Posted by Dave Arnold
This is the first of a series of posts on centrifugation that will eventually be a primer.
What is a centrifuge and why would we want one?
A centrifuge is a device that spins at high speed to separate products into parts based on density. The faster you spin it, the more separation power you get. This separation power is usually described in â€œgâ€™sâ€ (also called RCF)—the number of times the force of gravity the product is feeling.
In the kitchen we could use one to clarify juice by spinning the hell out of the juice and making all the cloudy parts slam into the bottom of the container forming whatâ€™s called a â€œpellet.â€ We could also take an emulsion, spin it, and demulsify it to remove the fat, etc.
Not all centrifuges are created equal. It isnâ€™t useful to have a centrifuge that canâ€™t develop the g forces needed to complete your task. A crappy knife will still cut, just poorly. A centrifuge that canâ€™t produce adequate g forces is useless—like trying to demolish the Great Wall of China with a whiffle-ball bat.
Centrifuges are expensive (duh). The bigger the centrifuge, the more product it can handle, which means it will cost more. Small centrifuges, on the other hand, only handle small amounts of product and are, therefore, useless. Speed also factors into cost—faster spinning centrifuges can separate a wider array of products, meaning that you will need to pay for that benefit as well.
Centrifuges are DANGEROUS. The spinny part of the centrifuge is called a rotor. Rotors have a finite lifetime. They undergo tremendous forces. Being made of metal, they can suffer from fatigue over time, causing the rotor to suddenly fly apart. The slower the rotor spins, the more inherently safe it is. Rotor failure at very high speeds can be deadly. Check out these photos: 1) Purdue; 2) MIT; 3) Cornell
Donâ€™t use an old rotor unless you know its history and it is within specifications. Also, using a centrifuge when you donâ€™t know what you are doing is REALLY BAD. You thought your unbalanced washing machine rattled and made noise…
The FCI and Centrifugation
When we first met up with Prof Kent Kirshenbaum at the Experimental Cuisine Collective, he asked us what we wanted to play with in his lab. The top two choices were his freeze-dryer (which we still havenâ€™t played with) and his centrifuge. We chose the centrifuge. Nils and I headed over to the lab with Johnny Iuzzini, who was also hanging out at the ECC, to check it out.
We wanted to use Kentâ€™s centrifuge to determine:
- Is centrifugation useful.
- What type of centrifuge would we need.
The test we decided was the clarification of lime juice. Because:
- Lime juice is delicious.
- Lime juice is difficult to clarify because it has a low pH, canâ€™t be heated, and doesnâ€™t store well at all, i.e. has to be used same day.
- At the time we were working on clear lime juice as part of our Rotovap work.
We were hoping that:
- A small, low-speed centrifuge would work.
- We could get one cheap.
Well, thatâ€™s not how it turned out. Kentâ€™s ‘fuge (we call them ‘fuges in the biz, pronounced like huge) is a superspeed type. Superspeeds are a step below an ultra-centrifuge, the butt-kickers of the centrifuge world. There was no point in trying an ultra-centrifuge because it would never, ever be practical in a kitchen. The rotors in an ultra undergo so much force that you have to keep track of how many hours they spin and throw them away after the hours are used up. Kentâ€™s superspeed spins a little over 20,000 rpm. He has a couple of rotors. The two we used are the GS-3, which holds 3 liters and can spin at 9000 rpm, producing about 13,800 gâ€™s, and the SS-34, which only holds 400 ml, but can spin at 20,000 rpm producing 48,000 gâ€™s.
The Bad News
Obviously, we were hoping that the slow, large capacity rotor would do the trick. If low-g forces worked, we could get smaller, cheaper, fuges to use in the kitchen (Kentâ€™s rotors ALONE cost $5k apiece). Nope. Turns out that it takes 27,000 gâ€™s before lime juice starts to clarify and that lime juice tastes TERRIBLE and metallic. If you up the centrifuge to 48,000 gâ€™s the lime juice is crystal-clear and delicious. This is an extremely interesting finding but also very depressing, because it meant that only an expensive, large centrifuge would do what we want.
The Good News
Using enzyme pre-treatments, we are able clarify some juices at considerably lower speeds (but not lime yet).
The Stupid News
After working in Kentâ€™s lab I decided to do an eBay search for superspeed centrifuges and ended up purchasing a Sorvall SS-1, the very first superspeed fuge they made. Itâ€™s from the late 1950â€™s. Let me just say this: BAD IDEA. We call it the dangerfuge. It has no protective casing at all â€“ just a 50 year old aluminum rotor spinning like a bat-out-of-hell in the middle of the room. It doesnâ€™t even have a switch, just a plug. It is the scariest piece of equipment I have ever fired up (and that’s saying A LOT) and I never intend on firing it up again. Sure, it seems fine. The whine, spin and wind coming off it are even strangely compelling; but at any moment, the rotor could fly apart sending shrapnel everywhere. As Kent says, â€œwhat youâ€™re paying for in a centrifuge is the protective armored casing that stops the rotor from going through YOU when it fails.â€ True.
Thatâ€™s where our centrifuge program stood until Unilever gave us a callâ€¦.