Effect of boiling time on nutritional characteristics and antioxidant activities of Lentinus edodes and its broth

ABSTRACT

The influence of boiling time (10 –120 min) on nutritional and antioxidant properties of Lentinus edodes pieces and broth was investigated. The results showed that the primary nutrients contents and antioxidant activities of L. edodes pieces decreased with the prolongation of boiling time, while ergosterol content increased. The nutrients contents in L. edodes broth presented an uptrend, and the antioxidant activities increased rapidly in the first 30 min, and then kept stable or decline, which was positively correlated with the change of total phenolic content. Microstructure observation showed that boiling treatment destroyed the cell wall of L. edodes, leading to the dissolution and loss of cell wall components and cytoplasmic contents. From the point of nutrition, the optimal boiling time for L. edodes pieces is 30 min and for broth preparation is 60 min. This study provides a useful reference for rational cooking or processing L. edodes and other mushrooms.

RESUMEN

El presente estudio investigó la influencia del tiempo de ebullición (10~120 min) en las propiedades nutricionales y antioxidantes de trozos de Lentinus edodes y su caldo. Los resultados permitieron constatar que el contenido de nutrientes primarios y las actividades antioxidantes de dichos trozos disminuyeron al prolongarse el tiempo de ebullición, mientras que el contenido de ergosterol aumentó. El contenido de nutrientes del caldo de L. edodes presentó una tendencia al alza, en tanto que las actividades antioxidantes se elevaron rápidamente en los primeros 30 minutos, manteniéndose estables o disminuyendo después, lo que se correlacionó positivamente con el cambio del contenido fenólico total. La observación de la microestructura confirmó que el tratamiento de ebullición destruye la pared celular de L. edodes, provocando la disolución y la pérdida de componentes de la pared celular y de contenido citoplasmático. Desde el punto de vista de la nutrición, el tiempo óptimo de ebullición para los trozos de L. edodes es 30 min y para la preparación de caldo, 60 min. Este estudio proporciona una referencia útil para la cocción o elaboración racional de L. edodes y otros hongos.

KEYWORDS:

• Lentinus edodes
• nutrition
• antioxidant activities
• boiling
• cooking
• mushroom

PALABRAS CLAVE:

• Lentinus edodes
• nutrición
• actividades antioxidantes
• ebullición
• cocción
• champiñón

1. Introduction

Lentinus edodes, also named shiitake, is one of the most popular mushrooms in the world. It is rich in polysaccharide, dietary fiber, protein, amino acids, phenols, ergosterol and other nutrients (Mattila et al., 2001), and has the bioactivities of immunoregulation, antitumor, antioxidant, anti-inflammatory and blood pressure lowering (Liu et al., 2019; Papetti et al., 2018; Puttaraju et al., 2006; Zhang et al., 2011). Plus the unique taste and flavor, more and more people like to eat this mushroom in recent years, especially in East Asian countries. In 2018, the total output of L. edodes in China is 10.43 million tons, being the largest yield variety of mushroom (China Edible Fungi Association, 2020).

As a popular table food, L. edodes can be cooked by many methods before consumption according to the recipes and culinary traditions of various countries (Lee et al., 2019). Nowadays, consumers prefer the foods being not only processed on the basis of convenience and taste but also the retention of nutrients and other health-promoting factors (Roncero-Ramos et al., 2016). Thermal processing, including boiling, induces great changes in the sensory qualities and chemical compositions of foods (including mushrooms and vegetables), such as texture, aroma, color, protein, sugar, phenols and vitamins, as well as the antioxidant activities (Morales et al., 2018; Tian et al., 2016; Yang et al., 2019). Studies have been reported that the cooked mushrooms were in lower nutritional concentration and lower antioxidant properties than that of natural mushrooms (Ng & Tan, 2017; Sun et al., 2014, 2011). But cooking treatments had also beneficial effects on the phenol content and antioxidant activity of mushrooms (Tan et al., 2015). Those changes may be related to the cooking duration. However, there are few studies on the changes in nutritional characteristics and functional properties of shiitake mushrooms caused by cooking time so far. In terms of production and consumption, the top three edible mushrooms were L. edodes, Auricularia auricular, and Pleurotus ostreatus in China in recent years, respectively, and each yield was more than six million tons (China Edible Fungi Association, 2020). As a popular table food and the largest yield mushroom in China, it is of great significance to scientifically and reasonably guide the cooking process of L. edodes. The aim of this work was to investigate the effects of boiling time on the nutritional composition and antioxidant properties of L. edodes and its broth, with a view to providing a useful reference for the scientific cooking and processing of this mushroom.

