1 Introduction

Malnutrition is a widespread problem that poses a public health issue due to the low levels of vitamins, protein and essential nutrients found in foods. This deficiency in macronutrients and micronutrients is caused by inadequate intake of nutrient-rich foods, unhealthy eating habits, infections, menstrual blood loss in women, increased metabolic requirements during early growth, pregnancy, and breastfeeding [1]. Food security has been a major concern in many nations for ages, with malnutrition being the primary contributor to global mortality and prevalence of diseases worldwide according to the World Health Organization (WHO). Recent statistics show that millions of people suffer from malnutrition, and responsible for approximately forty-five percentage of deaths among children below the age of five in low as well as middle-income countries [2]. Bangladesh, a highly populated country in South-East Asia, has one of the world's most severe levels of malnutrition, with approximately 15–16% of children under 5 years old being acutely undernourished [3]. Additionally, one-third of Bangladeshi women suffer from acute nutritional inadequacy, and malnutrition is a double burden in 6.5% of Bangladeshi households [4, 5]. Malnutrition impairs the body’s own immune system and increases oxidative stress in the cells, making individuals more vulnerable to infections and other diseases [6].

For thousands of years, mushrooms have been utilized for both their therapeutic properties and as a source of nutrition in various countries. While there is an estimated total of 140,000 mushroom species, only a small portion, around 10%, have been officially identified and categorized [7]. Out of these, approximately 3000 species are recognized as edible worldwide [8]. Mushrooms, along with other fungi, occupy a unique place in the natural world, as they do not fall strictly into the plant or animal category. Mushrooms exhibit diverse forms and are not constrained to being Basidiomycetes, aboveground, robust or safe to eat. They can also encompass Ascomycetes species, grow beneath the surface, and possess a non-robust texture. In reality, there exists a vast array of mushroom forms, showcasing immense diversity. Edible mushrooms are highly valued for their flavor, texture, medicinal qualities, and overall positive impact on health [9]

Therefore, the consumption of mushrooms is increasing worldwide as more people recognize their value as a nutritious food source. They are rich in carbohydrates, proteins, minerals, and vitamins (B, C, and D), and essential elements like iron, calcium, and potassium while being low in calories and fat [10]. Studies have shown that oyster mushroom extract can effectively reduce cholesterol levels, comparable to dietary supplements [11]. Moreover, mushrooms have been found to possess various health benefits, including anti-tumor, antiviral, antiallergenic, antibacterial, immunomodulatory, anti-inflammatory, hypoglycemic, and liver protecting properties [12]. As a result, both wild and cultivated edible mushrooms have significant potential to enhance the nutritional value of our daily diet.

The process of oxidation is crucial for energy production and various biological functions in the human body. However, excessive generation of ROS (reactive oxygen species) can lead to tissue injured and contribute to the development of numerous diseases, including chronic conditions that can be life-threatening [13]. Antioxidants play a vital role in inhibiting or delaying the oxidation process and removing harmful substances from the body. They help prevent radical chain reactions that occur during oxidation [14]. Macro fungi, such as Pleurotus sp., have a long history of being recognized as functional foods and sources of health-promoting nutrients. These mushrooms contain secondary metabolites that possess significant antioxidant properties. These bioactive components have been found to enhance the body’s defenses against oxidative stress by inhibiting lipid peroxidation, reducing levels of human low-density lipoproteins, and scavenging free radicals, among other mechanisms [9]. Furthermore, organisms necessitate specific trace quantities of heavy metals such as iron, copper, manganese, zinc and chromium to ensure their optimal functioning. However, elevated levels of these hazardous metals can be harmful to organisms [15].

The climate in Bangladesh is highly favorable for mushroom cultivation, making it one of the most suitable regions in the world for this activity. While mushrooms have a long history of consumption globally, their cultivation and usage in Bangladesh are quite new as people are still largely unrecognized for the nutritional and therapeutic benefits they offer [16]. The introduction of mushroom cultivation in the late 1980s by the Bangladesh Agricultural Research Council and the subsequent training provided by the Mushroom Development Institute (MDI) have contributed to its increasing popularity in recent years. Currently, there are around 20 mushroom species found in Bangladesh, including both edible and poisonous varieties. Of these, approximately 9 edible mushroom species are commercially cultivated by small-scale farms [4]. These mushrooms represent a possible alternative source of functional food that could greatly enhance nutritional status and aid in the prevention of prevalent ailments among Bangladeshis. Despite this, most of the species of cultivated mushrooms in Bangladesh remain unexplored. Antioxidant profiling of the Pleurotus species is not thoroughly investigated yet. Therefore, the aim of this study was to thoroughly assess the nutritive value and antioxidant capabilities of eight edible oyster mushroom species grown in Bangladesh, as well as to gain a better knowledge of the potential of oyster mushrooms in addressing malnutrition and combating diseases.

