What is the taste that stimulates your appetite?

As Reg Dwight comments, I don't think there's a particular taste which causes you to want more.

However, the food may be described as being moreish:

adjective
British • informal

So pleasant to eat that one wants more:
a moreish aubergine dip

It's derived from more+ish, which the ODO definition doesn't make clear, and pronounced /ˈmɔːrɪʃ/.

Something that stimulates our appetite could be said to be mouthwatering.
Appealing to the sense of taste; appetizing: the mouthwatering aroma of a baking pie.

Umami is one of the five basic tastes partially responsible for stimulating the salivary glands, i.e. the mouthwatering feeling we experience when we taste something savoury and to our liking.

moreish

http://dictionary.reference.com/browse/moreish

(of food) causing a desire for more: these cakes are very moreish