2. Materials and methods

2.1. Sample and reagents

Dry L. edodes Xixia 9608 were obtained from Xixia County, Henan Province, China. All mushrooms were harvested at the same time. All standards and reagents were of analytical grades.

2.2. Boiling treatments of L. edodes

L. edodes were carefully selected for uniformity of shape and size. 100 g of dried L. edodes were soaked in 2,000 mL of distilled water at 37°C for 75 min. The whole soaking process was carried out at a constant temperature of 37°C, in this way, the starting samples of each group were consistent, thus reducing the experimental error. The soaking time was determined after several preliminary experiments, and 75 min of soaking allows the dried L. edodes to fully absorb water and swell, which is conducive to the cutting and subsequent treatment. Too long soaking time will lead to the loss of nutrients from L. edodes and possible growth of bacteria. The swelling mushrooms were then cut into pieces (10 mm×10 mm×5 mm) and randomly divided into equal portions for further use.

Adding 100 g of L. edodes pieces into the 2,000 mL boiling water in a sealed pot under heating. The treating time was set as 10 min, 30 min, 60 min, 90 min, and 120 min, respectively. Suitable water was added every 15 min to keep the broth volume unchanged. When boiling treatment was completed, mushroom pieces and broth were quickly cooled to room temperature. Then, mushroom pieces were isolated from the broth, followed by freeze-drying and grinding through 80 mesh for analysis. The broth was centrifugated at 8,000 r/min and 4°C for 10 min, and the supernatant was stored at −18°C for subsequent determination.

2.3. Determination of nutritional components in L. edodes and broth

2.3.1. Polysaccharide content

L. edodes powder (1 g) was extracted with 25 mL at 90°C for 1 h, followed by centrifugation at 12,000 g for 10 min at 4°C. The precipitation was extracted one more time. Two supernatants were combined and diluted to 50 mL. The polysaccharide content was determined using a phenol-sulfuric acid method (Dubois et al., 1956). The mushroom broth was directly sampled for determination. Results were expressed as mg/g dry weight (DW) for L. edodes powder and mg/100 mL for mushroom broth.

2.4. Dietary fiber content

The contents of total dietary fiber (TDF), insoluble dietary fiber (IDF) and soluble dietary fiber (SDF) of L. edodes powder were assayed by the enzymatic-gravimetric method (Mccleary et al., 2012).

2.5. Protein content

The protein content was determined by the Kjeldahl method (AOAC Official Method 930.29 [17th ed.], AOAC, 2000).

2.6. Total free amino acids content

The content of total free amino acids was determined by the formaldehyde method, a Chinese official and standard approach (GB 5009.235–2016).

2.7. Total phenolic content

L. edodes powder was ultrasonic-assisted extracted with 50 mL 60% ethanol for 60 min, and the mixture was centrifuged at 12,000 g for 10 min at 4°C and diluted the supernatant to 50 mL. Total phenolic content was determined by the Folin-Ciocalteau method of Singleton and Rossi (1964) with some modifications. In brief, 1 mL of the diluted extract was mixed with 1 mL Folin-Ciocalteau reagent. After 10 min, 1 mL of 12% sodium carbonate solution was added to the mixture and was adjusted to 10 mL with distilled water. The mixture was incubated for 1 h in the dark and the absorbance was measured at 725 nm. Mixture without extract was used as control. Gallic acid was used as the standard to construct the calibration curve. The mushroom broth was directly sampled for determination. The results were expressed as mg of gallic acid equivalents (GAE)/g dry weight (DW) for L. edodes powder and mg GAE/100 mL for mushroom broth.