2 Materials and methods

2.1 Collection of mushroom species

Mushroom species, Pleurotus ostreatus (PO2), Pleurotus djmore (POP) Pleurotus sajor-caju (PSC), Pleurotus citrinopileatus (PY), Pleurotu florida (FLO), Pleurotus cystidiosus (PCYS), Pleurotus ostreatus-white snow (PO-WS), Pleurotus ostreatus-Oyster small (PO-OS) belonging to same family, (Fig. 1) Pleurotaceae were collected from Mushroom Development Institute (MDI), Savar, Dhaka, Bangladesh between December 2022 and March, 2023 based on their availability, scientific data availability and popularity in consumption among local people by the researcher. All samples were authenticated by MDI, Savar, Bangladesh. Samples of mushrooms were collected, dried and powdered to evaluate different contents. Samples that needed to dry were placed in airtight containers after dried in the shade.

Fig. 1
figure 1

Phenotypic appearances of different oyster mushroom species used in this study

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2.2 Preparation of mother spawn

Sawdust and wheat bran were combined in a 2:1 ratio with 0.1% calcium carbonate to prepare the mother culture substrate. Then, it was completely blended by hand, and the moisture content was kept at 55% by adding just the right amount of water. After that, an 18 × 25 cm polypropylene (PP) bag was used to tightly pack 200 gm of the mixture. Each bag was made by cutting off the top of a bamboo stalk, stuffing it with cotton, wrapping it in brown paper, and fastening it shut with a rubber band. For 1 h at 121 °C and 1.5 kg/cm2 pressures, the packets were sterilized in an autoclave and then stored for cooling. Next, aseptic placement of pure culture inoculums into the mother spawn packets was carried out. Following inoculation, the packets were once more blocked with cotton and maintained at 20–22 °C for the spawn run. Within 15–20 days, the entire substrate package turned white as a result of the expansion of fungal mycelia. Spawn packets were prepared by following the methods of [17].

2.3 Proximate composition analysis

To find out the amount of moisture, total ash, protein, fat, carbohydrate, and crude fiber in the mushrooms, proximate composition analysis was performed on them. The method outlined by AOAC was used to determine the mushrooms’ moisture and fat levels [18]. We used the micro Kjeldahl method to determine the total nitrogen content of the collected edible mushroom, and then calculated the crude protein content using the formula N × 6.25, where N represents the nitrogen content [18] where N is the average nitrogen content. In addition, the total ash and crude fiber were assessed using established procedures, and the total carbohydrate was calculated by subtracting the combined moisture, ash, total protein, fat and fiber contents from the sample weights. Dietary fiber was also measured by AOAC [19] method.

2.4 Mineral element analysis

An atomic absorption spectrophotometer (AAS) was used to determine the minerals, specifically calcium and iron, in the dried mushroom sample [20]. The samples were processed via the AAS apparatus using different lamps after being digested in an HNO3/HCl acid blend. To get rid of the turbidity, one drop of hydrogen peroxide was added to the mixture. After that, the mixture was put into a volumetric flask, and deionized water was added to get the volume up to 50 mL. Nitric acid was used to digest the sample and liberate Na, Ca, K and Fe. Corresponding standards were calibrated at varied concentrations for the various minerals under study. Sodium and potassium were assessed using flame emission photometry, while arsenic, lead, cadmium, and chromium were determined using an atomic absorption spectrophotometer [21].

2.5 Determination of vitamin (vitamin A and C)

The HPLC–PDA method outlined by Moyo et al. [22] was used to evaluate the vitamin A content of the mushroom samples by dividing β-carotene by 12 whereas the vitamin C was determined titrametrically using 2.6 Dichlorophenol Indophenol methods [23].

2.6 Determination of antioxidant components (total phenol, flavonoid, and tannin)

By utilizing a standard curve of gallic acid and the Folin-Ciocalteu technique, the total phenolic content of the mushroom samples was determined. In a volumetric flask, dilute 1 mL of the extract (1 mg/mL) with 46 mL of distilled water. Each 0.1 g extract was diluted with 5 mL of methanol. In a test tube, mix 200 µL of mushroom extract with 1 mL of Folin–Ciocalteau reagent. After 3 min, 3 mL of 2% sodium carbonate (Na2CO3) was added and allowed to stand for 2 h with intermittent shaking. The absorbance was measured at 760 nm with a spectrophotometer (UV–vis model 6305 Jenway UK).

Likewise, quercetin was used as a standard to calculate the samples’ total flavonoid content, which was then followed by [24]. Two mL of 2% aluminium trichloride (AlCl3) in methanol was combined with the same volume of extract solution, 0.1 mL of 1 maqueous sodium acetate. After 40 min of incubation at room temperature, the absorbance was measured calorimetrically at 510 nm using the same spectrophotometer. The total flavonoid concentration was determined using the quercetin standard calibration curve.

