Information from the official website of J.K.Rowling is generally considered canon. I remember at least one case when she 'changed her mind' after a particular rule has been published on her website (what happens with a secret if a Secret Keeper dies). Nevertheless, the information from the website can generally be considered reliable.
A rather obscure piece of information is J.K.Rowling's Wizards' Ordinary Magic and Basic Aptitude Test (W.O.M.B.A.T.) published on her website in 2007 and now accessible with a Time Turner.
In particular, we know that only one of the following options is correct (as the software allows you to choose only one option):
Which of the following is CORRECT?
a. Food can be conjured out of thin air.
b. Any object can be Transfigured into food.
c. Foodstuffs can be increased, Transfigured, summoned from a distance and magically cooked.
d. It is impossible to make the inedible, edible.
e. Food-related Charms are some of the simplest forms of magic.
W.O.M.B.A.T. Grade 3, 13 June 2007, question 8
The test appeared on J.K.Rowling's website before the release of Book 7, but we can almost safely assume that there was no time for J.K.Rowling to change her mind after compiling the test (though she must have submitted these questions weeks before they were eventually published).
As Hermione explained in Harry Potter and the Deathly Hallows:
Your mother can’t produce food out of thin air, no one can. Food is the first of the five Principal Exceptions to Gamp’s Law of Elemental Transfigura[tion]. It’s impossible to make good food out of nothing! You can Summon it if you know where it is, you can transform it, you can increase the quantity if you’ve already got some...
It means that the correct option is c, and therefore all other statements are false. So, apparently, you can make food out of something else, though not any object can be transfigured into food. Can you come up with any example of such transformation mentioned in the books?