(WASHINGTON POST) - At the Saturday farmers market in the college town of Bellingham, Washington, customers are lining up for the latest innovation at chef Gabriel Claycamp's bone broth cart. Equipped with an espresso machine, workers at Cauldron Broths are steaming a latte-like soup drink they call Froth Broth. Soup-baristas on a recent weekend offered a US$4 (S$5.50) special, infused with the flavours of tom kha gai, and recommended sparking the unseasoned US$3 broth-latte with condiments such as a tiny spoonful of sea salt.

If the idea makes (non-chicken-scented) steam come out of your ears, you are on one side of the bone broth debate. If it sounds as good as Claycamp's regular customers say, you're on the other.

The trend-topping drink, loved by celebrities, athletes and many humbler figures in search of better health, is generally made by simmering bones for hours in water with added vinegar. Acolytes say the resulting collagen-rich liquid reduces inflammation, cures leaky guts, nourishes the immune system, strengthens bones and promotes radiant hair and skin. Detractors think it is a ridiculous rip-off. Even Claycamp, whose past projects include a Seattle cooking school and a well-respected butcher shop, has friends who ask whether he is just peddling a fancypants rebranding of plain old soup stock.

Our backlash nation has a well-established excitement-to-ennui curve for every food trend, including US$4 cupcakes and coconut water. Bone broth, though, attracts an unusually - it has to be said - visceral divide. Mention it online and every word of praise for it as a magical elixir will be matched by a scoff that it is "hipster" hogwash.

"It makes sense that people are angry about it," said Ms Melissa Abbott, vice-president of culinary insights at the Hartman Group, a company that researches food- industry trends.

To understand why, it helps to look back - on some level, way back.

First, the people who say bone broth is nothing new are quite right. Boiled bones have been around as long as humans have cooked with fire. Its current form is documented at least to the 1500s, said cultural historian Libby O'Connell.

"In Europe, in the late Middle Ages, it was known as what was called 'restaurer.' It was a restorative broth" - and the source of the word "restaurant," said Ms O'Connell, author of the culinary history book The American Plate.

"People were very fond, especially in the wintertime, of buying what we would call bone broth... It was nutritionally dense, made essentially from leftover bones boiled until all the goodness and nourishment came out." Cooled, it became gelatinous. That calves-foot jelly of Victorian novels, the invalid's cure-all? "It's cold broth." And do not forget beef tea.

What we might call the first brodo bar - and what is popularly considered the first restaurant - came to France in 1765 when a certain Monsieur Boulanger opened shop near the Louvre (which was then a palace, not a museum). Boulanger sold the clear broth "as a cure for nervous exhaustion, people suffering from general fatigue and people who needed, essentially, a quick meal that you could drink on the run,"Ms O'Connell said.

Sound familiar?

In more recent years, bone broth was a low-key health food favourite, generally differentiated from soup stock by its ratio of meat to bone, or possibly the use of aromatics, or maybe the addition of vinegar. There is no formal consensus. Some serious fans were followers of the Weston A. Price Foundation, a Washington-based non-profit organisation that promotes "nutrient-dense foods." At that point, drinking bone broth was about as trendy and controversial as sprinkling nutritional yeast on your popcorn.

The gelatinous surge

Then, a few years ago, came mass popularity. It seemed sudden from the outside, but for Ms June Jo Lee, a food ethnographer who works with companies such as Google Food, it was a logical puzzle piece in the newly mainstream appeal of health and wellness. Fat was back, umami was in, sugar was out. Interest surged in "functional foods", to which "culinary medicinals" such as turmeric and ginger could be added in.

"By 2013, everybody just got it,"Ms Lee said. "It was a thing, it was real, it was part of our cultural norm."

Bone broth also, of course, was the poster child of the Paleo food movement, the most popular diet in the United States for the past few years, noted Sophie Egan, a programme director at the Culinary Institute of America and author of Devoured, an exploration of the modern American diet.

For Paleo followers, "meat and animal products really are the foods you can luxuriate in, and I think that's why you see this proselytising about bone broth's divine powers", she said. "'I can't have coffee or tea or dairy or grains, but I guess instead of going to Starbucks for my morning coffee, I can stop by the brodo counter to sip my morning broth.'"

That brodo counter was what brought the broth to full fad status, courtesy of chef Marco Canora of New York's Hearth restaurant, who claimed the drink had completely revitalised his health. In 2014, Canora opened Brodo, a takeout window selling broth by the cup.

Similar broth bars souped up the scene in other urban centres and college enclaves, along with bone broth home-delivery services and carts or freezer-packs. People bought into it, even at up to US$10 a cup. Except for the ones who did not.

