Claire McLoughlin

Science of Sunday Lunch - A Question of Taste

If you've never thought about all those chemical reactions taking place every time you cook food, here is a little something to whet your appetite. It's a mere million years since human beings began to apply fire to a variety of objects which had, for eons, been eaten raw. Cooked food prevailed with sushi and steak tartare being an acquired taste (or not).

The reason for this may well lie in the chemical changes food undergoes as a result of cooking.

The modern day chemistry of food flavour dates from the discovery of the browning reaction, also known as the Maillard reaction which, as its name implies, is responsible for the browning of all foods (i.e. meat fish and vegetables) at temperatures above 154°C.

The Maillard reaction, discovered in 1912 by the French chemist Louis Camille Maillard takes place between amino acids (the building blocks of proteins) and sugars. While grappling with the problem of how amino acids linked up to form proteins, he discovered that when he heated sugars and amino acids together, the mixture slowly turned brown. When heated together sugars and amino acids rapidly produce a whole range of highly flavoured molecules that that are responsible for the brown colour and flavour and aroma of foods cooked over a flame, in the oven, or in oil.

The Maillard reaction occurs most readily at around 148.9°C (300°F) to 260°C (500°F). When meat is cooked, the outside reaches a higher temperature than the inside, triggering the Maillard reaction and creating the strongest flavours on the surface. Food only goes brown, however, if it heats up to over 154°C (67.8°F). That is why boiling in oil makes things brown but water doesn't. Boiled foods and the moist interior of cooked meats and vegetables do not exceed 100°C (the boiling point of water) so therefore look and taste rather plain.

Interestingly, melanoidins (final products of the Maillard reaction) possess antioxidant activity and are also coloured.

So, now we've browned the meat, let's look at Mum's soggy vegetables and see how we can improve on things (with a little kitchen chemistry, of course). When plants, like vegetables or rice, are plunged into boiling water, their structure changes from crisp and firm, to soft, wilted, or mushy. All living things are made up of millions of cells, but plant cells differ greatly from animal cells. Firstly, they contain a substance called cellulose (a carbohydrate) in their cell walls. Cellulose (composed of carbon, hydrogen and oxygen) makes the plant rigid. However, when these cells are heated up, cellulose softens and the plant starts to wilt. The vegetable cell walls eventually collapse opening up their structure and releasing water and air. For most vegetables, this happens within 10 minutes of heating at 98°C - chemistry again!

Plants also contain starch granules inside their cells, where they store the energy they capture from the sun. Starch, a polysaccharide, is insoluble in cold water, but swells when it is cooked in warm water - this is known as 'gelatinisation'. For example, pasta and rice both contain a lot of plant starch, which is why they swell when cooked. Another little known fact is that vegetables also lose their appetising colours at temperatures between 66-79°C. So we now have explanations for why plants, such as vegetables or rice, when incorrectly cooked change from crisp and firm, to soft, wilted, or mushy and from coloured to faded.

So what is the solution?
To retain crispness, do not boil for too long and to maintain colour always put vegetables straight into boiling water rather than water which is heating up. When they have finished cooking, copy top chefs, who often plunge the vegetables straight into ice-cold water. This rapidly chills them to below 66°C, so they stop cooking and don't start to discolour.

So there we have it - the perfect Sunday lunch. But why do we like it? - It's a question of taste!

It is generally accepted that we detect four tastes: sweet, bitter, salt and sour. These tastes are due to the chemical components of what we eat. For example :

• Sweet - Receptors recognise hydroxyl (OH) groups on organic molecules including sugars and alcohols.
• Bitter - Receptors responds to organic alkaloids which are often poisonous.
• Salt - Receptors respond to ionic solutions dominated by cations (positive ions) such as sodium (Na). Many sodium salts are salty, but saltiness depends on size of an accompanying anion also. Hence sodium chloride (NaCl) is saltier than sodium acetate (NaCH2COO-) at the same concentration.
• Sour - Receptors respond to hydrogen ions (H+), and the metal ions in salts (such as Na+ in table salt).

