The Storage Ability of Broccoli after 1-Methylcyclopropene Treatment

The main commercial attributes of broccoli (Brassica oleracea L. var. italica) are green color and compactness of the flowering head. Softening and discoloration occur quickly after harvest, during storage at high temperatures such as 20 °C. Senescence symptoms result from chlorophyll breakdown (Gong & Mattheis 2003). At low temperatures (0 °C), the deterioration is delayed, and storage life is extended (Page et al. 2001). A decline in quality at low temperatures is generally limited by rotting and loss of compactness of the heads. Discoloration of stem scars also occurs during storage. These scars are formed where the leaves are cut off from the fleshy stem. During storage, these scars discolor, lowering the appearance of the shoot (DeEll et al. 2001). The storage length of broccoli was determined by Cantwell and Suslow (1997) to be 21–28 d at 0 °C, 14 d at 5 °C, and 5 d at 10 °C. Ethylene, both endogenous and exogenous, affects the loss of chlorophyll and, thus, the yellowing and aging of broccoli (Tian et al. 1994; Cantwell and Suslow 1997; Suzuki et al. 2004). Postharvest broccoli treatment with 1-methylcyclopropene (1-MCP) has been shown to reduce respiration and ethylene production, delaying greening reduction and broccoli senescence (Fan & Mattheis 2000). 1-methylcyclopropene (1-MCP) reduces enzyme activity responsible for ethylene synthesis and the expression of the enzymes’ genes (Ma et al. 2009). According to Wills et al. (1999), reducing ethylene concentration from 0.1 to <0.005 cm³·m⁻³ at least doubled the shelf life of broccoli at 20 and 5 °C. The authors suggested that the threshold level of ethylene action on non-climacteric horticulture produce is below 0.005 cm³·m⁻³. The effective concentration of 1-MCP for broccoli is 0.1–10 cm³·m⁻³ (Yuan et al. 2010). Broccoli branchlets after treatment with 1-MCP at the dose of 1.0 cm³·m⁻³ for 14 h at 20 °C showed delaying of yellowing during shelf life at 20 °C in dark conditions (Gong & Mattheis 2003). Ku and Wills (1999), comparing the effects of different doses of 1-MCP (0.02–50 cm³·m⁻³) and treatment times (2–6 h), found that the higher the concentration and longer the time, the greater the storage ability of broccoli heads was. Treatment with 2.5 cm³·m⁻³ for six h at 20 °C) extended shelf life at 20 °C from 2.5 to 5 d. In addition to delaying postharvest deterioration and chlorophyll degradation, the treatment inhibited the degradation of carotenoids, ascorbic acid, and glucosinolates during storage (Yuan et al. 2010). Xu et al. (2016) confirmed that the above treatment improves the shelf life of broccoli florets at 15 °C due to the maintenance of higher levels of chlorophyll and sugars. Zsom et al. (2020) recommended a low concentration of 1-MCP (0.625 cm³·m⁻³) for 24-h treatment. This treatment protects against the negative effects of ethylene applied at a concentration of 2.0 cm³·m⁻³ for 9 days at 5 °C. Broccoli florets stored at 10 °C after treatment with 1-MCP at 12 cm³·m⁻³, for 16 h, at 20 °C showed a 20% longer shelf life than untreated florets (Able et al. 2002). Also, in a study by Forney et al. (2003), treatment with 1.0 cm³·m⁻³ for 14 h at 10 °C delayed yellowing, and during 5 d of storage at 10 °C, treated florets maintained their green color, while untreated ones turned yellow. The storage period at 5 °C increased by 5–7 d of treated broccoli, and the percentage of commercial heads rose from 0 to 72–100 during simulated retail display (7 °C) for up to 2 weeks (Ekman et al. 2019). Wichrowska et al. (2021) confirmed that this treatment affects the maintenance of color and firmness during storage at 4 °C. However, the authors hypothesized that the response depends on the cultivar, and not all tend to store better after 1-MCP treatment.

There are a lot of reports on the postharvest treatment of broccoli with 1-MCP, which is usually applied at relatively high temperatures, and broccoli stored for a short time at high temperatures as well. This study aimed to test the effect of treating cooled broccoli with 1-MCP at 5 °C for 20 h and then storing them at 0–1 °C. The idea was to cool the broccoli as soon as possible after harvest, treat them with low temperature, and keep them under optimal conditions (0–1 °C and 98–100% relative humidity).

