A 5 Minute Steep on the Science of Tea Infusion

           If the instructions on your tea package call for an infusion of 2 minutes and 30 seconds at 180F… why is that? Why not 1 minute, or 5 minutes? And what if we chop that infusion up into many little ones, like gongfu style… how is the first infusion different from the 10th? Of course, we might guess that a 1-minute infusion would be weak, and a 5-minute infusion too strong, and the 1^st gongfu steep is more flavorful than the 10^th, but how and why? Let’s talk about at what types of molecules are extracted, what factors affect their extraction from the leaf into the liquid infusion, and how those extracted compounds affect flavor.

            First, we must introduce two groups of compounds in tea that are critical to understanding tea extraction; polyphenols and amino acids. These two major categories of bioactive molecules do a little dance every time we brew tea.

            Polyphenols. The backbone behind many of the reported health benefits surrounding tea. Anti-oxidant [1-4], anti-carcinogenic [5], anti-obesity [6], anti-diabetic [7], anti-inflammatory [8], anti-bacterial [9], anti-neurodegenerative [10-12], anti-depressive [13-15] (the list goes on). But, just like so many of the healthy foods we come across in life, polyphenols do not taste good… (why God?). Polyphenols account for the bitterness and astringency in tea. Slight bitterness/astringency is fine, and in fact necessary to give the tea “body,” or “structure” of taste. However, too many polyphenols will make our tea taste not so tasty.

            Amino acids. The crowd favorite, and the MVP of an excellent cup of tea. These provide the sweet and savory/umami flavors that offset the bitterness of polyphenols. They provide anti-stress effects [16-18], and they taste great. What more could ya want? While polyphenols can feel like an overpopulated species in tea (roughly one-third of total dry weight), amino acids are much less plentiful and more difficult to cultivate. But, the amino acid content is truly what separates the bottom shelf teas from the tippy top shelf. As several studies have shown, the amino acid content in tea correlates directly with the perceived flavor quality [19, 20], particularly in green tea where the amino acids are needed to offset the high content of polyphenols. Our bodies crave amino acids because they are the building blocks of life for nearly all living organisms. Tea plants use them for important life-maintenance functions, and so do we.

            An important observation is that amino acids infuse relatively easily into water, meaning that they do not need high-temperature water or long infusion time for successful extraction. They infuse well into the water at 80 Celsius/175 Fahrenheit [21], and need only a couple of minutes to get a majority of the compounds from leaf into cup [22]. This is because they are relatively light-weight molecular compounds compared to polyphenols. The image below shows results from a study that measured the percentage of 19 different amino acids extracted from tea leaves after a 1-minute infusion (a), and theanine and total AA extraction up to 5 minutes (b) [22]. 

            Polyphenols, being larger weight molecular structures, require the extra ‘umph’ of boiling hot water, leaf agitation, infusion time, etc. In the literature, you can find some interesting studies in which scientists try anything they can think of to maximize polyphenol extraction from tea leaves. Methods like ultrasound-assisted extraction, microwave-assisted extraction, high-pressure processing, supercritical fluid extraction (huh?), subcritical water extraction and so on and so forth [23]. This type of research explores ways to maximize tea polyphenol extraction for use in supplements (ever heard of an EGCG capsule?) [24]. However, using just water, peak polyphenols content in green tea was found in multiple studies using an infusion similar to 80 Celsius/175 Fahrenheit for 30 minutes (but unfortunately after 30 minutes the polyphenols start breaking down and antioxidant activity drops) [25, 26]. Also, different polyphenols extract efficiently at different temperatures. The polyphenol extraction topic is pretty messy, but you will see my point in a second. 

            So, I have not tried an ultrasound super-subcritical microwave infused green tea yet, but I don’t know if I want to. Furthermore, in the past, my accidental half-hour hot green tea infusions have not been great. My point is that tea leaves possess more bitter polyphenols than we could ever reasonably want to drink in a cup of tea. Moreover, they hang around in the leaf long after the savory amino acids have come and gone. To show this, I’ve attached an image from a study measuring the total polyphenol content of green, oolong, and black teas after 1, 2, and 3 infusions (5 minutes at 90 Celsius/194 Fahrenheit per infusion) [27]. Results show decreases in polyphenol content following each infusion (and directly correlative decreases in antioxidant activity, not included here but reported in the study).  

            However, notice how despite the significant decrease in polyphenols following each infusion, substantial portions of polyphenols remain in the leaves, anywhere from one half to two-thirds of total polyphenols remain after 3 infusions (3 strong infusions). Take a second and compare how quickly amino acids diffuse into water compared to polyphenols. 2 minutes to get 50% of amino acids, versus 15 minutes to get 50% of polyphenols. These are the critical ebbs and flows determining the taste of our tea. You can imagine that at a certain point in time, there’s going to be an optimal degree of extraction, where enough of the amino acids and polyphenols have come out to give the infusion a nice rich flavor, but the polyphenols have not yet come out in numbers that overpower the amino acids, a sort of impending doom for all too many cups of forgotten steeping tea. But no, we have the optimum balance of amino acids (AA) and tea polyphenols (TP), the golden AA/TP ratio.

