(Reuters) - Salad chain sweetgreen Inc said on Monday it had confidentially filed for an initial public offering in the United States, hopeful of strong investor interest as demand for plant-based food products surges globally. The company, which counts tennis star Naomi Osaka as its youngest investor, was valued at $1.8 billion after a funding round earlier this year, according to media reports. T.Rowe Price, Lone Pine Capital and D1 Capital Partners are among sweetgreen’s other investors. California-based sweetgreen, which was founded in 2007 and has more than 100 stores in the United States, did not reveal more details about the size of the proposed IPO. Plant-based food companies have attracted investor attention over the past few years, particularly as more people gravitate to healthy and environment-friendly food. Much of the demand is being led by millennials and generation Z consumers, who are more than willing to spend on sustainable products that are also healthy. About 65% of Gen Z consumers are in favor of plant-based foods, sweetgreen says on its website. Last year, plant-based retail sales in the United States hit $7 billion, up 27% year-on-year, according to a report by the Good Food Institute and the Plant-Based Foods Association (PBFA). Swedish oatmilk maker Oatly Group AB, which went public last month, closed nearly 53% above its IPO price on Friday. Plant-based burger maker Beyond Meat Inc was also up 16% this year. Reporting by Niket Nishant in Bengaluru; Editing by Shinjini Ganguli Our Standards: The Thomson Reuters Trust Principles.