Tag Archives: tea manufacture

Tea Talk – Withering

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When plucked fresh in the field tea leaves are crisp and if rolled in the hand will break up into flakes. Rolling machinery in the tea factory needs a soft pliable material to form into the stylish curled and twisted shapes that are typical of premium teas. Withering, at least in the sense that a teaman uses the word, means gently losing moisture from the fresh leaf until the resulting withered leaf is limp and ready for the rolling stage. This means losing some 25 to 50% of the weight of the fresh leaf as evaporated water, and the process takes around 18 hours.

Before we consider exactly what happens in withering we should be clear which teas do and don’t need withering to develop their character.

Green Teas – no wither. Pliable leaf is achieved by steaming (Japanese green teas) or by parching – dry heating in a wok (China green teas)

All other teas require withering, but the style of this varies.

White Teas – very slow and gentle wither often initially in sunlight and then under shade, in thin layers, for up to three days.

Oolong Teas – also begin with 1 or 2 hours of sun withering, then into shade for 16 hours during which time the leaf is “fluffed” many times by hand to gently bruise it.

Black Teas – leaf for these used to be thinly spread on tats (banks of mesh trays) but now it receives 16 to 18 hours of drying on special troughs that allow warmed air to be blown through the bed of leaf until the correct moisture content is achieved.
leaf plucking
Fresh leaf photosynthesizes in sunlight and produces glucose. Further biosynthesis produces a range of compounds that are temporarily stored in the leaf cells as starch and proteins and fats. Loss of water during withering causes the leaf to mobilize its stored reserves converting them to sugars amino acids and lipids – all of which are essential for the development of aroma volatiles and flavours during the rolling and oxidation processes.

This electron microscope photograph of two chloroplasts in a tea leaf cell after 1 hour of withering clearly shows large white starch grains – stored carbohydrate which will be fully solubilized into sugar within 8 or 10 hours. There are about 200 tiny chloroplasts in each leaf cell and each one is a biochemical powerhouse.

cell1

After 13 hours withering the chloroplast shows no starch grains but dark staining lipid globules can be seen as the leaf’s fat reserves are also solubilized.

cell2

At this stage the leaf is almost ready biochemically for oxidation, but withering usually continues to around 16-18 hours to ensure that the leaf is physically ready for rolling or cutting.

The amount of moisture lost from the leaf determines to a great extent the type of tea that will be produced:

Soft wither – little water loss – strong pungent and coloury black CTC teas of Assam and Africa

Medium wither – moderate water loss – orthodox black teas of Assam and Ceylon low country teas with more aroma than CTC teas

Hard wither – much water loss – Ceylon high grown teas with pronounced aroma and flavour

Very hard wither – extreme water loss to the point where some leaf is too dry to oxidize and remains green in the made tea – Darjeelings and Nepals with world famous aroma

Tea tasters can tell just by looking at and tasting the finished tea whether the withering process has been carried out correctly in the factory. Some of the clues to bad withering are:

Harsh metallic taste – wither too short, not enough moisture loss

Flakey leaf – if black and flakey then wither too short (too soft) and if green and flakey then wither too long or too hard

Acid fruity aroma and taste – often due to bacterial growth when rain wetted leaf is withered too slowly or not dried before withering

Plain tasting tea grey in the cup – over long or over hot wither

Tea Talk – Rolling

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Nearly all black teas are pressure treated to initiate oxidation which requires the miniscule chloroplasts in the green leaf cells to be “popped” to release the enzyme polyphenoloxidase that catalyses the oxidation of green catechins to form orange theaflavins (which then polymerize into brown thearubigins) – to make a typical black tea. Popping the chloroplasts requires a lot of pressure – this was originally done manually and now, in Orthodox processing, the pressure is provided by a rolling table.

rolling 1

Traditionally in China green leaf was hand rolled between the palms of the hands rotating them anticlockwise ninety degrees out of sync. In Japan – see photograph to the right – green leaf was hand rolled backwards and forwards between the hands and a flat surface. Thus in China hand rolling gives a twist and curl to the leaf – ultimately giving the well known gunpowder pellet. The orthodox rolling table, particularly the double acting type, is based on this rotating hand action and is used both for green and black tea manufacture. In Japan the forwards-backwards hand motion produces cylindrical needles, and modern Japanese green tea machinery mimics this hand action and product format.

rolling2

Rolling tables were first introduced by the British in the 1880’s as factory sizes grew, and to take advantage of developments in steam power and oil engines. The basic design has not changed to this day – a flat table some 2 metres across bearing an eccentrically rotating drum containing around 300 kg of withered leaf, and a pressure cap on the drum to squeeze the leaf – see photograph above of a British built Sirocco rolling table from the 1920’s: this would have been leather belt driven from overhead lay shafting probably powered by a central oil engine. It clearly shows the table, the drum and the pressure cap. Modern tables are very little different except that they are driven by electric motors – they are still hand loaded and – see photograph below of a new Indian built machine installed in Kenya in 2002 – and still called Britannia!

rolling3

Within the moving drum the leaf travels in a torroidal motion – imagine a doughnut ring with the leaf moving around the ring and also spirally from outer edge to inner and round again – this movement squeezes leaf hard against the table, the drum sides and the pressure cap in turn. Pressure is increased further in some rolling tables by putting crescent shaped battens on the table and a cone in the centre.

Typically a charge of leaf is rolled for 20-30 minutes and then sifted. Fine particles that fall through the sieves are collected for the next stage – Oxidation, while large unbroken leaf is returned to the rolling table for a further roll. In Sri Lanka it is not unusual for there to be four rolls in sequence, each under increasing pressure.

The rolling induced chloroplast popping action is helped by Withering (discussed in the May 2007 NBT Newsletter); withering weakens the cell membranes allowing the chloroplast to rupture with less pressure, thus black teas are withered, but green teas that do not require oxidation are not. The heavy rolling pressure required to induce oxidation – particularly with battens and cone as seen above – not only pops the chloroplasts but also breaks the shoot into pieces and curls and twists it giving character and style. The leaf pieces are ultimately sorted into the well known tea grades of commerce. Pekoe (P) may have some whole leaf but no tips. Orange Pekoe (OP) is primarily twisted stem, and no tips. Flowery Orange Pekoe (FOP) may have curled leaf pieces and some tip. Tippy grades come from the first sifting which removes the tender buds and tips; broken grades are smaller pieces derived primarily from the later rolls.