2.8. Ergosterol content

1 g of L. edodes powder was ultrasonic-assisted extracted with 50 mL methanol at room temperature for 1 h, and then centrifuged at 12,000 g for 10 min at 4°C and diluted the supernatant to 50 mL. The supernatant was passed through a 0.45 μm non-pyrogenic filter. A volume of 20 μL of filtered sample was injected into a Waters e2695 HPLC system equipped with Waters 2996 diode array detector tunable (Waters, Milford, MA, USA) and eluted through a reverse phase Waters SunFire C18 column (4.6 mm×250 mm, 5 μm) using methanol as the mobile phase at a flow rate of 1.0 mL/min. The detector tunable of the eluate was performed at 282 nm (Pastinen et al., 2017). The ergosterol was determined by comparing the retention times of standards obtained, and quantification was done by using a calibration curve. The results were expressed as mg/g dry weight (DW).

2.9. Determination of antioxidant activities of L. edodes and broth

2.9.1. Preparation of ethanol extracts

1 g of L. edodes powder was ultrasonic-assisted extracted with 50 mL 60% ethanol at 50°C for 60 min, and then centrifuged at 12,000 g for 10 min at 4°C and diluted the supernatant to 50 mL. The supernatant was diluted to four concentrations. Then, the antioxidant activities of the diluted extracts were measured.

2.10. DPPH· scavenging ability

DPPH· radical scavenging ability of extracts was determined by a modified method of Zhao et al. (2013). The reaction mixture containing 2 mL DPPH (0.2 mM) ethanol solution and 2 mL ethanol extracts was kept at room temperature in dark for 30 min. Distilled water was used instead of extracts as a control. The absorbance at 517 nm was recorded. The DPPH· scavenging ability was calculated according to the equation: scavenging activit (%) = (A_control-A_sample)/A_control × 100.

2.11. ABTS· scavenging ability

ABTS· scavenging ability was determined using the method of Re et al. (1999) with a slight modification. The ABTS· free radical reagent was prepared by mixing 7 mM ABTS and 2.45 mM potassium persulfate solution. The mixture was left in the dark for 16 hours to generate ABTS· free radical ions, and then diluted to an absorbance of 0.700 ± 0.005 (at 734 nm). 100 mL of extracts was added to 0.39 mL ABTS· reagent. The mixture was allowed to stand for 6 min and absorbance was measured at 734 nm. Trolox (water-soluble Vitamin E analogue) was used as a standard. The TEAC values were calculated based on final concentration and expressed as μmol Trolox equivalent per gram of mushroom powder samples (μmol TEAC/g) or μmol Trolox equivalent per 100 mL of mushroom broth samples.

2.12. OH· scavenging ability

OH· scavenging ability was assessed using a previous method (Yang et al., 2015) with some slight modification. 1 mL of ethanol extracts, 1 mL of 1.8 mM freshly prepared ferrous sulfate solution, 1 mL of 1.8 mM salicylic acid and 1 mL of 8.8 mM hydrogen peroxide were injected into the test tube and incubated for 30 min at 37°C. Distilled water was used instead of extracts as a control. Then the absorbance at 510 nm was recorded. OH· scavenging ability was calculated according to the equation: OH· scavenging activity (%) = (A₀-A_x)/A₀ × 100.

2.13. Reducing power

The reducing power of ethanol extracts was determined by the method of Oyaizu (2002). 1 mL of ethanol extract in 2.5 mL PBS (50 mM, pH = 7.0) were mixed with 2.5 mL 1% potassium ferricyanide. The mixtures were incubated at 50°C for 20 min. At the end of the incubation, 2.5 mL 10% trichloroacetic acid was added into the mixtures, followed by centrifugation. 2.5 mL supernatant was mixed with 2.5 mL distilled water and 0.5 mL ferric chloride, and the absorbance was measured at 700 nm. The increase in absorbance of the reaction mixture indicated the reducing power of extracts.

