Summer is full on in the Northern Hemisphere, bringing us at ACD/Labs’ HQ lots of sunshine, warm temperatures, and outside activities. Summer time also has many ties to chemistry – believe it or not – and thus, we present “The Science of Summer”, a chemistry exploration of some of the greatest summer traditions. Many thanks to Compound Interest, Sciencing, and The Scientific American for providing very helpful background reading, and note that all chemical structures were drawn using ACD/ChemSketch.
A beloved seasonal activity, the backyard cookout combines some of our favorite summer things: barbecues, ice cream, and swimming pools. Excuse the pun, but let’s dive in!
The key to any outdoor cookout, a barbecued meal is quintessential summer eating. The characteristic smoky fragrances and flavors of barbecue arise from burning charcoal to produce phenolic compounds like syringol, which contributes to the smoky scent, and guaiacol, which influences its smoky taste.
In addition to its smoky essence, barbecued meat is also known for its sweet and tangy flavor, resulting from the Maillard reaction. Specifically, amino acids and sugar in the meat are converted into a range of products depending on the acidity, temperature, and type of protein prepared. These classes of compounds include alkylpyridines, furans, furanones, pyrazines, and thiophenes.
Notably, grilling meat is also known to produce carcinogenic byproducts such as polyaromatic hydrocarbons (PAHs) and heterocyclic amines (HCAs). A variety of PAHs are generated from the combustion of meat drippings following direct contact with barbecue coals/burners, including known human carcinogens like benzo(a)pyrene. HCAs are formed in cooking meat, and are particularly concentrated in charred regions. Thankfully, research has demonstrated that marinating meat in beer prior to cooking can appreciably decrease the levels of HCAs, so make sure you have some extra pints on hand for your next cookout!
A classic summer cookout calls for delicious desserts, and nothing beats the heat quite like ice cream. Combining air, fat globules, ice crystals, and liquid syrup, ice cream is a colloid mixture—a solution containing suspensions of tiny insoluble particles. Fat globules constitute ~5% of ice cream volume and are largely responsible for its creaminess. They are formed when liquid triglyceride fat is surrounded by membranes of milk proteins and emulsifier molecules. Clusters of these globules help stabilize the air bubbles in ice cream, which make up the majority of mixture by volume (~ 50%).
The remaining 45% of ice cream volume is composed of ice crystals (~30%) and liquid syrup (~15%). During the freezing process most of the water in ice cream is converted into ice, but in addition to conferring the trademark sweetness of ice cream, the sugar in syrup also serves to lower the freezing point of the mixture. This reduces the amount of ice crystals and contributes to the overall creamy texture of ice cream. Additionally, artificial flavors are often used, such as vanillin for vanilla, as well as flavor enhancers like skatole in chocolate, and artificial coloring agents like the family of anthocyanins.
A perfect cookout always includes a convenient way to cool off and, as long as 30 minutes has passed since chowing down on barbecue and ice cream ;), the swimming pool reigns supreme. The primary chemical concern for swimming pools is preventing exposure to water-borne pathogens, so they are typically treated with chlorinating agents. These compounds react with water to form hypochlorous acid, which is a key bactericide. Hypochlorite salts are typically used as chlorinating agents, replacing chlorine gas based on hazards associated with its use and storage. Calcium chloride is frequently also added to pool water to prevent dissolution of calcium sulfate from the grouting between pool tiles.
No summer should go by without at least one visit to the beach, and chemistry is evident at the seaside as well. Whether collecting beautiful seashells along the seashore, or simply lathering on the sunscreen and/or sticking to the shade, chemistry is always at work.
What may seem to be standard mementos from a trip to the beach, seashells are in fact marvels of evolutionary biochemistry. They principally contain calcium and a small quantity of protein (at a maximum of 2%), and consist of three distinct layers: an uncalcified outer proteinaceous periosteum (similar in composition to human fingernails), a calcified prismatic middle layer, and an inner calcified layer of pearly nacre.
Mollusks grow their shells at the margins, adding material primarily at the central opening. First a natural strengthening polymer composed of protein and chitin called conchiolin is formed, then the highly prismatic middle layer, followed by the nacre. Notably, nacre is naturally iridescent due to its crystal aragonite platelets that function to disperse visible light similar to a diffraction grating. Mollusks are able to repair any damage to their shells by producing calcium carbonate/protein secretions that reinforce compromised regions.
Finally, the variability in seashell color across the globe is driven by the diverse diets of mollusks. Those in tropical regions have access to a wider variety of food sources, and consequentially greater availability of various pigments that contribute to their more colorful shells. While mollusks residing in colder climes typically have more limited food choices, which leads to darker colored, simpler shells.
Though the bright sunshine and warmer weather is one of the best aspects of summertime, it’s important to apply sunscreen to avoid sunburns and skin damage. Sunscreen represents a combination of specific inorganic and organic chemicals that prevent harm from solar UV radiation. The inorganic chemicals, such as titanium oxide and zinc oxide, both absorb and scatter UV light, while the organic constituents absorb UV radiation only, with their specific chemical structure determining whether they absorb UVA, UVB, or both.
As alluded to, solar radiation is classified into three types. UVA (wavelength 320-400 nm) accounts for 95% of the UV radiation that reaches Earth’s surface. It also exhibits the greatest skin penetration potential, causing indirect DNA damage that may lead to skin cancer. UVB (wavelength 290-320 nm) represents the remaining 5% of UV radiation that reaches Earth’s surface. UVB exposure is also a major contributor to skin cancer through its ability to cause direct DNA damage. UVC (wavelength 100-290 nm) does not reach the Earth’s surface, as it is completely filtered out by the ozone layer.
Organic UVA blockers in sunscreen include avobenzone and menthyl anthranilate, while UVB blockers include octyl methoxycinnamate and homosalate. Whereas oxybenzone and sulisobenzone are examples of organic sunscreen ingredients that possess the ability to block both UVA and UVB radiation. In short, don’t travel to the beach without some trusty sunblock!
Thanks for reading our Science of Summer post, and whether you prefer barbeques, swimming pools, beach vacations, or camping trips, now’s the time to go out and experience some summertime chemistry yourself!