QUALITATIVE CHARACTERISTICS OF HAZELNUTS

Hazelnuts are one of the most nutritious nuts that contain valuable amounts of nutrients, among which lipids predominate. The particular fatty acid composition, rich in monounsaturated fatty acids, has a recognized beneficial effect on human health. However, due to their high lipids content, hazelnuts are easily susceptible to rancidity. The aim of the post-harvest processing is to minimize metabolism, fungal, oxidative and enzymatic activity in the seed thanks to timely drying and proper storing: this, not only has fundamental impact on the quality of the crop but is of utmost importance for avoiding development of aflatoxins which could endanger human healthy.

Chemical composition of hazelnut 1.1.

Dried fruit has beneficial properties on human health that derive from its composition particularly rich in plant lipids and bioactive substances. These same substances allow the hazelnut a good shelf life, if the most appropriate and correct post-harvest management techniques are applied.

After drying, the seed has a moisture content of about 6%. The total oil content of the hazelnuts ranges from 58 to 65% on fresh weight, while the protein ranges from 8 to 15% (table 1.1). Hazelnuts also contain significant quantities of dietary fibers, mineral elements, and vitamins (table 1.2). Hazelnuts are an excellent natural source of the antioxidant vitamin E due to their tocopherol content, and are also rich in other biologically active compounds such as polyphenols.

Lipids have a fatty acid composition similar to that of olive oil, with 77-83% of oleic acid, 7-14% of linoleic acid, 5-7% of palmitic acid, 2-3% of stearic acid, with small quantities (<0.2%) of palmitoleic, linolenic and arachic acids. The propensity of the cultivars to rancidity of the oil is correlated to the percentage of linoleic acid which is a polyunsaturated fatty acid.

Compounds

Water 5-6 Oleic acid (77-83%), Linoleic acid (7-14%),

Total oil content (lipids) 58-65 Palmitic acid (5-7%), Stearic acid (2-3%)

Total protein 8-15 Arginine, Glutamic acid, Aspartic acid

Fiber 10-13

Sugars 3-5 Sucrose (90%)

Ashes 2.2-2.5

Phytosterols β-sitosterol (90-130g/100 g of oil)

Vitamins B1, B2, B3, B6 (0.4-0.6 mg/100 g), E (α-tocopherols 25-50 mg/100 g of oil)

Mineral elements K (500-800 mg/100 g), Ca (250-400 mg/100 g), P (200-400 mg/100 g), Mg (150-200 mg/100 g), Fe, Zn, Cu, Mg, Se

Phenolics Flavonoids (20-40 mg GAE/100 g)

Seed alterations during storage 1.2.

The main alterations that can occur during storage are due to lipids. The lipids consist of triglycerides which can release fatty acids during storage causing the product to become sour (acidification). The acidity upon harvesting is normally less than 0.1 g of oleic acid per 100 g of oil and gradually increases during storage; it is 1.5-2 times higher in roasted seeds than in raw ones.

The high presence of unsaturated fatty acids, can give rise to rancidity, a condition produced by oxidation of unsaturated fat with degradation of the molecule and formation of aldehydes (hexanal) and oxyacids, unpleasant at a sensorial level. The evaluation of the oxidation state of the lipids is given by the measurement of the peroxide content which is normally lower than 0.2 mEqO2 / kg of oil in the seed at harvest. Also in this case, roasting determines an increase in the peroxide content which, during storage, grows even more rapidly than is observed for acidity. There are no limits set by the legislation for the two parameters but it is preferable that in raw hazelnuts the acidity does not exceed 0.4% and the peroxide content of 0.5 mEqO2 / kg. Another variation that can occur during storage concerns the color of the perisperm which tends to darken due to the oxidation of phenolic compounds. In any case, if storage is not properly done, rotten issues and aflatoxins can easily seriously increase the damage to the quality of the lots.