close up of ice crystals on frozen raspberries

When & How to Use Frozen Berries

Fresh blueberries, strawberries, raspberries, they’re delicious and juicy. But, they’re also very seasonal and fragile. There’s a reason they tend to be sold in sturdy plastic containers, even a slight bruise ruins them.

The presence of these brightly colored fruits reminds me personally (being Dutch) of summertime. Even though you can buy them almost year-round nowadays. Fresh strawberries with a scoop of ice cream, while enjoying the outdoor sun, delicious! But, that does make them quite expensive and, in all honesty, out of season, they don’t taste as good!

Luckily, there’s an alternative to these delicate, expensive fruits: frozen straw/black/blue-berries! For a lot of uses the frozen version is just as good, if not better than the fresh berry.

Need some convincing? Let’s dig into the science of frozen fruits.

How fruit is frozen

Before being able to compare fresh vs frozen fruits, you have to know how fresh fruits are frozen. Fruit that is meant to be frozen, doesn’t even go to a store or a packing warehouse. Instead, the fruit goes to the freezing plant as quickly as possible after harvest. This prevents spoilage of the fruit and ensure that the final product has a good quality.

Pre-treating

Upon arrival the fruits are washed, rinsed, and things like sticks, stems and stones are removed. Larger fruits may be cut into pieces before freezing since smaller pieces freeze faster and more evenly. Berries, except for strawberries, tend to be small enough not to need any cutting.

In some cases fruit then receives a pre-treatment to help extend shelf life and improve the quality of the frozen fruits. This treatment could be a dip into ascorbic acid (vitamin C) and/or citric acid solution. These both reduce browning of fruits. Even in the freezer, fruits such as apples and bananas will turn brown over time (due to PPO enzyme activity). Berries, being red/black in most cases, don’t tend to change colour in the freezer.

Another possible pre-treatment bath could be a sugar solution. This can help them stay firmer over time.

A note on blanching: most vegetables are blanched shortly before they are frozen. This quick heat treatment deactivates the enzymes that cause browning. However, fruits are too delicate to be blanched so they aren’t, this is especially true for berries.

close up of ice crystals on frozen raspberries
Frozen raspberries, with ice crystals

Freezing

The freezing process itself is surprisingly simple. Nowadays, most fruits will travel through a freezer while on a conveyor belt. By positioning them apart on the conveyor belt they won’t block the airflow from one another and they won’t stick together. Fruits (and fish) frozen this way might say might say “individually quick frozen” (IQF).

At the very low temperatures, about -35°C/°F, well below your home freezer temperature (-18°C/0°F), the fruit freezes very quickly. It is important that freezing happens quickly. Freezing fast leads to the creation of a lot of tiny ice crystals, instead of just a few very large chunky ones. (It’s also why liquid nitrogen ice cream can be so smooth). Large crystals do more damage to the fruit, so you want to prevent those from forming.

Packaging and storing fruits

Once the fruits are fully frozen they can be packaged together in a bag/box again. Since they’re frozen already they won’t bruise one another and won’t stick (as much) either.

When storing frozen fruits the most important thing is to keep them frozen at all times. This may sound obvious, but is really important. Thawing and re-freezing cycles are never good for frozen products, especially fruit. If the fruit gets a chance to warm up the individual pieces might stick to each other again. Also, they will probably lose some moisture, which then recrystallizes into ice crystals within the bag.

frozen raspberries in pan
Notice all the ice crystals on these raspberries? Notice also that not all of them are separate from others anymore? This happens during prolonged storage and is sped up significantly by thawing and freezing the fruits during storage.

Properties of frozen fruits

As long as fruits remain frozen, they will hold onto their original shape. The hard ice crystals and the frozen water in the fruits ensures a raspberry still looks like a raspberry. However, once you thaw these fruits they don’t look like their fresh counterparts anymore. The strawberries don’t look plump anymore and even the raspberries are somewhat collapsed. Blueberries and pieces of mango come out looking quite nice, but even their textures are very different than of their fresh counterparts. This is because freezing fruits does quite some damage to their internal textures.

Whereas vegetables have a lot of fibrous textures, fruits barely have any of that. This texture helps a vegetable hold onto its shape, even if it frozen. Fruits don’t have this and rely on less freezer-proof structures to hold their shape.

Loss of turgor

Instead, fruits rely almost completely on turgor. Turgor is a mechanism in a plant that allows the cells to absorb water which causes the cells to press against the surrounding cell walls. Because of this pressure a salad leaf is crunchy (and becomes wilted once turgor is gone) and it is what keeps berry plump.

illustration describing turgor in a cell

However, if the water seeps out, the turgor is lost. It’s what happens when lettuce has wilted, the leaves have lost their turgor. It’s also what happens when fruits soften, the cells are losing water, even by drying out or because of damage to the cells allowing moisture to escape.

