Tea - the Wonder Beverage Part 3: The Chemistry of Tea

This is Part 3 of my series on the Wonder Beverage. Part 1 can be found here. Part 2 can be found  here.
  

Tea chemistry is based on the reactions of its polyphenols. Therefore it is important to select the part of the tea plant which is most abundant with polyphenols, for further processing.

Tea plants are trees.  Left in their natural state they grow tall with woody trunks.

 

But in tea plantations they are assiduously pruned so that they grow low, branching and bushy.

Tea pluckers pluck the surface of the bush. They harvest the first two leaves and the bud of the sprigs.

These immature vegetative portions, the rapidly growing shoot tips, are collectively called the tea flush. When we compare the composition of the tea flush with that of other parts of the tea plant, tea flush has the highest amount of polyphenols.

Plucking the tea flush fixes the chemical potential of the tea leaf, which is the raw material which goes in to the manufacturing process, to make the tea product. The quality attributes of the final product depend largely on the chemical composition of the raw material, namely the fresh green tea leaf.

Tea - the Wonder Beverage Part 2: The Colours of Tea

This is Part 2 of my series on the Wonder Beverage. Part 1 can be found here.

Tea is classified based on the agro climatic region in which it is grown, method of manufacture, and the different types of value added products. Ceylon Tea is grown at three distinct elevations and are classified as the low growns, medium growns and the high growns. These in turn can be divided into the Westerns and the Easterns according to the location of the estates. The best tea comes from the estates in the Nuwara Eliya region.

The Uva teas and the Dimbula teas are known as Seasonal teas. Lemon tea, Peach tea and many other spiced and scented teas are some of the value added products. It is said that like fine wine, fine tea is made under very difficult growing conditions. Each elevation, angle of slope, soil changes - all these affect the final quality attributes of the product.

Seasonal teas have a special superior quality in the flavour. In Sri Lanka there are two flavour seasons within the year; Dimbula season (January — March) and Uva season (July - September)

During this period the prevailing weather conditions are very harsh and stressful to the tea plant. There is no rain, it is windy, and the days are clear with a lot of sunshine while the nights are cold. These conditions are needed to produce flavour compounds in the tea leaf which give the characteristic high quality flavour to the Dimbula and Uva teas.

Depending on how tea leaves are processed we get Black tea, Green tea or Oolong tea. The difference in these teas is in the manufacturing process and it is the fermentation of the tea leaves that makes all the difference. Black tea is fermented, Green tea is not fermented, Oolong tea is partially fermented. Green tea is popular in South East Asia, mostly in China and Japan. Among the green teas the following types are popular:

• Japanese green tea (Sencha ar steamed green tea) — this has a characteristically refreshing aroma with a green note.
• Chinese green tea — (Lung Ching tea) — has an attractive flavour, characteristic floral, fruity and roasted aroma.
• Roasted green tea (Hajicha) — low grade of sencha tea is often roasted to about 180 °C for a few minutes to enhance the aroma to give a more brisk or light taste.

There are many other varieties and value added products of green tea available in the market. For example, white tea has a color that varies from pale yellow or light red to clear white. The flavor is very delicate. White Tea undergoes a minimal amount of processing and is made from the new leaves and young buds of a special variety of the tea plant. The leaves are harvested when they still have a coating of white fuzzy hairs. ‘Silver tips’ and ‘Silver needles’ are some examples that are marketed at a very special price!

The taste of milk tea

Boing Boing had a post which piqued my interest.

I think it's a matter of individual taste, some like their tea with a lot of milk, others only have a dash of it, with a stronger brew. Someone I know takes 5 spoons of sugar in his tea!  One of my research projects studies the interaction of milk with tea. The variation in taste may be due to the interaction between tea catechins and milk proteins.

