All too often restaurants, chip shops and chefs say "oh don't worry it has been deep fried so the gluten will have been killed". Frankly, this is nothing but bull poop, a great big sloppy pile of it.
To understand exactly what happens to gluten in a fryer, first we should recap on exactly what gluten is. Gluten is a general term for the proteins found in certain cereals; these two proteins are gliadin and glutenin. These proteins act as a glue (which is what gluten means) to hold food together, it makes bread soft and cakes spongey, however even before it hits the food we eat it nourishes the plants the cereals grow on so it is very important from the onset.
From this we know gluten cannot be killed, it isn't technically a living thing so we can't take a knife or a flame thrower to it and murder it, the gluten will still be very much "alive", so if you get one of the foolish chef's that use the term "oh the fryer kills the gluten" be sure to point out firstly that gluten isn't living.
Now the great theory that heat can destroy gluten molecules....this is FALSE.
Say you have gone into your local chip shop and they offered to make you gluten free battered cod but used the same fryer or went in a restaurant and your gluten free chips were cooked with the regular onion rings, when you get told this and query how much they actually understand about Gluten Free or Coeliac what you don't want to hear is the great The Fryer Destroys it theory.
So when your partners gluten filled cod has been put in the fryer to cook what happens? Well it cooks obviously. As it's doing that gluten is doing it's job, it is holding together the batter letting it end up nice and crispy, if that gluten was being destroyed the batter what would pretty much falling everywhere and not staying on the fish, think of making a collage and not sticking the paper down, it stays in one place until its forced to change (e.g cook in a fryer), but as soon as it's moved it all falls to pieces. Similar theory, no gluten, no glue. Put a pizza in the oven you won't get your nice crust (not in one piece at least). Bake bread without gluten...I'm sure most of us know how difficult it is to produce gluten free bread, with gluten it rises (and evidently isn't destroyed while baking), without gluten it's a pain in the bum trying to get it to rise effectively.
A pizza is cooked at 260°C in an oven, chip shop fryers are usually heated to 175°C, if gluten isn't destroyed in a nice fluffy pizza crust at a hotter temperature than a chip fryer, then gluten is most certainly not destroyed in a fryer.
Once your gluten filled fish is cooked it is removed, whatever you fry it will lose bits, be it batter, chips, onion rings, bits will end up floating round the fryer, with those you will get microscopic molecules of gluten. Put your gluten free battered fish in there now and lovely gluten bits are likely to stick to it as it fries since the heat won't destroy it....now you have a cross contamination problem, it only takes one molecule of gluten to effect a Coeliac, you are now potentially at risk of becoming ill.
Now I can hear you asking "but barley and alcohol can be made gluten free"...yes this is true, but that is due to the process they are put through not the heat par se, during distillation alcohol is separated from everything as a vapour, so unless gluten molecules can travel in a vapour then there is no way gluten can be in the alcohol. The same goes for vinegar.
So remember heat does not "kill" gluten, anything fried in a fryer used for gluten items are at high risk of contamination, make sure you go a chip shop that does gluten free nights when the oil is changed in the fryer or have dedicated fryers and make sure to query restaurants about their fryers when having chips or anything fried. The only thing that can prevent contamination is good cleaning!