Chilis, botanically known as Capsicum annuum L., belongs to the Solanaceae family and is considered one of the most widely cultivated crops used as an all-purpose spice (Batiha et al., 2020). The fruits are rich in vitamins such as vitamins A, C, E, and B6, beta-carotene, folic acid, and thiamine and are mainly cultivated for their greens, sauces, prickles, and special taste and aroma (Bosland and Votava, 2012). The aldehydes and volatile terpenes of pepper have also recently attracted the attention of many researchers and the food industry as they are excellent antimicrobial additives with very low residues for food preservation (Mercy, 2018). Dias (2012) stated that the capsaicin extracted from chili peppers increases heat production and oxygen consumption in the body, which promotes calorie burning and weight loss. In addition, the chili pepper plant provides an effective and inexpensive source with great economic and potential health benefits for controlling the development of various metabolic and antioxidant-associated diseases in contrast to treatments with synthetic drugs, which are not only expensive but also have side effects and become increasingly resistant with prolonged use (Mercy, 2018). Commercially, the fruit is of great importance as it is the most important element in a variety of cuisines around the world, adding flavor, aroma, and color to dishes (Chandra Sekhar Rao, 2014). The annual cultivated area of pepper in China is more than 13,000 km², which makes pepper one of the most widely grown vegetables (Ou et al., 2017).

“Hangjiao No.2” is a widely grown chili pepper cultivated in northern China, and its sources of various nutrients are crucial for dietary diversification and national food security. The plant is usually harvested and consumed at the green, mature stage. The fruits of pepper plants vary in pungency due to the presence of capsaicinoids in varying concentrations (Asnin and Park, 2015) and are mainly used for seasoning many Chinese cuisines. However, the market value of pepper fruits is limited by post-harvest conditions due to their short shelf-life. The shelf-life of pepper is within 3–5 weeks, especially when stored in moisture-retaining films at 7.5°C, and less than 2 weeks when stored at 5°C (Fernández-Trujillo et al., 2009). However, neither the distribution chain nor consumers usually have the ability to store a particular product under such ideal conditions, so compromises must be made in the choice of temperature and relative humidity. Due to the increasing consumer demand for products of superior quality and high nutritional content, the optimization of postharvest conditions to preserve the natural green color and quality of the pepper fruit is the focus of this study.

1-Methylcyclopropene (1-MCP) is a cyclic alkene and also an ethylene antagonist that binds to ethylene receptors in plant cells and inhibits ethylene binding, preventing signal transduction and ethylene action (Lurie, 2006; Serek et al., 2015). 1-MCP is one of the postharvest agents used to delay spoilage, thus improving the shelf-life, quality, and flavor of many climacteric fruit and vegetables. However, it was found that 1-MCP can also show a significant preservative effect in inhibiting the development of physiological disorders, discoloration, senescence, and color change of some non-climacteric fruit and vegetables such as broccoli, eggplant, pitaya, and jujube (Massolo et al., 2011; Zhang et al., 2012b; Fernández-León et al., 2013; Li et al., 2016). According to studies by Chae et al. (2009), fruits treated with 1-MCP had significantly lower lipoxygenase activities and electrolyte losses during storage than the control group, showing the potential benefits of 1-MCP. The effect of 1-MCP on storage disorders depends on the cultivar, the presence or absence of ethylene, and the concentration used (Salvador et al., 2005). Many studies have shown that 1-MCP protects agricultural products, especially fruits, from exogenous and self-generated ethylene, extends postharvest shelf-life, and provides more flexibility in storage, distribution, and sale (Watkins and Miller, 2005; Watkins, 2008). 1-MCP is non-toxic, leaves little residue, and is active at very low concentrations. Due to its availability and stable complexed formulation that releases 1-MCP when dissolved in water, it has attracted research and commercial interest worldwide (Watkins and Miller, 2005). Although there are several reports in the literature on the response of horticultural crops to different concentrations of 1-MCP (Zhang et al., 2012a,2020; Jiang et al., 2018; Hussain et al., 2019; Li et al., 2021), the available information on the effects of 1-MCP on the changes that occur in volatile aroma compounds, antioxidant enzyme activities, and internal qualities of peppers during storage is scanty. Considering that aroma, color, texture, and nutritional properties are the most important factors affecting the overall quality of pepper fruits, this study aimed to evaluate the mechanisms of different concentrations of 1-MCP on the stability of nutritional values, antioxidant enzyme activities, and volatile compounds of “Hangjiao No.2” pepper during storage.