Tea polyphenols, which include catechins found in the tea leaf, and their more complex derivatives formed during tea manufacture are responsible for the astringency (strong mouthfeel) of tea. These compounds bind to proteins on the tongue and lining the mouth —  that's how we experience the sensation of astringency. In a similar way, the catechins bind to the milk proteins and thereby reduce the astringency of the tea. People who don't like a 'strong' mouthfeel  will obviously prefer more milk in their tea. Interested readers should consult Tom Coultate's fine textbook  'Food: the Chemistry of its components'.

The full effect of milk proteins on tea is highly complex.  We understand the overall picture but the details are still the subject of active research by tea chemists, including myself. I hope to discuss this further in a future post.

Tea — the Wonder Beverage. Part 1: The History of Tea

This is the first in a series of posts about my favourite beverage - Tea! A version of these posts appeared as a single article in "Chemistry in Sri Lanka",  May 2008 Vol.25 (2). 

Part 2 can be found here.


Part 1: The History of Tea

Tea is produced from the plant Camellia sinensis (L) O.Kuntze which belongs to the family Theacea. It is a native of South East Asian countries such as Tibet, Western China and Northern India. The three main varieties of tea are the China variety, Assam and the Cambodian variety. The tea plant is a small evergreen shrub; a vast number of hybrids have been developed to obtain optimum quality parameters such as flavour, strength, colour and also adaptability to specific agro-climatic factors.

Though Sri Lanka is world famous for tea, the beverage did not originate in Sri Lanka. It was the Chinese Emperor Shen Nung, many centuries ago, who first introduced tea as a beverage to his subjects as having a ‘fine flavour and a soothing sensation’. Ever since, the praises of tea have been sung by millions of others down the ages. The Chinese Historian Lu Yu wrote in his classic work on tea, that "It is better to drink such a beverage than wine, which loosens the tongue".

Tea plants of the Assam and Chinese varieties were introduced to Sri Lanka in the early 19th century when the country was a British crown colony known as Ceylon. Tea plants were grown in the estates near Kandy and in Nuwara Eliya but tea cultivation remained a minor activity at the time because coffee was the country’s main export crop. In the l870’s, however, a dreaded fungal disease known as the ‘coffee blight’ struck the coffee plantations. It destroyed the coffee plants and with them the entire coffee industry. The local economy then shifted to the ‘new crop’ — tea — within a few years.

The rapid substitution of tea for coffee was due to the initiative and untiring efforts of a Scotsman, James Taylor. In 1851 Taylor had signed on as an assistant supervisor on a coffee plantation in Ceylon. His employers, highly impressed with his work there, put him in charge of the Loolecondera estate to experiment with tea plants. The Peradeniya nursery supplied him with his first seeds around 1860. By l867, he had planted tea in several hectares of forest land which had been originally cleared for coffee plantation and demonstrated the feasibility of tea as a plantation crop. A few years later he set up the first tea ‘factory’ in Ceylon. In 1872, Taylor invented a machine for rolling the tea leaves, an important step in the process in which green leaves are converted to black tea. Ceylon Tea became famous in the tea trade and its success led to the opening of an auction market in Colombo in 1883, and formation of the tea dealer’s association some years later.

Today, there are many hybrids which have been developed for optimum quality and for adaptability to specific agro climatic conditions.  Tea is the most popular non-alcoholic beverage in many parts of the world. It is a valuable foreign exchange earning agricultural crop and is one of Sri Lanka’s major export commodities. The chemical composition, especially the polyphenolic compounds, of the fresh tea leaves are responsible for the unique qualities of tea giving it its distinct aroma, superior flavor and therapeutic properties, making it a wonder beverage indeed.

Food facts

Food facts - We often assume that bacteria and substances added to food cause problems in our food, but natural and environmental toxins are also responsible for adverse health effects.
http://www.iupac.org/publications/ci/2011/3304/bw1_von-holst.html

Chemistry and Safety of Food Colours

Note: A version of this article appeared on Chemistry in Sri Lanka January 2011 Vol.28 No.1

Sensory properties of food such as colour, flavour and texture make food appetising and attractive to the consumer.  Colour is an important attribute of food. We judge the safety or freshness of a food based on its colour. Dinner guests compliment the hostess by looking at the food laid on the table, even before tasting! Our visual perception of food therefore is very important.

