Cleaning processes that help in the production of tea varieties comprise of three stages: cleaning after plucking, during manufacturing and during packaging. Tea manufacturing involves different stages like green leaf intake, withering, rolling, oxidation (fermentation), drying, sifting/grading/cleaning of grades, packing and dispatch. Besides these, moisture and temperature control also needs to be taken into consideration in the context of tea leaves, dust and bacterial contamination. When the leaf brought from the field is dry, there is little increase in bacterial growth. But if the leaf brought to the factory is wet, as it happens during the rains, the conditions become favourable for bacterial growth. The food material required for bacterial growth is drawn from the leaf itself. The bacterial growth is checked with the removal of surface moisture.
Tea leaf in the field carries on its surface a large population of naturally occurring bacteria. So when the leaf is carried to the factory, a considerable number of bacteria are likely to be carried along with it. These bacteria under favourable conditions develop very rapidly. The long interval of 16 to 18 hours between plucking & processing followed by oxidation periods of about 1.5 hours for cutting, tearing and curling (CTC) and around 2.5 hours for manufacture allow the possibility of a considerable bacterial build-up. During firing, all the bacteria are destroyed, but moulds have been known to persist even after drying.
Sources of Microbial Contamination
The possible sources of contamination of tea include the oxidized leaf, factory floor and air. The factory floor up to the stage of fermentation can be kept clean by washing properly, but the same thing cannot be followed in the drying room and beyond. If the drying and sorting rooms are not washed or if the atmosphere is warmer and dusty, then a different group of moulds start growing. These microbes affect adversely the tea quality. Warm, dusty atmosphere and moisture along with the changed substrate in the made tea provide ideal conditions for growth of the moulds.
Talking about hygiene in the manufacturing process,
Dr Mohan Kumar, Director, UPASI Tea Research Foundation, Tea Research Institute, Valparai, Coimbatore says, “The teas marketed in India have to pass Food Safety and Standards Authority of India (FSSAI) specifications that include black tea quality parameters and the amount of pesticides residues & heavy metals.”
Explaining the cleaning processes in detail Suresh Jacob, Vice president, Plantations, AVT Ltd, said, “The first stage of manual cleaning helps in removing only foreign bodies; all the bacteria get killed in the subsequent stages of drying. At green leaf stage, removal of foreign body like stones, leaves etc are done. In sorting and grading, removal of iron fillings at various stages and removal of stalk & fibre (tea fibre) are accomplished to have a clean black tea.” Withering and fermentation, which are entirely dependent on change in ambient weather condition need close supervision. In order to ensure that withering takes place in a favourable setting, cleaning of machineries is meticulously done.
Jacob goes on to explain the need for manual cleaning and why no sophisticated machinery is required to aid the cleaning of tea. “The bolder leaf grades are cleaned by a machine called colour sorter to clean and remove the long fibre and the smaller grades are cleaned through fibre extractors. The Wet processing area rollers/ fermenting area have to be cleaned well after every day’s manufacture – ideally with hot water to kill the bacteria.” In terms of the latest machinery that is available,
Dr Kumar opines, “The cleaning of tea machinery and floor in tea factories is done using high pressure water jet.”
During rolling, there is an immediate drop in the microbial population due to the antiseptic properties of oxidised polyphenols. But during this process the leaf juice is expressed which drips down to the floor with fragments of leaf and may become a source of bacterial growth. The juice and the fragments of leaf may also be found adhering to the roller caps, roller doors, ball breakers, CTC machines etc. If not cleaned properly, this infection is carried over to the following day. It is a common experience to see plenty of leaf particles adhering to the inside cover of the CTC machine for a long period if not properly attended to. This makes the leaf over fermented and also provides the source for bacterial growth. Similar is the case with the roller-doors where juice and fragmented leaf particles may be the source of contamination.
The floor of the rolling room can be a dangerous source of infection in the factory. The juice expressed on the floor should be washed away immediately. The floor should have a steep slope from the rear to the front of the rollers to facilitate washing and removing the juice to the side drain. However, a rung trolley placed under the rolling table is always preferred.
The use of all metal leaf trolleys reduces the bacterial growth possibilities in this stage. But these are to be kept thoroughly clean. Green leaf sifters can infect leaf with bacteria very easily. A high-pressure hose is suitable for cleaning these sifters. Although precaution may be taken during the daily cleaning at the end of the day’s manufacture, the infection tends to develop on sifters during the course of manufacture, as adequate cleaning is not possible unless the whole of the leaf has been rolled. It is, therefore, advisable to have more than one sifter so that one can be cleaned when the other is in use.