Capsicum (Capsicum annuum) is a non-climacteric fruit (Hou et al., 2018), while pepper has been shown to exhibit climacteric behavior (Paul et al., 2012; Hou et al., 2018; Dubey et al., 2019). Moreover, studies have shown that Capsicum peppers can exhibit both, making this classification inconclusive. Researchers have been studying the effects of 1-MCP on fruits and vegetables for decades, but the results are highly variable and often crop-specific. Several studies have been conducted on the use of 1-MCP in the storage and preservation of fruits and vegetables (Façanha et al., 2019; Massolo et al., 2019). The use of 1-MCP can minimize the need for low temperatures while extending shelf-life and maintaining quality. Many studies have also shown that 1-MCP treatment preserves the quality and extends the shelf-life of climacteric fruit such as “Bartlett” pear (Li et al., 2021), “Hayward” and “Qihong” kiwi (Park et al., 2014; Chai et al., 2021), strawberry (Yang et al., 2018), and tomato (Wang et al., 2016). However, no systematic, statistical analyses of the influence of different concentrations of 1-MCP on the nutritional properties, enzyme activities, and volatile compounds of “Hangjiao No.2” chili peppers during storage have been conducted. In this study, we focused on the effects of 1-MCP treatments on “Hangjiao No.2” chili peppers and specifically investigated which indicators were most affected by 1-MCP at different storage times at ambient temperature. It was also reported that 1-MCP treatment enhanced the activity of antioxidant enzymes in various plants such as broccoli florets (Yuan et al., 2010), coriander leaves (Hassan and Mahfouz, 2012), “Empire” apples (Fawbush et al., 2009), mangoes (Wang et al., 2009), and soybean plants (Djanaguiraman et al., 2011). The insignificant differences observed in MDA content between 1-MCP treatments after 3 days are in agreement with Hong et al. (2013), who discovered that 1-MCP prevented the increase in MDA content in Psidium guajava L. during early storage, but the effect was insignificant in the later days of storage. The activity of SOD varied among the different storage periods and 1-MCP treatments, as shown in Figure 1B. The results for SOD obtained in our study for pepper fruit are similar to the study on green asparagus exposed to 1-MCP during cold storage (Zhang et al., 2012b). The enzyme activities of POD and CAT increased before day 9 and then decreased at the end of storage regardless of the treatments, and the pepper samples treated with 0.5 μl L^–1 1-MCP had the highest value than the control and the treatment with 1.0 μl L^–1. The obtained results indicate that 1-MCP treatment can significantly increase the activities of POD and CAT in pepper fruits. The 0.5 μl L^–1 treatment was the most effective in increasing POD and CAT enzyme activities of pepper.

