Extreme weather with alternating periods of drought and heavy rainfall can negatively affect yield and quality of agricultural crops. Recent research into the impact of climate variations on the quality of tea has shown how dramatic the influence can be.
Second only to water, tea is by far the most widely consumed beverage in the world. Apart from actual or imagined physiological effects, the main and most important aspects for a tea drinker are thought to be flavor and enjoyment. There may be regularly recurring trends towards consuming only the healthiest beverages, but generally, consumers’ main concern is that their drinks taste good. However, sensory characteristics such as taste and aroma are exactly the qualities affected by weather, as Professor Albert (Al) Robbat from the Department of Chemistry at Tufts University in Medford, Massachusetts explains.
Tea — an ideal model plant for climate effect studies
In cooperation with a team of interdisciplinary scientists, Al Robbat investigated tea samples from Yúnnán province in Southwestern China, an area renowned for excellent tea. The aim was to determine why tea leaves picked during the rainy monsoon season are less aromatic and frequently have off flavor issues compared with tea leaves picked during the dryer spring season. This was found to be the case even when comparing leaves picked from the same plant. The chromatography expert explains that not only do tea drinkers suffer when exposed to inferior quality brews; the tea farmers incur significant losses since they are forced to sell crops grown during the monsoon season at much lower prices compared with the spring crop. There is an urgent need for more insight into causes and effects since the Yúnnán region is increasingly faced with extreme weather conditions that can affect quality and yield and in extreme cases lead to total loss of harvest. As Robbat explains, there has already been a noticeable shift in the start of the summer monsoon and its duration, which means periods with ideal harvest conditions are becoming shorter. The scientists set out to determine the root causes of changes in quality and flavor of Yúnnán tea as a function of rainfall, temperature, and elevation. Their findings are reported in Journal of Chromatography A . Initial investigations of tea harvested in the mountains of Yúnnán from spring until the onset of the monsoon period brought the following results: With increasing amounts of rainfall, the catechin compound concentrations dropped by more than 50 percent. Among these are: catechin, catechingallate, epicatechin-3-gallate, epigallocatechin, epigallocatechin-3-gallate, gallic acid, gallocatechin and gallocatechin-3-gallate. The same effect was found for methylxanthines (caffeine, theobromine and theophylline), Al Robbat reports . Although catechins and methylxanthines, astringent bitter compounds characteristic of poor quality teas, decreased in concentration, other polyphenolic compounds (also astringent and bitter) increased in concentration. “Initially, we assumed that the loss of quality was related to a kind of dilution effect, in other words that the plant growth would outpace the production of secondary metabolites to which the flavor compounds belong”, the scientist explains.
However, when they determined that the total concentration and activity of antioxidants in teas harvested in spring was lower than in comparable teas harvested during the monsoon period, Al Robbat and his colleagues concluded that the plant chemistry, i.e. metabolism and physiology had changed completely and had adapted to the change in precipitation. This had resulted in changes in metabolism and, therefore, in the associated flavor determining metabolites. The idea was obvious, Prof. Robbat explains, to not just focus on significant flavor compounds in tea, but rather look at the bigger picture, including determining the entire group of flavor relevant compounds and to generate a profile of as many metabolites as possible in order to understand how environmental conditions influenced tea plant chemistry. Taste and aroma of tea are a result of complex interactions between hundreds of chemical compounds. Al Robbat: “Extending our knowledge of potentially sensory relevant metabolites and monitoring them over a period of time is key if we want to develop an understanding of how environmental and climate factors influence tea quality.” The scientists know that seemingly unimportant compounds can significantly influence the organoleptic quality. Most studies listed in literature have focused on seasonal changes of non-volatile compounds, but we now know that volatile organic compounds (VOCs) with low odor thresholds contribute significantly to the total sensory impression, Al Robbat points out. This realization had led the group towards using GC/MS as the technique of choice for monitoring metabolites.
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