Botanical Characteristics of Hazelnut (Corylus avellana)
- Scientific Name: Corylus avellana
- Family: Betulaceae (Birch family)
- Common Names: Hazelnut, Filbert nut, Cobnut
- Native Region: Europe and Western Asia
Tree and Fruit Description
- The hazelnut tree is a deciduous shrub or small tree, typically 3–8 meters tall.
- The nuts grow inside a hard shell, surrounded by a leafy husk.
- Hazelnuts are widely used in food products, especially in chocolates, spreads (Nutella), and baked goods.
Hazelnut as a Food Allergen
Hazelnuts are classified as a major food allergen by the U.S. FDA, requiring mandatory labeling.
Allergenic Proteins in Hazelnuts
| Allergen | Protein family | Molecular weight (kDa) | Clinical relevance |
|---|---|---|---|
| Cor a 1 | PR-10 (Bet v 1-related) | 17 | Birch-pollen related; usually mild oral symptoms; destroyed by roasting |
| Cor a 2 | Profilin | 14 | Pollen-related panallergen; usually mild |
| Cor a 8 | Non-specific lipid transfer protein (nsLTP1) | 9 | Stable; linked to systemic reactions, especially in Southern Europe |
| Cor a 9 | 11S globulin (legumin) | 40 | Marker of severe or systemic hazelnut allergy |
| Cor a 11 | 7S globulin (vicilin) | 48 | Storage-protein allergen |
| Cor a 14 | 2S albumin | 10 | Marker of severe or systemic hazelnut allergy, especially in children |
| Cor a 12, 13, 15 | Oleosins (oil-body proteins) | 14–17 | May be missed by standard test extracts |
| Cor a 16 | Vicilin with alpha-hairpinin peptides | 47.5 | Recently described storage-protein allergen |
Detailed epitope (amino-acid) maps previously shown here have been removed because they could not be matched to published studies. Exact epitope positions vary between studies and between patients.
Stability of Hazelnut Allergens
| Allergen | Heat stability | Digestive stability | Effect of processing |
|---|---|---|---|
| Cor a 1 | Low | Low | Usually destroyed by roasting or cooking |
| Cor a 2 | Low | Low | Usually destroyed by cooking |
| Cor a 8 | High | High | Survives cooking and digestion |
| Cor a 9 | Moderate to high | Moderate | Largely survives roasting |
| Cor a 11 | Moderate to high | Moderate | Largely survives roasting |
| Cor a 14 | High | High | Survives roasting and digestion |
- Hazelnut allergens like Cor a 8, Cor a 9 and Cor a 14 remain stable, meaning roasted hazelnuts can still cause allergic reactions.
- Cor a 1 is heat-sensitive, so cooking may reduce its allergenicity.
Cross-Reactivity of Hazelnuts
| Source | Related hazelnut protein | What it means |
|---|---|---|
| Birch pollen | Cor a 1 (PR-10) | Most common reason for hazelnut sensitization in birch regions; usually mild oral symptoms |
| Peach, apple and other LTP foods | Cor a 8 (LTP) | Can cause systemic reactions |
| Walnut and other tree nuts | Cor a 9, Cor a 14 (storage proteins) | Co-allergy is common; assessed individually |
| Peanut | Storage proteins | Co-allergy is common; mostly separate sensitization |
Symptoms of Hazelnut Allergy
Reactions can range from mild to life-threatening.
Mild Symptoms (Oral Allergy Syndrome - OAS)
- Itchy mouth, lips, throat.
- Mild swelling of tongue.
- Occurs mostly in birch pollen allergy sufferers (due to Cor a 1).
Moderate to Severe Symptoms
- Hives, skin rash.
- Vomiting, nausea, diarrhea.
- Swelling of face, eyes (angioedema).
- Wheezing, difficulty breathing.
Severe Reactions (Anaphylaxis) Epinephrine (auto-injector or nasal spray) is required for anaphylaxis
- Drop in blood pressure (hypotension).
- Severe airway constriction (wheezing, asthma).
- Throat swelling (laryngeal edema).
- Cardiac arrest (in rare cases).
Diagnostic Testing for Hazelnut Allergy
Skin Prick Test (SPT)
- Detects IgE sensitivity to hazelnut extract.
- A positive test shows sensitization; it does not confirm clinical allergy or predict severity.
Serum IgE Testing
- Measures IgE antibodies to hazelnut components. Cor a 9 and Cor a 14 are the main markers of risk for severe reactions; sensitization to Cor a 1 alone usually reflects milder, birch-pollen-related allergy.
Oral Food Challenge (OFC)
- Gold standard for confirming allergy.
- Conducted under medical supervision.
Anaphylaxis: act fast. After an exposure, any breathing symptom (wheeze, repeated cough, throat tightness, hoarse voice), fainting or dizziness, or symptoms in two or more body systems (for example hives plus vomiting) is anaphylaxis — even without hives.
- Give epinephrine right away (auto-injector or nasal spray). Do not wait to see if symptoms get better.
- Call 911, even if symptoms improve.
- Lie flat with legs raised (sit up if breathing is hard). Give a second dose after 5–15 minutes if there is no improvement.
Antihistamines and steroids do not treat anaphylaxis and must never delay epinephrine. Carry two doses and follow the written emergency action plan from your allergist.
Management and Prevention
Avoidance Strategies
- Read labels carefully → Hazelnuts are found in chocolates, Nutella, baked goods.
- Beware of cross-contamination → Especially with walnuts, almonds, and peanuts.
Emergency Treatment
- Epinephrine (auto-injector or nasal spray)
- Antihistamines may ease mild skin symptoms such as itching or hives, but they do not treat anaphylaxis and must never delay epinephrine.
- Corticosteroids are sometimes given by doctors after a reaction, but they do not treat anaphylaxis and have not been shown to prevent a second-wave (biphasic) reaction.
Immunotherapy
- Oral Immunotherapy (OIT) is being researched for hazelnut desensitization.
Conclusion
- Hazelnut allergens (Cor a 1, Cor a 8, Cor a 9, Cor a 14) can cause mild to severe reactions; Cor a 9 and Cor a 14 are linked to the most severe reactions.
- Cross-reactivity with birch pollen (Cor a 1) is common. Co-allergy with other tree nuts and peanut is also common but mostly reflects separate sensitization, and Cor a 8 (LTP) can link hazelnut to fruits such as peach.
- Heat and digestion stability vary: the LTP (Cor a 8) and the storage proteins (Cor a 9, Cor a 14) are the most stable, while Cor a 1 is destroyed by roasting.
- Strict avoidance and emergency preparedness are necessary.