If 40 °C in the water bath of the rotary evaporator is not a danger for your compounds (usually it is not) then the rest ist physics. The effect may be caused by saponins, that cause foaming. With a standard membrane pump going down to ca. 10 mbar  even the removal of water should not be a problem at all.

A practical hint for foaming solutions is to concentrate them in a rather LARGE flask, e.g. then you use a 2 L flask for 500 ml solution, i.e. a rather "empty" flask instead of a up to 50% filled one.

Note: Freeze-drying is only a good idea for purely aqueous solutions without any organic liquids, hence: for 80% ethanol and 20% water ist is not to recommend.

 What is the temperature of the water bath of your eveporator? I would suggest to reduce the temperature, as high heat may destroy some of the heat-labile compounds. However under reduced pressure, the boiling pt of a solvent can be lowered. Another suggestion is, why not you use higher percentage (95 or 97%) of ethanol to extract? As it is troublesome to get rid of water content in your sample if you use 80% ethanol. 

Is it that the temperature of the water bath of the rotar vapor is set above the recommended temperature of 40oC for plant secondary metabolite analysis? Or is it just that the boiling is as a result of frothing which normally occur for plant material that is rich in saponin?

Dear;

It's prefered to use 40°c for secondary metabolites from plants. At boiling several molecules should be destroyed and some alterations may occur destroying your metabolites, and at the other hand new molecules may appear. 

Use 40°c for secondary metabolites from plants. At boiling several molecules should be destroyed and some alterations may occur destroying your metabolites, and new molecules may be seen. I agree with Mr. Bachir Benarba.

the rotary temperature is normal thing in isolation reasearch so may not use high temperature than 60 oC for ethanol hope it will not effect

At reduced pressure liquids boil at lower temperatures, thus with a rotary evaporator, you have a combined effect of increasing the surface area, through rotation, and reducing the boiling point temperature, under vacuum. In your case it is not possible to say if the boiling will 'harm' your extract. All the compounds in your extract may or may not be stable at the temperature you have set your water bath at. It is most like that some are and some are not. So ideally you want to use as little heat as possible to dry your extract. A rule of thumb for plant extracts, where we are interested in the secondary metabolites, not higher than 40 degrees C is recommended. If your extract boils excessively you do run the risk of it jumping over to the solvent collection flask. This is not ideal as you will have to start again and you would also have to clean your apparatus afterwards as you may have contaminated the cooling column etc. with your extract. You will also have a little trouble driving off the water from your 80% extract. Freeze-drying this would be the best option if available.  

hi 

thanks for your reply. Greatly appreciate your help my sample is bitter gourd which consists of high saponin levels and my rotary vap is set at 38 C .  I try to monitor the process during the run from beginning to end the second i see that it goes for boiling i increase the pressure. I can see some change in the color of my extract how ever I think its due to sedimentation

regards 

I think its all right to run at 38C, the possibility of lossing extra compounds due to heat is very low in your case. 

If 40 °C in the water bath of the rotary evaporator is not a danger for your compounds (usually it is not) then the rest ist physics. The effect may be caused by saponins, that cause foaming. With a standard membrane pump going down to ca. 10 mbar  even the removal of water should not be a problem at all.

A practical hint for foaming solutions is to concentrate them in a rather LARGE flask, e.g. then you use a 2 L flask for 500 ml solution, i.e. a rather "empty" flask instead of a up to 50% filled one.

Note: Freeze-drying is only a good idea for purely aqueous solutions without any organic liquids, hence: for 80% ethanol and 20% water ist is not to recommend.

I agree with Dieter Sicker. I suggested freeze drying as an option only after the organic solvent had been removed. Using a larger flask with less extract is a good solution to 'foaming saponin rich' extracts. The colour of your extract may change as it gets more concentrated. 

for plant extract its always better to keep the temp. below 40°C. It is temp which is more important than boiling. As under reduced pressure, boiling occur at low temp. and thats why we use rotary evaporator. Further, froth may occur due to presence of saponins. so if the temp was less than 40°C, you need not to worry...

if you're evaporating at maximum temperature of 40 under vacuum ( at least 10 mmHg) which is achievable with a good water pump then there should be no worries of decomposition

thx everyone your comments were very useful

Excessive boiling may affect the heat labile components of your extract. And you never know, these components might be the only active components as far as your study is concerned.

Be careful, please, it is not the VISIBLE effect of "boiling" that may be dangerous to compounds contained in the solution because this effect takes places at only 40 °C (but we tend to connect it with the boiling of HOT water in the kitchen that we know to occur at 100 °C) the "boiling at 40 °C however is just a MILD one at a distinctly lower temperature.

Likely, it is no danger, especially when we take into account that a temperature of 40 °C is "known" to many plants from their former life in the environment, e.g. in the hot sunlight  of high noon or in the djungle. That means compounds present in your sample are resistant enough to withstand this temperature.

the distilled water extract of pomegranate leaves obtained after boiling for 15 minutes could not be concentrated even at 100 degree Celsius under vacuum in rotary evaporator. Hence, another method should be applied to obtain crude powder.

What you report is physicochemically impossible. Water should evaporate under this conditons, and also under milder ones.

You may have a technical problem, may be there ist no vacuum at all in your evaporator. Check your vacuum source (pump OK?), check the set up if there is a leakage, check if all valves are closed.

In case of possible foaming the hints of Gary Ivan Stafford are very good ones, see above.