Welcome to another post about vegan food chemistry. Today, we will learn about why water and oil don’t mix and how we can use the knowledge we gained in the previous post on pH and tofu to flip proteins inside out and force oil and water to mix into yummy mayonnaise and meringues. We will also learn why water is wet, why ice floats, and why these questions are actually not weird. We’ll also see how 108° is essential to life as we know it. Vegan food chemistry sure is involved in a lot of things.
But first, can you tell what is going on in this video?
Hydrophobicity
If you have ever made a vinaigrette for your salad or if you ever touched something oily and tried to wash your hands with nothing but water, you know that oil and water don’t mix. This is what you see in the short video above: I mixed oil and vinegar vigorously to create tiny vinegar (basically water) droplets in oil. As time goes by, vinegar droplets appear, merge and grow as the vinegar leaves the oil. Usually people say that oil is afraid of water, it is hydrophobic. This causes a problem in the kitchen – how can we mix the two? (The answer to why we would want to is that we can create rich, creamy foods like cream and mayonaisse.)
I think the separation of oil and water is something everyone has experienced before but why is oil hydrophobic?
Water really likes water
Instead of describing oil as hydrophobic and placing the blame on the metaphorical shoulders of the oil, we should take a look at the real culprit: water. The fact is that il doesn’t really care about water and is certainly not afraid. Instead, the truth is that the water molecules, H2O, love each other very, very much, and want to be together as much as possible. As a result, they crowd out the oil and force it away. This is a very anthropomorphic* description (see below). What really happens is that the system (your salad dressing jar in this case) minimizes its total energy by bringing as many water molecules into contact with each other as possible. This has the result of minimizing the water surface area. If you mix oil and water in a glass, you will get a clean separation with as little water surface as possible. If you throw water in the air, the water will create spheres (like raindrops) which is the smallest area possible.
* Anthropomorphism is a dangerous habit in science where you give different phenomena human feelings and desires, using words like “want”, “love”, and “care”. It is useful to describe what happens in understandable terms but one needs to keep a distance and not start thinking of water molecules as though they actually have desires of their own.
Why does water like water?
Above we saw that oil is not really afraid of water but the water molecules are so attracted to each other that the system (jar of salad dressing) can minimize its energy by separating oil and water from each other. The next questions then becomes – why do water molecules like each other so much? (Or in non-anthropomorphic terms: why do water molecules feel such strong attraction to each other?)
Why can’t you trust atoms?
They make up everything
The answer is that water molecules are polar. As you know, everything is made up of the 100+ atoms on the periodic table. The difference between each element (type of atom) is the number of positively charged protons in the nucleus and the number of surrounding electrons. The difference in proton number means that some atoms are better at attracting electrons than others. We call this electronegativity [1].
Atoms with a large electronegativity are good at attracting electrons from atoms that are less electronegative. When two atoms meet, they play tug of war with the electrons to see who gets them. Sometimes, the difference is so great that one atom completely steals an electron from another atom in which case you get ions. This is for instance true for table salt, where chlorine is so much stronger than sodium that it tears its electron clean of and we get Na+ and Cl-. (Since opposites attract, these two ions will build beautiful crystals together until dropped in a soup.)
Water is polar
Most of the time though, the difference in electronegativity is small and one atom does not quite manage to steal an electron from its neighbour. This is the case for carbon dioxide, as shown in the figure below. The two oxygen atoms flanking the lone carbon atom pull on the electrons so much that the electrons spend most of their time around the oxygen atoms.
In the case of water, the electrons spend most of their time around the lone oxygen atom rather than the two hydrogen atoms. The difference between water and carbon dioxide is that, while carbon dioxide is straight, the water molecule is bent. The slight 108° bend of the water molecule makes all the difference in the world and cannot be understated. It is the reason life exists at all. This slight bend creates one slightly positive side and one slightly negative side. This is called a dipole and is the reason water molecules attract each other so strongly. For carbon dioxide, the molecule is straight and the electron imbalance cancels out; carbon dioxide molecules do not have a dipole and the molecules feel a very minimal attraction to each other.
