Effects of Chlorine Dioxide on the Postharvest Storage Quality of Fresh-Cut Button Mushrooms (Agaricus bisporus)

Abstract

The application of chlorine dioxide (ClO₂) in microbial safety and quality maintenance of fresh produce has received extensive attention. Fresh-cut button mushrooms (Agaricus bisporus) are prone to spoilage, resulting in a short shelf-life. In this study, ClO₂ treatment was used to preserve fresh-cut button mushrooms, and its effect on maintaining the postharvest quality was investigated using sensory evaluation, weight loss, color, firmness, respiration rate, electronic-nose (E-nose) analysis and microbial analysis. During the 8 d storage, both the 50 and 100 mg·L⁻¹ ClO₂ treatment reduced the aerobic bacteria count on the surface of fresh-cut button mushrooms. However, the results showed that the 50 mg·L⁻¹ ClO₂ treatment but not the 100 mg·L⁻¹ ClO₂ treatment significantly inhibited the deterioration of comprehensive quality. The 50 mg·L⁻¹ ClO₂ treatment maintained the high sensory evaluation and pleasant volatile odor of fresh-cut button mushrooms as well as reduced the decreasing of fresh weight and firmness. Contrary to expectations, the application of ClO₂ treatment showed worse appearances in color (lower L^*, higher a^*, b^* and browning index) of fresh-cut button mushrooms. Significant differences were observed between CK and ClO₂-treated groups from day 0 to day 6, while there were no significant differences of a^*, b^* and browning index between CK and the 50 mg·L⁻¹ ClO₂ treatment at the end of storage (on day 8). In conclusion, the 50 mg·L⁻¹ ClO₂ can maintain the good quality of fresh-cut button mushrooms.

1. Introduction

Mushrooms are highly popular in the global food market for their deliciousness and valuable nutrients [1,2,3]. Moreover, mushrooms are famous for their medicinal properties, such as the treatment of psoriasis, the inhibition of the inflammatory reaction caused by lipopolysaccharides, and antioxidant properties [4,5,6]. Agaricus bisporus (button mushroom) is the most cultivated mushroom around the world, and the global mushroom market consumption was around 12.74 million tons in 2018 [6]. Its cultivation market was valued at USD 16.7 billion in 2020 and is expected to reach USD 20.4 billion in 2025 [7]. However, as a kind of edible macro fungus, the rapid quality deterioration and short shelf life (1–3 days at ambient temperature, 5–7 days at 0–2 °C) of mushrooms can cause serious economic losses [1,8]. Thus, lots of researches have focused on the preservation of postharvest button mushrooms in recent decades. Most current reports (from 2019 to 2024) were focused on the biocomposite film packaging technique [9,10,11,12,13,14,15]. Different kinds of biochemical substances, such as oregano essential oils and caffeic acid-grafted-chitosan/polylactic acid, were integrated into the packaging materials to improve the preservation of button mushrooms. The second most extensively studied topics were modified atmosphere packaging (MAP) technologies, including anaerobic conditions and high oxygen, high N₂/CO₂ and high O₂/CO₂ situations [16,17,18,19,20]. In addition to the above-mentioned two types of quality preservation methods, washing, irradiation and electric field methods were also developed and reported [21,22,23]. However, there were few reports about the application of chlorine dioxide (ClO₂) on postharvest button mushrooms [24], and how it affects the quality of button mushrooms remained a mystery.

