These molecules are neutral, and do not interact with water because they are not attracted to the
water 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 (which
is less dense) will float to the top and not mix with the water. The hydrophobic oil molecules will “stick
together” with special bonds (called hydrophobic interactions), and the hydrophilic water molecules will
“stick 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 is
stopped, the oil droplets are free to move around and combine, and as the droplets become bigger
again 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 be
added. These molecules are called tensioactive molecules. They possess a hydrophilic head and a
hydrophobic tail, or in simpler terms they have one end of their molecule that is soluble in water, and
one end that is soluble in oil. The principal is that these molecules surround the tiny oil droplets by
making contact with their hydrophobic parts, leaving their hydrophilic parts to contact the water part
and keep the fat droplets very dispersed. Many foods contain such tensioactive molecules which are
commonly used to stabilise oil/water mixtures.
Role in food:
Water is the most abundant molecule in nature. Most foods contain primarily water (eg. vegetables
contain very high amounts of water, as do meat and fish, and diary products like milk and eggs). Many
of these substances therefore lose mass if they are cooked at high temperatures, because the water
contained 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 the