"What I want is to create a great sushi experience without the tuna," Corwell tells The Salt.
To make this Tomato Sushi, he skins and removes the seeds from fresh Roma tomatoes. Then he vacuum seals them in sturdy plastic bags and cooks them in hot water for about an hour — a technique called sous-vide.
The process firms up the tomatoes and creates a texture similar to tuna. Corwell throws in a few more ingredients (he won't divulge what they are), and slices them up. When eaten with sushi rice, nori, ginger, soy sauce and wasabi, they're delicious.
Corwell is not the only entrepreneur experimenting with fish-like alternatives to seafood. (His product is so far available at one retail market in San Francisco and via mail order.) But with issues like overfishing, bycatch and high mercury levels gaining traction with consumers, it may only be a matter of time before demand kickstarts a faux-fish movement on the heels of the plant-based protein revolution already underway.
So far, a handful of sushi chefs and food manufacturers are testing the waters. Sophie's Kitchen makes vegan calamari, scallops and fish fillets, with most products breaded and ready to bake or fry. Their VeganToona actually comes in a can; it's made from pea protein, potato starch, seaweed powder and olive oil.
Garden Protein International uses soybeans and other plant material to make a line of vegan meat substitutes, including a "Fishless Fillet."
But in spite of these trailblazers, the alt-seafood market is still a few steps behind plant-based alternatives to meat, egg and dairy products.
Beyond Meat, a company based in Los Angeles, makes vegan ground beef and chicken strips with soy and pea protein. "We take [amino acids, fats, carbohydrates and minerals] and, with heat and pressure, stitch them together in the architecture of meat protein," he says. "We do what it takes a cow two years to do in three minutes."
Impossible Foods is making plant-based beef, complete with bioengineered plant-based blood, and cheese. Hampton Creek Foods makes a vegan mayonnaise with pea protein in place of eggs.
Muufri is a small firm still in the research-and-development stages of creating milk — without the cow. The production process, explains one of the company's founders, Isha Datar, involves genetically modified yeast cells, which reassemble amino acids into milk-like form.
Datar says that her company is not inventing new versions of dairy products. "We're just learning new ways to make the same product," Datar says.
So far, Brown of Beyond Meat, who has been a vegan for about 15 years, has not seriously attempted making vegan fish. But he says he is thinking about it.
"I'm not interested in making [plant protein] behave like meat," he says — a reference to tofu burgers, Tofurkey and myriad other meat alternatives made primarily with soybeans and other legumes. He wants to make real fish, and he believes tuna flesh will not be hard to approximate with his meat-making machinery. "Tuna has a similar architecture to the proteins we're already working on."
He says he was once approached by a canned tuna distributor interested in an odor-free albacore. But overall, Brown says, the demand for manufactured fish is not great enough yet. He says most vegan food tech is aimed for now at using plants to replicate meat products because of negative media portrayals of factory farming.
San Franciscans, it seems, have a few options for vegan alternatives to seafood. Earlier this month, restaurateur and sustainable fisheries advocate Casson Trenor opened a restaurant there called Shizen Vegan Sushi Bar and Izakaya.
"So much of sushi is visual, and using vegetables gives us the opportunity to use so many beautiful colors," Trenor says. "We have some dishes that feature the bright color of tuna meat. Are we trying to mimic maguro? No. Are we trying to put purple and red into the menu? Yes."
Corwell of Tomato Sushi was first convinced of the need to shift away from eating the bigger tuna species after visiting Tokyo's celebrated Tsukiji fish market in 2007. He was stunned by the hundreds of frozen bluefin carcasses sprawled across the warehouse floor.
"The way I learned to cook with big slabs of meat [and fish] isn't going to be possible in the future, and that's nothing to be scared of," Corwell says.
Tuna isn't his only focus. Corwell has created an eggplant-based rendition of unagi and a granular seasoning blend meant to taste like dried, salted bonito flakes. Through the use of fermented ingredients and yeast, caramelization and lots of stovetop test runs, Corwell says he hopes to develop many more vegan sushi products.
"[Tomato Sushi] is the just the tip of the iceberg," he says.
Alastair Bland is a freelance writer based in San Francisco who covers food, agriculture and the environment.