Thanksgiving: A Food Chemist's Holiday

With Thanksgiving rapidly approaching you're surely interested in knowing the science behind roasting the tastiest turkey. The Maillard reaction is one of a few fundamental chemical processes that relates to cooking. While I've long been aware of this I've never delved deeper than casually noting that it causes the browning of toast and the dark streaks on grilled food. Besides changes in appearance the reaction involves a number of well-documented chemical steps that result not only in the darkening of food but also the formation and release of aroma compounds. Just think about this: does raw meat have the same flavor as grilled? Right. So, that's a very clear example of just how much the Maillard reaction affects what we eat.

Raw fish does NOT taste like cooked fish

Just over a hundred years ago Louis Camille Maillard wondered what could make cooked food turn brown and release carbon dioxide. Although he was unable to discover the specific mechanisms involved he did realize that the reaction produced a class of previously unknown compounds which he called melanoidins, i.e. compounds that are produced through a reaction of sugar(s), amino acid(s), and heat.

Louis Camille Maillard

The reaction itself has now been illustrated in a variety of ways. In The Chemistry and Technology of Flavors and Fragrances Liam O'Hare and John Grigor chose to represent the process as the simplified  scheme proposed by John Hodge in 1953. His schematic goes something like this:

A carbonyl group, such as glucose, and an amino compound, such as lysine or glycine, react when heated together. Condensation occurs to produce a Schiff Base. A glycosylamine forms and either an aldose or ketose results. If an aldose forms the Amadori rearrangement creates an unstable intermediate while when a ketose forms the Heyns rearrangement leads to an unstable intermediate. The next step in either case is a dehydration followed by a deamination leading to a variety of flavor and aroma products like furans and thiazoles.

Hodge Schematic from The Chemistry and Technology of Flavors and Fragrances

In this system, water acts as a mobility-enhacing solvent. Additionally, analogous reactions can occur with thiols. Sulfur-containing compounds, while acrid or foul-smelling in high concentrations, are surprisingly critical to flavor of cooked meat.

I found this video helpful in understanding the reaction: 

And that brings us back to Thanksgiving. Why does only the surface of a roasted turkey turn that gleaming golden-brown color? That's due a different - but also important - reaction: caramelization. When sugars are heated above 150 degrees Celsius, anhydrides form. All meat contains ribose and therefore sugar is available for a reaction to occur. The high temperature and lack of water on the surface of say, a turkey in the oven create ideal conditions for caramelization. The anhydrides break down into furfural or 5-hydroxymethylfurfural and with continued application of heat become furans, aldehydes, ketones, aliphatic and aromatic hydrocarbons. Some furans present in cooked meat are critical to their flavor. For example, bis-2-methyl-3-furanyl is a key "beefy" component and has one of the lowest odor thresholds known being detectable at 0.00002ppb.

White on the inside, browned on the outside: caramelization 

"Flavor moderators" also impact the taste of cooked meat. The degradation of phospholipids leads to fatty aldehyde - key components of "meaty"flavor - formation. One experiment to prove their importance extracted all the lipids from a piece of meat and upon cooking the lipid-less tissue a biscuit-like smell was produced.  A high proportion of fatty aldehydes limits the production of heterocyclic aroma compounds during cooking (i.e. the Maillard Reaction).

There's a balancing act going on at the chemical level: too many high-impact aroma compounds can cause strong and unpleasant sulfurous odors while too few aroma compounds will leave your turkey tasting like the biscuits it's served with.

Ashley



Sources

"100 Years of the Maillard Reaction: Why Our Food Turns Brown," J. Agric. Food Chem. 2013. 61. p.10197. http://pubs.acs.org/doi/pdf/10.1021/jf403107k 

Hodge, John. "Chemistry of browning reactions in model system." J.Agric. Food Chem. 1953, 1, 928−943.

Rowe, David J (ed.) (2005) Chemistry and Technology of Flavors and Fragrances. Blackwell Publishing: Poole. Ch.3.