Media playback is unsupported on your device Media caption Members of the Park Slope Co-op explain why they're happy to put in the hours

Chris Agee is loading boxes of food and drink products on to a conveyor belt down in the basement of a grocery store in Brooklyn, New York.

One floor above him, shoppers push trolleys up and down the aisles, picking up their weekly groceries.

But this isn't Chris's full-time job. In fact he's not getting paid a wage at all. Chris actually teaches political science at City University of New York.

He is one of the 16,000 members of the Park Slope Food Co-op, who volunteers at the store.

The co-operative is one of the few in the US that still has a member labour requirement - members have to work there if they want to shop there.

Image caption Chris Agee usually teaches political science but says he is proud to work in the Co-op

Cheaper products

Chris admits that some people find it hard to believe that he stacks shelves at the weekend, but he says he does it for two main reasons - the quality of the food and the pricing of the food.

"We hire 80 people to select the best quality produce, so when you shop here you really feel like you're getting food that's healthy and organic.

"The other reason why I do it is there's no profit margin that goes to some unknown group of people. We end up paying a third less because there's no profit margin because it's a co-operative," he explains.

"People ask me, 'What were you doing?' I say, 'Stocking tomatoes.' But it's fun, it's a change of rhythm and actually I'm quite proud of it."

Image caption Ann Herpel says Park Slope members range from bus drivers to people who work for hedge funds

'The world's changed around us'

All members of the Park Slope Co-op are required to work 2.75 hours every four weeks. Jobs vary from working on the check-out to unloading deliveries to working in the office upstairs.

As co-ops have grown in their sales volume, there was less of an actual need. They could [afford to] hire trained staff Adam Schwartz, The Co-operative Way consultants

When you enter the shop as a customer, people on the entrance desks will scan your membership card to ensure that you are keeping up with your scheduled shifts.

Ann Herpel, one of the general co-ordinators at the Co-op, says the member labour requirement was a decision made when the Co-op was founded.

"We're 40 years old this year. It was decided that a co-operative at its base means working together," she says.

"So in order for us to be a true co-operative [it was decided that] working together was the best way of both owning and making a business you own sustainable."

She says that in the 1970s Park Slope wasn't the only working co-op in the US, and that though it might be unusual today, it is only because others have moved away from the original model.

"We don't really feel we've changed, we feel the world's changed around us."

Natural evolution

That's certainly true. The National Co-operative Grocers Association (NCGA) says that of the 170 stores their members operate, none have a mandatory labour requirement. (Park Slope is not a member of the NCGA.)

What is a food co-op? A co-operative business is owned and run by and for the group's members

A food co-op is a food distribution outlet, organised as a co-operative

It typically offers natural foods

Decisions on production and and distribution of food are taken by members

Consultant Adam Schwartz from The Co-operative Way - a firm dedicated to improving the operations of co-ops - says there are many reasons why most other co-operatives in the US dropped the requirement for members to work.

"Part of it is a little bit of natural evolution - as co-ops have grown in their sales volume, there was less of an actual need. They could [afford to] hire trained staff."

Paid staff also bring more professionalism to an organisation, he says.

"When you're paying someone you have a very good expectation that they're going to be there that you might not have with volunteers. Plus paid staff are going to develop more expertise with the products."

So what is it that makes Park Slope different?

The neighbourhood became increasingly gentrified in the 1980s and 1990s but never lost its sense of community, according to Ms Herpel.

"Community is something lacking in many ways in America," she says.

'Good for the health'

Musician Shawn Onsgard agrees. He's been a member of Park Slope for six years and writes articles for the Co-op's newsletter about how to grow food safely in Brooklyn and the specific issues that people face gardening here.

Image caption Vegetarian Shawn Onsgard says the variety and quality of the food at the Co-op is much better than at its rivals

He says in many ways community can actually be developed through labour.

"Putting produce on the shelves, taking boxes off trucks, the kinds of conversations that you have with people - it becomes fun and you come to the Co-op and it's hard not to bump into people that you know and to have friendships.

"I really like the relationships I've made with people through the process of doing work here. It's been a real community-building, friendship-building experience for me."

And as Chris Agee puts it, working at the Co-op is "good for the health, it's good for the pocketbook, it's good for social relations".