Written by: Emily L. Eberhardt
Edited by: Olivia Pifer Alge, Jessica Li, Jeremy Chen, and Ryan Schildcrout
Illustrated by: Paola Medina-Cabrera
“Vaccines are full of chemicals! Wait… so are apples?” While scrolling through social media the other day, I came across an old meme where a cheeky individual attempts to pass off the chemical makeup of an apple as that of a vaccine. While looking at the list of hyphenated, long-winded names, I fact-checked the chemical composition of an apple, expecting to find a simple source. Instead, I discovered decades of research dedicated to thousands of species of apples and their chemical complexities. Surprisingly, the study of apples happens to be a (don’t mind the pun) fruitful field of study.
Before delving into what’s under the peel, let’s discuss how apples are grown. Growing apples is not as simple as digging a hole, dropping in a seed, and providing water and sunlight. Only 1 in 80,000 apple seeds will produce a tree that meets growth quality standards. Across the world, there are thousands of different cultivars (varieties derived from consistent horticultural growth) dependent on environmental factors that affect the apple’s flavor, texture, and shape. In the U.S., only a few hundred cultivars are actively produced, with the most popular being Gala, Red Delicious, and Honeycrisp. Apples are grown through grafting––a practice that dates back to Ancient Greece (at least 300 B.C.)––that reliably produces plump and appealing fruit. Grafting involves joining tree cuttings with a rootstock (base of a tree) to produce trees of the desired size and growth speed. Apples with desirable characteristics are plucked and distributed to grocery stores, offering you the choice between a sour Granny Smith or a sweet Fuji. With that in mind, let’s sink our teeth into the flesh of the apple and discuss what is inside.
Seed to peel: What is an apple?
Apples contain many nutritious compounds crucial for a healthy human diet. Some of these compounds include phytochemicals––chemicals derived from plants––including various metabolites necessary to promote and maintain plant health. But before we get into the core of how scientists have determined what is in an apple, let’s cover the basics––from seed to peel.
To begin, apple seeds are primarily made up of oil, protein, and fiber (~52–84%). The predominant oils include fatty acids such as linoleic and oleic acids, and the most abundant proteins are storage proteins, which provide nutritional value to developing plants. The fiber in the seeds is mostly indigestible, in the form of cellulose. Additionally, apple seeds contain sugars that can react with enzymes to produce cyanide. But don’t worry! You would have to eat anywhere from 150 to several thousand apple seeds (crushed, not whole) to be at risk for cyanide poisoning, and apples typically contain only five to eight seeds.
Housing the seeds is the core, a tough membranous layer of cells rich in structural molecules. Next, the juicy bite of an apple comes to fruition with the flesh, as the flesh of an apple is mostly water (~85%). Additionally, the flesh of the apple contains trace vitamins (A, B₃, B₉, and C, etc.) and minerals (iron, magnesium, calcium, etc.) and <5% fiber (primarily digestible pectin) which provide nutritional benefits and support bodily functions. The flesh also contains flavor elements such as sugars and malic acid, providing a sweet or sour bite.
Finally, apple peels typically contain more vitamins and minerals and twice as much fiber as the flesh. The peel also contains the highest proportion of phytochemicals, including phenolics, flavonoids, tannins, and other bioactive compounds. A medium-sized apple with the peel contains ~20% more magnesium, ~75% more Vitamin E, and ~75% more Vitamin K than without. This might be a good reason to put down the peeler!
All together, each part of an apple has a unique chemical composition. Adding to the complexity, environmental conditions can affect the chemical makeup. Time spent in the shade or sun, time to harvest, or storage time can affect the color, shape, or metabolite content.
A lot to swallow: Digesting the study of apples
If an apple is water, fiber, and a bunch of different phytochemicals, what are all those chemicals from the meme? Well, the world around us is a chemical hodgepodge of a variety of matter, substances, and mixtures. While the word “chemical” often invokes a negative connotation, everything––all things natural and artificial––is made up of chemicals.
Complicated naming conventions were implemented to describe chemical structures (and what they look like) internationally. Describing chemicals based on their structure allows scientists to identify unique compounds. For example, we know common chemicals like “β-D-arabino-hex-2-ulofuranosyl α-D-gluco-hexopyranoside” and “2-(acetyloxy)benzoic acid” as sucrose (sugar) and aspirin, respectively. So, understanding what an apple is “made of” is the first step to demystifying chemical complexity. Scientists can use a combination of laboratory techniques to understand the chemicals in anything (and everything!). As I type this sentence, I tap computer keys made from acrylonitrile butadiene styrene and sip a mixture of chlorogenic acids, 1,3,7-trimethylxanthine, and dihydrogen monoxide (coffee).
So, how do scientists identify what is present in an apple? Well, apples must be broken down into their chemical building blocks. The first step is extraction, where compounds of interest are separated from the rest of the fruit using solvents (chemicals used for dissolving substances). Extraction techniques are always evolving, becoming more eco-friendly and less cost and time prohibitive. These methods incorporate low-frequency wavelength assistance mechanisms (microwave and radio waves), pressurization, or electric field stimulation (zapping with electricity) to increase output.
After preliminary screening for total phytochemical content, the extract is further characterized using separation techniques. Phytochemicals are separated depending on physical characteristics such as size, shape, or stickiness. Then, using an instrument called a mass spectrometer, the extract is converted into ions (charged particles) that hit a detector. The detector communicates with a computer to generate a graph that can be interpreted to determine the individual compounds present in the extract. After identifying its chemical composition, in vitro experiments (studies done in a petri dish) can be used to determine the effects of different compounds at the cellular level. To date, scientists have identified thousands of phytochemicals!
Picking out chemicals in our daily lives
Beyond the peel, apples present us with much to digest. Something as simple as an apple is quite chemically complex, containing a variety of compounds from seed to peel. Regardless, complicated chemical names are arbitrary, functioning only to indicate differences in the structure or properties of any molecule, allowing for easy identification by scientists. Think about how you ask someone to pass you the “salt,” not the “sugar,” two substances that look remarkably similar, but are chemically distinct. By studying the chemical makeup of various items in the world around us, scientists can determine which compounds are beneficial, even helping to prevent or reduce disease risk. This also allows consumers to be able to choose products that support a healthy lifestyle, giving credibility to eating an apple a day!
Emily is a researcher, artist, and scicommer. She currently conducts research in pediatric pulmonology at the University of Michigan. As a creative, she makes “bioart,” a fusion of biology and art. Her portfolio can be found on her website: www.emilyleberhardt.com
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