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Mineral composition of different species of Pleurotus commercially grown on rice straw--based formulation in Central Luzon, Philippines.

INTRODUCTION

Mushrooms have been traditionally used as culinary ingredients due to their aroma and flavor. They have been reported to contain appreciable quantities of amino acids, crude fiber, carbohydrates, minerals, and fats [1, 2, 3]. Minerals such as iron, copper, zinc and manganese are also present in mushrooms, which are essential since they play an important role in biological systems. Aside from these important nutrients, mushrooms also contain several biologically or medicinally active components responsible for their functional activities including polysaccharide, proteins, fats, glycosides, flavonoids, carotenoids, folates, alkaloids, volatile oils, tocopherols, phenolics, ascorbic acid, enzymes and organic acids [4].

Besides being used as food flavorings, mushrooms nowadays are regarded as nutraceuticals [5]. Several studies have confirmed that mushrooms exhibit functionality and bioactivity against lifestyle diseases such as hypertension [6, 7], diabetes [8, 9] and cancer [10, 11]. Among the different mushrooms which are commercially grown in the Philippines, different species of Pleurotus are the most popular due to a number of reasons. These include their acceptability among the Filipino consumers because of their flavor, technical suitability for propagation in the prevailing climatic conditions and financial feasibility of production while utilizing locally available materials. Our research team encourages the efficient utilization of rice straw [12] for the production of these ligninolytic mushrooms while discouraging the traditional farmers' practice of burning rice straw in the open field which oftentimes poses hazards to the environment. Pleurotus sajor-caju for instance grows efficiently on rice straw--based formulation [13].

In this paper, the mineral composition of five species of Pleurotus (P. florida, P. pulmonarius, P. djamor, P. salmoneostramineus and P. cystidiosus) which have been commercially grown on rice straw--based formulation is hereby presented. The aim of our investigation is to provide baseline data about the mineral profile of the different commercial species of Pleurotus in the Philippines. Previous papers on the mineral composition of mushrooms dealt on an array of wild edible species [14, 15] and this is the first report about the mineral composition of commercially grown Pleurotus spp. on rice straw-based formulation particularly in the Philippines.

[FIGURE 1 OMITTED]

MATERIALS AND METHODS

Cultivation and fruiting of different Pleurotus species:

Following the protocol of Reyes et al [16], fruiting bodies of Pleurotus were produced as follows: Pure cultures of five different commercial species of Pleurotus namely P. florida, P. pulmonarius, P. djamor, P. salmoneostramineus and P. cystidiosus which were obtained from the Center for Tropical Mushroom Research and Development, Department of Biological Sciences, College of Arts and Sciences, Central Luzon State University, Science City of Munoz, Nueva Ecia, Philippines were revived using potato sucrose gelatin (i.e. 250 g potatoes, 10 g sucrose, 20 g gelatin, 1000 ml distilled water) as the propagating medium for mycelial growth. Grain spawn made from unmilled rice grains were prepared to inoculate the previously sterilized rice straw based formulation consisting of 7 parts composted rice straw and 3 parts of sawdust (v/v) contained in a heat resistant 7 x 14 inches polypropylene bags (i.e. 750 g of formulated substrates per bag). After a month of incubation, the fruiting bags of the different Pleurotus species were transferred to the mushroom growing house to allow the emergence of fruiting bodies. Fruiting bodies (Fig 1) at marketable size were harvested 3-5 days after opening the fruiting bags. Composite samples of fruiting bodies derived from the different bags of each species were gathered and air-dried at 25-30[degrees]C.

Analysis of mineral composition:

Dried samples of fruiting bodies of P. florida, P. djamor, P. pulmonarius, P. salmoneostramineus and P. cystidiosus were analysed using Thermo EDX System (Noran System 6, Ultra Dry, 10mm2 SDD crystal, 129eV resolution, NORVAR window, LN2-Free Type Detector) installed in Hitachi SU1510 Scanning Electron Microscope.

RESULTS AND DISCUSSION

Mushrooms are known to contain minerals [17]. Minerals are simple inorganic nutrients usually required in small amounts--from less than 1 mg to about 2500 mg per day [18]. In our investigation, six mineral elements were detected in the different species of Pleurotus where magnesium, phosphorus, sulfur, chlorine, iron and potassium were found in all the species under study (Fig 2). Chlorine was only detected in P. salmoneostramineus. Silicon was recorded to be present in P. pulmonarius, P. djamor and P. cystidiosus.