2.14. Microscopy

The microstructures of L. edodes were observed using a Quanta 200 environmental scanning electron microscope (FEI, Hitachi, USA). L. edodes was cut into pieces (5 mm×5 mm×10 mm) and freeze-dried. The dried L. edodes pieces with natural fracture surfaces were fixed by conducting resin and coated with gold and then observed at a voltage of 20 kV and high vacuum condition.

2.15. Statistical analysis

Data analysis was performed using the SPSS package program version 22.0. All data were expressed as the mean ± standard deviation (SD) (triplicate). Statistical significance of the nutritional components and antioxidant activities of different samples were determined by using one-way ANOVA with Turkey post-hoc test. Pearson’s correlation and regression test was used to determine the relationship between the biochemical assays. P-values < 0.05 were considered statistically significant.

3. Results and discussion

3.1. Effect of boiling time on nutritional components of L. edodes pieces and broth

3.1.1. Effect of boiling time on polysaccharide content

Polysaccharide is one of the primary nutritional and bioactive components of L. edodes and other mushrooms (Zhang et al., 2011). The effect of boiling time on polysaccharide content of L. edodes pieces and broth was shown in Figure 1. The results shows that polysaccharide content in mushroom pieces decreased with the prolongation of boiling time. Polysaccharide content declined rapidly in the first 10 min, then decreased gently, and kept stable after 60 min. The reason might be that part of polysaccharides in L. edodes were easy to dissolve in hot water at the initial stage of boiling. With the increase of boiling time, the amount of water-soluble polysaccharides in mushroom pieces gradually decreased, and the dissolution rate and adsorption rate of polysaccharides gradually reached the balance, so the decline rate of polysaccharide content slowed down and gradually tended stable. Figure 1 also shows that polysaccharide content in mushroom broth gradually increased with the prolongation of boiling time, reached the maximum at 90 min, and then decreased slightly. The increase of polysaccharide content in the broth came from the dissolution of polysaccharides in mushroom pieces. Furthermore, due to the Maillard reaction between polysaccharides and nitrogen-containing substances under high temperature for a long time (De Oliveira et al., 2016), polysaccharide concentration in the broth decreased at 120 min. The result suggests that the optimal boiling time for L. edodes broth preparation is 60 ~ 90 min, and too long time boiling will cause the loss of polysaccharide in the broth.

Figure 1. Effect of boiling time on polysaccharide content in L. edodes pieces and broth. Different letters with same color on the error bars indicate significant differences among treatments (p < 0.05).

Figura 1. Efecto del tiempo de ebullición en el contenido de polisacáridos de los trozos de L. edodes y del caldo. Las distintas letras del mismo color en las barras de error indican diferencias significativas entre los tratamientos (p < 0.05).

3.2. Effect of boiling time on dietary fiber content

The dietary fiber in L. edodes mainly includes β-glucan, hemicellulose, chitin, mannan, etc. (Manzi et al., 1999), which has an important effect on maintaining normal physiological functions of the human body. TDF can be divided into SDF and IDF according to the difference of solubility. Figure 2 shows that TDF in L. edodes was mainly composed of IDF, and SDF content was relatively low. With the cooking time extending, TDF content in L. edodes pieces presented a decreasing trend. TDF declined rapidly at the beginning (0 –10 min) of boiling time, and then decreased gradually slow and kept stable after 30 min. The variation trend of IDF content was similar to that of TDF. The SDF content increased slightly in the first 10 min, and then maintained stable. It can be seen that the decrease of TDF content was mainly due to the IDF. The IDF in L. edodes is mainly composed of cell wall polysaccharides. Their structure is compact and difficult to dissolve in water. During boiling treatment, part of cell wall structure was destroyed, the dense and orderly arrangement of IDF was broken, and some IDF was degraded and turned into soluble state (Su et al., 2017). Therefore, the contents of IDF and TDF in mushroom pieces decreased rapidly in the initial stage of boiling process. Because the left IDF was very stable, the reduction of IDF and TDF tended to be gentle and stable after 10 min. This changing trend was similar to that of polysaccharide in mushroom pieces.

