Can you carry hydrated sourdough starter in the luggage and take them on a plane?

put it in your checked baggage instead of your carry on luggage.  Label it and include your name on the label.  Be sure to use a container that can cope with the expanding starter and the reduced air pressure in the hold.  Put the whole thing in a ziploc bag as secondary containment.  

I've done this with dried starter, which is easier to handle.  I also put several packets in my luggage, in the hopes that at least one would survive the gauntlet.  As it turned out, all of them made it. 

Paul

I've carried flour, sugar, jellies from the UK, France and Italy to the US in checked luggage with no issues.

I've also brought back tulip bulbs from Holland. I made sure I purchased the ones marked as approved for export to the US. I declared them, went through agricultural inspection as part of the customs process. I showed the agent the 'approved for US export' and I was on my way quickly.

The only issue I could see with a sourdough starter is that it's a 'biologic' containing living organisms. I'd suggest labeling it clearly as sourdough starter and declaring it if asked on the customs/immigration form.

Turn a portion of your liquid sour into a really stiff small ball and pack it in a sealed plastic container rolling it in flour first. You only need a really small amount, then let it down and feed it soon as you get to your destination.

We do have great range of flours in the UK, just wondering why you are even thinking to bring flour, unless it's to give to people over here as presents?

Andy

Carried about 30g of stiff starter in a small plastic container with screw-top lid, inside a small ziplok bag in my checked bag. California to Pennsylvania; total travel day of about 12 hours including >6 hours by air. I made bread with this starter (after feeding) about three days later and it worked normally.