Contrary to popular opinion however, taste is not experienced on different parts of the tongue. The lumps on the tongue generally called taste buds are really called papillae and all can respond to all types of taste although there are small differences in sensation. Papillae have several pores in them. These pores are the end of taste buds which contain active cells. When a taste is in the mouth, it moves down the pores and stimulates the taste receptor.

Let's not forget the importance of smell in flavour perception however. Approximately 80-90% of what we perceive as 'taste' is actually due to our sense of smell. Taste receptors exist in the mouth, but flavour is registered in our olfactory bulb, behind the bridge of the nose and while our mouths are sensitive to only a few tastes, our noses can detect thousands of smells. Just think about how dull food tastes when you have a head cold with a stuffy nose or try eating raw, grated onion. It's quite palatable as long as your nose is blocked. Not so if you can also smell it.

Smell and taste can nevertheless be over-ruled by our primary sense - sight! We naturally associate taste with what we see, so when faced with an array of flavours we expect strawberry to be red, apple to be green and so on. If strawberry ice-cream is offered to us in different colours, even though the flavour of each is identical, each will 'taste' different.

Taste receptors have already been identified for sweet and bitter tastes, but research has also unearthed a possible fifth taste receptor. The receptor for Umami! It has been suggested that this taste is triggered by compounds of some amino acids (the building blocks of proteins), such as glutamates or aspartates. Particularly implicated is the flavour-enhancing substance monosodium glutamate. Monosodium glutamate is the sodium salt of glutamic acid, an amino acid present in most proteins. In its bound form, glutamate is linked with other amino acids to form proteins and does not produce a flavour enhancing effect. In order to enhance, the glutamate not only must be in its free form, but be present in its L-configuration rather than its D-configuration. (Most flavour-enhancing substances have a sole isomer that is active while other structural arrangements of the same chemical formula have no enhancing properties whatsoever.)
In 1825, the French gastronome Brillat-Savarin, in his book The Physiology Of Taste, used the word "osmosone" to describe the "meaty" taste. The term umami was first coined by the scientist Kikunae Ikeda of the Tokyo Imperial University way back in 1908. There is still no direct translation for it in English, but umami is best described as savoury, meaty and broth-like. Ikeda is quoted as saying: "There is a taste which is common to asparagus, tomatoes, cheese and meat but which is not one of the four well-known tastes of sweet, sour, bitter and salty." Anyone who has eaten a Chinese meal has experienced the Umami taste! This is due to the addition of monosodium glutamate which gives this Chinese food its unique flavour.

Nutritional and scientific journals continue to address the new 5th taste sense down to the molecular level. What they are finding is that umami rich foods taste good because they are good for you. Fish sauce and anchovy are fermented protein products. During the salt cure, the protein breaks down into a wide variety of free amino acids and nucleotides. This assortment of active compounds provides a rich full umami taste and nutritional benefits that other products cannot match. Foods naturally high in umami content include Parmesan, shiitake mushrooms, soya sauce (the naturally fermented variety) and all the fermented oriental fish sauce products. Japanese scientist, Shizuko Yamaguchi who researches the Umami taste extensively has discovered a synergy between foods that are high in nucleotides (nitrogen containing chemicals) and foods high in natural MSG. It has been found that, when combined in the right proportions, the taste intensity can be magnified by up to five times. Even more interestingly, this synergy actually drives us to eating a balanced diet! Take the glutamates found in a tomato-based pasta sauce and combine them with the protein in meatballs. Add a dash of aged cheese e.g. parmesan and the carbohydrates in pasta and hey presto, you have an extra delicious umami taste and a nutritionally balanced meal! Could it therefore be the case, that the reason why we love our Sunday roast so much and occasionally crave a Chinese take-away, is due to our taste for umami?

You do the taste test!

- March 2005

About the Author

Claire McLoughlin is from the press and publications department of the Royal Society of Chemistry.