Key Quality Parameters in Black Tea Evaluation: What Makes a Good CTC Tea?

Every cup of black tea carries a story written in chemistry. The rich amber color, the sharp brisk bite, the lingering strength – none of these happen by accident. They are the outcome of precise biochemical transformations during tea manufacture, and evaluating them is a science in itself. For CTC (Crush, Tear, Curl) black tea – the dominant form produced and consumed in India – quality evaluation relies on a combination of chemical indices, liquor characteristics, and regulatory compliance. Understanding what these parameters mean, and why they matter, is key to appreciating what separates a truly good cup of tea from an average one.

The chemistry behind black tea quality 🔗

When fresh tea leaves go through the CTC process, a critical biochemical transformation takes place during fermentation (oxidation). The oxidation of catechins in the presence of polyphenol oxidase leads to the formation of two major pigment groups – theaflavins (TF) and thearubigins (TR) – which together determine nearly every sensory attribute of the final brewed tea.

During fermentation, catechins are acted upon by oxidative enzymes – polyphenol oxidase and peroxidase – to form theaflavins and thearubigins, with theaflavins appearing as golden yellow pigments and thearubigins as orange-brown ones. These two compound groups are responsible for the taste, strength, and color of black tea. Theaflavins constitute roughly 0.5-2% of dry weight depending on manufacturing parameters, while thearubigins make up 6-18% of the dry weight of black tea.

Theaflavins: the marker of brightness and briskness 🔗

Theaflavins (TF) are widely regarded as the primary quality index for black tea. The presence of TF contributes to increased astringency and brightness in CTC tea, and a positive correlation is normally observed between the TF content and quality scores of finished tea. In practical terms, a tea with high theaflavin content pours bright and delivers the lively, refreshing sensation that tea tasters call briskness.

According to the UPASI Tea Research Foundation, TF has a direct correlation with quality and price realization – it contributes to both briskness and brightness of tea liquor. During fermentation, TF progressively increases until an optimum point, after which it begins to decline. Getting the fermentation timing right is therefore critical to locking in peak TF levels.

Beyond total TF content, researchers note that using individual TF fractions is more appropriate than total TF content for quality assessment, because the contribution of TF to astringency varies with the extent of galloylation, and the relative abundance of simple theaflavin, theaflavin monogallates, and theaflavin digallate each affect the brew differently.

Thearubigins: the backbone of strength and color 🔗

Thearubigins (TR) are complex condensation products formed as oxidation continues beyond the theaflavin stage. Thearubigins, constituting 6-18% of dry weight, are responsible for taste, total color, and body of the liquor – contributing around 35% of total color and playing a significant role in the brown color, strength, and mouthfeel of tea.

However, thearubigins are a double-edged compound. While TR contributes to mouthfeel and color, increased TR is associated with an ashy taste and a reduction in brightness. This is why very high TR levels – typically a sign of over-fermentation – are undesirable. Together with highly polymerised substances (HPS), TR increases color depth and body, but the balance with TF must be carefully maintained.

The TF:TR ratio – the most critical quality index 🔗

No single parameter captures the quality of CTC black tea as efficiently as the ratio of theaflavins to thearubigins. According to UPASI Tea Research Foundation, in addition to achieving maximum TF, there must be proper balancing of liquor parameters, for which the guideline states that TR should be ten to twelve times the value of TF. This translates to an ideal TF:TR ratio of 1:10 to 1:12 for good quality CTC tea.

When the ratio deviates from this range, quality suffers in predictable ways. A ratio skewed toward too much TR (indicating over-fermentation) produces a dull, ashy cup with poor brightness. Conversely, insufficient TR means the liquor lacks strength, body, and the characteristic reddish-brown color. The ideal ratio ensures brightness, briskness, strength, and color all come together in a well-rounded cup.

Chemically, the optimum fermentation point can be assessed by monitoring TF content and identifying when TF formation reaches its maximum value – at this point, the balance between TF and TR is also most favorable for a good cup of tea.

The four key liquor characteristics 🔗

When tea tasters evaluate CTC black tea, they assess the brewed liquor against four primary sensory benchmarks. These aren’t arbitrary descriptors – each maps directly to a biochemical reality.

Brightness 🔗

Brightness refers to the clarity and visual vibrancy of the tea liquor. While theaflavins are responsible for the attractive golden color and brightness in tea liquor, thearubigins are reddish-brown and tend to reduce brightness when present in excess. A bright, sparkling liquor is a reliable visual cue of proper oxidation and good theaflavin content.

Briskness 🔗

Briskness is the lively, astringent quality that gives black tea its refreshing character. The briskness index, developed at UPASI TRI, is expressed as the percent ratio of TF to TF plus caffeine, and the normal range for south Indian CTC tea is above 23. Caffeine and theaflavins act together – caffeine combines with theaflavin to contribute to this briskness in the black tea liquor.

