Incomplete Conversion

[mrcej23]

If there is an incomplete starch-to-sugar conversion in the mash, what is the impact, if any, to the finished beer other than less alcohol?

I read somewhere that starch is a breeding ground for bacteria, but if my wort is around 1.07 isn’t that enough to kill off any bacteria anyway?

Incase any of you are wondering, I put a 17 lb grain bill in my unibrau v.3 with 7.5 gal of water and the mash was so thick I could barely stir it, which I think contributed to an incomplete conversion. I’m not sure if this is a limitation of the machine or if I need to learn how to brew big beers effectively on it. It’s supposedly able to take up to 20 lb of grain. This is my second brew on it and first high gravity one. It was supposed to come out at 1.086 and instead was 1.07.

 

[bracconiere]

It was supposed to come out at 1.086 and instead was 1.07.

usually do get less efficiency with bigger brews.....i assume because you'd have to sparge more and boil off more....

did you do a iodine test, to make sure it was incomplete conversion? and not just a bad sparge?

 

[bucketnative]

Missing your target gravity is not necessarily a sign of incomplete conversion. There are several factors that can affect the gravity after the mash.

Also, the specific gravity of your wort really has nothing to do with preventing bacteria, except for the fact that it is an indicator of the potential alcohol in your finished product, which could inhibit bacteria.

 

[Gnomebrewer]

To answer the first part of your question, incomplete conversion (starch in finished beer) can lead to permanent haze and poor shelf stability. It can also be a source of food for bacteria/wild yeast (but, as with any beer, good sanitation should prevent it).

As others have said though, your issue is more likely poor extraction efficiency. This may be (partly) due to incomplete conversion in the centre of the 'bits' of grain, but is not likely to result in starch in the finished beer. Your mash should (generally) be the same thickness regardless of the intended OG, but have less sparge water with a higher OG (a mash 'so thick you can barely stir it' is not ideal). Less sparge = lower efficiency (all else being equal), so bigger beers have poorer efficiency (the liquid left with the grain has more sugar still in it). You can get around it to some degree by boiling for longer (more boiloff = more sparge needed). I prefer the lazy method of bumping up with extract or syrups (but rarely brew big beers).