Rolling of the withered leaf was perfected over centuries, a process which brings out flavour in tea. Rolling assists in establishing proper conditions for enzymatic oxidisation of the flavanols by atmospheric oxygen. This is achieved by disruption of cell structure which facilitates enzyme substrate contact.
The purpose of rolling is to achieve the final curved appearance and to break the leaf cell walls so as to release essential oils to start a chemical reaction of fermentation. The leaf is rolled several times. The first roll, known as the pre-conditioning roll is very gentle, of which the aim is to cover the twisted particles with the juices from the leaf. The rolling process is what releases the colour, strength, aroma and the taste of the liquid we ultimately pour into our cups.
When the leaf cells are ruptured following the rolling of the withered leaf, the enzymes in the leaf come in to contact with the oxygen in the air which initiates chemical reactions that are necessary for the production of black tea.
Roll breaking has two primary objectives. To remove the twisted leaf off the rolled shoots that clogs and impedes circulation, and to facilitate further twisting action on the large leaf. Additionally, it also cools the leaf which would have risen in temperature during the process of rolling.