To some,  decoction mashing is superfluous, outdated, eccentric and provides no real benefits… Well, to some.  I would argue that for some styles it’s unnecessary, but for others it’s not.  For Bavarian Helles it’s unavoidable, whereas for British Ales it’s not necessarily the right choice (although I do make a very yummy Old English Ale this way… my D.O.A – Didsbury Old Ale).

Arguably, it is a bit of a strange way of doing things when one learned infusion mashing first. Why do things keep to continue to work and convert although the grain has been boiled previously?

The reason is simple. At temperatures above 80C, more starch is released as the cell membranes are broken down.  Apart from releasing unwanted tannins, this is also one of the reasons that one should not mash-out and sparge at temperatures higher than that.

It’s quite easy to see for oneself. A simple iodine test of the decoction before the boil and after saccharification is complete, shows that no more starches are present.  This is show by this brown tint.
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However, after the 10 or 15 minute boil (I do that at 95-96C – its unnecessary to go to 100C).  The iodine test shows a deep blue colour, almost like ink.  This proves that fresh starches are released and are now ready to be converted.

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This simply cannot be replaced by other methods.  More starch is available, and this leads to a higher mash efficiency (up to 10%) and a brighter, lighter in colour,  and stronger beer.

For me this is a very important consideration when reading or designing a decoction mash schedule.  After every boil fresh starches are available, and these need to be converted.  It’s like mashing two or three times over and over (for a double or triple decoction mash).  That’s why they take so long.  A brew-day from end-to-end lasts a whooping 12 hours!  That from milling the grain to the wort pitch-ready in the fermenter.  Many breweries split this over two days and I vividly remember the smell of the hop boiling during lunch hours when going to school in Munich as a child…

I hope this sheds some light on why decoctions are so important for certain styles.   It allows for much maltier beers such as those found in Bavaria:  the Helles.  Without a decoction mash they would turn out way darker with a lower attenuation — which is not desirable.  I found that the 10% improvement in mash efficiency contributes to colour about 2EBC brighter.

It’s also important to appreciate that the sugars created by the α-amylase at temperatures above 70C in the preceding decoction, are able to be further broken down by the β-amylase at temperatures lower than 70C after the decoction is returned to the mash (“Aufmaischen” – this time German is more succinct and shorter… a rare occasion).  Again, this benefits the final attenuation and contributes to a higher mash efficiency.

This means my mash efficiency is at 72% and my overall brewhouse efficiency comes at a whooping 88% using the schedule below.

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