Elementary analysis in banana samples using X-Ray Fluorescence

This work aims to characterize the elemental concentrations of two banana types gold and silver. In both were analyze in the pulp and peel by X-Ray Fluorescence technique. The results showed that the elements in both types are more concentrated in the peel than in the pulp. In addition, it was also observed that the normalization of the peel and pulp concentrations for the elements K, Fe, Zn, Br, Rb, and Sr is higher in the silver banana compared to the gold banana. The results indicate that banana peels can be used to supplement nutritional deficiencies. The work also demonstrates the potential that X-Ray Fluorescence has in investigation of elemental composition of foods. As the technique has easy instrumentation and data interpretation, it can be implemented as a routine in the investigation of the elemental composition of foods. In addition, the technique has the advantage of carrying out in situ analyzes by portable instruments. These in situ investigations can even be applied to make quality control of bananas at the time of harvest.


Introduction
Banana is one of the most versatile fruits when it comes to the culinary area. Due to its sweet flavor, that pleases palates, and nutritional value banana is one of the most consumed fruits in the world, making it an ingredient in food (Du et al., 2016;Yang, Tu, et al., 2019). These characteristics have sparked several studies about this fruit in several areas, such as culinary preparation (Akubor, 2003;Akubor et al., 2003;Almeida et al., 2017;Carneiro et al., 2020;L. O. Ribeiro et al., 2018; J. M. , transportation and storage (Mugula et al., 1994;Nsumpi et al., 2020;Pathak et al., 2017;Yang, Zeng, et al., 2019), packaging (Fernando et al., 2020), and conservation (Chowdhury et al., 2020;Kritchenkov et al., 2020). According to the Food and Agriculture Organization of the United Nations -FAO, banana is produced in more than 130 countries, resulting in 18 million tons produced worldwide in 2014 (Cordoba et al., 2018). Brazil is the fourth largest banana producer in the world with approximately 7 million tons in 2017, representing 6% of the world production (de Sá et al., 2019). The banana is the most consumed fruit in Brazil in 2016 (G1, 2017) and the fifth in Chine in 2017 (Ji et al., 2020), with emphasis on the types silver banana (scientific name musa paradisiac) and banana gold (another variety of musa paradisiac). The basic characteristics of a healthy diet with the full use of food are the high nutritional value of the products, the low cost to purchase them, and the quick and easy preparation (SESC, 2003). With a view to making full use of the fruit in the areas of gastronomy and nutrition, research has been carried out related to enriching diets and contributing to minimizing food waste (Almeida et al., 2017;Ortiz et al., 2017;J. M. Santos, 2014).
Based on this idea, this work aims to characterize the concentrations of elements present in two banana types both in the pulp and in the peel using the X-Ray Fluorescence (XRF) spectroscopic technique to show that the banana peels have a high nutritional value. In 1996 Selema and Farago published a study on bananas comparing the skin and trunk mineral elements (Selema & Farago, 1996). Later in 2007, Emaga et al. compared the chemical composition of peels of different types of bananas (Happi Emaga et al., 2007). And in 2012 Ho, Aziah and Bhot investigated the mineral composition of banana pseudo-stem flour (Ho et al., 2012). However, to the best of our knowledge, no study has compared the elements present in the banana pulp, which is usually consumed, and in the peel, usually discarded as waste.

Theoretical foundation
X-Ray Fluorescence (XRF) is a non-destructive technique that allows for a qualitative and quantitative analysis in which it establishes the proportion of each element present in the sample. The XRF is based on the characteristic X-Rays, that is, a source of ionizing radiation emits a beam of X-Rays towards the sample. When interacting with its atoms, it transfers energy to the electrons of these atoms to the point of removing them from its orbit, generating vacancies in the orbitals of each affected atom. To stabilize the atom again, electrons from more energetic orbitals jump into less energetic orbitals to fill the void. When carrying out this orbital transition, they emit this energy difference between one orbital and another, in the form of electromagnetic radiation, more specifically as a characteristic X-Ray. The energy distribution spectrum of the characteristic X-Rays is discrete, as they are dependent on the energy levels of the electrosphere (Ashida & Tsuji, 2014;dos Anjos et al., 2000;Grieken & Markowicz, 2001).
In order to carry out the analysis by X-Ray Fluorescence to characterize samples, it is necessary to carry out three steps: excitation of the elements that constitute the sample, dispersion of the characteristic X-Rays emitted by the sample, and detection of these X-Rays (Gama Filho, 2015; Grieken & Markowicz, 2001). Its main advantages are: non-destructible analysis, fast qualitative analysis, little interference between lines, simplicity in sample preparation, a wide range of elements that can be analyzed, easy operation and in situ analysis, in this case using portable equipment. These advantages give the technique a wide range of use (Ávila et al., 2021;A. V. C. Braga et al., 2020;A. V. de C. Braga et al., 2019;Valter S. Felix et al., 2018;Valter Souza Felix et al., 2021;Freitas, Ribeiro, Calza, Oliveira, Silva, et al., 2016;Freitas et al., 2019;M. O. Pereira et al., 2021;Pimenta et al., 2020;I. M. N. Ribeiro et al., 2016;Rocha et al., 2021;Tavares et al., 2020). This technique is well consolidated in the elementary analysis in plants and vegetables (Marguí et al., 2009;Richardson et al., 1995;Spolnik et al., 1999).
The concentration (Wi) of each element was obtained as a function of the average of the peak areas of each element divided by the counting time (Ii), the sensitivity of each element (Si), the surface density of the sample (D) and the total mass absorption coefficient (Xi) according to equation (1) (R. S. . (1) Xi being determined as a function of the sample's mass absorption coefficient for energy E0 of the incident photons µ(E0), the angle of incidence of radiation with energy E0(inc), the sample's mass absorption coefficient for energy Ei of the fluorescent radiation produced by the i-th element of the sample µ(Ei) and the angle of incidence of radiation with energy E0(r) according to equation (2) (R. S. Santos, 2014). (2)

