The chemistry of flavor
The Maillard reaction
Did you know that the golden brown crust on a loaf of fresh bread shares some chemistry as the crispy brown sear of a well-grilled steak? Not only are both delicious, the Maillard reaction is responsible for their enticing aromas. In fact, we can thank the Maillard reaction for some of cooked foods’ more appealing flavors and the brown color of roast coffee, bread crusts, soy sauce and barbecue, to name a few. Considering how billions of meals are cooked each day, the Maillard reaction is easily the most practiced chemical reaction in the world.[1]
Every time I bake a loaf of bread at home, my stomach starts rumbling when the bread is nearly ready because the aroma of 2-acetyl-1-pyrroline from Maillard reactions fills my kitchen as the crust browns. The scent of various other browned foods leads to a similar reaction. Why do we like the smells of the Maillard reaction’s products so much?
Humans long ago learned that cooking food makes it easier to digest and boosts nutrient uptake in a big way.[2] A 2012 study connected the invention of cooking about 1.8 million years ago to a ~3x surge in brain size that occurred at roughly the same time.[3] The increase in the number of neurons required an additional 360 Cal/day of energy, which would not have been available to our ancestors eating raw foods alone. By evolving a penchant for the distinct aromas of cooking food, humans preferentially ate cooked foods. This increase in caloric uptake is enough to satisfy the brain’s higher energy demands.
Mechanism
Discovered by the French chemist Louis-Camille Maillard in 1912, the Maillard reaction is the reaction between simple sugars and amino acids or proteins [4]. The mechanism was first described in 1953 by the African-American chemist John E. Hodge, and consists of three stages [5]:
Stage 1: Reaction of a sugar with the amino group of an amino acid or protein:
Note that loss of water occurs in this stage when forming the glycosylamine product.
Stage 2: Amadori rearrangements form aminoketose compounds:
Stage 3: Additional dehydration and deamination reactions form dicarbonyls [6]
Dicarbonyls undergo a variety of further conversions, additions, rearrangements and/or polymerizations to form hundreds of molecules including those affecting flavor, color and aroma. For example the dicarbonyl can react with another amino acid to form a Strecker aldehyde [7]:
Maximum Deliciousness
Based on the above mechanism, three things must be available for the Maillard reaction to proceed: amino acids (or amino groups on proteins), reducing sugars and water. Many if not most foods naturally contain all three constituents required to allow the reaction to proceed. Different foods have different potential flavors because they contain different types of amino acids that can react with the sugars. Glycine reacts with sugars to produce beerlike odors, while the amino acid cystiene is responsible for meat and cracker aromas.[1]
The rate of reaction, the extent and the product distribution are all affected by cooking conditions. Increasing surface temperature, cooking time or pH will generally increase reaction rates. While the Maillard process can occur at room temperature, it is very slow, so heat is usually required to accelerate the reaction.
Temperature targets
The optimal surface temperature for a faster Maillard reaction is around 300 °F (149 °C), with the rate increasing with temperature. Unfortunately there is a limit: pyrolysis reactions start “burning” the food above 355 °F (180 °C).[8]
Moisture management
Since the Maillard reaction occurs on the surface of the food, the surface needs to reach 300 °F while having access to water. This is above the boiling point of water, so the interior needs to stay below 212 °F (100 °C) or the food will be dry and terrible. If the surface is too moist, steam will form between the pan and the food, which means the surface temperature won’t rise above 212 °F (100 °C). The key is to ensure the surface of the food is dry before applying heat. Some helpful suggestions listed in Reference 8 are:
Use paper towels to dry the surface of the food before cooking
Air dry meat and vegetables on a plate or tray in the refrigerator overnight
Salt foods right before cooking to prevent excess moisture on the surface due to osmosis
Alternatively, salt meat at least 45-60 minutes before cooking. At first, the salt will draw out moisture from the meat through osmosis. Over time, the meat will reabsorb that salty brine and become more tender and moist. Remove excess moisture with paper towels immediately before cooking to get that brown sear.
Reverse sear beef in the oven before pan-searing to dry the surface for faster browning
Adjusting pH
Since some steps in the Maillard reaction are promoted by base, controlling surface acidity is important to quickly achieve a perfect brown on your food. If the pH is below 6, the browning process will be slow. Adding a small amount of an alkaline ingredient like baking soda boosts the browning reaction, increasing flavor and aroma compounds; the brown crust formed in this manner will be crunchier than it would be if acidity were not controlled.
Summary
The Maillard reaction is a reaction between reducible sugars and amino acids or proteins that occurs when browning food, producing new flavors, aromas and colors. Food becomes more enticing thanks to those aromas, triggering an evolutionary response that makes cooked food more appealing. This is beneficial since cooked food is easier to digest and extract more nutrients from than uncooked food. The increased nutrient uptake helps humans consume more calories per unit mass of food to satisfy the high energy demands of the human brain. Understanding the Maillard reaction can help us cook more delicious food to share with friends and family.
References
Everts, S. “The Maillard Reaction Turns 50,“ C&EN, 2012, 19(20)
Carmody, R.N.; Weintraub, G.S.; Wrangham, R.W. “Energetic consequences of thermal an d nonthermal food processing,” PNAS, 2011, 108, 19199.
Herculano-Houzel, S. “The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost,” PNAS, 2012, 109, 10661.
Maillard, L.C. “Action des acides aminés sur les sucres: formation des mélanoïdines par voie méthodique“ Compt. Rend. 1912, 154, 66
Hodge, J.E. “Chemistry of Browning Reactions in Model Systems“ J. Agric. Food Chem., 1953, 1, 928
Maillard Reaction, Wikipedia, accessed February 2023
Lund, M.N.; Ray, C.A. “Control of Maillard Reactions in Foods: Strategies and Chemical Mechanisms“ J. Agric. Food Chem. 2017, 65, 4537
Gavin, J. “Maillard Reaction: The Key to Flavor Development,” jessicagavin.com, accessed February 2023.
