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I cooked up my first Double IPA recipe over the weekend but have seriously missed the mark with the target gravity. So much so it most definitely won't be a Double.

Target O.G. = 1.085
Actual O.G. = 1.050

Malt bill

The malt bill was as follows:

75%    Maris Otter  
12.5%  Malted flaked oats  
10%    CaraHell  
2.5%   CaraPils

A volume of 30L gave me a rough required extract for 1.085 gravity of 6.66kg. After adjusting the weights for their efficiencies/max yield I came out with:

                               Weight      Max Yield    Actual weight
Maris Otter           75%      4.995kg     82.5%        6.055kg
Malted flaked oats    12.5%    0.833kg     70.0%        1.190kg
CaraHell              10%      0.666kg     74.0%        0.900kg
CaraPils              2.5%     0.167kg     71.4%        0.234kg

This gave me a total weight of 8.401kg. My mash and sparge volumes were:

Mash = 26L
Sparge = 16L

Mash

The mash started off a little too hot and so I ended up doing a 150 minute mash. The readings went as so:

T = 0    temp = 72.7ºC
T = 20   temp = 71.6ºC
T = 40   temp = 70.1ºC
T = 60   temp = 68.0ºC
T = 80   temp = 66.5ºC
T = 100  temp = 65.8ºC
T = 120  temp = 65.4ºC
T = 150  temp = 64.9ºC

After transferring the wort to the boiler, where it sat on at a low setting for a couple of minutes while I added the sparge water to the mash tun, I had a wort temperature of 68.4ºC and a gravity reading of 1.034. Adjusted for temperature using brewers friend it came out with a reading of 1.051. I was a bit despondent at seeing this.

My sparge after finishing had a reading of 1.017 and a temperature of 73.1ºC. Brewers friend only goes to 71ºC but it gives a rough gravity of 1.035.

There were 400g of hops in the boil/flame out. After boiling and cooling the wort I topped it up with about 3.5L of the leftover sparge water and a further 2.5L of water to top it up to 30L (I also used some of that sparge water to top up the boil). My end gravity was 1.050.

Brewhouse efficiency
Writing this out I realise I did not adjust my recipe for brewhouse efficiency. However, assuming I did nothing wrong, that gives me an appalling efficiency of 58.8%. That is so bad I think there was something more to it. Had the gravity been in the region of 1.064 I could safely write this off as a failure to accommodate for the brewhouse efficiency.

Question
So while I aimed for 1.085, after adjusting for a rough brewhouse efficiency of 75% I should have hit 1.064. I actually hit 1.050.
- Why was I so far off the mark?
- Are there any tips for consistently hitting high target gravities?

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Mostly the high temperature. You started out at 73° C, which is already at the high side for the alfa-amylase enzymes, which convert the starches into complex sugars. While at this moment, the beta-amylase enzymes, which convert the complex sugars into simple sugars, are not yet denatured, two things work against them:

  • The high temperature, which will denature them very fast
  • The fact that they work better between 62°C and 64° C

So, in the hour between start and finish, you are still at 68° C. This means that at this moment, the beta-amylase enzymes are pretty well all destroyed. At the same time, the denaturation of the alfa-amylase enzymes has probably stopped, but now you have entered a temperature range where they do not work well.

Mashing this long time has probably made the alfa-amylase enzymes do enough work to have at least some modicum of gravity.

If you had waited from topping up your wort after the boil, you would have had wort with an OG of 1.063. Before adding water after the boil, first measure your gravity. Then you could still have added some sugar to increase your OG (I calculated that you could have added 1.5 kg of sugar, which would be about 15% of the total fermentable bill. Since it is a Double IPA, that should not have been a problem).

  • Exactly. Starting at the temperature at the lower end, and increasing on halfway point, would have worked better. – Philippe Sep 17 '18 at 14:35
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    This is a great answer. If this happens again, dump water, ice, whatever, to get that mash temperature down as soon as possible. It's a common practice to start with a low temperature and bring it up. – Kingsley Sep 18 '18 at 0:42
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    Agreed: high temp denatured too much enzyme. In the future, if this happens. Once cooled you can always add a couple pounds of 6-row or crush some Beano (basically beta-amylase) – Evil Zymurgist Sep 19 '18 at 14:04
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More bad news: If you didn't like the OG, you'll probably like the FG even less. This is because higher mash temps produce more complex (and less fermentable) sugars. You want to be making disaccharides like the maltose which will be converted to alcohol, but you probably made a lot of trisaccharides and unfermentable dextrins like maltotriose, etc.

If you overshoot your mash temps again, throw some cold water in immediately. It's way better to have a mash that is too thin than a mash that is too hot.

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