By Se Ri Lee '19
Imagine ordering a 10 oz New York strip at your favorite steakhouse. Your mouth waters as you watch the browning flesh sizzle and inhale the nutty aroma infusing the room. You are also witnessing one of the most complicated chemical reactions in the word: the Maillard reaction.
The Maillard reaction is a type of non-enzymatic browning. It occurs when there’s a reducing sugar and an amino acid above a certain temperature threshold. Careful not to confuse this with enzymatic browning, such as the spontaneous browning of fruits, and caramelization, which is another type of non-enzymatic browning that requires the presence of sugars only.
Most foods that are cooked at 250–300 °F undergo the Maillard reaction, caramelization, or often a combination of the two. Cooked onions, toasted bread, sugar-added roasted coffee beans, all of which are foods that go through this reaction, taste nuttier and richer and appear brown because of melanoidin, a nonvolatile compound created in the reaction.
Though the reaction mechanism is complex, John Hodge was able to simply it into three steps:
Step 1: A reducing sugar, such as glucose and fructose, reacts with an amino acid to form water and glycosylamine.
Step 2: Due to its instability, the glycosylamine then forms either an Amadori or Heyns compound, depending on the type of the initial sugar molecule, whether it is an aldehyde or ketone.
Step 3: The Amadori or Heyns compounds collide with each other to form numerous different products that give foods their nutty flavor and brownish color.
Certain conditions make the Maillard Reaction highly favorable. First, temperature. Whether it’s cooking steak or toasting bread, the Maillard reaction occurs rapidly at high temperatures. Lack of water and higher pH also quickens the pace of the reaction. A piece of dry bread toasts faster than does a piece of wet bread. Cooked onions brown more rapidly with the addition of baking soda, an alkaline substance.
Maillard products enhance the flavor and aroma of foods, but some give foods an unappetizing flavor and can be hazardous to our health. For example, 2-acetyl-1-pyrroline, a compound that can be formed in Hodge’s “third” step, enhances the flavor of bread and popcorn. However, this same compound makes pasteurized milk taste bad at high temperatures. The Maillard reaction can also yield acrylamide, a carcinogenic substance, found in french fries and chips.
Researchers are continuing to find ways to control this mechanism, varying initial conditions at times, to obtain only the desirable products. This multistep mechanism, however complex and magical it may be, continues to fascinate scientists and food-lovers alike.
Fun fact:
Ironically, when French chemist Louis Camille Maillard discovered this reaction, he was not in his kitchen but in his laboratory studying the effects of glycerin and sugar on the amino acids of kidneys. In 1913, Maillard published his thesis on the complex, multi-step reaction mechanism to propose a viable cure for kidney diseases. Scientists soon found out that Maillard’s proposed mechanisms created products accountable for the appetizing aroma of food and often some undesirable products that are formed in stored or canned foods.
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