Rotronic

Water Activity

Water activity (Aw) measures the energy status of water in a system. It quantifies the amount of “free” or “unbound” water available to participate in chemical reactions or support the growth of microorganisms like bacteria, yeast, and mould. It is measured on a scale from 0.0 (completely dry) to 1.0 (pure water).

Unlike moisture content, which tells you the total mass of water in a product (how much water is there), water activity tells you the status of that water (what that water can do). For example, salami and fresh meat might have similar moisture contents, but salami has a much lower water activity because its water is bound by salt and curing agents.

Why is Water Activity Important?

Controlling water activity is a critical quality control metric across the food, pharmaceutical, and cosmetic industries for several major reasons:

Microbial Safety (Shelf-Life)

Microorganisms need a certain level of available water to survive and multiply. By lowering Aw, you effectively “starve” microbes of the water they need.

Most bacteria stop growing below 0.91 Aw.
Molds and yeasts can tolerate lower levels but generally stop growing below 0.60 Aw.
Product Safety: If a product’s water activity is kept below 0.60, it is microbiologically stable and will not spoil from mould or bacteria.

Chemical and Physical Stability

Water acts as a solvent and a reactant. High water activity accelerates unwanted chemical reactions:

Browning & Oxidation: Lipid oxidation (which makes oils go rancid) and non-enzymatic browning (Maillard reaction) are highly dependent on Aw levels.
Texture Degradation: It dictates whether a product stays crispy, becomes soggy, clumps together, or turns into a hard lump. For example, keeping potato chips below 0.40 Aw ensures they retain their crunch.

Moisture Migration in Multi-Component Foods

Moisture naturally moves from regions of high water activity to regions of low water activity, regardless of the moisture content.

Example: In a breakfast cereal with raisins, the raisins have a higher moisture content but a lower water activity than the dry flakes. If not properly formulated, water will migrate from the flakes into the raisins, making the raisins rock-hard and the cereal soggy. Matching or controlling the Aw prevents this.

The Main Hardware

HygroLab (Benchtop Indicator): The central command benchtop unit features a large touchscreen interface. It independently monitors up to four digital probes simultaneously, allowing you to run separate tests side-by-side to save time. It includes an AwQuick function that uses an algorithm to predict the final equilibrium value in typically under 5 minutes.
HygroPalm 23-AW (Handheld Kit): The portable alternative designed for field work, production floor checks, or incoming goods inspection. It is a rugged, battery-operated handheld meter that connects to a single digital probe. Despite its mobility, it features the exact same AwQuick prediction algorithm as the benchtop HygroLab, allowing you to get laboratory-grade results directly on the factory floor without waiting for full equilibrium.
HC2A-USB-AW USB Probes: These are a cylindrical measurement probe with USB Cable. The probe sits directly on top of the sample holder, minimising the internal air volume so the sample reaches equilibrium rapidly.

Accessories

Sample Holders (WP-14 or WP-40): These are solid, heavy-duty metal blocks designed to hold the disposable sample containers. Because temperature stability is critical for accurate Aw calculations, the massive steel or aluminium body acts as a thermal mass to shield the sample from rapid micro-climatic temperature shifts.
Disposable Sample Containers (PS-14 or PS-40): Small, medical-grade plastic cups available in two depths (14mm or 40mm). They prevent cross-contamination between batches. You fill these cups with your sample (powders, gels, tablets, or liquids) and drop them directly into the matching metal sample holder.
Mechanical Clamping Kit (AW-KHS): For very light samples, low water activity levels, or environments prone to vibration, a simple friction fit might not seal the probe tight enough. The mechanical clamping mechanism physically locks the HC2-AW probe down onto the sample holder with mechanical leverage. This ensures an airtight, hermetic seal, completely eliminating ambient room air leakage that could skew the sensor’s readings.

HygroLab

The Rotronic HygroLab is a high-end benchtop laboratory analyser designed to measure water activity (Aw) in food, pharmaceuticals, and cosmetics. Featuring a 7-inch touchscreen, it supports up to four probes for simultaneous or asynchronous testing. Its signature "AW Quick" function utilizes an advanced algorithm to deliver highly accurate results within 4 to 5 minutes.

Water Activity Probes

Portable Water Activity Probes. Ideal for sampling products in storage, or testing goods arriving before accepting.

Water Activity Accessories

Accurately measuring water activity requires sampled are tested in perfect isolation. A specialised ecosystem comprising heavy-duty sample holders, disposable containers, and physical clamping mechanisms that lock out the outside world are critical for consistent sampling.

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