Shelf Life: Introduction to Water Activity

The interior of a brightly-colored truffle with a thick shell

This fifth article in our shelf life guide will offer a concise introduction to the concept of water activity (AW) as it relates to shelf life. Subsequent articles will explore how AW is measured and best practices that will extend the shelf life of your products.

Whether you are a cottage chocolatier just getting started or a successful shop owner ready to start shipping your products or selling them wholesale, the shelf life of your products is an important consideration when creating your business plan, planning a product assortment, and thinking about future goals. Whether your ganache lasts for just a few days or is shelf-stable for months is, in part, a question of water activity (AW).

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Extending Shelf Life: Controlling Moisture

In balancing a chocolate ganache recipe, a chef strives to use as many water-biding ingredients as possible while not compromising taste or texture.
Extending Shelf Life: Controlling Moisture

Explore the entire Shelf Life library here

Chef Russ Thayer pours ganache into a confectionery frame

What is Water Activity (AW)

Water Activity is a measurement of bound vs free water

It indicates how much water is available in a food product for all manner of reactions:
FREE WATER = ACTIVE WATER
Aw is a better measure of the risk of spoilage than the water content itself.
The Aw value is an abstract number and is always greater than 0 and less than 1.
Aw is measured at the temperature at which the product is distributed and sold (20°C).

The AW value is expressed on a scale from 0 to 1:
0 = completely dry mass or solid ingredients
0.5 stands for a solution corresponding to the activity of 50% free water
1 = 100% water

Do Not Confuse Moisture Content with Water Activity

As we saw in the previous article, two recipes with the same moisture content can have different water activity levels depending on the amount and water-absorbing power of the ingredients used.

Water content = Total amount of water

free water + water fixed by other components

Water activity = The free water in a product

A chart showing Aw values and their corresponding anticipated shelf life

An Overview of Water Activity

The following table lists the average water content for some common ingredients.

Product	Water Content	Aw
Milk	88%	.993 - .995
Dry Milk Powder	2 - 4%	.100 - .300
Butter (unsalted)	16%	>.970
Butter (salted)	14%	.910 - .940
Evaporated Milk	73%	.980 - .990
Condensed Milk	27%	.770 - .850
Glucose Syrup	20%	.680 - .720

These values are a general guideline. It is impossible to know these values without scientifically verified data, and, as much as manufacturers strive for consistency, variations do occur.

A chart showing at which water activity level different bacteria form

Aw Ranges: Common Products and the Development of Microorganisms.

To help you better understand the meaning and consequences of free water, this chart places different products on a scale of water activity according to where the average product would fall; on the right-hand end of the scale, we see the Aw levels that encourage various microorganisms.

In an example from the previous article, we looked at two ganaches with identical moisture contents. However, one had an Aw of .750, the other an Aw of .900. Note the significantly higher amount of microbial activity that could occur in the second ganache vs the first - all due to free water.