Because freeze drying removes the solvent at a very low temperature, it should be the gold standard if you want to know which compounds are present in their original state in the plant. The rate of chemical changes for thermochemical reactions usually double with every 10 degree C the temperature is raised.  Similarly reducing the temperature by 10 C would half the rate of chemical change.  The higher the temperature the more likely it is that chemical artefacts can be formed.  

Another important aspect of freeze drying is that it limits oxidative changes of metabolites taking place because the oxygen concentration is very low under vacuum.  

There is one very important exception; freeze drying will remove volatile compounds from your extract.  

If you use volatile solvents such as hexane or acetone drying your extracts under a cold air stream is the easiest.  Remember that not all extracts will yield a dry product.  Some extracts especially with non-polar extracts will lead to a oily product.  When the mass remains constant all the solvent has been removed.

If you have water extract and you do not use freeze drying there is a good chance that you will have difficulties to dissolve the dried extract again because many plants contain carbohydrates that can dry to form a film that is not well solible.

Dear Salma

Check the following thread on the same subject and informative publications

http://www.researchgate.net/post/Can_we_use_a_freeze_dryer_for_drying_of_herbal_extracts_instead_of_a_spray_dryer

http://www.ifrj.upm.edu.my/20%20%2802%29%202013/22%20IFRJ%2020%20%2802%29%202013%20Farooq%20%28230%29.pdf

http://nutriweb.org.my/publications/mjn005/mjn5n12_005.pdf

http://www.academia.edu/5333811/Effect_of_freeze-drying_and_extraction_solvents_on_the_total_phenolic_contents_total_flavonoids_and_antioxidant_activity_of_different_Rabbiteye_blueberry_genotypes_grown_in_New_Zealand

Thank you so much Sir B.R. Rajeswara Rao 

Because freeze drying removes the solvent at a very low temperature, it should be the gold standard if you want to know which compounds are present in their original state in the plant. The rate of chemical changes for thermochemical reactions usually double with every 10 degree C the temperature is raised.  Similarly reducing the temperature by 10 C would half the rate of chemical change.  The higher the temperature the more likely it is that chemical artefacts can be formed.  

Another important aspect of freeze drying is that it limits oxidative changes of metabolites taking place because the oxygen concentration is very low under vacuum.  

There is one very important exception; freeze drying will remove volatile compounds from your extract.  

If you use volatile solvents such as hexane or acetone drying your extracts under a cold air stream is the easiest.  Remember that not all extracts will yield a dry product.  Some extracts especially with non-polar extracts will lead to a oily product.  When the mass remains constant all the solvent has been removed.

If you have water extract and you do not use freeze drying there is a good chance that you will have difficulties to dissolve the dried extract again because many plants contain carbohydrates that can dry to form a film that is not well solible.

much informatibe Jacobus Nicolaas Eloff .. Thanks Alots

You can please follow the links below to obtain valuable informations:-

1. http://www.academia.edu/1445141/The_effect_of_freeze_drying_and_its_implications_for_botanical_medicine_a_review

2. http://www.ncbi.nlm.nih.gov/pubmed/12164280

Hi prof.

Can we dry plant latex by using freeze dryer