Before the implementation of food laws and regulations, colours were used indiscriminately by food vendors.  The results were disastrous.  Lead, copper and arsenic salts were used to colour food.  Meat and fruit products were coloured using azo dyes, originally used in the textile industry.  Some of these dyes are now known to be carcinogenic.  Today, the list of permitted food colouring is regularly updated so that consumer safety is adequately looked after. 

The trend now is to adopt new colours derived from natural pigments that occur in food, such as turmeric, paprika and carotene. However, natural food colours are not easy to isolate and use as colouring material because of their instability outside the natural environment and under processing conditions.

Natural Food Colours
Chlorophyll
The structure of chlorophyll (a) and (b) is shown below.

Chlorophyll a, X = CH3
Chlorophyll b, X = CHO

The presence of magnesium in the molecule is very important in producing the bright green colour of the pigment. Cooking and processing conditions can result in the loss of magnesium which will impart a ‘dirty’ brownish colour due to the formation of pheophytin. This occurrence is highly undesirable in green peas. When peas are canned the acidity formed results in the replacement of Mg 2+ with protons, i.e. the formation of pheophytin. Frequently artificial colouring is added to make the peas look green.  One must be cautious that poisonous dyes are not used for this purpose.

Synthetic dyes should always be used with caution.  Food chemists are constantly encouraged to examine the possible use of natural derivatives as alternatives.

In the case of chlorophyll, it is known that in the past, cupric sulphate was used, but now a mixture of tartrazine and Green S is most frequently used. In the 18th and 19th centuries vegetables were pickled in vinegar, in copper pans. This gave a bright green colour to the vegetables due to the leaching of Cu 2+ ! Legislation now ensures that pickles are free from copper.

Sodium copper chlorophyllin is now used as an artificial colouring material. It is more soluble and heat resistant than natural chlorophyll, and the amount of copper is negligible: too small to be harmful

Carotenoids

The bright yellow and orange colours in  vegetables such as carrots and pumpkin, the bright red colour of tomatoes, and the attractive pink of watermelon are due to the presence of carotenoids. 

Carotenoids are generally insoluble in water, being soluble only in non polar organic solvents. Though very stable in their natural environment, heating and extraction into organic solvents make them unstable. Exposure to sunlight, and dehydration can cause fading of colour. Isomerisatin of the trans double bonds to cis causes this loss of colour.

TABLE 1 CAROTENOIDS COMMONLY USED AS FOOD COLOURANTS.

Pigment	Commercial source
Beta carotene, mixed carotenes	Palm oil, carrots, and algae
Cis-bixin, norbixin	Annato
Capsanthin	Red peppers (Capsicum annum)
Lycoopene	Tomato
β -Apo- 8’-carotenal	Chemical synthesis
Ethyl ester of β -Apo- 8’-carotenal	Chemical synthesis
Lutein	Marigold, alfalfa
Canthaxanthin	Chemical synthesis
Crocin, crocetin	Saffron (Crocus sativus)

Anthocyanins

A range of pink, red, mauve, purple and  blue colours of fruits leaves and flowers are due to the presence of anthocyanins.

The different glycosilation patterns and acylation result in a wide range of natural anthocyanins.  The aglycones are known as anthocyanidins,  the common ones being cyanidin, pelargonidin, delphinidin, and malvidin.  Most anthocyanidins are found in black grape, cyanidin in red fruits such as strawberries and delphinidin in black current, dark grapes and aubergines.  At low pH of fruits in the natural environment anthocyanins are stable.  However they are very sensitive to pH changes and processing conditions such as treatment with sulphur dioxide and sulphites resulting in bleaching.  Anthocyanidins form dimers and polymers with certain flavanoids present in fruits such as grapes, especially during the fermentation of wine, where procyanidins are formed.  Procyanidins are more stable than anthocyanidins and give a brown red colour to wines.  Some sources of anthocyanins used as food colourants are grape skins and red cabbage.

