Food stability and water activity

What is food stability? 

Pioneer herbal provides a picture that gives a simple example of free water and water binding to other foods. Water is a major component of most foods, influences texture, taste, appearance, and spoilage of food products. This brings us to the topic of food stability – also called shelf life – which is the life expectancy of a food or product that we consume every day. For example, there is more free water in fruit like an apple than a jam. This is due to sugar binding to the water in the jelly vs. free water in the fruit. Despite the difference, both have high water content. Another good binder to water is pectin and glycerol. Keep in mind, salt is 6 times better binder to water than sugar. Also, the more free water available, the more microbial growth occurs in the product or food.

Another thing to consider in food is moisture and water activity. Each one does not correlate with the other. For example, pasta products have low water activity and higher moisture compared to other foods. Though, we can group foods from low, medium to high water activity. This, in turn, leads us to the next topic. Food stability can be measured by water activity between 0 to 1.00 – is the water activity of water. This term is important for food technologist because it measures shelf stability and food safety. Keep in mind, water activity doesn’t measure the water content of food but predicts the shelf life of certain food items. Another way to explain this; the lower the water activity, the higher the shelf life, and this leads to inhibiting microbial growth in the product or food item.

As I explained in the beginning; water affects the texture of food. For example, a cookie that has added sugar traps moisture which gives it a chewy texture.  As you can imagine, cookies have different textures based on how much sugar or liquid is added. This is why texture and moisture change so much with each new recipe you try online or in your favorite cookbook.

Microbial growth and food 

Meter group gives a list of water activity measurements that will support different microorganisms; such as, salt tolerant bacteria, yeasts, and molds as you can see from the table above.

What kind of unit to use? 

Meter group continues by providing a list of water activity machines. You can get trained by a professional in many places and it is easy to use. Most machines come with calibration solutions that you use every time you test products or food.

References

1. Mitchell Mattes (October 8, 2011). Food Studies: the science of cookie texture. Grist Magazine; Website: https://grist.org/food/2011-10-06-food-studies-the-science-of-cookie-texture/
2. Website: https://www.selectscience.net/products/aqualab-dew-point-water-activity-meter-4te/?prodID=113875
3. Website: http://aqualab.decagon.com.br/educacao/key-concepts-in-water-activity-basics-for-safety-quality-in-food-products/
4. Lynda Searby (March 15, 2016). Colours and flavours: the rise of foodstuffs. Food manufacture – William Reed Business Media Ltd 2019; Website: https://www.foodmanufacture.co.uk/Article/2016/03/15/Natural-food-colourings-on-the-rise?utm_source=copyright&utm_medium=OnSite&utm_campaign=copyright