeasts, and moulds use for their growth and metabolism. This is a major reason why fresh, high-moisture foods spoil quickly at room temperature.

Affects texture: The juiciness of fruits, the tenderness of meat, and the moist crumb of fresh bread are all largely due to the presence of free water.

Bound water 🔗

Bound water is the opposite of free water. It is tightly associated with food components – proteins, carbohydrates, starches, pectins, and minerals – through hydrogen bonds, ion-dipole interactions, and other molecular forces. This water is so firmly held that it behaves almost like part of the solid food itself.

Several properties set bound water apart from free water:

Cannot act as a solvent: Because bound water molecules are locked onto food molecules, they cannot dissolve salts or sugars or participate in biochemical reactions.

Extremely difficult to freeze: Bound water does not freeze at the normal freezing point of water (0°C). It typically requires temperatures as low as −40°C to solidify. In animal tissues, roughly 8-10% of the total water remains unfrozen even below 0°C.

Has negligible vapour pressure: Since the molecules cannot easily escape into the air, bound water exhibits almost no vapour pressure – which is why it cannot be removed easily by conventional drying.

Does not support microbial growth: Microorganisms cannot access bound water, which makes it a critical factor in food stability. Foods with a high proportion of bound water relative to free water have inherently longer shelf lives.

Even fully dried foods retain some bound water. Wheat flour, for example, holds roughly 14% moisture – most of which is bound water associated with starch granules and gluten proteins. Hard cheeses like Parmesan, dried legumes, and spice powders all contain bound water despite feeling dry to the touch.

Entrapped water 🔗

Entrapped water (sometimes called imbibed water) occupies an intermediate position between free and bound water. It is physically trapped within capillaries, cells, or gel structures in the food, but it is not chemically bonded to food molecules.

Behaviour: Entrapped water does not flow freely under normal conditions, but once the food structure is damaged – through cutting, pressing, or cooking – it is released and then behaves exactly like free water.

Freezing: Unlike bound water, entrapped water can be frozen, though typically at temperatures slightly below 0°C.

Microbial implications: While not as readily available as free water, entrapped water can still serve as a medium for microbial growth and chemical reactions once released.

Good examples of entrapped water include the moisture held within pectin gels, the water inside intact plant cells of fruits and vegetables, and the moisture in bread crumb structure. When you cook rice, the starch granules absorb water and swell – the resulting softness comes from water that has been imbibed into the starch structure.

What is water activity (aw)? 🔗

Knowing the total moisture content of a food tells you how much water is present, but it doesn’t tell you how much of that water is actually available for microbial growth and chemical reactions. That’s where water activity comes in.

Water activity, written as a_w, is defined as the ratio of the vapour pressure of water in a food to the vapour pressure of pure water at the same temperature. The U.S. FDA defines it using the formula:

a_w = P / P₀

Where P is the vapour pressure of water in the food and P₀ is the vapour pressure of pure water under the same conditions. It can also be expressed as the equilibrium relative humidity (ERH) divided by 100.

The scale ranges from 0 (completely dry) to 1.0 (pure water). Most fresh foods fall between 0.95 and 0.99, while dried products can be as low as 0.20.

Water activity is not the same as moisture content 🔗

This is a common point of confusion. Two foods can have the same moisture content percentage but very different water activity values. For example, honey and fresh meat may contain similar amounts of water by weight, yet honey has an a_w of around 0.6 while fresh meat sits at about 0.99. The difference is that honey’s water molecules are tightly bound to its high sugar content, making them unavailable. This explains why honey stays shelf-stable for years while fresh meat spoils within days.

Why water activity matters for food preservation 🔗

Water activity is arguably the single most important factor in predicting microbial stability of foods. Every microorganism has a minimum a_w threshold below which it simply cannot grow. This principle forms the basis of many preservation strategies that have been used for centuries.

Microbial growth thresholds 🔗

Different types of microorganisms tolerate different levels of water activity:

Bacteria are the most demanding. Most bacteria need a_w above 0.91 to grow. Pathogenic bacteria such as Salmonella and E. coli generally require even higher levels, above 0.95. The dangerous Clostridium botulinum has a minimum a_w of approximately 0.93.

Yeasts can survive at somewhat lower water activity levels, generally down to about 0.88. Some osmophilic (sugar-tolerant) yeasts can grow at a_w values as low as 0.61.

Moulds are the most tolerant. Many common moulds can grow at a_w levels as low as 0.70-0.65, which allows them to colonise foods like dried grains, nuts, and even certain spices that bacteria and yeasts cannot spoil.

A key regulatory benchmark: the FDA uses a_w of 0.85 as the threshold below which certain food safety regulations (specifically 21 CFR Parts 108, 113, and 114) do not apply. Below a_w 0.60, virtually no microorganism – bacterium, yeast, or mould – can grow.

Impact on chemical reactions 🔗

Water activity doesn’t just affect microbes. It also influences the rate of chemical and enzymatic reactions that degrade food quality. Lipid oxidation and Maillard browning, two major causes of off-flavours and colour changes in stored foods, are both influenced by water activity levels. Lowering a_w slows these reactions, helping maintain flavour, colour, and nutritional quality over time.