Fig 2. Transitions in the anthocyanin structure

Fig. 3 Structure of malvidin (3-coumaryl gluscoside)


Betalains

Betalains are divided into two groups, the purple red betacyanins and the yellow beta xanthins.  Pigments of significance in food are those present in beet rood (red beet) Beta vulgaris. Ninety percent of beet root betalaines are betacyanins.  These are quite different from anthocyanins chemically and in their stability, though earlier they were mistakenly named as ‘nitrogenous anthocyanins’.  Their colour is not affected by pH normally found in food and are more resistant to processing.  However, they are unstable to heat at neutral pH.

Fig 4.    Betanidin

Curcumins

These are derived from turmeric (Curcuma longa).  The powdered dried root of the plant is used as a spice and a food colourant especially in oriental cookery.  Curcumin is obtained by solvent extraction of the powdered turmeric and is a bright yellow pigment.  Due to increasing concern about the potential toxicity of synthetic food dyes, curcumin has become useful in colouring desserts, pickles, icecreams etc.   Turmeric also has many medicinal, antioxidant and antiseptic properties.

Fig. 5 Curcumin
   
Colours formed during processing and manufacturing

In addition to natural colours of plants and synthetic dyes added to food, colours are formed during food processing and manufacture. Browning due to enzyme activity, sugar caremalisation and Maillard browning (initiated by the reaction between reducing sugars and amino acids) are such examples.

The most important attribute of black tea, for which Sri Lanka is famous internationally, is its charatersitic colour and flavour.  The orange-red compounds in tea, the theaflavins, are formed during the fermentation stage of tea manufacture.  This is in fact  an enzymic oxidation and not a microbial fermentation. The tea catechins form dimmers which have a benzotropolone ring system responsible for the colour.  Thearubigins are the brown pigments in tea.  The ratio of theaflavins to thearubigins determine the final colour of the tea brew.  Tea polyphenols cause the astringency of the beverage and are known to have antioxidant properties as well as many beneficial therapeutic effects.

Fig.6 Theaflavin

OR = OH or gallate

Artificial food colourants

As mentioned previously, artificial colouring of food has given opportunities to the unscrupulous vendor to cheat the consumer.  In the past, as far back as 1857, a survey of adulterants detected in food revealed that substances such as lead chromate, mercuric sulphide, red lead, and even copper arsenite were used to colour sweets.  Aniline dyes  which were subsequently developed  eliminated the use of metallic compounds.

Synthetic dyes are cheaper, more stable and brighter with a wider range of shades than natural colours.  The use of synthetic dye stuffs to colour food therefore steadily increased but at the same time strict legistlation removed protentially toxic chemicals from the list of food dyes, limiting the permitted dyes to only eight in Sri Lanka. These are listed below

1. Carmoisine
2. Ponceau 4R
3. Sunset Yellow FCF
4. Tartazine
5. Erythrosine
6. Fast Green FCF
7. Indigo Carmine
8. Brilliant Blue

Food colours are mostly used in fruit containing products such as desserts, confectionery, icecreams etc, and in colouring canned peas. Erythrosine is used to colour canned meats. Addition of colour to raw or unprocessed food (meat, poultry, fish, vegetables, tea, coffee..) is prohibited.

Testing food colourants is often confined to looking at evidence for carcinogenicity . However, food colours such as tartrazine are known to cause intolerance in some individuals, manifesting as allergic reactions such as asthma or eczema.

Despite the growing concern about synthetic dyes, colour remains an important attribute by which the consumer is guided when selecting food. Food chemists and technologists therefore play a vital role in ensuring the safety of consumers, while looking after their culinary preferences.

References:
Tom Coultate , Food The chemistry of its components (5th Edition) RSC Publication (2009)
S. Carmen Food colorants: Chemical and Functional Properties CRC Press Boca Raton, Florida (2007)

A Test Post

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