If there's one number I wish every confectionery maker — from the home kitchen to the production floor — would learn to pay attention to, it's water activity.
Not moisture content. Not Brix. Not pH (well, pH matters too, but that's another post). Water activity.
I've been working with water activity for most of my career, first through my PhD research on sugar thermal stability in Dr. Shelly Schmidt's lab at the University of Illinois — one of the world's leading water science programs — and then through decades of formulation and product development work across confections, bakery, fillings, and more. At Bellis Food Solutions, my R&D company, we measure water activity every single day. It's not optional. It's how we know a product is done, stable, and safe.
This post is about why water activity deserves that level of respect, what it actually tells you, and how understanding it can save you from the kind of shelf life failures that are expensive, embarrassing, and almost always preventable.
Water activity (abbreviated aW) is a measure of how much of the water in your product is "free" — available to participate in chemical reactions, support microbial growth, or move around within or between product components.
It's expressed on a scale from 0 to 1, where 0 is completely dry and 1 is pure water. Most food products sit somewhere in between, and where they sit determines a lot about how they'll behave over time.
Here's what's important to understand: water activity is not the same as moisture content. Two products can have the same moisture content and very different water activity values — and very different shelf lives as a result. It depends on how tightly the water is bound to the other molecules in your formula. Sugar, for instance, binds water very effectively, which is part of why high-sugar confections are shelf-stable even at relatively high moisture levels. Replace that sugar with other ingredients and suddenly the rules change entirely.
This is why we measure water activity directly rather than relying on moisture content or solids readings. A refractometer gives you a number, but it's telling you about total dissolved solids — not about how that water is behaving. A water activity meter tells you something much more fundamental about your product's stability.
Water activity is the single best predictor of shelf life we have — both from a quality standpoint and from a microbiological safety standpoint.
On the microbiology side, most bacteria, yeasts, and molds have minimum water activity thresholds below which they simply can't grow. Most bacteria require aW above 0.90 to thrive. Many molds can grow down to around 0.70–0.80. Xerophilic molds — the really stubborn ones — can sometimes push below that, but they're the exception. Understanding where your product sits relative to these thresholds tells you a lot about its inherent microbial stability.
On the quality side, the relevant processes are different but the principle is the same. Enzymatic reactions, oxidation, non-enzymatic browning (the Maillard reaction), crystallization, textural changes — all of these are strongly influenced by water activity. Too high and your gummy gets sticky, your hard candy weeps, your chocolate blooms. Too low and your soft chew turns into a brick.
The target water activity for your product isn't arbitrary. It's the value at which your specific formula is stable — chemically, physically, and microbiologically — for the duration of its intended shelf life. Finding that target, and then consistently hitting it during production, is one of the most important technical decisions in product development.
At Bellis, we don't dry candy to a target solids content. We dry it to a target water activity. That shift in thinking — from measuring what's in the product to measuring how the water is behaving — changes everything about how you approach process control.
It's also more consistent. Handheld refractometers are subjective. Bench-top versions depend on sample prep. A water activity meter gives you a direct, reproducible measurement every time. You put the sample in, you wait, you get a number. That number tells you whether your product is where it needs to be.
We also use water activity for multi-component products — anything with more than one distinct layer, filling, or coating. And this is where things get really interesting.
Water always wants to be at equilibrium. If you have a gummy with a sanded sugar coating, and the water activity of the gummy is higher than the water activity the sanding blend wants to be at, moisture is going to migrate. From the gummy into the sand. And when that happens, the sand gets sticky, the texture changes, and you've got a product that's failing on the shelf before it should be.
We see this all the time. And the fix isn't just "dry the candy out more" — it's understanding why the transfer is happening and addressing both sides of the equation.
One of my favorite examples to teach with is a case we worked on at Bellis involving a functional gummy — the kind with added ingredients like vitamins or botanical extracts. The gummies were sanded, but not with plain sugar. The client had a custom sanding blend, and the gummies were getting sticky during shelf life.
The instinct when gummies get sticky is to blame the gummy. Dry it out more, reformulate, add more humectant — and those might all eventually help through trial and error. But before you start changing things, you need to understand what's actually happening.
We asked the client for a sample of their sanding blend and ran a moisture sorption isotherm on it. An isotherm maps out how a material absorbs or releases moisture across a range of relative humidities — it gives you a complete picture of the material's hygroscopic behavior. And critically for this case, it tells you the deliquescence point.
The deliquescence point is the water activity (or relative humidity) at which a crystalline material — or a blend of crystalline materials — absorbs so much moisture that it dissolves into a syrup. Below that point, crystals absorb a little moisture on their surface but remain stable. At or above it, they just... go liquid.
What we found was that the deliquescence point of the sanding blend was too low — lower than the water activity of the gummies themselves. Which meant the gummies were essentially feeding moisture into the sand continuously, until the sand reached its deliquescence point and became syrupy. Sticky gummies, every time, no matter what.
