Understanding Rotary Evaporators

BEAT Geek: Understanding Rotary Evaporators

What is a Rotary Evaporator?

A Rotary Evaporator (or ‘Rotovap’), is a device that is used primarily by chemists to remove and/or separate solvents from one another in a mixture.

The Rotovap was invented in 1950 by Lyman C. Craig, a Chemical Researcher who worked at the Rockerfella Institute for Medical Research. Working in Pharmaceuticals, Craig’s research primarily focused on separating solvents from one another that had formed a compound. He was instrumental in pioneering a technique for separating solvents of different densities, which came to be known as ‘Countercurrent Distribution’.

Following this early work, Craig realized that it was possible to identify different boiling/evaporation points in varying solvents, and use this to his advantage in order to achieve separation. It was at this point that he created the Rotary Evaporator, a piece of equipment that is found in most Laboratories and many Bars to this day!


How does it work in principle?

Rotary Evaporators apply the same principles of basic principles of distillation, however differ from a regular pot still due to the variables that can be controlled. Unlike a regular still, Rotary Evaporators allow the user to control the temperature of the heating element and the atmospheric pressure of the liquid being distilled to a finite degree.


Why is this important?

Referring to the laws of physics known as “Boyle’s Law” and “Charles’ Law”, we know that the boiling point of a liquid is related to the atmospheric pressure that said liquid is exposed to. As atmospheric pressure is increased, the boiling point of a liquid will also increase. Similarly, if you decrease atmospheric pressure, the boiling point will decrease alongside it. To put this in perspective, I will use the example of boiling water upon Mount Everest.

Atmospheric pressure is effectively measuring how much our earth’s atmosphere pushes down on the objects on the ground. In theory, the further you get away from the earth, the less the atmospheric pressure will be. At sea level, the atmospheric pressure of our earth hovers around 1012mbar (Millibar), or ONE ATM PRESSURE. At sea level, water will boil at roughly 100°c. Atop Mount Everest, the atmospheric pressure is about a third of the pressure at sea level (around 330mbar). Due to Boyle/Charles’ Law, a pot of water will boil at around 72°c. The graph below will show you this relationship.


How does the machine physically work?

1. Water Bath

The water bath is effectively your heating element to warm the liquor to be distilled. The pot (See 3.) is submerged into the liquid and spins at a consistent rate to maintain the temperature of the liquid being distilled.


2. Water Bath Temperature Control

The temperature of the water bath is typically controlled using a digital dial. The advantage to using a water bath as a heat source as opposed to an open flame or steam for example, is that you can accurately control and maintain a consistent temperature (much like cooking sous vide in a baine marie).


3. Pot

The pot is the chamber in which your neutral alcohol and botanicals are placed to be distilled. This chamber is subject to changes in pressure, based on how your vacuum is set (See Point 5).


4. Speed Control

This dial is used to control and maintain how fast the pot spins whilst in the water bath. By rotating the pot, the user can ensure a consistent thin layer of liquid on the outside of the glass pot, which helps with evaporation. If you increase the surface area of the liquid by fast rotation, you will create a quicker rate of evaporation. Slow it down, and this will result in a slower distillation. It is important to rotate the pot flask consistently to achieve an equal distribution of heat and allow the botanicals to mix with the alcohols.


5. Vacuum

Most modern rotary evaporators use a vacuum pump in order to reduce or increase the amount of pressure inside the pot and subsequent chambers. These are often controlled by digital monitors which let you know the atmospheric pressure inside the chamber. Setting the atmospheric pressure will allow you to distill at lower temperatures. Modern rotovaps are capable of achieving pressures as low as around 6mbar. When operating a rotovap, the user has to be careful not to drop the atmospheric pressure too quickly, as this can result in foaming of the distillate, causing it to shoot over into the reservoir, ruining the distillation.


6. Condenser

Much like in regular distillation, once you have evaporated your liquid into a gas, you need to change it back into a liquid again for collection. The condenser is a cold coiled pipe which reacts with the heated alcohol vapour, reducing its temperature and turning it back into a liquid. This liquid will then drip from the pipe and down into the reservoir (See Point 7).


