Mushroom growing is an ecologically clean and wastefree production (Varoquaux et al., 1999). There are whole mushroom industries were created in France, England, Holland, Germany, and the USA (Yoo et al., 2005). The world leaders in mushroom growing are China, the USA, and France (Zhang et al., 2014). The industrial production of cultivated mushrooms is one of the most perspective industries of agriculture in Ukraine. The main types of mushrooms that are officially allowed to grow in our country are AB and PO (Dubinina and Timofeeva, 2009; Nesterenko, 2011). Mushroom is a rich source of good quality proteins (Simahina, 2008), having most of the essential amino acids, fatty acids, minerals, and vitamins with low calories (Mattila et al., 2002; Lee et al., 2011). Consumption of mushrooms has a positive effect on the human body. Their components enhance immunity, have hepatoprotective, antitumor, antidiabetic, cardiac effects, help reduce the level of "harmful" cholesterol, improve the functional state of certain organs and systems of the body (including nervous, sexual) (Wasser, 2002; Lindequist, Niedermeyer and Julich, 2005; Patel and Goyal, 2012; Kalač, 2013).

The shelf life of the fresh mushrooms is limited to 1 – 3 days at ambient temperature and it may be increased up to 4 – 7 days at the 4 ºC. Browning and texture changes are the main processes that responsible for the loss of the sensory quality of mushrooms. The free water high level in the fruiting bodies of mushrooms promotes their high IR and causes dehydration and metabolic activity.

The IR of the harvested mushrooms is high in comparison to the other horticultural crops and this main cause their shorter postharvest life. Many short-term storage measures are followed to retard the deterioration in quality at the level of mushroom grower till it reaches the consumer. By following proper packing, cooling, PHH, and transportation, the shelf life of mushrooms can be extended (Kumar et al., 2014).

Good results in extending the shelf life of mushrooms have been obtained as a result application of the Controlled Atmosphere Storage (CAS), Modified Atmosphere Packaging (MAP), and Modified Humidity Packaging (MHP) (Barron et al., 2002; Kim et al., 2006; Mahajan et al., 2008; Jiang et al., 2010).

In the case of CAS, the product is stored in cold storage chamber where created atmospheric composition with certain relative humidity (RH) and concentration of O₂ and CO₂ that is maintained constant throughout storage. CAS reduces IR of mushrooms, their texture changes, and brown discoloration (enzymatic browning) as a result shelf life is extended (Djekic et al., 2017; Park et al., 2020). A major disadvantage of CAS is the significant cost of equipment and its maintenance.

MAP is a method of storage when fresh product is in a sealed package and is creating a modified atmosphere by respiratory gas exchange, namely oxygen intake and carbon dioxide evolution (Zhang, Pu, and Sun, 2018; Zalewska et al., 2018). Equilibrium concentrations of O₂ and CO₂ are consequently established in the case the rate of gas permeation through the packaging material equals respiratory gas exchange (Gantner et al., 2017; Gholami, Ahmadi and Farris, 2017). The balance of the gas exchange depends on product weight, temperature, the respiration rate of a certain product, permeability O₂ and CO₂ through packaging material, free volume in the package, and film area. MAP helps to extend the shelf life and keep quality as a result creation of a corresponding atmosphere around the products that are packaged in plastic films (Ozturk, Havsut and Yildiz, 2021; Vunduk et al., 2021). There are two methods of creating a modified atmosphere: active and passive modifications. The product is only sealed in a polymeric package at the passive modification. The atmosphere is modified as a result of the fresh product breathing and permeation of gases into the package. It takes a long time to reach the steady-state conditions within the package in passive modification. In the case of the active modification, air initially compulsorily is pushed into the package. Therefore, a stable state of the atmosphere is reached after packaging quickly (Li et al., 2014; Han Lyn et al., 2020). MAP of mushrooms has been shown successfully to delay senescence and keep quality after harvest.

Many publications report that reduced O₂ and elevated CO₂ concentration have beneficial effects on the shelf life of mushrooms (Lin et al., 2017; Iqbal et al., 2009; Singh et al., 2010; Simón, González‐Fandos and Tobar, 2005; Hosseini and Moradinezhad, 2018).

The atmospheres with a high concentration of CO₂ can potentially reduce respiration rate, production of ethylene, sensitivity, decay, and physiological changes in the mushrooms (Farber et al., 2003).

The IR depends on the product, its developmental stage, CO₂ concentration, and time of exposure (Fonseca, Oliveira and Brecht, 2002). CO₂ has significant and direct antimicrobial activity. Aerobic bacteria, such as Pseudomonas, are inhibited by moderate to high levels of CO₂ (10-20%) (Farber et al., 2003; Lee et al., 1995). A combination of 3% O₂ and 10% CO₂ is reported to extend the storage life of mushrooms up to 12 to 15 days at 0 °C (Cantwell and Suslow, 2002). Another report says that 10 to 15% CO₂ concentration reduces cap opening, browning, and stripe elongation (Zhang, Pu and Sun, 2018).

