Cooking with magnets - induction hobs

You can read the story of how I got interested in the use of induction hobs on the sous-vide cookery page. This page follows my tests of this remarkable technology to the conclusion that it has nothing to offer for sous vide and goes on to explain what it is good for.

It’s clever stuff, this. Instead of generating heat in the form of a flame, a red-hot electric element or microwave radiation, an induction hob uses a powerful electromagnet to generate heat directly in the bottom of the cooking pan by inducing (hence ’induction’) electric currents to flow in the metal - thankfully not the kind that might electrocute you if you touch the pan! This has to be made of a magnetic metal, so aluminium is out, and I was a bit concerned that our Bourgeat pans from Nisbets, which are stainless steel but have an aluminium sandwich in the bottom to spread the heat more evently, might not work. No worries: Nisbets actually specify them in the bits about induction hobs.

When I got Johnny’s spare hob home, I discovered that it was a Kenwood one made for the domestic market, and that - unlike the commercial ones - it had variable power but no temperature control. I immediately plugged it in, stuck a pan of water on top, switched it on to the lowest power setting and waited. Not for long: while the top of the hob stayed stone cold, the water quickly began to steam. Out came the trusty digital thermometer. The temperature stabilised at about 70°C. Disappointing as sous-vide can require temperatures as low as 43°C for fish. Still, I figured that, at constant power, the more water I had in the pan the lower the temperature would be.

On went a bigger pan with a lot more water - but I still couldn’t get it to settle much below 70°C. Thwarted, I consoled myself with using the hob to make stock from the chicken carcasses I’d bought from Johnny, and which we’d roasted and picked at with such enjoyment at dinner time.

Patricia loathes the smell of chicken stock cooking, which really isn’t very pleasant. So out went the stockpot and Johnny’s hob to the garage, where the stock could simmer away all day without offending anybody.

Next stop: Ebay

So clearly I needed a hob with temperature control as well as power control. These work using some sort of sensor to measure the temperature of the pan-bottom.With a week to wait - even I wasn’t going to trek all the way to Nottingham just for the hob - I did some research. Our Nisbets’ catering catalogue had three hobs: one at £599.99, one at £639.99 and a Buffalo one at a more reasonable £159.99. I learned from the specifications that these hobs have both power and temperature controls.

I was loath to part with £160 (let alone £600) on spec, so as usual I resorted to Ebay, where I found just one seller offering professional-quality hobs. I decided the middle one would suit me best - the cheapest (in terms of starting bid) was underpowered and the most expensive seemed identical to the middle one except that it was even beefier to suit a busy commercial kitchen. Here’s the picture off Ebay of the one I decided to go for:

This is a 2.7kW unit with both power and temperature controls, offering a range from 60 to 240°C, in a robust stainless steel case (the Kenwood is plastic). It was offered with a starting bid of £65 (about what the Kenwood costs new) and a retail price of £360 was mentioned. The beefier one with the same specification quoted a retail price of £580, which chimes pretty well with what I found at Nisbets.

I had to be poised to bid at 8:30am on Sunday morning, but I was awake long before then (that’s the Ebay bug for you!). There was one bid of £65, so I entered a maximum of £85 and watched the clock. At 15 seconds before the end of the auction I hit the button and moments later the hob was mine for £65 (no - I don’t know why, either!). Thank goodness there weren’t any of the daft early bidders who haunt auctions for more popular consumer items, jacking the prices up for a couple of days before the auctions end.

On the morning of the 19 March 2008 I got an email from the people I bought the hob from - ChefKing (UK) Ltd - saying that there was VAT to add to the price I’d paid. That’s a first on Ebay. I phoned and paid the VAT with a credit card, making it clear that I wasn’t impressed and that this would be reflected in my feedback. I couldn’t afford to tell them to take a running jump as this is a rare bargain. To be fair, I got an email shortly afterwards to say the item had been dispatched and will be here the following day.