Whilst, the tannin content was assessed using the Folin-Ciocalteu's method with a standard curve of tannic acid [25]. In each sample, 25 μL mushroom extracts were combined with 0.5 mL of Folin–Ciocalteau reagent. The well-combined solution was incubated at room temperature in the dark for 5 min before adding 1 mL of 10% Na2CO3, which was thoroughly mixed with a vortex machine for a few seconds before being left for 20 min to develop the color. The absorbance against the blank was measured at 725 nm.

The findings for the total phenolic, flavonoid, and tannin contents were presented as gallic acid equivalent (mg GAE/100 g dry matter), quercetin equivalent (mg QE/100 g dry matter), and tannic acid equivalent (mg TAE/100 g dry matter), respectively, derived from the established standard curves.

2.7 Determination of total antioxidant potential

The mushrooms' overall antioxidant capacity was assessed by modifying the ammonium molybdate reduction method [26]. In contrast to a previous study that used a 1:1 ratio of test sample and reagent mixture, we opted for a 1:10 ratio to ensure full reaction of antioxidant components in the sample with the reagent solution. During this procedure, reagent blend (3 mL) containing 4 mM ammonium molybdate, 0.6 M H2SO4, and 28 mM Na3PO4 was combined with 0.3 mL of the test sample, which ranged in concentration from 100 to 500 μg/mL. The mixture was then stored at 90 °C for ninety minutes. The mixtures’ absorbance at 695 nm was determined after they had cooled to room temperature. Using ascorbic acid (AA) as a positive reference, the IC50 value was determined by comparing the blank absorbance with the sample concentration function.

2.8 Determination of free radical scavenging activity and reducing power activity

To assess the mushrooms' capacity to scavenge free radicals, we used 2,2-diphenyl-1-picryl-hydrazyl (DPPH). The methodology used was almost exactly the same as that of earlier studies with slight modification [27]. The cuvettes were loaded with 1 mL of the extract and 2 mL of a methanol solution containing DPPH radicals at a concentration of 0.05 mg/mL. The mixture was shaken and then allowed to sit at room temperature for half an hour. Next, the absorbance at 517 nm was measured with a spectrophotometer. As positive references ascorbic acid was used. The following formula was used to determine the DPPH radical concentration:

\(\text{DPPH scavenging effect }(\text{\%})\hspace{0.17em}=\hspace{0.17em}({\text{A}}_{0}-{\text{A}}_{1})/{\text{A}}_{0}\hspace{0.17em}\times \hspace{0.17em}100,\)

where A0 is the absorbance of the control and A1 is the absorbance of reaction mixture or standards. The metric utilized to compare the radical scavenging activity was the inhibition concentration at 50% inhibition (IC50). A reduced IC50 indicated increased radical scavenging efficacy.

To evaluate the extracts of collected mushrooms in terms of their ability to decrease ferric to ferrous ions using known methodologies [24]. Different concentrations (0.2–1 mg/mL) of the extract (1 mL) were combined with 2.5 mL of phosphate buffer (0.2 M, pH 6.6) and 2.5 mL of potassium ferricyanide (1%), then incubated in a water bath at 50 °C for 20 min. The reaction was then terminated by adding 2.5 mL of 10% trichloroacetic acid. The upper portion of the solution (2.5 mL) was combined with 2.5 mL of purified water and 0.5 mL of FeCl3 solution (0.1%). After holding it at room temperature for ten minutes, the absorbance was measured at 700 nm against an adequate blank solution, with AA as a standard.

2.9 Statistical analysis

Using the Statistix 10 software, a one-way ANOVA was used to statistically analyze the data collected for each parameter in the trials in order to ascertain the variations arising from various experimental treatments. In triplicate, the tests were performed on each of the three samples utilized in each preparation. The mean values and standard deviations (SD) of the results are shown. Duncan’s multiple range tests were used in a one-way analysis of variance to assess differences in mean values amongst distinct groups. The mean differences were evaluated by the Least Significant Difference (LSD) test [28].

3 Result

3.1 Proximate composition status

The macronutrient values of Bangladeshi edible oyster mushrooms were determined using proximate analysis in this study (Table 1). One of the most critical factors influencing mushroom nutrient content is moisture content. According to our observations, the moisture content of the eight oyster mushroom species varied from 4.40% (FLO) to 9.54% (PY) (Table 1). Furthermore, our findings revealed that the examined mushrooms had a lower moisture content.

Table 1 Proximate composition of selected Bangladeshi cultivated oyster mushrooms
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In this study, our results showed that the crude fat content of the selected mushroom samples ranged from 0.91 (FLO) to 2.60 (PO-WS) g/100 g (Table 1). On the other hand, our data revealed that PO-OS had the highest carbohydrate content (63.22 g/100 g), whereas PY had the lowest (45.25 g/100 g). Furthermore, the crude protein content of the eight oyster mushroom species sampled in this study ranged from 23.48 to 33.16 g/100 g (Table 1) whilst lowest one was PO-OS and highest amount of crude protein found in PY.

