Pumpkin Spice and Everything Nice

The Wonderful World of Flavor Chemistry

Artificial flavors aren’t always as scary as they’re made out to be

Forget the autumnal equinox; Americans have come to associate the beginning of fall with the annual roll-out of all things pumpkin spice. Even if you’re like me and find Starbucks’ famous PSLs too cloying for your taste, you probably enjoy the aroma of pumpkin spice in some other form, whether that be in a homemade baked good, mulled wine, or a scented candle. Pumpkin spice’s popularity is inextricably linked to the holiday-time nostalgia that many people, particularly Westerners, associate with the aroma of a freshly baked pumpkin pie. Our perception of flavor is heavily influenced by memories of past experiences involving that flavor, such as where we consumed it, what we were doing, who we were with, even the color and appearance of the food. When that flavor is then isolated and mixed into your Starbucks latte, all of those warm fuzzy feelings associated with the pumpkin spice flavor come rushing back, resulting in a positive reaction to the taste of the drink. The psychology of pumpkin spice’s popularity is more broadly understood than the actual chemistry that goes into creating pumpkin spice flavoring. What exactly is in pumpkin spice flavoring that makes it taste so much like the real thing?

Secret (Pumpkin) Sauce: Volatile Compounds

Despite the negative connotations associated with PSLs for being “artificial”, they taste so much like the real thing because chemically, they pretty much are the real thing. The main flavor notes in a pumpkin pie, as well as many other autumnal treats, are cinnamon, nutmeg, clove, and allspice. These four spices are the same main flavor notes incorporated in industrially produced pumpkin spice flavor packets. Rather than use spices in their ground form, food industry-grade pumpkin spice flavor is created using nature-identical flavor molecules. “Nature-identical” molecules are compounds that are chemically identical to an ingredient found in nature, but are produced synthetically in a laboratory¹.

In their natural form, ingredients that create flavors and aromas aren’t just one compound; they’re composed of hundreds of chemicals. For example, take cinnamon. The cinnamon plant, like any other plant, is a conglomeration of hundreds of chemical compounds. Even hydrodistilled cinnamon bark oil isn’t just “cinnamon”; it’s composed of over 17 chemicals. But when we think of “cinnamon”, we don’t think of bark oil, or even the cinnamon plant as a whole. We think of the warm, rich flavor of cinnamon. In order to replicate that flavor, not all 17 chemical compounds of cinnamon bark oil are essential. Of the many chemicals found in the natural ingredient, only a couple contribute to the overall flavor or aroma of the ingredient². In the case of cinnamon, cinnamic aldehydes are the chemical compounds that give cinnamon its characteristic flavor and smell.

An original drawing by yours truly…currently accepting volunteers to sketch my horrible puns

Similarly, just a small handful of compounds make up the major flavor notes that we associate with the natural ingredients of nutmeg, cloves, and allspice. The cute little chemical structures of eugenol and sabinene (pictured below) are mainly responsible for the big flavors we know as nutmeg and cloves, respectively³.

Eugenol | Bacterial Inhibitor | MedChemExpress — Eugenol

Sabinene natural, 75% | 3387-41-5 — Sabinene

How can these little chemicals pack such a big punch when it comes to flavor? One thing that many flavor-contributing compounds have in common is that they are volatile compounds. Volatile compounds are compounds that vaporize easily⁴.

When we describe a person as “volatile”, we mean to say that they are unstable, or have a quality that rapidly and unpredictably changes. Volatile chemicals can be described in the same way; they have a characteristic that is unstable, which contributes to their ability to rapidly change from liquid to gaseous form. Take another look at the chemical structures of cinnamaldehyde, eugenol, and sabinene. The configuration of atoms and chemical bonds in a structure informs us of the chemical’s intermolecular forces, or forces holding a molecule together. Bonds between two hydrogen molecules are particularly strong.

What does hydrogen have to do with your PSL, you say? Bonds between hydrogen bonds are attractive forces that hold a molecule together. The fewer potential hydrogen bonds that a chemical can form, the less energy it takes to break those bonds and allow the molecule to vaporize. Chemicals that are easily vaporized, or emitted from a food matrix, in turn easily contribute to the major aroma notes of a food product.

