Hazelnut

The hazelnut is a widely available nut that can be found in many supermarkets, health food stores and at weekly markets. You can buy it fresh in the shell, already shelled, roasted or ground as flour or paste. Its popularity comes from hazelnuts' slightly sweet, nutty flavor and their use in many sweets, cakes and spreads. Depending on the season there is fresher produce: the main harvest time is from late summer to autumn, so fresh hazelnuts in the shell are especially easy to find in autumn.

Hazelnuts grow on shrubs or small trees called hazel. These plants like a temperate climate, neither too hot nor too cold. Major growing areas include:

You can think of it like apple growing: some regions have the perfect weather and soils, so especially many and good hazelnuts grow there.

There is not just a single hazelnut, but many varieties. These varieties differ in size, flavor, frequency of harvest and how well the nuts suit particular recipes. Important differences include:

If you want to buy hazelnuts, look for a fresh smell (not musty) and firm shells on whole nuts. For recipes where an intense flavor is important, roasted hazelnuts or hazelnut paste are a good choice. For allergy sufferers: hazelnuts are among the common tree-nut allergens, so always read the ingredient list or ask if in doubt.

In summary: hazelnuts come from regions with mild climates, there are many varieties and processing methods, and they are available year-round in various forms – from fresh in the shell to ground flour or paste for cakes and spreads.

Hazelnut (Corylus avellana) is the edible seed fruit of the hazel shrub and is botanically classified as a nut. It is an oil-rich shell fruit with a characteristic round to slightly flattened shape and an average size of 1–2 cm in diameter. Hazelnuts are cultivated mainly in temperate regions; important producing countries are Turkey, Italy, Spain and the USA. Varieties differ in fruit size, shell hardness and oil content, which influences uses in the food industry.

Botanical and physical properties: The hazelnut develops from the ovary wall and has a woody shell (pericarp) that encloses the oil- and fat-rich kernel. The seed coat (testa) is thin and inside usually light brown to reddish. Hazelnuts are borne in catkins and are released at maturity by softening of the fruit stalk. Physically they are relatively sensitive to pressure; mechanical processing requires suitable calibration methods, cracking and sorting machines as well as gentle drying to avoid quality losses due to shell cracks and oxidation.

Chemical composition and nutritional values: Hazelnuts consist mainly of fat (approx. 60–66%), protein (approx. 12–15%) and carbohydrates (approx. 6–10%), supplemented by fiber and small amounts of free sugars. The main components of the lipids are unsaturated fatty acids, primarily oleic acid (C18:1, monounsaturated) and linoleic acid (C18:2, polyunsaturated). In addition, hazelnuts contain vitamins such as vitamin E (tocopherols), B vitamins (especially folate), and minerals such as magnesium, copper, manganese and phosphorus. Typical average values per 100 g of roasted hazelnuts are about 628 kcal, 62 g fat, 15 g protein and 9 g fiber, although concrete values vary by variety and processing.

Bioactive components and secondary plant compounds: In addition to macronutrients, hazelnuts contain phenolic compounds, flavonoids and proanthocyanidins, which have antioxidant properties. Tocopherols act as fat-soluble antioxidants and contribute to the stability of the oils. Sterols and triterpenes are also present in small amounts. These bioactive substances are associated with health-promoting effects such as reduction of oxidative stress and potential cardiovascular benefits.

Processing and extraction: Industrial processing steps include harvesting, drying, shelling, roasting, grinding and oil pressing. Drying, ideally to 6–8% residual moisture, prevents mold formation and lipid oxidation. During roasting hazelnuts develop their typical aroma through Maillard reactions and thermal degradation processes of amino acids and sugars; roasting time and temperature influence flavor and color. Oil is obtained by cold pressing or extraction; cold-pressed oils retain more flavor and nutrient components but are less productive in yield.

Health aspects: The high proportion of monounsaturated and polyunsaturated fatty acids makes hazelnuts a beneficial component of a heart-healthy diet, provided they are consumed in moderation. Numerous epidemiological studies show associations between regular nut consumption and reduced risk of coronary heart disease, improved lipid profiles and reduced inflammatory activity. Hazelnuts also provide vitamin E, which protects against lipid peroxidation, as well as magnesium and fiber that can support metabolic health. At the same time hazelnuts are a known trigger for food allergies; hazelnut allergens (e.g. Cor a 1, Cor a 8) can cause severe IgE-mediated reactions. Furthermore, improper storage carries the risk of mold contamination and mycotoxin formation, which is why drying and storage are important.

Safety, storage and sustainability: Fresh hazelnuts should be stored cool, dry and protected from light to avoid oxidation and rancidity of the oils. Vacuum packaging and refrigeration extend shelf life. Sustainability aspects concern plant protection, water management and variety selection; integrated cultivation systems and resistance breeding can reduce the use of plant protection products. The valorization of by-products, such as shells as biomass, contributes to resource efficiency.

In summary, the hazelnut is a nutrient-rich, chemically complex shell fruit that is nutritionally valuable because of its oily profile, antioxidant constituents and contribution to plant protein supply. Processing and storage influence aroma, nutrient content and safety; allergic risks and quality controls are therefore central in production and trade. Scientific engagement with hazelnuts covers aspects from plant breeding to food technology and nutritional medicine research.