DETERMINATION OF THE CONTENT OF MACRO- AND MICROELEMENTS, AS WELL AS FLAVONOIDS, IN THE KERNEL AND SHELL OF COMMON HAZELNUT (Corylus avellana L.)

ABSTRACT

As we know, macro- and microelements need to be at normal levels for human health. They enter the body through the consumption of food products. This article examines the macro and microelement content, as well as the flavonoid levels, of common hazelnuts grown in Uzbekistan through experimental analysis. The results are compared with data presented in the literature.

АННОТАЦИЯ

Как известно, поддержание нормального уровня макро- и микроэлементов имеет важное значение для здоровья человека. Эти элементы поступают в организм через потребляемые пищевые продукты. В данной статье исследуется содержание макро- и микроэлементов, а также уровень флавоноидов в обыкновенном лесном орехе, выращенном в Узбекистане, на основе экспериментального анализа. Полученные результаты сравниваются с данными, представленными в литературе.

Keywords: common hazelnut (Corylus avellana L.), polyphenolic compounds, macro elements, microelements.

Ключевые слова: фундук обыкновенный или лесной орех (Corylus avellana L.), полифенольные соединения, макроэлементы, микроэлементы.

Introduction. The main regions where nut-bearing trees grow include Asia Minor, the Balkan Peninsula, Iran, China, Moldova, Ukraine, Krasnodar, the Caucasus, and Central Asian countries. These include Greek nuts, Brazil nuts, macadamia nuts, hazelnuts, cashews, pistachios, and almonds. Nut-bearing trees are long-lived, and their nuts contain 96–98% fats, proteins, and carbohydrates. In addition, they are rich in all essential vitamins, including B-group vitamins as well as vitamins A, E, and PP [1].  The production and export of nuts are increasing yearly. According to statistical data, over 17 million tons of nuts were harvested in 2021 alone, with more than half being exported [2]. For example, global hazelnut production reaches approximately 1 million tons annually, with 45% of it produced in Asia, 30% in Europe, and 20% in North America.

Hazelnut (Corylus) belongs to the birch family (Betulaceae) and comprises deciduous shrubs or tree species. To date, around 20 species have been identified, primarily found in the forests of North America, Europe, and Asia. The height of hazelnut plants ranges from 2 to 5 meters (sometimes up to 8 meters, and in tree form, up to 30 meters). The flowers are bisexual, and the fruit is a one-celled, single-seeded nut. Hazelnuts thrive in moist and fertile soils and are sensitive to heat and humidity. They begin yielding crops 5–6 years after planting. A single plant can produce up to 8 kg of nuts per year. The nuts contain 60–80% oil. Hazelnuts propagate through seeds, suckers, and layering, and the plants can live for over 80 years [3]. Among the species, the common hazelnut (Corylus avellana), or hazel, is the most widespread. It is a shrub that grows 2–5 meters tall and is found in Europe, the Caucasus, the Near East, and Asia. The tree bark is smooth, light brown-grey with transverse lines, and the buds are brown-grey and hairy. The root system is shallow but strong. The buds are oval or round, slightly compressed, and up to 5 mm long. The leaves are rounded or oval, 6–12 cm long, 5–9 cm wide, often tapering to a point or short apex. The plant flowers in May, and its fruit ripens in August–September [4].

Composition of Common Hazelnuts (Corylus avellana L.). The fruits of the common hazelnut contain up to 77% fat, up to 18% protein, and 11% sugar. They are rich in vitamins B, C, D, E, PP, and K, as well as macro and microelements such as calcium, potassium, magnesium, phosphorus, sodium, iron, copper, zinc, manganese, and others. Hazelnut oil contains valuable unsaturated fatty acids, including up to 30% linoleic acid and up to 85% oleic acid, which are highly beneficial for human health [5].

Through this experiment, we determined the macro and microelement content, as well as the levels of flavonoids, in the kernel and shell of common hazelnuts. The results obtained were compared with data reported in the literature.

Macro and microelements are essential minerals required by the human body to ensure the proper functioning of various vital processes. These elements enter the body through the consumption of food and beverages. Like vitamins, they perform numerous functions, such as maintaining a healthy metabolism, strengthening bones and teeth, and regulating nervous and muscular activities [6].

Both types of elements are crucial for the body and help maintain the balance of all bodily functions. A healthy and varied diet can provide sufficient amounts of these elements daily.

Flavonoids are a class of naturally occurring phenolic compounds widely found in plants, and known for their antioxidant properties. They provide colour to plants and protect them from ultraviolet radiation, diseases, and harmful insects. Flavonoids are beneficial for human health as they exhibit antioxidant activity, combat inflammation, and strengthen the immune system [7].

