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Copyright © 2025 Foodies Only
Copyright © 2025 Foodies Only
7 min
28/01/2026
All of you must have experienced how onions gradually turn from white to soft and then to golden after frying them. But did you ever wonder what leads to this? The process called the Maillard reaction plays an important role in this. This article looks at the science behind caramelising and makes you understand the steps that turn the onions from white to golden. Dive deeper to know more.
There are many magical things that happen in the kitchen if you witness them carefully, and one of them is the caramelisation of onions. Many of you have a love-hate story with onions; you start with tears when you chop them, but end up loving the aroma and taste of it as it changes its shape, colour, and flavour. But behind this transformation is science and chemistry. When onions cook slowly over heat, their natural sugars and proteins react in a process called the Maillard reaction. This is the same reason behind a bread turning golden after you put it in the toaster or how meat browns when you fry it in a pan. Along with the Maillard reaction, sugar caramelisation takes place, which is a slower process where sugars break down and develop sweet and rich flavours. When these two reactions take place together, it results in caramelisation of the onions. Read this article to understand the entire process and what makes it so special in cooking.
The Maillard reaction is the secret behind browning in all kinds of cooking, from toasted bread to grilled meats. In onions, this reaction begins once the temperature in the pan reaches around 120°C. The amino acids in the onion’s cells start to react with reducing sugars, creating a cascade of flavour molecules, adding that familiar brown colour. What is fascinating about it is how this reaction adds layers and flavours. The Maillard process does not create only one type of flavour, but hundreds of them. That is the reason why caramelised onions taste butty, roasted and even smoky. These flavours are not because of the ingredients but because of the mixture and chemistry behind it. However, the Maillard reaction only works properly under the right conditions. You have to make sure there is not too much water, the temperature is not too high, and the sugars do not burn before the flavours are formed. The golden rule of caramelisation is patience. Slow and even heat allows the Maillard reaction to unfold gradually, which adds flavours, richness and colour to the onions over 30 to 45 minutes.
While the Maillard reaction is about the savoury flavour, caramelisation is about sweetness. Onions contain natural sugars like glucose, fructose, and sucrose. When these sugars are exposed to heat, usually above 150°C, they begin to break down and rearrange themselves into new compounds. This process of breaking and rearranging is called caramelisation. Unlike the Maillard reaction, caramelisation does not need proteins or amino acids; this process is purely about sugar and heat. As the reaction progresses, onions darken from pale gold to deep brown, while their flavour transforms from mild and buttery to sweet, rich, and slightly bitter at the edges. This is the reason why caramelised onions can taste almost dessert-like, even though they are completely savoury. This sugar breakdown also affects the texture. The natural sugars thicken into a syrupy glaze, coating each onion strand in a glossy finish. This is the moment cooks love, when the onions become soft, sticky, and sweet enough to spread on toast or blend into a curry base.
Achieving perfect caramelised onions is not about fancy ingredients or special tools; it is about control. The process starts with slicing onions evenly so they cook at the same pace. A wide, heavy-bottomed pan helps water evaporate quickly, allowing the temperature to rise steadily. Begin with low to medium heat and a little oil or butter to prevent sticking. As the onions start to soften, sprinkle in a pinch of salt, as this draws out more moisture, speeding up the initial stage. Once the water has evaporated, the Maillard reaction begins quietly, building flavour from within.
Avoid the temptation to increase the heat. High temperatures might seem faster, but they will burn the sugars before the flavours can form. Keep stirring occasionally, not constantly, to give the onions contact with the hot surface while preventing them from catching. In the process, if brown bits form at the bottom, deglaze with a splash of water, stock, or even vinegar. Those bits are concentrated flavour, and scraping them up and mixing them back in adds richness. For a shortcut, some chefs also add a tiny pinch of baking soda, which raises the pH levels and speeds up browning. But make sure you add just a pinch of it, as too much of it will make the onions mushy.
After about 40 to 45 minutes, you will have golden-brown onions that smell sweet and taste complex, which is the perfect blend of the Maillard reaction’s savoury notes and caramelisation’s natural sweetness.
Caramelised onions are more than a garnish; they are the foundation of flavour. In Indian cooking, they form the base of countless dishes: biryanis, kormas, curries, and even kebab marinades. In European kitchens, they enrich soups, tarts, and sandwiches. The secret behind all these dishes lies in the deep balance of sweet and savoury that only caramelisation can bring.