The difference in attraction can be seen in the two molecules’ different boiling points: water boils at +100°C and is hence wet on our planet. Liquid carbon dioxide doesn’t even exist (under normal air pressure): it goes from solid (dry ice) to gas at -78.5°C (going from solid to gas is called sublimation rather than boiling). (The melting and boiling points of a substance is determined largely by the attraction the molecules feel toward each other.)
A flowing mesh
The charge imbalance in the water molecule causes water to be polar. The negative oxygen side of the water molecule will be attracted to the positively charged hydrogen atoms of nearby water molecule. The dipole in the water molecule is stronger than in most molecules which causes the attraction between water molecules to be very strong. It is in fact so strong and important that we have given it a special name, hydrogen bond.
In a glass of water, the water molecules will arrange themselves to have one negative oxygen end interact with two hydrogen on another molecule. The result is a very strong mesh or network. If you add energy in the form heat (in other words, you boil the water), the water molecules will start moving around and the hydrogen bonds will begin to break. The individual water molecules will lose their attraction to each other and fly away just like the non-polar carbon dioxide – the water boils and evaporates.
Hydrogen bonds are not unique to water molecules but occur any time you have a weak hydrogen bonded to a strong atom like oxygen or fluoride. We also encounter hydrogen bonds in the DNA molecule where it binds the two spiraling backbones to each other. We also saw them in the previous post about protein folding where I compared it to Velcro.
Ice floats, or does it?
If you instead remove energy (put the water in the freezer), the water molecules become free to arrange themselves as they wish without the molecular movement (heat) interfering with their preferred bonding. They will create beautiful ice crystals where most water molecules have that perfect arrangement where they bind to four other water molecules. The resulting ice crystals expand and the frozen water is larger than the liquid water. In other words, the density of ice is lower than the density of water which causes water ice to float on liquid water. For most other substances, the solid form is heavier than the liquid form: solid iron sinks in molten iron, solid gold sinks in molten gold, frozen canola oil sinks in liquid canola oil. Link to video.
How to solve the oil problem
See what I did there? Solve? Solution? Applies to problems and liquids…
So far on today’s journey into the realm of vegan food chemistry, we have learned that oil is not really hydrophobic but that water pushes out the oil to increase the number of hydrogen bonds between water molecules. To get around this issue, we can add molecules that can interact with both oil and water at the same time [2]. These molecules are said to be amphiphilic – they “love both” [oil and water]. Amphiphilic molecules have one end that is hydrophilic (water-loving) and one that is lipophilic (fat-loving). Since they help reduce the cost of creating surfaces between oil and water, they are also called surfactants. I hope you have come into contact with these before as they are the component in soap that makes soap work.
Learning from the past
In a previous post on vegan food chemistry, we saw that we could use low pH to destroy proteins and cause them to expose hydrophobic patches while maintaining parts of their hydrophilic surface. If you think this sounds like exactly what we need to create a mixture of oil and water you are correct. By destroying proteins we can build a bridge between oil and water.
Why do we want to do create a mix of oil and water though? Because it can be very tasty. Depending on how you make it, you will either end up with plant-based dairy (almond milk, coconut cream, soy yoghurt…), mayonnaise or meringues. Dairy consists of small oil droplets dispersed (chem-speak for spread out) in water, stabilized by either proteins or other surfactants. Making your own dairy product is very straight forward; soak nuts or almonds in water and blend them [3]. Optional: strain out the solid pieces to create your own almond flour. Mayonnaise on the other hand is small water droplets dispersed in oil while meringues are air bubbles trapped in a protein mesh.
Make your own mayo or meringues with chickpeas
You can make your own mayonnaise [4]or meringues with aquafaba [5]–[7] – the slimy liquid you get when you buy or boil chick peas. It is a protein-rich water solution. If you add a bit of acid (e.g. lemon juice or tartaric acid), the proteins will start to denature – just like we saw in the tofu post. The denatured proteins will become amphiphilic and be able to act as a mediator between oil/air and water and allow water to do their precious hydrogen bonding without forcing the oil/air away.