Experimental procedure
The research was carried out according to the methodology described in this references (Koche, 2011;Lüdke & André, 1986;Yin, 2014).Initially, some silver banana and gold banana samples were selected from an open market in Rio de Janeiro, Brazil. The pulp was separated from the peel, and the masses were measured separately. The Research, Society andDevelopment, v. 10, n. 11, e565101119667, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i11.19667 4 samples were placed in an oven at a temperature of approximately 60°C, to dehydrate (Celestino, 2010). Table 1 shows the evolution of the dehydration process of only one of the silver banana and gold banana samples: Each 2.54 cm diameter insert. Figure 1 shows the formed inserts. Tables 2 and 3 show the masses and densities of the tablets.   To obtain the spectra (which will be presented in the results section) the following parameters were used: the X-Ray tube (AMPTEK, model Mini-X) was adjusted under the following conditions: voltage 35 kV, current 50 µA, and counting time of 1000 seconds, and a Si-DRIFT detector (AMPTEK, model XR -100SDD) (Amptek, 2021). Three spectrum were obtained for each sample.
The spectra obtained from the AMPTEK DppMCA program provided the chemical elements of the samples. With the aid of the AXIL program, we obtained the sum of the area of all peaks for each spectrum. Afterwards, we calculated the average of the three measurements for each element's peak area divided by the counting time (Ii). Figure    Source: (Spolnik et al., 1999).

Results and Discussion
From the obtained spectra, we verified the presence of the elements Cl, K, Ca, Mn, Fe, Cu, Zn, Br, Rb, Sr in the gold and silver banana peels that according to Hassan et al., 2018, are part of most of the minerals that make up the banana peel (Hassan et al., 2018). Some of these elements have an essential function, such as prosthetic groups of some enzymes (Happi Emaga et al., 2007). In the pulp, the element Ca does not appear in any of the types, and the Cl does not appear in the silver banana. Table 6 shows the concentrations of the elements in part per million (ppm) in the banana peel and pulp and the concentration ratio between the peel and the pulp.  Source: Authors.
The vitamins, dietary fibers, minerals and many health-promoting bioactive phytochemicals from banana presents: antioxidant properties, reduce the risk of cardiovascular diseases and cancer, potential hepatoprotective effects, cholesterol reduction, precursors of vitamin A, antioxidant, antimicrobial, anti-inflammatory, antiallergic, among others (Sidhu & Zafar, 2018) Potassium was the main element present in the pulp and peel. Other studies performed only on the peel also found potassium as the main element present (Happi Emaga et al., 2007). Manganese had an essential role in the photosynthesis process; 20 to 500 ppm is the usual range for this element (Selema & Farago, 1996;Valkovic, 1978). Small amounts of Rb have been detected. Rubidium can be a nutritional substitute for potassium (Valkovic, 1978). The Sr presence is not considered essential for plants (Selema & Farago, 1996). However, the quantities found are within the expected range; 1-200 ppm (Valkovic, 1978).
The consumption of vitamins and minerals has a fundamental role in human health and well-being (Ji et al., 2020).
The consumption of minerals present in fruits is beneficial to the nervous system's functioning (Stefańska et al., 2014). Some elements, such as Fe (Sheikh et al., 2017) and K (Fullana et al., 2019), can alleviate depressive symptoms. A study conducted in mice revealed that banana consumption is associated with a decrease in symptoms of depression (Samad et al., 2017). Figure 3 shows the spectra of the banana peel and pulp samples for and gold. Figure 4 shows the normalization of the concentrations between the pulp and the peel for the gold and silver bananas, making the ratio between the concentrations of the peel and the pulp.

Conclusion
The results showed that not all the elements found in the peel were observed in the pulp, like Cl and Ca. We found that in both banana types, the concentration of all elements found is higher in the peel in both. Banana types as seen in Table 6.
As a reference for a source of potassium (Carneiro et al., 2020), it is worth mentioning that for silver bananas the concentration is about 4 times higher in the peel than in the pulp, and for gold bananas is about 2.5 times higher in the peel.
The normalization of the concentrations illustrated in Figure 4 shows that for the elements K, Fe, Zn, Br, Rb, and Sr the relationship between the concentrations for the peel and for pulp is higher in the silver banana compared to the gold banana.
That is, these elements are more concentrated in the peel than in the pulp in the silver banana compared to the gold banana.
Research on fruit and vegetable peels is a promising field in the food area. Investigating the nutritional value of the peels is a crucial step to spread the use in cooking of a part of the food that is usually neglected. It is recommended to investigate other foods and the nutritional value of the components found in this research.