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Water Activity Effect on D-Value

Printed From: Dairy Science and Food Technology
Category: Thermal processing
Forum Name: Thermal processing
Forum Description: Effects of heat on food, enzymes and microorganisms
Printed Date: 01 Dec 2020 at 12:16pm
Software Version: Web Wiz Forums 12.03 -

Topic: Water Activity Effect on D-Value
Posted By: cordyceps
Subject: Water Activity Effect on D-Value
Date Posted: 07 May 2012 at 4:43pm
How does Water Activity affect the lethality curve?
Do we need to consider it in calculation of D-
and F-values?
(Please note that Water Activity is not merely the moisture content of the product.)

Posted By: Admin
Date Posted: 07 May 2012 at 8:51pm

Welcome to the forum.

In general as the water activity (aw ) decreases (solids increase e.g. sugars and fats) in a food the heat resistance of microorganisms and their D-values increase. As a consequence more heat (and higher F values) is required to obtain a similar log reduction in microbial numbers. The opposite occurs when the water activity increases.

Because the D-value generally increases (sometimes by a large factor) when aw increases, the number of log cycles that a pathogen or spoilage organism is killed will reduce significantly potentially resulting in public health or spoilage problems. I will try to explain this.

Let’s say we have a heat process that has a Fo of 3 and a pathogen that has a D-value of 0.21 minutes. The Fo process will reduce the number of pathogens in the final product by 3/0.21 =14.3 log cycles. A very large reduction.

If the aw is reduced e.g. by the addition of sugars, milk fat and protein the new D-value might  increase to say 0.70 minutes. It might even be higher! Since the Fo remains the same (unless we change it) the number of log cycles the pathogen will be reduced is 3/0.7 = 4.3. This would not be an acceptable process.

Does this explain your query?

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