7. Reservoir

The reservoir is where you collect your distilled liquid post-distillation.


How can it be used in the Drink Industry?

Below is a list of a number of ways in which a rotary evaporator can be used in relation to drink:


Distilling/Re-Distilling with Alcohol

Most commonly, rotary evaporators are used in the drink industry to distill or re-distill alcohols in order to add a particular flavour. Usually, a neutral alcohol is used along with botanicals to create a specific flavour. A good example of this would be 69 Colebrooke Row’s Horseradish Vodka, a key component of their signature Bloody Mary.

N.B. When working with alcohols to distill or re-distill, it is imperative to obtain the correct licences to do so as laid out by the UK Government.


Creating an Essence or Hydrosol

Rotary evaporators can also be used to create infusions of high or low concentration using alcohol or even water. Dependent on the amount of botanical you use, you can flavour liquids lightly or intensely to create a specific flavour. Rotary evaporators are useful for extracting flavour from botanicals that are hard to obtain through techniques such as maceration (e.g. Peppercorns). However, it is important to watch for contamination in your still when distilling a robust botanical.


Creating a Concentrate

An obtuse way of looking at distillation with a rotovap, it is possible to extract the alcohol from a liquid, yet retain a concentration of flavour that is left over in the pot flask afterwards. These types of preperations are useful for creating a flavour to add to drinks without adding too much alcohol. The Port Reduction which is added to the Zetter Townhouse’s ‘Master at Arms’ cocktail is a good example of this.


Clarification of Drinks

When used correctly, a rotovap can be used to remove colour from liquids or mixed drinks, whilst still retaining all the flavour through distillation. A good example of this would be Bramble’s ‘Clarified Mojito’.

Why is it useful in the Drink Industry?!

When distilling or extracting flavour using heat, you need to be careful not to burn the botanical or ingredient that you are using, especially in the case of delicate botanicals such as mint. The main advantage of using a rotary evaporator when distilling is that by reducing the atmospheric pressure of the liquid quite significantly, you can distill alcohols along with their botanicals at a much lower temperature. In doing this, you give the botanical time to infuse with the alcohol, without burning or ‘overcooking’ in the process.

In addition, by being able to control each of the variables in distillation to a finite degree, distilling using a rotary evaporator allows the user to create a consistent product with a precise flavour profile on each run. Due to distilling under pressure, Rotary evaporators allow a user to create a ‘carbon copy’ of flavour, without any loss of intensity.


Where can I get one?

Rotary evaporators are available to be purchased online, largely from chemistry equipment suppliers or straight from the manufacturer. The original Rotary Evaporator was designed in partnership with a company called Buchi, who are still widely regarded to be leaders in the technology to this day. It is advisable to shop around, looking for a machine that will fit your needs. It is important to maintain quality within a machine, but there is no need to break the bank if it is not required. Machines vary quite considerably in price, from around £2500 all the way up to about £12000.


Does my bar need one?

Although an extremely interesting piece of kit, rotary evaporators don’t come cheap, so it is worth giving some serious thought as to whether or not your bar will benefit from one. There are ways to create infusions and obtain flavour without paying out a large sum of money (e.g. cooking liquids sous vide with ingredients/maceration). However, if you are part of an organization that is progressive and keen to push the boundaries of flavour, it can be quite a useful addition to your arsenal.

Additionally, rotary evaporators require a lot of care and attention, not just whilst distilling, but also in terms of maintenance. I would only recommend purchasing a rotovap if you have ample time and commitment to experimentation as well as maintaining the upkeep of the machine. Poor upkeep can lead to breakages, which can be mighty expensive to fix!

Lastly, you need to assess whether or not the machine will be profitable in future. It is all well and good distilling weird and wonderful botanicals at a high cost of goods to achieve an interesting flavour, but if this product isn't appreciated by your customers, your P&L is certain to take a dent in the long run!

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