ClO₂ is an oxidative gas with a pungent odor, and its oxidative capacity in hypochlorous acid is 2.5 times that of chlorine [25,26]. Better than chlorine, it does not form trihalomethanes and other halogenated disinfection by-products [27]. The generation and application of gaseous ClO₂ is a little troublesome. Therefore, the application of aqueous ClO₂ has attracted more attention. ClO₂ is well known for its broad antimicrobial effects on bacteria, fungi and viruses such as Listeria monocytogenes and Escherichia coli O157:H7 [28]. According to the International Chemical Safety Cards (ICSC), the occupational exposure limits of ClO₂ (ICSC No: 0127) is 0.1 ppm (TLV, ceiling value), and it is assigned to peak limitation category I. ClO₂ may be used as an antimicrobial agent in water to wash fruits and vegetables that are not raw agricultural commodities in an amount not exceeding 3 ppm of residual chlorine dioxide (21CFR173.300, USA). After being treated with ClO₂, the pathogenic bacterial in spinach and tomato were inactivated and the total number of aerobic bacteria colonies decreased effectively in fresh-cut coriander [29,30]. The respiration rate was reduced and fruit senescence was delayed in postharvest tomato and fresh-cut ‘Hami’ melon, respectively [31,32]. ClO₂ significantly increased ascorbic acid and total phenolic content in fresh coriander, maintained the quality of fresh spinach in terms of ascorbic acid and total soluble solids, alleviated the yellowing of ready-to-eat broccoli and prolonged the shelf-life of fresh-cut asparagus lettuce [33,34,35,36]. In our previous study, ClO₂ treatment was found to affect ethylene-related signal pathways and might enhance the stress resistance of fresh-cut cauliflower [37]. Ku et al. reported that 5, 10 and 50 ppm of ClO₂ treatment delayed weight loss, inactivate polyphenol oxidase and reduce the microbial growth of button mushrooms [24].

The objective of this study was to investigate the efficacy of ClO₂ treatment on the postharvest quality of fresh-cut mushrooms. The detection of weight, color, texture, respiration rate, volatile odor and microbial growth as well as a sensory evaluation were conducted on fresh-cut mushrooms during low temperature storage. The results are expected to enrich the theoretical basis for the application of ClO₂ on fresh-cut mushrooms.

2. Materials and Methods

2.1. Materials and Treatment

Fresh button mushrooms (Agaricus bisporus) were purchased from Yonghui supermarket (Wenzhou, Zhejiang) and were transported directly to Southern Zhejiang Key Laboratory of Crop Breeding within 1 h. Samples with uniformity of size (approximately 5 cm in diameter), absence of weight-loss, defect, mechanical injury or decay were selected for further experiments. The cutting board, knife and hands were sterilized with 50 mg·L⁻¹ sodium hypochlorite solution (Guangjian Testing Technology Co. Ltd., Guangzhou, China), and samples were cut into slices of 5–8 mm thickness.

The mushroom slices were divided randomly into three groups and were immersed in different solutions (CK, the control group, deionized water; T1, 50 mg·L⁻¹; T2, 100 mg·L⁻¹ chlorine dioxide (Beijing Startech Science and Technology Co. Ltd., Beijing, China)) for 10 min and then air dried. Subsequently, each group of mushrooms was weighted (approximately 200 g) and packaged into a polyethylene film bag (thickness, 6 μm; size, 40 cm × 35 cm) and then stored at 4 ± 1 °C and 85 ± 5% RH in a refrigerator. The permeability of the bags to water vapor, oxygen and CO₂ was 5–10 g·m⁻²·d⁻³, 600 and 220 g·m⁻²·d⁻³, respectively. Samples of nine bags were randomly selected at 0, 2, 4, 6 and 8 d after cold storage for later analysis. The determination of weight loss was conducted before that of the respiration rate using the same samples. The sensory evaluation, color, firmness and microbial analysis were conducted using the same three bags from the selected nine bags.

2.2. Sensory Evaluation

The sensory evaluation was conducted by ten panelists from the Institute of Food Science, Wenzhou Academy of Agricultural Science (Wenzhou, China), using the method described in [38,39] with minor modifications. The assessment criteria were based on a 0–10 points scale (as shown in

Table 1. Sensory assessment criteria of fresh-cut button mushrooms.

Score	Color	Off-Odor	Texture Stretchy	Decay	Consumer Acceptance
10–8	White	No	Stretchy	No	Very satisfied
8–6	Slight browning	Slight	Slight soft	Slight	Satisfied
6–4	Mild browning	Obvious	Mild soft	Obvious	Grudging acceptance
<4	Severe browning	Severe	Severe soft	Severe	Unacceptable

) using fresh mushrooms as the control (score = 10).

2.3. Weight Loss

Three bags of fresh-cut button mushrooms were randomly selected from each treatment group and weighed on the subsequent sampling days. The weight loss was calculated using the following equation:

$$\mathrm{Weight}\mathrm{loss}(\%)={\displaystyle \frac{m_0-m_i}{m_0}}\times 100$$

(1)

Note: m₀ is the fresh weight (g) of the fresh-cut mushrooms at 0 d; m_i is the fresh weight of the fresh-cut mushrooms on the sampling date.

