Production of Oyster Mushroom (Pleurotus ostreatus) in Peach Palm By-Products: Effects on Composition and Maximization of Antioxidants Extraction

Abstract

The aims of this study were to investigate the use of the peach palm by-products - PPBs (shell and sheaths) as substrate to Pleurotus ostreatus cultivation, evaluate its composition, and the antioxidant properties. The cultivation conditions were defined varying the C:N ratio and moisture content, using as substrate PPBs, and for comparison, a control medium of sawdust was used. P. ostreatus produced in these substrates and a commercial sample were evaluated as composition and antioxidant properties; in these last analyzes it was previous defined how to extract the highest content of phenolic compounds (TPC). The highest yield of P. ostreatus (24.47% wet basis, w.b.) was obtained using the PPB with C:N of 69.14 and moisture of 86.2% (w w^-1); yield higher than that found in the control medium (17.15% w.b.). The protein content of commercial mushroom (28 g 100 g^-1, dry basis - d.b.) was higher than the cultivated samples (16 g 100 g^-1, d.b). In the mushroom samples the contents of ashes, lipids and dietary fibers ranged between 6.09 and 6.92 g 100g^-1, d.b, 1.22 to 1.72 g 100g^-1, d.b and 39.46 and 43.24 g 100g^-1, d.b, respectively. The best condition for TPC extraction was: 38.8 min, using 29.4% (v v^-1) of ethanol. P. ostreatus cultivated in PPBs showed the highest contents of TPC, phenolic acids (gallic, vanillic, trans-cinnamic, ferulic and coumaric), and antioxidant activity. Therefore, PPB is a low-cost alternative to mushrooms cultivation with high nutritional value, avoiding environmental problems of its incorrect disposal.

Keywords:
Black shimeji; Bactris gasipaes; Nutritional value; Bioactive compounds

HIGHLIGHTS

Peach palm by-products (PPB) are low-cost substrates for P. ostreatus cultivation.

PPB with C:N ratio of 69 and moisture of 86% reached highest P. ostreatus yield (24%).

Phenolics and antioxidant activity were higher when using PPB as substrate.

Gallic, vanillic, trans-cinnamic, ferulic and coumaric acids were found in mushrooms.

This proposal can avoid environmental problems of PPB discard.

INTRODUCTION

Brazil is one of the largest world’s producer, consumer and exporter of peach palm (Bactris gasipaes); the country is responsible for 95% (w w^-1) of peach palm consumed worldwide. Peach palm, locally known as pupunha, is a tropical palm which have become a crop that may be cultivated in several regions. In canned foods, about 80% of the peach palm stem is classified as by-product - including shells and sheaths - and then discarded or used as feed. The peach palm by-products (PPBs) are composed of proteins (~10 g 100g ^-1 dry basis - d.b.) cellulose (~13 g 100g ^-1 d.b.), lignin (~12 g 100g ^-1 d.b.), hemicellulose (~28 g 100g ^-1 d.b.), and soluble sugars (~22 g 100g ^-1 d.b.) [¹]. These nutrients can be used as source of nitrogen and carbon to edible mushrooms growth, allowing the use of PPBs as a low-cost substrate. After mushrooms cultivation, the substrates are mainly used as fertilizer.

Pleurotus sp. mushrooms are advantageous by comparison with other fungi regarding their cultivation, since they require short time to grow, have enzymes that are capable of degrading cellulose, lignin and hemicellulose, use lignocellulosic substrates that do not need to be composted and are not very demanding in terms of environmental conditions [²].

The so-called oyster mushrooms have variable protein content (11 - 42 g 100g ^-1 d.b.), with all essential amino acids, mainly lysine and tryptophan, which are not common in cereals [³]. Besides, minerals (1.03 -2.20 g 100g ^-1 d.b.), vitamins, fibers (0.98 - 2.1 g 100g ^-1 d.b.) and low lipid contents (0.2 - 8 g 100g ^-1) [⁴, ⁵, ⁶].

Even though different substrates used for produced mushrooms may impact on contents of secondary metabolites and proteins in the fruiting body, the profile of amino acids is not affected [⁷]. The ratio of carbon and nitrogen (C:N) in the substrate is an important factor in cultivation, since excess of the latter may inhibit fungus fructification while little amount may result in mushrooms with low protein content and yield [⁸, ⁹, ¹⁰].

Mushrooms exhibit several secondary metabolites, such as phenolic compounds, which contribute to sensory characteristics of food, such as aroma, flavor and color. They may be defined as compounds that have at least an aromatic ring with one or more hydroxyl groups (OH) and other functional groups. In addition, they exhibit antioxidant activity. Koutrotsios and coauthors [¹¹] evaluated phenolic profiles of different Pleurotus ostreatus strains and found high variation in phenolic compound contents (1.27 to 8.62 mg GAE100g^-1 wet basis, w.b.). p-OH-benzoic and p-OH-phenylacetic were the main hydroxybenzoic acids while cinnamic was the main hydroxycinnamic acid in the samples under analysis. Natural extracts of antioxidants in food formulations may replace synthetic additives whose use has been restricted [¹²].

Therefore, mushroom production is important due to its nutritional and functional potential since it has been acknowledged as a sustainable protein source. According to Siqueira and coauthors [¹³], sugarcane bagasse is the most common agro-industrial by-product in P. ostreatus cultivation in Brazil, mainly in São Paulo state, where this material is abundant and where mushroom production is intense. However, its availability may become limited because of production of second-generation ethanol and electrical energy, besides other uses for the by-product. Thus, it is very important to study sources of substrates which derive from agro-industrial by-product and are available in all regions to produce Pleurotus sp.

