Stapletons Technical Bulletin:
Water Activity in Dried Fruits

Introduction

Water Activity (Aw) is the measurement of available (hence, "active") water in a substance.  The concept of "Available" is important because the microbial and chemical stability of a food product is directly related to how much water is available for biological or chemical reactions.  Water Activity is defined as follows:

Aw = % Relative Humidity / 100

A substance with a Aw approaching 1.0 has a very high water activity (example, juices).  Substances such as dehydrated foods have a very low Aw, for example a typical dehydrated food such as pasta or dried milk would have Aw in the 0.2 to 0.6 range.  

The chemical and microbial stability of a food product is directly related to its water activity.

Most Food Technologists agree that food items having a Aw of less than 0.80 as being stable to almost all organisms except for a few rare cases of halophiles and extreme osmophiles.

The typical water activity of dried fruits typically range from 0.4 to 0.8 (depending on moisture content and type of fruit).  Click here to see the relationship between moisture content and Aw.

Water Activity of Prunes

The standard method for measuring water in prunes is using a DFA moisture meter.  This is a modified conductivity measurement device that this made and calibrated by the DFA.   We have tested prunes with a water activity meter and have found that the results closely match literature values.  Here are literature* values for Aw in prunes:
 

Moisture

 Aw

20%

 0.67
25% 0.76
30% 0.83
 
 
In general, we consider a prune with a moisture of less than  25% as microbially stable and does not require the need for added preservatives (such as potassium sorbate).  As an additional note, Natural Condition Prunes are typically 18-20% moisture.
 

Water Activity of Raisins

Raisins are typically packed at 15-18% moisture for consumer use.  Aw at these moistures range from 0.51 - 0.56.

Water Activity of Fruit Juice Concentrates

Stapleton-Spence supplies both prune and raisin concentrate at 70 Brix.  Typical Aw for concentrates at this Brix level are roughly 0.82 which makes them stable for most common microorganisms.



Sources:

*Dauthy, M. E., "Fruit and vegetable processing", FAO AGRICULTURAL SERVICES BULLETIN No.119, Food and Agriculture Organization of the United Nations, Rome, 1995

California Prune Board

California Raisin Marketing Board


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