[FIGURE 2 OMITTED]

Though Konuk et al [19] reported that iron, as a micronutrient, is common in all the wild mushrooms (Sarcosphaerea crassa, Morchella costata, Helvella leucopus) that they have investigated, iron was not prevalent among the species of Pleurotus under study. It was only detected in P. djamor. Similarly, iron was not traced in Pleurotus tuber-regium, Pleurotus squarrosulus and Auricularia auricula [20].

Among the mineral elements, potassium was the most abundant (65.14%, 63.16%, 50.49%, 48.66% and 33.53% in P. pulmonarius, P. salmoneostramineus, P. djamor, P. cystidiosus and P. florida in decreasing order of abundance across species, respectively). The dominance of potassium in the different species of Pleurotus in this investigation supports parallel reports of Gyorfi and his colleagues [21] who demonstrated that four strains of Agaricus subfrufescens were found to have high and remarkable concentrations of potassium. Similarly, mushrooms across species have been reported to possess an appreciable amount of potassium [22] which emphasizes that mushrooms are very good sources of this important mineral. Among the species of Pleurotus, P. djamor contains all the seven mineral elements. P. florida, on the other hand, had the least composition of minerals including nitrogen, phosphorus, sulfur and potassium. However, different species of Pleurotus under investigation did not contain metal elements as micronutrients such as zinc, manganese and copper as previously reported by other researchers [23, 24]. Similarly, heavy metals such as cadmium, lead and mercury were also not detected. This significant finding suggests that Pleurotus spp grown in rice straw--based formulation are safe for human consumption. The unpasteurized rice straw--based formulation which was analyzed prior to its used in this study in propagating the different species of Pleurotus possessed the following minerals: silicon (42.17%), potassium (2.64%), calcium (2.13%), magnesium (0.79%), sodium (0.60 %), and sulfur (0.49%). These results imply that different species of Pleurotus may not necessarily uptake the available nutrients from their growing medium and ultimately become part of their system. Phosphorus was not detected in rice straw-based formulation but present in the fruiting bodies of the 5 species of Pleurotus under investigation. Moreover, iron was also not present in the formulation but was recorded in P. djamor. The disparity in the mineral compositions of mushrooms may not necessarily be attributed to environmental factors such as climate and growing conditions. Bio-absorption of minerals was only recorded in silicon by 3 out of 5 species of Pleurotus namely P. djamor, P. pulmonarius and P. cystidiosus. This significant observation therefore does not conform with the reports of previous investigators [24] regarding the ecological role of mushroom as myco--accumulators [25]. The differences in the mineral composition of the different species under investigation may be ascribed to their inherent biological attributes.

In conclusion, the five Pleurotus mushrooms analyzed for their mineral contents serve as sources of magnesium, phosphorus, sulfur, chlorine, iron and potassium. Among the mineral elements, potassium was the most abundant among the species; P. djamor contains all the mineral elements detected in the present study. Accordingly, these species are good supplementary healthy foods with great contribution in human nutrition.

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(1) Michael R. Umagat, (2) Rich Milton R. Dulay, (2) Sofronio P. Kalaw, (2) Marcelino D. Abon and (1,2) Renato G. Reyes

(1) Electron Microscopy Laboratory, Department of Biological Sciences, College of Arts and Sciences, Central Luzon State University, Science City of Munoz, Nueva Ecija, Philippines

(2) Center for Tropical Mushroom Research and Development, Central Luzon State University, Science City of Munoz, Nueva Ecija, Philippines

Address For Correspondence:

Renato G. Reyes, Department of Biological Sciences, College of Arts and Science, Central Luzon State University, Science City of Munoz, Nueva Ecija 3120, Philippines.

Tel: 63-44-4560-9405; E-mail: renezoilo@gmail.com

Received 12 February 2016; Accepted 12 March 2016; Available online 22 March 2016
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Article Details
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Author:Umagat, Michael R.; Dulay, Rich Milton R.; Kalaw, Sofronio P.; Abon, Marcelino D.; Reyes, Renato G.
Publication:Advances in Environmental Biology
Article Type:Report
Geographic Code:9PHIL
Date:Feb 1, 2016
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