Cooking is an Art, Baking is a Science

If pumpkin spice is a combination of cinnamon, nutmeg, cloves, and allspice, then why is it so difficult to create that specific PSL flavor at home? That’s because pumpkin spice flavor mimics the aroma of a baked pumpkin pie. A pumpkin pie doesn’t smell or taste nearly as good when it’s just uncooked batter sitting in a pan, as opposed to when it’s fresh out of the oven. Baking a pie is a chemical reaction that manipulates the chemical structure of food ingredients, releasing aromatic products in the process. Your kitchen is basically just one big science lab. Any type of heat-based cooking process, such as frying, baking, or broiling, physically changes the chemical composition of the ingredients being subjected to said cooking process. This increase in temperature alone is enough to vaporize volatile compounds found in food, and release the aroma of the heated ingredient. Additionally, baking a pumpkin pie is a chemical reaction that releases flavor notes that can’t be extracted from the raw ingredient as it’s found in nature. Baking a pie is a heat-facilitated reaction between the sugars and proteins of the pie ingredients. This reaction, known as a Maillard reaction⁵ in the chemistry world, releases products that give off aromas associated with caramelization or freshly baked bread.

Image source: https://www.chemistrylearner.com/chemical-reactions/maillard-reaction

The Maillard reaction is essentially the chemical process of caramelization. As an example, in order to create the flavor and smells associated with brown butter, butter and brown sugar need to be heated for a given amount of time as opposed to just being added in with the rest of the ingredients. Similarly, in order to truly capture the aroma of a freshly baked pumpkin pie, Maillard reaction products are just as critical to include in the pumpkin spice flavor packet as the core ingredients of cinnamon, nutmeg, and cloves.

Image source: https://www.mdpi.com/2072-6643/9/8/835/htm

Maillard reaction products can be synthesized in a lab and are added alongside cinnamon, nutmeg, cloves, and allspice, to food industry-grade pumpkin spice flavoring. In order to recreate a natural flavor in synthetic form, flavor chemists need to consider the high-level essence of the flavor. The incredibly nuanced profession of flavor chemist is just as creatively complex as it is technical.

Gas Chromatography

Flavor chemistry as we know it today is possible because of the 1952 invention of the gas chromatograph⁶, an instrument that separates compounds that can be vaporized without decomposing.

Chromatography is a general term that refers to the process of separating parts of a mixture. Specifically, gas chromatography is the process of converting a mixture into gaseous form, and then separating the components of that gaseous mixture. Because so many flavors and aromas come from volatile compounds (compounds that easily vaporize), gas chromatography is a great analytical tool to apply to the study and production of flavors and fragrances.

Chromatographs (machines that perform chromatographic separation) separate mixtures by forcing the mixture to move through a medium in which the components move at different speeds, due to different components binding more or less strongly to the medium. All types of chromatography have two types of phases, or parts that aid in separating mixtures; stationary phases and mobile phases. Stationary phases are materials, often resins or packed beads inside of a column, that attract different components at different rates. Mobile phases are gasses or liquids that flow through the instrument at the same time as the sample, and help carry the sample through the stationary phase. The balance between the stationary and mobile phases causes certain components to bind more strongly to, or move more slowly through, the stationary phase, which ultimately separates the components by causing components to come out of the column at different times.

In the case of gas chromatography, a mixture is injected into a hot sampler port. The high temperature of the port vaporizes any samples that aren’t already in gaseous form.

The mobile phase is an inactive carrier gas, like helium or nitrogen, that transports the gaseous sample into a column.

Inside the column is the stationary phase, usually coated with an adsorbant solid. As the carrier gas pushes the sample molecules through the column, different components of the mixture adsorb onto the stationary phase at different rates, eventually reaching the end of the column at different times.

At the end of the column is a detector which records the time and relative amount of each component after it is separated.