Figure 2. Effect of boiling time on dietary fiber content in L. edodes pieces. Different letters with same color on the error bars indicate significant differences among treatments (p < 0.05).

Figura 2. Efecto del tiempo de ebullición en el contenido de fibra dietética de los trozos de L. edodes. Las distintas letras del mismo color en las barras de error indican diferencias significativas entre los tratamientos (p < 0.05).

3.3. Effect of boiling time on protein content

The protein content is about 20% of the dry weight of edible mushrooms (Barros, Cruz, Baptista, Estevinho & Ferreira, 2008), which is many times higher than that of ordinary vegetables. Figure 3 shows that with the prolongation of boiling time, the protein content in L. edodes pieces presented a downtrend, which was more obvious (9.3% reduction) in the first 30 min, and then tended to be stable. Erjavec et al. (2012) reported that the protein in L. edodes was stable for thermal process and about 90% of protein was water-insoluble, so most of protein was remained in mushroom pieces after boiling treatment. The protein content in mushroom broth presented an uptrend with the prolongation of boiling time and tended to be stable after 60 min (Figure 3). The results indicated that boiling process can only dissolve the water-soluble proteins, peptides and amino acids, which accounted for about 11.0% of the total protein of L. edodes.

Figure 3. Effect of boiling time on the protein content in L. edodes pieces and broth. Different letters with same color on the error bars indicate significant differences among treatments (p < 0.05).

Figura 3. Efecto del tiempo de ebullición en el contenido de proteínas de los trozos de L. edodes y del caldo. Las distintas letras del mismo color en las barras de error indican diferencias significativas entre los tratamientos (p < 0.05).

3.4. Effect of boiling time on free amino acids content

Free amino acids are the important source of the desirable flavor of mushrooms (Chen et al., 2015). Figure 4 shows that the content of free amino acids in L. edodes pieces significantly (p < .05) decreased 58.6% (from 5.03 mg/g to 2.09 mg/g) in the first 10 min of boiling process, and then continue to reduce significantly (p < .05) to 30 min of boiling, but the reduction tended to be gentle (p > .05) after 30 min. Another significant (p < .05) decrease occurred after 90 min, and the content of free amino acids was only 0.68 mg/g at 120 min. The content of free amino acids in mushroom broth increased sharply at the beginning of boiling, reaching the maximum at 60 min, and then decreased gradually (Figure 4). Most of free amino acids was easily dissolved into hot water, so its content in mushroom broth presented an uptrend. After 60 min of boiling process, due to the Strecker degradation reaction of free amino acids and/or the Maillard reaction (Li et al., 2011), the content of free amino acids in the broth decreased instead.

Figure 4. Effect of boiling time on free amino acids content in L. edodes pieces and broth. Different letters with same color on the error bars indicate significant differences among treatments (p < 0.05).

Figura 4. Efecto del tiempo de ebullición en el contenido de aminoácidos libres de los trozos de L. edodes y del caldo. Las distintas letras del mismo color en las barras de error indican diferencias significativas entre los tratamientos (p < 0.05).

3.5. Effect of boiling time on total phenolic content

Mushrooms contain a certain amount of phenolic substances, which are the important components of antioxidant activity (Sánchez, 2017). Figure 5 shows that the content of total phenol in L. edodes pieces decreased significantly (p < .05) during the first 10 min of boiling process, from 4.32 mg GAE/g to 2.02 mg GAE/g. The decline tended to be gentle after 30 min (p > .05), and the content became stable after 90 min, about 27.9% of the initial. Sun et al. (2014) and Tan et al. (2015) reported that the total phenolic content of mushrooms were significantly reduced after cooking, which is consistent with our results. The content of total phenol in mushroom broth increased gradually with the prolongation of boiling time, reaching 5.46 mg GAE/100 mL at 120 min (Figure 5). The results suggested that most of free phenolic substances in L. edodes could dissolve into boiled water. Moreover, some bound phenols were converted to free states, which also led to the constantly increasing phenolic content in mushroom broth (Choi et al., 2006).

