1. The picture above shows five hexagonal pancakes, in different shades of brown thanks to different cooking times, arranged to resemble benzo[a]pyrene, a polycyclic aromatic hydrocarbon (“PAH”). This is a very common component of incomplete combustion, found in grilled meat, coal tar and tobacco smoke. Last week we discussed caramelisation, a process which turns sugars brown by making simple sugar molecules react together to form longer chains. It is termed a “non-enzymatic browning process” as it is caused only by heating, unlike when apples and bananas turn brown, and, you guessed it, enzymes play a role. The Maillard reaction is also a non-enzymatic browning process in which amino acids or proteins react with sugars to give a complex mixture of compounds giving browned foods their colour and flavour.
Foods are often browned during roasting and frying, as shown by the different colours of bread and toast in the photo on the right, but not when they are cooked in boiling water. This illustrates a key feature of the Maillard reaction – it generally occurs quickly above 150°C, but since water boils at 100°C, boiled foods do not get hot enough for the reaction to occur significantly. Sugars tend to caramelise at 160-180°C so the flavours and colours of some foods, like roasted coffee and caramelised milk, in fact arise from a combination of Maillard and caramelisation processes happening at the same time.
The Maillard reaction may well be the most commonly performed chemical experiment in the world since it gives the brown colour of many everyday foods, including bread crusts and toast. It was first described in 1912, though it could be argued that it was actually discovered the first time prehistoric humans cooked food over a fire. In more recent decades the food industry has researched the reaction in great detail to try to optimize the processes for making foods which look and taste appealing to consumers. This research is summarised in a number of review articles.
2. Foods contain a huge range of proteins and sugars (carbohydrates) capable of undergoing the Maillard reaction, so trying to describe this reaction in detail is very hard. The first mechanism was published in 1953 and, though it has since been shown to be incomplete, this scheme is still used today to describe the formation of the “melanoidins” which give the brown colour of roasted and fried foods. As there are several connected processes happening at once, a “curly arrow” mechanism, illustrating the changes in electron distribution and bonding for the entire system would be extremely lengthy, but in general, carbohydrate chemistry involves lots of simple steps rather than a few complicated ones.
The first step of the reaction is one of the most important as it joins the amino acid to the sugar. This happens by a nucleophilic amine group of an amino acid attacking a carbonyl group of the sugar. Amines are basic (proton acceptors), so in biological systems and foods they are likely to be protonated and therefore not nucleophilic.
Adding a stronger base to the reaction will take away some of these protons, accelerating the reaction. This can be seen in the making of pretzels and lye rolls in Germany; before baking, these are glazed with a solution of the strong base sodium hydroxide (for which “lye” is an old name) which encourages the Maillard reaction on the surface, giving the breads a rich brown crust but a white centre. For keen home bakers, a solution of baking soda (sodium bicarbonate) is a safer alternative to sodium hydroxide.
The brown colour associated with the Maillard reaction comes from compounds called “melanoidins” which are large nitrogen-containing reaction products with a range of structures. This review article gives examples of some of the small molecules formed during cooking, as well as the polymer products which they can go on to form by reacting with proteins. These molecules are highly unsaturated, containing sets of conjugated (connected) double bonds which make them absorb light in the visible region. As each molecule has a different structure, it will absorb a different wavelength of light and so have a different colour – the combination of all these chromophores results in a brown colour.
The Maillard reaction is called a browning reaction but it also strongly influences the flavour of foods. Just as with formation of melanoidins, many reaction steps are involved and many products are possible; the most important difference is that the flavour molecules are much smaller. This recent review paper outlines the formation pathways for aroma compounds based on fairly simple model systems of sugar and amino acids reacting together, and while this is a good start point, the paper notes that very little research has been done into how the reactions proceed differently in real food. The flavour compounds are likely to be related to those found in caramels, but, as amino acids are present, they can also contain nitrogen, which further complicates the system.
In summary, the apparently straightforward process of a yorkshire pudding or churro turning brown as it cooks is in fact an intricate network of connected chemical reactions, over which the bakers in the tent display impressive control.
Contributors: Harry Morgan