Microfiltration of high concentration black tea streams for haze removal using polymeric membranes

摘要

Black tea used for ready-to-drink beverages suffers from an inherent haze problem affecting the appeal of products in terms of colour and appearance; the effect also diminishes health-giving properties. Microfiltration of black tea streams up to 10.0wt.% has been carried out in an attempt to remove this haze as a replacement to current alkali solubilisation methods which can damage the product. The three commercial polysulphone membranes tested show superior haze removal in tea when filtered at ambient temperature, with turbidity values of below 5.0 nephelometric turbidity units (NTU) being recorded for all membranes over all processing conditions tested. In addition, haze reoccurrence has been slowed following membrane filtration, with turbidity values generally remaining below 50 NTU after 4weeks of cold storage. At the cross flow velocities used for experimentation (0.5-1.5ms(-1)), fluxes were below what would be classified as acceptable for industrial applications although flux improvements were shown to be apparent for elevated tangential velocity and transmembrane pressure (4.0bar) with membrane resistance during filtration dominated by concentration polarisation, a result of the high solids load in the feed. There is a loss of polyphenolics and colour in the permeate product although a trade-off between haze and stability over time and levels of key polyphenolics (responsible for colour and flavour) was expected. The rejection coefficients for polyphenolic species was much greater compared with total solids rejection for all operating conditions. Tea permeate appearance showed strong dependency with operating conditions, which shows the potential for tailoring of tea appearance, a useful capability in terms of maintaining product consistency given natural variability, a result of growing conditions and other factors. Overall, the potential for microfiltration of black tea has been shown to be effective in removing haze with this article highlighting the research efforts required to make this process a scalable industrial possibility.