The Microbial Reasons Why the Impossible Burger Tastes So Good

May 16, 2019

What makes meat delicious? This arguably understudied question is the scientific focus of the research team at Impossible Foods, the company Pat Brown founded and whose mission is to completely replace animals as a food production technology. Discovering and imitating the molecular interactions that make meat delicious “is the most important scientific question in the world right now,” Brown said. This is the story of how 2 different microbes help scientists at Impossible Foods mimic the delicious taste of meat.
Part of what makes meat delicious is its high concentration of heme, a family of iron-containing molecules that serve a variety of functions, including oxygen transportation and involvement with electron transport and redox reactions. Every living plant or animal cell contains heme proteins, and animal muscle has a high concentration of hemoglobin. When Brown set out to develop a recipe for a meat replacement, he was pretty sure heme would play an important role.
These patties contain no animal product, but plenty of heme.
These patties contain no animal product, but plenty of heme.

It turns out that heme is an integral part of why meat tastes good, but not only because of the iron it provides: heme plays a central role as a chemical catalyst during cooking. According to Brown and his research team, many meat-specific aromas rely on heme interactions with cellular proteins and biomolecules as it cooks. “Heme catalyzes very specific types of chemical reactions,” said Brown, “that transform abundant, simple nutrients into this explosion of hundreds of diverse volatile odorant molecules. When you experience them, together they add up unmistakably to the smell and taste of meat.”
To recapitulate the delicious taste of meat, Brown and his scientific team needed to find an alternative source of heme.

Rhizobia to the Rescue: a Role for Microbes in Meat 

Brown’s inspiration for a heme source came from a high school biology teacher, who had taught him that the chemistry of nitrogen fixation is sensitive to oxygen concentrations. This is particularly important for legumes, a group of plants that form symbiotic relationships with nitrogen-fixing bacteria called rhizobia. The rhizobia live in root nodules on the plants, where they convert atmospheric nitrogen into ammonia that can be incorporated into bacterial or plant metabolic pathways. To verify the high concentration of legume heme proteins, yourself, slice open the nodules on clover (a legume) and note the bright pink color associated with the presence of heme.
Every legume has its own rhizobial species with which it has an evolved relationship, and Brown turned to one of the largest crop legumes as a potential heme source for Impossible Foods: soybeans. The roots of these crops have long-studied relationships with Bradyrhizobium and Sinorhizobium species, and soybeans produce a heme protein called leghemoglobin to regulate the oxygen for their nitrogen fixation activities. Given that soybeans constitute 89.6 million acres of crops in the U.S., Brown suspected using the roots, leftover after harvesting, would be a sustainable source of heme for his product.
Soybean roots (L) and those of other legumes have root nodules, which harbor nitrogen-fixing symbionts like the Bradyrhizobia (R). Soybeans produce leghemoglobin to regulate oxygen levels for this nitrogen fixation.
Soybean roots (L) and those of other legumes have root nodules, which harbor nitrogen-fixing symbionts like the Bradyrhizobia (R). Soybeans produce leghemoglobin to regulate oxygen levels for this nitrogen fixation.

Soybean roots became a major focus for the Impossible Foods team. For a year and a half, scientists and engineers concocted a number of Rube Goldberg-like contraptions designed to separate the nodules from the root and soil, as well as attempting various methods to purify the leghemoglobin. While this generated enough leghemoglobin to make a proof-of-concept burger, the process was too labor intensive and inefficient to scale up for mass production. They needed a better way to produce heme proteins.

