Concentrate Fruit Juice with Rotovap

[MAKPac]

Any recipes/experience creating a concentrated fruit sauce in a rotovap? I've seen some savory recipes (one is in modernist cuisine), but haven't been able to find a successful approach to concentrating a fruit juice. The idea is to create something like a sauce or coulis, and my thought was to use the rotovap to gently evaporate the water to concentrate the juice at low temperature, thereby resulting in a sauce with more intense flavor than could be achieved using heat, which would destroy certain flavor compounds in the fruit. What's happening when I try this is that all the flavor and aroma is being evaporated into the distillation flask, so I end up with a tasteless sludge that has no aroma or flavor, and then I end up with flavored water that has a beautiful aroma, but if I put the flavored water back into the distilled sauce, then I'm back where I started, with the aroma + the water that I was trying to eliminate. Would appreciate hearing from anyone who's successfully done this! 

[blue_dolphin]

Dave Arnold describes making a syrup with blood orange juice (see over here on Cooking Issues) and says it can be done with other juices as well.  Though he is only making a concentrated syrup, not going so far as to get a sludge. He gives the temps he uses at that link.  You could try asking him on Twitter or Instagram for more info.

[gfweb]

Rotovap is kind of a no heat distillation. Anything volatile (ie flavor) will come off and probably at a lower temp than the water you are trying to remove

 

I think I'd experiment with shorter times to hopefully concentrate flavor , but leaving a usable liquid behind.  Its not obvious to me that this would work.

 

 

[Bernie]

Not sure what a Rotovac looks like, but I have had some experience with vacuums and liquids.

Most liquid have a set boiling point at atmospheric pressure. Water its 100C/212F (this is where the water turns to steam, it evaporates)

 

But as you drop the pressure the boiling point also drops. At vacuum levels the water boils off, the "hotter" molecules first then the rest.

In fact as the vacuum drops most chemicals "boil off" including things like impurities in the surface of metals and plastics etc.

It is why it is VERY hard to get perfect vacuums.

This happens to all the different components in the liquid. But they reach their individual boiling points at different vacuums, much like at different temperatures at atmospheric pressures.

 

I would think you need to capture and distill the "evaporates" to add back.

But the vacuum you use has to be low enough to boil off the volatiles without boiling off the water.

 

But you will still get some water even at low vacuum.

This is temperature of water is the AVERAGE temperature (energy level) of all the molecules that make up the liquid. Not all the molecules need to reach boiling point to change state.

We call that drying when we talk about water and wet things like clothes.