The low level of O₂ and very high CO₂ in the atmosphere may cause the development of off-flavors, veil opening, and stripe elongation (Farber et al., 2003; Lee et al., 1995).

Besides, the excessive level of CO₂ can cause cell membrane damage and physiological damages to the product, such as strong enzymatic browning and loss of mushroom density (Briones et al., 1992).

So, low concentrations of CO₂ did not allow obtaining the desired effect but high has negatively affected the quality and shelf life of the mushrooms.

To establish the optimal regime of AB and PO mushrooms treatment by carbon dioxide, we were kept them in sealed chambers with 20% CO₂ in the environment for 2, 12, and 22 h, and then stored with temperature 1 °C for 6 days in the normal atmosphere.

The influence of parameters and regimes of the PHH on the quality of mushrooms was determined by organoleptic and chemical indexes. Organoleptic evaluation of AB and PO showed that PHH by 20% CO₂ for 2 h had almost no effect on changes in color, smell, and consistency of fruiting bodies. Prolonged treatment by carbon dioxide during 22 h significantly deteriorated the color of mushrooms (Figure 2). In our opinion, prolonged contact of mushroom tissues with high concentrations of CO₂ has caused respiratory disorders and inhibited the biochemical processes associated with oxygen deficiency. This caused the death of individual cells, enzymatic browning, and a decrease in the fruit density of mushrooms, which corresponding with the results obtained by other researchers (Briones et al., 1992). Besides, products had a slight characteristic smell of carbon dioxide.

A positive effect on the organoleptic characteristics of the mushrooms was established in the variant of treatment by 20% CO₂ for 12 h. There was no foreign smell; the fruiting bodies remained whole and dense. As can be seen in Figure 2, the mushrooms of AB had advantages over control in appearance at this regime.

However, it should be noted that photos do not fully demonstrate changes in the appearance of mushrooms. The difference is more clearly visible in the organoleptic evaluation products in the boxes. The better appearance of mushrooms at this regime, compared with the control, can be explained by inhibition of the activity of the enzyme tyrosine as a result of CO₂ treatment. As a consequence, the formation of melanins slows down, which causes enzymatic browning of AB. A similar result in terms of inhibition of the enzyme tyrosine activity as a result of treatment by high concentrations of CO₂ was obtained in the investigations of straw mushrooms (Volvariella volvacea) (Jamjumroon et al., 2013; Jamjumroon et al., 2012).

PO in contrast to the AB did not change in appearance at different regimes of CO₂ treatment.

Preservation of AB and PO depending on the regime of PHH was determined by the results of studies of three cycles of cultivation (repetitions). The highest yield of marketable products was observed at the regime of treatment by 20% CO₂ within 12 h (Table 1).

For mushrooms AB of the IBK-25 strain, on average, was 94.9% that is 1.4% more than the control variant (93.5%). For strain IBK-15, this index was 94.2% (in the control variant – 92.5%). The duration of treatment during 2 h did not significantly affect the output of marketable fruit bodies. Depending on the growing cycle, they were 93.3 – 93.7%, while in the control variant – 93.0 – 93.9%.

Negatively affect had a variant of treatment by CO₂ for 22 h on the yield of marketable products. On average, during three growing cycles, their quantity decreased relative to control by 1.8-1.9%, depending on the strain of the AB. The regime of treatment by 20% CO₂ during 2 h had little effect on the natural LW (Table 2) and quantity of non-marketable fruiting bodies of mushrooms. Such a shortterm treatment cannot significantly change the vital processes that take place in the mushrooms after harvest.

The treatment by carbon dioxide also affected the DC of mushrooms (Table 1). There best values were observed for treatment during 12 h (5.56 and 5.65 g.cm^-1), and in the control variant – 5.48 and 5.58 g.cm^-1, respectively.

The regime of the mushrooms treatment during 22 h had positive effects on the natural LW (Table 2). In this regime, a natural loss is slightly lower (4.0 – 4.2% depending on the strain), compared with the control (4.8 – 5.0%), but at the same time in the experimental version significantly increases the quantity of non-marketable fruiting bodies (from 1.8 – 2.5 to 4.4 – 5.1%, respectively).

The fruiting bodies of AB of the strain IBK-15 have larger fruiting bodies compared to the strain IBK-25, respectively, and a larger area of moisture evaporation. This, in turn, affected the natural LW during storage (4.2 – 5.0% for strain IBK-15) against 4.0 – 4.8% for strain IBK-25, depending on the regime of PHH.

The tendencies concerning the influence of PHH of 20% СО₂ on the natural LW, quantity of a commodity, and noncommodity fruit bodies in the AB were characteristic and for PO (Table 3). There was a slight increase in the yield of marketable products – by 0.6 – 0.9% relative to the control for the twelve-hour treatment by carbon dioxide. This is due to the breaking of vital processes in mushrooms as a result of CO₂ treatment. The total LW during storage of PO mushrooms in these conditions was formed mainly due to natural losses. For strain NK-35 their value was 5.2%, for Amycel 3000 – 4.7%. The quantity of non-commodity fruit bodies was 0.5 and 1.5%, respectively (Table 4). PO of strain Amycel 3000 had a higher number of non-marketable fruiting bodies that explain the botanical features of the formation of their clusters. The clusters of this strain had a large number of small fruit bodies, which after storage for 6 days dried up or became watery.

