Tuesday, January 1, 2008

When will it be cooked?

This posting is a bit technical, but it is all to do with predicting when something will be cooked/ready to serve. Let's assume that we want to eat at 8pm, and the dish is some giant thing like a turkey which is to be roasted. It's going to need to rest for about 30 minutes for the juices to redistribute and while you do the other things that have to get done. That means you want it leaving the oven at 7:20 - to allow for time to carve it. Now the big question, how do you know whether it will be done at 7:20?
I use a "rate of change" based method. I use a probe thermometer, and look at what rate the temperature is rising. Because the probe is deep inside the roast, the temperature will rise very slowly at first. So for the first hour of cooking, you may only see a 5-10 degree increase in temperature. However, now imagine it is 6pm - you want to take the thing out of the oven in 1 hour and 20 minutes, but you notice that its current temperature is 140 and it has risen 30 degrees in the last hour. A quick, back of the envelope calculation suggests that it will be ready in about 20 more minutes - a whole hour sooner than you would like. So, what to do? Turn the heat down - in this case quite a lot! So if you were cooking it at 350, back the heat off to 275 and pay attention. After 30 minutes longer, see what has happened. If it has reached its target temperature, take it out and let it rest longer. If not, see how much it has moved. If it has risen another 15 degrees or so, you are well on track.
So the thinking process is, don't just look at the instantaneous temperature, look at how fast the temperature has been rising - remembering that the temperature rises faster the nearer the thing is to being cooked. If it looks like the temperature is rising too fast, turn the heat down some. If it is rising too slowly, turn it up.
It is better to use a permanently inserted probe than one of the pop-ups. A pop-up will tell you that something is done. What you need is to be able to predict is when it will be.

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