Proselytising and polarising

America's growing class divide fuels some of the tension around bone broth, as with its Instagrammable friends, such as avocado toast.

"There are a lot of young people who, because of the nature of their work, can afford it all of a sudden," Ms Lee said. It's not just the perception of luxury spending that annoys more frugal generations, she thinks, it's that today's broth-sipping 20somethings aren't spending that money on mortgages or retirement plans or other traditional means of long-term stability. "They're spending it on experiences. I think that makes people angry."

Then, there's the proselytising that Egan mentioned - and the idea that broth is promoted as the simple answer to so many problems. Paleo is an unusually strict and hard-to-follow regimen, she said, encouraging "diet evangelism", where people try to convert others to their way of eating.

"Being on a diet really is like adopting a religion," she said. And miracle cures of the sort that Paleo promises are also "very polarising".

The divide has intensified along with bone broth's popularity, said Ms Abbott, because of an accompanying "misappropriation" of the authentic high-quality drink. "The challenge here is that we have modern food culture trying to create a convenience item out of it."

Last year, in what Eater.com called "peak bone broth", a company began selling bone broth K-cups for Keurig machines. "Bone broth powder" is discounted by the pound on Amazon.com. Bottled refrigerated juices boast added bone broth. Supermarkets sell aseptic, pasteurised, shelf-stable boxes of it in the soup section - "not an authentic product", Ms Abbott said - and sales are even rising of a highly processed bouillon soup base that includes corn syrup solids and caramel colour.

"To me, that was a good indication that consumers weren't sure what the whole trend was about," she said.

So, where do we go from here?

The fad aspect of bone broth, Ms Abbott thinks, will die down, while progressive customers keep simmering up batches at home and "passionate" businesspeople will still find customers in neighbourhoods such as Bellingham and Brooklyn. Repeat customers will drop off if they do not see miraculous results.

Perhaps broth's polarisation will shrink along with its reach, when it is no longer pushing boundaries or offending purists.

And indeed, recent customers kept a steady line five-deep at the Cauldron broth cart sandwiched between a strawberry farmer and a farmstand brewery. Some were already committed regulars, while others accepted samples with apparent delight.

Should they be angry? Is it just expensive stock?

Expensive is relative, said Claycamp; organic bones alone cost him nearly US$2 per kg, and his profit margins aren't high. But yes, he said, it is just stock - high-quality stock.

And to his view, stock is the thing that was misappropriated in the first place; a fresh, nourishing staple that devolved over the generations into a mass-market processed food. They couldn't reclaim the name no matter how good their product was, he said.

"So it had to become something else," he said. "Or at least a new name."

Golden Bone Broth

Makes about 7 litres

You will need a 15-litre pot or divide the ingredients between two 7-litre pots. Note: This spends many hours on the stove top. Make sure the chicken parts are thoroughly defrosted (if they have been frozen).

Make Ahead: The broth needs to be refrigerated overnight and can be held that way for up to five days or frozen for up to six months. If you are freezing it, it is best to transfer each litre to a freezer-safe zip-top bag, seal with as much air pressed out as possible and frozen flat.

Adapted from Brodo: A Bone Broth Cookbook by Marco Canora (Clarkson Potter, 2015).

Ingredients

1kg chicken feet (see headnote)

2kg chicken wings

3kg chicken backs and necks (see headnote)

Water

3 large onions, coarsely chopped

6 ribs celery, coarsely chopped

2 large carrots, scrubbed well and coarsely chopped

5 bay leaves

1 Tbsp whole black peppercorns

1 bunch flat-leaf parsley

Fine sea salt

Method

1. Place all the chicken parts in the pot(s). Add enough cool water to cover by 5 or 6cm. Bring to a boil over high heat, skimming off foam/impurities from the surface every 15 to 20 minutes. (Discard what you skim off.)

2. Once the liquid is at a full boil, reduce the heat to low and pull the pot(s) partially off the burner, if you can. Cook for 1½ hours, skimming once or twice, as needed.

3. Add the onions, celery, carrots, bay leaves, peppercorns and parsley, pushing them down into the liquid. Cook for three to five hours, checking the broth once or twice to make sure the chicken parts remain submerged. Add water, as needed.

4. Use a Chinese spider or skimmer to remove the solids; discard them. Strain the broth through a fine-mesh strainer. Taste and add salt, as needed - although if you are using this for more than a sipping broth, you may want to add the salt later. Let cool.

5. Transfer the broth to separate 1-litre containers, leaving any sediment in the pot(s). Cover and refrigerate overnight; it should partially gelatinise.

6. Uncover and skim/discard any solidified fat on the surface before using or freezing for longer-term storage.