Dear Mr. Wizard:
I have been brewing for about seven years. I started with kits, which gave me the confidence to move on to the all-grain brewing process. So far, all of my batches have been drinkable with no major mishaps.
Here's something I have been pondering: Would it be possible to mix your grains with cold water, then put them in the oven at 150 degrees and leave it overnight? A friend of mine did this and it didn't convert. Do you have an idea why not? Also, is it possible to get too much tannin out of the husk with this method? If it goes from 100 degrees to 150 degrees in a period of say, eight hours, wouldn't all the temperature breaks be covered, to make a complete mash?
Mr. Wizard replies:
The oven-mash method can be effective. A very slow increase from 100° to 150° F will give all mash enzymes an opportunity to convert the starches in malted barley into fermentable sugars. But the problem with such a long, slow temperature increase is that the wort may turn out to be too fermentable and will result in a very dry beer. This is how some commercial brewers produce light and dry beers. For example, Anheuser-Busch uses a very long rest (about 3 hours) at around 140° F to make Bud Light. This is the optimal temperature for beta-amylase activity. Extending this rest, coupled with a slow increase to 150° to 158° F for conversion, results in a highly fermentable wort.
I would set my oven at about 162° F to get a slightly higher conversion temperature of about 156° to 158° F. Setting the temperature a bit higher than your target will assure the mash makes it up to temperature. Since the rate of temperature change decreases as the air temperature and mash temperature get closer, it could take a long time to hit your target temperature. Remember that thermometers and thermostats require calibration. My guess is that your friend's oven is out of calibration. This is a common problem with household ovens. If you want to give the oven-mash method a try, you should begin by calibrating your oven with a calibrated thermometer.
The easiest way to calibrate a bi-metallic thermometer (the most common type of household food thermometer) is to make an ice bath. The temperature of an equilibrated mixture of ice and water resulting from the ice melting is 32° F. If the thermometer does not read 32° F, you can calibrate by using its calibration screw or turning the dial. After checking it on the low end, boil some water and check the boiling point. The boiling point of water should be around 212° F. If it's not exactly 212° F, don't worry because boiling point is affected by altitude and atmospheric pressure. Once you know your thermometer is accurate, check the oven dial thermometer against it.
I have never gotten around to trying this method myself. However, I have used a variation on the theme. When I first started homebrewing, I used to mash in a big kitchen pot and would stick the pot in a pre-heated oven to prevent the grains from cooling off during the mash.Your method would be handy if you were staying up late on Friday tasting some good brews. You could mix up your mash, toss it in the oven and get up the next day and finish brewing by noon.
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