Food turns black when it’s burnt because heat drives off the water, oxygen and hydrogen in it and leaves behind a residue that is mostly carbon—char. The char is dominated by amorphous carbon and graphite-like nanostructures whose disordered atomic arrangement absorbs essentially all visible light, so the surface reads as black to your eye.
You may have seen food turn black when it gets overheated, or when it’s left on the stove longer than it should have been. Have you ever wondered what brings that black hue to burnt food?
Recommended Video for you:
Presence Of Carbon In Almost All Organic Matter
Not just food, but most organic matter, is made of carbon, along with a few other elements, including oxygen, hydrogen, nitrogen, phosphorus etc. Thus, when you burn food, it usually turns black, as the carbon present inside it undergoes combustion and leads to the formation of carbon dioxide gas and burnt carbon, which is black in color.
Overheating Vs Burning
It is also worth noting that food does not jump straight from raw to charcoal. As the surface temperature climbs, sugars and amino acids first react together in the Maillard reaction (roughly 140—165°C), giving toast its golden-brown colour and savoury aroma. Pure sugar follows a parallel route called caramelisation, mostly between about 160—180°C. Push past around 180—200°C and pyrolysis takes over: the organic molecules break apart, volatile compounds escape as smoke, and what is left is the carbon skeleton we recognise as char. So "burnt" is really the end of a temperature ladder, not a single event.
Not All Forms Of Carbon Are Black
A carbon atom has 4 valence electrons. In diamond, every carbon atom is connected to 4 other carbon atoms in a tight tetrahedral lattice; in graphite, each carbon is linked to only 3. Thus, the extra electrons that don’t participate in any bond formation remain ‘delocalized’, and are sort of shared among all carbon atoms via a shared electron cloud.
Color Of Carbon-based Substances
Everything gets its color based on how light rays interact with it. When light rays hit an object, they may be reflected, refracted, scattered or absorbed. Which color (of light) is absorbed by an object is totally dependent on the latter’s atomic structure.
Diamond is shiny because its carbon atoms form a regular, well-ordered lattice, which is why diamond absorbs very few frequencies of light. Consequently, diamond does not absorb/scatter light in the visible spectrum; hence, it appears transparent and shiny. (Note: diamond brilliance is also related to total internal reflection, which is another optical phenomenon).
On the other hand, if you consider materials such as graphite, which do not have as well-ordered a structure as diamond, and have free delocalized electrons, it absorbs/scatters a broader range of frequencies, including colored light. This causes graphite to have a characteristic blackish hue, since it doesn’t reflect almost any color back to your eyes.
Carbon And Burnt Food
It’s now clear that burned food contains carbon. However, a noteworthy thing is that burned food is mostly amorphous carbon, graphite and other miscellaneous random carbon nanostructures. These structures absorb/scatter all visible light falling on them, thereby imparting a blackish color to burnt food, or any other completely burnt organic material, for that matter.
So, it’s not just the stuff that’s left behind that gives burnt things their black color, but also how the constituent atoms inside that stuff are organized, as that plays a major role in determining the color of burnt organic materials.