Ash is the sample’s totally non-burnable inorganic salts; in mushrooms, the principal ash constituents are potassium (K) and phosphorus (P). Lowest proportion of total ash content was recorded in PO-OS (7.25 g/100 g) which was followed by PO (7.28 g/100 g) as well as they were statistically similar and the highest in PY (9.41 g/100 g). Whilst, total crude fiber in Pleurotus spp. ranged from 0.16 to 1.91% (Table 1). The total amount of dietary fiber in 100 g of the collected oyster mushrooms ranged from 40.71 g (PSC) to 52.31 g (POP), respectively (Fig. 2).

Fig. 2
figure 2

Dietary fiber content (%) in eight oyster mushrooms

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3.2 Vitamin contents in the samples

A vital ingredient for some animals, including humans, is vitamin C. Animals, including humans, need vitamin C as a cofactor in several enzymatic activities that mediate a number of crucial biological processes. Vitamin C content in eight cultivars of Pleurotus sp. ranged from 0.119 to 0.416 mg/100 g (Table 2). Whilst vitamin A was not detected in selected oyster mushrooms (Table 2).

Table 2 Vitamin, mineral elements and heavy metal compositions of oyster mushrooms
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3.3 Mineral element contents

The growth conditions of a mushroom can influence its mineral composition. Table 2 presents the results of the mineral element contents of the mushroom sample, which were measured based on their concentration in dry weight.

The sodium content obtained from different cultivars of Pleurotus spp. was appreciably the highest in PY (138.65, mg/100 g) while the lowest was found in PO2 (42.07, mg/100 g) (Table 2).Calcium content was the highest in POP (1110.11 mg/100 g) and PCYS (155.21 mg/100 g) had the least (Table 2).According to the findings presented in Table 2, oyster mushrooms had higher levels of potassium. PO2 exhibited high potassium content (1139.40 mg/100 g) whereas, PO-OS showed lowest potassium content (722.24 mg/100 g). In the present study, highest iron amount was recorded in PO-WS (83.077 mg/100 g) followed by POP (61.29 mg/100 g) and PO2 (50.98 mg/100 g). The lowest iron content found in FLO (1.9833 mg/100 g) (Table 2). The findings of the study are presented in Table 2, which highlights the concentrations of various metals in mushrooms. Arsenic (As) is a metal that can be found in both marine organisms and mushroom fruit bodies, as well as in the environment. Arsenic was found in PO2 (0.008 mg/kg), FLO (0.05 mg/kg), PSC (0.04 mg/kg), PO-WS (0.009 mg/kg) and PY (0.05 mg/kg) whilst POP, PO-OS and PCYS had no As in their fruiting body. However, in the mushroom sample analyzed in this study, no detectable levels of cadmium or chromium were found. Lead was found in FLO (0.07 mg/kg), PO2 (0.04 mg/kg), PSC (0.02 mg/kg) and PCYS (0.05 mg/kg) which were lower than tolerance limits (0.3 mg/kg).

3.4 Profile of antioxidant components (total phenol, flavonoid, and tannin)

In this study, the total phenolic, total flavonoid, and total tannin contents of the collected mushroom samples were measured and compared. The analysis of antioxidants was conducted using a regression equation derived from calibration curves of respective standards. The total polyphenols, total flavonoids, tannin contents and total antioxidant content of Pleurotus sp. are shown in the table. Table 3. Phenolic, flavonoid, and condensed tannin contents were found in the ranges 279.96 mg (PO-OS) to 644.75 mg (POP) per100 g., 80.500 mg (PCYS) whereas, 24 mg (PY) per 100 g and 402.13(PSC) to 920 (POP) mg per 100 g, respectively.

Table 3 Bioactive antioxidant components of the collected Bangladeshi edible Oyster mushrooms
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3.5 Total antioxidant contents

In this study, the total antioxidant activity of the oyster mushroom extracts was assessed by their capacity to reduce Mo (VI) to Mo (V). The results, presented in Table 3, values which varied between 798.41 mg/100 g (FLO) to 1627.3 mg/100 g (PSC).

3.6 Free radical scavenging activity and reducing power capacity

The DPPH free radical scavenging assay was performed in this investigation to determine the amount of antioxidant contained in mushroom samples that can scavenge free radicals. The results of our analysis of the DPPH radical scavenging capacity of mushroom samples are listed in Table 4. The highest antiradical activity of the extract of PO2 (75.4%) against DPPH noted at 0.2 mg/mL (Table 4). However, PSC (69.57%) and PO-OS had also high scavenging capacity (64.56%) at a concentration of 0.2 mg/mL whereas, PY showed (37.27%) lowest scavenging activity against 0.2 mg/mL among all the species.

