Freeze drying remove the water from food while keeping the food from turning into a liquid. Because freeze drying removes water from food without allowing the food to turn into a liquid, the cell in that food remain intact. Because the cells in the food remain intact, the food retains its original quality after the freeze drying process is complete.
A temperature chart is used to show the exact temperatures that must be utilized during the process; using the incorrect temperatures during freeze drying will lead to a loss of the shelf life of the food or the texture of the food will be ruined. The first stage of the freeze drying process is the deep freeze stage. During this stage, the food product must be frozen to the temperature at which all of the water within the food become solid ice crystals.
Freeze Drying Steps and Temperatures
Should there be still liquid water within the food during the freeze drying process, the expansion of that water will tear the food apart. Most home freeze dryer reach temperatures between negative thirty degrees and negative fifty degrees during the deep freeze stage. This range is included in the temperature chart for this stage of the process because the food must be solid by the time vacuum pump and shelf temperature begins to rise.
The second stage of freeze drying is primary drying. During this stage, the process of sublimation occurs; ice crystals within the food sublimate to become vapor and the vacuum pump removes them. The shelf temperatures between seventy and one hundred degrees during primary drying; temperatures that is too low will require many days to complete the process, and temperatures too high will melt the outer layer of the food before the center of the food is freeze dried.
The freeze drying process and the temperature chart both indicate the various temperature range needed for different types of food. The final stage of freeze drying is secondary drying. During this stage, the cook increases the shelf temperature to temperatures of one hundred twenty-five degrees; this increased temperature aid in the removal of the last traces of moisture from the food.
Food is already light and porous during secondary drying. The goal is to reduce the moisture within the food to levels of two or three percent; if the moisture levels of the food are below two percent, the food will retain a shelf life of twenty-five years. Should the moisture content of the food be higher than two or three percent, mold could grow on the food; if the shelf temperature during secondary drying is too high, the food may lose it flavor during the process.
Different foods will require different amount of time to complete the freeze drying process. The freeze drying process includes a reference grid within the temperature chart that indicates the length of time required to freeze dry fruits, vegetables, meats, and prepared meals. Food products that contain high amounts of sugar or fat will require more time to freeze dry than items that have been blanched prior to freeze drying.
The reference grid will allow an individual to determine the proper amount of time to freeze dry their food without guessing. The size of the freeze dryer will impact the freeze drying process. A small countertop freeze dryer will not be able to move heat and vacuum as fast as a large commercial freeze dryer.
The setting for the freeze drying process will have to be adjusted according to the size of the freeze dryer. For instance, ninety degrees is a common temperature for a countertop freeze dryer, while larger freeze dryers can reach higher temperatures. Vacuum level are the same for all sizes of freeze dryers, but the batch times will be shorter for larger freeze dryers.
Small freeze dryers should always be set to the same settings as indicated in the chart to avoid freeze drying food with the settings created for commercial machine. The preparation of food prior to the freeze drying process is important. Food should be cut to uniform thickness.
Vegetables should be blanched for one or two minutes to preserve the color of the vegetables. Food should be spread out in a single layer on the freeze drying trays; it should not be stack on top of itself during the process. Following these steps will ensure the food will properly dry according to the temperatures indicated on the chart.
A rehydration test can be performed after the freeze drying process is complete. Properly freeze dried food will rehydrate to its original texture within ten to twenty minutes. If the center of the food is still leathery after twenty minutes, the food need to be placed into the freeze dryer for additional time.
If the edges of the food are brown and brittle, the shelf temperature during the secondary drying stage was too high. Using the temperature chart ensures that the freeze drying process will be consistent. Using the chart, an individual will know for how long to cook the food to reach the proper shelf life, and what settings to use to protect the food.
Should an individual use the chart regularly to assist in the freeze drying process, the chart will ensure that the food is stocked in the pantry in a dependable manner.