That’s a lot of cold brew. Amazon announced early Friday that it will buy upscale supermarket chain Whole Foods for $13.5 billion. Under the proposed deal, Amazon will pay $42 a share, a 27 percent premium to Thursday’s closing price. It will open the door to Amazon, which has been experimenting with food and grocery delivery, as well as billions of products from every category. John Mackey, the co-founder and chief executive of Whole Foods, said the partnership is an opportunity to maximize the company’s value “while at the same time extending our mission and bringing the highest quality, experience, convenience and innovation to our customers.” The deal has not been approved by Whole Foods shareholders, and must also receive regulatory approval. It is expected to close in the second half of the year.