DETERMINATION AND EVALUATION OF THE MINERAL COMPOSITION OF LEMON PEEL USING ICP-OES

ABSTRACT

In this study, the macro- and microelement composition of lemon peel was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). The results revealed that lemon peel contains high concentrations of essential macroelements, particularly potassium (K), calcium (Ca), magnesium (Mg), and phosphorus (P), which play a crucial role in metabolic processes and the maintenance of physiological functions in the human body. In addition, significant amounts of important microelements such as iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu) were detected. These elements are known to contribute to enzymatic activity, immune system support, and hematopoiesis.

АННОТАЦИЯ

В данном исследовании макро- и микроэлементный состав кожуры лимона был определён методом индуктивно связанной плазменно-оптической эмиссионной спектрометрии (ICP-OES). Результаты показали, что кожура лимона содержит высокие концентрации основных макроэлементов, таких как калий (K), кальций (Ca), магний (Mg) и фосфор (P), играющих важную роль в обмене веществ и поддержании физиологических функций организма человека. Кроме того, были выявлены значительные количества микроэлементов, включая железо (Fe), цинк (Zn), марганец (Mn) и медь (Cu), которые участвуют в ферментативных процессах, поддержании иммунной системы и кроветворении.

Keywords: lemon peel, ICP-OES, mineral composition, macroelements, microelements, heavy metals.

Ключевые слова: кожура лимона, ICP-OES, минеральный состав, макроэлементы, микроэлементы, тяжёлые металлы.

INTRODUCTION. In recent years, the efficient utilisation of agricultural and food industry by-products has become an increasingly important research area due to environmental and economic concerns [1]. Citrus fruit peels, particularly lemon peel, are commonly treated as waste; however, numerous studies have shown that they contain a wide range of biologically active compounds and essential mineral elements [2,3]. Mineral elements play a crucial role in maintaining physiological functions in the human body. Macroelements such as potassium, calcium, magnesium, and phosphorus are involved in bone formation, cardiovascular function, and nerve signal transmission [4]. In addition, microelements including iron, zinc, manganese, and copper are essential for enzymatic reactions, immune system support, and hematopoiesis [5]. Although the chemical composition of citrus fruits, especially their bioactive components, has been extensively investigated, the mineral composition of lemon peel has not been sufficiently studied, particularly using modern analytical techniques [6]. Inductively coupled plasma optical emission spectrometry (ICP-OES) is a powerful and reliable analytical method that allows accurate and sensitive multi-element determination in complex matrices [7]. Therefore, this study aims to determine the macro- and microelement composition of lemon peel using ICP-OES and to evaluate its potential biological and practical significance.

MATERIALS AND METHODS

For the present study, lemon fruits of the “Toshkent tong‘ichi” cultivar grown in Uzbekistan were used as raw material. The peels were manually separated, thoroughly washed with distilled water to remove surface impurities, and dried under laboratory conditions. The dried samples were then finely ground to obtain a homogeneous powder. Approximately 1.0 g of the prepared sample was accurately weighed using an analytical balance (±0.001 g) and subjected to dry ashing in a muffle furnace (Nabertherm, Germany). The temperature was gradually increased: initially to 95 °C for 30 minutes, followed by 120 °C for 60 minutes, 300 °C for 120 minutes, and finally up to 550 °C, where the sample was maintained for 5 hours until complete ash formation was achieved. The obtained ash was treated with 3 mL of concentrated nitric acid (HNO₃, 70%, Sigma-Aldrich, USA) and 2 mL of hydrogen peroxide (H₂O₂, 60%) and heated on a hot plate under a fume hood until the formation of white fumes ceased. After cooling, the solution was quantitatively transferred into a 100 mL polypropylene volumetric flask and diluted to the mark with ultrapure water. The resulting solution was filtered through a 0.45 μm syringe filter before analysis.

RESULTS AND DISCUSSION

The mineral composition of lemon peel was determined using ICP-OES, and the obtained results are presented in Table 1. A total of 69 elements were analysed; however, only those with concentrations above the limit of detection (LOD) are included in the table for clarity. The analysis revealed that lemon peel contains a wide range of macro- and microelements in varying concentrations.

Table 1.

Mineral composition of lemon peel (µg/100 g)

Note: Elements with concentrations below the detection limit (<LOD) were excluded from the table.

Among the macroelements, potassium (K) was found to be the predominant element (1,244,843 µg/100 g), followed by calcium (Ca) (879,623 µg/100 g), phosphorus (P) (90,628 µg/100 g), and magnesium (Mg) (60,502 µg/100 g). The high levels of potassium and calcium are particularly important, as these elements play a crucial role in maintaining electrolyte balance, supporting cardiovascular function, and contributing to bone mineralisation [8,9].

In addition to macroelements, several essential microelements were detected. Iron (Fe) was present at 1246 µg/100 g, zinc (Zn) at 603 µg/100 g, manganese (Mn) at 312 µg/100 g, and copper (Cu) at 215 µg/100 g. These trace elements are involved in numerous biological processes, including enzymatic activity, oxygen transport, and immune system regulation [5]. Their presence highlights the nutritional and functional potential of lemon peel as a natural source of micronutrients. Furthermore, minor elements such as strontium (Sr), lithium (Li), chromium (Cr), and silicon (Si) were detected in relatively low concentrations. Although required in trace amounts, these elements may contribute to metabolic regulation and structural functions in biological systems [10]. Importantly, toxic heavy metals such as lead (Pb) and cadmium (Cd) were not detected in the analysed samples (<LOD). This indicates that the lemon peel used in this study is environmentally safe and does not pose a risk of heavy metal contamination. Similar findings have been reported in previous studies on citrus fruits, where toxic metal levels were found to be negligible or below permissible limits [11]. Comparative analysis with previously published data suggests that citrus peels are generally rich in essential minerals, particularly potassium, calcium, and magnesium, although their concentrations may vary depending on cultivar, soil composition, climatic conditions, and agricultural practices [6]. The relatively high mineral content observed in this study may be attributed to the specific characteristics of the “Toshkent tong‘ichi” cultivar as well as the geochemical properties of the cultivation region. Overall, the obtained results confirm that lemon peel, commonly regarded as agricultural waste, represents a promising and sustainable source of essential mineral elements. Its utilisation may contribute to the development of value-added products in the food, pharmaceutical, and agricultural sectors, as well as to the reduction of agro-industrial waste.

CONCLUSION. In this study, the mineral composition of lemon peel was comprehensively analysed using ICP-OES. A total of 69 elements were investigated, among which a significant number of macro- and microelements were successfully detected and quantified. Overall, the obtained results suggest that lemon peel, commonly regarded as an agricultural by-product, can be considered a promising and sustainable source of essential mineral elements. Its utilisation may contribute to the development of functional food ingredients, nutraceutical products, and environmentally friendly agricultural materials, thereby supporting waste valorisation and resource efficiency.

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