No hydrogen bonding? You’re uninvited!
Every water molecule ever
To make mayonaisse, you simply mix aquafaba with an acid, add some seasoning and whisk in oil. Mayonaisse is so non-fuzzy that you can use many other surfactant sources as well, such as soy milk [8].
Making meringues follows the same formula: trap air in water by adding surfactants/amphiphilic molecules as a bridge. A standard recipe is aquafaba with an acid (e.g. tartaric acid) and sugar. Whisk in lots of air. The sugar improves flavor and helps trap air by increasing viscosity. You can increase the viscosity further by adding large molecules like xanthan gum [7] but that’s a story for a future post. After you trap the air in the liquid, you simply dehydrate the foam in the oven for an hour and you get a crunchy solid foam.
The similarity in the recipes for mayonnaise and meringues explains why you should not use a plastic bowl for making meringues: the plastic bowl often has oil clinging to it which will compete with the air to be incorporated. You will make a mayonnaise – meringue hybrid with some oil instead of air. (If this sounds appealing, by all means, make it.)
Conclusion
I hope you enjoyed reading this piece on vegan food chemistry- Today, we learned that oil is not really hydrophobic but that water really likes itself and expels the oil from the party, and we learned how to overcome this water self-love and create cream, mayonnaise, and meringues.
And now you know why the vinegar ‘falls’ out of solution in the video below. Can you see how the vinegar falls small drops that grow? In the next science post, I will write about how this works and how it is related to chocolate, champagne, ice cream and electronics. If you don’t want to miss it, subscribe using this link and I’ll send you an email when my next post is out.
Cheers!
- [1]P. Flowers, K. Theopold, R. Langley, W. R. Robinson, and et al., “Chemistry 2e, Chapter 7,” openstax, 14-Feb-2019. [Online]. Available: https://openstax.org/books/chemistry-2e/pages/7-2-covalent-bonding. [Accessed: 19-Jul-2020]
- [2]P. Flowers, K. Theopold, R. Langley, W. R. Robinson, and et al., “Chemistry 2e, chapter 11,” openstax, 14-Feb-2019. [Online]. Available: https://openstax.org/books/chemistry-2e/pages/11-5-colloids. [Accessed: 19-Jul-2020]
- [3]M. Baker, “How to Make Almond Milk,” Minimalist Baker. [Online]. Available: https://minimalistbaker.com/how-to-make-almond-milk/. [Accessed: 19-Jul-2020]
- [4]M. Baker, “Easy Vegan Aquafaba Mayo,” Minimalist Baker. [Online]. Available: https://minimalistbaker.com/easy-vegan-mayo-with-aquafaba/. [Accessed: 19-Jul-2020]
- [5]A., “Vegan Meringue (Airy and Perfect!),” Loving it vegan, 19-Apr-2018. [Online]. Available: https://lovingitvegan.com/vegan-meringue/#tasty-recipes-7358. [Accessed: 19-Jul-2020]
- [6]S. Turnbull, “Vegan Meringue Cookies,” It doesn’t taste like chicken, 05-Jul-2015. [Online]. Available: https://itdoesnttastelikechicken.com/vegan-meringue-cookies/#wprm-recipe-container-13761. [Accessed: 19-Jul-2020]
- [7]M., “Vegan Meringue (How to Make it Three Ways),” Namely Marly, 18-Nov-2015. [Online]. Available: https://namelymarly.com/how-to-make-vegan-meringue-three-ways/. [Accessed: 19-Jul-2020]
- [8]I., “Vegan Mayonnaise,” Simple vegan blog, 22-Dec-2019. [Online]. Available: https://simpleveganblog.com/vegan-mayonnaise/. [Accessed: 19-Jul-2020]