Strength 🔗

Strength refers to the robustness and fullness of the tea’s flavor – the sensation of a thick, rich taste that lingers on the palate. As described by Harler (1963), strength is a combination of thick liquor with pungency and taste. Thearubigins and HPS are the primary contributors here. A tea that is strong but lacks briskness will feel heavy; good CTC tea strikes a balance between both.

Color 🔗

Color is both an aesthetic and a diagnostic parameter. Total liquor color is a measure of overall liquor color and should be above 3.0 for CTC tea. The color index (CI), expressed as the percent ratio of TF to TR plus HPS, has a normal range of 5-11 for south Indian CTC teas. A bright, reddish-brown liquor signals proper oxidation, while a dull or muddy color can indicate either under- or over-fermentation.

Organoleptic evaluation: the human dimension 🔗

Chemical indices tell a quantitative story, but the final verdict on tea quality has always rested with skilled human sensory evaluators. During organoleptic evaluation, parameters such as dry leaf appearance, infused leaf appearance, color, strength, taste, and briskness of tea liquor are all assessed by the tea taster – and based on these tasters’ scores, market prices of commercial teas are ultimately determined.

Organoleptic evaluation of tea is carried out by skilled and experienced tasters who determine market value on a subjective basis, and the four important liquor characteristics assessed are brightness, briskness, strength, and quality. Research has confirmed that both TF and TR show positive correlations with these tasters’ evaluations, though the impact of theaflavins on tasters’ scores is greater than that of thearubigins.

Tea is also evaluated on a 10-100 scale in many research frameworks. Studies from Tocklai Tea Research Institute show clear regional variation in quality scores, with teas from upper Assam, south bank, and North Bank regions consistently scoring higher than those from Barak Valley, Dooars, and Terai – differences traced directly to variations in TF and TR fractions.

Regulatory standards: the role of the PFA Act, 1954 🔗

Quality evaluation in India is not purely a technical exercise – it also has a legal dimension. The Prevention of Food Adulteration (PFA) Act, 1954 provides the statutory framework for ensuring that food products, including tea, meet minimum quality requirements and are free from adulterants.

Under the Tea Board of India’s framework, “tea waste” is defined as any article purporting to be tea that does not conform to the specification for tea laid down under the Prevention of Food Adulteration Act, 1954. This creates a clear regulatory boundary between genuine tea and substandard or adulterated material.

The PFA Act was passed in 1954 and came into effect in June 1955, replacing earlier state-level food adulteration laws that lacked uniformity – a critical step toward standardizing food quality across India’s diverse states and union territories. The Act mandates a Central Committee for Food Standards and authorizes Central Food Laboratories for analysis and testing. It also defines food adulteration as the addition of contaminants to food items to increase quantity and decrease cost – a major concern in the tea industry where mixing with tea waste or inferior material can compromise product integrity.

For tea producers and manufacturers, PFA compliance is non-negotiable. Tea that does not meet the prescribed chemical and physical standards – whether through adulteration, improper processing, or contamination – is classified as substandard or adulterated under the Act, with serious legal consequences for those in the supply chain.

Putting it all together: what makes a good CTC tea? 🔗

A good quality CTC black tea is the result of precise biochemical management, sensory balance, and regulatory compliance working together. At the chemical level, it needs sufficient theaflavin content (typically reflected in a briskness index above 23), a well-maintained TF:TR ratio of 1:10 to 1:12, a total liquor color above 3.0, and a color index in the 5-11 range. At the sensory level, the liquor must be bright, brisk, strong, and well-colored. And at the regulatory level, it must conform to the specifications laid down under the PFA Act, 1954.

These parameters are not independent – they reinforce each other. A tea that is biochemically balanced will naturally perform well in sensory evaluation, and one that meets PFA standards will already satisfy the minimum quality thresholds needed for safe, honest commercial sale. Understanding all three dimensions – chemical, sensory, and regulatory – is what equips producers, processors, and buyers to consistently evaluate and deliver quality black tea.

What do you think? Given that both chemical indices and human sensory evaluation are used to assess CTC tea quality, which do you think should carry more weight in setting market prices – the objective biochemical measurements or the experienced judgment of a trained tea taster? And with the PFA Act now largely superseded by FSSAI regulations, do you think India’s tea quality standards need a more modern, science-based statutory overhaul?

References

1. https://www.journalijdr.com/sites/default/files/issue-pdf/16658.pdf
2. https://www.sciencedirect.com/science/article/abs/pii/S0308814611006078
3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3551171/
4. https://www.sciencedirect.com/science/article/abs/pii/S0260877411006492
5. https://www.upasitearesearch.org/tea-quality-parameters/
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC4648934/
7. https://www.researchgate.net/publication/309252237_A_machine_vision_system_for_estimation_of_theaflavins_and_thearubigins_in_orthodox_black_tea
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC3791237/
9. https://teaauction.gov.in/teaboardadmin/(S(azr1nynfpyntot1ysxq52rhm))/Home.aspx
10. https://byjus.com/neet/the-prevention-of-food-adulteration-act-1954/