Water Activity and Food Preservation

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Food science was by far one of my favorite classes. Scheduled in the last year of college, it felt like a reward for making it through all my other coursework . Although we learned a lot of cool, applicable things, my favorite concept was about water activity and food preservation. It was like everything I knew about food took on new meaning.

Now, water activity plays a role in lots of areas of cooking, but today we are going to focus on jam. If you have ever made your own jam, you know that there is indeed about twice as much sugar as there is fruit. Why, why all this sugar?

According to sugar.org (bear with me) sugar has three main functions in jam:

Gelling

Preservation

Color Retention

Let me tell you, one time I threw caution to the wind and attempted to make very low-sugar strawberry jam (without a recipe). It didn’t taste particularly pleasing, and midwinter it had turned a very unlovely redish gray. There is a reason for sugar in jam.

Water Activity

Like I said, this was one of my favorite concepts in food science class, mostly because I felt I understood it well. In all three functions of sugar in jam – gelling, preserving, and color retention – water activity is the principle at work.

You see, sugar lowers the water activity in the solution. Imagine it’s December and you have baked up an unthinkable mass of Christmas cookies. Or a thinkable one, either way. Now, you planned to give these away to whomever you run into – dentist, pastor, vet, coworker. But then you get a house guest. And then another. Soon your house is filled with sugar. I mean house guests.

Instead of you taking your cookies and giving them away to outsiders, those cookies stay at home and your guests eat them. In the case of food preservation, bacteria are the outsiders, and you don’t want to give them your cookies (water) or they will start multiplying and spoil your food. So you can say, “sorry, I have all these house guests (sugar) at home, and they need all my cookies (water).”

For things like jam, the whole solution needs to be 60-65% sugar in order to be shelf-stable. This means that there is enough sugar to hog all the water so bacteria can’t have any. If bacteria doesn’t have water, it can’t thrive and multiply.

One sentence diversion:

Hygroscopicity is the tendency of something to draw moisture from the environment; fructose is the most hygroscopic sugar.

There are alternatives to traditional jam, such as sugar-free spreads or low/no sugar needed pectin for making your own. This type of pectin has calcium phosphate added to help the jam to set up with less/no sugar.

As for me, I typically use traditional pectin in my jam, though I have enjoyed the lower sugar jams because they aren’t so terribly sweet. I regard jam as a sugar, just like honey or maple syrup – something you want to go easy on. However, for people with very limited carbohydrate intake, lower sugar alternatives may be necessary.

This year I purchased this book on canning. There are some great pictures and very creative ideas. We tried the red onion marmalade and were quite pleased. It is also a wonderful resource for someone who wants to get started canning or has an interest in entering canned goods in a fair.

In regards to food science, I’d love to hear what you would love to hear. Fill me in!

 

This post contains affiliate links. Please visit my disclosure page to learn more about how all of that works.