The ice crystals that form in a fruit are hard solid ice crystals that grow into spiky shapes. These crystals are hard enough to actually break the cells within the fruit. The damage they do causes thawed frozen fruits to be so soft. Damage by ice crystals is worse if the crystals are larger, but also if they melt and regrow. Regrowing will go slowly and as a result, these regrown crystals are larger and more damaging.

Once cells are damaged, not just the turgor is lost. Instead, the release of all these molecules from the cell can cause all sorts of breakdown reactions to occur.

Why are not all frozen fruits rock solid?

If you eat a frozen berry you will notice that this berry is a lot softer than lets say an ice cube. This is due to the presence of all that sugar in the fruit. Sugar lowers the freezing point of water. As a result, not all of the water in the berries actually freezes, some of it remains liquid.

This same mechanism is essential for making ice cream and it is why a frozen caramel often doesn’t turn rock solid either!

Shelf life of frozen fruits

When you store fruits in a regular freezer (-18°C/0°F) the fruits will stay good for at least a year. However, from a safety perspective, they can survive way longer storage times. Spoilage and pathogenic micro organisms (those that make your sick) don’t grow at these low temperatures.

Instead, the fruits mostly decrease in quality during storage. ‘Freezer burn’ may occur for instance. These are the dry white spots you may see on frozen foods over time, especially if they’ve not been packaged well. Freezer burn is not harmful, but your fruit will be less juicy. Freezer burn is nothing more than evaporation of water (yes, that can still happen in the freezer!) causing the fruit to dry out.

Also, as you could see on some earlier photos, consumer freezers often aren’t of high enough quality to prevent the formation and growth of new ice crystals. The frequent opening and closing of a freezer door for instance isn’t particularly good for a frozen berry.

strawberry mirror cake
The strawberry mousse in this cake can be made with frozen strawberries for sure!

Comparing fresh vs frozen berries

Now that you know how frozen berries are made and what processes occur, let’s return to comparing those fresh and frozen berries.

The qualities of fresh berries

Berries and strawberries can be found in stores in abundance during summer. But even then, most berries tend to be expensive fruits. These fruits are very fragile. A slight bruise is enough for molds to get access to all the fruit juices in the fruit and start growing. Berries simply don’t have the strength of an apple or a banana. As such, growers need to be careful when handling them, they can only be packed in small poxes and only keep for a few days. That all drives the price up.

Their delicate structure though is also what makes them so delicious to eat. You can just pop them in your mouth and they break apart with no to little effort.You don’t need to peel them or cut them, they’re perfect for snacking.

However, if you’re not eating them fresh, the fruits quickly lose their added value. If you use the berries in a sauce, a mousse or a puree, you lose the most attractive part of a berry: its texture.

When to use: eat fresh berries fresh, don’t cook them, blend them, freeze them, enjoy their texture as is!

The qualities of frozen berries

The main quality that frozen berries lack that fresh ones do: the texture. Once you freeze berries the ice crystals that grow within the berries, no matter how small, puncture the delicate texture of the fruits. It causes them to leak moisture and thus lose their turgor, which is the force that keeps them plump and firm.

If you thaw frozen berries most of them will be very soft and almost puree like. They aren’t that pleasure to eat anymore as such. However, they do still contain all the sugars and flavors from the fresh berries. As a matter of fact, they likely contain a bit more of it. Frozen berries are frozen within hours or just 1 or 2 days after harvest. They don’t even have time to spoil.

When to use: for any application where you need to break down the texture of the berries anyhow through blending or heating for instance. Smoothies, mousses, cakes, etc. are all perfect for using frozen berries.

Using frozen berries

If you’re using frozen berries in an application where you’ll be blending the fruits, you can go ahead as you would for fresh berries and just follow the instructions for the recipe. For instance, for making this strawberry mousse or if you’re using them in a panna cotta.

If your application doesn’t start with blending or heating a bunch of berries, you do need to adjust your way of working slightly. For instance, if you’re using frozen blueberries in muffins, they will cool down the batter. As such, your baking time maybe just a tab longer. Also, if you want to spread the individual fruits throughout a dish, you’d best do so when they’re still quite cold. This way they hold their shape well and you won’t break them when mixing your batter.

Overall, frozen fruits taste great and it allows you to always have a stack of berries at hand, for unexpected baking/cooking projects!

aerated raspberry bavarois on spoon
A Bavarois (an aerated panna cotta with a lot of fruits) made using frozen raspberries.

Sources

Dole Fruit, How it’s made – Frozen fruit, Oct-11, 2013, YouTube, link

Gustavo V. Barbosa-Cánovas, Bilge Altunakar, Danilo J. Mejía-Lorio, Freezing of fruits and vegetables, FAO, 2005, link

Newsletter subscription box

Add comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.

subscribe
sign up for a food sceince course
Find a book to read
Subscribe