The most distinctive sensory characteristic of high-grade shaded tea is not its umami taste or its deep green color. It is its aroma: a complex, seaweed-like, marine scent that Japanese tea professionals call ooika (covered aroma). This aroma is absent in unshaded teas and intensifies with longer shading duration. It is the olfactory signature of premium matcha and gyokuro, and it commands the highest prices in the market.

The molecule primarily responsible for ooika is dimethyl sulfide (DMS), a volatile sulfur compound with a characteristic marine odor. DMS is also the molecule that gives the ocean its smell and contributes to the aroma of cooked corn and certain cheeses. In tea, its presence depends on the accumulation of a precursor, S-methylmethionine (SMM), which converts to DMS during the heat processing that follows harvest.

The chemistry from SMM to DMS is well understood. What remains unsolved is the mechanism by which shading triggers the accumulation of SMM in the first place. This article presents what is known, what is not, and why this gap represents the last great unsolved problem in matcha biochemistry.

1. The DMS/SMM Pathway: What We Know

The pathway from precursor to aroma molecule follows a two-stage process:

Stage 1: SMM accumulation in living leaves. S-methylmethionine is synthesized from L-methionine by the enzyme methionine methyltransferase (MMT). MMT transfers a methyl group from S-adenosylmethionine (SAM) to methionine, producing SMM. This reaction occurs in the cytoplasm of leaf cells and is part of the broader methionine cycle.

Stage 2: Thermal conversion of SMM to DMS. SMM is thermally unstable. When tea leaves are subjected to heat during processing (steaming in Japanese-style processing, or pan-firing in Chinese-style processing), SMM decomposes to release DMS and homoserine. The DMS is volatile and contributes directly to the aroma of the finished tea. The thermal conversion is not enzymatic; it is a straightforward chemical decomposition driven by heat.

The efficiency of this thermal conversion is well documented: 44 to 80 percent of the SMM present in fresh leaves is converted to DMS during standard steaming conditions. The wide range reflects differences in steaming temperature, duration, and leaf water content. Longer, hotter steaming converts more SMM but also drives off some of the DMS as vapor before it can be captured in the finished product.

2. Why Shading Increases SMM

The critical observation is that shaded tea leaves contain far more SMM than unshaded leaves. This difference is the direct cause of ooika: more SMM means more DMS upon processing, which means a stronger covered aroma. The question is why shading causes SMM to accumulate.

Several hypotheses have been proposed:

• Increased methionine availability. Shading increases free amino acid concentrations generally (as discussed in Parts 6 and 7). If methionine levels rise, more substrate is available for MMT, and SMM production could increase proportionally. This hypothesis is plausible but does not fully explain the magnitude of SMM increase, which often exceeds the general amino acid increase.
• Upregulation of MMT. Shading could specifically induce MMT gene expression, increasing the amount of enzyme available to convert methionine to SMM. This would produce a targeted increase in SMM beyond what substrate availability alone could explain.
• Reduced SMM consumption. SMM participates in the methionine cycle and can be reconverted to methionine by homocysteine methyltransferase (HMT). If shading suppresses HMT activity, SMM would accumulate because it is being produced but not consumed.
• Altered sulfur metabolism. Shading changes the overall sulfur balance of the leaf, potentially redirecting sulfur-containing metabolites toward the SMM pool.

None of these hypotheses has been conclusively confirmed. The molecular trigger that connects the absence of light (or UV-B specifically) to the upregulation of SMM production remains unknown. This is the missing link.

3. The Unknown MMT Activation Mechanism

MMT is the enzyme at the center of the mystery. In order for shading to increase SMM accumulation, either MMT activity must increase or SMM consumption must decrease (or both). The most direct explanation would be transcriptional upregulation of MMT in response to shade.

However, the signaling pathway from light deprivation to MMT gene expression has not been identified. Unlike the catechin cascade (Part 6), where UVR8, HY5, and MYB12 provide a clear chain from UV-B to enzyme expression, no equivalent chain has been mapped for MMT. The transcription factors that regulate MMT in tea are not definitively identified. The promoter elements of the tea MMT gene have not been fully characterized. Whether the same UVR8/HY5 system that controls catechin biosynthesis also influences MMT is unknown.

This gap is not for lack of interest. The covered aroma is the most commercially valuable aroma characteristic in Japanese tea, and identifying the molecular switch that controls it would have immediate practical applications. But the sulfur metabolism pathway in tea is less well studied than the flavonoid or amino acid pathways, and the tools for gene function analysis in tea (a recalcitrant species for genetic transformation) are limited.

4. The "Second Source" of DMS

Complicating the picture further is the observation that fresh, unprocessed tea leaves already contain measurable DMS, in concentrations ranging from 180 to 1,700 micrograms per kilogram. This DMS is present before any thermal processing, which means it was not produced by SMM decomposition during steaming. It exists in the living leaf.