The solution had two parts: raise the deliquescence point of the sanding blend (by adjusting the blend composition), and bring the water activity of the gummies down slightly so the two components were in equilibrium. Once both were in the right place relative to each other, the stickiness problem went away.
What I love about this example is the elegance of the approach. Instead of weeks of trial-and-error reformulation — change something, wait three weeks to see if it sticks, change something else, wait again — a few days of targeted testing told us exactly what to fix and why. That's what understanding water activity makes possible.
A single water activity reading tells you where your product is right now. A moisture sorption isotherm tells you how it will behave across a range of conditions — which is what you actually need to predict shelf life.
An isotherm plots moisture content against water activity across the full range from dry to saturated. The shape of that curve tells you a lot. Some materials have a nice flat curve — they don't absorb much moisture even as relative humidity increases, which means they're inherently stable across a wide range of storage conditions. Others have curves that rise steeply at a certain point, meaning there's a critical water activity above which the material starts absorbing moisture rapidly. That's the zone you need to stay out of.
For product developers, isotherms are useful in several ways:
Formulation decisions. When you're choosing between different sweeteners, bulking agents, or hydrocolloids, their isotherm behavior can tell you a lot about how they'll affect shelf stability. Two ingredients that look similar on paper can behave very differently at 65% relative humidity. Running isotherms on your candidate ingredients before you commit to a formula can save you a lot of trouble downstream.
Packaging selection. Every packaging material has a water vapor transfer rate — how much moisture it lets through over time. If you know your product's isotherm and you know the average relative humidity of your storage environment, you can model how your water activity will change over the shelf life period with different packaging options. This lets you select packaging based on actual predicted performance rather than assumption.
Storage environment mapping. This matters more than people often realize. A product stored in a climate-controlled distribution center in Arizona is living in a very different world than the same product stored in a retail warehouse in the Pacific Northwest in February. Understanding your product's isotherm lets you anticipate how it will behave in different environments and design your shelf life claims accordingly.
If water activity is complicated in conventional confections, it gets significantly more complicated when you start removing sugar.
Confectionery systems are essentially built around sugar's properties. Sugar's ability to bind water, depress water activity, resist crystallization under certain conditions, and contribute to texture and stability is so fundamental that when you replace it — even partially — with something else, everything shifts.
The challenge with low-sugar formulations is that the replacement ingredients don't all behave the same way. Polyols, fibers, high-intensity sweeteners, bulking agents — each has its own water activity and isotherm behavior, and when you blend them, the resulting system can be difficult to predict without actually measuring it.
Our approach at Bellis is to run isotherms on each ingredient candidate individually before we start formulating. That data tells us how each one responds to moisture, what its deliquescence point is (if it's crystalline), and how it's likely to interact with the other components in the system. From there, we can start building a formula with a much better idea of where we're going to land on water activity — rather than formulating by intuition and then measuring afterward and hoping for the best.
This doesn't eliminate iteration. Low-sugar product development is inherently iterative because you're balancing molecular weights, textures, sweetness profiles, processing behavior, and water activity all at the same time with a limited ingredient palette. But it dramatically reduces the number of iterations you need, and it helps you understand why something isn't working when it isn't — which is half the battle.
Whether you're developing a new confection from scratch, troubleshooting a shelf life problem, or trying to understand why your product behaves differently at different times of year — water activity is almost certainly part of the answer.
Here's what I'd encourage you to take away from this:
Measure water activity, not just moisture content. If you're not already using a water activity meter as a routine quality tool, it's worth the investment. It will tell you things about your product that moisture content simply can't.
Think about equilibrium in multi-component products. Whenever you have a product with more than one distinct component — a filled confection, a sanded product, a chocolate-enrobed center — those components are going to reach for equilibrium with each other over time. If you design for that equilibrium intentionally, you get a stable product. If you ignore it, you get surprises on the shelf.
Use isotherms when you're formulating. Especially if you're working on low-sugar or novel formulations where you can't rely on conventional intuition. The data you get from an isotherm in the early stages of development will pay for itself many times over in avoided reformulation cycles.
Connect water activity to your storage and packaging decisions. Your product doesn't live in a controlled environment. It lives in warehouses, trucks, retail shelves, and consumers' homes. Understanding how its water activity will change in those environments — and designing your packaging accordingly — is how you build a shelf life claim you can actually stand behind.
Water activity isn't glamorous. It doesn't have the drama of a tempering curve or the romance of a new flavor ingredient. But it quietly determines whether your product makes it to the consumer in the condition you intended. That makes it one of the most important tools in a confectionery developer's toolkit.
And once you start measuring it seriously, you'll wonder how you ever made decisions without it.
Questions about water activity in your confectionery formulation? This is exactly the kind of technical deep-dive Candy Mentor is here for. Drop a comment or reach out — I'd love to hear what you're working on.
Also check the episode I did about Water Activity in Confections with Zachary Cartwright for the Water and Food Podcast. You can find it here:
https://www.youtube.com/watch?v=sZdT3PekQwU
Water In Food - Episode 39 - November 21, 2023
Talk soon!
— Michelle
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