Vitamin C content in pepper can range from 43 mg 100 g^–1 to 247 mg 100 g^–1 of fresh fruits and can represent 50 to over 100% of the recommended daily intake (Wahyuni et al., 2013). The postharvest loss of nutritional values such as vitamin C can be substantial due to physical damage, relative humidity, temperature, light intensity, and prolonged storage time (Lee and Kader, 2000). Therefore, the decrease in vitamin C content observed in treated pepper fruit with increasing storage time is consistent with the fact that vitamin C, like any other antioxidant, is susceptible to degradation during the later storage period (Kalt, 2005). The highest vitamin C content in fruit treated with 1.0 μl L^–1 and 1.5 μl L^–1 1-MCP compared to the control and 0.5 μl L^–1 1-MCP treatments during storage suggests that higher concentration of 1-MCP may have a suppressive effect on peppers (Win et al., 2006). The highest vitamin C content obtained under the 1.5 μl L^–1 treatment during 12 days of storage was in agreement with the previous study by Jiang et al. (2018), who reported that a high concentration of 1-MCP maintained the vitamin C content of yardlong beans after 21 days of storage. According to Lee and Kader (2000), vitamin C is an antioxidant that reacts with and neutralizes singlet oxygen and other free radicals. Therefore, maintaining the vitamin C content of pepper fruits after harvest can improve fruit quality while reducing damage from reactive oxygen species. According to Barroso (2016), the average consumption of vegetables is 400 g day^–1 per person as stated by WHO. Therefore, the health risk of a person is estimated based on the amount of vegetable consumption of a person based on 400 g day^–1. Based on the analysis, “Hangjiao No. 2” chili peppers can be considered to be a good source of vitamin C. The acceptable daily intake of nitrate given by Gruszecka-Kosowska and Baran (2017) is 3.7 mg kg^–1. The average nitrate levels obtained in our study in the different treatments were far below the permissible nitrate levels. This could be due to the type of fertilizer used in the production process of pepper plants (Hsu et al., 2009). The application of 1-MCP treatments inhibited the decline in protein content more than the control, especially at 1.5 μl L^–1, which further retarded the decline in protein content at the end of storage by (34.22%) compared to the control (53.69%) (Table 1). These results support the findings that treatment with 1-MCP significantly retarded the activities associated with senescence such as protein and chlorophyll degradation in coriander leaves (Hassan and Mahfouz, 2012) and spinach leaves (Gergoff et al., 2010). The content of the total chlorophyll also decreased as storage progressed in this study. Moreover, 1-MCP at a concentration of 1.0 μl L^–1 further retarded the degradation of chlorophyll in pepper fruit compared to the control during storage. 1-MCP has been reported to retard chlorophyll degradation in broccoli florets (Fernández-León et al., 2013), S. melongena, L. (Massolo et al., 2011), Yardlong beans (Jiang et al., 2018), and strawberries (Li et al., 2016). This could be the reason for the observed delay in chlorophyll content in pepper fruit treated with 1.0 μl L^–1 during the storage period.

The aroma as a secondary metabolite is an important fruit property that is attracting the attention of a growing number of consumers and researchers. It is reported that pepper fruits contain over 200 volatile compounds (Kollmannsberger et al., 2011; Pino et al., 2011; Bogusz et al., 2012). The quality of a fruit’s aroma is determined by the type and concentration of volatiles that affects its organoleptic properties. Aromatic changes occur as a result of postharvest metabolic processes and can be influenced by a variety of pre- and postharvest factors (Sdiri et al., 2017; Fellman, 2019). The gas chromatography-mass spectrometry method was used to detect 88 volatile compounds in chili pepper “Hangjiao No.2” (Figure 2). Terpenes dominated in the pepper fruit, followed by aldehydes, esters, alkanes, alcohols, acids, phenol derivatives, ketones, and other compounds. During storage, the composition and concentration of volatile aroma compounds in the pepper fruit treated with the different treatments showed significantly different patterns with increasing