2.4. Chromatic Analysis

The chromatic values were measured with a colorimetric analyzer (Spectro colorimeter CS-411, Hunan Gonoava Instrument Co., Ltd., Changsha, China). A total of 10 individual slices of mushroom were randomly selected from each group. The L^*, a^* and b^* values were measured on the top position of the caps of mushroom slices. The browning index (BI) was calculated by the following equation:

$$\mathrm{BI}=100\times (x-0.31)/0.17$$

(2)

In Equation (2), x = (a* + 1.75L*)/(5.645L^* + a* − 3.012b*).

2.5. Firmness

The firmness was determined by a TMS-PILOT texture analyzer (Food Technology Corporation, Sterling, VA, USA) with a 5 mm diameter cylindrical flat head probe (P/5), according to the method described in [38] with minor modifications. The pretest speed, test speed and upward speed after testing were 300 mm·min⁻¹ with a 5 s pause between the two times of compression. The compression ratio of the samples was 50%, and the trigger force value was 1 Newton (N). A total of 10 individual slices of mushroom were randomly selected from each treatment group for testing.

2.6. Respiration Rate

The respiration rate was detected using a handheld O₂/CO₂ meter (Checkpoint, Hangzhou, China). Three bags of fresh-cut button mushrooms from each treatment group were sealed in the container for 1 h before being measured. The respiration rate was calculated by the following equation:

$$\mathrm{Respiration}\mathrm{rate}(\mathrm{mg}{\mathrm{CO}}_2·{\mathrm{kg}}^{-1}·{\mathrm{h}}^{-1})={\displaystyle \frac{∆c\times ∆v\times 44\times 1000}{m\times t}}$$

(3)

In Equation (3), Δc is the change in the volume fraction of CO₂ in the container (%), Δv is the volume of gas in the container (L), m is the mass of the samples (g) and t is the test time (h).

2.7. Electronic-Nose (E-Nose) Analysis

The PEN3 portable electronic nose (AIRSENSE, Mecklenburg, Germany) was used for analysis. The detector is comprised of 10 metal oxide sensors, each of which corresponds to a different type of sensitive substance (

Table 2. Sensor type and corresponding representative sensitive substances of E-nose.

Array Number	Sensor Type	Substance for Sensing
1	W1C	aromatic benzenes
2	W5S	nitrogen oxides
3	W3C	ammonia
4	W6S	hydrogens
5	W5C	alkanes
6	W1S	methane
7	W1W	terpenes, hydrogen sulfides
8	W2S	alcohols
9	W2W	aromatic components, organic sulfides
10	W3S	aromatic alkanes

). The sampling and test procedure were conducted according to the reported method [40] with slight modifications. Samples (about 100 g) from each group were added to a 250 mL glass beaker and sealed and placed at room temperature (25 °C) for 30 min. The test procedure was as follows: cleaning time of 120 s, a waiting time of 5 s, and a test time of 60 s using a flow rate of 300 mL/min. Three biological replicates were performed for each group. The response values were expressed as the ratio G/Go, representing the resistivity ratio between the sample gas and pure air. The average response value (50–52 s) for each sensor was extracted as the latent variable.

2.8. Microbial Analysis

The total count of the aerobic bacteria was determined according to the report [9] with minor modifications. The fresh-cut mushrooms (25 g) were weighed and mixed with 225 mL of 0.9% sterile normal saline, placed in sterile bag and homogenized for 2 min. After being mixed thoroughly, a gradient dilution by 10 times was carried out in turn. Microbial counts were counted by utilizing the pour plate approach in plate count agar. Each sample (1 mL) was used to coat uniformly in the plate count agar medium and incubated at a constant temperature of 37 °C for 48 h.

2.9. Statistics

All of the measurements were completed in three replicates, except for the 10 repetitions when measuring texture and color. The WinMuster software (version 1.6.2.18) with E-nose was used to process the E-nose data for analysis. SPSS 19.0 (IBM, Armonk, NY, USA) was used for all data analysis, and the significance of differences between groups was evaluated using a one-way analysis of variance (ANOVA) and the least significant difference (LSD) method (p < 0.05). A correlation analysis was also conducted on the different indicators using Pearson’s correlation (two-tail tests) by SPSS 19.0. The data were plotted using Origin 2021 (OriginLab Corporation, Northampton, MA, USA).

