Tea Leaf Compounds: The Chemistry of Bitterness and Sweetness
There’s an old Chinese saying that a good cup of tea offers a “returning sweetness”—hui gan—a soft sweetness that follows a faint bitterness or astringency. This beautiful transformation begins in the leaf itself: catechins lend that initial bitterness, while theanine and a touch of caffeine round it out with clarity and calm. It’s one of the most beautiful moments in tea tasting: the bitterness fades, and a clean, lingering sweetness gently unfolds.
But this transformation isn’t just poetic—it’s chemistry at work. Within each tea leaf lies a complex world of compounds. Among them, the three mentioned above—catechins, theanine, and caffeine—play an especially vital role in shaping the flavor, energy, and character of the cup.
Tea Table Setting
Catechins (Tea Polyphenols)
In freshly plucked tea leaves, tea polyphenols make up about 18% to 36% of the dry weight. But they’re not just one thing—the term refers to a group of plant-based compounds, including catechins, flavonoids, anthocyanins, and phenolic acids.
Their levels vary based on everything from the tea variety and harvest season to how mature the leaves are and how the tea is processed. Generally speaking:
Camellia sinensis var. assamica contains more than Camellia sinensis var. sinensis
Younger buds contain more than older leaves
Summer and autumn harvests have more than spring
Higher altitudes and longer oxidations lead to less tea polyphenols
But more isn’t always better. High polyphenol content can make a tea overly bitter or harsh—and potentially irritating for the stomach. The real magic lies in balance.
Catechins, which make up 70–80% of all tea polyphenols, are the stars here. In heavily oxidized teas, these transform into theaflavins and thearubigins—the very compounds that give black tea its rich color and mellow, rounded taste.
Green Tea
Caffeine (Alkaloids)
Tea naturally contains eight types of alkaloids, with caffeine being the most dominant—making up about 2% to 5% of the dry leaf. Other alkaloids like theobromine and theophylline are present in smaller amounts.
Caffeine levels are usually highest in fast-growing, tender leaves—especially the second leaf from the bud, which is why black and oolong teas often call for slightly mature leaves.
Factors like tea variety, season, shade-growing, and even fertilizer use can all affect caffeine levels. So can processing—high-heat methods like pan-firing and roasting tend to lower it.
Caffeine contributes a subtle bitterness, but it also gives tea its clean, energizing “snap.” It binds with theaflavins and thearubigins, lifting the flavor while mellowing its own sharpness. And good news—caffeine is typically flushed out of your system within 4–6 hours.
Oolong Tea
Theanine (Amino Acids)
One in particular—theanine—is the soul of tea’s calm.
Found only in the tea plant, theanine is most concentrated in young stems and leaves (especially tender stems, which can have up to 3x more than buds). It gives tea its sweet and savory (umami) notes and contributes to aroma, texture, and that famously smooth mouthfeel.
Theanine is what brings the “ahh” to your sip. It naturally balances caffeine’s stimulating effects, creating a gentle, sustained alertness rather than a jittery spike.
Interestingly, theanine is only made in the roots—and shady, humid environments slow its conversion into catechins. That’s one reason why tea grown in misty mountain gardens often tastes so refined and smooth.
White Tea
So, What Makes a Tea Truly Good?
A good tea is more than just a tightly rolled leaf or a clear liquor. It’s about nuance and balance: floral, fruity, or woody aromas; elegant structure; and a flavor that’s smooth, layered, and deeply satisfying.
A perfect cup doesn’t shout with bitterness or overwhelm with sweetness—it reveals itself slowly, with a gentle invitation to pause and savor.