storage time, especially the content of aldehydes, acids, esters, terpenes, and other aroma compounds decreased, while alcohols increased. Interestingly, alkanes showed an increasing or decreasing trend with advancing storage dates. From Figure 2, of the seven acids detected, acetic acid, oxalic acid, and hexanoic acid were the most abundant in “Hangjiao No.2” pepper fruit. Overall, the fruit treated with 1.0 μl L^–1 treatment exhibited higher concentrations of these volatile acids than the control pepper fruit throughout the storage period. According to Wang et al. (2011b), the most important volatiles in mature fruits is aldehydes. As the second most abundant volatiles in the aroma profile, aldehydes and alcohols are known to impart the “green” notes to the aroma of fruit and are present in a wide variety of plants (Jaeger et al., 2003). The results in Figure 2 show that pepper fruit treated with 1.0 μl L^–1 1-MCP had a higher production of aldehydes than the fruit treated with the other treatments, suggesting that this is a consequence of the treatment. (E)-2-hexenal, which has a strong influence on the aroma of sweet peppers (Eggink et al., 2012), and Shimatogarahi (Manikharda, 2018) was also predominant in “Hangjiao No. 2” fruit. The content of (E)-2-hexenal, which exhibits fresher notes, was higher in the 1.0 μl L^–1 treatments than in the other treatments in this study, indicating that the 1.0 μl L^–1 treated fruit might exhibit fresher notes compared to the control. The results for aldehydes are consistent with the findings of Zhang et al. (2009), who found that the concentration of aldehydes typically decreases as the fruit ripens. Esters, the most common type of volatile, are known to impart “sweet” and “fruity” characteristics to the fruit aroma (Wang et al., 2011a). In kiwifruit, 1-MCP affected the synthesis of esters, especially ethyl butanoate and methyl butanoate, which were found to be the major esters in “Hayward” and “Qihong.” The effects of 1-MCP on volatile esters have been previously documented for apples (Lurie, 2006) and pears (Li et al., 2016). The results of Huan et al. (2020) indicated that the majority of esters increased with the ripening of kiwifruit during storage at room temperature in the control group, but 1-MCP at a concentration of 0.5 and 1.0 μl L^–1 retarded the formation of esters in this study (Figure 2). The diverse varieties of esters in the volatile fraction of “Hangjiao No.2” was also reported in Malagueta and paprika peppers (Bogusz et al., 2012), Tabasco, Laotian, and Pebrera peppers (Rodríguez-Burruezo et al., 2010). Our results showed that all ester contents decreased or diminished under the different treatments during the storage period. A similar result was observed in apples, suggesting that the application of 1-MCP may lead to a decrease in ester production (Defilippi et al., 2005). A decrease in ester content was also observed in kiwifruit during cold storage (Günther et al., 2015). However, the treatment with 1.0 μl L^–1 1-MCP maintained a decrease compared to the other treatments in the present study (Figure 2). This indicates that the use of “Hangjiao No.2” treated fruit as a condiment can complement dishes due to the pleasant and fruity flavors resulting from its ester content. Terpenes are the most abundant and diverse bioactive compounds of plants in nature (Cox-Georgian et al., 2019). Terpenes are widely distributed in higher plants such as citrus, conifers, and eucalyptus and are detected in their leaves, flowers, stems, and roots (Ninkuu et al., 2021). According to Bueno et al. (2020) and Fischedick et al. (2010), terpene profile is influenced by a variety of factors such as genotypes, age of inflorescences, and environmental, production, and harvesting conditions. Although terpenes were most abundant in chili pepper “Hangjiao No.2” after harvest, their levels were relatively low compared to aldehydes in the present study, and their levels increased or diminished after treatment during the storage period, especially after day 3 of storage in the 1-MCP treatments compared to the control. 