The relationship between water activity and moisture content at a given temperature is called the moisture sorption isotherm. This relationship is complex and unique for each product due to different interactions (colligative, capillary, and surface effects) between the water and the solid components at different moisture contents. An increase in aw is almost always accompanied by an increase in the water content, but in a nonlinear fashion. Moisture sorption isotherms are sigmoidal in shape for most foods, although foods that contain large amounts of sugar or small soluble molecules have a J-type isotherm curve shape. There is a difference (called hysteresis) in the adsorption (wetting) and desorption (drying) isotherm curves.
Decagon is pleased to announce the development of an automatic moisture isotherm generator. It will be available for a much lower price than currently available isotherm instruments. The user will place their sample into the isotherm chamber, set test parameters, and then walk away. In one to three days, a complete isotherm curve (both adsorption and desorption) will be generated. It will be possible to choose from several isotherm equations to characterize the isotherm graph. The isotherm coefficients will be displayed on the screen and transferable to an additional application software package. This instrument will facilitate sorption isotherm development for any company that would like to better understand the water in their product.
Moisture sorption isotherms provide valuable information to a company. For anyone who dries or wets their product, the sorption isotherm serves as a drying and wetting curve. It shows the water activity of a product when wetted and dried to a specific moisture content. For anyone who still relies on a moisture content specification instead of a water activity specification, the sorption isotherm will clearly illustrate the potential problems that can occur when you wet to a moisture content vs. dry to a moisture content. The moisture content associated with a safe water activity on the drying curve will not be associated with the same water activity on the wetting curve due to hysteresis. If the production technician controlling drying of a product over-dries beyond the specified moisture content and decides to re-wet the product back to the moisture content specification, the results could be disastrous. The product's response to moisture is no longer following the drying curve, but has switched to the wetting curve. The moisture content specification may correspond to a safe water activity on the drying curve, but on the wetting curve, the moisture content may be related to a very unsafe water activity level.
An additional function of the isotherm is moisture content prediction. Although water activity is a much better predictor of safety and quality than moisture content, there are times when it is necessary to know both water activity and moisture content. Moisture content measurements can be inaccurate, time-consuming, and require a precision balance. Thankfully, the sorption isotherm can be used to predict moisture content based on water activity, usually with better accuracy than actually running a moisture content analysis and in much less time. Thus, you will have the complete water picture for your product in less than 5 minutes.
Isotherms are very valuable for formulation and product development. By comparing the isotherms of different formulations, it is possible to determine if a product can be adjusted to allow higher moisture content at a given water activity or a lower water activity at a given moisture content. The result can be a moister product that is still shelf stable. For those producing multi-component products, it is possible using the isotherms of the two components to determine what the final water activity will be of the mixture without actually making the product. For dried products, the isotherm will predict the moisture content of the product when it is dried to a shelf stable water activity level.
Finally, sorption isotherms are valuable for shelf life prediction. A product's isotherm can be used to determine package requirements depending on the products sensitivity to moisture gain or loss. Products with a crystalline type isotherm will be less susceptible to caking than products with an amorphous type isotherm. However, the water activity of a crystalline material will increase very quickly until the saturation point is reached-as evidenced by the isotherm. Temperature can also impact shelf life; sorption isotherms conducted at different temperatures help determine the impact of temperature abuse on moisture content and water activity.
Isotherms are very valuable to anyone concerned with product quality and safety. With the release of Decagon's affordable isotherm generator, it will be possible for many more companies to begin developing sotherms for their products. For more information about isotherms and isotherm generation, contact any of Decagon's Application Engineers.