Table 4 DPPH free radical scavenging activity of eight oyster mushrooms (Pleurotus sp.)
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All oyster mushrooms extracts showed an increase of absorbance with the increase of concentrations and absorbance compared with vit C absorbance (Fig. 3) except FLO which absorbance remain constant after 0.2 mg/mL and PY which showed lower absorbance and after 0.4 mg/mL concentration. OS had approximately similar absorbance at 0.6 mg/mL concentration.

Fig. 3
figure 3

Reducing power ability of the eight-oyster mushroom extract

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4 Discussion

The consumption of mushrooms is increasing worldwide as more people recognize their value as a nutritious food source. The present experiment was undertaken to study the proximate composition, selected minerals level and antioxidant properties in oyster mushroom (Pleurotus sp.). The moisture level of food is crucial in terms of consumption, freshness, and food stability. Furthermore, our findings revealed that the examined mushrooms had a lower moisture content. These findings are consistent with a recent study of Indian edible Pleurotus species, which had approximately 90% dry matter and 10% moisture [29]. Moreover, the moisture content of the flour at 50 °C is below 10%, they inhibit microbial growth so the product can be considered stable in storage under suitable packaging conditions [4]. Similar to that obtained in this study at the same temperature as recorded by Vélez-Uribe et al. [30].

Natural lipids have important functions in storing energy, creating cell membranes, and regulating intracellular signaling and local hormones [31]. Consuming fatty acids, such as omega-3, from plant or animal sources has been linked to various health benefits, including infant development, cancer prevention, cardiovascular disease prevention, and mental illness prevention [32]. Sifat and his associated [4] experimented with oyster mushrooms and their results agreed with our findings that Pleurotus species cultivated in Bangladesh showed lower fat content as well as Mutuku et al. [33] findings. Fats primarily provide the energy necessary for bodily functions and a low-fat diet can help with weight loss and reduce the risk of coronary heart disease and certain cancers.

On the other hand, all species were high in carbohydrate which makes them an appealing source of energy gained from food. Carbohydrates are macronutrients that the body requires for growth, energy provision, and the maintenance of other physiological functions [33]. In contrast, the highest percentage of carbohydrate was secured for GL3 (57.85%) and the lowest was for PY (46.01%) was observed by Sifat et al. [4]. Effiong et al. [34] studied Pleurotus ostreatus and revealed that it contains 43.42% carbohydrate, and 1.21% lipid in dry samples of species, respectively which was correlated with this finding.

Moreover, in this study, crude protein content was higher for all mushroom species., these findings are consistent with prior research on Bangladeshi mushrooms [35]. All the data of eight mushrooms were statistically different from each other. Obodai et al. [36] experimented with P. ostreatus cultivars and found the highest protein value. Li et al. [37] conducted an experiment on different oyster mushrooms and found approximately similar result like crude protein (28.3%) in P. citrinopileatus. Another study on P. ostreatus was done by researcher in Ethiopia. They found protein of the mushroom cultivated from all the utilized substrates in the range of 17.30–21.5 which was lower than our findings [38]. Bangladeshi mushrooms are an excellent source of protein due to their high protein content. Proteins are required for growth and muscle formation. Furthermore, mushrooms were found to be higher in protein than green veggies. According to Rashidi and Yang [39], protein contents of mushrooms vary depending on numerous aspects, including mushroom type, the medium of growth composition, the time of harvest, handling style, and substrate conditions.

The amount of the nutritionally significant inorganic minerals that are present in the mushroom can be determined by looking at its ash content. Ash content was lower than carbohydrates and proteins in this experiment. Mutuku et al. [33] and Li et al. [40] found similar results in Auricularia polytricha and P. ostreatus. The present values were supported by Johnsy et al. [41] in Pleurotus species, Ash (10.17 ± 0.35%). The amount of the nutritionally significant inorganic minerals that are present in the mushroom can be determined by looking at its ash content. In a similar fashion, total crude fiber in Pleurotus spp. was lower. However, Mutuku et al. [33] conducted an experiment to carry out nutritional analysis of the fruiting bodies of Auricularia polytricha. The mushroom was found to contain 43.12% crude fibre which was higher than our results. According to Johnsy et al. [41]; fiber content in Pleurotus ostreatus (4.2 ± 0.4%), Pleurotus sajor-caju (4.9 ± 0.57%) and Auricularia auricula (8.4 ± 0.5%). Meanwhile, dietary fiber content was found high in this study. Okwulehie and Ogoke [42] also reported high dietary fiber in four species of mushrooms (Pleurotus sp., Cheimonophyllum candidissimus, Auricularia sp. and Russula sp.). Sifat and his associates, [4] also found high fiber in oyster mushrooms. According to the findings, due to their high fiber content, mushrooms should be a regular part of diet to prevent atherosclerosis and high cholesterol.

