There’s one chemical reaction that, whether you have an interest in chemistry or not, we all carry out on a regular, maybe even daily, basis. That reaction? The Maillard Reaction. This is a process that takes place whenever you cook a range of foods – it’s responsible for the flavours in cooked meat, fried onions, roasted coffee, and toasted bread. The reaction’s name is a little deceptive, because it’s really an umbrella term for a number of reactions that can produce a complex range of products. The main stages, and some of the different classes of products, are summarised in this graphic.
The Maillard reaction takes its name from French chemist Louis-Camille Maillard, who originally described the reaction between amino acids and sugars in 1912. His study did not offer much in the way of analysis on the reaction’s impact on flavour and aroma in cooking, however; it was not until the 1950s that its mechanisms and culinary contributions would become more clearly understood.
In 1973, American chemist John E Hodge published a mechanism for the different steps of the reaction, categorising its stages and identifying a range of the different products produced as a result of these. He identified the first stage as being the reaction between the sugar and the amino acid; this produced a glycosylamine compound, which in the second step rearranged to produce a ketosamine. The final stage consists of this compound reacting in a number of ways to produce several different compounds, which can themselves react to produce further products.
Melanoidins are one of the potential end products. These are long, polymeric compounds, which act as brown pigments, giving the cooked food its brown colouration. The Maillard reaction is referred to a non-enzymatic browning reaction, as these melanoidins are produced without the aid of enzymes; this differs from enzymatic browning, which is what turns fruits such as avocados brown.
Hundreds of other organic compounds are formed. A subset of these can contribute to the food’s flavour and aroma, and some of the different families of these compounds are detailed in the graphic. As a consequence of the complexity of the Maillard reaction, different amounts of different compounds can be formed in different foodstuffs, giving the wide variety of potential flavours. Cooking conditions can also influence the flavours produced; temperature and pH, amongst other factors, can have an influence.
The products of the Maillard reaction aren’t all good news, however. The carcinogenic compound, acrylamide, can also be produced as a result of the reaction, and the levels of it rise as food is heated for a longer period of time. A 2002 study found that fast food can contain particularly high levels of acrylamide, though measures have since been taken to try and reduce these levels. This gives some perspective to the discussion of carcinogens in food products; whilst, of course, we’d prefer to limit our exposure to these types of chemicals, in many cases carcinogenic compounds are already present as a natural consequence of cooking.
It’s not just in your kitchen that the Maillard reaction is taking place. It also occurs at a much slower rate in our bodies, and researchers have suggested that it may have a role in the formation of some types of cataracts. It’s also been linked as a contributor to other medical conditions.
Due to its complexity, there’s still plenty we don’t know about the Maillard reaction. Whilst we know that factors such as pH and temperature can affect the course of the reaction, we still know little about how to adjust these to specifically influence the final products. As we learn more about it, we learn more about the reactions that make cooked foods taste so good – not a bad application of chemistry!
References & Further Reading
- Formation of flavour compounds in the Maillard reaction – M A J S Van Boekel
- The Maillard reaction turns 100 – S Everts, C&EN
- Analysis of acrylamide in heated foodstuffs – E Tareke & others
- Food – The chemistry of its components – T Coultate