Masa Shiroki is busy bottling his latest batch of Junmai Ginjo Genshu. The Granville Island artisan sake maker will fill 1,800 bottles by the end of the day – but that’s not all he will be left with.

There will also be 170 kilograms of leftover rice mash, known as kasu sake – an unprepossessing byproduct that looks more like putty than the latest star ingredient on menus across the city.

In Japan, the traditional uses of kasu include stirring spoonfuls into soup, feeding pigs or as a cosmetic skin product. In Vancouver, it’s used as a marinade, a cure for prosciutto, a mayonnaise and the base for a sorbet.

“I started giving chefs a taste of the kasu when they were in the store looking at the sake,” Mr. Shiroki explains. “Many of them were intrigued enough to take a tub away with them and begin experimenting.”

Kasu is recovered by filling traditional long, thin sacks with the moromi (fermented sake and residual solids) and gently pressing them to separate the alcohol from the mash.

The result is a smooth, grey paste, similar in texture to cream cheese, that boasts an impressive nutritional makeup. It is rich in protein – 100 grams is roughly the equivalent of 70 grams of beef – and vitamins B1, B2 and B6. Though the alcohol burns off when the kasu is heated, the protein and vitamins remain intact.

If you’re sampling it raw, the first thing that hits you is the residual alcohol (about 7 per cent); then the rice flavour comes through. It’s a perfect example of the sought-after umami – the fifth taste produced by protein-heavy foods, of which the closest English translation would be “savouriness.”

“I love the creamy, mild flavour of sake and rice,” says Quang Dang, chef de cuisine at C Restaurant. “Texturally, it has a wonderful mouth feel.”

At C, the kasu is made into a mayonnaise-style sauce that accompanies spring salmon fishcakes. “It is really good for making sauces and mayo without eggs, because it emulsifies beautifully,” Mr. Dang says.

A more traditional approach is taken at Aurora Bistro, where chef and owner Jeff Van Geest uses kasu as a marinade for sablefish.

“It’s just an amazing flavour,” he says. “I marinade the fish overnight – it’s supereasy to use and I love that although the rice Masa uses is not local, we can claim the kasu as a local product.”

Mr. Van Geest has also mixed the kasu with salt to cure a ham and must wait another eight months to try the results. (Impatient locals can head to Oyama Sausage Co. in Granville Island market where they make their own kasu prosciutto.)

Angus An, chef and owner of Gastropod, has embraced the possibilities of kasu just as enthusiastically. Throughout the month, he is offering a tasting menu paired with different sakes (both imported and Mr. Shiroki’s). The kasu features in two dishes. Braised pork cheek with scallops and spot prawns is set in an organic barley miso soup. The pork is marinated in the kasu for an hour then cooked overnight sous vide.

The result is butter-soft meat with a background accent of sake – eaten with the miso soup, it’s an umami explosion. More radically, Mr. An’s second application is a kasu sake sorbet, where the kasu is mixed with yogurt and sugar to create a refreshingly boozy ice.

“I even tried to make pasta with it,” Mr. An says with a laugh. Though he is pushing the envelope, the chef says the home cook can easily use kasu and shouldn’t feel intimidated.

“You can simply brush it on meat or fish before grilling, or mix it with cottage cheese and eat it with fruit. Some people make oatmeal and add a scoop on top.”

Mr. Shiroki is developing a range of kasu-based products he hopes to market later in the year. “We’re looking at condiments – salad dressing and sauces – and we’d like to come up with a beverage, too,” he says. For now, he’s simply selling it raw from his Granville Island store for $3.50 a tub.

[via The Globe and Mail]