The not-good kind of browning is called enzymatic browning, and it occurs when a fruit or vegetable is cut open and the enzyme polyphenol oxidase within the fruit reacts with the oxygen in air and manifests as a brown pigment on the cut surface.
This browning indicates a degrading of flavour, texture and nutrition in the produce. (Think of apples, bananas, avocados.)
You can deactivate the enzyme by blanching vegetables in hot water soon after cutting. Adding an acid such as lemon juice to the cut surface can also help, as it blocks the enzyme from reacting with the oxygen.
The Maillard reaction is a kind of non-enzymatic browning browning. It occurs when sugars and proteins interact in the presence of heat to create a host of delicious new flavour and aroma compounds. This is the reaction you see when you make dosas, fry sliced potato, bake bread.
You can even add simple sugars (glucose, fructose) for a darker crust.
What happens when pure sugars meet heat? Caramelization.
This is the browning of sugar molecules at high heat. When you heat sugar, initially it melts into a thick syrup and then gradually gets darker and stickier. There’s a point at which it develops rich, caramel-y notes and flavorful aromas that were not present in the original syrup. This is the point of caramelisation. The exact temperatures for caramelisation vary for different types of sugar. For example, fructose caramelises at 110 degrees Celsius while glucose and sucrose need to reach temperatures of 160.
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