The flavor of roasted meats depends on their fat content.
DO FATS IMPART A DISTINCTIVE FLAVOR to meats? If so, which one? It was long believed that lipids were capable only of dissolving odorant compounds, many of which are water insoluble. They have also been accused of giving meat a bad taste, turning rancid, or oxidizing during cooking. Nonetheless cooks have long known that the flavor of meat is affected by the fats it contains or the fats that are added to it during cooking. Today chemists can confirm that fats play a decisive role in Maillard reactions, whose products are the chief aromatic components of heated foods.
There are hundreds of odorant compounds, which vary according to the type of meat, the age of the animal, its diet, and the mode of cooking. Moreover, compounds present in minute quantities may be aromatically preponderant. One of the principal reactions responsible for generating tastes is the Maillard reaction between sugars (such as glucose) and amino acids. Named after Louis-Camille Maillard, a chemist in Nancy who first identified the reaction in 1912, it contributes to the flavor of bread crust as well as the roasted aroma of meats, beer, and chocolate, among other foods. This reaction also leads to the formation of the dark compounds called melanoidins, which give cooked foods their characteristic color.
Chemists have been investigating the precursors of the volatile compounds of meats for several decades. They first observed that these compounds have a low molecular mass. In addition to the reactive agents typical of Maillard transformations (amino acids and sugars), they found phosphate sugars, nucleotides, peptides, glycopeptides, and organic acids.
The role of lipids, in particular, long resisted explanation. It was known that phospholipids (fatty acids linked to a hydrosoluble group that are very sensitive to oxidation) were responsible for the appearance of fatty and rancid notes, but in 1983 Donald Mottram and his colleagues at the AFRC Meat Research Institute in Bristol, England (now the Institute for Food Research), were the first to observe that they are also indispensable to the development of the characteristic taste of cooked meat. In 1989, their colleague Linda Farmer showed that lipids are involved in the unfolding of Maillard reactions, not only through their degradation products but also on their own account, changing the odorant profiles of roasted meats.
The first studies showed that extracting triglycerides (molecules consisting of a glycerol molecule bound to three fatty acid molecules) from a meat did little to change its odor after cooking, whereas eliminating phospholipids replaced its characteristic aroma with one of roasted meat and biscuit. It is thought that triglycerides are scant in polyunsaturated fatty acids, which gives them a relative degree of chemical stability; many phospholipids are rich in polyunsaturated fatty acids, however, which explains their sensitivity to oxidation. Their hydrosoluble part can react with oxygen as well.
Proof by Reaction
At the Institut National de la Recherche Agronomique station in Nantes, Gilles Gandemer, Anne Leseigneur, and their colleagues studied the role of phospholipids in triggering Maillard reactions in simplified systems. To an aqueous solution of cysteine (an amino acid chosen because it contains a sulfur atom and creates molecules crucial for the formation of the aroma of cooked meat) and ribose (a sugar known for its activity in cooking that can be released in nucleotides) they added either fatty acids found in phospholipids (linoleic acid, palmitic acid, and ethanolamine) or the principal phospholipids in meats (phosphatidylcholine and phosphatidylethanolamine), producing concentrations of various molecules comparable to those found in meats. These mixtures were then heated to 140°C (284°F).
Because the products of Maillard reactions are too numerous to be tested in a controlled way, chemists have sought instead to study changes in chromatographic profiles, focusing on the heterocyclic compounds, which have a meaty taste, and on the products of lipid oxidation. Observing the appearance of new peaks on the chromatograms and the falling off from certain peaks generated by systems modeled without lipids, the chemists were able to confirm that phospholipids have a greater effect than triglycerides. They also showed that the aromas of cooked meat caused by phospholipids arise mainly from two effects: a fatty note created by the presence of carbonylated compounds (which contain the C = O chemical group), the result chiefly of the oxidation of fatty acids, and the interaction of lipids and their degradation products with the direct and intermediate products of Maillard reactions, which leads to the synthesis of a few new molecules and a reduction in the formation of other compounds.
It was also known that the nonvolatile products of Maillard reactions impede the oxidation of lipids. Further analysis showed that the odors of the modeled systems resulted more from a disturbance of Maillard reactions than from lipid oxidation. Although lipids do not come into contact with compounds dissolved in the aqueous phase, phospholipids, because of their polar head, are partially soluble and can react with the intermediate products of Maillard reactions.