            The AA/TP ratio gives us the basic framework for understanding tea extraction. But, not all teas get us to that ideal ratio with the same infusion parameters. There are some other factors to consider when estimating how long, or how many times you should infuse your tea leaves.

            Tea type is important. Polyphenols themselves become less bitter as they oxidize or ‘ferment’ from green tea (non-fermented) into oolong or white tea (semi-fermented), and eventually into heavily fermented tea (black or ripe pu’er). This means that 100mg of black tea polyphenols will taste less bitter than 100mg of green tea polyphenols. A 10/100 AA/TP ratio might be overly bitter in a green tea, while that same AA/TP ratio might be just right in a black tea, because black tea polyphenols taste less bitter. Remember, the structures of tea polyphenols themselves change with tea type, and those structural changes affect how polyphenols taste.  

            Another important factor to consider when brewing tea is how much the tea has been rolled. Some tea leaves get rolled as a step of the production process, which serves to break down plant cell walls, and ease the extraction of flavor compounds later when we brew the tea. All tea types get rolled except for white tea. This means that white tea requires you to go through greater lengths to extract the flavor compounds out of the leaves. 2 minutes 30 seconds at 180F might extract 70% of compounds out of a green tea, but only 20% of compounds out of a white tea. As a result, those 2 minutes and 30 seconds you used for the green tea ain’t gonna cut it for the white tea.

            One recent study reported that 51% of panelists on a panel of trained tasters found the optimal infusion parameters for a white tea to be 98 Celsius/208.4 Fahrenheit for 7 minutes, 24% liked 98 Celsius/208.4 Fahrenheit for 10 minutes, and 25% preferred 98 Celsius/208.4 Fahrenheit for 15 minutes (the highest time/temperature combination tested in the study) [28]. To clarify my point, that near-boiling temperature was preferred significantly over any of the lower temperatures tested in the study. Since white tea is not rolled, it needs more time and higher temperatures to release its true flavor. It is a common misconception that white tea is the ‘fragile’ tea type. The tea seems delicate only because we’ve been extracting it at ‘2 minutes at 180F’ when it’s best at 7 minutes at 208F! With unrolled tea, you are battling a cell wall (millions of cell walls). Stay in the fight and turn up the heat if you want the best AA/TP levels.

            The flip side of unrolled white tea is low-grade CTC teas, which are not rolled, but instead crushed, torn, curled…sliced… diced… and sometimes bang/crackle/popped. They have been broken down extensively, leaving little to prevent flavor compounds from infusing into water. CTC teas present the opposite end to white tea on the “ease of extraction spectrum.” It moves from CTC, to whole leaf non-white teas, to white teas. And within non-CTC teas, different farmers roll their leaves to varying degrees, meaning that it’s hard to know how intact the cell walls of your tea are, and thus how quickly your flavor compounds will infuse into water. Leaf rolling is one of many intricacies of premium tea production, and one with considerable implications for the time and temperature necessary to get an ideal AA/TP ratio.

            Finally, it’s important to consider that we don’t actually know the total quantity of amino acids or polyphenols present in the leaves until we start infusing the leaves and getting a feel for them. Dry tea leaves with a strong sheen or light reflectivity can sometimes indicate abundant AA content, but not always. Knowing how to extract an optimal ratio of flavor compounds ultimately comes down to trial and error. Whoever went through the process of trial and error and ended up with the 2 minutes 30 seconds at 180F you see on your tea package probably did not get it totally wrong, but the chances are slim that they got it exactly right. If not for any other reason, you can almost guarantee their water was somehow different than yours, which significantly affects polyphenol extraction and flavor quality [29].

            I believe that when we acquire a new tea, we begin the process of getting to know that tea. It is indeed a process to understand the content and ratio of the leaves’ amino acids and polyphenols, and how those compounds interact with your water and steeping equipment. Unlike CTC tea bags that are designed to be identical, loose-leaf teas have character. They are unique and require time to understand their subtleties.

            Within the same varietal of premium loose-leaf tea, there will always be variation based on season, cultivar, your steeping gear of choice, water source, etc. The knowledge we should seek is not anyone else’s recipe, but rather an understanding of the process of how to figure it out for ourselves. We should see 2 minutes and 30 seconds at 180F on the package, and consider that a starting point from where the exploration begins. Tea polyphenols may have some bite to their taste, but they don’t bite. Don’t be afraid to experiment, and remember that it’s a learning process that takes practice. 

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