What Are the Protease Inhibitors in Eggs and How Do They Affect You?

November 22, 2025 •

4 min read

Research has shown that the protein digestibility of raw eggs can be significantly lower compared to cooked eggs due to natural antinutritional factors. This difference is primarily caused by protease inhibitors in eggs, which are proteins designed to protect the developing chick embryo from bacteria.

Quick Summary

Egg whites naturally contain protease inhibitors, such as ovomucoid and ovoinhibitor, that can hinder the activity of digestive enzymes. Cooking effectively deactivates most of these compounds, improving protein digestion and safety.

Key Points

Key Inhibitors: Ovomucoid and ovoinhibitor are the primary protease inhibitors found in egg whites.
Digestive Impact: These inhibitors can reduce the digestibility of proteins in raw or undercooked eggs by blocking digestive enzymes.
Cooking's Effect: Heat treatment denatures most protease inhibitors, significantly improving the body's ability to absorb egg protein.
Avidin Risk: Raw egg whites contain avidin, which can bind to biotin and prevent its absorption, but cooking inactivates avidin.
Food Safety: Cooking eggs is crucial for eliminating the risk of Salmonella bacterial contamination, a concern associated with raw eggs.
Allergen Stability: Ovomucoid is a major egg allergen that is heat-stable, meaning it can still cause allergic reactions even after cooking.
Nutrient Absorption: Eating cooked eggs improves overall nutrient absorption and makes them a more complete and beneficial protein source.

What Are Protease Inhibitors?

Protease inhibitors are naturally occurring proteins found in eggs, particularly the egg white (albumen), that can block the function of proteolytic enzymes (proteases). In nature, these inhibitors serve an important protective role for the egg, defending against harmful microorganisms by inhibiting the digestive enzymes they produce. For humans, however, these compounds can reduce the bioavailability of proteins if eggs are consumed raw. The primary types of protease inhibitors in eggs include:

Ovomucoid: This glycoprotein is a multidomain, Kazal-type inhibitor that targets serine proteases like trypsin. Notably, ovomucoid is highly resistant to heat, which is why it remains a potent allergen even in cooked eggs.
Ovoinhibitor: Another Kazal-type glycoprotein, ovoinhibitor can inactivate multiple serine proteases, including trypsin, chymotrypsin, and elastase.
Ovostatin (also Ovomacroglobulin): A large homotetrameric protein that belongs to the alpha-2-macroglobulin family. It is known for its ability to inhibit a broad range of proteases, including serine, cysteine, and metalloproteases.
Cystatin: This non-glycosylated protein specifically targets and inhibits cysteine proteases, such as papain.

The Effect of Protease Inhibitors on Human Digestion

When a person consumes raw or undercooked egg whites, the protease inhibitors present can interfere with their own digestive enzymes. The digestive process relies on enzymes to break down large proteins into smaller, more easily absorbed peptides and amino acids. The inhibitors bind to and block these enzymes, reducing their efficiency. This is why the protein digestibility of raw egg whites is significantly lower than that of cooked eggs. However, this effect is largely mitigated by proper heat treatment.

Neutralizing the Inhibitors Through Cooking

Cooking eggs is the most effective and widely practiced method for neutralizing most protease inhibitors. Heat causes the protein structures to unfold and change shape, a process known as denaturation. This structural change prevents the inhibitors from effectively binding to and blocking digestive enzymes, thereby improving protein bioavailability. While cooking denatures most of these proteins, the heat-stable nature of ovomucoid means it retains its allergenic properties even after extensive heating.

