Sweet, milky and colourful — bubble tea is wildly popular in China, where people sipping through straws from large plastic cups is a common sight in high streets and shopping malls across the country. But there’s fresh competition brewing in the vast market, characterised by ultra-cheap products that are striking a chord with China’s increasingly frugal young consumers. Bubble tea — which classically includes tapioca balls and comes in a wide range of flavours, with or without milk — has gained huge popularity in China, coinciding with an economic boom in recent decades that propelled living s…