Author: Renee Cao
Editors: Galiba Anjum and Ken Saito
Artist: Kaitlyn Stanton
Have you ever wondered why toast tastes better than plain bread? Or what gives foods like grilled meat, french fries, cookies, and roasted coffee their distinctive flavors, colors, and aromas? The answer is the Maillard reaction, the chemical reaction between amino acids and reducing sugars.
Amino acids are the organic compounds that makeup proteins, and they consist of an amine group (-NH₂), an acidic carboxyl group (-COOH), and an R group, which are chains of carbon and hydrogen atoms that are specific to each amino acid. Reducing sugars, such as glucose and lactose, are simple sugars that are reducing agents. In other words, they can oxidize because they have a free aldehyde or a free ketone group. Reducing sugars are vital for Maillard reactions because complex sugars such as sucrose (table sugar) and starch have molecules that are too big to react with Maillard proteins.
French physician Louis-Camille Maillard first described the Maillard reaction in 1912. The Maillard reaction is one of the two forms of non-enzymatic browning, browning that does not involve a chemical reaction between enzymes and oxygen. The other type of non-enzymatic browning is caramelization, which is often confused with the Maillard reaction since they can co-occur and can result in similar appearances and flavors. However, caramelization only involves the oxidation of sugar and has more of a sweet and nutty flavor. Enzymatic browning, on the other hand, involves chemical reactions between enzymes and oxygen. When the enzyme phenolase oxidase is exposed to oxygen, the phenolase changes the organic compound phenol into melanin, which causes foods such as fruits and vegetables to be brown. Enzymatic browning is usually not desirable since it causes a lot of food to rot and go waste; half of all fruit market losses are caused by enzymatic browning. However, it can help develop flavor in tea, and color and flavor in figs and raisins.
For the Maillard reaction to occur, heat is necessary, so temperatures at around 285-445°F (140-230°C) will allow the reaction to be noticeable and occur at fast rates. This is the reason why many recipes require ovens that can reach these temperatures. Though the Maillard reaction can occur at lower temperatures, it won’t be nearly as noticeable and will take a significantly longer time. Dryness is also essential because if the food has a lot of moisture, the temperature won’t be above boiling point and thus, temperatures will be very low; more energy will be put into evaporating the water, and less into the Maillard reaction. Baking soda is often added to foods to simulate higher pH levels (basic) since it makes more amino acids available to react.
The Maillard reaction is a quick and complex process with 20 to 30 steps. Though food scientists don’t know all of them, the process can be summarized into three main stages. In the first stage, the carbonyl group (C=O) of a sugar reacts with an amino group to create an N-substituted glycosylamine and a water molecule. In the second stage, the glycosylamine rearranges into either an Amadori or Heyns compound. If the sugar involved in the Maillard reaction is an aldehyde group, it will undergo the Amadori rearrangement; if the sugar is a ketosamine, it will undergo the Heyns rearrangement. After the glycosylamine goes through either process, a ketamine will be formed. The ketosamine then polymerizes into melanoidins and flavor compounds. Melanoidins are brown heterogeneous polymers responsible for the brown colors during the Maillard reaction. The thousands of flavor compounds are responsible for the taste and aromas derived from the Maillard reaction. For instance, the compound furan is responsible for meaty, burnt, and caramel-like flavors; the compound thiophene produces meaty and roasted flavors. Acylpyridine produces cracker-like cereal flavors, and pyrazine produces roasted and toasted flavors.
All in all, the Maillard reaction is an essential process that happens every day when cooking. It kills potentially dangerous microorganisms with heat, provides us with extra nutrition, and makes food look appetizing. Without it, we would not have the delicious flavors and smells in foods.
Feiner, Gerhard. “4 - Definitions of Terms Used in Meat Science and Technology.” Meat
Products Handbook, edited by Gerhard Feiner, Woodhead Publishing, 2006, pp. 46–71.
ScienceDirect, doi: 10.1533/9781845691721.1.46.