by Dave Arnold
The Buchi corporation invented rotary evaporation, and rotary evaporators. They design their machines to remove solvents from chemicals in a laboratory setting. Because rotovaps distill gently at low temperature under vacuum, they happen to also be good at preserving flavors — much better than traditional distillation. The rotovap lets you make uncooked port wine reductions and the freshest herb liquors in the world. It helps you remove specific unwanted flavors from any liquid, and keep the ones you want. It’s your aroma thief, removing and retaining volatiles so you can drink them. (For in-depth information, see the rotovap primer.) Buchi visited me several weeks ago to discuss rotovapping in the kitchen. I showed them my in-progress prototype (my long-time back-burner project, as loyal readers know: see here). I’m happy to say that Buchi was receptive to a lot of my ideas. Rob Crotchfelt, our regional sales rep (and Cooking Issues’ newest best buddy), Herve Lacombe, President of Buchi Corporation, and Matthias Schacht, Product Manager for Rotary Evaporators at the main factory in Switzerland (a real rotovap engineer!) engaged me in a great discussion, then left behind a spanking-new Buchi R210. It’s still built for the lab, but even in the kitchen it is one sweet machine.
A quick history of my rotovap lineup. About five years ago I built a rudimentary vacuum still, a bootlegvap that was a finicky pain in the rear. Fed up, I bought an early 80’s EL131 Buchi rotary evaporator on eBay the next year (they’re cheap at auction – check it out). I customized it to better suit my kitchen needs, adding a peristaltic pump to remove distillate as it was produced. My pride and joy, that rotovap pumped out many bottles of delicious booze. It earned its retirement, which was effective the day I got my state-of-the-art R210.
My new Buchi evaporator is much easier to take apart and clean than its predecessor– which means I’ll be much calmer when Habanero pulp sprays all over the inside. It’s sturdier than the EL 131, and it has a motorized lift mechanism to move its glassware up and down – a feature I initially thought was extravagant, but now realize is essential: it keeps you from slipping and breaking another $350 worth of glassware. The new machine leaks a hell of a lot less air, so no more hours testing and fixing vacuum leaks.
My sit-down with Buchi
Good news: Buchi showed interest in developing a more kitchen friendly (read: cheaper) machine. They aren’t really up for something as different as I proposed — a ground-up, purpose-built kitchen tool – but they see the logic in making some chef-and-bartender-friendly modifications to their design. Buchi agrees that the food world could use its own rotovap, because:
1. Kitchens and fragile, expensive lab-quality glass don’t mix
Lab-vaps must withstand a wide range of corrosive chemicals, so they’re made with expensive (and fragile) lab glass. In the three years I’ve run the tech intern program we’ve destroyed about $1500 worth of lab glass –ouch. Kitchens don’t require the purity and chemical resistance of glass. We are better off with more durable materials, a fact I demonstrated to the Buchi guys by banging my fist repeatedly on the polycarbonate/stainless-steel skeleton of my prototype (the machine won). The Buchi team was convinced.
Accurate variable speed rotation is another pricey rotovap feature essential for labs but not kitchens. Buchi agreed it could be done away with.
2. Vacuum control is great, but really expensive the way it is done now.
Distillation is boiling. The boiling point of a substance is a combination of a temperature and pressure. In traditional distillation, pressure is constant (atmospheric pressure) and temperature is adjusted to control the distillation. In rotary evaporation, on the other hand, temperature is the constant and pressure is the variable, controlled by the level of vacuum. Old-school rotovap junkies (and lab paupers) fly their machines by adjusting the vacuum pressure with a bleeder valve. A bleeder valve and an inexpensive refrigeration-style vacuum pump costs less than $120. That system is cheap, but it requires constant monitoring, is loud, and takes lots of practice to master. Modern rotovap vacuum pumps and vacuum control units solve these problems, but are very expensive –several thousands of dollars.
Modern lab vacuum systems use quiet, dry vacuum pumps. They are a joy to use and don’t make nasty oil fumes, but they will set you back at least a thousand dollars. We couldn’t think of a cheaper alternative to the loud, oily refrigeration pumps that would both work and be affordable. I guess we’ll deal with noise and fumes.
Modern vacuum controls let technicians walk away from the machine while it’s running. These controls are not cheap. Their vacuum sensors are expensive, and so is the software development they require: vacuum gauges measure only the level of vacuum, and require sophisticated microprocessor control to determine the optimal pressure for a distillation based on extrapolations from the pump’s current vacuum-pumping rate.
And adding insult to injury, these pricey automated distillations are not really good for kitchens. Kitchen distillation, unlike most lab distillation, needs to be quick (for flavor, and for practicality) and involve multiple solvents (usually ethanol and water) whose boiling points are constantly changing. The automated lab systems tend not to be speedy enough for kitchen distillations and don’t handle multiple solvents well.