3. Results

3.1. Sensory Evaluation

The sensory quality of fresh-cut button mushrooms was evaluated according to the criteria in Table 1. Due to the selection of fresh mushrooms without defects, the samples on day 0 performed well in appearance and smell, and each item received a score of 10. The detailed evaluation of the five sensory items on 2, 4, 6 and 8 days after treatment (DAT) are shown in Figure 1a–d, respectively. The most noticeable sensory change in samples within the first two days was color. The colors of the T1 and T2 groups were not expected to be darker than that of CK, and this phenomenon was observed throughout the whole storage. The flavor, texture quality and consumer acceptance of each group rapidly deteriorated from 0 DAT to 8 DAT (Figure 1a–d). A small portion of the mushrooms in the CK group was rotten, while no rotted ones was observed in the treated group at 8 DAT (Figure 1d). In total, it was obviously that the T1 group obtained the highest sensory evaluation during the late stage of storage (Figure 1c,d).

3.2. Weight Loss

As shown in Figure 2, the weight loss in both of the ClO₂-treated (T1, T2) and CK fresh-cut button mushrooms increased along with the storage time. The CK group showed the most serious weight loss within the first two days and then increased slowly. However, the T2 group increased rapidly and showed the most serious weight loss from 4 to 8 DAT. Compared with the T2 group, the CK group showed a lower weight loss rate from 4 DAT to 8 DAT, and a significant difference was observed at 6 and 8 DAT. The lowest weight loss was found in the T1 group during storage, with a significant difference from 4 to 8 DAT compared to the T2 group (p < 0.05) (Figure 2).

3.3. Chromatic Analysis

The color of mushrooms is one of the important factors that affect the interior quality and thus the consumer purchasing behavior. Chromatic values including L^*, a^* and b^* were detected to evaluate the color change of fresh-cut button mushrooms during the low temperature storage (Figure 3). The L^* value in all of the groups of fresh-cut mushrooms decreased gradually from 92.1 to 77.5 along with the storage time (Figure 3a). It was unexpected that the L^* value of the CK group was higher than that of T1 and T2, and the significant differences in the L^* value between CK and T1 and T2 was observed from 2 DAT to 8 DAT. The L^* value of T2 group was the lowest among the three groups during storage.

The a^* value in all of the groups increased gradually during the storage, which ranged from 4.1 to 11.5 (Figure 3b). In total, the lowest a^* value was found in the CK group, while the highest a^* value was observed in the T2 group. The significant differences of a^* value between CK and T2 were observed from 2 DAT to 8 DAT, while the significant differences of the a^* value between CK and T1 were observed from 2 DAT to 6 DAT. There was no significant difference between CK and T1 at 8 DAT. Similar results were found in the b^* value during storage (Figure 3c).

Similar to the results of a^* and b^*, the T2 group showed the highest level of BI while CK showed the lowest (Figure 3d). Significant differences of BI were found in the three sample groups from 4 DAT to 6 DAT.

Altogether, the above results show that the CK group exhibited the best appearance of color, while the T2 group exhibited the worst. At the end of storage (8 DAT), there was no significant differences in a^*, b^*, and BI between CK and T1.

3.4. Firmness

The firmness in both of the CK and ClO₂-treated (T1, T2) fresh-cut button mushrooms decreased sharply within the first 2 days and then decreased slowly from 4 DAT to 8 DAT (Figure 4). The firmness of the CK group was significantly lower than that of T1 and T2 at 2 DAT and 8 DAT (p < 0.05) but showed no significant difference to that of T2 from 4 DAT to 6 DAT. In total, the T1 group showed the highest level of firmness and significant differences when compared to the other two groups (except 6 DAT).

3.5. Respiration Rate

The respiration rate in all groups initially increased from 0 DAT to 4 DAT and then decreased (

Table 3. Respiration rate (mg CO₂·kg⁻¹·h⁻¹) of fresh-cut mushrooms.

Storage Time (d)	CK	T1	T2
0	86.81 ± 7.07 a	86.81 ± 7.07 a	86.81 ± 7.07 a
2	96.73 ± 4.06 a	91.22 ± 3.47 a	94.71 ± 6.68 a
4	98.28 ± 2.21 ab	92.81 ± 2.16 b	101.85 ± 4.96 a
6	70.95 ± 4.04 b	84.22 ± 5.18 a	83.25 ± 4.35 a
8	56.54 ± 5.55 b	78.35 ± 3.53 a	77.76 ± 10.21 a

^ab Different lowercase letters represent significant differences at 0.05 level. CK: deionized water; T1: treated with 50 mg·L⁻¹ ClO₂; T2: treated with 100 mg·L⁻¹ ClO₂.

