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More about culinary ingredients: Water

Water structure?

Pure water is made of water molecules which are constantly moving around when water is liquid. Eachof these water molecules is composed of two hydrogen atoms and one oxygen atom, organised in aV shape as shown below.

O

H H

Although this molecule is neutral, and is not charged overall, its electrical charge is actually evenlydistributed throughout its structure the oxygen (O) atom carries a weak negative charge, whereaseach hydrogen (H) carries a weak positive charge.This makes the water molecules attract one and other. In water, which contains many of thesemolecules, the negative oxygen atom on some water molecules will be attracted to the positivehydrogen atoms on other water molecules (in a similar way to the north and south poles of magnets)and this attraction is called a hydrogen bond.

Water properties

When water is heated, the molecules get more energy and speed, so the water molecules will escapefrom the liquid (this is water vapour). At 100 C, under atmospheric pressure, the energy is such aswater is no longer a liquid and it becomes a gas. In contrast, when water is cooled, the watermolecules will have less and less energy to move around, so bonds between one another are moreimportant that kinetic energy. The water molecules will be no longer free to move, so the water will

become solid. This is ice.

When water is liquid, water molecules always distribute so that their energy is equal in allcompartments that can communicate.In particular, when a drop of colored syrup is put in a glass of water, molecular motions will dispersethe syrup molecules in water, so that finally the water concentration is the same in all the glass.This diffusion phenomenon is important in gherkins production: when the plant samples are put firstinto salt, their water molecules move so that they go out of the plant tissue, and dissolve salt. Thisprocess is called osmosis.For example, if a strong coffee is added to a glass of water, the large number of water molecules inthe water will redistribute themselves evenly in the coffee, and the resulting solution will be an evenlydistributed mixture of coffee and water.or a much weaker coffee!

Solubility in water

Molecules are often classified by how they interact with water. Molecules that like to interact with waterare called hydrophilic, or water-loving. These molecules like to interact with water because, likewater, they are charged, so are attracted to the water molecules and can form links with them. This iswhat occurs when a substance dissolves. For example, when salt is added to water, it will dissolve.Salt is made of Na+ ions and Cl- ions held together, and in the presence of water, these ions willseparate and be attracted to and form bonds with the water molecules. Because the salt has beensplit into individual Na+ and Cl- ions, it is no longer visible to the naked eye because the separatedions are too small. However, if the mixture is heated to evaporate all the water, only the Na+ and Cl-ions will be left, which will then be free to rejoin with each other to form the original salt.The boiling point of water can be changed if substances added to it, depending on the relative boilingpoint of the added substances. For example, water will boil at a slightly higher temperature if salt ispresent because salt has a much higher boiling point than water. The more salt contained in the water,

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the higher the boiling point of the solution. However, if alcohol, which has a boiling point lower thanthat of pure water, is added to water, it will decrease the boiling point of the final mixture.The freezing point of water is also changed by adding substances to it, however all substances addedto water will act to decrease its freezing point. This is because any other substance present will act toget in the way of the water molecules as they try to make bonds with each other to form ice, sotemperatures lower than freezing are required to freeze the water.This explains why salt is often added to ice on the roads to encourage it to melt it causes the ice will

melt at a lower temperature.

Non-solubility in water

Many molecules however do not interact with water. They are called hydrophobic, or water-hating.These molecules are neutral, and do not interact with water because they are not attracted to thewater molecules, so they will not preferentially mix.For example, oil does not mix with water. This can easily be seen if oil is added to water, the oil (whichis less dense) will float to the top and not mix with the water. The hydrophobic oil molecules will sticktogether with special bonds (called hydrophobic interactions), and the hydrophilic water molecules willstick together due to the hydrogen bonds.If the mixture is vigorously shaken, the force of shaking will break the two liquids into smaller droplets,and the tiny droplets of oil will temporarily disperse in the water. However, as soon as this force isstopped, the oil droplets are free to move around and combine, and as the droplets become biggeragain the two layers separate out.

In order for a mixture of oil and water to stay stably dispersed, a special sort of molecule needs to beadded. These molecules are called surfactant molecules. They possess a hydrophilic head and ahydrophobic tail, or in simpler terms they have one end of their molecule that is soluble in water, andone end that is soluble in oil. The principal is that these molecules surround the tiny oil droplets bymaking contact with their hydrophobic parts, leaving their hydrophilic parts to contact the water partand keep the fat droplets very dispersed. Many foods contain such tensioactive molecules which arecommonly used to stabilise oil/water mixtures.

Role in food

Water is the most abundant molecule in nature. Most foods contain primarily water (eg. vegetablescontain very high amounts of water, as do meat and fish, and diary products like milk and eggs). Manyof these substances therefore lose mass if they are cooked at high temperatures, because the watercontained will evaporate, making the cooked mass less than the raw mass.In food, water is often perceived as tenderness. Hard cheese contains much less water than soft

cheese, and is therefore less tender. A rare steak (where little liquid has been evaporated during theshort cooking time) is much more tender than a well-done steak (where water evaporation is muchmore significant).