I would love the luxury of walking away from the rotovap while it is running. I have long maintained that good automated control of distillation would be possible using inexpensive thermocouples rather than expensive vacuum gauges. Controlling distillation using the temperature difference between areas with and without distillation vapors in the condenser would ensure an aggressive distillation is maintained throughout the distillation cycle – and it would be cheap to build. I was elated to find that Buchi offers this option on their high-end controller. Unfortunately, it’s only offered as an addition – not an alternative — to their high-end vacuum controller. After I test it, I’ll tell you how it works. Maybe they can make a cheap controller that uses only temperature.
2. Lab rotovaps lack features kitchens need, like:
Ability to pump out the distillate:
To make my rotary evaporator kitchen-worthy I add have added a pump that constantly pumps out distallate. I have gone through several versions of this pump. I eventually settled on a peristaltic pump. Peristaltic pumps work by squeezing liquids or gasses through lengths of tubing. Most tubing won’t work. I tested dozens of varieties and ended up with semi-rigid polyurethane tubing; it’s connected to the rotovap using a spherical glass adapter. On my old EL131 rotovap, the pump was separate– inconvenient when raising and lowering the machine. I bolted the pump on the side of my new rotovap, so the automatic lift raises and lowers it– very swanky. I developed this system for two reasons: 1. you want to taste the distilled product as it’s being made, so you know when to cut distillation off; and 2. You want to remove product as you make it so the distillate doesn’t sit in the machine over the whole run – I believe that doing so kills the flavor.
It’s my hunch that in the unaltered rotovap, flavor compounds are always re-distilling a little bit in the flask, and they aren’t always re-condensing 100% –they get lost through the vacuum pump. Matthias, the engineer, made an interesting counter-observation on this point. “Assume,” he said, “that your rotovap had no leaks in it whatsoever. You would be able to suck an initial vacuum, turn off the pump, and do the entire distillation without turning it back on. In that scenario there could be no flavor loss because the system is sealed.” I had to agree.
In a leak-proof rotovap, flavor won’t be lost in the flask. I now believe that leaky rotovaps and continuously running vacuum pumps are the prime cause of flavor loss in an otherwise properly done distillation. In fact, I have noticed that leaks in different places have different impacts on flavor. Maximum flavor loss occurs when the air leak is located at the distillate collection point. The incoming air strips out the flavor.
The guys from Buchi understood why I added the pump, but I didn’t get the feeling they wanted to go in that direction. They say they currently have a system that, with a little modification, will allow the operator to remove the distillate in small batches for testing without breaking the vacuum — I need more information before I can weigh in on that one.
Ability to pump out reduced product or waste:
The lab rotovap’s distillation flask is constantly rotating, so you can’t pump the spent product out as you are working. There’s no way to drain from a rotating vacuum flask, so waste or reduced product builds up inside the distillation flask. In our system you can only rotovap about three liters of product before you must break the system down and empty the flask. In my ideal rotovap design, the distillation area doesn’t rotate, so it can be drained with a pump similar to the one I use to pump out distillate. You can operate continuously if you can pump out product, pump out waste, and add fresh product without stopping the machine. If you can develop a good automatic distillation system, you could run continuously and unattended. The Buchi guys weren’t really willing to go down this path with me.
Easily cleaned flasks:
The Buchi guys left me with a distillation flask that I really like: it is built like a wide mouth beaker with a screw-on cap. It is half the capacity of my narrow-necked 3 liter flask, but it is so easy to clean that it makes me smile just thinking about it. Cleaning out the distillation flask is typically a crummy job because it’s very hard to scrub the inside. Additionally, when we make thick reductions it’s hard to recover all of the product from the narrow necked flasks. The beaker flask fixes those problems. If only they made a three liter beaker…
One of the big problems with the rotovap is boil-over –you start a distillation, and – blammo- the trapped air inside your product foams up and sprays all over the inside of the rotovap. Big drag. I talked with Matthias about using a fritted material in the distillation path to prevent bubbles from getting into the condenser side of the machine. He said Buchi makes something similar, but it is problematic because the fritted material restricts too much airflow. The vapor duct that separates the distillation side from the condensing side is fairly narrow, and the speed of the vapor shooting through it is quite high. Restricting airflow through that passage isn’t such a great idea –which is why they nixed another of my anti-boilover ideas: an electrical sensor that tests for boiling liquid. They make those sensors for their larger units, where constriction at the vapor duct isn’t so severe.
Happily, the beaker flask Buchi dropped off has made possible a cheap and simple boilover remedy –Scothbrite pads. I bought some fine pads (the white ones) and cut them to fit just inside the head of the beaker. It barely impedes the flow of vapor and is difficult to clog (although I have managed to do it.) But you must be careful not to let the pad fibers get on the sealing surface of the beaker.
Next up in the rotovap chronicles:
What I learned in England; some experiments with mint; a refreshing summer drink.