The source of this pre-existing DMS is not established. Several possibilities have been considered:

• Enzymatic SMM degradation. An enzyme other than heat might catalyze SMM to DMS conversion at ambient temperature in the leaf. No such enzyme has been identified in tea, but analogous enzymes exist in marine organisms.
• Direct methylation of hydrogen sulfide or methanethiol. DMS could be produced through a pathway entirely independent of SMM, involving methylation of volatile sulfur precursors. This would represent a second biosynthetic route to DMS.
• Microbial production. Endophytic or surface-associated microorganisms could produce DMS from sulfur-containing substrates. Tea leaves harbor diverse microbial communities, and some bacterial species are known DMS producers.

The wide range of pre-existing DMS concentrations (180-1,700 ug/kg) suggests that whatever produces it is highly variable, influenced by cultivar, growing conditions, microbial community, or some combination. This variability is itself a clue: it argues against a tightly regulated plant metabolic pathway and may point toward an environmental or microbial contribution.

5. Why This Mystery Matters

The covered aroma is not an academic curiosity. It is the aroma that distinguishes premium matcha from ordinary powdered green tea. Producers who can reliably produce strong ooika command significantly higher prices. Currently, the only tool for enhancing ooika is extended shading, which is costly and stresses the plant. If the molecular switch that triggers SMM accumulation were identified, it might be possible to:

• Select or breed cultivars with naturally higher MMT expression, producing stronger ooika with less shading.
• Optimize shading protocols more precisely by targeting the specific wavelength or signaling pathway responsible for MMT activation.
• Develop agronomic treatments (sulfur supplementation, microbial inoculants) that enhance SMM production.
• Optimize processing conditions to maximize the conversion of the "second source" DMS in addition to SMM-derived DMS.

Until the missing link is found, producers rely on empirical knowledge accumulated over generations: shade longer for more aroma, use cultivars known for strong ooika, and process quickly after harvest to preserve volatile precursors. This knowledge is effective but imprecise. The molecular mechanism, when discovered, will transform it from craft intuition into quantifiable science.

6. The Current State of Research

Recent advances in tea genomics and metabolomics have brought researchers closer to the answer without yet reaching it. Transcriptomic studies comparing shaded and unshaded tea leaves have identified hundreds of differentially expressed genes related to sulfur metabolism, but none has been definitively linked to MMT regulation. Metabolomic analyses have confirmed that SMM accumulates preferentially in shaded leaves and that the SMM-to-DMS conversion during steaming follows predictable kinetics.

The tools now exist to solve this problem: CRISPR gene editing (though difficult in tea), heterologous expression systems for testing candidate enzymes, and high-sensitivity volatile analysis for tracking DMS production in real time. What is lacking is the identification of the regulatory gene or signaling molecule that connects light deprivation to MMT activation. This is the experiment that remains to be done.

Summary

• The covered aroma (ooika) of shaded tea is primarily caused by dimethyl sulfide (DMS), produced when S-methylmethionine (SMM) decomposes during thermal processing with 44 to 80 percent conversion efficiency.
• Shading dramatically increases SMM accumulation in tea leaves, but the molecular mechanism linking light deprivation to MMT (methionine methyltransferase) activation is unknown. This is the missing link in matcha aroma science.
• Fresh tea leaves contain a "second source" of DMS (180-1,700 ug/kg) that exists before any thermal processing, produced by an unidentified mechanism that may be enzymatic, metabolic, or microbial.
• Solving the MMT activation mystery would enable cultivar breeding, optimized shading, and processing improvements for enhanced aroma production.

Frequently Asked Questions

Why does matcha smell like seaweed?

Both the ocean and shaded tea produce dimethyl sulfide (DMS) as a significant volatile compound. In the ocean, DMS is produced by marine algae and bacteria from dimethylsulfoniopropionate (DMSP). In tea, DMS is produced primarily from S-methylmethionine (SMM) during heat processing. The chemical is the same molecule in both cases, which is why the aroma is similar. The seaweed-like or marine quality of matcha's aroma is a direct consequence of DMS production, not a coincidence of description.

Does longer shading always produce more covered aroma?

Within the practical range, yes. SMM accumulation increases with shading duration, so longer shading produces more DMS precursor and therefore more covered aroma after processing. However, the relationship has limits. Beyond approximately 30 days, the additional SMM accumulation is marginal while the negative effects on the plant (excessive etiolation, reduced vigor, compromised photosynthetic capacity) become significant. Furthermore, the processing step has its own efficiency ceiling: only 44 to 80 percent of SMM is converted to DMS, and some DMS is lost as vapor during steaming. There is a practical maximum aroma intensity that cannot be exceeded by shading alone.

Could the covered aroma be artificially enhanced by adding DMS to matcha?

Technically, DMS is a simple volatile compound that could be synthesized or obtained from other sources. However, the aroma of premium matcha is not determined by DMS alone. It is a complex mixture of hundreds of volatile compounds including linalool, geraniol, indole, ionones, and numerous other molecules that contribute to the overall aromatic profile. Adding DMS to low-quality matcha would produce a one-dimensional seaweed smell without the complex supporting aromatics. Moreover, such addition would constitute adulteration and would be detectable by trained sensory professionals and by gas chromatography. The covered aroma cannot be faked.