All the cultivars showed low amount of vitamin C in this experiment. Li et al. [40] reported that oyster mushrooms had low vitamin C content. Dimopoulou et al. [43] conducted an experiment with sixteen edible mushroom species and found all the species were rich in dietary fiber and protein, although they were shown to be poor in vitamin C. Gebru and his associates [44] found vitamin C (10.61 mg/100 g) which was higher than our findings in oyster mushrooms. Although plant foods are typically a rich source of vitamin C, it can be another option. The quantity in mushrooms varies according to the type, the soil, the climate where it was grown, the duration of time since it harvested, the storage environment, and the preparation technique. Whilst vitamin A was not detected in selected oyster mushrooms. Agboola et al. [45] found vitamin A in A. bisporus, on the other hand, Dimopoulou et al. [43] also reported that A. bisporus contained very low levels of vitamin A but no vit A in Pleurotus sp.

High amount of sodium, potassium, calcium and iron content was observed. Sodium, which is found in the crust of the earth, aids in regular plant metabolism and helps plants concentrate carbon dioxide. However, too much sodium can cause plant tissues to become dehydrated [45]. Similar findings were found for Agaricus bisporus that showed higher amount of calcium, sodium and potassium present in it [45]. Okwulehie and Ogoke [42]; also found higher amount of macro mineral Pleurotus sp., Cheimonophyllum candidissimus, Auricularia sp. and Russula sp. The sodium concentration in the current study was much greater than the maximum ranges of sodium in agricultural soils of 0.75–7.5 mg/g and potassium in agricultural soils of 0.4–30 mg/g [46]. Potassium is one of the three most significant nutrients in soil.

Potassium and sodium are essential for maintaining the balance of osmotic pressure between cells and interstitial fluids [47]. According to the findings presented in Table 2, oyster mushrooms had higher levels of potassium compared to sodium. Consuming a diet rich in potassium and low in sodium has been linked to the prevention of high blood pressure. Additionally, a high potassium diet has been shown to counteract the negative effects of excessive sodium intake. It is recommended to maintain a Na/K ratio of less than 1 in order to prevent high blood pressure [33]. Therefore, based on the Na/K ratio of approximately 0.25 for all the cultivars in this study, it suggests that oyster mushrooms could be beneficial in lowering blood pressure.

The study also revealed significant amounts of calcium in eight Pleurotus sp. cultivars. Calcium is essential for bone formation and maintenance, blood clotting and the transmission of nerve impulses to muscles [48].

The order of abundance for the mentioned elements is potassium > calcium > sodium > iron (Table 2). These findings matched with Obodai et al. [36] findings in mushrooms in terms of mineral composition. Other findings revealed the same result; they found the macro-elements were abundant in P. ostreatus in the order of potassium, magnesium, calcium, and sodium [38].

The body requires macro elements (such as potassium, sodium, magnesium, calcium, and phosphorus) in larger quantities compared to trace elements. These macro elements play crucial roles in functions like transportation, homeostasis, storage, absorption, and excretion. They also work in conjunction with vitamins to stimulate hormone production and support metabolic processes [33].

Iron content was varied among eight oyster mushrooms in this experiment, a value which by exceeded the set limit of 15 mg/kg by WHO [36]. Previous research suggested that up to 90% of the iron in Pleurotus sp. is accessible and absorbable because mushrooms lack phytates, which decrease the body's ability to absorb iron [33]. Li et al. [37] studied oyster mushrooms for mineral analysis. They found that iron and zinc could be heavily enriched in P. ostreatus. The iron content of the mushrooms in this study was lower compared to values reported in previous literature by Sesli and Tuzen [49] who reported a range of 31.3 mg/g to 1190 mg/g. Similar results were also reported by Agboola et al. [45]. Iron is also important for plants, as it plays a role in photosynthesis, chloroplast development, and chlorophyll biosynthesis. On the contrary, the concentration of iron in the body should be tightly regulated as it can form free radical and may cause several neurodegenerative disorders. In the present study, Humans require iron for a variety of functions, but excessive amounts can be harmful to the body [50].

The findings of the study are presented in Table 2, which highlights the concentrations of various metals in mushrooms. Arsenic was found in some oyster mushroom species. Arsenic (As) is a metal that can be found in both marine organisms and mushroom fruit bodies, as well as in the environment. It is important to note that consuming food with high levels of arsenic over a long period of time can have serious health consequences, including an increased risk of cancer and the development of skin lesions [51]. It is crucial to be aware of the potential risks associated with consuming food that may contain high levels of arsenic Obodai et al. [36] experimented with mushrooms for heavy metals and reported negligeable amount of arsenic. Sinha et al. [52] also reported that Agaricus and Pleurotus sp. had lower amount of As. Acceptable level for arsenic in food as well as vegetables according to FAO/WHO (2015) is 0.2 mg/kg [51]. The arsenic amounts in the samples were below the FAO/WHO tolerance levels.

