Starbucks will close more than 8,000 shops in the US on Tuesday to conduct anti-bias training, the next of many steps the company is taking to try to restore its tarnished image as a hangout where all are welcome.

After the arrests of two black men in Philadelphia last month at one of its stores, the coffee chain's leaders apologised and met with the two men, but also reached out to activists and experts in bias training to put together a curriculum for its 175,000 workers.

That has put a spotlight on the little-known world of "unconscious bias training," which is used by many corporations, police departments and other organizations to help address racism in the workplace. The training is typically designed to get people to open up about implicit biases and stereotypes in encountering people of color, gender or other identities.

The Perception Institute, a consortium of researchers consulting with Starbucks, defines implicit bias as attitudes - positive or negative - or stereotypes someone has toward a person or group without being conscious of it. A common example, according to some of its studies, is a tendency for white people to unknowingly associate black people with criminal behavior.