Have you ever noticed that some dried fruits stay fresh for months while others quickly develop fungus? The secret lies in a scientific concept called water activity (aw) — a key factor that determines how long food remains safe and stable. Understanding water activity is essential for anyone involved in food drying, processing, or storage. It explains why products like dried pomegranate arils, raisins, and dehydrated snacks stay fungus-free even without preservatives or nitrogen flushing.
What Is Water Activity (aw)?
Water activity doesn’t measure how much total water is present in food — it measures how much of that water is available for microbes like bacteria, yeast, and mold to use.
In simple terms:
Moisture content tells you how much water is in food.
Water activity tells you how free that water is for microbial growth.
When water is tightly bound to other molecules such as sugars, acids, or salts, microorganisms cannot access it — and that’s what keeps food safe.
Scientific Definition
Water activity is scientifically defined as the ratio between the vapor pressure of water in a food and the vapor pressure of pure water at the same temperature:
aw = P / P₀
Where:
- P = vapor pressure of water in the food
- P₀ = vapor pressure of pure water
Pure water has a water activity of 1.0, while dried or concentrated foods have much lower values — typically between 0.1 and 0.6.
Water Activity Levels and Microbial Growth
| Food Type | Typical Water Activity (aw) | Microbial Growth Risk |
|---|---|---|
| Fresh fruits | 0.95 – 0.99 | Very high – bacteria and molds grow easily |
| Semi-dried fruits | 0.70 – 0.85 | Moderate – some yeasts can grow |
| Dried fruits (like pomegranate arils) | 0.45 – 0.60 | Safe – no fungal growth possible |
| Crackers, powders | 0.10 – 0.30 | Completely stable |
👉 Most molds require a water activity above 0.70, while bacteria typically need levels above 0.85.
That’s why properly dried fruits, with an aw around 0.55, remain safe for months when stored properly.
Why Dried Fruits Don’t Catch Fungus
Dried fruits like pomegranate arils, dates, and figs have high sugar and acid content, both of which naturally reduce water activity. Sugars and acids bind water molecules so tightly that they become unavailable for microbial growth.
Even when moisture content is around 12–14%, the water is not “free” — it’s bound within the food structure. This is why dried fruits remain fungus-free without the need for chemical preservatives.
Water Activity vs. Moisture Content
Although the two are related, they are not the same. Moisture content measures the total amount of water in food, while water activity measures how much of that water is accessible to microbes.
| Parameter | Meaning | Impact |
|---|---|---|
| Moisture Content | Total water present (%) | Affects texture and weight |
| Water Activity (aw) | Free water available for microbes | Determines safety and shelf life |
| Ideal for Dried Fruits | 12–14% moisture, aw ≈ 0.55 | Safe and stable from fungal growth |
In short, low moisture + low water activity = long shelf life.
How to Measure Water Activity
Water activity is measured using specialized devices called water activity meters, which determine aw accurately within minutes. These devices are used by food industries to ensure consistent quality, safety, and shelf life — especially for dried fruits, powders, spices, and snacks.
When Does Fungus Grow?
Fungal growth occurs when:
- The product absorbs moisture from humid air (inadequate packaging),
- The drying process is uneven (some areas retain hidden moisture),
- Relative humidity during storage exceeds 60%, or
- The product is stored in warm, unventilated conditions.
Airtight packaging and low-humidity storage are essential to prevent reabsorption of moisture.
Practical Example: Dried Pomegranate Arils
When dried pomegranate arils are dehydrated to 12–14% moisture:
- Their water activity drops to about 0.55,
- The acidic pH (around 3.0–3.5) further inhibits microbial growth, and
- The sugary composition binds remaining water molecules.
As a result, the product stays fungus-free naturally, even without nitrogen flushing or preservatives.
Water activity is one of the most critical yet overlooked parameters in food science. It’s the reason some foods stay fresh for months while others spoil in days. Controlling water activity — not just reducing moisture — ensures microbial safety, flavor stability, and long shelf life.
By understanding and managing water activity, food producers and home processors alike can create safe, naturally preserved, and delicious dried products that last longer and retain their quality.