This study aimed to investigate the viability of the use of by-product from peach palm as substrate to Pleurotus ostreatus production, evaluate its composition and maximize the conditions of antioxidants extraction.

MATERIAL AND METHODS

Collection of peach palm by-products

By-products of peach palm processing, sheath and shell, were provided by Sabores da Graciosa, a company located in Antonina, Paraná (PR) state, Brazil (25°27'18.0"S 48°43'59.3"W), registered with IBAMA n. 7552805. The material was dried by an oven with air circulation (MA035/5, MARCONI) at 70 ºC for 24 h. Afterwards it was ground by a knife mill (MA340, MARCONI), sieved (0,15 mm) and stored in sealed plastic bags at room temperature. Moisture of the dried material was determined by the oven-drying method (NOVA ÉTICA) at 105 ºC [¹⁴]. Carbon and nitrogen contents of the medium sheath and shell of peach palm by-product were determined by an elemental analyzer (LECO) [¹⁵].

Mushroom cultivation

The species under analysis, Pleurotus ostreatus, was purchased from a local producer and kept in a liquid medium composed of 10 g L^-1 glucose (C₆H₁₂O₆), 2 g.L^-1 sodium nitrate (Na₂NO₃), 1 g L^-1 dipotassium phosphate (K₂HPO₄), 0.4 g L^-1 monosodium phosphate monohydrate (NaH₂PO₄.H₂O), 0.5 g L^-1 magnesium sulfate heptahydrate (MgSO₄.7H₂0) and 2 g L^-1 yeast extract [¹⁶]. The inoculum for the following experiments was prepared with wheat grains as the substrate. They were cooked in distilled water for 10 min. Water was then drained from the cooked grains, which were placed in polypropylene bags (every bag held 250 g substrate). Bags were sealed and sterilized by an autoclave at 121 ºC for 15 min. After sterilization, cultivation bags were inoculated and kept at controlled temperature up to through grain colonization [¹⁷].

To determine conditions of high yield in mushroom production, optimization of the substrate composition was carried out by the Central Composite Rotational Design (CCRD) with the use of the C:N ratio and moisture of the substrate as independent variables (Table 2).

Different ratios of PPBs (shell and sheath) were used to reach C:N ratios defined by the experimental design. The amount of water added was determined using Equation 1. To buffer the substrate, 1% gypsum (w w^-1), as the calcium source, and 1% (w w^-1) shelly limestone were added. Substrates were placed in polypropylene bags (22x15 cm), which were sealed with D28 foam and sterilized at 121 ºC for 15 min. Afterwards inoculation of 2% (w w^-1) of fungus was conducted in a laminar flow cabinet [²].

$M_{water}=M_s.\left(\left[\frac{100-Mn}{100}\right].\left[\frac{Mreq+100}{100}\right]-1\right)$ (Eq. 1)

*Note: M_s: Substrate mass; Mn: moisture of substrate; Mreq: moisture required to prepare the substrate

Inoculated bags were incubated at 25 ºC in a BOD incubator (SPLABOR, SP-500) for 30 days in the dark. When there was uniform mycelium growth all over the substrate, an “X” cut (about 5 cm) was made to enable fungus fructification, which took place at 18 ºC, on average, and 12-hour photoperiod. After fructification, mushrooms were harvested and weighed. Yield (Y) was determined according to the Equation 2, being the values in wet basis (w.b.) [¹⁸].

$Y=\frac{fruitingbodyweight}{initialsubstrateweight}.100$ (Eq. 2)

For comparison, cultivation was also carried out in a medium whose C:N ratio was 70, with the use of sawdust and wheat bran. One % (w w^-1) gypsum, 1% (w w^-1) shelly limtruestone and water were added to the mixture to reach 54% (w w^-1) of moisture. The other cultivation conditions were similar to those applied to cultivation in PPBs [²].

Figure 1 shows the flowchart with the steps of oyster mushroom production in peach palm by-products.

Determination of mushroom proximate composition

Proximate composition of dehydrated mushrooms was analyzed by official methodologies proposed by the Association of Official Analytical Chemists [¹⁴], which included moisture (method 925.09), ashes (method 923.03), lipids (method 920.85) and proteins (method 920.87, using 4.38 as the conversion factor). Dietary fiber contents were determined by the enzymatic-gravimetric method (method 985.29). Carbohydrate content was found by the difference among the other constituents.

Determination of antioxidant activity and profile of phenolic acids of mushrooms

To be extracted, P. ostreatus samples were lyophilized (LD 1500, TERRONI), ground by an analytical mill (TE633, TECNAL) and passed through a 28-mesh sieve. Extractions were carried by an ultrasonic bath (UNIQUE USC 1400, 40 kHz) at 30±2 ºC; sample:solvent ratio was 1:53.57 mL [¹⁹]. Ethanol concentration in aqueous solutions and extraction times ranged from 29.4 to 38.8% (v.v^-1) and 55 to 90 min, respectively, according to the CCRD (Table 5). After extraction, the samples were centrifuged 2050 x g.force (CELM-COMBATE) for 15 min and supernatants were removed, stored in Falcon tubes and kept under refrigeration up to the analyses.

