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  • Writer's pictureMarco Aguilera

What is Freeze-Drying?



Freeze-drying is a dehydration method by which solvents can be extracted from various samples without the need to expose them to high temperatures.

One of the significant advantages of freeze-drying is that, by operating at low temperatures during the dehydration process, the physical and chemical structures of the element being dried are not significantly altered. This includes important compounds such as anthocyanins in the case of agraz, blueberries, and generally, red fruits. Consequently, there are numerous literature reviews that attest to the differences between freeze-drying and other traditional drying methods (sun drying, oven drying, hot air drying, etc.) in relation to nutrient preservation.

Another point to consider is that freeze-drying achieves residual moisture levels below 5%. It is worth mentioning that, in general, fruits contain moisture percentages above 80%. This means that for every kilogram of a fruit's weight, approximately 800 grams will be water.

It is important to consider that the water content in a fruit, or any food in general, largely determines how quickly it degrades under normal storage conditions (e.g., in the pantry). This phenomenon is due to the ease with which microorganisms reproduce when there are high moisture levels in foods. Consequently, freeze-dried fruit, having a very low moisture content, will have a longer shelf life compared to fresh fruit.

A Bit of History

The Incas and their current descendants often freeze-dry potatoes using the low temperatures and pressures of the Andes. There, the potatoes freeze, and slowly, thanks to the low pressures due to the altitude of the mountains, the sublimation phenomenon occurs. The obtained product is known as Chuno, which can be defined as a precursor to modern instant mashed potatoes.

Regarding the industrial application of the freeze-drying process, the following historical facts have been presented:

  • In 1933, researchers from the University of Pennsylvania managed to freeze-dry blood.

  • During World War II, freeze-drying was used for the first time to preserve penicillin and plasma. Years later, this technique would become a standard for vaccine preservation.

  • In 1930, during Brazil's coffee boom, the government of that country was looking for a way to preserve coffee and process it in such a way that it could offer consumers an economical and easy-to-use product. This task was entrusted to Nestlé, who developed freeze-dried coffee (equivalent to the instant coffee you drink every day) in 1938. This development was pioneering in the use of this technology for the food industry.

How Does It Work?

Given that the ultimate goal of freeze-drying, which is generally the same for any dehydration method, is to extract the water contained within a sample, it is essential to understand that there are two types of "waters": free water and bound water, defined as follows:

  • Free Water: This is the water that circulates freely in the sample. It is the liquid that comes out when you squeeze an orange or lemon. This type of water represents approximately 90% of the water present in fruits.

  • Bound Water: This is the water that is strongly bonded to another molecule within the sample (proteins, pectins, starches, etc.) and is not as easily extracted as in the case of free water.

Components of a Freeze-Dryer

  • Drying Chamber: A hermetically sealed chamber subject to temperature and pressure changes.

  • Support Trays: Trays placed inside the drying chamber on which the samples to be dehydrated are placed. These trays are usually in direct contact with the thermal element.

  • Thermal Element: Elements that will transfer temperature differentials to the sample through radiation and conduction.

  • Condenser: In this element, the water vapor from sublimation is condensed. It must have sufficient area and cooling capacity to capture all the vapor from sublimation.

  • Refrigeration System: Cools the condenser and, depending on the equipment design, can also cool the product support trays.

  • Vacuum System: Consists of a vacuum pump that regulates the pressure inside the drying chamber. Through the vacuum, the sublimation effect is achieved, as well as the suction of vapors from the chamber to the condenser.

Starting from the above information, the freeze-drying process can be understood as the following sequence of steps:

  1. Freezing: The sample is taken and frozen until both solids and liquids are fully crystallized (this can be done outside the freeze-dryer, as not all equipment includes the option to function as a freezer). Similarly, the manner in which freezing is carried out is crucial in defining the characteristics of the product after freeze-drying.

  2. Primary Drying: Based on the previously mentioned concept of free water and bound water, it can be stated that the main objective of primary drying is to maximize the elimination of free water through the sublimation process. This is achieved by applying a medium vacuum level and increasing the temperature of the thermal element to indirectly heat the sample to a temperature close to the eutectic point (the temperature at which the sample remains in the solid phase). Under these parameters, a significant pressure differential between the condenser and the sample will be obtained, thus achieving shorter cycle times (faster sublimation).

  3. Secondary Drying: This stage begins when the product temperature is above the eutectic point. Its goal is to remove the bound water still present in the sample. At this point, the vacuum level should be increased, as well as the product temperature by modifying the operating parameters of the thermal elements. Each manufacturer must determine the duration of this stage based on their acceptance criteria regarding the desired moisture percentage in the product.

  4. Packaging: Once the secondary drying stage is completed, the product should be vacuum-packed. It should be noted that freeze-dried products are highly hygroscopic (absorb moisture from the environment) and therefore it is ideal to have controlled humidity levels when packing and using airtight containers.

Conclusions

Freeze-drying proves to be an ideal technique for food preservation as it significantly improves shelf life, does not require the addition of preservatives, and maintains nutrients, flavors, and other qualities. Despite this, freeze-drying is a process with very high energy consumption, high investment in machinery, and long cycle times.

Moreover, given that more than 80% of the total weight of a fruit is composed of water, every time you consume a kilogram of freeze-dried THANI product, you are actually consuming 5 or more kilograms of fresh fruit (depending on the fruit and its moisture content).

All of the above leads to the price of freeze-dried products being significantly higher than the cost of products dehydrated using other technologies. Despite this, we believe that this technology is the way forward to offer our customers a product of the highest quality!

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