Most foods contain protein.
Proteins are made up of amino acids, and one of those amino acids is called “glutamate”.
Glutamate is responsible for the savoury flavour that we call “umami”. If a food tastes savoury, it contains some level of glutamate.
Foods vary massively in the amount of glutamate they contain. An apple, for example, has a really small amount of glutamate in it, and so doesn’t taste very umami, whereas a mushroom has much more glutamate and so tastes much more umami.
So: less glutamate, less umami; more glutamate, more umami.
But glutamate can also either be bound or free.
This tomato, for example, contains lots of glutamate, but a fresh tomato doesn’t taste particularly umami. That’s because most of its glutamate is bound up in proteins, and we can’t taste bound glutamate.
But if we age and dry out the tomato – like we do to get a sun-dried tomato – the proteins break down, and all of that bound glutamate gets released. It becomes free glutamate, which we can taste. That’s why a sun-dried tomato has a deeper, more savoury, more umami flavour than a fresh tomato. It’s not because we’ve added some glutamate to it; we’ve just released the umami that was already there in the proteins.
It’s the same with cheese. Fresh cheeses like mozzarella don’t have much umami flavour, but they have a lot of umami potential. If we age them, the proteins in them break down, releasing the glutamate and making the cheese taste more umami. That’s why parmesan cheese tastes more umami than mozzarella.