According to storage recommendations (Cantwell & Suslow 1997; Pogson & Morris 1997; Toivonen & Forney 2016), the broccoli was placed in a cold chamber and cooled overnight at 5 °C immediately after harvest. The 1-MCP treatment was applied at the earliest possible time, i.e., on the second day after harvest (as suggested by Able et al. 2002). Weight losses after 30 d at 0–1 °C were small and ranged from 0.66–0.73% (0.66% – control, 0.73% – 1.0 cm³·m⁻³ 1-MCP, and 0.67–3.0 cm³·m⁻³ 1-MCP). During the next 30 d, the losses increased slightly, respectively: 1.08% for control, 1.01% for broccoli treated with 1.0 cm³·m⁻³ 1-MCP, and 1.04% – 3.0 cm³·m⁻³ 1-MCP. Low weight loss was indicative of the lack of wilting and maintenance of good compactness of the heads. As in the study of Ekman et al. (2019), no significant differences in weight loss were found between 1-MCP treated and untreated broccoli.

After 30 d of storage, broccoli crowns, both treated and untreated, represented excellent visual quality. They retained deep green color and good compactness, and only very small black spots were noted on a few crowns (Table 1). In the study by Pogson and Morris (1997), broccoli also maintained perfect color during 35 d at 1 °C and showed only a slight loss of chlorophyll. In contrast, Sabir (2012) found slight crown yellowing after 28 d at 0 °C. Treatment with 1-MCP did not affect quality parameters. In the presented study, no bud opening was observed at all. The fleshy stem was more deteriorated than the crown due to the darkening of the scarred surface when the leaves were cut off. This darkening negatively affected the appearance of the broccoli and reduced its commercial value (Table 1). However, the suitability for consumption was still high, as the stem, after peeling, presented well. A practical solution to avoid this problem is to leave 2–5 cm stumps of petioles on the stem for cold storage. The stumps would be trimmed at the stem only when preparing the material for trade, making the new cut areas fresh and green.

By extending the storage period by another 30 d, the quality of broccoli deteriorated. The most instant contributor to this was rotting (Table 2). A slight loosening of the crowns and the beginnings of their yellowing also contributed to the decline in quality. Makhlouf et al. (1989) noted severe rotting and unpleasant odours after six weeks of broccoli storage at 1 °C. In contrast, Pogson and Morris (1997), after 70 d at 1 °C, found a significant loss of chlorophyll (25%) and signs of head yellowing. In the presented study, the untreated broccoli significantly softened and yellowed more than those treated with 1-MCP. In the quality of the crowns, there was a trend toward higher rating for broccoli treated with 1-MCP. In the case of the stems, those treated with 1-MCP maintained a significantly better appearance. In addition to the stronger darkening of the surface of the scars, the bases of the petioles remaining on stems showed signs of flesh yellowing and senescence (Table 2). For 60 days at 0–1 °C, broccoli, both treated and untreated with 1-MCP, on average, still maintained the quality above a threshold of commercial suitability. It should be considered that this was possible due to the use of a low storage temperature of 0–1 °C. Treatment with 1-MCP had only a minor effect on the storage ability of broccoli (Table 2). Also, according to Able et al. (2002), storage temperature can extend the shelf life of broccoli to a great extent, and this effect is much more significant than that induced by 1-MCP treatment.

Under retail conditions, which occurred after 30 d of refrigerated storage, weight loss increased quite rapidly, despite the heads being covered with PE film. After two days, losses were 0.25% for all objects, 4 d from 0.60 to 0.64%, and 6 d from 1.04 to 1.09%. No significant differences were found between broccoli treated with 1-MCP and untreated. For two days, the heads continued to look very good, their color remained unchanged, and only a slight decrease in compactness and very little rotting was observed. A slightly worse appearance presented in the stems due to the darkening of the scar areas of the leaves, which was already visible after refrigerated storage. After another two days, the crowns were found to be slightly discolored, compactness decreased somewhat, and rotting increased significantly. The quality of the crowns reached a fairly good level, but the shoots were only satisfactory.

The bases of the trimmed leaves on the control broccoli shoots showed deepened signs of yellowing and even abscission from the shoot. The appearance of the control shoots was significantly worse than those treated with 1-MCP. During the next two days, the crowns of the broccoli turned intensely yellow, but this process was particularly fast in the untreated objects (Table 3–6). In treated objects, yellowing also occurred but at a much slower rate, and there was a trend that yellowing was slowest in broccoli treated with 3.0 cm³·m⁻³ 1-MCP. This is partially consistent with the findings of Yuan et al. (2010) that significant differences in color and visual quality were noted after 1 day at 20 °C following refrigerated storage. In the current study, the crowns loosening and rotting proceeded similarly in both treated and untreated broccoli (Table 4–5). Also, according to Ekman et al. (2019), the effect of treatment on the development of rotting is small, and no differences in weight loss are found between treated and untreated broccoli.