Comparison of Raw vs. Cooked Eggs


Feature	Raw Eggs	Cooked Eggs
Protein Digestibility	Significantly lower, with limited absorption due to active protease inhibitors.	Protein structure is denatured, allowing for up to 91% absorption.
Protease Inhibitors	Active, binding to and blocking digestive enzymes like trypsin.	Largely inactive, with structural changes preventing them from inhibiting digestion effectively.
Avidin Activity	Active, binding to biotin and hindering its absorption, potentially causing biotin deficiency with high intake.	Inactive, as the heat breaks the avidin-biotin bond, freeing biotin for absorption.
Salmonella Risk	Presents a potential risk of bacterial contamination.	Cooking eliminates harmful bacteria, making them safer for consumption.
Overall Bioavailability	Lower absorption of protein and biotin compared to cooked eggs.	High absorption of protein, biotin, and other nutrients.

Avidin: Another Antinutritional Factor

In addition to protease inhibitors, raw egg whites contain another antinutritional protein called avidin. Avidin has an extremely high affinity for biotin, a water-soluble B vitamin, effectively binding it and preventing its absorption by the body. While it would take a large quantity of raw egg whites consumed regularly to induce a biotin deficiency, the risk exists. Cooking, however, completely deactivates avidin, allowing for unimpaired biotin absorption. This makes cooked eggs a highly reliable source of biotin.

The Importance of Cooking for Safety

Beyond just maximizing nutritional benefits, cooking eggs is essential for food safety. Raw and undercooked eggs carry a risk of contamination with the bacteria Salmonella. Infection with Salmonella can cause a range of symptoms from mild gastrointestinal distress to severe illness requiring hospitalization, particularly in vulnerable populations like the elderly, children, and those with weakened immune systems. Cooking eggs thoroughly until the whites and yolks are firm eliminates this bacterial risk. For those who wish to consume raw eggs, it is crucial to purchase pasteurized egg products, which have been heat-treated to kill harmful bacteria without cooking the egg.

The Final Word on Protease Inhibitors in Eggs

In conclusion, the protease inhibitors in eggs are a natural defense mechanism that primarily affects the digestibility of raw egg protein. While they can reduce nutrient absorption, this issue is easily and effectively addressed through cooking. The simple act of heating eggs denatures most of these inhibitors, enhancing the bioavailability of protein and other nutrients like biotin. Cooking also eliminates the risk of Salmonella contamination, making it a safer and nutritionally superior choice. For individuals seeking to maximize the nutritional benefits of eggs, cooking is the recommended preparation method. While raw egg consumption carries risks, pasteurized egg products offer a safer alternative if needed for specific recipes. The study in PubMed [9117993] provides further detail on these fascinating proteins and their inhibitory mechanisms.

Frequently Asked Questions

The main protease inhibitors in eggs, specifically the egg white, are ovomucoid, ovoinhibitor, ovostatin (or ovomacroglobulin), and cystatin. These proteins can hinder the action of various digestive enzymes.

Cooking effectively denatures most protease inhibitors in eggs, causing them to lose their ability to inhibit digestive enzymes. However, one key allergen, ovomucoid, is known for its thermal stability and can remain active even after extensive heating.

Protease inhibitors can interfere with the activity of digestive enzymes, such as trypsin and chymotrypsin, reducing the body's ability to break down and absorb protein from raw egg whites. Cooking overcomes this effect by inactivating the inhibitors.

While consuming raw eggs can lead to reduced protein absorption due to protease inhibitors, the most significant danger comes from potential contamination with Salmonella bacteria. Cooking eliminates this risk and is the recommended way to consume eggs safely.

Studies show that the human body can absorb around 91% of the protein from cooked eggs, while protein absorption from raw eggs is only about 51%. The difference is primarily due to the inactivation of protease inhibitors by heat.

Avidin is a protein found in raw egg whites that can bind to the vitamin biotin, making it unavailable for absorption by the body. This can potentially lead to biotin deficiency if large quantities of raw eggs are consumed frequently. Cooking, however, denatures avidin.

Yes, some egg allergens, particularly ovomucoid, are heat-stable and can cause an allergic reaction even after cooking. People with ovomucoid sensitivity typically cannot tolerate either raw or cooked eggs.