Freeze Drying vs. Thermal Drying, What Are The Differences And Which Is Better?
Freeze drying vs. thermal drying — introduction
Freeze drying is the sublimation of water, and thermal drying is the evaporation of water. The former is the direct change of solid into a gaseous state, and the latter is the direct change of a liquid into a gaseous state.
Freeze drying vs. thermal drying — freeze drying
During the drying process, in order to maintain the physical structure of the sample to the maximum, a preferred method is vacuum freeze drying. Because the physical structure of the sample is basically unaffected under low temperature and low pressure, and the sample will not be shriveled due to surface pore constriction or internal loss of water caused by the moisture gradient within and outside the cell wall. At low pressure, the sublimation point of the sample is increased (towards normal temperature). And this speeds up the sublimation process and improves the drying efficiency.
Freeze drying vs. thermal drying — thermal drying
Thermal drying is a drying process that uses the cooperation between a heat carrier and a wet carrier. Usually, hot air is used as heat and a wet carrier at the same time. That is, the air is heated and then the air heats the sample, and the evaporated moisture of the sample is taken away and discharged by the air. Because the relative humidity will decrease by 4.5% every time the air rises by 1 degree. So this is a very common method.
Freeze drying vs. thermal drying — which is better?
Freeze drying in most cases is better than thermal drying. Here are the reasons. And these are also part of the answer to the question “what are the differences between freeze drying and thermal drying”.
1. Drying speed
The effect of thermal drying on the sample is the temperature from the outside to the inside, and the humidity from the inside to the outside. And there is a limit to the speed, which is the maximum speed at which the sample can diffuse water from the inside to the outside at the limit temperature that the sample can withstand. Excessive speed will cause the skin shrinkage and reduces the drying speed; while too-slow speed will affect efficiency. At the same time, too high temperature will cause the internal moisture to vaporize too quickly, and it will expand before it is discharged through the capillary tube, causing cell rupture.
Freeze-drying is a sublimation effect, and the speed depends on the external pressure (also water vapor pressure or absolute humidity). It has little relationship with temperature, so it can ensure that the physical structure of cells (microorganisms) is not destroyed to the greatest extent.
2. Product quality
Product quality is also one of the differences between freeze drying and thermal drying. The food dried by a freeze dryer lyophilizer is usually healthier and tastier than that by thermal drying device. Therefore, when judging from the performance of the dried products, freeze drying win over thermal drying.
But there are still some special advantages that thermal drying owns. For example, costs. Though freeze-drying is a preferred method for drying materials, a lyophilization machine usually costs more than a thermal drying machine. What’s more, you can put fruit or other common materials in the sunshine to let it dry. But if you want to freeze drying them, you need to prepare a freeze dryer.
What are the differences between freeze drying and thermal drying? Freeze drying vs. thermal drying, which is better? Now I believe you should have a clear answer to such questions.