Figure 1
Steps of substrate preparation for cultivation of Pleurotus ostreatus in peach palm by-products

Contents of total phenolic compounds (TPC) were determined by the Folin-Ciocalteu reagent. The reading was carried out in a spectrophotometer (UVmini-1240, SHIMADZU) at 725 nm, and an analytical curve was constructed with gallic acid (0 - 100 μgmL^-1) [²⁰].

The extract that exhibited the highest content of TPC was subjected to evaluation of antioxidant activity. The method of free radical ABTS (2,2'-azino-bis3-ethylbenzothiazoline-6-sulfonicacid) was performed according to Thaipong and coauthors [²¹]. The extract was also analyzed by the method of ferric reduction power (FRAP), following Benzie and Strain [²²] recommendations. For both methods, it was used analytical curves obtained with Trolox solutions (from 0.20 to 0.75 mM).

The profile of phenolic acids was determined by an HPLC 20A (Prominence, Shimadzu), which consists of a UV detector (SPD-20A, Shimadzu), whose wavelengths are 280 and 320 nm. A column oven (CTO-20A, Shimadzu) kept the C-18 column (Shim-pack CLC-ODS (H)™, 25 cm x 4.6 mm x 5 mm, Shimadzu) at 25 ºC. A manual injector (SIL-10A, Shimadzu) injected 20 µL of every extract at a time while a quaternary pump (LC-20AT, Shimadzu) operated at flow rate of 0.8 mL min^-1. In chromatographic separation, mobile phases were ultrapure water acidified 0.05% (v v^-1) with formic acid (A) and methanol acidified 0.1% (v v^-1) with formic acid (B) in a gradient elution mode: from 0.01 to 5.00 min - 5% (v v^-1) B, from 5.00 to 15.00 min - 20%(v v^-1) B, from 15.00 to 35.00 min - 80% (v v^-1) B and from 35.01 to 42.00 min - 100% (v v^-1) B. To carry out quantification, solutions of gallic, coumaric, vanillic, ferulic, caffeic, syringic, trans-cinnamic, hydroxybenzoic and chlorogenic acids (1 to 10 mg mL^-1) were prepared to obtain analytical curves (R²≥ 0.99).

Statistical analysis

Results of composition, antioxidant compounds and properties were expressed in dry basis (d.b.), as mean ± standard deviation. In the statistical analysis of data, the Statistica program, version 7.0, was used for analyses of variance (ANOVA), followed by the Fisher’s test to identify significant differences among means, considering p<0.05. In the experimental designs, the response surface methodology (RSM) was used by the previously mentioned program [²³].

RESULTS

Substrate characterization and mushroom cultivation

Carbon and nitrogen contents of the peach palm by-products - sheath and shell - are shown in Table 1.

Table 1
Carbon and nitrogen contents of dehydrated and ground peach palm by-products (sheath and shell)

	C (%, w w-1)	N (%, w w-1)	C:N
Sheath	51.29	2.04	25.14
Shell	52.73	0.71	74.27

Table 2 shows resulting yields at different C:N ratios and moisture values.

Table 2
Yield of Pleurotus ostreatus mushroom produced in peach palm by-product at different moisture (M) values and C:N ratios

Assay	C:N	M (%, w w-1)**	Y (%, w w-1, w.b.)*
1	55.00	65.00	0***
2	45.00	80.00	7.78
3	55.00	65.00	0***
4	69.14	65.00	8.29
5	40.86	65.00	0***
6	65.00	80.00	21.62
7	55.00	86.21	11.79
8	45.00	50.00	10.50
9	55.00	43.79	0***
10	65.00	50.00	0***
11	55.00	65.00	0***

• Note: *Y= Yield, **M = Moisture, ^***yield in wet basis equal to zero is attributed to the condition without fructification.

Figure 2 shows the response surface and the resulting equation (R² = 0.89). Determination of the optimal point used the desirability function, i.e., C:N ratio was 69.14 (+1.41), moisture was 86.21% w w^-1 (+1.41), and expected yield was 28% (w w^-1, w.b.). The analysis of variance (Table 3) shows that moisture influenced yield (p<0.05) more than the C:N ratio, which was only significant in the quadratic term (p<0.10). Cultivation carried out in both conditions of C:N ratio and moisture exhibited up to three fructifications and maximum yield of 24.47% (w w^-1, w.b.).

Figure 2
Effect of variation in the C:N ratio and moisture of the substrate and the equation generated by the desirability function; C:N is the ratio between carbon and nitrogen of the substrate, M is the moisture of the substrate and Y is the yield of Pleurotus ostreatus produced in peach palm by-product

Table 3
Analysis of variance of results of the Central Composite Rotational Design in Pleurotus ostreatus produced in peach palm by-products

	SS	Df	MS	F	P
C:N	28.39	1	28.39	2.43	0.18
C:N²	61.91	1	61.91	5.30	0.07
M	158.21	1	158.21	13.53	0.01
M²	98.95	1	98.95	8.46	0.03
C:N x M	148.06	1	148.06	12.66	0.02
Error	58.46	5	11.69
Total SS	518.82	10

• Note: C:N is the ratio between carbon and nitrogen of the substrate and M is the moisture of the substrate; SS: sum of squares; Df: degrees of freedom; MS: Mean square; p: significance level.

Proximate composition

Proximate composition, on a dry basis, of P. ostreatus samples is shown in Table 4.