Cadmium and chromium are known to be highly toxic elements that can pose a threat to food safety and human health when present at elevated levels in soil and drinking water [42]. These metals can have detrimental effects on various biological activities and can negatively impact the digestive, respiratory, and immune systems. However, in the mushroom sample analyzed in this study, no detectable levels of cadmium or chromium were found. This is reassuring as it suggests that the mushrooms in this study are not contaminated with these hazardous heavy metals. Mutuku et al. [30] also found similar result for mushrooms. In contrary, Okwulehie and Ogoke [42]., Bhattacharjya et al. [53] found Cd and Pb Pleurotus sp. which were above the tolerance limit set by WHO.

Lead is known to be toxic in nature and is used in the manufacture of drugs and paints. Lead can harm the kidneys, induce miscarriages, anemia, and raise blood pressure. It can be absorbed directly from water as well as, to a lesser amount, from air and food [54]. Leas found in four species of oyster mushroom among studied samples which was lower than permitted limits set by WHO. Sinha et al. [52] observed Pb (0.04–0.06) in mushrooms lower than permitted limits set by WHO and FAO. According to FAO/WHO guidelines, an adult's permissible Cd and Pb intakes are 0.42–0.49 mg/week and 1.5–1.75 mg/week, respectively [51].

The differences in heavy metal concentrations in mushrooms can be influenced by various factors, including the methods used for analysis and the substrate on which they are grown. Mushrooms have a unique ability to absorb heavy metals from their environment, making them susceptible to the metal concentrations present in their surroundings [45]. Environmental factors like soil composition, pH levels, and metal concentrations, as well as fungal factors such as species, fruiting body morphology, developmental stages, mycelium age, intervals between fructifications, and biochemical composition, can have an impact on the accumulation of heavy metals in mushrooms [4].

The antioxidant activity of mushrooms is attributed to secondary metabolites such as phenolics, flavonoids, and tannins. These compounds play a crucial role in protecting the body against oxidative stress and scavenging harmful free radicals. In this study, the total phenolic, total flavonoid, and total tannin contents of the collected mushroom samples were measured and compared. The phenolic component in Pleurotus species varies and is most closely associated to antioxidants and phytonutrients [55, 56]. Phenolic compounds are extremely soluble in water and organic solvents [57]. Sifat et al. [4]; reported high phenol content in oyster mushrooms and found Phenolics: 10.50–83.50 mg GAE/g. Yilmaz et al. [58] also found similar result in Pleurotus sp. The total phenol content in Pleurotus djamor var. roseus varied from 6.75 ± 0.29 to 1.48 ± 0.23 mg/g [59]. The consumption of foods rich in phenolics and polyphenols has been linked to various health benefits, including protection against heart disease and cancer. Phenolic compounds, which are abundant in our diet, possess antioxidant properties and can scavenge harmful free radicals. In this study, the mushroom samples were found to contain significant levels of phenolic compounds. According to the findings, the phenolic content of the mushroom samples is as follows the order: POP > PCYS > PY > PO-OS > PO2 > FLO > PO-WS > PSC.

Tannins, which are present in various plant species and are a part of our regular diet, have been widely recognized for their antioxidant properties and other beneficial effects. These compounds have been associated with a reduced risk of cancer and cardiovascular diseases due to their strong antioxidant activity [60]. The findings indicated that tannin content in mushroom samples followed the order: POP > PCYS > PY > PO-OS > PO2 > FLO > PO-WS > PSC. Sifat et al. [4] found lower amount of tannin in oyster mushrooms as compared to present findings. Okwulehie and Ogoke [42] experimented with Pleurotus sp. for their antioxidant activities. They found tannins in the ranged from 0.44 ± 0.09–1.38 ± 0.6%.

Flavonoids are a group of natural phenolic compounds that are known for their diverse range and widespread presence. They are highly regarded for their strong antioxidant activity. Flavonoids are believed to play a protective role in membranes by localizing within artificial and biological membranes. Their interactions at the lipid bilayer surface can limit the entry of harmful molecules, such as oxidants, and safeguard the structure and function of membranes from oxidative damage [4]. The flavonoid contents found in this study followed the order: PCYS > POP > PO-WS > PSC > PO-OS > PO2 > FLO > PY. Raman et al. [56]; also observed similar results in Pleurotus sp. Agboola et al. [45] studied white button mushrooms and found high amount of flavonoid content. Nevertheless, it is worth noting that there was a notable difference in the total flavonoid content between the Malaysian mushroom samples (PD and PO) from the Pleurotus genus, with a content of approximately 14 mg QE/g of dry extract [61]. This significant difference in Bangladeshi species, could be attributed to various factors such as the cultivated medium, climate of the cultivation country and the specific extraction procedure used in each study.