3-Carene has a pleasant aroma that translates as a rubbery odor and is common in a number of Capsicum cultivars (Oruña-Concha et al., 1998). The highest content of 3-carene, a monoterpene observed in the control, may contribute to the sweet and pungent odor with a woody character of “Hangjiao No. 2” fruit. Beta-element is one of the important terpenes produced by Jabuticabas, and it is extensively studied for its biological functions (Freitas et al., 2020). It is known for its fresh, herbaceous, and waxy aroma and ranked third among the emitted terpenes from “Hangjiao No. 2” peppers in this study. The content of terpenes was higher in the control than in the other treatments. Our results suggest that “Hangjiao No. 2” may be a natural source of β-elements, which can be used as a potential raw material for its extraction (Wang, 2012). A study by Zhu et al. (2015) revealed that nerolidol is one of the most aroma-active compounds contributing to the aroma profile of tea infusions as well as rose and apple aromas. Therefore, nerolidol could be an important contributor to the characteristic floral aroma note found in the “Hangjiao No. 2” fruit. The presence of alkanes was highlighted in Figure 2 and the relative content of alkanes showed considerable variation during the 12 days storage. Aliphatic alkanes as well as 2-methyl branched alkanes found in Shimatogarashi (Manikharda et al., 2018) and Cupsicum frutescens (Liu et al., 2009) domesticated in China were also present in “Hangjiao number No.2” in this study. 2-Methylpentadecane, heptadecane, 2-methyltridecane, and pentadecane were higher in “Hangjiao No. 2”. The levels of alkanes detected in this study initially increased and then declined or diminished with increasing storage time in all treated pepper fruit. The highest levels of these alkanes were detected in the treatment groups when treated with 1.0 μl L^–1 1-MCP. According to Rodríguez-Burruezo et al. (2010), aliphatic and methyl branched alkanes are associated with capsaicin biosynthesis, as they have been found in chili pepper species. As shown in Figure 2, “Hangjiao No.2” chili fruit started to form new alcohol on day 6 during the storage period. Beekwilder et al. (2004) stated that, the availability of alcohol, in particular, is considered as a barrier for the synthesis of esters. In this study, the 1-MCP treatments and the control significantly altered the alcohol emission of the pepper samples during the 12 days storage period. No common trend was observed for any of the detected alcohols, suggesting that these metabolites have different metabolic origins (Ortiz et al., 2010). Higher alcohol production was observed for some of the volatile compounds detected after 3 days of storage, particularly in pepper samples treated with 1.0 μl L^–1 1-MCP compared to 0.5 μl L^–1 treated fruit (Figure 2), and concomitantly, with higher production of esters. A similar result was reported that 1.0 μl L^–1 1-MCP treatment can lead to significant changes in the alcohol production of “Tardibelle” peach fruit during storage at 20°C (Ortiz et al., 2010). The aroma compounds of Tabasco pepper include some alcohols such as 3-methyl-1-pentanol, (E)- 3-hexen-1-ol, and 1-hexanol (Azcarate and Barringer, 2010), which were also detected in “Hangjiao No. 2” fruit. 1-hexanol and 3- methyl-1-pentanol are known for their green, grassy, fresh aroma (Azcarate and Barringer, 2010; Zhu et al., 2015). This suggests that the highest concentration of these compounds detected in 1.0 μl L^–1 of the treated “Hangjiao No. 2” fruit (Figure 2) is likely an important contributor to flavor. Bogusz et al. (2012) reported that α-ionone and β-ionone were detected as degradation products of carotenoids in Brazilian chili peppers with low aroma threshold and characterized as fruity or floral. However, only β-ionone was detected in the pepper “Hangjiao No.2” after harvest, and it decreased in all treatments during the storage period. According to Zimmermann and Schieberle (2000), β-ionone plays a significant role as an odor-active component in pepper odor, which is still developing at all three stages. This implies that the highest β-ionone content observed in the control treated fruit may contribute a fruity or floral aroma to “Hangjiao No.2”. The group of phenolic derivatives was identified by the presence of methyl salicylate, a compound found mainly in unripe S. lycopersicum L. (Lewinsohn et al., 2005). Although methyl salicylate was detected in “Hangjiao No.2” pepper fruit in all treatments throughout the storage period, its characteristic sweet aroma