Hard-boiled eggs

Harold McGee’s fascinating On Food and Cooking suggests, with a full scientific justification, that hard-boiled eggs are better cooked for a longer period at a lower temperature than I was taught by my Mum. He recommends a temperature of just 85ºC for 25 to 35 minutes rather than a full boil for ten minutes. The argument is that you get enough heat to the yolk to cook it without making the white too hard.

I couldn’t resist trying this. I put two large free-range eggs, straight from the fridge and well ahead of their best-before date, in a medium-sized saucepan of water at 85ºC (using one of two laboratory thermometers which have survived from my Dad’s days as an industrial chemist - he retired in 1970). By juggling the gas control and checking every three or four minutes I managed to keep the temperature stable to within a degree. (For anyone without a kitchen full of laboratory equipment, at 85ºC the water was still with tiny bubbles forming on the bottom of the pan and only two or three reaching the surface at any one moment. I wouldn’t even call this ’simmering’.)

I removed the first egg after 25 minutes and immersed it in running cold water. After ten minutes I did the same with the second egg, then rescued the first and peeled it.

The first thing I noticed was that the membrane between the shell and the white seemed much tougher than usual. Once I’d managed to tear the membrane (no mean feat), I was able to remove the shell very easily and cleanly.

The next was that the hydrogen sulphide smell so typical of hard-boiled eggs was much less powerful than usual. I had always assumed (presumably because of its yellow colour) that the gas came from the yolk, but in fact it comes from one of the proteins in the white and is released by heating it. Presumably keeping the cooking temperature low reduces this effect. Slicing the egg with a very sharp knife, I found the white ususually tender, as McGee had led me to expect. The yolk was fully cooked, and there was no sign of the dark greenish-black surface on it that one often sees. McGee says that this layer is ferrous sulphide, a harmless compound formed by the hydrogen sulphide reacting with iron on the surface of the yolk. Less H₂S - less FeS, it seems.

The egg was very pleasant to eat - quite the nicest hard-boiled egg I had ever cooked, with a white that was indeed much more tender than I’m used to. The only surviving problem was this: how do you get the white and the yolk to stick together when you slice the egg? My yolk was, if anything, a touch overcooked.

The 35-minute egg, also cooled for exactly ten minutes for consistency’s sake, was more of a problem. The membrane had stuck to both the shell and the white, so large pieces of white came away when I peeled the egg. The yolk was, inevitably, even more overcooked.

I wouldn’t dream of claiming that this tiny and unrepresentative selection of eggs has led to conclusive results, but the next egg I try will be cooked at 85ºC for 20 minutes.

Much later, we ate the famous ?Duck Egg 62ºC? (a TV star in its own right) at Sat Bains?s wonderful restaurant in Nottingham. This was, as you?d assume, cooked 23 degrees cooler than Harold?s hard-boiled job - but for two hours! The white was wonderfully tender and the yolk, astoundingly, was still runny, though thick and unctuous. It?s all about the temperatures at which different proteins coagulate, apparently. Maybe the hard-boiled egg could be cooked at a temperature lower than 85ºC...