Mash efficiency and aged Malts

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JerseyBrewer

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So I'm an all grain brewer - I've been brewing on the same system essentially the same way for 3 years (probably close ~20 batches) and have dialed in my efficiency pretty well. Although, I've never been too happy with the efficiency of the system (~61%), I compensate with more grain and it's all good. However, today I hit 43%! (calc at the pre-boil value).

As I said, nothing has changed - same water profile, same equipment, same mill size (mine), same volume. So, I looked back at this year's brews (only 5 this year :-( ) and I noticed a startling trend: The efficiency has steadily dropped. 61%, 55% and now 43%.

I had a friend over today and he was asking about the grain and it dawned on me: The maris otter that I was using (62% of the malt bill) was from 4/2010... Is it possible that the older grain has lost some of it's enzymatic power? Would a conversion test indicate this problem - (I did a conversion test and it didn't show any black so I assumed it was ok).

The Maris Otter from 4/2010 was part of a pallet order - I only have about 10 lbs of pilsen malt left, but a crap load of crystal malts and munich. Is that ruined? Assuming it is the problem, could I mix it with some new to restore the needed enzymes?

Any help is very appreciated.
 
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