Freeze-drying has been around more than 100 years. But how does freeze drying work? And why is it better than simply dehydrating?
People have developed many food preservation methods to extend life and nutrition of their edibles during seasonal changes or travel. Anthropologists have identified some of the first methods of preserving food as curing and fermentation. These included drying meat and plant products with heat and airflow, smoke, or salt, to remove moisture. Fermentation includes making cheeses and yogurt, vinegars, and alcoholic beverages. Scientists found evidence of curing as early as 12,000 BC and cheesemaking at 6,000 BC.
Many preservation techniques developed by location: Civilizations in colder climates, such as northern Europe and the homesteads of the Old West, used cooling methods such as freezing, root cellars, and burying food within clay jugs. Warmer locations learned, early on, how to ferment; anthropologists found strong evidence of fermentation within Babylon, ancient Egypt, Sudan, and Mexico.
Then came the modern methods: Nicolas Appert invented home canning in 1806, Louis Pasteur developed pasteurization in 1862. Now we have irradiation, chemical preservatives, and hurdle technology: combining several modern approaches to control pathogens.
How freeze-drying works?
Simply put, freeze-drying is the removal of water from a frozen product using a process called sublimation. Sublimation occurs when a frozen liquid transforms directly to a gaseous state without passing back through the liquid phase. The process of freeze-drying consists of three phases: prefreezing, primary drying, and secondary drying.
Prefreezing:
Food items that are freeze-dried must first be prefrozen below its eutectic temperature, or simply put, freezing the materials (solute) that make up the food. Although a product may appear to be frozen because of all the ice that is present, in actuality it is not completely frozen until all of the solute is frozen as well.
Primary Drying:
After prefreezing, ice must be removed from the product through sublimation. This requires careful control of two parameters; temperature and pressure. The rate of sublimation depends on the difference in vapor pressure of the product compared to the vapor pressure of the ice collector. Molecules move from the higher pressure sample to the lower pressure sample. Since vapor pressure is related to temperature, it is also necessary for the product temperature to be warmer than the ice collector temperature.
Secondary Drying:
After primary drying, all ice has sublimated but some liquid is still present in the product. Continued drying is necessary to remove the remaining water. The process for removing this excess water is called isothermal desorption. The excess water is desorbed from the product by making the product temperature higher than the ambient temperature.
During the entire freeze-drying process, the exact freezing methodology and proper storage is very important.
