Anyone have definitive info if diastatic power is reduced with grain age? If so, any data to estimate current DP?

Brewing with some pretty old 2-row today. It has been stored properly for 1.5 years, temperature never got above 80°F, it never got damp, and the grain still smells and tastes fresh; it has been stored in air tight container.

I'm having beta-amylase conversion issues. Time seems to be letting it work though it, but it's been mashing for 3 hours now and the iodine test is still slightly dark. In the past with this same recipe I've had clear tests in 30 minutes.

24lb 2-row,

2lb flaked maize in the mash,

0.31gal water/grist ratio

Dough in had a rough start, forgot to read grain temp before strike water calc, estimated 80°F but, must have been closer to 70°F. 142 hit, 148 was target. Fired up RIS to bring up, applied some mash run flame while stirring. Brought it up to 148 and held for 1.5h, stepping up to 158 slowly now. To see if alpha-amylase is pooched too.

I guess it's possible some amylase denatured from direct flame, but I can't see it reducing the DP below 30 for this grain bill, I was really stirring well.

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    Update: I was able to get alpha-amylase to complete the conversion. 3 hour mash. Hit 90% efficiency, pre boil was 1.063, post 1.076 added 1.25g water to drop to 1.067 still high for my 1.062 target. Oh well we'll see what a 7.5abv cream ale is like. Also .5g short of 12g target. Commented Aug 27, 2017 at 23:28
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    I also used old 2-row lately (also 1+ year old) and my conversion was done in 1.5 hours, I added a very small amount of fresh smoked grain, so perhaps it helped? If it took 3h instead of 1.5h, we can estimate a loss of 50% DP in 1.5 years.
    – Philippe
    Commented Aug 28, 2017 at 13:58
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    @Philippe specialty grains generally have no DP. I brewed with this same 2-row 50/50 with similar age maris otter with no issues, conversion was complete in 45 minutes, on an alpha-mash. I'm thinking beta-amylase is more pron to an age issue. Or it may be possible the grain hit beta denature temps in a cargo container or something before it came into my care for storage. Also this was a really fine grain crush, it's possible it was too compacted to convert, though I did stir a lot. Commented Aug 28, 2017 at 14:17
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    If it helps, Briess data sheets state Whole kernel diastatic and preground malts are best when used within 6 months from date of manufacture. Whole kernel roasted malts may begin experiencing a slight flavor loss after 18 months.
    – Wyrmwood
    Commented Sep 1, 2017 at 19:23

2 Answers 2


The only definitive information I could find specific to your question was in the book Malts and Malting:

'[Malt] must be stored cool and dry in sealed stores [...] to arrest the decline in enzyme levels'

One brief and somewhat vague sentence in 750 pages may give you an idea of how little professional maltsters and brewers seem to concern themselves with loss of enzymes/diastatic power throughout the course of proper storage.

Here is another study published in the Journal of the Institute of Brewing which investigates changes in malts over a period of 12 months at different temperatures and humidities. In it, the conclusion is drawn that:

'The sugar content in the sweet wort was constant throughout malt storage, showing that the major components in the malt and the starch-degrading enzymatic activity were intact.'

However, from a theoretical standpoint enzymatic activity (and therefore DP) absolutely does decline with time. Thanks to entropy, we can be relatively assured that enzymes will have a tendency towards denaturing (a less ordered state) and not spontaneously "re"nature themselves, resulting in a net decline in activity. Luckily, when stored cool and dry this will be an incredibly gradual process, on the order of years, maybe even decades. But we also know we have to deal with two factors which accelerate enzymatic loss: elevated temperature and elevated moisture.

Let's think about this a bit more, as it applies to your malt-in-question. The study I linked above looks at the effects of malt storage at either 10 or 20 degrees C (50 or 68 degrees F) at a different relative humidities which equate to roughly 8% and 12% malt moisture, showing no significant loss of enzymes. You say maybe your grain hit 80 degrees F. Could this account for a significant loss of DP? That depends on how long it sat there, but it certainly will be worse off than if it hadn't gotten that warm.

You also say it was stored air-tight but, over the course of a year and a half and with (presumably) multiple openings, your malt could easily have achieved (and almost certainly did) a much higher moisture content than it started with. For example, given my local climate, assuming a year-round average relative humidity of 65% (for simplicity) I can safely guess that malt stored for a significant length of time here might eventually reach ~12-14% moisture, as it comes into equilibrium with the environment. For reference, you'd expect a fresh and properly stored base malt to have a moisture content of ~4-6%.

It's well established that enzyme destruction is much more significant at higher moisture contents, given a certain temperature, so I think it's safe to say that with the conditions you describe there was certainly the possibility for significant loss of diastatic power. Also, as you point out, beta amylase is a more sensitive enzyme and will be prone to a greater degree of destruction in a given set of conditions. This seems to be born out in the specific issues you are having with the malt. Regarding loss of enzymes to direct-fire heat, it stands to reason that a malt which is already enzymatically 'compromised' will suffer a more drastic loss of DP than a more sound, fresh malt.

Basically, it's easy to say that, theoretically, yes DP should reduce with time (albeit very slowly), but is incredibly difficult to say to what degree it is lost in any given situation, for any given malt, even if you know as much as you can about the storage conditions and the state of the grain. I think the only fool-proof way to answer that question would be to perform an actual test of the diastatic power on the grain in question.

  • Thank you. It seems there is evidence of the decline of DP but it hasn't been measured and documented to establish predicted declines or even an ideal storage environment to halt all decline. In sure the data is out there, the world seed bank probably depends on it. Commented Sep 4, 2017 at 22:34
  • Also I do believe moisture played a roll in the decline I saw, I do use evaporative cooling in my home in the summer, that's why I use air tight chemical drums. But yes they do get exposed from time to time. Commented Sep 4, 2017 at 22:36

Best I can do this evening is this, but I will dig further: http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2005.tb00646.x/full

Here is PDF link: http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2005.tb00646.x/pdf

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    This paper deals with the storage of barley prior to malting and the effects that storage has on the enzymes developed during malting. Unfortunately it doesn't touch on the fate of those enzymes during the post-malting storage period, which, if I read it correctly, is what the question is concerned with. Still a good read, though. Commented Sep 4, 2017 at 22:36
  • I know :( I will not investigate further as you seem to have done a cracking job of answering this one.
    – Mr_road
    Commented Sep 7, 2017 at 20:39

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