). Both the T1 and T2 treatments had no significant effect on the respiration rate of fresh-cut button mushrooms in the first 2 days. All samples reached the respiration peak at 4 DAT, while the respiration rate of the T1 group was significantly lower than that of the T2 group. From 4 DAT, the respiration rate of the CK group decreased rapidly from 98.28 to 56.54 during later storage, showing a significantly faster decrease than that of the respiration rate of the T1 and T2 groups. In total, the respiration rate of T1 group was lower than that of CK and T2 at the early stage (2–4 DAT) but higher than that of CK and T2 at the late stage (6–8 DAT). Therefore, the T1 treatment down-regulated the peak of respiration at the earlier stage and inhibited the fast decrease of respiration at the later stages in fresh-cut button mushrooms.

3.6. E-Nose Analysis

The E-nose can quickly provide the overall information of a tested sample through specific sensors and pattern recognition systems, indicating the implicit features of the sample. In order to better evaluate the volatile characteristics of fresh-cut button mushrooms during storage, an E-nose analysis was conducted. The response values of the 10 sensors of each mushroom group varied during storage (Figure 5a,c,e). Samples of each group on day 0 and 2 showed high response to W1C (aromatic benzenes), W3C (ammonia), W5C (alkanes) and W3S (aromatic alkanes). On days 4 and 6, the situation changed as samples showed high response to W1C, W5S (nitrogen oxides), W1W (terpenes, hydrogen sulfides) and W2W (aromatic components, organic sulfides), especially W5S and W1W, indicating a change in the composition of volatile compounds during storage. A significant difference was observed in the CK group on day 8, where samples showed high response to W6S (hydrogens) and W1S (methane), while the other two groups did not.

A principal component analysis (PCA) can reduce the dimensionality of E-nose data and extract the main characteristics of mushroom samples for linear analysis. As shown in Figure 5b, PC1 of the CK group contributed 76.8% while PC2 contributed 11.3%, with a total contribution of 88.1%. The total contribution was 93.4% and 92.0% in the T1 group and T2 group, respectively (Figure 5d,f), while the total contribution was 86.9% in all three groups (Figure 5g,h). The high contribution suggests that it was adequate for identifying the similarities between different treatment and storage times.

Overlap regions were observed among different treatments in Figure 5g, which mean that there were no significant differences of flavor odor among three groups. Considering the combination of treatment and storage time, the flavor odor of samples can be clustered into three types (Figure 5h). Type I included samples from 0 to 2 DAT of each group and type II included samples from 4 to 6 DAT of each group and 8 DAT of both ClO₂-treated groups, while type III included only samples from the CK group on day 8. Significant differences were observed between type I and the other two types, while no significant difference was observed between type II and type III. For further understanding, the detailed information of each group is analyzed in Figure 5b,d,f. Figure 5b shows that the volatile compounds of the CK samples can be clearly divided into three types: 0–2 d, 4–6 d and 8 d. Unlike the CK group, the T1 group can be clearly divided into three types, 0 d, 2 d and 4–8 d, according to Figure 5d. The same result was found in the T2 group (Figure 5f).

These results suggest a distinct change in volatile odor from day 4 among all groups and another significant change in the CK group on day 8, indicating the deterioration of flavor in fresh-cut button mushrooms. The PCA results validate the results of the radar chart (Figure 5a,b,e) and provide a more in-depth analysis.

3.7. Microbial Analysis

The invasion of microorganisms can cause the decay of fresh products. As shown in Figure 6, the total number of bacteria on the surface of fresh-cut button mushrooms increased with storage time. The aerobic bacteria count in the CK group increased from 4.24 to 8.87 log₁₀cfu/g throughout the whole storage, while the counts in the T1 and T2 groups were significantly lower than that in the CK (p < 0.05). The results show that the ClO₂ treatment effectively inhibited the microbial infection in fresh-cut mushrooms. The aerobic bacteria count in the T2 group was significantly lower than that in the T1 during storage, except for 8 DAT. This means that the effect of ClO₂ treatment on inhibiting microbial infection was not completely concentration-dependent in long-term storage (Figure 6).