Figure 5. Effect of boiling time on total phenolic content in L. edodes pieces and broth. Different letters with same color on the error bars indicate significant differences among treatments (p < 0.05).

Figura 5. Efecto del tiempo de ebullición en el contenido fenólico total de los trozos de L. edodes y del caldo. Las distintas letras del mismo color en las barras de error indican diferencias significativas entre los tratamientos (p < 0.05).

3.6. Effect of boiling time on ergosterol content

The ergosterol content in L. edodes is relatively high, and it can be converted into vitamin D under ultraviolet irradiation (Czub & Baginski, 2006). Figure 6 shows that ergosterol content in L. edodes pieces presented an uptrend with the prolongation of boiling time, especially after 90 min ergosterol content increased significantly (p < .05) and reached 2.73 mg/g at 120 min. A long-term boiling treatment can cause great damage to the cell membrane, leading to more ergosterol being easily extracted during the determination process. Ergosterol could not be detected in mushroom broth, probably because it is difficult to dissolve in water.

Figure 6. Effect of boiling time on ergosterol content in L. edodes pieces. Different letters on the error bars indicate significant differences among treatments (p < 0.05).

Figura 6. Efecto del tiempo de ebullición en el contenido de ergosterol de los trozos de L. edodes. Las distintas letras del mismo color en las barras de error indican diferencias significativas entre los tratamientos (p < 0.05).

3.7. Effect of boiling time on the microstructure of L. edodes

The L. edodes with different boiling time were observed by environmental scanning electron microscopy (ESEM). Figure 7 shows that the interior of raw L. edodes presented a uniform and dense cross-network structure, and the cell wall and plasma membrane were relatively complete. With the prolongation of boiling time, the internal network structure of L. edodes was gradually destroyed, and the cell wall turned into loose, even broken and dissolved. The destruction of cell structure resulted in the dissolution and loss of the polymers in the cell wall and the soluble substances in the cytoplasm, which caused many alveolate holes (red circles in Figure 7) to be left in the mushroom tissue. When boiling time was 30 min, the destruction of cell structure of L. edodes began to be significant, which resulted in larger alveolate holes (Figure 7c). The alveolate holes in the mushroom tissue got larger and larger with increasing of the boiling time (Figure 7d–f), as more and more damages to the cell walls and membranes produced by heating (Vallespir et al., 2019). The nutrients contents in L. edodes pieces were also significantly different from the initial value after 30 min, suggesting the cell structure of mushroom was seriously damaged after cooking for 30 min and caused the loss of substances in cell wall and cytoplasm (Wang et al., 2008).

Figure 7. Micrographs of L. edodes boiled for 0 min (a), 10 min (b), 30 min (c), 60 min (d), 90 min (e), and 120 min (f), respectively.

Figura 7. Micrografías de L. edodes hervidas durante 0 min (a), 10 min (b), 30 min (c), 60 min (d), 90 min (e) y 120 min (f).

3.8. Effect of boiling time on antioxidant activities of L. edodes pieces and broth

The antiodidant activities of L. edodes pieces and broth were detected by DPPH· scavenging ability, ABTS· scavenging ability, OH· scavenging ability and reducing power. Figure 8 shows that the boiling time had a less impact on DPPH· and OH· scavenging abilities and a great effect on ABTS· scavenging ability and reducing power of L. edodes pieces. The minimum values for DPPH· and OH· radical scavenging rates were 71.5% at 90 min (Figure 8a) and 65.3% at 120 min (Figure 8e), respectively. ABTS· scavenging ability and reducing power presented a similar trend, i.e. the decrease rate was fast in the first half hour of boiling time, about one third of initial value at 30 min, and then the values tended to be stable (Figure 8c,g). The antioxidant activities of mushroom broth increased rapidly in the first 30 min, and then kept stable for DPPH· scavenging ability and reducing power (Figure 8b,h), or decreased to some degree for ABTS· and OH· scavenging abilities (Figure 8d,f).

