Peter Greweling, Culinary Institute of America

Water Activity

Appears in

Chocolates and Confections

By Peter Greweling and Culinary Institute of America

Published 2007

About

The same amount of total water can have widely varying amounts of free water depending on sugar content.

Theory: Equilibrium Relative Humidity

Equilibrium Relative Humidity (ERH) is another method used to express water activity. While water activity is expressed as a decimal, ERH is expressed as a percentage. To translate from ERH to A_w, divide by 100, or simply move the decimal point two places to the left. For instance, pure water has an ERH of 100 percent and an A_w of 1.00. A soft caramel might have an ERH of 55 percent, corresponding to an A_w of 0.55. The two methods of assessing water activity—ERH and A_w—express exactly the same thing: the unbound water in a product. ERH is defined as the relative humidity at which a substance will neither gain nor lose weight due to moisture migration. Free water always moves from a system of higher water content into a system of lower water content. If a center has a high ERH and is exposed to an atmosphere in which the relative humidity is low, the moisture in the center will evaporate into the air, resulting in a dried center. Conversely, when a center with a very low ERH is exposed to an atmosphere with higher relative humidity, the water in the atmosphere may migrate into the center, resulting in a softened center that, in the case of hard candies or toffees, is likely to crystallize during storage. The only confections that are not prone to moisture migration are those that are fat systems—gianduja, for example. Because these are fat-based products, they are impervious to moisture migration and will neither pick up nor lose moisture to the atmosphere even when there is a large discrepancy between their ERH and the relative humidity of the atmosphere. Enrobing in chocolate, wrapping, and packaging are all methods that effectively protect confections from the moisture migration that can result from disparities between relative humidity of the environment and the ERH of confections. Chocolate enrobing creates a layer of fat (the cocoa butter in the chocolate) that is impervious to moisture migration. Wrapping individual pieces tightly in plastic wrap or wax paper accomplishes exactly the same thing, except that the barrier is not designed to be eaten with the confection! Packaging such as boxes creates and contains a micro-atmosphere, which quickly reaches the ERH of the confections contained in the box. Once this equilibrium is reached, there is no further moisture migration between the atmosphere in the box and the confections.