Racking to secondary on frozen fruit

Hi all! Quick question. I am racking 5 gallons of sour mashed Berliner Weiess onto 3 lbs of mixed berries tonight and I was wondering if I need to thaw them or rack onto them still frozen? I was thinking that the beer would thaw them as it was added.

Also, should I put the fruit in a paint strainer or throw them right in?
I am using a bucket.

Thanks and cheers!

Unless you have a lot of dissolved CO2 from fermentation, racking onto the frozen fruit should be fine. I say strainer bag for berries to avoid the mess later.

How do I know if I have disolved co2?

Transfer a small sample into a hydrometer tube, cover the open end with your hand and shake gently. If it foams over, then that's your sign to be careful with frozen fruit. However, if the transfer is a slow siphon it won't likely be an issue as the fruit will thaw pretty quickly once it's covered in the liquid.

Ok. Could you please explain a little further?
1- if the beer is vented the atmosphere via air lock, why would co2 disolve?
2- how does the co2 affect fruit? Frozen or otherwise?

The airlock helps vent the majority of the CO2 created by fermentation, but the beer will always maintain some level of CO2 in solution if CO2 is present in the head space. The amount of gas dissolved depends on the temp of fermentation and if it has been agitated. The lower the temp of the liquid, the more gas will dissolve into it.
Frozen fruit usually has a fairly large surface area, much of which has small ice crystals on it. Those crystals are nucleation points, places where the gas will be invited to come back out of solution suddenly. When a liquid that is has volatile gas like CO2 dissolved in it comes in contact with those crystals, a lot of that gas can rapidly come out of solution and create a foam-over.

Ok so a blowoff tube would be advised depending how cool the fermentation was? Is that correct? The other option is to defrost the fruit and any issues would be minimized?

Yes and yes

Awsome thanks!!!!!

+1 on the bag. I did a sour mash Berliner and racked over onto frozen raspberries (that I thawed and mashed up in the bottom of the bucket with a potato masher - probably not necessary but would speed up extraction vs leaving them whole). awesome raspberry flavor, but racking and bottling we're a huge pain because apparently raspberries don't settle well... fruit issues aside, one of the best beers I've ever made! enjoy

I am going to bottle it this weekend. I cant wait!!