Table 4
Proximate composition (dry basis) of Pleurotus ostreatus produced in peach palm by-products, sawdust with wheat bran and a commercial sample

Component	P. ostreatus (peach palm by-products)	P. ostreatus (commercial)	P. ostreatus (sawdust)
Moisture (g 100g-1)	11.61a±0.56	7.60c±0.28	10.16b±0.16
Proteins (g 100g-1 d.b.)	16.09b±0.04	28.44a±0.71	16.25b±0.61
Lipids (g 100g-1 d.b.)	1.23b ±0.00	1.72a±0.02	1.22b±0.00
Ashes (g 100g-1 d.b.)	6.92a±0.16	6.87a±0.28	6.09a±0.002
Dietary fibers (g 100g-1 d.b.)	42.92a±1.55	43.24a ±1.86	39.46b±0.47
Total carbohydrates (g 100g-1 d.b.)	32.84	19.73	36.98

• Note: Different letters on a line show significant difference (p<0.05).

Phenolic compounds, antioxidant activity and profile of phenolic acids

The ethanol concentrations and the extraction times used to obtain TPC in the P. ostreatus, cultivated in PPBs and in the commercial sample, are shown in Table 5. The analysis of response surface showed that values were not predictive; thus, it cannot be stated that variables under analysis exerted some effect on extraction.

Table 5
Experimental conditions of extractions and total phenolic contents of Pleurotus ostreatus produced in peach palm by-products and from a commercial sample

Experimental conditions		Total phenolic content (mgGAE 100 g-1, d.b.)
Extract	Ethanol concentration (%, v v-1)	Extraction Time (min)	P. ostreatus cultivated in peach palm by-products	P. ostreatus commercial
1	40.0	90.0	2.07f± 0.03	1.29c ± 0.09
2	50.6	38.8	2.15d± 0.02	1.47b± 0.08
3	29.4	81.2	2.37b± 0.02	1.62a± 0.06
4	25.0	60.0	2.40b± 0.04	1.65a ± 0.03
5	40.0	60.0	2.29c± 0.02	1.55ab ± 0.10
6	40.0	60.0	2.28c± 0.06	1.57ab ± 0.13
7	40.0	30.0	2.29c± 0.05	1.56ab ± 0.10
8	29.4	38.8	2.57a± 0.02	1.55ab ± 0.13
9	55.0	60.0	2.26c± 0.01	1.64a ± 0.05
10	40.0	60.0	2.25ce± 0.01	1.44bc ± 0.10
11	50.6	81.2	2.20de± 0.04	1.63a ± 0.04

• Note: Different letters in a column show significant difference (p<0.05) by the Fisher’s test. GAE: gallic acid equivalent

The condition whose ethanol concentration was 29.4% (v v^-1) and extraction time was 38.8 min was the one chosen to evaluate antioxidant activity by both ABTS and FRAP methods and to analyze phenolic acids since it exhibited the highest content of total phenolic compounds and the lowest extraction time (Table 6).

Table 6
Evaluation of antioxidant activity and composition of phenolic acids of extracts from commercial Pleurotus ostreatus, Pleurotus ostreatus produced in peach palm by-product and Pleurotus ostreatus produced in sawdust by both ABTS and FRAP methods.

Parameter	P. ostreatus (peach palm by-product)	P. ostreatus (sawdust + wheat bran)	P. ostreatus (commercial)
Antioxidant activity - ABTS (nmolTE g-1 d.b.)	31.43a±0.00	23.1b±0.00	24.3b±0.00
Antioxidant activity - FRAP (µmolTE g-1 d.b.)	622.52a±2.70	536.71b±8.86	439.17c±11.54
Phenolic acids (mg 100g-1 d.b.)
Gallic acid	21.98a±0.44	17.80b±0.99	15.32b±0.81
Vanillic acid	111.85a±0.21	84.17c±0.90	95.13b±0.17
Trans-cinamic acid	29.48a±1.23	17.86c±0.40	22.81b±0.29
Ferulic acid	23.46a±0.05	9.09c±0.24	21.74b±0.05
Coumaric acid	16.89a±0.96	8.95b±0.09	8.04b±0.12

• Note: Different letters in a column show significant difference (p<0.05) by the Fisher’s test. TE: Trolox equivalent

DISCUSSION

The shell and sheath of peach palm have a C:N ratio of 74.27 and 25.14, respectively. Some common substrates used for mushrooms production are sugarcane bagasse, coffee grounds and sawdust, whose C:N ratios are 37, 22 and 115, respectively [²⁴]. According to Cueva and coauthors [²⁵], the ideal range to grow Pleurotus ostreatus is between 38 and 58 of C:N ratio.

In this study, moisture ranged from 50 to 86.21 g.100g^-1, while C:N ratio ranged from 45 to 69.14, in agreement with previous studies [²⁵]. There was no fructification in the lowest and highest C:N ratios (25.14 and 74.27) under investigation. Regarding moisture, values above 86.21 g 100g^-1 were used, but frequent contamination made their use impracticable.

Cultivation in sawdust and wheat bran as the substrate, with C:N ratio of 70 and moisture of 54 g.100g^-1, led to 17.15% (w.b.) yield. Jin and coauthors [²⁶] used corn cobs supplemented with by-product from herbal medicines and reached 39.12% (w w^-1) yield, when the C:N ratio was 36.85, and 33.78% (w w^-1) yield was observed in the C:N ratio of 48.43, i.e., lower C:N ratios resulted in higher yields.

