Imagine you’re looking for dried apples that still look and taste like real fruit. You would have two options: freeze-dried apples or air-dried apples. These two drying methods can retain the look and shape of fresh product in a way that other processes cannot.
Because they have this in common, these products tend to be confused for one another. A freeze-dried apple slice and an air-dried apple slice can look deceivingly similar at first glance. Both methods remove moisture and prolong shelf life, so they may even seem interchangeable.
In reality, though, freeze-drying and air-drying differ in both process and several characteristics of the finished product (we’ll look at 7 specifically). Each comes with certain benefits that can make or break your application, and you should be intentional about choosing the right fit.
Before we dive into the details of this process, let’s make sure we’re on the same page when we refer to the term air-drying. Many call this process dehydration, and the terms are often considered synonymous.
We’re using the term air-drying over dehydration for two reasons: 1) the process very literally dries with air, and 2) dehydration means different things to different industry sectors. Dehydration could potentially describe any process that removes moisture. So for the purpose of this post, we’ll avoid confusion and use the term air-drying, as we also do on each product page throughout our website.
Air-drying can be performed effectively with some variety from processor to processor. In one common method, product runs on a conveyor through a chamber circulating with hot, dry air. The moisture evaporates out of the product. Other processors choose to spray product with hot air, operating more like a fan system, but producing the same effect. Some also choose a more stationary method and dry product on racks rather than conveyors.
In any case, the application of hot air removes the majority of the moisture from the product, producing a dried product that still looks relatively like the raw product, although a little shriveled.
Freeze-drying removes moisture from raw, frozen product through a vacuum system and process called sublimation.
The processor begins by cutting frozen raw product down to the desired piece size, and spreading it evenly onto trays that are stacked and stored in freezers. During this time, product freezes at an even colder temperature, reaching an optimal state at which the original shape of the product can be retained.
To freeze-dry the product, the processor loads trays into a refrigerated freeze-drying chamber that seals tightly shut. The chamber’s vacuum system acts on the frozen material, removing ice from the product and converting it directly to water vapor. The process of converting a solid to a gas, without going through the liquid state, is called sublimation. Since the water is removed from the product in a frozen state, cell structure remains intact and finished product appears less shriveled.
Finished product characteristics: Freeze-dried vs. air-dried
Both processes result in dried solids that retain the flavor and general appearance of the fresh product. Let’s compare 7 specific characteristics of the finished product. Each of these varies from product to product in both categories, but these guidelines are beneficial as a general rule of thumb when determining which process may be right for your application.
As stated previously, both freeze-dried and air-dried products can retain piece identity – that is, the general appearance and color of the original fresh product. Both can also offer a wide range of piece sizes – whole, sliced, diced, flaked, powdered, etc. Favorable piece size for an application can be determined based on goals for how the product will function. For instance, if flavor is the key goal, a powder could be used rather than a piece.
Overall appearance between any given freeze-dried apple piece and air-dried apple piece, for instance, may look similar at first glance. But the sublimation process in freeze-drying keeps cell structure intact more effectively than the air-drying process. Finished air-dried products tend to have a more shriveled appearance.
Due to higher moisture content, however, an air-dried product can have a higher color saturation than its freeze-dried counterpart.
Both freeze-dried and air-dried products are made up 100% of the original raw product. Nothing is added during the process itself, and only water is removed. After that initial process, some processors may choose to add carrier for certain products to keep it free-flowing or to benefit the intended application.
3. Water Content
Water content is lower for freeze-dried products, although it varies from product to product in both categories. An average for freeze-dried is around 2% water content, and an average for air-dried is around 5%.
Because cell structure remains intact throughout the freeze-drying process, those cells remain ready to take on water again. Freeze-dried products may rehydrate in a couple minutes, whereas air-dried products may take closer to thirty minutes – once again depending on the specific product.
Rehydration time affects different applications differently. In a soup mix, for example, several elements must hydrate for the same length of time, in which case a longer rehydration time may be beneficial. A drink mix with fewer ingredients, on the other hand, may be conducive to a shorter rehydration time.
5. Shelf Life
The shelf-life of any given product directly links to the moisture content of that product. Removing water removes potential for bacterial growth. Since air-dried products contain more moisture, they have a shorter shelf life. Actual length of shelf life for any given freeze-dried or air-dried product depends on packaging, storage temperature and the product itself.
6. Flavor / Texture
Freeze-dried and air-dried products can have similar flavors, and both can retain the flavor of the raw product to a considerable extent. Air-dried products tend to be a bit denser due to higher water content, and freeze-dried products maintain a lighter, crunchier texture.
Neither process cooks the raw product, so both retain nutritional value. Freeze-dried product – often considered the closest to fresh product – retains most of its nutrition. The heat applied throughout air-drying may break down certain nutritional elements, but preserves many as well.
Please comment below with any questions raised about freeze-dried products vs. air-dried products, or which may work better for your application. We’d love to hear what you’re up to!