We Had a Cake and ate it too!
You may have started to wonder if meat was the only food with any water. Today is the dawn of a new day where we look at not only water in meat, but also water in other foods as well. Looking for inspiration, I drove down to our local Safeway Grocery Store to find something to examine and post on the blog. I got to the Bakery section and looked no further.
I found a cake in an open refrigerated case, which I thought was interesting. Do cakes need to be refrigerated? This particular cake was a chocolate 1/8 sheet Penguin Bistro cake with white icing and chocolate fudge filling. I asked one of the bakers why this cake was refrigerated and some of the other cakes were not. She said “It’s just to keep it fresh.” I also asked for a little bit of extra icing and some of the chocolate fudge and they were happy to get it fresh from the sealed bucket for me.
As I pulled up to METER, I couldn’t wait to see what the results of the lab tests would be. If you have ever read the ‘Water Activity Tools’ pamphlet you know that there are two case studies about cakes. The first study is about a cake that only had a 3 day shelf life and the second study is about moisture migration in a fruit cake. The two stories emphasize the importance of controlling water activity in baked goods to increase shelf-life, prevent microbial growth and minimize moisture migration. While the bakery in ‘Water Activity tools’ was shipping their cakes to grocery stores and had a 9-12 day shelf life, the cake at Safeway was made at the local store and had a sale by date 7 days after the cake was made.
Moisture migration occurs when two ingredients, like cake and frosting, are at different water activities and moisture moves from the higher water activity ingredient to the lower water activity ingredient. I knew that there was a potential for moisture migration between the cake, frosting, and filling and that if the water activity in one of those items was much different than the others, it would lead to problems down the road.
Excited to see if this cake was the poster child for short shelf-life, I cut off a piece, separated out the frosting, cake, and filling, and took a water activity measurements on each. Just to prove a point, I also took a slice into the lab to measure the moisture content of all three parts, the cake, the frosting and the filling. A quick aside: tests in our the lab are done faster if your products taste good. If you are testing wood chips, it’s going to take a while. If you test cake, you get your oven dried moisture content results the next day.
The results are as follows:
Chocolate Cake: 0.866 aw and 31.06% Moisture Content
Vanilla Icing: 0.816 aw and 6.58% Moisture Content
Chocolate Fudge Filling: 0.816 aw and 12.81% Moisture Content
Fresh Chocolate Fudge Filling: 0.811 aw and 8.18% Moisture Content
Fresh Vanilla Icing: 0.829 aw and 6.27% Moisture Content
You can see that the water activity on the fudge filing went up a tiny bit, but not enough to cause any problems. The aw on the icing also went down some, but also not enough to make a difference in the cake quality. I would assume that the frosting dried out a little bit because the cake was not in an airtight container. These are pretty good results, especially from walking into a store, picking a cake off the shelf and testing it.
Looking at the results, could you imagine trying to formulate a product like this cake without using water activity and only using moisture content? It would be nearly impossible to make a shelf stable cake like this one without using water activity.
The Twinkie is often referred to as the gold standard as far as shelf-life and cakes go. Although a Twinkie only has a shelf-life of 25 days, it has been inflatable park rumored that they last much longer. This begs the question, is the Penguin Bistro cake from Safeway as bomb proof as a Twinkie? Twinkie bread tests at 0.789 aw and the filling tests at 0.786. Since we only get to see the product once it’s been in the package for days, the two components may not have always been that close in water activity. It’s a safe bet, though, that both the cake and the filling have a low water activity because of their sugar content. Compared with the cake, the Twinkie has a lower water activity, and probably a longer shelf life. Both the cake and the Twinkie appear to be solid when it comes to moisture migration. It looks like the bakers and scientists at Safeway understand the implications of water activity and are pretty good at building a cake that can withstand the test of time.
After leaving the cake in the original packaging for four days, I divided tested the three main parts of the cake and here is what I found:
Chocolate Cake: 0.835 aw
Vanilla Icing: 0.795 aw
Chocolate Fudge Filling: 0.808 aw
The cake had dried out a little bit, but not enough to really end shelf life. The only real difference that had occurred is that the inner filling had migrated slightly. If I had left the cake in the refrigerator, it could have possibly prevented this. The bottom line is: equilibrium amongst cake, icing and filling means a better product.