A worker checks the temperature of fermenting tea at the Gisovu Tea Factory. This important process, where tea reacts with oxygen at carefully controlled temperatures, gives the tea its characteristic aroma and colour, and turns it from green to brown. As Rwanda's biggest export earner tea is a very important part of the country's development process. Gisovu, Mata and Kitabi, the three mountainous tea estates which border the Nyungwe Forest in south western Rwanda, consistently command the top three prices of any African-grown teas, and supply the tea that is exported to Britain and blended to produce Yorkshire Tea.