Figure 8. Effect of boiling time on antioxidant activities of L. edodes pieces and broth. (a)(c)(e)(g) DPPH·, ABTS·, OH· radical scavenging ability, and reducing power of L. edodes pieces, respectively; (b)(d)(f)(h) DPPH·, ABTS·, OH· radical scavenging ability, and reducing power of L. edodes broth, respectively. Different letters on the error bars indicate significant differences among treatments (p < 0.05).

Figura 8. Efecto del tiempo de ebullición en la actividad antioxidante de los trozos de L. edodes y del caldo. (a)(c)(e)(g) Capacidad de eliminación de los radicales DPPH, ABTS, OH y potencia reductora de los trozos de L. edodes, respectivamente; (b)(d)(f)(h) Capacidad de eliminación de los radicales DPPH, ABTS, OH y potencia reductora del caldo de L. edodes, respectivamente. Las distintas letras del mismo color en las barras de error indican diferencias significativas entre los tratamientos (p < 0.05).

L. edodes contains a variety of antioxidant components such as phenolic compounds and polysaccharides (Zhu et al., 2018). Because some phenolic compounds can be dissolved in boiled water, the antioxidant activities decreased for L. edodes piece and increased for L. edodes broth during the boiling process. There was a significant positive correlation (p < .01) between antioxidant activities and total phenolic contents (r² = 0.861, 0.803, and 0.951 for DPPH· scavenging ability, ABTS· scavenging ability, and reducing power, respectively), indicating that phenolic compounds might be the main antioxidant component for L. edodes (Özyürek et al., 2014). Because some phenols, polysaccharides and other antioxidant compounds were not stable in boiled water for long time, the antioxidant activities of L. edodes broth appeared decline (for ABTS· scavenging ability) or fluctuation (for OH· scavenging ability and reducing power).

4. Conclusion

This aritcle studied the effects of boiling time on the nutritional components and antioxidant activities of L. edodes pieces and broth. The results showed that the contents of main nutrients (polysaccharide, dietary fiber, protein, amino acids and total phenols) and antioxidant activities of mushroom pieces decreased with the prolongation of boiling time, and the inflexion points were different for different nutrients, while ergosterol content increased. With the extension of the boiling time, the contents of main nutrients in mushroom broth presented an uptrend, and the inflection point occurred at about 60 min. Ergosterol was not detected in the broth. The antioxidant activities of mushroom broth increased rapidly in the first 30 min, and then kept stable or decreased to some degree, and the changing trend was positively correlated with the total phenolic content in the broth. Boiling process destroyed the cell wall structure, leading to the dissolution and loss of cell wall components and cytoplasmic contents, thus causing the change of nutritional components and antioxidant activities of L. edodes pieces and broth. In addition, long-term high-temperature boiling treatment can destroy the combination of phenols, ergosterol and other substances, which is conducive to their release and dissolution. Meanwhile, amino acids, phenols, polysaccharides, etc. dissolved in the broth can react with other substances due to long-term heating, resulting in their losses and producing some new flavor compounds. From the points of nutritional value and antioxidant activity, the boiling time should not exceed 30 min for mushroom pieces and 60 min for mushroom broth when cooking or processing L. edodes. In addition, the L. edodes broth contains a considerable amount of nutrients and antioxidant activity, which should be recommended into the daily diet. This study provides a theoretical basis for the rational cooking, processing and consumption of L. edodes.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Innovation Scientists and Technicians Troop Construction Projects of Henan Province [2017JR0006];High Level Talents Research Start-Up Project of Henan Institute of Science and Technology [2017019];Postdoctoral Research Grant in Henan Province [001803039];