Dehydration is a type of freeze-drying commonly referred to as lyophilization. To remove the water content of food, it is first frozen.
The best drying technique is generally agreed to be freeze-drying. As a result, the food and pharmaceutical industries frequently use it.
You can easily purchase many freeze-dried foods in your neighborhood supermarket, from ready-to-eat meals to fruits or snacks.
This page details freeze-drying, including what it is if it is beneficial, and how to do it at home.
What is Freeze Drying?
Freeze drying also called lyophilization, removes water frequently used to preserve perishable goods to increase their shelf life and make them travel-ready. By first freezing the material, lowering the pressure, and increasing heat, freeze drying causes the material’s frozen water to transform immediately into vapor.
This indicates that the product’s water content changes directly from a solid to a gaseous state or from ice to vapor without moving through a liquid condition.
Because it works at low temperatures, freeze-drying preserves a product’s nutritional value, taste, look, and heat. This is one of the reasons it is regarded as a high-quality dehydration technique.
-sensitive compounds
The product’s deterioration is greatly postponed by freezing, which inhibits chemical and microbiological activities (thereby extending its shelf life).
The Basics of Freeze-Dry Foods
Before freezing, the product must undergo all necessary preparations—like freezing the product in a particular shape, altering cryoprotectant concentrations, or adding more.
Freezing
The product is chilled until it is completely frozen. A simple chest freezer, a commercial blast freezer, specialized laboratory freezing equipment (such as a glycol bath or LN2, for example), or some types of freeze dryers (like our HS/HSL and F series freeze dryers, which can freeze products in situ), can all be used for this.
Slowly freezing a product will result in bigger, interconnected ice crystals that speed up the elimination of vapor during sublimation. In consequence, this permits a quicker freeze-drying cycle.
Large ice crystals, however, are not always ideal, especially regarding food or organic cell structures, where they might damage the final result by rupturing the cell walls. In these circumstances, it is advisable to freeze the substance to produce smaller ice crystals quickly.
Primary Drying
During this stage, the condenser cools to its operating temperature, and a vacuum pump lowers the chamber pressure to a few millibars.
The product receives heat energy to speed up the sublimation process (the necessary heat input can be calculated with the latent heat of the sublimation of ice or other sublimating material). After the primary drying phase, the water content will normally be 5 to 8%.
For example, the heat can be the ambient temperature in the case of basic laboratory equipment, heated conductive shelves, or radiative heat from shelves/product chamber walls. There are many different ways to introduce heat energy for sublimation.
In many situations, the procedure’s pace is crucial. While some products can be dried as quickly as the machine can handle without significantly compromising quality, others need a long, progressive drying process to minimize heat damage.
It’s crucial to remember that the product dries from the outside. Due to the difficulty of releasing vapor from the product from the center ice core, the thickness of the material will therefore play a role in determining how long this stage lasts. In the worst-case situation, the decrease in sublimation cooling and subsequent rise in product temperature may compromise the product’s structural integrity. As a result, the drying process will be slower and less effective.
Secondary Drying
In this stage, fluid that has not yet frozen but is sorbed (bonded) to the product is removed. The pace at which this sorbed water is eliminated through desorption depends on the product’s temperature. Once the primary drying phase is finished, the secondary drying phase product temperatures can be fine-tuned to accelerate the pace of desorption either by a controlled gradual temperature rise or just by setting the temperature at maximum for the quickest rate. Be careful not to overheat your product or accelerate vapor production to the point where the vacuum level drops below the product’s eutectic point.
Which Equipments are Used in Freeze Drying?
The fundamental components of freeze-dryers are refrigeration, vacuum, control system, product chamber or manifold, and condenser. The refrigeration system cools the condenser inside the freeze-dryer. To freeze the goods, it can cool the shelves in the product chamber. The vacuum system comprises an airtight condenser, an attached product chamber, and a separate vacuum pump. The intricacy of control systems might vary, but they frequently have temperature and pressure sensors. Products are typically dried in a manifold with attached flasks or a bigger chamber with shelves or trays.
The condenser, usually located adjacent to the trays, is housed inside the drying chamber, which is normally a horizontal cylinder. Heat is applied to the food by radiation or conduction while spread out on the trays. The product’s vapors are drawn to the condenser, where they are transformed back into a solid form (ice). The condenser in shelf freeze-dryers can be either inside the product chamber (internal condenser) or a different chamber (external condenser) connected to the product chamber via a vapor port.
Three different commercial setups are used for industrial freeze-drying. The first method involves large-scale internal freezing procedures that normally deal with just one or a small number of items. Instead, a small to moderate amount of freeze-drying can be done internally to produce a variety of freeze-dried ingredients that are then combined to create the finished commercial items, such as soups. Third, facilities for copacking and freeze-drying can freeze-dry meals on demand for outside parties.
