Acid, Retort and Water Activity: A Cheat Sheet on Shelf-Stable Food Production

There are three kinds of food products in this world: 1) the shelf-stable product that can sit on the shelf for several months, 2) the refrigerated product that usually tastes better, tastes fresher and has retained its original bright colors because the product has been minimally processed, and 3) the frozen product that can last for months.

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In a perfect world, all products could be manufactured and sold in the refrigerated and/or frozen section of the supermarket. Unfortunately, due to the high cost of placing the product in that space, cold transportation and the need to move the product quickly (refrigerated products have a short shelf life) many new gourmet specialty food producers have to go down the shelf-stable path in order to make the product economical and profitable. Shelf-stable food (sometimes called ambient food) is processed so that it can be safely stored in a sealed container at room temperature for an extended shelf-life.  

Specialty food producers are often disappointed when they realize that they have to do some severe processing to their gourmet food product in order to make it shelf-stable. Below are a few of those methods that can have a serious effect on the flavor and texture of the product.

 

Dry/Low Water Activity

Water activity is the amount of “available” water in a food system. In order for a product to be stable and not support yeast and mold growth it has to have a very low water activity of around .65 (water activity is measured by using a water activity unit sold by Aqualab). For example, a delicious, moist energy bar with a water activity around .75 will only last a few days on the shelf at room temperature before yeast and mold start growing on it. In order to sell this bar on store shelves, you'd have to lower the water activity. This is done via dehydration, adding more salt, or adding more sugar. This is why products like beef jerky, jams and freeze dried fruits can last forever. The water has been tied up with salt, sugar or just simply sucked completely out of the product. 

 

Retort

A retorted food product is essentially a “canned” food. Products that are canned are basically cooked to about 250’F and are made sterile. A can of food can last forever on the shelf. Canned meat, canned stews, canned green beans — those are all retorted. Of late you can also get flexible pouch retorted products (tuna fish pouches, for example) but they are still heat treated. So if you developed an amazing chicken soup and want it to be shelf-stable, you may have to retort/sterilize it if you want it to last on the shelf and it may not taste as great as what you made in the kitchen.

 

Acid or Acidified

Acidified foods are products that use a combination of heat and acid to generate shelf stability. For example, apple sauce is naturally acidic (pH below 4.6) and may be subjected to a heat treatment of around 195F. In this case, the acidity is a bacterial inhibitor so it's not necessary to cook it as hot as the retorted samples. These types of products are usually tangy and sour and include fruit purees, juices and other shelf-stable pasteurized products.

It’s important to think about shelf stability and how to achieve it when you are developing your specialty food product. Many entrepreneurs don’t consider the processing conditions that will have to take place to take their concept from the kitchen to the room temperature supermarket shelf.

 

Rachel Zemser is a Certified Culinary Scientist with a BS in Food Science from the University of Massachusetts, a MS degree in Food Microbiology from the University of Illinois and a culinary arts degree from the New York Restaurant School/Art Institute. She has been working in the food industry for 17 years in both technical and creative roles. She spends most her time all over the U.S. working with both large and small start up companies assisting them with their food science and R&D. 

 

Follow Rachel on Twitter for updates about the food industry or check out her Local Food Lab portfolio.

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