Yeast Make Heme Production More Efficient

The scientific team turned to yeast to explore recombinant protein production. Using a recombinant system freed the team to explore other heme sources, including hemoglobin and myoglobin from animal sources. Though the goal is to generate a sustainable protein source that replaces the ecologically taxing way people currently raise livestock, Brown had no qualms about sourcing genetic material from animals, if the result is a better-tasting, more environmentally-friendly burger: “The problem we’re trying to solve here is the catastrophic effect of covering the planet with cows. If we’re producing a heme protein that’s encoded normally by the cow genome but we’re producing it in a way that isn’t an environmental catastrophe, that would be fine. We’re fixating on the best heme protein for performance in meat.”
The heme production machine became Pichia pastoris, a yeast used for large-scale production of a number of recombinant proteins. Developed in the 1980s and 1990s, Pichia has been used as a factory for proteins used in therapeutics and food preparation, and was a good fit for housing the complex metabolic pathways that constitute heme protein production. After testing a wide variety of heme proteins, however, it turned out the first one was the best fit: it was leghemoglobin that created the best-tasting burger.
Pichia pastoris (L) are now used to produce soybean leghemoglobin (R).
Pichia pastoris (L) are now used to produce soybean leghemoglobin (R).

It may seem counterintuitive that a plant hemoglobin would have superior performance to one from the animal whose tissues the scientists wanted to mimic. Brown says the reason lies in the purpose of this particular hemoglobin. “For us, (the protein) doesn’t need to do everything that it did in the cow. The enzymes and catalysts and small molecules in the system have to be able to reproduce the biochemistry that produces flavor in meat.” Those other roles of hemoglobin in a normal cell–function in the electron transport chain, muscle contraction, or glycolysis–are dispensable as long as the key properties of meat are reproduced. Though other components of the Impossible Burger recipe have changed over the years, leghemoglobin produced by yeast has remained an essential ingredient.

Why Replace Meat? 

“The whole reason the company exists, and the reason we have so many amazingly great scientists working here, is that we are working on the most important and urgent problem in the world right now. Arguably, the most important and urgent problem our species has ever faced is the catastrophic meltdown in biodiversity and the relentless progression of climate change,” said Brown. The food system has contributed to both of these problems, he argues.
Many are familiar with the ecological impacts of raising livestock: it requires a lot of freshwater, it produces pollutants that contaminate soil, water and air, and it eliminates the natural biodiversity that would otherwise fill the land. “The single biggest reason it’s so destructive is because it’s land-intensive,” said Brown. “Something like 45% of the entire area of land on earth is actively in use raising animals for food right now.” This includes both land for the livestock pastures as well as land spent growing feed crops. As global meat production nearly doubled between 1980 and 2004, so did the land required for its production.
The solution isn’t to use these numbers to criticize people for eating meat, but to make a better tasting burger that meat-lovers prefer. “The only way to do it is to create products that consumers prefer that can compete successfully in the marketplace against those products.”
Brown hopes the taste, nutrition, and smaller environmental footprint will convince meat lovers to switch to the Impossible Burger.
Brown hopes the taste, nutrition, and smaller environmental footprint will convince meat lovers to switch to the Impossible Burger.

A competitive Impossible Burger must be more delicious, nutritionally superior (“which we already are,” said Brown), and economical compared to traditional beef burgers. Making Impossible Burgers uses one-tenth the water, less than one-twenty-fifth the land, less than one-tenth the fertilizer input, which basically means the fundamental drivers of cost are vastly lower than using animal production. Lower resource use means lower cost at scale.
So what makes meat delicious? As Brown has learned, heme goes a long way toward re-creating the indescribable umami of beef. But perhaps the lower environmental and financial costs associated with the Impossible Burger will also make it more palatable. As Impossible Foods becomes increasingly available at restaurants around the country, Brown hopes the nutritionally superior, lower cost, and better-tasting burger choice will make an argument for itself.

Listen to Pat Brown speak at the Keynote Address during ASM Microbe 2019.
Register by May 22 2019 to get a discounted rate!


Press play to listen to Pat Brown's appearance on Meet the Microbiologist

Author: Julie Wolf

Julie Wolf
Dr. Julie Wolf is in science communications at Indie Bio, and was a former ASM employee. Follow Julie on Twitter for more ASM and microbiology highlights at @JulieMarieWolf.