Weird Under Carbonation Issue

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Been bulk priming for years and I've never had this before..
However 2 weeks ago i bulk primed an ESB.
Using 1768PC which I know from experience likes to start and stop.
Bulk primed with 110g table sugar to give me around 2.2vols carb according to beersmith.
Usual method - boiled a cup of water with the sugar. Put the sugary water in the bottom of clean fermenter.
Siphon the cold conditioned beer into that sugary mixture.

Two weeks on and open a bottle slight psst sound - very few bubbles and almost no head.
Anyway never done this before but I had 27 and a half bottle worth.
Usually I have just poured that last bit out. This time I filled a half bottle and capped it.
The other night I opened that bottle and the cap blew off and hit the ceiling no sooner than I touched the lid with the bottle opener.

Thought today I might get one of the other bottle down and add half a teaspoon of sugar to it.
You know maybe I didn't mix it well enough or maybe I didn't tare the scales..
Opened the bottle (Normal half assed psst) put the half teaspoon in. (Also never tried this before)
As soon as the sugar touched the surface the bottle started frothing like mad..
Anyone have any ideas first of all why that happened? I need to understand. Perhaps that's completely normal I don't know.

Also what should I do with said under carbed beer. Is adding an extra half teaspoon of sugar a bad idea?

For the current batch - just wait longer. It has some carb now, your priming rate seems adequate, time will sort it out.

As for why it foamed when you put the sugar in - try the same thing with a commercial beer and see what happens. There isn't anything wrong with your batch. I'd guess it has something to do with all the teeny-tiny nucleation points you're pouring into the beer - I'm sure a more sciency brewer will explain it correctly soon enough.
Adding dry sugar on top of beer in a bottle will froth. That's normal.

The fact that the beer is starting to carbonate suggests it's just taking longer.

I have found that half filling a bottle with that last bit will result in very similar pressure to that which you describe. As well as heaps of yeast, the last bit has trub/hop debris etc which act as nucleation points. A full bottle from that end bit will often gush (my last bottles, I mark with an 'X' so I know not to give it away or open without chilling first). When half full, the pressure build up seems enormous. Better to run that last bit into a glass and sample the beer or run into the garden.

In short - all seems normal. leave the bottles, warmish for another week or so and see if there's improvement.
The high pressure in the half filled bottle I expect is associated with the availability of additional oxygen to the yeast. As previously stated the sugar grains provide nucleation points for the dissolved co2.
As always a wealth of knowledge.

Thanks.. I guess I will just have to be patient.

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