So, between barrowing loads of rubble from the terrace to the skip out front (we’re having a conservatory built), I did some research on sous-vide cooking with induction hobs. After a few false starts I managed a search string for Google that got me lots of chefs? forums, loads of relevant stuff and hardly anysites advertising equipment:

"sous-vide cooking" +induction

If you type it in just like that, with the quotation-marks and searching the whole web rather than just UK sites, you’ll find masses of useful stuff. I even found a blog by a female chef who has cooked sous-vide on an induction hob at Ferran Adria’s legendary El Bulli restaurant. Authoritative source - or what?

The most useful was a monster web-page from an American scientist, Douglas Baldwin, that explains all this, together with real recipes.

I was very disappointed with my purchase. First, it only allows temperature-control from 60 to 240°C in 20-degree steps - not exactly a precision tool for sous-vide where, if you believe the chefs, one degree can make all the difference. If you’re a Blumenthal, a Bains, a Keller or an Adria, I’m sure this is true. For the rest of us mortals it may not be quite that bad - but 20 degrees? Also, I noticed that the hob heats one side of the pan far better than the other, which can’t be right. Maybe that’s why it was £65 instead of £360. I spoke to the vendors, who turned out to be a large commercial operation with a full service department, who were sceptical but said I could send the unit back for checking if I wished. As things turned out, I didn?t bother.

I used the hob in the garage to cook and reduce some chicken stock and the uneven heating didn’t seem too bad. I was impressed that, with the temperature set at 100°C, it maintained a modest boil with hardly any change as the thermostat cut in and out. I also experimented with the 1-15 heat setting and found that at the lowest setting a medium-sized pan held quite steady at around 55°C. So, if the temperature control wasn’t fine enough, I thought the way forward could be to try different quantities of water at different power settings.

The three variables would be the hob’s power setting, the quantity of water (keeping the same pan) and the room temperature. Our kitchen has a radiator with a thermostatic valve, so the last should be quite steady. Assuming that it is, then for a given power setting it should be possible to manage the temperature by varying the quantity of water, Obviously a large quantity of water should also provide greater stability, so I guessed that one of our big Bourgeat pans should be used with plenty of water. Tests would then find the temperature at which this stabilises for each of the 15 power settings. If this didn’t find enough temperatures, adding or removing water should provide a means of fine-tuning the system.

The first trial wasn’t too encouraging. With 5 litres of water in the smaller stewpan, power setting 1 settled at around 47°C but 2 got to around 70°C. And things got worse: I tipped the water into our bigger stockpot, from the same Bourgeat range, and got an error from the hob. Even without water in the pan the error persisted, and I discovered that, while the bottoms of the rest of the pans have a core of aluminium sandwiched between two layers of stainless steel, the biggest one has the aluminium as the contact surface. The hob just won’t deal with this - somehow it ’knows’ that there isn’t a magnetic layer in contact with it. So I needed a layer of iron or steel to stand the pan on.

Meanwhile, I managed to get another litre of water into the smaller pan, but this didn’t help. I decided to try standing the bigger pan in our large black-iron paella pan. This obviously convinced the hob’s computer that it was magnetic, but I found that the paella pan got very hot without transmitting much heat to the stockpot. This was because the bottom isn?t flat. I tipped half a litre of warm water into the iron pan: it boiled vigorously but soon settled down. Maybe this would act as an additional buffer...

Or not! There was still a huge gap between temperatures with the power set at 1 and 2.

I’m just about out of ideas here - thinking that the hob will be useful for doing smelly cooking outdoors or in the garage, but no use at all for sous-vide. Other options are to try to maintain temperature on the gas hob, which I did for chickens last summer with reasonable success (above), or in the oven. Luckily the temperature range I’m after is low enough to leave the digital thermometer probe in the pot even in the oven for monitoring.

I’m disappointed, though. If the induction hob had continuously-variable temperature control instead of 20°C steps, I’m sure I could crack it. I’m almost tempted to take it to bits and investigate the electronics!

Anyway, for now the induction hob will be used for smelly cooking outdoors or in the garage, and the sous-vide experiments will go in different directions.