Consumers are increasingly willing to pay more for foods they believe were sustainably produced, like free-range chicken, fair-trade coffee and pesticide-free wine. But what does 鈥渟ustainable鈥� actually mean?

鈥淭here鈥檚 not necessarily just one certification that people are going to find stamped on their food that says, 鈥楾his is a sustainable food product,鈥欌€� said Rob Myers, who is one of the directors of the U.S. Department of Agriculture鈥檚  (SARE) program. 鈥淲e don鈥檛 have that yet and we鈥檙e not likely to have it any time soon.鈥�

The USDA鈥檚 , which dates back to the 1980s, includes three broad goals: Sustainable farming means using practices that aren鈥檛 harmful to the nation鈥檚 land, air and water; that are economically viable; and that improve the quality of life for farmers, ranchers and their communities.

That definition doesn鈥檛 necessarily include 鈥渘atural,鈥� 鈥渙rganic鈥� or 鈥渓ocal,鈥� according to Myers.

鈥溾€楲ocal鈥� in itself does not necessarily connotate 鈥榮ustainable,鈥欌€� Myers said. 鈥淏ecause as you鈥檙e implying, you could have a lot of pesticides applied, or you could have a farm that is not preventing their soil from washing away or contaminating local water sources, or doing other things that most people would say, 鈥楾hat鈥檚 not very good from a sustainability standpoint.鈥�

Some retailers are capitalizing on the consumer demand for foods produced in environmentally friendly ways and using labels to indicate which of their products are sustainably produced. Like Whole Foods, which is  on all of its produce and flowers in September. They will rate these items on labels as 鈥済ood鈥�, 鈥渂etter鈥� or 鈥渂est鈥� and each rating will correspond to the way the product was raised.

鈥淲e have had customers asking about sustainability: 鈥楬ow is this particular kale raised or where do these flowers come from? What is the sustainability of our earth? How do the plants work into that? How does this affect my health?鈥欌€� said Margaret Wittenberg, global vice president of Quality Standards and Public Affairs at Whole Foods. 鈥淓specially as people are told to eat more fruits and vegetables, that's an important question to ask. So we want to be able to give the tools to our customers to make these better decisions for themselves.鈥�

Whole Foods already has a  in place, along with a  implemented for meat that鈥檚 鈥淩aised with Care.鈥�

Many other retailers now include sustainability in their missions, including Safeway, which . Chipotle is trying to  from suppliers that don鈥檛 treat their birds with antibiotics or include additives in feed like arsenic. Wal-Mart is gauging sustainability in part through  to develop fertilizer management plans, with the goal of reducing the amount of fertilizer applied to farmers鈥� fields.

With retailers looking to provide more food they can market as produced with environmental impacts in mind, food producers have taken notice.

On his farm in Rocheport, Mo., Bill Heffernan remembers when Whole Foods approached his Ozark Mountain Pork Cooperative five years ago and asked farmers to begin selling the company humanely-raised meat. To meet the first step in Whole Foods鈥� 5-Step Animal Welfare Rating Standards, farmers had to prove they did not house their sows in metal stalls called farrowing or gestation crates.

鈥淲hen Whole Foods came to us, they asked us to meet five steps,鈥� Heffernan said. 鈥淪ome of our members still are at the Stage 1. Many of us are up at Stage 3. That means our animals do have access to outside space. We don鈥檛 use farrowing stalls.鈥�

Heffernan says getting 鈥渟tep-certified鈥� by a Global Animal Partnership auditor takes time though, and costs him about $1,500 in annual fees. That can be a barrier to entry for small farmers.

鈥淔irst of all, they look around the property for about two hours and then sit down, and you鈥檝e got this booklet to fill out like as if it鈥檚 never going to end,鈥� Heffernan said. 鈥淎nd I鈥檓 saying, 鈥楯ust for 300 pigs, I鈥檝e got to answer all these questions?鈥欌€�

But he agrees that these quality audits and piles of paperwork allow Whole Foods to charge more for his humanely-raised meat.

鈥淭he consumer needs the guarantee or the whole system falls apart,鈥� Heffernan said.

In truth, no one wants their food to damage the environment. And now shoppers can put a price on what they care about. But with so many definitions of sustainability out there, how do consumers know what is and isn鈥檛 worth paying extra for?

鈥淭hat鈥檚 the million dollar question,鈥� said Eric Cartwright, executive chef for campus dining services at the University of Missouri in Columbia. Cartwright鈥檚 in charge of making sure 2.5 million meals a year get served and in a perfect world, all of them would be sustainably produced.

"Our approach is to really look at what practices are being used on the farm: Are they good sound practices that allow that farm to continue without having to put a lot of artificial resources into it? Whether that be 鈥� 5 miles away or 500 miles away. And especially as we work with a lot of our smaller farmers, there鈥檚 the relationship that happens,鈥� Cartwright said.

But since it鈥檚 not possible for everyone to have that kind of close relationship with the farmers and ranchers who grow and raise our food, consumers will have to rely on information that retailers and restaurants provide on labels and menus.