3.8. Correlation Analysis

A Pearson correlation analysis of all the indicators of fresh-cut button mushrooms was conducted (Figure 7). Significant correlations were observed between most indicators. The sensory evaluation gradually declined with the extension of storage time in the present study. The correlation analysis showed that sensory evaluation was positively correlated with L^*, firmness, respiration rate, W1C, W3C, W5C and W3S, while it was negatively correlated with a^*, b^*, BI, aerobic bacteria count, W5S, W1S, W1W, W2S and W2W. It is clear that the respiration rate, W6S, W5C and W2S showed no correlation with nearly half of the indicators.

4. Discussion

Fresh-cut A. bisporus is prone to deteriorate after harvesting and slicing. Tissue browning and softening are the main visible quality deteriorations. In addition, the microbial growth promoted by the mechanical injury of slicing aggravates the shortening of shelf-life and the reduction of commercial value [14,22].

The application of ClO₂ in microbial safety and quality maintenance of postharvest products has received extensive attention [28,41]. The effects of ClO₂ treatment depend on the application form (gas or solution), treating time, concentration, temperature, the type of fresh produce and other factors [25]. When ClO₂ acts, it exhibits significant differences in oxidative capacity towards biomolecules with different chemical structures, mainly reflected in the oxidation rate [28]. In this study, fresh-cut button mushrooms were treated with 50 and 100 mg·L⁻¹ of aqueous ClO₂ to investigate preservation efficacy. The results show that the T1 treatment (50 mg·L⁻¹ ClO₂) maintained good quality, including high sensory evaluation, low weight loss, good volatile odor and low microbial growth. According to the previous report, mushrooms have a rotten odor after 12 d storage at 4 °C [22]. Herein, a small portion of the mushrooms in the CK group was rotted at 8 DAT and ClO₂ treatment delayed the rotting process.

Harvested mushrooms have a high respiration rate, including button mushrooms and straw mushrooms (Volvariella volvacea) [8]. All samples showed a trend of first increasing and then decreasing (Table 3). The trend of changes in respiration rate here was similar to that reported by Yan et al. and Sun et al. [9,17] but different from that reported by Xu et al. [21]. From 6 DAT, the respiration rate of the control group declined significantly while that of the treated groups was maintained at some level, indicating that ClO₂ treatment delays the senescence progress of button mushrooms.

The weight loss of fresh produce is mainly due to respiration, moisture evaporation, and dry matter consumed during storage [42]. The weight loss of fresh-cut button mushrooms during 8 d of storage was delayed after 50 mg·L⁻¹ of ClO₂ treatment (Figure 2), which was similar to the report [24] where the weight loss of button mushrooms treated with 5, 10 and 50 ppm ClO₂ was lower than that of the control samples. However, samples of the 100 mg·L⁻¹ ClO₂ treatment showed significantly higher weight loss, which might be due to its significantly higher respiration rate.

How to prevent tissue browning is a technical difficulty associated with mushrooms in postharvest preservation. According to previous reports, when the L^* value was below 80 and 69, the whiteness of mushrooms at the wholesale and consumer levels, respectively, was considered unacceptable [31,41,43]. In the present study, the L^* value in all of the groups of fresh-cut mushrooms decreased gradually from 92.1 to 77.5 along with the storage time, which meant that the whiteness of mushroom slices after 8 d storage was acceptable for consumers (Figure 3a). Visible color changes were observed on day 2 in all groups (Figure 1a and Figure 3), while the L^* value declined significantly and the values of a^*, b^* and BI increased significantly. In the previous report, the lac dye-based intelligent colorimetric indicator showed a significant color change in the fresh button mushrooms on day 4 [43], which was later than our results (day 2). It might be because slicing accelerated the color deterioration of button mushrooms. It was unexpected that the CK group showed the best appearance of color and T1 was better than T2, though there was no significant difference between CK and T1 at the end of storage (Figure 3b–d). It meant that the treatment of 50 and 100 mg·L⁻¹ ClO₂ did not prevent tissue browning of button mushrooms and even negatively affect it. Browning is the result of the oxidation and polymerization of phenolic compounds [42]. The negative effect of ClO₂ treatment on mushroom color was similar to that of lotus and bok choy, which might be because the high oxidation capacity of ClO₂ destroyed the active sites or structures of enzymes [44,45]. Similar results were also reported on other fresh produce. Both romaine and iceberg lettuce treated with 200 ppm chloride ion also showed a negative effect on color [46]. The white blushing of mulberries was more severe in fruit treated with 80 mg·L⁻¹ of ClO₂ compared to that with lower concentration (60 mg·L⁻¹) [47]. Likewise, the calyx of strawberries was discolored after treatment with 5 mg·L⁻¹ ClO₂ [48]. However, the detailed mechanism of the color changes still remains unclear.

