This is why independant testing is needed. Companies have every reason to not want test results like this be public.

Americans wouldn’t know about it, but Ribena is a popular (blackcurrant) fruit drink in much of the world, produced by GlaxoSmithKline. For decades they advertised how it was high in vitamin C, until in 2007 some school kids in New Zealand were doing a project to show how it was healthier than cheaper brands, when they found out that it contained no vitamin C.