Sometimes a term might be so familiar to you that you use it all the time, whereas another person, who doesn’t know that term, has no idea what you’re talking about. When talking about science that can be quite a common problem. And even though we try to keep the level of our posts such that they’re also accessible to non-scientists, there will be moments when there’s just too many terms in one go. That’s why we’re dedicating quite a lot of posts to the basics of food science. And this is another one of them, discussing the terms hydrophilic and hydrophobic, along the way introducing polarity and electronegativity.
Importance of water in food
In food water is pretty common, although that’s an understatement, it’s very very common. And the interaction of molecules with water is thus something we often describe. Some molecules might dissolve in water, whereas others don’t, some components love to sit in water, whereas other don’t. In order to describe these behaviours with water proper terminology is required. The most basic relationship with water especially, is a molecule water loving or water hating. This is where hydrophilic and hydrophobic come in.
Water, electronegativity and polarity
Whether or not a molecule likes to sit in water has a lot to do with the polarity of a molecule. Water is a so-called polar molecule.
Atoms all have electrons ‘floating’ around the core of their center. These electrons are negatively charged. The center of the atom will pull the electrons towards itself. The strength with which an atom does this differs per atom and is expressed using the term electronegativity. Oxygen and nitrogen are examples of atoms that pull quite hard on atoms, they have a higher electronegativity value.
When atoms connect to one another they won’t all pull as strongly on the atoms around them. They won’t only pull onto their own electrons, but all electrons in their surrounding. Thus an atom with a high electronegativity will pull all electrons around it just a little more towards them. As a result the cloud of electrons can be shifted a little and not be spread evenly. If this occurs a molecule is called polar.
In order for the cloud to be moved a little off-center the different forces of an atom shouldn’t cancel each other out. For instance, if there is an atom that pulls strongly on both the left and right side of a molecule, with the rest being even, those two forces will probably cancel each other out. However, if both of these sit on one side, they won’t.
Water is polar
Water is an example of such a polar molecule. Water is made up of one oxygen atom and two hydrogen atoms. Oxygen pulls onto the electrons a little harder than the hydrogen atoms does. What’s more, the water molecule is not fully symmetric. In other words the forces don’t cancel out. In the picture below that’s been illustrated, since the hydrogen atoms are pointed slightly downwards the upper side has slightly more electrons than the lower side. This is indicated using the Greek delta symbol and the type of charge on each side.
When a molecule likes to sit in water, it is called hydrophilic (= water loving). Hydrophilic molecules tend to be polar molecules, just like water, but they can also be ions (e.g. dissolved salts). Most of them have in common that they have some sort of non-zero electric charge. These charges can interact with one another.
In food common examples of hydrophilic molecules in food are sugars (they have oxygen group which make them polar and they tend to be asymmetric) but also salt or certain amino acids.
On the contrary, when a molecule does not want to sit in water, it is called hydrophobic. If a hydrophobic molecule is mixed with water the two will tend to separate and form two individual phases.
Hydrophobic molecules are generally non polar molecules, they do not have this slightly uneven charge distribution. The most common example of hydrophobic molecules in foods are fats and oils.
Hydrophilic and hydrophobic in one
Some molecules have both hydrophilic as well as hydrophobic sections in their structure. This is especially common for proteins, but is also a common trait for so-called emulsifiers. Emulsifiers can be used to mix oil and water and in this case they do that by sitting both in the water and oil phase with a different part of the molecule. That way, they keep the two together and prevent them from separating! Egg yolk in mayonnaise is a great example of this.