Food Molecule Change

Certainly hydrocarbon breakdown occurs depending on the type of cooking that one is doing. I don't believe though that at anytime but a quick transitory condition does elemental hydrogen (H₂) ever get produced. Hydrogen and hydronium ions do constantly appear and form water in acid/base reactions that change our taste perception from a sour or bitter taste. I recently heard a cooking chemist claim that a mildly basic pH is what causes that mouth watering savory taste. The Chemistry of Cooking is a long, complicated, intriguing, tasty study.

Well I was wondering if the H2 breaks off and joins with oxygen to form water, because the food turns from a bright colour to a dark colour to black. Doesn't that mean that it's then almost pure carbon instead of hydrocarbons?

You present here another case of pronoun abuse. I'm not sure what you refer to by the pronoun "it". Most of the molecular weight of a hydrocarbon does come from carbon but there are so many hydrogen atoms in each molecule that most hydrocarbon molecular diagrams don't include the hydrogen bonds. Now the hydrogen to carbon bonds are the weakest bonds so in cooking these will break before the carbon to carbon or carbon to other atom (oxygen, nitrogen, etc.) breaks. But pure carbon in any of its elemental forms are not digestible and don't taste very good. So most cooks try not to burn the meal.