How to control water activity 🔗

Food processors and home preservers have several practical methods to reduce water activity and extend shelf life. These methods work either by physically removing water or by binding it so tightly to solutes that microorganisms cannot use it.

Adding salt 🔗

Salt (sodium chloride) is one of the most effective water activity-lowering agents. It works by binding with free water molecules, making them unavailable for microbial use. On a weight basis, a 13% salt solution reduces a_w to approximately 0.91 – low enough to suppress most common bacteria. This is the science behind salt-cured meats, pickled vegetables, soy sauce, and salted fish.

Adding sugar 🔗

Sugar also binds free water, though it is less effective than salt on a weight basis. Reaching an a_w of 0.91 with sugar requires a concentration of about 55%. This is why jams, jellies, candies, and condensed milk – all with very high sugar levels – remain shelf-stable despite containing significant moisture. The high sugar content of honey (a_w ≈ 0.6) is a classic example of sugar-based preservation.

Drying and dehydration 🔗

Removing water directly through sun-drying, hot-air drying, spray-drying, or freeze-drying is the most straightforward way to lower a_w. Dried fruits, jerky, and powdered milk typically have a_w values below 0.75, well under the threshold for most microbial growth.

Freezing 🔗

Freezing immobilises water molecules and dramatically slows both microbial metabolism and chemical degradation reactions. While freezing doesn’t technically lower a_w in the same way as drying or solute addition, it effectively makes water unavailable for biological processes.

Using other humectants 🔗

In modern food manufacturing, ingredients like glycerol, sorbitol, glucose syrup, and pectin are used to bind water and reduce a_w without making the product excessively salty or sweet. Intermediate-moisture foods such as soft pet foods, some sausages, and certain bakery products use these humectants to stay soft and palatable while resisting spoilage.

Water activity values of common foods 🔗

To put these concepts into a practical context, here are the typical a_w values of some everyday foods, as documented by the FDA and Kansas State University:

Fresh meat and fish: 0.99 – highly perishable, requires refrigeration.

Raw fruits and vegetables: 0.97-0.99 – very high water availability, quick to spoil.

Cheese spread: 0.95 – still supports bacterial growth.

Salami: 0.82 – below most bacterial thresholds, resistant to common pathogens.

Soy sauce: 0.80 – preserved primarily by salt content.

Jams and jellies: 0.75-0.80 – preserved by high sugar content.

Peanut butter: 0.70 – only certain moulds can grow at this level.

Dried milk: 0.70 – shelf-stable at room temperature.

The relationship between bound water and water activity 🔗

There is an inverse relationship between the proportion of bound water in a food and its water activity. As the percentage of bound water increases, a_w decreases – because bound water molecules do not contribute to vapour pressure. This relationship is fundamental to understanding why dried, salted, and sugared foods last longer.

When salt or sugar is added to a food, these solutes form bonds with free water molecules, effectively converting them into bound water. The result is a lower vapour pressure, a lower a_w, and a food environment where microorganisms struggle to find the available water they need for growth. The same principle applies when water is physically removed through drying – fewer free water molecules remain, and the overall a_w drops.

Food scientists use moisture sorption isotherms – graphs that plot moisture content against water activity at a given temperature – to understand and predict how a specific food will behave during processing and storage. These curves are essential tools in designing shelf-stable food products and in setting up Hazard Analysis and Critical Control Points (HACCP) programs.

Practical significance in food processing 🔗

Understanding water types and water activity is not just academic – it has direct applications across the food industry:

Product formulation: Food manufacturers calculate exactly how much salt, sugar, or other humectants to add to achieve a target a_w that ensures safety and desired shelf life.

Quality control: Water activity is routinely measured during production. Many food facilities use a_w as a critical control point in their HACCP plans.

Packaging design: Moisture can migrate from high a_w areas to low a_w areas. A composite product like a cracker-and-cheese snack pack will eventually become soggy if the moisture migration from cheese to cracker is not accounted for in packaging design.

Texture management: Controlling the balance of free, bound, and entrapped water directly affects whether a product is crispy, chewy, moist, or dry. Potato chips, for instance, lose their crispness when they absorb atmospheric moisture, converting bound water back toward free water.

What do you think? How might the growing demand for clean-label, preservative-free foods push the food industry to rely even more on water activity management? And in your own kitchen, have you ever noticed how adding salt or sugar to a food changes not just its taste but also how long it lasts?

References

1. https://ebooks.inflibnet.ac.in/ftp03/chapter/water-in-food-systems/
2. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/bound-water
3. https://www.sciencedirect.com/science/article/abs/pii/S1466856416305161
4. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
5. https://ucanr.edu/program/uc-master-food-preserver-program/article/water-activity-and-its-role-food-preservation
6. https://www.food-safety.com/articles/4420-water-activitye28099s-role-in-food-safety-and-quality
7. https://bookstore.ksre.ksu.edu/download/water-activity-of-foods_MF3674
8. https://pmp.errc.ars.usda.gov/WaterActivity.aspx