The overall quality of the crowns was excellent after 2 d at 15 °C, but in the following days it decreased very fast. After 4 d, they reached quality close to good, but after 6 days, only broccoli treated with 3.0 cm³·m⁻³ 1-MCP was rated slightly above commercial suitability (Table 6). Generally, the fast yellowing and quality decreasing during shelf life are in line with the findings of Tian et al. (1994), which also claimed that in high temperatures, broccoli florets senesce rapidly, which is typical for crops harvested before a full mature stage.

After six days, both crowns and shoots looked worst in the untreated broccoli (Table 6). An opposite conclusion was reached by Fernandez-Leon et al. (2013). In their study, 1-MCP had the effect of maintaining better color in broccoli, but only during refrigerated storage at 1–2 °C, up to 27 d. The authors claimed that using 1-MCP reduced the loss of chlorophyll pigments, but only during cold storage; during shelf life at 20 °C, yellowing developed faster on treated than on control broccoli. It seems that the response of broccoli to 1-MCP may still depend on many factors such as cultivar, maturity stage, application method, and the length of time between harvest and treatment, as previously reported by Watkins (2006, 2008). In contrast, Ma et al. (2010) confirmed that the yellowing process is suppressed by the treatment with 5.0 cm³·m⁻³ 1-MCP at 20 °C.

The intensity of broccoli respiration is expressed as the amount of CO₂ released. It was significantly higher in broccoli transferred after refrigerated storage to 10 °C than those transferred to 5 °C (Fig. 1). The same relationship was found earlier by Cantwell and Suslow (1997). These authors also showed that the respiration intensity at 10 °C is more than double that at 5 °C. After one day of acclimatization at 5 °C, the highest respiration intensity was displayed by untreated broccoli. After three days, the respiration intensity in all combinations decreased and equalized for treated and untreated broccoli. In a study by Forney et al. (2003), respiration rates decreased during five days of storage at 12 °C. In the current study, the next three days brought no significant changes in the intensity of broccoli respiration. At 10 °C, treated and untreated broccoli respired with similar intensity, and on days 3 and 6 there was a rising trend in this intensity. Thus, in the present study, there was no effect of 1-MCP treatment on the respiration intensity of broccoli (Fig. 1). There was also no increase in respiration intensity for broccoli that turned yellow faster. Likewise, Pogson and Morris (1997) found that respiration intensity is not necessarily related to chlorophyll breakdown, and, thus yellowing and aging of the broccoli. However, most researchers claim that 1-MCP inhibits the increase in respiration rate (Fan & Mattheis 2000; Sabir 2012; Thi Hand Phuong et al. 2022). In the current study, the effect of 1-MCP treatment on respiration intensity can be suppressed by low storage temperature, which is the main factor limiting biochemical reactions in plant tissue.

The broccoli in this study generated very low amounts of ethylene, and despite the lack of statistical differences, slightly less at 5 °C than at 10 °C. It is noteworthy that at both temperatures, there was a tendency for greater ethylene production by broccoli treated with 1-MCP than untreated. The obtained results contrast with the statement of Wills et al. (1999) and Thi Hand Phuong et al. (2022), and are consistent with Porter et al. (2005), who claimed that treatment of Chinese cabbage with 1.0 cm³·m⁻³ 1-MCP stimulated ethylene more than 0.01 cm³·m⁻³ 1-MCP. According to Lomaniec et al. (2003), 1-MCP can increase ethylene biosynthesis in parsley. In the current study, broccoli that released more ethylene turned yellow and aged at retail at a slightly slower rate than broccoli that produced less gas. This indicates that alternative factors interfere with the effect of 1-MCP on ethylene production and plant tissue yellowing. It can be hypothesized that other phytohormones may play an important role in the yellowing and aging of broccoli during storage. Tian et al. (1995) have previously demonstrated that cytokinins cause stimulation of ethylene production on the one hand and simultaneous retardation of broccoli yellowing on the other hand. In addition, the response of plant tissue to hormones depends on the number of hormone receptors, and it is possible, as previously stated by Sisler and Serek (1997), that ethylene receptors become active again or new ones are formed during extended storage. This research needs to be continued to clarify the above hypotheses.

Figure 1.

Figure 2.

The results obtained in this research led to the conclusion that the use of 1-MCP to improve the storability of broccoli had only marginal benefits. There was a significant delay in yellowing when the heads were moved from refrigerated conditions (30 d at 0–1 °C) to a higher temperature of 15 °C. 1-MCP treatment delay fleshy stem scar discoloration and senescence during long refrigerated storage and subsequent shelf life. There was no reduction in respiration and ethylene production in broccoli treated with 1-MCP, during shelf life at 5 and 10 °C after 30 d of refrigerated storage.