How can I Know When the First Drying Stage is Finished?
Track the product temperature using a thermocouple. Once primary drying is finished, the product temperature will rise and get close to the shelf or chamber temperature since sublimation cooling is no longer present. The primary drying phase can be finished when the product and shelf temperatures are similar (temperature differential, T around 3-5°C). The area around the thermocouple will dry faster than most of the product due to heat conduction via the thermocouple; therefore, a safety margin for additional primary drying should be incorporated.
Since the product will dry from top to bottom, the thermocouple should be positioned toward the bottom.
Pressure Rise Test – A pressure rise test may be done if the freeze dryer has an external condenser. The pressure rise in the product chamber is measured while the isolation valve between the condenser chamber and the product chamber is closed. The primary phase can be finished if the pressure rise approaches zero and there is little to no vapor produced by sublimation.
Convergence test between a capacitance manometer and a Pirani gauge.
To ascertain the primary drying endpoint, installing a capacitance manometer and a Pirani gauge is necessary because the latter gives falsely high values when vapor is present. The primary drying phase is over once the two readings have converged (and no further change in the readings is noticed).
How Much Time is Required to Freeze Dry Food?
Depending on your technique, your food will freeze dry between 20 hours and a month. Additionally, it is based on the food you are freeze-drying. For instance, whereas watermelon and squash take longer to dry, items like corn, beef, and peas do so more quickly. The time required to freeze dry food will also depend on the thickness of the food slices. 20 to 40 hours are needed if you have a freeze-dryer.
However, household use of this freeze-drying apparatus is rather expensive. Although there are numerous possibilities under $2,000, the most efficient dryers cost between $2,000 and $5,000. The least expensive method, however, requires up to a month to thoroughly freeze-dry your food.
Even though it requires more effort than a regular freezer, dry ice is a quick alternative.
Can you Freeze Dry Food at Home?
You can freeze-dry food at home if you know how to do it. Using a freeze-dryer, freezer, dry ice, or vacuum chamber, your food can be freeze-dried. Just adhere to the directions above to freeze-dry your food for later use. Home freeze-drying is far less expensive than employing professional services. If you’ve never frozen-dried food before, start with straightforward foods like apples, bananas, and berries. You can practice on vegetables like peppers and broccoli and move on to other foods once you’re satisfied with the results. Remember that food that has been properly frozen doesn’t change color.
What are the Health Benefits of Consuming Freeze-Dried Food?
According to research by Gary Stoner, Ph.D., and the American Institute for Cancer Research, the antioxidant phytochemical content of fresh fruits is roughly equivalent to that of their freeze-dried counterparts. However, ascorbic acid levels and the quantity of polyphenol, a substance in berries that protects cells, were shown to be noticeably reduced by freeze-drying in both Stoner’s study and the Chilean blueberry study.
In a study published in “Seminars in Cancer Biology” in October 2007, Stoner employed powdered freeze-dried berries that demonstrated promise for preventing and reducing colon cancers. Stoner also participated in a University of Connecticut study that was revealed to be successful in treating ulcerative colitis using freeze-dried black raspberry powder in “Carcinogenesis” in 2011. Researchers in Oklahoma found that women with metabolic syndrome, a condition that raises risks for coronary artery disease, stroke, and type two diabetes, had lower levels of harmful LDL cholesterol and total cholesterol after consuming freeze-dried strawberry powder. The “Nutrition Journal” published its findings in 2009.
Reference: Functional Dehydrated Foods for Health Preservation
The most popular methods for fruit or vegetable powder production are freeze drying, foam drying, drum drying, and spray drying. Free drying allows for the optimum preservation of phytochemicals and their bioactivity in powdered fruits and vegetables. Although it is the most popular method for drying probiotics, spray drying appears to be a good alternative for producing probiotic powder on an industrial scale due to its low productivity and high operational costs.
Conclusion
There are many methods for preserving food without refrigeration. Freeze drying, dehydrating, and pressure canning are some of them. These methods can preserve various foods, including fruits, vegetables, meat, and dairy. While freezing-drying does not destroy bacteria, you should still use healthy produce. Below is a list of some foods that can be safely freeze-dried.
Freeze-drying is the most difficult step in the process. Some types of raw foods are more challenging to freeze-dry. The water content of raw foods is usually eighty to ninety percent. It is important to remember that these foods can be separated into two categories: free water and bound water. Free water must remain frozen throughout the process, while bound water must remain in the freezer. The best way to freeze-dry food is to cut the food into uniform portions.