The role of phytochemicals with strong antioxidant properties in preventing the development of chronic diseases caused by oxidative stress is well-established [62]. In addition to their nutritional value, many vegetables and fruits are considered functional foods due to their ability to act as antioxidants. In this study, the total antioxidant activity of the oyster mushroom extracts was assessed by their capacity to reduce Mo (VI) to Mo (V). Egra et al. [63] reported high TAC in white oyster mushroom (368.78 mg/g). Differences in the total antioxidant capacity were observed among different species of Pleurotus mushrooms. Pleurotus ostreatus [64]. These variations in antioxidant activity could be attributed to various factors, such as differences in cultivation conditions, age of the mushrooms at the time of collection, storage conditions, and the method used to measure antioxidant activity. These findings suggest that these mushrooms could be used as a dietary supplement to enhance antioxidant defenses and reduce oxidative stress levels.

The DPPH assay is used to assess the antioxidant's ability to scavenge free radicals in a sample. The DPPH free radical scavenging assay was performed in this investigation to determine the amount of antioxidant contained in mushroom samples that can scavenge free radicals. Mutuku et al. [33] reported high scavenging activity in methanol extracts of Auricularia polytricha. Li et al. [40] analyzed Pleurotus sp. for scavenging activity and reported higher scavenging activity found in oyster mushrooms. Hossen et al. [65] reported that D. confragosa had an IC50 value of 51.21 g/mL, which is nearly identical to the IC50 value of ascorbic acid (49.19 g/mL). They also found antioxidant action due to the low IC value.

The presence of phenolic compounds, such as flavonoids and tannins, in mushrooms contributes to their ability to scavenge free radicals. The stable and synthetic free radical DPPH is commonly used to assess the antioxidant activity of substances. DPPH accepts an electron or hydrogen atom, resulting in the formation of a stable diamagnetic molecule. This molecule absorbs light at a wavelength of 515–517 nm, giving the solution a purple color in methanol. When an antioxidant substance capable of donating an electron or hydrogen atom is added to the solution, the purple color fades, indicating the scavenging of the DPPH radical. This change is reflected in the decrease in absorbance at 517 nm. The lower the absorbance, the higher the radical scavenging activity of the extract [66].

Reducing capacity is considered as a significant index of antioxidant activity [67]. In this study, all mushroom extracts were evaluated for their reducing power through their ability to reduce ferric ions (Fe3 +) to ferrous (Fe2 +). All extracts showed increased reducing power with the increase of concentrations. The presence of antioxidants in mushrooms suggests that they may have potential health benefits, including the ability to slow down aging and combat oxidative stress, which is associated with various diseases and complications such as diabetic neuropathy [68]. Therefore, incorporating mushrooms with antioxidant properties into the diet may be beneficial for preventing and managing diseases.

5 Conclusion

Results from the present investigation demonstrated that out of the eight oyster mushroom species, most of all showed higher percentage in terms of carbohydrate, protein, crude fiber and dietary fiber with low fat content. First of all, for ash, PY and PSC scored the highest levels. Moreover, PO-OS and PO2 had higher amount of carbohydrate than others, besides PY and POP had the highest protein content. Furthermore, FLO scored low in fat content along with POP and PO-OS. However, dietary fiber was high in POP along with PCYS while PO2 and POP had higher amount of crude fiber than others. PY scored the highest level in terms of vit C. The mushroom's high crude fiber and low-fat content make it useful for maintaining weight loss and lowering the risk of cardiovascular disease, diabetes, and colon cancer. The mushroom also had a significant number of mineral components that are essential in the human diet. The minerals, potassium, calcium, sodium and iron were present in good amounts. Cadmium and Chromium, which are hazardous metals, were not found whilst, Arsenic was detected in PO2, FLO, PSC, PO-WS and PY along with lead in FLO, PO2, PSC and PCYS but lower than tolerance level by WHO. Thus, these oyster mushrooms can be considered safe for the consumption of humans. With regard to antioxidant assays, recorded the highest level of phenolic, flavonoid, and tannin contents in POP and PCYS which might contribute to its free radical scavenging capacity. Highest percentage of total antioxidant capacity was observed in PSC and POP. In methanol extracts, the mushroom also demonstrated dose-dependent antiradical action against DPPH, with the highest radical scavenging capacity. In PCYS, PO-OS and PO2. Results obtained clearly indicate that these eight edible oyster Bangladeshi grown mushroom species are rich in protein, fiber, carbohydrate and minerals as well as a good source of antioxidants. The findings indicate that oyster mushrooms, which contain adequate amounts of the necessary nutritional properties, minerals, and antioxidant elements, are a potentially nutritious and safe to eat. Therefore, these oyster mushrooms can be promoted as a potential source of nutrients and antioxidants to combat malnutrition and other diseases.