Kolsch with Bottle bombs Galore

I brewed a kolsch with the below recipe. Let fermentation go for 4 weeks down to 1.010 then for clarity lagered in a fridge for 20 days. I bottled the batch of 4 gallons with 4oz of priming sugar, maybe a shade less then 4gallons due to trub loss but it was not dramatic

These beers after 3-4 weeks turned into firecrackers. Extremely over carbed and 9 of the batch exploded so i tossed the rest as I didn't want anybody getting hurt.

When I tasted there was no hint of sourness or infection but due to that gravity reading over a few days I am puzzled at why they turned out as they did the bill was very simple.

I might have primed a bit more then needed but maybe .5oz more then needed.

Any ideas?

malt & fermentables
% LB OZ MALT OR FERMENTABLE PPG °L
94% 8 0 Pilsner Malt 38 2 ~
6% 0 8 Munich Malt 37 9 ~
8 8


Batch size: 5.0 gallons

Original Gravity
1.054 measured
(1.048 estimated)

Final Gravity
1.010 measured
(1.012 estimated)

Color
3° SRM / 5.91° EBC measured
(4° SRM estimated)
(Yellow)

Mash Efficiency
84% measured
(75% used for O.G. estimate)

hops
USE TIME OZ VARIETY FORM AA
boil 60 mins 0.2 Horizon pellet 11.1
boil 30 mins 1.0 Ultra leaf 9.0
Boil: 4.0 avg gallons for 60 minutes

Bitterness
29.4 IBU / 2 HBU
ƒ: Tinseth
BU:GU
0.61
yeast

White Labs German Ale / Kolsh (WLP029)
ale yeast in liquid form with medium flocculation and 75% attenuation

Alcohol
5.9% ABV / 5% ABW
(4.8% est. ABV / 4% est. ABW)

Calories
158 per 12 oz.

misc
USE TIME AMOUNT INGREDIENT
boil 15 min 1 tsp Irish Moss

What was the temperature of your batch when you bottled? Did you mix the sugar into a solution and then mix that in or rack onto the solution before bottling?

At, say, 70F, for a 4-gallon batch, using corn sugar, 4oz will get you to 2.7 volumes of CO2. That's not overly much, I wouldn't think, and seems to be just right for a Kolsch. I've read that normal beer bottles should stand up to around 4 volumes of CO2, assuming no flaws in the glass.

On the other hand, if you took it straight from the fridge and bottled it, it was at a lower temperature. At 40F, and assuming maybe you did put too much sugar in by 0.5oz, you'd be looking at something like 3.5 volumes of CO2 if using corn sugar - closer to the danger zone, especially when they warm up to room temperature and more of that CO2 comes out of solution. Bottle bombs usually come either from infection (you say you ruled that out, probably) or from uneven mixing of the solution so a larger proportion of sugar ended up in some bottles. Knowing what you did and under what conditions will help.

I always use a calculator and my actual bottling volume and temperature to figure out the amount of sugar to use. (BTW, and I may be wrong on this, but it doesn't really matter what kind of beer you started with, you could bottle straight water with yeast and a certain amount of sugar and still get bottle bombs if the CO2 produced is too much for the vessel. Right?)

Why get rid of them when you can refrigerate them?

I had one batch that I used too much priming sugar and had bottle bombs. I put the remainder I'm the fridge and none of the refrigerated ones blew up. They were still over carbonated but still tasty. I thought about dumping them just because I didn't want anyone hurt but like others have stated the fridge will help. Not sure why yours blew unless you had cheap glass. I have had some bottles break during bottle conditioning due to thin glass. Most broke where the seam was. I try to only use heavier glass now.

They were seriously dangerously overcarbed judging from the volcano when opened 4-5 volumes probably.

When I racked it was probably in the 50s. I would have kept the batch but had 6 go off one night and was left with just 4 so didn't want to screw with it and tossed.

I did stir the solution as I was racking to the bottling bucket.

but i gather the cold temps could have been the kicker as I've never really lagered before