Skip to 0 minutes and 6 secondsAmazingly, for hundreds of years tea makers have produced drinkable teas, from tealeaves, using principles of withering and oxidation, with no understanding of the underlying chemistry. Today, we now know the most important organic compounds in fresh tealeaves, responsible for producing teas with the most desirable appearance, aroma, flavour, and taste. These include polyphenols, largely responsible for astringency, and amino acids, which give tea its brothiness. During tea processing the organic compounds undergo changes to produce a 'finished' or 'made' tea - that is, one that has been processed and is ready for packaging or steeping. In steeped tea, polyphenols are mainly responsible for astringency (the 'drying' sensation experienced in the mouth after drinking tea).
Skip to 0 minutes and 52 secondsThese compounds are produced as a defence against insects and other animals and are the most abundant compounds in tea comprising as much as 30-40% of both freshly plucked tealeaves and solids in tea liquor. A tea drinker typically consumes 180 to 240 mg of polyphenols from a strong cup of tea. There are an estimated 30,000 polyphenolic compounds in tea, including (-)-epicatechin, a member of a class of polyphenolic compounds called flavanoids, which have similar structures, all containing 15-carbon atoms. One for the key distinctions between black teas and green teas is the production of another type of polyphenol, called theaflavins, which includes theaflavin and related compounds.
Skip to 1 minute and 43 secondsTheaflavin gives a bright red-orange appearance to the tea, due to the presence of a benzotropolone ring system, and is formed from two molecules of a flavonoid. Polyphenol oxidase and peroxidase are the most important enzymes in tealeaves. They are responsible for the enzymatic browning of tealeaves that occurs when the cell walls in the leaves are broken and the polyphenols exposed to oxygen. These enzymes may be deactivated using heat so that browning cannot occur; this is one of the first steps in green tea production and explains why finished green tea leaves remain green. Amino acids give tea its savoury taste. Tealeaves contain many amino acids, the most abundant one being L-theanine.
Skip to 2 minutes and 30 secondsL-Theanine is responsible for promoting alpha brain wave activity, which can promote relaxation. Plant pigments give leaves their colour and are responsible for absorbing light for photosynthesis. The two major groups of pigments in fresh tealeaves are chlorophylls and carotenoids. During oxidation, the green colour of tea chlorophylls is converted to black pigments called pheophytins. This conversion leads to the dark appearance of finished oxidized teas. Chlorophyll comprises two organic compounds with similar structures; one is called chlorophyll a. When chlorophyll a loses the magnesium ion from the centre of the ring, and it is replaced with two hydrogens, it forms an olive-brown coloured compound called pheophytin a.
Skip to 3 minutes and 19 secondsThe aroma of tea is made up of hundreds, perhaps thousands, of flavour and aroma compounds that exist in trace amounts. Many of these aromatic compounds do not exist in fresh tealeaves, but are formed during processing. The flavour and aroma of each tea depends on a wide variety of combinations of these compounds, hence it is called aroma complex. So, how do we know that these organic compounds are present in tea? Analytical techniques, particularly high performance liquid chromatography (HPLC) has proved very useful in this respect. A pump is used to pass a pressurized liquid solvent through a column filled with a solid called the stationary phase.
Skip to 4 minutes and 4 secondsA sample is injected into the solvent stream and each component in the sample interacts slightly differently with the stationary phase, causing different flow rates for the different components and leading to the separation of the components as they flow out the column. The presence of an organic compound can be detected using an ultraviolet absorbance detector, which produces a UV spectrum. An HPLC-UV spectrum is a plot of UV absorbance versus time. The retention time can be used to identify the organic compound by comparison with standards (comparing the retention times of known compounds injected into the same type of column, operated under the same conditions) and the area under the peak can identify its concentration.
Skip to 4 minutes and 43 secondsSo, this method is extremely useful in separating, detecting and quantifying the components in tea, including caffeine.
The organic flavour components of tea
A Sweet Solution
Sugar is such a popular additive, and understanding how it interacts with water and other molecules is important when influencing the flavours of food and drinks. Research at York has shown that sugar has an important effect in reducing the bitterness of tea and coffee, not just by masking it but by affecting the fundamental chemistry.
Caffeine, as well as acting as a stimulant, is partly responsible for the bitter taste in tea and coffee. In water, caffeine molecules tend to stick to each other, and this is further enhanced by the addition of sugar. As sugar causes the caffeine molecules to clump together, it takes away the bitterness of tea and coffee – as they clump together there is less surface area to arouse our tastebuds and so we find it harder to taste them.
For the first time, work at York suggests that the underlying cause of the clumping is the affinity between sugar molecules and water, which in turn makes the caffeine molecules stick together (or aggregate) in order to avoid the sugar. This work, published in the Royal Society of Chemistry journal 'Food and Function' (S. Shimizu, Food Funct., 2015, 6 , 3228), will help food scientists in their development of new tastes.
In the UK, a levy on sugar-sweetened drinks has been introduced to combat child obesity. It was argued the NHS could save billions of pounds and also thousands of lives in a generation by weaning the public off its sweet tooth.
Today, research has shown that children and teenagers are consuming three times the recommended level of sugar. But, some people object to being taxed for something that causes no harm if eaten in moderation and they don't want the 'nanny state' interfering in their choices.
What do you think?
For those of you interested in practical aspects and would like to know how to separate a mixture using High Performance Liquid Chromatography (HPLC), you might like to see this video, made to help prepare our undergraduates before their practicals.
This is an additional video, hosted on YouTube.
© University of York