Re: Why does Marmite turn white when patted repeatedly with a spoon?

I wouldn't trust a breakfast spread that does this! Perhaps you should 
change to vegemite! ;-)

Seriously, though, this is an example of quite a general phenomenon. And 
curiosity driven experimentation and observation is a GOOD THING, even when 
it involves playing with your food.

Have you looked at a lump of rosin that a violinist uses to prepare the 
bow? It is a block of yellow or orange material. But where the violinist 
has rubbed the bowstrings along it, it looks white. And the powder it 
produces definitely looks white. This is another example of essentially the 
same phenomenon.

Marmite is a material that has visco-elastic flow properties, somewhere 
between a viscous liquid and a soft gel. When you pat the surface 
repeatedly with a spoon, what you are doing is generally roughening up the 
surface into a tangle of microscopic peaks and valleys. Some marmite 
adheres to the spoon, and is lifted into a sharp peak as the spoon is 
withdrawn. Because the material is not truly liquid, the peak stays there.

Optically, marmite (as usually encountered with a smooth surface) is an 
indescribable brownish-black colour with a significant surface lustre. It 
can look quite shiny. What this means is that the light which reaches our 
eyes from a sample of marmite has travelled through a fair amount of light 
absorbing material before we get a little bit of it reflected back (the 
brownish-black colour that we see). But is also means that a small but 
significant proportion of the light -- perhaps 10 to 15% -- is reflected 
directly back from the surface (the lustrous or shiny appearance). When we 
roughen the surface, we increase the amount of that surface reflection, and 
we change things so that the reflection will be a diffuse one from several 
surfaces rather than a direct one. The result is that the material looks 
white, because most of the light we see coming from it has been reflected 
off its front surface in a diffuse way, without ever penetrating deeply 
into the material where it can be absorbed.

There are many materials, like the rosin, that look quite highly coloured 
when in the form of large crystals or glasses, but lose most or all of 
their colour and look white, when ground to a fine powder. 

As a postscript, I really should direct North American readers to 
information on the products concerned. I am told that they may be 
unfamiliar to you. Both Vegemite and Marmite are different forms of yeast 
extract. They are brownish materials that are usually spread thinly on 
bread or toast, but can also be diluted with hot water to make drinks, or 
used as a flavouring in certain sorts of cooking.
For information about Marmite  check here.

Where "some English children are addicted to Marmite", which is a "strong 
flavoured, slightly salty" material, most Australian children are addicted 
to Vegemite, which is stronger flavoured, saltier, and somewhat sweeter. 
Vegemite is an Australian cultural icon, and a bit of an in joke. Marmite 
is available here, but does not sell well. There is also web information 
available about  Vegemite