Spotlight Food & Beverage Analysis


The Influence of Water Activity (aw) in Bakery Products Markus Bernasconi, Sales Engineer & Product Manager, Novasina AG, markus.bernasconi@novasina.ch


Water activity - a widely unknown or underestimated quality parameter offers many more benefits than just normal moisture content determination. It is the key parameter of gathering information about product shelf life, texture and taste or microbiological and chemical stability. Nowadays, where foodstuffs are shipped around the globe, a tough control of a product's water activity helps to reach the shelf-life goals and provides support for safe and uncontaminated foods to the consumer.


Especially bakery products are very difficult to handle because in most of the cases, they contain different types of ingredients such as sugar, fats/oils, milk proteins etc. This paper sets out to discuss the influence of those kind of ingredients on water activity measurement and how the determination of aw helps to reach the required quality goals for bakery products.


What is Water Activity (aw)?


Water activity is defined as the current volume and availability of ‘free’ water in a sample and should not be directly compared with the water content (g water/ g substance). The water activity is given as the aw – value and ranges between 0 (absolute dryness) and 1 (100% relative humidity).


Only this component takes an active part in the exchange of moisture with the ambient air and can possibly form the ideal medium for microbiological growth on the surface, which influences the microbiological stability. The water activity also has an important effect on the chemical reactions in food.


To determine the aw-value the relative humidity over a sample is measured after reaching the equilibrium humidity (partial water vapour pressure). This relates proportionally to the aw-value. An accurate and significant aw-measurement is only possible, if the sample shows a constant temperature during the measurement, thus a temperature controlled measuring chamber in the range of 0°C to 50°C is absolutely mandatory.


The time for establishing the equilibrium between the free water in and water vapour over the sample is the key for an accurate, reliable and reproducible aw measurement.


A temperature pre-conditioning of the sample reduces the measurement time.


Water activity measurement methods Different measurement methods can be found in currently available instruments:


Resistive electrolyte cell Dew point mirror Bakery Products


The expression bakery products also covers the product categories of long-life bakery products such as biscuits, crackers, cake, waffles, gingerbread etc., which are durable without cooling or freezing over a longer period (6 to 12 months) at ambient temperature (18°C – 25°C). In long-life bakery products the water content but is reduced severely.


The main criteria for the quality of a durable product, is the hygiene during processing and handling as well as the optimal sensory properties.


Capacitive measurement


Both factors, such as quality and durability of a product, are affected by the raw materials, quality, formulation and storage conditions. The durability of bakery products of middle and high moisture content is limited by the growth of molds. Some species grow at a water activity around 0.8 aw , while xerophilous species still can grow down to an aw-value of 0.6.


Some measuring sensors have in-built calibration data memory at a number of points across the range or aw. The accuracy of available instruments in the market is +/- 0.003 aw.


Bakery products can be filled with various ingredients such as cream, nuts, nougat, fruit and jam. These ingredients change different factors of the product, which again change the microbial and sensory properties of the bakery product. Combined food that consists of one or several layers differs in its composition. In such food there is the possibility for moisture to migrate from one component to another. This migration happens from regions of high water activity to regions of lower water activity. The water activity is a physical parameter that indicates the ‘energy status’ of the moisture in a material. Thus it is better to qualify the moisture migration tendencies of the combined food product, using aw, than simply to control the absolute water content of the components.


The Influence of Water Activity in Foods


The equilibrium relative humidity value of a product, which is ascertained through its partial pressure of water vapour on the surface depends on the chemical compound, temperature, water content, storage environment (T/rh), absolute pressure and packing.


‘Free’ water in products is jointly responsible for the growth of unwanted micro organisms such as bacteria or fungi, which produce “toxins” or other harmful substances. But also chemical/biochemical reactions (for example, the Maillard reaction) increasingly take place and can change the following factors of a product:


• Microbiological stability (growth) • Chemical stability • Content of proteins and vitamins • Colour, taste and nutritional value • Stability of the compound and durability • Storage and packing • Solubility and texture


The optimisation and stabilisation of the product properties require a partially narrow aw value margin. The aw – value of a product can be changed by the adding of so called ‘Humectants’ such as sugar or polymeric polyols, lactic acid or natural extracts which normally bound water and thereby reduce the amount of free water and finally the water activity. Nowadays the measurement of water activity in the food industry is established in research, development, quality control and production.


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