Creamy berry crumble – On using frozen fruits in a dessert

While making a delicious strawberry/raspberry pie the other day, I made far too much of the pie crust. It was about twice the amount I needed for making my pie. So I had to get creative and come up with something else, preferably a dessert since we still had to eat dinner that day.

That’s how I came up with a fabulous dessert recipe that I’m bound to try again another time, I call it: ‘creamy berry crumble with ice cream and fruit syrup’. Luckily we still had some frozen fruits in the freezer that I could use to give this dessert just that little extra tweak. And a light bulb went of in my head, I had to read more about frozen fruits!

The dessert itself

But before diving into the world of frozen fruits, I’ll share my creation with you. As mentioned, I had too much of the pie dough, see recipe here. So I decided to bake the other half in the oven as well. It came out slightly crunchy and tasting great. I’d also made my creme patissiere (some sort of a custard) for the pie, so I decided to use that as well.

Here I went, crumbling up the crust in an oven tray and covering it with creme patissier. I had learned my lessons though by watching Masterchef Australia that you should always have some different textures as well as flavours. A very rich and heavy dessert, as it would be with only crust and custard, could do with some sweetness and a little hint of sour. It could also do with some colour. Here’s where the frozen fruit comes in!

I heated the frozen fruit in a pan with some sugar until it was all melted. I then dropped the fruit over the creamy crumble, as elegantly as I could. I thought it made a great appealin dessert. Luckily we still had some ice cream in the freezer as well which finished it all of perfectly!

Frozen fruit vs fresh fruit

Berries and strawberries can be found in stores in abundance during summer. But even then, blueberries and raspberries tend to be expensive fruits. It makes sense since these fruits are very fragile. I imagine it is hard work to harvest, package and transport them to the supermarkt. Their short shelf lifes will probably increase the loss of product in the chain, all increasing the prices.

Therefore, out of the seasons and even during the summer season frozen fruit is perfect. It’s a lot cheaper, can be stored for a long period of time (even when all of a sudden you decide on a different recipe) and still has a great flavour.

When to use frozen fruit

That said, frresh and frozen fruit do have some significant differences. The main differences sit in the structure of the fruits. Freezing fruits, no matter how well done, will always damage the fruits’ texture (read more below). When thawing the fruit it will never have as firm a texture as the fresh product does. Berries will be soft and can be deformed easily and strawberry look slightly mushy.

Nevertheless, the flavour is still there. Therefore, use frozen fruit whenever it’s more about the flavour than about the texture. Using frozen fruits in pies or cakes is perfect (take care that you probably have to thaw them before using), but eating just frozen strawberries with cream is something I would advise doing. Whenever you need warm fruit frozen fruit is also perfect. Heating/cooking fruit will ruin its structure anyway, so don’t feel bad about using frozen fruit.

fruity crumble with ice cream

Freezing damages fruit to some degree

In a previous post I’ve talked about the texture of fruits and vegetables. Discussing their cellular structure, turgor and thus why they might wilt or soften over time. Freezing fruits is done on a very large scale, as can be seen in this YouTube video.

The process of freezing fruit isn’t complicated. Besides clearning and cutting the fruits (if necessary), fruits are transported into a freezer pretty quickly. When freezing fruits the main trick is to do it fast, we’ll come back to the why later.

So most fruits are frozen at temperatures below -18°C. There are a lot of freezer types. A common and relatively simple one is the use of a belt. The fruits are transported on a belt into the freezer. Once in the freezer the temperature of the fruits is decreased as fast as possible. This can be done by a lot of air flow which transports the warmer air around the fruits away. The temperature of the air can be as low as -30°C, but is lower than -18°C for sure.

It depends on the freezer design, but often the air some from below, through the belts. This also move the fruits around slightly, preventing them from sticking to one another, so freezing properly individually.

Freezing temperature of fruits

When freezing a product it will have a certain freezing temperature, the temperature at which the material changes from liquid to solid. For water this is 0°C. However, once other components are added, such as sugar or salt, the freezing temperature will drop down. This phenomenon is called the freezing point depression and I’ve zoomed in on this phenomenon in a post on making ice cream, where this is also very important.

Most fruits are in essence water with several solutes dissolved in them, especially sugars. This means that the temperature at which the water in the fruits starts to freeze up is a few degrees below 0°C.

Crystals damage the fruit

Once the temperature is low enough water can start to freeze, when doing this it will form crystals. The first crystals are formed by so called nucleation. Once a crystal has been formed it will continue to grow into a larger crystal since more water will sit on it.

It’s the crystals that actually damage the fruit. Ice crystals can grow in such a way that they will cause breakage of cellular structures. So cells themsleves might break or the bonds keeping cells together can be broken by these ice crystals. Fruits are especially vulnerable to this since they have quite a fragile structure. Meats on the other hand, don’t get damaged as much.

So, when freezing fruits, it’s most important to try to keep the crystals as small as possible. You can’t really do anything about the total amount of crystals (that’s mainly determined by thermodynamics and this freezing point depression), but you can try to get a lot of small crystals instead of a few large ones.

The way to get a lot of small crystals is by lowering the temperature well below the nucleation point, thus the temperature at which nuclei start forming. The lower you go the higher the nucleation rate, as a consequence a lot of crystals can be formed. If you would cool more slowly nucleation (thus the start of crystal growth) would be slower and crystal growth itself as well. That makes it ideal for making a few large ones.

Freshness and safety of frozen fruits

When fruits are kept at a temperature of -18°C they can be kept for just about forever, that is, from a safety perspective. At this low a temperature no spoilage bacteria or moulds will be able to grow on the product. The same goes up for pathogenic micro-organisms, those that make you sick.

That said, frozen fruits cannot be kept forever from a quality perspective. Even when frozen in a very well controlled manner, the quality will decrease over time. One of the possible causes is ‘freezer burn’. These are dry white spots you can see on frozen foods over time. I’ve seen them several times with meats that I’ve kept in the freezer far too long. Freezer burn is not harmful at all, it has only dried out part of the fruit. The fruit will be less juicy when eaten, but if you heat the fruit with a little moisture or use it in a cake or pie, there’s no issue at all.

Freezer burn is actually nothing more than dried out food. Water has evaporated, even while the product stays in the freezer! At these low temperatures evaporation is very slow, but it does occur. Since products are kept in the freezer for quite some time even this slow process can be seen. Freezer burn can never be fully prevented, but try opening and closing the freezer only when necessary. Temperature changes increase the chance of freezer burn.

 

All in all, frozen fruits are perfect to use for loads of desserts. Let me know what you prefer to use them for!

Sources

For those looking for a more deep-dive into freezing fruit, there are a couple of sources I found that will be helpful. The FAO has written an extensive book on freezing vegetables and fruits. The United Nations has also published an interesting article, also discussing world wide trends, as did the GCCA.

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