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When making our seasonal beer we ended up getting a pre-boil gravity sample that was way higher than expected, so we added water to dilute the mash. A new sample was taken and the first one must have been polluted because now we ended up with a to low gravity.

This lead us to a three hour boil to get the sought after OG. What can we expect from the fermentation? Is there any risk that the sugars we got are un-fermentable?

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Boiling would caramelise the sugars to some degree and darken the solution. So for a Christmas beer the taste might even improve. Caramel does not necessarily count as fermentable sugar and so the useful sugar content of the wort may be lower than desired and thus the final ABV may be lower without some adjustment of the wort. I presume the boiling for some hours did not include boiling with the hops....

However if the wort has a suitable OG then I would ferment it anyway and see what happens. At worst it will be a caramel flavoured weaker festive beer - which may be not a bad thing!

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    This goes along the same lines as our reasoning as well, cheers! And for the record, the hops where added within the last half an hour of the boil. – dotmartin Nov 23 '16 at 9:45
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    The final sugar content hasn't changed really, but the ratio of fermentable to non-fermentable has changed, which may lower ABV slightly. And to be technically accurate caramelization doesn't happen in wort. Maillard reactions are what are causing the browning/darkening and they would have the same effect on the ratio however. – brewchez Nov 23 '16 at 11:21
  • "And to be technically accurate caramelization doesn't happen in wort." Do we have any evidence or source for such a comment? Fructose will begin to caramelise at 110 degrees C and a boiling wort solution will approach this temperature as the water boil's off. Fructose is usually present in most worts,so I would suppose that some caramelisation will happen with prolonged boiling of the wort. I agree that Maillard reactions will also occur and produce some colouring of the solution but I doubt this is the only reason the wort would become darker. It would be interesting to boil wort under N2. – barking.pete Nov 24 '16 at 11:24
  • Caramelization requires temps in excess of 360F. You cannot reach that in a kettle of liquid. – Denny Conn Nov 24 '16 at 16:47
  • Others do mention caramelisation in boiling worts. I cannot comment on their findings but have some sympathy with their observations, eg: homebrew.stackexchange.com/questions/418 Fructose will caramelise at 110C in a slightly acidic (eg pH5.5) aqueos solution. A boiling malt solution can be hotter than 103C so some caramelisation should happen at that temperature.. Sucrose, maltose and glucose (etc) caramelise at the higher temperature quoted in the comment above so would play a lesser part - but even then some caramelisation should occur. – barking.pete Nov 29 '16 at 9:22

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