The treatment OP by CO₂ during 22 h harmed the quantity of non-marketable products.

Their numbers increased up to 3.4 and 3.6%, compared with controls – 0.8 and 1.8%, depending on the strain. Some mushrooms had brown spots. In our opinion, the reason for this is the burns of the tissues of the mushrooms by carbonic acid that was formed by the interaction of condensate, which appeared as a result of respiration of products and carbon dioxide of high concentration.

Natural losses of mushrooms during storage were formed due to the evaporation of moisture and loss of chemical substances (Tables 5 and 6).

The postharvest handling by carbon dioxide during the storage of AB mushrooms contributes to the preservation of DM. It explains by the inhibition effects of CO₂ on the respiration processes and development of the mushroom’s fruiting body that decreased sugar consumption. Thus, at the end of storage for the mushroom of strain IBK-25, the amount of DM in the fruiting bodies of the experimental variants were 8.5, 8.6, and 8.4%, while in the control variant – 8.3%.

Carbon dioxide in most cases did not significantly affect the changes in the amount of PN and AA compared to the control. Only after treatment duration by 22 hours, the amount of PN partially changed.

For strain IBK-25 its amount was 3.5% for the experimental variant and 3.7% for the control; for strain IBK-15 – 3.4 and 3.5%, respectively. This may be due to oxygen deficiency and protein breakdown. Similar trends in the effect of postharvest handling by carbon dioxide on the content of chemical substances were observed for OP mushrooms. The amount of DM in mushrooms of strain NK-35 during the storage period decreased from 12.5 to 11.3 – 11.7% depending on the exposure and in the control variant – up to 11.4%.

Most DM was preserved in fruiting bodies after twelve hours of treatment: in strain NK-35 – 11.7% and in the strain Amycel 3000 – 11.5%, that it is significantly more than the control variant.

There are similar to the AB in PO mushrooms wasn’t observed significant changes in the quantity AA during the treatment by CO₂.

The amount of PN changed only after treatment during 22 h (3.1 and 3.2% against 3.3% in the control variant).

Analysis of the IR of mushrooms indicated that after harvesting quantity of carbon dioxide released during respiration begins to increase sharply, and then decreases during storage due to low temperatures (Table 7 and 8).

Significantly affected the amount of CO₂ released had postharvest treatment by carbon dioxide, especially in the first 3 days.

As can be seen from Table 7 and Table 8, there is a tendency that with increasing duration of treatment by CO₂, the process of respiration is suppressed. It is the least intense respiration was as a result of treatment CO₂ during 22 h. It was typical for both AB mushrooms and OP.

Thus, for AP of strain IBK-25 after a day of storage, the amount of CO₂ released in the experimental variant was 7.5 but in the control – 9.2 mL.kg^-1.h^-1 for CO₂; after two days – 8.1 and 11.3 mL.kg^-1.h^-1 for CO₂; after three days – 4.0 and 5.3 mL.kg^-1.h^-1 for CO₂, respectively. In the future, the IR of the control and experimental variants was gradually equalized and at the end of storage did not differ significantly (2.1 – 2.3 mL.kg^-1.h^-1 for CO₂ in the experimental variants and 2.5 in the control) (Table 7 and 8).

Based on the data on the intensity of respiration, calculations of the HR of mushrooms were performed (Tables 9 and 10).

Postharvest short-term treatment from 2 till 22 h by carbon dioxide at a concentration of 20% affected the average HR of mushrooms during storage.

Since the intensity of HR directly depends on the IR, so the trends of its change are similar.

Thus, for the OP of the strain Amycel 3000, these indexes were: after two hours of treatment – 1.6 kJ. kg ^-1.day^-1; after 12 h – 1.4; after 22 hours – 1.3; control – 1.8 kJ.kg ^-1.day^-1.

Postharvest short-term handling mushrooms of AB and PO by carbon dioxide at a concentration of 20% is an effective method that reduces the level of natural losses, increases the yield of marketable products, helps preserve their chemical substances, and inhibits the intensity of physiological processes in the product (IR and HR). According to all the obtained data, the most effective is the application of the regime of mushrooms treatment by CO₂ during 12 h. Increasing the time of treatment by carbon dioxide up to 22 h most effectively inhibits the processes of respiration (on the 0.4 – 3.2% for AB and the 0.6 – 5.4% for PO) and HR (on the 0.1 – 0.8% for AB and the 0.1 – 1.4% for PO), compared with the control and depending on the mushrooms’ strain and duration of their storage. However, this regime promoted sharply increases the quantity of nonmarketable products (on the 2.6% for AB and the 1.8 – 2.6% for PO) compared to the control and depending on the strain and wave of fruiting.