I carry yeast, flour, and oil with me to the airport to demonstrate proving if necessary. On the off-chance that the white powder is not in fact baking powder -

I carry yeast, flour, and oil with me to the airport to demonstrate proving if necessary. On the off-chance that the white powder is not in fact baking powder - after a trip to South America for example - I use self-raising flour (harina leudante) cleverly switched into a plain flour packet.