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When eating your cereal, whether it’s early in the morning, or late at night, have you ever thought about the texture of the soft raisins and hard crunchy bits? If you did, by any chance, you might have been wondering how those raisins remain soft, while the rest remains crunchy!
Unknown to most of us, it’s one of the major challenges for cereal manufacturers: how to keep the soft ingredients soft, and the crunchy ones crunchy. Because, over time, soft ingredients tend to go hard, whereas crunchy ones tend to go soggy. Turns out, there’s a simple trick: lowering the water activity of the raisins by adding glycerol. Did that go too fast, let’s dive in a little deeper.
- Soft turns hard, crunchy turns soggy
- Keeping raisins soft – several strategies
Soft turns hard, crunchy turns soggy
Store crunchy cookies together with soft moist cookies in a container and within a matter of days, if not hours, the crunchy cookies will have turned soft. Add a moist filling on top of a crunchy pie crust and again, within no time, the crust will become soggy. Mix regular raisins with crunchy cereals or dry oat flakes and within days the raisins will have gone dry and hard, whereas the cereals might have lost some of their crunch.
Water is a food scientist’s friend and enemy. It’s crucial for making all sorts of delicious products but has a way of making things challenging as well. Water doesn’t just stay in place (unless it’s frozen, and even then it still moves slowly). Instead, it continuously moves around within food. It prefers to move from an area with a high concentration (for instance: raisins) to one with a low concentration (such as crunchy cereal pieces).
Water activity measures how available the water is
To complicate matters, ‘it’s not just the amount of water in a food that’s important. It’s how ‘available’ this water is, that counts. Some foods with the exact same concentration of water can still have a different amount of ‘available’ water. That’s because some ingredients are great at ‘holding on’ to water. They bind it, and keep it in place, making it unavailable for moving around.
This amount of ‘available’ water is described by the water activity of a food. A product that’s pure water has a water activity of 1, the highest possible score. A product that does not contain any free water has a value of 0. Fresh foods such as fruits, vegetables, and meat have a water activity >0.9. Dried and crunchy foods tend to have a water activity of <0.5. Water will always want to move from a place of a high water activity to a low one.
Cereals have a water activity <0.5
In order for food to be crunchy, it can’t have too much water. Water will make it moist and soft. Instead, most crunchy products are relatively dry, with a water activity <0.5. The same goes for cereals. There is a huge variation between different types of crunchy cereals, but generally speaking, crunchy cereals have a water activity <0.5 and they tend to turn soggy at a water activity >0.5.
Raisins have a water activity >0.5
Recall that raisins are simply dried grapes. Grapes start out with a water activity close to 1, but by removing water, the water activity of raisins goes down significantly, though tends to stay above 0.5. As you can imagine, water loves to seep out of the raisins, into the cereals. And whereas cereals get soggy when they get too moist, raisins get very hard and dry when they get too dry, especially when their water activity <0.5.
The water activity of raisins does not depend on whether they’re brown or yellow. However, it does depend on how they’ve been dried and packaged during storage.
Keeping raisins soft – several strategies
Unless there is an impenetrable barrier, water will move from a high water activity to a low water activity product. To prevent this from happening, and to keep raisins soft, there are a few possible strategies to follow.
Even out water activities to prevent water movement
If the water activity of the crunchy cereal and the raisins is the same, water will no longer move in between the two. It’s a very commonly used strategy in a lot of products. Candy bars for instance are often designed in such a way that the different components that touch each other have the same water activity.
In the case of cereals, we’d have to either increase the water activity of the crunchy bits, or decrease that of the raisins. It is very hard, if not impossible to increase the water activity of most crunchy bits. Once you increase the water activity, which essentially means you’re adding water, you inherently soften the pieces. It’s why almost every cereal will use the opposite strategy: lower the water activity of the raisins.
Add glycerol to the raisins
Glycerol is a small molecule, comparable to glucose, but not as sweet, and very water loving. That is, it works as a humectant, it absorbs and holds on to water very well. This way, it can keep raisins nice and soft. But, since it binds that water, it does lower the water activity of the raisins.
Soak in sugar water
Another commonly used method to keep raisins soft is to add sugar. Sugars are great at holding onto water and lowering water activity. It’s the reason for fruit jam‘s long shelf life. The added sugar holds onto water, making it unavailable to spoilage micro organisms. This way keeps the jam well over time. In a similar way, the sugar can hold onto water in raisins and make it unavailable for moving into the cereal bits.
There are a few ways to add this sugar to the raisins. One is to soak the raisins in concentrated sugar water, increasing the sugar content in the raisins. Another is to coat the raisins in some sugar, protecting them from moisture increases this way.
Add an impenetrable barrier
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The other way to prevent moisture migration in and out of raisins is to add a barrier through which water can’t travel. Chocolate is a commonly use barrier. Chocolate contains a lot of fat and water can’t travel through easily. It’s why chocolate coated fruits stay good for so long! But, in cereals, you tend to just want a very thin layer of fat, to not impact the flavor of the cereal as a whole. And that’s pretty hard to accomplish since raisins tend to be wrinkly. As a result, it’s easy to have gaps in this fatty layer, through which water will still be able to travel.
Store in air tight packaging
Another great barrier is plastic that doesn’t allow water to travel through. It’s why you should never store raisins as is in a paper bag. Water can easily travel through paper, drying out the raisins. A plastic bag though prevents that movement of water quite well, same for glass and plastic containers.
If you plan on making your own raisin cereal, this is probably your best strategy. Pack the raisins in an air tight container and only mix them in with the crunchy cereal bits at the last minute. It’s your best chance for success, without any complicated measures. Because water is strong, it will move, whether you’d want it or not ;-).
Water activity in foods – Fundamentals and applications, 2007, IFT Press link
Ted Labuza, Karl Roe, Camy Payne, Fern Panda, Theodore J. Labuza, Peter S. Labuza and Laura Krusch, STORAGE STABILITY OF DRY FOOD SYSTEMS: INFLUENCE OF STATE CHANGES DURING DRYING AND STORAGE, Drying 2004 – Proceedings of the 14th International Drying Symposium (IDS 2004)
São Paulo, Brazil, 22-25 August 2004, vol. A, pp. 48-6 link