Friday, May 27, 2011
"The Chemistry Of ...Cooking"
A Biochemist Explains The Chemistry Of Cooking
January 1, 2009
A biochemist and cook explains that cooking is all about chemistry and knowing some facts can help chefs understand why recipes go wrong. Because cooking is essentially a series of chemical reactions, it is helpful to know some basics. For example, plunging asparagus into boiling water causes the cells to pop and result in a brighter green. Longer cooking, however, causes the plant's cell walls to shrink and releases an acid. This turns the asparagus an unappetizing shade of grey.
You love to cook, but have you whipped up some disasters? Even the best recipes can sometimes go terribly wrong. A nationally recognized scientist and chef says knowing a little chemistry could help.
Long before she was a cook, Shirley Corriher was a biochemist. She says science is the key to understanding what goes right and wrong in the kitchen.
"Cooking is chemistry," said Corriher. "It's essentially chemical reactions."
This kind of chemistry happens when you put chopped red cabbage into a hot pan. Heat breaks down the red anthocyanine pigment, changing it from an acid to alkaline and causing the color change. Add some vinegar to increase the acidity, and the cabbage is red again. Baking soda will change it back to blue.
Cooking vegetables like asparagus causes a different kind of reaction when tiny air cells on the surface hit boiling water.
"If we plunge them into boiling water, we pop these cells, and they suddenly become much brighter green," Corriher said.
Longer cooking is not so good. It causes the plant's cell walls to shrink and release acid.
"So as it starts gushing out of the cells, and with acid in the water, it turns cooked green vegetables into [a] yucky army drab," Corriher said.
The culprit here is ethylene gas. Given off by apples and even the bananas themselves, it can ruin your perfect fruit bowl -- but put an apple in a paper bag with an unripe avocado, and ethylene gas will work for you overnight.
"We use this as a quick way to ripen," Corriher said. Corriher says understanding a little chemistry can help any cook.
"You may still mess up, but you know why," she said. When it works, this kind of chemistry can be downright delicious.
WHAT ARE ACIDS AND BASES? An acid is defined as a solution with more positive hydrogen ions than negative hydroxyl ions, which are made of one atom of oxygen and one of hydrogen. Acidity and basicity are measured on a scale called the pH scale. The value of freshly distilled water is seven, which indicates a neutral solution. A value of less than seven indicates an acid, and a value of more than seven indicates a base. Common acids include lemon juice and coffee, while common bases include ammonia and bleach.
WHY DOES FOOD SPOIL? Processing and improper storage practices can expose food items to heat or oxygen, which causes deterioration. In ancient times, salt was used to cure meats and fish to preserve them longer, while sugar was added to fruits to prevent spoilage. Certain herbs, spices and vinegar can also be used as preservatives, along with anti-oxidants, most notably Vitamins C and E. In processed foods, certain FDA-approved chemical additives also help extend shelf life.
Chemistry in Cooking
By Gilly French
Out of the fire into the frying pan....if you've never thought about all those chemical reactions occurring every time you cook some food, here is a little enlightenment to whet your appetite....
The modern day chemistry of food flavour dates from the discovery in the nineteen-teens of the browning reaction, also known as the Maillard reaction, which generates much of the characteristic colour and aroma of foods cooked over a flame, in the oven, or in oil. The 'simplest' browning reaction is the caramelisation of sugar, but it is not a simple reaction. Glucose produces at least a hundred different products, including organic acids, fragrant molecules and brown-coloured polymers. This takes place at a relatively low temperature of about 154°C, which is why most foods brown on the outside during the application of dry heat. Maillard reactions proper occur between amino acids (found in proteins) and sugar molecules: when these are heated together they produce, rapidly, a whole range of highly flavoured molecules that are responsible for the brown colour and distinctive taste of cooked meat. Foods that have been boiled, and moist interiors of meat and vegetables, do not exceed 100°C and will therefore look, and taste, plain. To make a rich tasting stew the meat, vegetables and flour must be browned before adding any liquid; conversely, if a cook wants to highlight natural flavours, high temperatures should be avoided. The food industry uses purified sugars and amino acids to approximate to otherwise costly flavours: for example, a well-known coffee substitute is simply a mixture of roasted wheat, bran and molasses.
High temperatures are also used, of course, to increase the rate of chemical reaction: if the rate doubles with every 10°C rise in temperature, which is a reasonable approximation, then a reaction which would normally take a day can be over in a matter of seconds. Frying, for example, is possible at temperatures of up to 250°C. The first stage of the cooking allows for heat transfer; the second, at a higher temperature, encourages Maillard reactions. At this temperature the fat is no longer a heat transfer agent and starts to influence the taste of the resulting Maillard compounds.
However, we have still not answered the question of why we should prefer our food cooked. To do this, we have to look at what happens during the caramelisation of sugar. Plain crystalline sugar has no odour. Heated to 160°C it will melt; at 168°C it begins to colour and to develop a rich aroma. At this point several hundred molecules have been formed, as the carbon, hydrogen and oxygen atoms interact with each other and with oxygen in the air at high temperatures. The volatile products include acids, aldehydes, alcohols and esters (those familiar fruity smells).
The interesting point is that several of these flavours are contributed by chemical families of compounds that are common in nature, particularly in fruit. Alcohols, esters and the suchlike are found throughout the living world because they are all associated with the process of energy production. Cooks generate them by breaking up sugar molecules under the influence of heat; fruits generate them during the ripening process. So some of the components of the caramelised aroma would have been familiar to our ancestors in the form of fermented fruit.
The important families of aroma compounds produced in the Maillard reaction (which occurs between amino acids and sugars at a lower temperature than caramelisation) include pyrroles, thiophenes, thiazoles, pyridines and pyrazines. Several of these contribute a nutty flavour, some a 'roasted' impresion, even with hints of chocolate. And several contribute floral odours, or are reminiscent of green leaves and vegetables: flavours our ancestors would have encountered long before they had 'discovered' fire. For example the compound 2-methyl thiazole, which is reminiscent of green vegetables, is found in cooked beef. Pyrazines have been identified in molasses, coffee, green peas, Gouda cheese, red beans, asparagus and other green vegetables. The American food scientist Harold McGee, whose work has inspired this article, suggests that 'fruits probably provided our evolutionary ancestors with refreshing sensory interludes in an otherwise bland and dull diet...perhaps cooking with fire was valued in part because it transformed blandness into fruitlike richness'. Our ancestors have been encountering molecules characteristic of the roast for probably hundreds of millions of years.
Some animals, eg ants, produce their own cyclic aroma molecules by way of chemical communication - pheromones. It is suggested that, in terms of smell, there is not that much difference between us and some insects. It has been important for all animals to detect a wide variety of aroma molecules, particularly those generated by plants and other animals. McGee suggests that' our powerful response to odours may in part be a legacy of their prehistoric importance to animals, which have used them to recall and learn from experiences'.
Raymond Blanc: 'Blanc Mange', BBC books
Harold McGee: 'On food and cooking', Unwin paperbacks;
'The Curious Cook', Collier Books
T.P. Coultate: "Food, the chemistry of its components' Royal Society of Chemistry
Hooke Magazine - Issue 10
ScienceDaily's Article's Link
Camille and Henry Dreyfus Foundation INC. https://homepages.westminster.org.uk/hooke/issue10/chemcook.htm
The Chemistry of Cooking Webpage