The Impossible Burger is a popular plant-based alternative that closely mimics the experience of eating ground beef. Unlike earlier meat substitutes, this product achieves its uniquely meat-like qualities through a single, specialized ingredient: heme. Heme, specifically soy leghemoglobin, is the molecule responsible for the signature color, aroma, and savory taste that sets this burger apart from other plant-based options.
The Role of Heme in Sensory Experience
Heme is included in the Impossible Burger because it is a powerful catalyst for the chemical reactions that occur when the plant-based patty is cooked. When the patty hits a hot surface, the iron-containing molecule accelerates the breakdown of other ingredients, particularly fats and amino acids. This process generates a complex mixture of volatile compounds that are nearly identical to the aroma molecules produced by cooking animal meat.
The presence of heme also dictates the visual experience of the burger, beginning with its appearance before cooking. Soy leghemoglobin imparts a vibrant red color to the raw patty, visually mimicking uncooked ground beef. As heat is applied, the molecule undergoes a transformation, causing the pigment to transition from red to a rich, cooked brown color. This change provides the familiar visual cue that the burger is fully cooked.
Beyond visual and aromatic changes, heme contributes significantly to the savory taste described as umami. This deep, metallic flavor associated with meat is intensified by the iron atom within the heme structure. The interaction of the heme iron with other ingredients during heating creates the flavor complexity and mouthfeel that consumers desire in a burger.
Understanding the Heme Molecule
Chemically, heme is a metalloporphyrin, a ring-shaped organic compound with an iron atom coordinated at its center. This specific structure, known as heme B, is identical across nearly all living organisms. The central iron atom binds to oxygen and facilitates the molecule’s various biological functions.
In the animal kingdom, heme is bound to proteins like hemoglobin in blood and myoglobin in muscle tissue, primarily transporting and storing oxygen. In the plant kingdom, the heme molecule is carried by leghemoglobin, found in the root nodules of leguminous plants like soybeans. Leghemoglobin regulates oxygen levels for the nitrogen-fixing bacteria that live symbiotically in the roots.
The Impossible Burger uses soy leghemoglobin, a plant-derived protein, as its heme source, rather than animal myoglobin found in traditional meat. This plant-derived protein provides the exact same heme molecule. This ensures it can perform the necessary flavor and color catalysis despite its plant origin.
Producing Soy Leghemoglobin for Mass Consumption
Harvesting leghemoglobin directly from soybean root nodules would be inefficient for large-scale production. Instead, Impossible Foods uses synthetic biology and fermentation to produce the ingredient. The process begins by isolating the specific gene sequence from the soybean plant that codes for soy leghemoglobin.
This isolated gene is then inserted into a strain of genetically engineered yeast, Pichia pastoris. The yeast uses the new genetic instructions to create the desired protein. This use of a modified organism to produce a specific compound is known as precision fermentation.
The engineered yeast is grown in large fermentation tanks, where it multiplies and produces soy leghemoglobin, similar to the process used in brewing beer. Once fermentation is complete, the leghemoglobin protein is isolated and purified from the yeast culture. This purified preparation is then added to the plant-based patty mixture.
Safety Assessment and Regulatory Approval
Because soy leghemoglobin expressed through genetically engineered yeast is considered a novel food ingredient, it underwent a comprehensive safety review. The manufacturer first submitted data to a panel of external food safety experts, who concluded the ingredient was Generally Recognized As Safe (GRAS). This self-affirmation process was then voluntarily submitted to the U.S. Food and Drug Administration (FDA).
The FDA reviewed the extensive safety data, which included results from various toxicity tests, such as a stringent rat feeding study. Following this assessment, the agency issued a “no questions” letter regarding the GRAS status, indicating no challenges to the manufacturer’s safety determination. Additionally, because the protein imparts a red color, the FDA approved it for use as a color additive.