Image source: https://en.wikipedia.org/wiki/Gas_chromatography#/media/File:Gas_chromatograph-vector.svg

The detector creates a picture of the relative separation of components in a mixture. Is it an ugly picture that looks like a four year old just scribbled a bunch of lines on a paper? Yes, but properly resolved chromatograms (think “Instagram”; chromatograms are pictures of chromatographic data) can give us lots of valuable information. Below is a gas chromatogram of cinnamon essential oil. Each peak is a different component or molecule inside the larger mixture that is cinnamon essential oil. This is why flavor chemists focus on the handful of molecules that contribute to a food’s flavor, as opposed to recreating the entire thing; can you imagine synthesizing all 26 compounds in this chromatogram⁷?

Image source: https://ijbms.mums.ac.ir/article_8841.html

However, at the end of the day, chromatograms are just pictures. From images alone, there’s no way to know exactly what each peak and line represents. Once a mixture is separated, flavor chemists (as well as scientists in the many other fields that utilize gas chromatography) identify each separated component, most often through a mass spectrometer detector. Mass spectrometry is an incredibly powerful tool with mechanisms that would be incredibly difficult to describe without putting all of you readers to sleep. It also isn’t necessary to understand the physics of the inner workings of mass spectrometry in order to understand how mass spectrometer detectors work on a high level (fine print: physics is not the author’s strong suit).

After gas chromatography separates a mixture into its individual components, a connected mass spectrometer ionizes and fragments these separated components. The fragments can then be identified by their mass-to-charge ratio. Large mass spectral libraries containing mass-to-charge ratios of known compounds are used as references to identify the components in the analyzed mixture. (If you’re interested in learning more about the basics of mass spectrometry, Explain that Stuff has a great article to get you started: https://www.explainthatstuff.com/how-mass-spectrometers-work.html ).

The Nose Knows

There is one more detector that is commonly used for industry-grade identification of aroma-inducing compounds. The olfactory detector, when coupled with gas chromatography, forms the highly sophisticated analytical assay known as GC-O (gas chromatography-olfactory)⁸. Some say that the olfactory detector is even more sensitive and valuable than the strongest mass spectrometers.

The olfactory detector is literally just a human nose.

Sometimes the molecules that contribute to an appear in trace amounts that are below a GC-MS’s level of detection. This is when olfactory detectors, or professionally trained sniff-testers, use their super-smell to identify aroma components. In GC-O, a chamber with a sniffing port is connected between the output line of the gas chromatograph and a signal generator. The scientist stands at the sniffing port and puts their nose to work.

Once the trained flavorist identifies the aroma-attributing compound, that compound is diluted in incremental amounts and re-sniffed until the flavorist can no longer detect the smell⁹. The dilution at which the flavorist can no longer detect the aroma of the compound is valuable information that is used to determine the level at which to formulate that aroma-attributing compound into a product.

Synthetic Flavors Are the Spice of Life

This is just a high-level overview of the complexities of flavor chemistry. If it takes years of education and training to learn how to properly synthesize artificial flavors, is it worth all of that effort?

The short answer is yes, absolutely. Compared with their natural ingredient counterparts, synthetic flavors are significantly cheaper, easier, and more sustainable¹⁰ to formulate in large-scale productions.

If every time someone wanted a pumpkin spice latte, they went out and used naturally extracted cinnamon and vanilla, there would be a lot less of those ingredients left in the world, and therefore a lot less pumpkin spice flavored things for all of us to enjoy. Sourcing natural ingredients in this way is inefficient, expensive, and often unethical. It’s much cheaper and ecologically sustainable to chemically synthesize the top flavor notes of those natural spices.

Synthetically produced flavors are also much more soluble in water than their ground spice counterparts (cinnamon or nutmeg sprinkled atop your coffee often tends to sink to the bottom of the cup). This makes synthetic flavors easy to incorporate into a variety of food products. Ease of formulation combined with a highly regulated and quantitative synthesis allows for more consistent flavoring from batch to batch.

The next time you swing by Starbucks for your daily dose of seasonal caffeinated goodness, do so shamelessly, knowing that you’re supporting a marvel of modern and sustainable chemistry. The only shame in indulging in a PSL is knowing that a bunch of chemistry nerds can make a better cup of coffee than you.

Cited Sources