This blog post was written by guest Jimmy Burridge, PhD in Plant Science and tea aficionado, with a burgeoning interest in the intersection of tea agronomy, chemistry and terroir (you can blame him for the confusing science parts!).
Climate change can affect the unique flavor, or terroir, of a tea from a particular place . This is because the aroma and taste profile of a tea results from a couple dozen different compounds that the tea plant makes in response to environmental conditions , . Changes in maximum and minimum temperature, temperature pattern over the course of a day or a year, amount and distribution of rain, and changes in weed or insect pressure can all affect the delicate balance of secondary metabolites, phenolic compounds and chlorophyll content that give a fine tea its characteristic balance of umami and astringency. Fog and cloud cover protect tea from intense sunlight, which affects the balance of protective chemicals the tea plant uses to protect itself and can result in subtle flavor and aroma changes.
A previous post mentioned how tea farmers in Uji are observing and responding to climate change by changing some of their shading practices. Many organic farmers, like Ayumi Farms (Cyittorattu) apply straw mulches or waste from soy sauce, sake, or miso processing, to their fields. These inputs increase the soil carbon content and improve the soil’s ability to buffer temperature and precipitation variation. Recently, we spotlighted a farm (Tarui Farms in Shizuoka Prefecture) that uses a goat to help control weeds. Today we will talk about some of the details of how environmental conditions can influence what tea scientists call the "functional quality" of tea.
Wazuka tea farmer Nishiyama-san inspects his tea field in the hills of Wazuka Village, Kyoto Prefecture. Photo by Jimmy Burridge.
Thus far, the effects of climate change for many tea farmers in Japan have not been as severe as in places like India, where temperatures can become high enough to literally damage the tea plant. Some Japanese farmers actually see positive effects in terms of a longer growing season and greater total volume of harvested tea. However, there may be a tradeoff between quality and quantity. Firstly, evidence exists of a simple dilution effect, meaning that when growth is excessively vigorous, the tea produced has a less pronounced flavor due to relatively fewer secondary metabolites associated with good flavors, such as theanine, catechin and methylxanthine .This could be part of the reason why organically farmed tea, which typically has less total available soil nutrients, and tends to grow more slowly, can have a better flavor.
Jimmy helps Wazuka tea farmers during their autumn harvest. Photo by Moé Kishida.
There are also conditions that result in changed ratios of chemicals, notably polyphenols and amino acids, within the tea leaves that can negatively affect the flavor, aroma and “functional quality” of the tea . In broad chemical terms, the ratio of amino acids, principally L-theanine, to polyphenols determines taste and quality. L-theanine is a type of amino acid that gives the umami flavor to tea. It generally occurs in higher quantities in shaded and first flush teas. L-theanine is naturally converted to various types of polyphenols and catechins, especially at higher temperatures and with more sunlight. Catechins contribute the important shibumi (astringency) and antioxidant properties to tea.
Many of the compounds we associate with superior quality are actually secondary metabolites, meaning chemicals that the plant produces in order to protect itself from different environmental conditions. For example, if a tea bush senses more beetles than normal eating leaves it may increase production of a certain compound to make itself less palatable. This can add a desirable flavor for humans and some farmers welcome a certain amount of pest pressure, but anything in excess can be harmful!
Similarly, a certain amount of cold weather can be good for flavor, in part because of slower growth rates. If you’ve ever noticed how good kale or salad greens taste after a frost, then you’ve appreciated how temperature affects plant chemistry. The taste of homegrown tomatoes or cheese made from mountain pastured cows are other examples of how environment can affect flavor.
The complex chemistry of photosynthesis, whereby the plant uses energy from the sun to split a water molecule and transform carbon dioxide into a basic sugar, is highly dependent upon cell membrane dynamics, which are very sensitive to temperature. High temperatures will cause the plant to produce a different class of chemicals to protect itself from being damaged and this can in turn affect aroma and flavor. It is for this reason that the altitude at which tea is grown, which is related to temperature, has a strong influence on the ratio of polyphenols to amino acids, which is a major determinate of taste. Even the ratio of individual catechins, a class of polyphenols, to total polyphenol content, is affected by altitude and influences taste .
Figure 2. Simple schematic showing how high temperature and exposure to sunlight leads to more theanine (left) being converted to catechins (right). Theanine is responsible for the umami flavor and catechins for astringency. How proportions of individual catechins respond to temperature and sunlight is incompletely understood, as are the impacts of these changes on flavor and aroma.
It is perhaps because of the negative effect of high temperature on tea quality that India is developing agroforestry tea production systems to shade the tea plants and create a micro-climate under accompanying trees, some of which are nitrogen fixing . Some of the most traditional tea production systems in China also involve agroforestry systems in which larger tea bushes grow under a sheltering tree canopy . These types of bushes produce high quality tea but must be hand harvested, which highlights the challenge of how to respond to climate change in a way that ensures top quality tea but doesn’t make the tea too expensive.
Japanese tea farmers are exploring options using organic farming methods and shading that create favorable micro-environments for the leaves of tea bushes and for their root systems . Tea farmers may also have to experiment with new cultivars of tea bushes. The future of high-quality tea farming looks to have its share of challenges but the community of Japanese tea farmers has shown its ability to creatively integrate traditional methods with modern tools. We hope they will continue to experiment and produce outstanding tea for the rest of us to enjoy!
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