Flavonoids are mainly found in fruits and vegetables, particularly in citrus fruits, berries, grapes, apples, onions, tea (especially green tea), and cocoa. They help strengthen blood vessels, reduce the risk of cardiovascular diseases, boost immunity, and lower the risk of cancer.

Flavonoids are classified into the following types:

Flavones (Luteolin, Apigenin, Chrysin), flavonols (Quercetin, Kaempferol, Galangin, Rutin, Myricetin), isoflavones (Genistein, Daidzein, Glycitein, Formononetin), flavanones (Hesperidin, Naringenin), anthocyanins (pigments) [8].

Consuming flavonoid-rich foods regularly as part of a balanced diet strengthens the body's defence mechanisms and helps improve overall health.

The micro and macro elements, as well as the flavonoid content identified in the kernel and shell of the common hazelnut, highlight their importance for human health. These substances play a significant role in preventing and treating various diseases occurring in the body, including disruptions of the antioxidant defence system, cardiovascular diseases, and weakened immunity. Our goal is to analyze the nutritional composition of the common hazelnut cultivated locally in Uzbekistan and, based on this, develop food additives.

Experience part. For the experiment, fruits and shells of common hazelnuts grown under local conditions in Uzbekistan and harvested in 2023 were used. The shells of the common hazelnuts were manually cleaned and separated from the kernels.

Table 1.

Weights of hazelnuts, kernels, and shells from hazelnut cultivar produced in Uzbekistan

1. Determining the Chemical Element Content in the Sample Using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

Required Equipment and Reagents: 2% HNO₃ solution with a standard concentration of 10 mg/l (High Purity Standards, USA), 60% H₂O₂ solution, Balance (Navigator™, OHAUS®), Muffle furnace (Nabertherm, Germany), iCAP PRO X Duo ICP-OES inductively coupled plasma optical emission spectrometer (Thermo Fisher Scientific, USA), ground hazelnut kernel and shell samples.

Preparation of Standard Solutions: Standard solutions with concentrations of 500 μg/l, 100 μg/l, and 10 μg/l were prepared by diluting the 10 mg/l standard solution of 63 elements in 2% HNO₃ (High Purity Standards, USA). Calibration curves were created using ultrapure water.

Preparation of the Sample Working Solution: To obtain ash from the pre-dried, ground sample, 1 g of the sample was weighed with an accuracy of 0.001 g using a balance (Navigator™, OHAUS). The sample was placed in a porcelain crucible and heated in a muffle furnace (Nabertherm, Germany) at 500°C. The temperature was initially increased to 500°C at a rate of 100°C/hour, and the sample was held at 500°C for 5 hours.

The resulting ash was treated with 6 ml of concentrated HNO₃ (ICP-MS grade) and 2 ml of 60% H₂O₂. The mixture was heated on a hot plate under a fume hood until the emission of white fumes ceased. The cooled solution was transferred to a 100 ml polypropylene volumetric flask and diluted to the calibration mark with ultrapure water. The working solution was then filtered using a syringe filter and used for analysis.

2. Determining the Phenolic Compound Content in the Extract Using the YUSSX Method

Reagents and Equipment Used: Gallic acid from "Macklin" (China), salicylic acid from "Rhydburg Pharmaceuticals" (Germany), quercetin, apigenin, and kaempferol from "Regal" (China), and rutin extracted using superior chromatography methods from natural sources. HPLC-grade solvents: water, acetonitrile, chemically pure vinegar acid, and sodium hydroxide.

The determination of polyphenol content in plant extracts was performed on a high-performance liquid chromatograph (HPLC) manufactured by Shimadzu Corporation (Japan), model LC-40 Nexera Lite.

Preparation of Standard Solutions: Gallic acid (5.2 mg), salicylic acid (5.2 mg), rutin (5 mg), quercetin (5 mg), apigenin (5 mg), and kaempferol (5 mg) were dissolved in 96% ethanol for 20 minutes in an ultrasonic bath. The solution was transferred to a 50 ml volumetric flask and diluted to the mark with ethanol. 200 µl was taken from each solution, mixed, and diluted to create four different solutions. Each solution was transferred to vials for analysis.

