It is depend on your antibacterial agent structure, for example it can active in oven drying?

By the way freeze-drying is the best way but it can be expensive method. 

I agree, the properties of the antimicrobial component to be recovered might, to a large extent, inform the choice of drying method. However, if the target antimicrobial is unknown (general screening), try avoiding direct sunlight, it could affect the activities of many bioactive components.

Dear Mac,

I agree with Alireza's suggestion. The method of drying would depend on your bioactive compound of interest with antimicrobial properties. If you could do a phytochemical screening of both the starting material and your extract, it will give you an idea of the phytochemicals present in them.

However, I dont think either of the drying techniques would cause a lot of damage since the suggested temperatures (50 and 60) are tolerable.

Freeze drying is most prefered, however the rest should work fine in the absence of the former.

Best wishes,

Chigozie

Dear Chigozie Okolie and Others

Can we do freeze-drying without the addition of water?. My plant sample is a fruit which is quite moist. Freeze-drying is suppose to remove water content is it? Is there any benefit of adding water to the extract in the freeze-drying process?

I completely agree with others. Freeze drying is the best method for concentrating or drying any water based bioactive crude sample. Though this procedure is time-consuming and costly. If your molecules are thermostable, then only you perform the  heat involved process. For safer side, it is always better to go for Freeze drying. There are no benefits of adding excess water to your sample as this will only increase the time consumption of your drying process.

Dear Abhirup Mookherjee

Thanks for your answer.

Does adding excess water to the extract have any positive effect on phytochemical stability or bioactivity of phytochemicals during freeze-drying?

Dear Mac,

I don't think adding excess water will make a positive effect on phytochemical stability. You can try different level of alcohol (20,40,60,80,100) for fractionating the compound. 

Dear K.A. Martin Xavier 

Thank you. Will you be able to show me an article that compares the antimicrobial activities of air dried vs freeze-dried plant extracts?

Please refer this,

Effect of different drying procedures on the bioactive polysaccharide acemannan from Aloe vera ( Aloe barbadensis Miller)

Article in Carbohydrate Polymers 168:327-336 · July 2017

DOI: 10.1016/j.carbpol.2017.03.087

Freee-drying is the best technique to remove water from extract but if you have an organic extract you should use rotary evaporator.

Though freeze-drying may be the best technique but drying using rotary evaporator at control temperature e.g 50 or 60oC will be fine and fast. However, the disadvantage of rotary evaporation technique is the difficulty in removing the extract if allowed to dry totally. Air-drying of extracts will allow contamination of extract with fungal spores and bacteria.

using rotary evaporator at 45oC

First you have to take account solvent used for extract:

1.       Rotavapor to concéntrate metabolites and evaporate solvent used (40°C-50°C).

2.       Most of compounds are labils to temperature and light exposition (oxidation) high temperatures will decrease bioactivity and contamination for microorganisms.

3.       If you have carbohydrates(sugars) in the extract will not have a tipic dry sample it will have honey apperance and you can promote non enzymatic reactions (Maillard) for temperature exposition time (no more than 50°) and polymerizing structures that could in some cases benefit bioactivity.

My Rank is:

Freeze drying

Oven drying 50°C

Oven drying 60°C

Air drying in shade

Air drying under sun

Dear Armando Solari

Thanks for your response.

Why is oven drying better than air drying in shade?

The options are:

1st:Freeze drying; 2nd: Air drying under a shade; 3rd: Oven drying at very low temperatures of 30 to 50 degrees Celsius. All you should aim at is the maximum retention of bioactive compounds as well as essentials oils

Air drying in shade

My rank is

Freeze-drying

Air drying in shade (room temperature)

Air drying under sun

Oven drying at 50 °C

Oven drying at 60 °C

when increase the temperature during drying process  it may affect the activity of extracts.

Dear G.Dayana Jeyaleela

Thanks for the answer.

But will UV light destroy bioactive phytochemical compounds?

Air draying at room temperature 

is good method to avoid the active ingredients distorting 

Absolutely the number one is freeze-drying, and it may continue with oven drying at 50 °C, oven drying at 60 °C, air drying in shade (room temperature) and air drying under sun. Actually depending on your extraction solvent you may not able to dry your extract and could possibly end with a mould production at air drying both at shade at room temperature and under sun. I don't recommend you try to dry your extract under sun too, due to the effect of light on the active substances. Since you have asked a ranking between these options, I have added the air drying in the list, but I normally won't recommend them for drying extract. If you have a special case you can contact me through messaging. I would be glad to help you.

Dear Ergin Murat Altuner

Can I freeze-dry my extracts without adding extra water?

We got higher antibacterial activity in air dried (room temperature) plant extract than sun dried and oven dried. It may depend on your phyto ingredient of concern even though most may get lost or depreciated at high or very high temperature.

Air drying under shade (Laboratory bench)at room temperature is good.