The next set of common reactions that could catch the fancy of a chemist are of the Maillard type. With the addition of heat, the amino group (NH2) of amino acids attacks the carbonyl group (C=O) of a reducing sugar eventually leading to a range of brownish and appetizing compounds. Here are some examples of baking and cooking products that include compounds from Maillard reactions:1) Bread crust2) Boiling of maple syrup3) Roasting of almonds, coffee or cocoa nuts4) Beer-making (where’s the heat you may wonder? It’s the heat of fermentation.)5) Baking of cookies6) Browning of sauce on meat.The first compound formed from the amino attack is an N-substituted glycosylamine. But the hexagonal ring of this molecule then breaks up, undergoes another rearrangement with the help of a pH-change to produce an Amadori compound.What happens to this type of compound depends on pH, but in either case the NH2 group is lost forming ketones.(A compound with C=O group sandwiched between carbons.) In the next stage, these compounds are split into some of the brown compounds that we taste and smell. While Maillard reactions are taking place, amino acids can decompose into Schiff bases that eventually produce cereal like flavours and those of roasted nuts, bread and meat.Unless a sweet sauce is added to meat, the browning seen upon cooking is not a Maillard reaction. Rather it results from the oxidation of the Fe2+in myoglobin to the Fe3+ state. This is part of the reaction where the myoglobin protein is denatured to hemichrome.And when a cooked leaf loses its green colour, it is because chlorophyll has lost its Mg2+ ion. Like most reactions it depends on enzymes. In this case if you want to maintain the green colour, a little baking soda can be added. (not too much or you’ll gain both colour and bitterness) The higher pH from HCO3 – prevents the enzyme from converting chlorophyll into pheophytin.If you are a non-meat eater, you won’t mind me switching the topic from meat to seafood. Astaxanthin is a compound related to the carotene in carrots. It is pink and found in shrimp. Normally while the shrimp is alive, the pink colour of astaxanthin is not evident because it is bound to a protein, which changes its colour. But the heat of cooking uncoils the protein, unsheathing the same pigment that keeps flamingos feathers pink.A similar explanation applies to the blue/green to red colour change for cooked lobsters. Astaxanthin’s structure is similar to that of carotene. But the extra C=O group in astaxanthin increases the alternating single-double bond network, which makes it easier for electrons to get excited to higher energy levels. Compared to carotene, astaxanthin needs less energy or that of a longer wavelength for electronic excitation. This is consistent with the fact that astaxanthin reflects color of a longer wavelength: pink instead of orange. By the way, if you find a flamingo feather, keep it. Astaxanthin sells for $ 7000/kg! Of course a feather will have a negligible fraction of astaxanthin, so look for a lost flamingo instead.We often distort reality by trying to operate by simple rules of thumb. This certainly applies to vitamins in fruits and vegetables. Cooking certainly reduces the concentration of vitamins in food, but it does not destroy them, as shown in the table below:Effect of Heat on Vitamins(Source: USDA)
|Food(100g)||Vit A in Raw(IU)||Vit A in Cooked(IU)||
|Vit C in Raw(mg)||Vit C in cooked(mg)||
And where in the fruit and vegetable are the vitamins located? Does the peel contain a lot of nutrients? The answer varies. In the case of the potato, the peel does have fiber and minerals, and baking with the peel prevents the escape of some vitamin C during the cooking process. I have not been able to verify the claim that most of the vitamin C is in the flesh just underneath the peel. In the case of mangos, vitamin A is distributed evenly throughout the orange pulp, which is coloured by similar beta carotene molecules. But the peel, especially when ripe, has antioxidants, carotenes and vitamin C. Apple peels are not devoid of nutrients either: they contain minerals (K+, Mg2+), antioxidants and fiber.
Here’s a great example of another endothermic reaction in the kitchen: cooking avocados. Some compounds in the avocado will be converted into bitter alkaloids with heat. But if the avocados are added towards the end of a recipe to minimize the amount of heat absorbed, then the amount of bitter-tasting products will be kept to a minimum.
Finally an update following some experiments I did in the kitchen after reading Herbert This’ book, Molecular Gastoronomy.
-McGee, Harold. On food and cooking: the science and lore of the kitchen. Simon&Schuster. 2004
-C.M. Ajilaa, S.G. Bhata and U.J.S. Prasada Rao. Valuable components of raw and ripe peels from two Indian mango varieties. Central Food Technological Research Institute
-United States Department of Agriculture usda.org – http://www.plantphysiol.org/cgi/reprint/39/6/1056.pdf