The mycelium tends to grow faster, but less vigorously, in substrates deficient in nutrients [²⁷,²⁸]. Some studies have shown that substrates with high C:N ratio favor mycelium growth, while low ratios stimulate development of fruiting bodies [²⁷, ²⁸]. Nitrogen in the substrate is essential for mushrooms to synthesize amino acids and proteins. Higher C:N ratios, due to N deficiency may accelerate the mushroom metabolism, releasing energy that heats the substrate, and consequently, there is an inhibition of the mycelium growth [²⁵]. However, high nitrogen content may inhibit synthesis of enzymes that degrade lignin; thus, it has negative effect on mycelium growth, delaying the formation of the fruiting body [²⁹].

Bernardi and coauthors [³⁰] reached 19.96% (w w^-1) yield when Pleurotus ostreatus was grown in Napier grass (C:N ratio was 162:1). Duprat and coauthors [³¹] evaluated P. ostreatus yield in peach palm leaves supplemented with rice bran and reached yields between 20.6 and 42.3% (w w^-1).

Donini and coauthors [²⁸] evaluated P. ostreatus produced in Napier grass supplemented with soybean, wheat, rice and corn bran and observed that the yield (23.59%, w w^-1) was favored when wheat bran was used at 10%, w w^-1 (C:N 60:1), while yield with no supplementation (C:N 162:1) was 9.30% (w w^-1). On the other hand, cultivation in soybean bran as the supplement exhibited lower yield than the one of cultures grown with no supplementation. This study suggests that different nitrogen sources may supply other important nutrients to enable mushroom development and yield improvement. It should be mentioned that nutrient availability may affect mechanisms of absorption and synthesis of mushroom.

The composition of P. ostreatus are affected by the chemical composition of substrates and cultivation conditions. Protein content of P. ostreatus produced in peach palm by-product was within the values found by Sales-Campos and coauthors [³²], between 11.96 and 21.16 g 100g^-1, using different by-product from the Amazon region, and similar to that found by Patil and coauthors [³³] who produced P. ostreatus in wheat straw (21 g 100g^-1). Otherwise Duprat and coauthors [³¹] reached high protein content (24.1 g 100g^-1) in P. ostreatus produced in peach palm leaves. The protein content in the mushrooms is very important to human health, especially when used in plant-based diets, due to the restrictions in the consumption of sources of proteins from animal origin.

Wang and coauthors [³⁴] observed that supplementation with wheat bran favored protein content in P. ostreatus more than rice or corn bran. Even though protein contents of substrates with rice, wheat and corn bran are similar, the one of mushrooms was different, a fact that shows that the nature of the nitrogen source also influences protein content of mushrooms. In this study, the protein content of mushroom grown in peach palm by-product did not exhibit any significant difference from the one of mushroom produced in sawdust supplemented with wheat bran.

Lipid contents of mushroom samples under analysis ranged from 1.22 to 1.72 g 100g^-1, in the ranges reported by Lavelli and coauthors [³⁵], which was from 1.18 to 4.4 g 100g^-1, by Sales-Campos and coauthors [³²], which was between 1.27 and 2.14 g 100g^-1, and by Fernandes and coauthors [³⁶] which was from 1.18 to 1.68 g 100g^-1 (all values in d.b.). In lipid composition of mushroom, the fact that it has unsaturated fatty acids but no trans-fatty acids should be emphasized [³⁷]. Hossain and coauthors [³⁸] identified palmitic (C16:0), stearic (C18:0), oleic (C18:1), linoleic (C18:2), linolenic (C18:3) and arachidonic (C20:4) acids in mushroom.

Ash contents in P. ostreatus samples were similar to the ones reported by Sales-Campos and coauthors [³²] (from 6.10 to 8.97 g 100g^-1 d.b.) and Duprat and coauthors [³¹] (6.0 g 100g^-1 d.b.). Fernandes and coauthors [³⁶] reached conflicting ash values in samples produced in oat straw, white paper and printed paper, i.e., 5.68, 15.9 and 10.5 g 100g^-1 (d.b.), respectively. Lavelli and coauthors [³⁵] found ash values that ranged between 4.60 and 10.3 g 100g^-1 (d.b.). According to Carrasco-González and coauthors [¹²], the most abundant minerals in P. ostreatus, in ascending order, are potassium, phosphorus, magnesium, sodium, calcium, iron, zinc, manganese and copper.

Carbohydrates are found in Pleurotus sp. as polysaccharides and glycoproteins, mainly chitin, α- and β-glucans and hemicelluloses. Values of dietary fibers were higher than the ones found by Bach and coauthors [¹⁹], who reached 32.72 g 100g^-1 (d.b.). Lavelli and coauthors [³⁵] found fibers contents between 13.0 and 47.3 g 100g^-1 (d.b.) in P. ostreatus.

Extracts from P. ostreatus produced in peach palm by-product exhibited higher total phenolic content (TPC) than the one of the commercial samples, i.e., maximum contents were 2.57 and 1.65 mg GAE g^-1 sample, respectively. According to Gasecka and coauthors [³⁹], substrate composition affected TPC of Pleurotus ostreatus and Pleurotus eryngii, a fact that may be due to the decomposition of lignin by mycelium of fruiting bodies.

