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 (NaCH<tt>2</tt>COO-) 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.