Sandpaper & Tea: The Science of Astringency

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A slightly unripe banana. A glass of Cabernet Sauvignon. The feeling of your mouth drying up as you take a sip from your cup of black tea. Astringency. An effect produced by polyphenols, metals, and acids, as well as dehydrating agents¹, the familiar puckering feeling is still at the forefront of scientific research. However, the question of its working mechanism—or rather, mechanisms—is still unsettled.

Contrary to intuition, astringency is actually not a taste, but rather a tactile perception². It’s similar to the effect experienced when tasting Sichuan peppercorns³. To understand this phenomenon, scientists blocked the taste nerves of human subjects and found that their “taste” of astringency was undiminished². Depending on the specific polyphenol (a category which encompasses tannins from wines and oak as well as catechins present in tea), the perception is different. Some polyphenols bind to proteins in the saliva, leading to the loss of saliva’s the lubricating effect¹. This is the cause behind the drying effect of astringent polyphenols, and is complimented by a sandpaper-like feeling created by the friction of the precipitated protein-polyphenol complexes⁴. However, there are polyphenols which do not bind to salivary proteins at all. For example, EGCG (the main polyphenol in tea) directly activates the trigeminal nerve, which is normally responsible for facial muscles². All this to say, we “feel” the tannins in a red wine more than we actually “taste” them.

As anyone that has ever tasted a very astringent wine can testify, the perception of astringency needs some time to fully develop (about 15 seconds) and lingers for far longer (about 5 minutes)⁵. To differentiate between two astringent beverages, a wine taster might be tempted to wash away the astringent perception with oil or milk, or by eating crackers. While this does work very effectively, the astringency of the next drink will now be under-experienced. The best solution, therefore, is to wait, and maybe drink some water⁶. Since astringency is very much about saliva, people with a higher saliva flow rate and a higher salivary protein concentration have a much weaker perception of astringency^7,8.

Astringency is influenced by many factors. Increasing the alcohol concentration of a cocktail decreases the intensity of astringency because of an increase in viscosity⁹. Likewise, an increase in temperature will lower the viscosity and thereby increase the intensity of the astringency⁹. Even your astringent source material can be highly variable, as we perceive different polyphenols with different astringency intensities even if they are at the same concentration¹⁰. On top of that, the polyphenols themselves are rarely at the same concentration. Oolong tea for example, has a lower polyphenol content when it’s cultivated at higher altitudes¹⁰.

Spearheaded by Dave Arnold from Booker and Dax¹¹, milk- or egg-washing astringent infusions—such as black tea vodka—have become reliably successful ways to modify astringency. In the case of milk-washing, the milk proteins (most notably casein) bind to the astringent polyphenols from tea¹². Once acid is added to the mixture, they begin to curdle, and can be filtered out together with the bound polyphenols. As noted above, polyphenols can also come from oak, such as in barrel-aged spirits, and can be removed by the same technique. This not only removes astringency from the cocktail, but also leaves something behind.

Whey proteins (mostly beta-lactoglobulin) also bind polyphenols, but they don’t curdle in the presence of acid. Therefore, they can act as foam stabilizers in milk-washed cocktails¹³. Apart from that, the milk used for washing contains far more than just protein: it also contains lactose and fats, which can impart some flavor to the final product¹⁴. Although cow milk got the whole “milk-washing” thing started, there is no inherent reason to limit the technique to that. In fact, Gabriella Mlynarczyk from the Birch restaurant in Los Angeles uses goat milk to wash spirits, which she found gave her the best results in terms of flavor and clarification speed¹⁴. On a more exotic note, Paul Bradley of Gold on 27 even tried to use camel milk in one of his cocktails¹⁴.

Apart from flavor, one of the most notable differences between different kinds of milk is their protein content. While camel milk has slightly less protein than cow milk, the milk of goats, buffalos, sheep and reindeers contain progressively more protein^15,16. Thus, by mixing them with spirits, you could achieve a more thorough wash with interesting flavors; alternately, you could use less milk, and thereby keep your milk-washed spirit at a higher alcohol concentration. Other factors also come into play: as goat milk is more alkaline than cow milk¹⁷, it needs more added acidity to bring it to curdling (which might lend a citrusy flavor if you use lemons to achieve that). However, not every milk is suited for this. Different nut milks, for instance, typically do not contain casein¹⁸, and therefore cannot bind polyphenols. In addition, scientific studies found that the slightly sweet dietary fiber polydextrose efficiently bound to polyphenols and masked the astringency of beverages¹⁹, making it an interesting candidate for a cocktail ingredient.

Far from being a villain, astringency can be a wonderful taste to be savored. Even though polyphenols can be healthy for us, the ability of our salivary proteins to bind polyphenols is actually a clever protection mechanism, as some polyphenols can inhibit digestive enzymes⁴. Bartenders even have started to experiment with new ways to add astringency to their cocktails with ingredients such as the Japanese herb Shiso²⁰, the Middle Eastern spice sumac²¹, or through creating an infusion with tea to generate new and interesting flavors. Only if the astringency proves to be overpowering, or is an unwanted feature in the final drink, should one feel the necessity to remove it by washing with milk or eggs.

That being said, I do wonder what a cocktail made with reindeer milk would taste like.

Sources:
1) https://www.ncbi.nlm.nih.gov/pubmed/22515235

2) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4064959/

3) https://www.smithsonianmag.com/science-nature/why-szechuan-peppers-make-your-lips-go-numb-5668606/

4) http://onlinelibrary.wiley.com/doi/10.1111/jtxs.12022/pdf

5) http://www.ajevonline.org/content/37/3/184

6) https://www.sciencedirect.com/science/article/pii/S0950329309001207

7) http://www.sciencedirect.com/science/article/pii/0950329394900086

8) http://europepmc.org/abstract/AGR/IND23286005

9) http://www.sciencedirect.com/science/article/pii/S0924224414001654

10) http://www.sciencedirect.com/science/article/pii/S1021949816301417

11) https://books.google.ch/books/about/Liquid_Intelligence.html?id=UrPmngEACAAJ&redir_esc=y