Yilmaz and coauthors [⁴⁰] found phenolic compound contents of P. ostreatus between 2.03 and 2.51 mgGAE g^-1, which are similar to values found by this study. However, higher contents were found by Chowdhury and coauthors [⁴¹] - 3.20 mgGAE g^-1 - and Mircea and coauthors [⁴²] - from 5.47 to 6.17 mgGAE g^-1 - in P. ostreatus samples. Variation in phenolic compound contents of mushroom may be related to the substrate, temperature, moisture, culture time and solvent used in the extraction process [⁴³]. According to Barros and coauthors [⁴⁴] phenolic compounds are mainly responsible for antioxidant activity of mushrooms, whereas lycopene, ascorbic acid and beta-carotene are only found at very low amounts.

Results of analyses of antioxidant activity showed that compounds found in mushroom samples under investigation may act either in free radical scavenging (ABTS method) or in reduction of ferric ions (FRAP method), with benefits to human health as examples, preventing the occurrence of degenerative diseases and premature aging. The highest values of antioxidant activity (FRAP: 622.52 µmolTE g^-1 sample; ABTS: 31.43 nmolTE g^-1 sample) were found in the P. ostreatus sample grown in peach palm by-product. Results highlighted benefits of P. ostreatus consumption, a fact that had already been shown by other mushroom species, such as P. florida, P. citrinopileatus and P. pulmonarius. Jayakumar and coauthors [⁴⁵] (2009) also found high antioxidant activity in ethanolic extract of P. ostreatus, which may become easily accessible food rich in natural antioxidants, a food supplement and even a pharmaceutical agent.

P. ostreatus produced in peach palm by-product enabled high phenolic acid contents identified by this study to be reached, i.e., gallic, vanillic, trans-cinnamic, ferulic and coumaric acids (Table 5). Alterations in their contents depended on the media in two cases: when the same conditions were applied to different substrates (peach palm by-product and the mixture of sawdust and wheat bran) and in terms of contents of the commercial sample. Such differences may be related to the activation of the phenolic acid synthesis due to different metabolites in the culture medium [³⁹, ⁴⁶].

Gasecka and coauthors [³⁹] and Bach [¹⁹] also found ferulic, coumaric, trans-cinnamic and vanillic acids in P. ostreatus. Even though they did not find gallic acid, they found different compounds, such as p-hydroxybenzoic, syringic, caffeic, di-hydroxybenzoic and protocatechuic acids [¹⁹]. Gallic acid exhibited significant correlation with both ABTS (r=0.95, p<0.05) and FRAP (r=0.98, p<0.05) antioxidant methods. Gallic acid consists of an aromatic ring, a carboxylic acid group and three phenolic hydroxyl groups bound in an ortho position, which favors antioxidant activity [⁴⁷]. The phenolic compound found at the highest amount in the samples under analysis was vanillic acid, which exhibited positive correlation with the ABTS method (r=0.99, p<0.05). The other phenolic acids did not show any positive correlation with the antioxidant activity methods under evaluation. The action of each phenolic compound can be affected by solvents, pH and temperature used in these in vitro antioxidant methods, as also their reaction mechanisms, as free radical scavenger (ABTS) or electron donators (FRAP). Bach and coauthors [¹⁹] showed positive correlation (p<0.05) not only between the gallic acid content and both DPPH and ABTS antioxidant activity methods but also the ferulic acid content and the DPPH, ABTS and FRAP methods. Such results reinforce the importance of increasing phenolic compound synthesis, depending on the substrate in cultivation, to increase antioxidant activity of oyster mushrooms. Extracts from oyster mushroom produced in peach palm by-product exhibited higher phenolic compound content and higher antioxidant activity than the other samples.

CONCLUSION

The condition that enabled the highest yield of oyster mushrooms in peach palm by-product, i.e., 24.47% (w w^-1, w.b.), was the C:N ratio of 69.14 and moisture of 86.21 g 100g^-1. Regarding protein contents, there was no difference between both types of cultivation (16 g 100g^-1, d.b.), but the commercial sample showed the highest content (28 g 100g^-1, d.b.). Extraction of phenolic compounds was higher when ethanol concentration was 29.4% (v.v^-1) and time was 38.8 min. Phenolic compound contents of P. ostreatus extracts produced in by-product ranged from 2.07 to 2.57 mgGAE g^-1, d.b., while they ranged from 1.29 to 1.64 mgGAE g^-1, d.b. in the commercial sample. Thus, the highest antioxidant activities, evaluated by FRAP and ABTS methods, were found in mushrooms cultivated in PPBs. Five phenolic compounds were identified in the samples: gallic, vanillic, trans-cinnamic, ferulic and coumaric acids. Pleurotus ostreatus produced in peach palm by-product is a viable alternative to use the material and may become an extra income source to farmers.

Acknowledgments

CAPES, Araucária Foundation, CNPQ, Sabores da Graciosa Indústria de Palmito Pupunha.