Preparation of Plant Extract: To extract phenolic compounds, 1 g of the sample was weighed with a precision of 0.01 g on an OHAUS NV222 balance (USA) and placed in a 50 ml conical flask. Then, 25 ml of 96% ethanol was added to the sample. The mixture was extracted for 20 minutes at 60°C in a GT SONIC-D3 ultrasonic bath (China). The mixture was then cooled, filtered, and made up to 25 ml with ethanol. 1.5 ml of the extract was centrifuged at 7000 rpm using a Mini-7 centrifuge (BIOBASE, China). After centrifugation, the extract was filtered through a 0.45 µm syringe filter and used for analysis.

Chromatographic Conditions: For detecting phenolic compounds, standard solutions and sample extracts were analysed using a Shim-pack GIST C18 (150 × 4.6 mm; 5 μm, Shimadzu, Japan) reverse-phase column. The mobile phase was a gradient consisting of acetonitrile (A) and a 0.5% aqueous solution of acetic acid (B) (Table 2). Injection volume: 10 µl, flow rate: 0.5 ml/min., column thermostat: 40°C. The analytical signal for phenolic compounds (peak area) was recorded at 300 nm (Fig 1).

Table 2.

Dynamic phase gradient program

Figure 1. Chromatogram of standards at 300 nm

The analysis was performed using an iCAP PRO X Duo ICP-OES inductively coupled plasma optical emission spectrometer manufactured by Thermo Fisher Scientific (USA). Method development and analysis of the results were carried out using the Qtegra ISDS software. The analysis parameters are presented in Table 3.

Table 3.

Parameters of the analysis

Result and Discussion. The amounts of macro and microelements in the kernel and shell of the common hazelnut grown in Uzbekistan were determined using the ICP-OES method in the laboratory of the Chemistry Department at Andijan State University. The obtained results were compared with those presented in Reference 9 and are summarized in Table 4. Our findings revealed that the kernel of the common hazelnut (per 100g) contains high levels of phosphorus (350 mg), magnesium (150.7 mg), and potassium (903.7 mg).

Table 4.

Results of determination of chemical elements in the sample by the ICP-OES method and data from the literature (mg/100 g).

In the obtained results, it was found that the amounts of elements such as phosphorus, potassium, magnesium, iron, manganese, and zinc in the kernel and shell of common hazelnut grown in our climatic conditions are several times higher than the quantities of these elements reported in the literature. This further proves that common hazelnuts are rich in macro- and microelements.

When phenolic compounds in the sample were analyzed using a chromatographic method, it was determined that the total polyphenol content in the common hazelnut kernel was 2.513 mg, while the shell contained 3.908 mg (per 100 g of sample).

The obtained results. The amount of phenolic compounds in the sample extract was determined. The chromatogram of the 1 g sample extract was obtained (Figures 2-3), and based on the results, the amounts of phenolic compounds in 100 g of the sample were calculated using the following formula and presented in Tables 5-6.

Here,

X – the amount of phenolic compounds in 100 grams of the sample, mg;

C_phen – the concentration of phenolic compounds in the extract determined by the HPLC method, mg/l;

V_extract – the volume of the sample extract, l;

m_sample – the mass of the sample taken for extract preparation.

Figure 2. Chromatogram of polyphenols in the extract of common hazelnut kernel

Figure 3. Chromatogram of polyphenols in the extract of a common hazelnut shell

Table 5.

The amount of polyphenols and retention times in the common hazelnut kernel extract

Table 6.

The amount of polyphenols and retention times in the common hazelnut shell

Conclusion. The results obtained by us showed that the amounts of macro- and microelements such as phosphorus, sodium, potassium, iron, manganese, and zinc in common hazelnuts are several times higher than the values reported in the literature. This suggests that consuming 100 grams of common hazelnuts daily helps partially fulfil the body's needs for essential macro- and microelements. Currently, various parts of the hazelnut tree, including its leaves, fruit, and shell, are widely used in traditional medicine. The leaf infusion is used as a treatment for varicose veins, venous thrombosis, and ischemic heart disease. Furthermore, the leaf and fruit have hepatoprotective and antibacterial properties. The methionine amino acid in the oil is beneficial for treating liver diseases [10].

The dry extract of the hazelnut has been used to create gelatin capsules containing rutin and quercetin, as well as a syrup ("Cardial") for heart diseases. A liquid obtained from the dry distillation of the tree wood has been used as the basis for the "L-2 Lesovaya" medicine, which is used to treat eczema and skin diseases. The oil is widely used in cosmetics and pharmaceuticals, while the fruit and flour derived from it are extensively utilized in confectionery.

Our main goal is to create new biologically active food supplements that are rich in chemical elements using inexpensive raw materials. For this purpose, the chemical and nutritional properties of the product were studied.

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