The shelf life of bakery products simplified with water activity by Dr. Brady Carter, Senior Research Scientist with Carter Scientific Solutions.
The baked snack industry is a constantly growing market valued at $440 billion. These products typically cover a range of textures, colours, and flavours, and it is critical that they remain safe while maintaining their expected texture and sensory properties. One of the most important factors influencing the quality and shelf life of bakery products is water activity. Water activity control can help prevent or minimise various degradative events such as rancidity, microbiological spoilage, staling, or changes in texture due to water migration.
Theory of water activity
Water activity is defined as the energy status of water in a system and is rooted in the fundamental laws of thermodynamics through Gibb’s free energy equation. It represents the relative chemical potential energy of water as dictated by the surface, colligative, and capillary interactions in a matrix. Practically, it is measured as the partial vapour pressure of water in a headspace that is at equilibrium with the sample, divided by the saturated vapour pressure of water at the same temperature. Water activity is often referred to as the ‘free water’, but since ‘free’ is not scientifically defined and is interpreted differently depending on the context, this is incorrect. Free water gives the connotation of a quantitative measurement, while water activity is a qualitative measurement of the relative chemical potential energy. Rather than a water activity of 0.50 indicating 50% free water, it more correctly indicates that the water in the product has 50% of the energy that pure water would have in the same situation. The lower the water activity then, the less the water in the system behaves like pure water.
For baked snacks, water activity is measured by equilibrating the liquid phase water in the sample with the vapor phase water in the headspace of a closed chamber and measuring the Equilibrium Relative Humidity (ERH) in the headspace using a sensor. The relative humidity can be determined using a resistive electrolytic sensor, a chilled mirror sensor, or a capacitive hygroscopic polymer sensor. Instruments from Novasina, like the Labmaster NEO, utilise an electrolytic sensor to determine the ERH inside a sealed chamber containing the sample. Changes in ERH are tracked by changes in the electrical resistance of the electrolyte sensor. The advantage of this approach is that it is very stable and resistant to inaccurate readings due to contamination, a particular weakness of the chilled mirror sensor.
The resistive electrolytic sensor can achieve the highest level of accuracy and precision with no maintenance and infrequent calibration. Sampling for water activity testing can be particularly challenging for bakery products because they tend to be too large to fit in the sample cup and are often multi-component. While water activity is an intensive property that provides the energy of the water in a system, moisture content is an extensive property that determines the amount of moisture in a product. Water activity and moisture content, while related, are not the same measurement.
Moisture content is typically determined through loss-on-drying as the difference in weight between a wet and dried sample. For baked snacks, moisture content provides a standard of identity and an expected mouthfeel but does not determine if the product is microbially safe. Water activity and moisture content are related through the moisture sorption isotherm. The moisture content associated with a safe water activity will be different for each product and as will be demonstrated in the next section, should never be relied on as an indicator of microbial safety.
Water activity and texture/crispiness
Bakery products are known for their pleasing texture and taste and each product has its own unique organoleptic requirements. The most common mode of failure in baked products is an unexpected change in texture or flavour. For example, a butter biscuit should be crispy, and a cake should show a good mixture of firmness and moisture. Figure 1 shows the water activity range for baked snacks and the expected texture for each region. Changes in water activity will result in changes in texture and each baked product has an optimal water activity range where the texture and taste will be ideal. The key then to prolonging the shelf life of these products is to manufacture them to that ideal water activity and maintain that water activity during storage and transport with effective packaging. Many bakery products contain multiple components such as a cream filling or an icing covering. For these products, moisture migration between components can lead to undesirable texture changes or even susceptibility to microbial growth. Moisture moves from high water activity to low water activity, so the way to prevent moisture migration and its accompanying consequences is to match the water activity of the components. The icing, cake, and cream filling of a snack cake all need to be manufactured to the same water activity before being combined and then, even if their moisture content is different, there will be no moisture migration between the components.
