Moisture in food and water activity

Food moisture meters and water activity meters are instruments for measuring total moisture content (mass of water over total mass of the product) and water activity (aw) in food. Both are fundamental parameters for quality control and food safety: total moisture affects many properties of the product (texture, weight, price for products sold by weight); water activity instead determines the availability of free water for chemical reactions and microbial growth, and is the microbiological safety parameter for many foods. They are used in the food, bakery, confectionery, canning, dairy, cured meat and ice cream industries.

Thermogravimetric balances

Thermogravimetric balances for measuring the total moisture of food combine: a high-resolution precision balance (0.001 g), an infrared or resistance drying chamber with thermal control, and software with programs specific to each food category (temperature, end-of-test criterion). The sample (5-10 g) is heated until the weight loss stabilises (an asymptote); the percentage difference gives moisture with excellent accuracy (±0.1%). Typical analysis time is 5-30 minutes. They are used for pasta production, baking (checking the degree of bake), confectionery, cured meats (monitoring curing), food powders (milk powder, coffee, spices) and dehydrated products.

Water activity meters

Water activity meters measure water activity (aw), a dimensionless quantity between 0 and 1 that represents the vapor pressure of the water in the food divided by the vapor pressure of pure water at the same temperature. The instrument uses a sealed temperature-controlled chamber in which the sample is placed: after equilibrium (minutes), a relative humidity sensor (capacitive or chilled-mirror) measures the RH of the air in equilibrium with the sample. The aw equals RH/100. Typical accuracies are ±0.02 aw, with a resolution of 0.001 aw. Measurement times are 5-30 minutes. They are used for validating the microbiological safety and shelf life of food.

What water activity means

Water activity determines the microbiological stability of a food: pathogenic bacteria such as Salmonella, Listeria and E. coli only grow above aw=0.86; yeasts above aw=0.80; molds above aw=0.60-0.65. Foods with aw < 0.60 are microbiologically stable at room temperature (dried pasta, biscuits, powders). For foods of intermediate aw (0.60-0.85, such as tarts, dried fruit and cured meats) correct management of aw is fundamental to food safety and shelf life. Product specifications in many cases set maximum aw limits in the company’s HACCP plan.

Applications

The applications include: the pasta industry (moisture control of the dry product, aw for stability); baking (bread moisture, monitoring the drying of biscuits); the cured meat industry (monitoring the curing of hams and salami – aw is a critical maturation parameter); confectionery (quality control of chocolate, sweets and ice cream); the canning industry (checking the stability of preserves, jams and dried fruit); food powders (milk powder, instant coffee, spices); infant and baby products (high microbiological requirements); and laboratory analysis to verify compliance with HACCP plans.

Reference standards

The reference standards are: ISO 21807 (microbiology of food – determination of aw); ISO 712 (cereals); AOAC 925.45 (USA – moisture in food); ASTM E1142 (terminology); UNI 12153 (Italy – determination of aw). Thermogravimetric balances must be calibrated periodically with certified weights; water activity meters are calibrated with saturated salt solutions of known aw (for example LiCl at 11.3%, MgCl2 at 33%, NaCl at 75%, K2SO4 at 97%). ISO 17025 calibration is recommended for food safety applications.

Selection criteria

The choice of instrument depends on the parameter of interest (total moisture versus aw – they are different and complementary parameters), on the range of typical products, on the accuracy required, on the acceptable analysis time, and on integration with the company’s LIMS systems for managing quality data. Multi-product laboratories choose modular instruments with preset programs for each food; small specialised producers choose compact dedicated instruments. Compliance with 21 CFR Part 11 is recommended for FDA-regulated applications.