I have taken frozen water and yogurt on flights many times now with no issues. Been through Canadian, American, EU, NZ, AU, and Japanese security and not had any problems either. In 1994 I worked with UAL and Air Canada to provide safe gluten-free meals on their flights especially the international ones as they were receiving more requests and I, as a traveler for business, flying 100k a year, was having problems getting consistant meals. I was the top consumer of gluten-free meals for both airlines. Meals at that time were also served domestically for travel. In the USA and Canada they take allergen meal prep very seriously in flight kitchens and it is all done in a sterile area where no cross contamination can happen. I cannot speak for the overseas kitchens, however the ones I toured in both counties here were better than any restaurant environment. In fact, UAL, took it one step farther, primarily due to all the issues I had with meals and substitutions, they had a central commissary in Chicago, under sterile and allergen control conditions, prepare all their GFML's and ship them frozen to their flight kitchens to put on planes so each kitchen would no longer make mistakes such as subsitute rice with orzo pasta... Happy travels!