decreases with increasing storage time. However, the highest levels of this phenol ester were found in the 0.5 and 1.5 μl L^–1 1-MCP treated fruit at the end of storage. This means that the fruit treated with 0.5 and 1.5 μl L^–1 1-MCP could have a more pronounced sweet-green mixed aroma than the control and the treatments with 1.0 μl L^–1 1-MCP. In addition, 6 other volatiles were detected in the fruit of chili pepper “Hangjiao No.2” after harvest, including methyl benzhydroximate, 2-pentyl-thiophene, dihydro-5-pentyl-2(3H)-furanone, 2-methoxy-3-(1-methylpropyl)-pyrazine, m-xylene, and 2-methoxy-3-(2-methylpropyl)-pyrazine. Among these compounds, methyl benzhydroximate was the most abundant, followed by 2-methoxy-3-(2-methylpropyl)-pyrazine and m-xylene while 2-pentylthiophene, dihydro-5-pentyl-2(3H)-furanone and 2-methoxy-3-(1-methylpropyl)-pyrazine were present in small amounts. Most compounds belonging to this group were relatively stable on day 9 of storage, but decreased dramatically or disappeared after day 6 of storage. In addition, methyl benzhydroximate and 2-methoxy-3-(2-methylpropyl)-pyrazine appeared in 1.0 and 1.5 μl L^–1 of pepper fruit treated with 1-MCP throughout the storage period, but treatment with 0.5 μl L^–1 1-MCP inhibited the production of methyl benzhydroximate after treatment. Mazida et al. (2005) and Pino et al. (2006) found that methoxypyrazines are the major flavor components in green peppers and are responsible for its distinctive green aroma. Analysis of volatile aroma compounds of fresh chili (C. annuum) at different stages of maturity using solid phase microextraction (SPME) have also shown that methoxypyrazines, especially 2-methoxy-3-(2-methylpropyl) pyrazine, have a stronger influence on the quality and odor intensity of many varieties of C. annum, baccatum, and pubescens (Rodríguez-Burruezo et al., 2010). According to Mazida et al. (2005), 2-methoxy-3-(2-methylpropyl) pyrazine and hexanal decreased with fruit ripening during storage, and the color also changed. These results suggest that the further delay in the decrease of 2-methoxy-3-(2-methylpropyl) pyrazine observed in pepper fruit treated with 1.0 and 1.5 μl L^–1 1-MCP may be due to the fact that the treatments with 1-MCP at 1.0 and 1.5 μl L^–1 suppressed the degreening process with increasing storage time.

In this study, the positive effects of 1-MCP treatments on the nutritional properties of pepper fruit during storage at ambient temperature were found. We demonstrated that the application of 1-MCP inhibited the increase of nitrate content and delayed the degradation of chlorophyll, vitamin C, and protein of “Hangjiao No.2” peppers. In addition, 1-MCP had the greatest effect on the activities of antioxidant enzymes such as SOD, POD, and CAT and decreased the accumulation of MDA in pepper fruit. However, 1-MCP treatments did not have any effect on the shelf-life of this pepper cultivar during the 12 days storage period. The study also revealed that terpenes dominated the “Hangjiao No.2” chili fruit, followed by aldehydes, esters, alcohols, alkanes, acids, phenol derivatives, ketones, and other volatile compounds. During storage, significant changes were observed in the pepper samples treated with the different treatments with increasing storage time. The desired volatile compounds including aldehydes, esters, alkanes, and alcohol classes in the “Hangjiao No.2” chili fruit treated with 1.0 μl L^–1 1-MCP showed higher retention during the storage period as compared to the other treatments applied. 1-MCP at a concentration of 1.0 μl L^–1 was found to be the most suitable concentration of all the applied treatments. This study shows that the application of different treatments and storage times can affect the aroma composition, nutritional properties, and enzyme activities and provide important information about “Hangjiao No.2” that could be useful in promoting pepper fruit for future marketing and processing.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

EB and JZ: conceptualization. KH, YM, and YC: methodology. EB: software, formal analysis, investigation, data curation, writing—original draft preparation, and writing—review and editing. JX and JZ: validation, visualization, and supervision. JX: resources, project administration, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.