Freeze Drying 3 Stages
A freezing drying recipe includes many steps. Generally, there are mainly 3 stages in a freeze drying process:
Freezing. Is the first step also is the most important step of a success freeze drying.
Primary Drying. Remove physical water, which ratio >90% in raw product. This stage takes quite long time.
Secondary Drying. Remove bound water, by use of ultra vacuum and highest allowable Temp.
Process Time Chart
Freeze drying recipes or freeze drying time chart is the relations between shelves temperature and lasting time. Of course, the latest technology also allow operators control the drying chamber pressure. Which means a complete drying curve includes shelves temperature, lasting hours and chamber pressure setting.
To program the right drying curve, operator should know the basic data of raw product include: Eutectic point, Collapse temperature, Max allowable drying temperature etc. Here are the key considerations for a perfect recipe:
During freezing stage: Freeze the raw product lower than its freezing point and keep the temperature at least 1hr. Proper control the ice crystal growth, to get same size.
In primary drying stage: Ensure frozen product temperature always/at least 5C lower than its eutectic point. Meanwhile, control the vacuum degree to 20%~30% of the saturated vapor pressure which product temperature corresponds to.
In this stage, various steps may need set, but keep in mind of above 2 key points. It is not so hard.
In secondary drying stage: Increase shelves temperature, to max allowable drying temperature. And pull chamber pressure down to ultra. Keep this situation untill chamber pressure and product temperature not change any more.
At last, proper reduce shelves temperature, to the most suitable range. Prepare for dried harvest.
