Sorting and Grading of Tea
Sorting is a mechanical process. In the early days of the industry, it was believed that sorting did not require the same degree of management as other operations of tea manufacture. However, it has been found that in order to get good prices at the auction, proper sorting is essential for all grades of tea in general, and broken grades in particular.
One of the objectives of sorting is to classify tea according to the size of the particles. In the past, the object of sorting was merely to remove foreign matter and big leaves. This process was carried out by hand, with the only implement used being a small pincer used to pull out long stalks. The process of grading was known as sifting at that time. Today, no distinct division can be made between sorting, cleaning and grading since the processes are carried out simultaneously and procedures are common to all. All processes are based on the four important characteristics of made tea. These include:
- size,
- shape,
- specific gravity, and
- surface characteristics of the particles.
Modern Methods of Sorting
Modern methods of sorting were introduced along with mechanisation in the 1870s. Mechanical sorters were developed to utilise power from steam engines and other sources. In 1880, George Reid introduced a mechanical breaker into the sorting operation to reduce the size of leaves. In one stroke, a very tedious process is greatly simplified.
More modern sorting machines incorporate different sieves with meshes of size 8, 10, 12, 16, 24, 30 or greater. Tea passed over a number 8 mesh is cut and sifted again, and so on. As the tea passes through the various meshes, it is classified into grades such as whole leaves, broken leaves, fanning, and dusts.
In modern factories, a Myddleton machine is used at the first stage of the operation for orthodox teas. For CTC manufacture, an electrostatic stalk extraction unit is used first. Usually, in orthodox manufacture, the spillover from the Myddleton is passed through a ghugi or rotary hexagonal sifter; from the trays, the leaf is passed into the Arnott and Macintosh sorters. But in CTC and leggcut manufacture, the ghugi is not used. (Leggcut manufacture was another old method of black tea manufacture where Legg cutter was used to bruish the leaves and leaves were not withered. The method is not followed nowadays. Thereafter, the process incorporates the Britannia Tea Sorter, the Macintosh sorter and the wind tunnel. A number of hand sievings are also essential to produce some quality grades. In wind tunnels and waterfalls, fibre and dust grades are separated according to the specific gravity of the particle. Tea breakers and cutters are extensively used in modern sorting operations to reduce particle sizes for subsequent classification.
In orthodox manufacture, the first classification of tea takes place at the time of sifting. BOP, BOPF and OP grades are produced from the first and second fine teas. The coarse leaf that is first broken or cut goes into the production of BP, PF and souchong grades. In CTC manufacture, sorting is based wholly on the size of the leaf particles.
Precaution to be Taken While Sorting
Sorting should not be carried out directly on hot particles discharged from the dryers; this can destroy the bloom of the made tea. After drying, the particles should be cooled on the floor or on a wooden platform. The tea should not be bulked hot. At the same time, care should be taken to see that the leaf does not cool to the point where it can pick up atmospheric moisture.