How to prevent tissue softening is also a technical difficulty associated with mushrooms in postharvest preservation. The softening of fresh produce is often related to cumulative weight loss, cell wall structure and ethylene biosynthesis [42]. The decline of firmness was delayed more effectively by 50 mg·L⁻¹ of ClO₂ treatment than by 100 mg·L⁻¹ of ClO₂ treatment (Figure 4), which might be because the higher oxidation capacity of the 100 mg·L⁻¹ ClO₂ treatment disrupts the cell wall structure.

To our knowledge, this was the first time E-nose equipment was used on A. bisporus for volatile odor analysis. Samples of Hericium erinaceus showed high response to W1C (aromatic benzenes), W5S (nitrogen oxides) and W1W (terpenes, hydrogen sulfides), indicating different flavor characteristics from A. bisporus (Figure 5a,c,e) [40]. Samples of each group on day 0 and 2 showed high response to W1C (aromatic benzenes), W3C (ammonia), W5C (alkanes) and W3S (aromatic alkanes), which was consistent with a previous report that hydrocarbons were the dominant group of aroma compounds in fresh button mushrooms on day 0 [43]. Significant differences were found in A. bisporus between the later (4–8 d) and early (0–2 d) stages of storage, suggesting a deterioration in volatile odor from day 4. However, it was not until day 8 that a significant difference was found between the CK group and ClO₂-treated groups, indicating that the effect of ClO₂ on volatile flavor in fresh-cut A. bisporus occurred later. ClO₂-treated citrus fruits retained some unique volatile substances and exhibited better flavor compared with the CK group [49]. Nevertheless, the efficacy of ClO₂ on volatile odor requires more research.

ClO₂ is famous for its strong antimicrobial ability. In the current study, both treatments of ClO₂ effectively declined the aerobic bacteria count in fresh-cut mushrooms. However, the effect of ClO₂ treatment was not completely concentration-dependent in the long-term storage, which might be because ClO₂ treatment may not cause irreversible damage to microbes [25,50].

In summary, the results showed that the 50 mg·L⁻¹ ClO₂ treatment was beneficial for the postharvest storage of button mushrooms while the 100 mg·L⁻¹ ClO₂ treatment not. It was reported that 50 ppm of ClO₂ treatment had no effect on the mass of shredded yam and 3 mg·L⁻¹ of ClO₂ treatment did not alter the respiration rate of fresh-cut iceberg lettuce [51,52]. Due to the fact that the effect of ClO₂ is not only related to the concentration but also to the type of fresh products [25], the kinetics of ClO₂ action on products should be clearly studied before commercial use. For further understanding of the efficacy of ClO₂ treatment, more physiological changes need to be investigated, such as enzymatic activity (especially browning and softening related enzymes), total phenolic content, water status and detailed composition of volatile compounds (especially amino acids).

5. Conclusions

Many methods have been reported and applied to the preservation of mushrooms, but the reports about the effect of postharvest ClO₂ treatment on the quality of mushrooms were limited. Our results showed that the 50 mg·L⁻¹ ClO₂ treatment had a significant effect on preventing the quality deterioration of the button mushrooms during 8 d storage but not the 100 mg·L⁻¹ ClO₂ treatment. The 50 mg·L⁻¹ ClO₂ treatment delayed the process of microbial infection and maintained the high sensory evaluation and good flavor of fresh-cut A. bisporus as well as reduced the decreasing of fresh weight and firmness, which are important for food safety and consumer acceptance. In summary, our research affirms that the appropriate concentration of ClO₂ can be used in the preservation of mushrooms. Chlorine dioxide treatment with unreasonably high concentrations might damage the quality of postharvest mushrooms, which might be due to its great oxidation capacity that destroys the structures of cell walls or related enzymes. The possible mechanisms involved in this phenomenon require further systematic analysis.