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  • Why You Have to Shake Italian Salad Dressing: Where Food Meets Science

    We all know that when you pick up a bottle of Italian Salad dressing, you have to shake it vigorously in order to get the dressing to emulsify before you pour it over your salad. What you might not realize is that there are chemical properties inherent to the two main ingredients, water and oil, which prevent them from continual unity.

    Hydrophobic interactions- that’s why these two ingredients do not mix. Although it appears to be a big, scary word, the concept is relatively simple. Let’s take a brief trip down Memory Lane, or perhaps down Nightmare Lane depending on your grades, back to your high school chemistry class wherein you learned a principle chemistry phrase, like dissolves like.

    Water, H2O, is comprised of two hydrogen atoms and one oxygen atom. Hydrogen atoms have a +1 charge whereas oxygen atoms have a -2 charge. Oh yes, I’m about to reference the periodic table of elements here. Feel free to click this link if you’d like a refresher on ionic charge.

    Ultimately, this means that water is a polar molecule. This also means that water will bond with other polar molecules- hence the phrase, like dissolves like.

    Simply illustrated, salt dissolves in water because both salt and water are polar molecules. When put together, water molecules bond with salt molecules and as a result, the salt crystal begins to dissolve. Any substance that dissolves in water is referred to as hydrophilic, meaning water-loving.

    Lipids, on the other hand, are nonpolar. Therefore, they are hydrophobic, or water-fearing. As a result, water and oil do not bond. Sure, you can shake the bottle of salad dressing and see that the two have “mixed” momentarily but the oil will not dissolve in the water. Rather, the two ingredients will separate after some time.

    So what accounts for the momentary combination of the water and oil in your standard bottle of Italian salad dressing? The Second Law of Thermodynamics. This law essentially states that all things tend from order to disorder.

    My biochemistry professor once likened the inability for water and oil to exist cohesively to that of White Migration. Think of water as the white people of various European decent and the oil as the minority race.

    We all live in a bottle of salad dressing. On one side, you have the water molecules and on the other side, you have the oil molecules. Next, we all try to shake things up and suddenly, the water and oil molecules are living together! For a brief moment in time, all of the water molecules are neatly arranged around the oil molecules. Things appear to be in order.

    Initially, the water molecules say,

                          Yeah, we’re super open-minded! We’re totally ok with this! We can live in harmony with other types of molecules. No problems here, everything is peachy keen and dandy…                  

    But as time goes on, the water molecules begin to realize that maybe they prefer to live in neighborhoods filled with molecules of similar origin. Maybe they like seeing other water molecules around and could do with seeing less oil molecules in their neighborhood.

    Eventually, a few water molecules move out of their oil-filled neighborhood. And once a few start to leave, then other water molecules follow. The next thing you know, all of the water molecules are on one side of the bottle and all of the oil molecules are on the other side of the bottle. The ordered life of water molecules neatly situated around the oil molecules was not preferable. Rather, the disordered existence of water on one side of the bottle and oil on the other side is more desirable for the water molecules.

    And that’s why you will always have to shake that bottle of Italian Salad Dressing before pouring it on your salad.

    #chemistrywin #thermodynamics #entropy #chaos #foodscience

    Not understanding the relevance of this post to dietetics? Stay tuned as I next discuss the difference between water soluble and fat soluble vitamins.



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