REFERENCES

• ¹ Spacki KC, Correa RCG, Uber TM, Barros L, Ferreira ICFR, Peralta RA et al. Full Exploitation of Peach Palm (Bactris gasipaes Kunth): State of the Art and perspectives. Plants. 2022 11(22):3175.
• ² Sharma S, Yadav RKP, Pokhrel CP. Growth and yield of oyster mushroom (Pleurotus ostreatus) on different substrates. J New Biol Reports. 2013 2(1):3-8.
• ³ Valverde ME, Hernández-Pérez T, Paredes-López O. Edible mushrooms: improving human health and promoting quality life. Int J Microbiol. 2015 2015:1-14.
• ⁴ Khan MA, Tania M. Nutritional and medicinal importance of Pleurotus mushrooms: an overview. Food Rev Int. 2012 28(3):313-29.
• ⁵ Atri N, Sharma SK, Joshi R, Gulati A. Nutritional and nutraceutical composition of five wild culinary-medicinal species of genus Pleurotus (higher Basidiomycetes) from northwest India. Int J Med Mushrooms. 2013 15(1):49-56.
• ⁶ Corrêa RCG, Brugnari T, Bracht A, Peralta RM, Ferreira IC. Biotechnological, nutritional and therapeutic uses of Pleurotus spp. (Oyster mushroom) related with its chemical composition: A review on the past decade findings. Trends Food Sci Technol. 2016 50:103-17.
• ⁷ Dundar A, Acay H, Yildiz A. Effect of using different lignocellulosic wastes for cultivation of Pleurotus ostreatus (Jacq.) P. Kumm. on mushroom yield, chemical composition and nutritional value. African J Biotechnol. 2009 8(4):662-6.
• ⁸ Kim D, Jeong SW, Lee CY. Antioxidant capacity of phenolic phytochemicals from various cultivars of plums. Food Chem. 2003 81(3):321-6.
• ⁹ Bernas E, Jaworska G, Lisiewska Z. Edible mushrooms as a source of valuable nutritive constituents. Acta Scientiarum Technologia Alimentaria, 2006 5(1):5-20.
• ¹⁰ Silva EG, Dias ES, Siqueira FG, Schwan RF. [Chemical analysis of fruiting bodies of Pleurotus sajor-caju cultivated in different nitrogen concentrations]. Ciência e Tecnologia de Alimentos. 2007 27(1):72-5.
• ¹¹ Koutrotsios G, Kalogeropoulos N, Stathopoulos P, Kaliora AC, Zervakis GI. Bioactive compounds and antioxidant activity exhibit high intraspecific variability in Pleurotus ostreatus mushrooms and correlate well with cultivation performance parameters. World J Microbiol Biotechnol. 2017 33(5):98.
• ¹² Carrasco-González JA, Serna-Saldívar SO, Gutiérrez-Uribe JA. Nutritional composition and nutraceutical properties of the Pleurotus fruiting bodies: Potential use as food ingredient. J Food Compos Anal. 2017 58:69-81.
• ¹³ Siqueira OAPA, Zanon AR, Martins OG, Andrade MCN. New substrates for the cultivation of Pleurotus ostreatus using exhausted compost. African J Agric Res. 2016 11(26):2295-301.
• ¹⁴ AOAC. Official Methods of Analysis of AOAC International. (2005). (18th ed). Gaithersburg: AOAC International.
• ¹⁵ Fontana A, Bianchi SR. [Total Carbon and nitrogen: elementar analyzer]. EMBRAPA Soils 2017 (INFOTECA-E).
• ¹⁶ Milovanović I, Brčeski I, Stajić M, Korać A, Vukojević J, Knežević A. Potential of Pleurotus ostreatus mycelium for selenium absorption. Sci World J, 2014 68:1834.
• ¹⁷ Bonatti M, Karnopp P, Soares HM, Furlan SA. Evaluation of Pleurotus ostreatus and Pleurotus sajor-caju nutritional characteristics when cultivated in different lignocellulosic wastes. Food Chem. 2004 88(3):425-8.
• ¹⁸ Bisaria R, Madan M, Bisaria VS. Biological efficiency and nutritive value of Pleurotus sajor-caju cultivated on different agro-wastes. Biol Wastes. 1987 19(4):239-55.
• ¹⁹ Bach F, Helm CV, Bellettini MB, Maciel GM, Haminiuk CWI. Edible mushrooms: a potential source of essential amino acids, glucans and minerals. Int J Food Sci Technol. 2017 52(11):2382-92.
• ²⁰ Singleton VL, Orthofer R, LamuelA-Raventós RM. Analysis of total phenols and other oxidation substrates and antioxidants by means of Folin-Ciocalteu reagent. Methods Enzymol. 1999 299 152-79.
• ²¹ Thaipong K, Boonprakob U, Crosby K, Cisneros-Zevallos L, Byrne DH. Comparison of ABTS, DPPH, FRAP and ORAC assays for estimating antioxidant activity from guava fruit extracts. J Food Compos Anal. 2006 9 669-75.
• ²² Benzie IFF, Strain JJ. The ferric reducing ability of plasma (FRAP) as a measure of “antioxidant power”: the FRAP assay. Anal Biochem. 1996 239 70-6.
• ²³ Novaes CG, Yamaki RT, De Paula VF, Do Nascimento Júnior BB, Barreto JA, Valasques GS et al. [Optimization of analytical methods using response surface methodology - Part I: process variables]. Rev Virtual Quim, 2017 9(3): 1184-215.
• ²⁴ Rodrigues TA, Leandro-Neto J. [Agricultural Production and Environmental Conservation Practices]. Atena Editora. 2019: 20-5.
• ²⁵ Cueva MBR, Hernández A, Niño-Ruiz Z. Influence of C/N ratio on productivity and the protein contents of Pleurotus ostreatus grown in different residue mixtures. Rev Fac Cienc Agr, 2017 49(2): 331-44.
