Expo Milan 2015, the first world fair since Shanghai's groundbreaking event five years ago, opens its doors on Friday amid optimism that the six-month nutrition and gastronomy-themed fest will help lift the host country out of the economic doldrums.

"We can say with great satisfaction that the Expo gamble has paid off," Italian Prime Minister Matteo Renzi said on Monday, underlining the high hopes invested in an event expected to draw more than 20 million people.

With more than 50 countries hosting exhibitions in purpose-built national pavilions and more than 140 involved in some way, Italy is counting on long-term benefits in terms of revamping the country's brand image and attracting inward investment.