• ²⁶ Jin Z, L Y, Ren J, Qin N. Yield, nutritional content, and antioxidant activity of Pleurotus ostreatus on corncobs supplemented with herb residues. Mycobiol. 2018 46(1):24-32.
• ²⁷ Hoa HT, Wang CL. The effects of different substrates on the growth, yield, and nutritional composition of two oyster mushrooms (Pleurotus ostreatus and Pleurotus cystidiosus). Mycobiol. 2015 43(4):423-34.
• ²⁸ Donini LP, Bernardi E, Minotto E, Nascimento JS. Cultivation of shimeji on elephant grass substrate supplemented with different kinds of bran. Scientia Agraria. 2009 10(1):67-74.
• ²⁹ Figueiró GG, Graciolli LA [Influence of the chemical composition of the substrate on the cultivation of Pleurotus florida]. Cienc Agrotecnol, 2011 35: 924-30.
• ³⁰ Bernardi E, Donini LP, Minotto E, Do Nascimento JS. [Cultivation and nutritional characteristics of Pleurotus in pasteurized substrate]. Bragantia. 2009 68(4):901-7.
• ³¹ Duprat MFLB, Rampinelli JR, Silva DAK, Furlan SA, Wisbeck E. [Nutritional potential of Pleurotus ostreatus mushrooms grown on peach palm leaves]. Boletim Centro Pesq Proc Alimen, 2015 33(1).
• ³² Sales-Campos C, Araujo LM, Minhoni MTDA, Andrade MCND. Physiochemical analysis and centesimal composition of Pleurotus ostreatus mushroom grown in residues from the Amazon. Food Sci Technol. 2011 31(2):456-61.
• ³³ Patil SS, Ahmed SA, Telang SM, Baig MMV. The nutritional value of Pleurotus ostreatus (Jacq.: Fr.) kumm cultivated on differents lignocellulosic agrowastes. Innov Rom Food Biotechnol. 2010 (7):66-76.
• ³⁴ Wang D, Sakoda A, Suzuki M. Biological efficiency and nutritional value of Pleurotus ostreatus cultivated on spent beer grain. Biores Technol. 2001 78(3):293-300.
• ³⁵ Lavelli V, Proserpio C, Gallotti F, Laureati M, Pagliarini E. Circular reuse of bio-resources: the role of Pleurotus spp. in the development of functional foods. Food Funct, 2018 9(3):1353-72.
• ³⁶ Fernandes A, Barros L, Martins A, Herbert P, Ferreira ICFR. Nutritional characterization of Pleurotus ostreatus (Jacq. ex Fr.) P. Kumm produced using paper scraps as substrate. Food Chem. 2015 169 396-400.
• ³⁷ Kalač PA review of chemical composition and nutritional value of wild‐growing and cultivated mushrooms. J Sc Food Agricult. 2013 93(2):209-18.
• ³⁸ Hossain MS, Alam N, Amin SR, Basunia MA, Rahman A. Essential fatty acid contents of Pleurotus ostreatus, Ganoderma lucidum and Agaricus bisporus. Bangladesh J Mushroom. 2007 1: 1-7.
• ³⁹ Gąsecka M, Mleczek M, Siwulski, M, Niedzielski P, Kozak L. The effect of selenium on phenolics and flavonoids in selected edible white rot fungi. LWT - Food Sci Technol. 2015 63(1):726-31.
• ⁴⁰ Yilmaz A, Yildiz S, Çelik A, Çevik U. Determination of heavy metal and radioactivity in Agaricus campestris mushroom collected from kahramanmaraş and Erzurum proviences. Turkish J Agric - Food Sci Technol. 2016 4(3):208-15.
• ⁴¹ Chowdhury MMH, Kubra K, Ahmed SR. Screening of antimicrobial, antioxidant properties and bioactive compounds of some edible mushrooms cultivated in Bangladesh. Ann Clin Microbiol Antimicrobials. 2015 14(1):1-6.
• ⁴² Mircea C, Cioanca O, Iancu C, Tataringa G, Hancianu M. In vitro antioxidant activity of some extracts obtained from Agaricus bisporus brown, Pleurotus ostreatus and Fomes fomentarius. Farmacia. 2015 63(6):927-33.
• ⁴³ Furlani RPZ, Godoy HT. [Nutritional Value of Edible Mushrooms: A Review] Rev Inst Adolfo Lutz. 2005 64(2):149-54.
• ⁴⁴ Barros L, Ferreira MJ, Queirós B, Ferreira ICFR, Baptista P. Total phenols, ascorbic acid, b-carotene and lycopene in Portuguese wild edible mushrooms and their antioxidant activities. Food Chem. 2007 103(2):413-19.
• ⁴⁵ Jayakumar T, Thomas PA, Geraldine P. In-vitro antioxidant activities of an ethanolic extract of the oyster mushroom, Pleurotus ostreatus. Innov Food Sci Emerg Technol. 2009 10(2):228-34.
• ⁴⁶ Gąsecka M, Mleczek M, Siwulski M, Niedzielski P. Phenolic composition and antioxidant properties of Pleurotus ostreatus and Pleurotus eryngii enriched with selenium and zinc. Eur Food Res Technol, 2016 242(5): 723-32.
• ⁴⁷ Badhani B, Sharma N, Kakkar R. Gallic acid: a versatile antioxidant with promising therapeutic and industrial applications. Royal Soc Chem Adv. 2015 5(35): 27540-57.

• Funding:
  This research was funded by CAPES, grant number 88882.461670/2019-01.

Edited by

• Editor-in-Chief:
  Bill Jorge Costa

• Associate Editor:
  Bill Jorge Costa

Publication Dates

• Publication in this collection
  11 Oct 2024
• Date of issue
  2024

History

• Received
  15 May 2023
• Accepted
  16 Aug 2024