Ethylene Measurement and Postharvest Pepper Storage Quality

Peppers, a popular vegetable, are rich in phytochemicals with antioxidant, antimicrobial, and anti-inflammatory properties. Peppers are also a source of red pigment capsicum, a high-quality food and textile dye. To serve their purpose, peppers need suitable postharvest treatment, storage, and packaging to maintain their quality.

Factors Affecting Pepper Quality

Peppers are harvested and eaten at green immature and red mature stages. Being perishable, peppers require careful postharvest handling to decrease losses and maintain nutritional quality.

It is possible to enhance shelf life and produce quality by managing storage temperature and humidity, treatments, ethylene levels, and using suitable packaging materials since they limit the physiological processes that cause shriveling, softening, and physiological disorders.

At low temperatures, respiration rate and ethylene production are decreased. Packaging material can reduce water loss, respiration rate, chilling injury, and physiological disorders. Chemical, physical and gaseous treatments can delay ripening and prevent microbial infections. However, identifying the exact conditions is crucial, and achieving the combination that produces the best results requires research.

Tests Conducted on Sweet Delilah

To improve the shelf life of a commercially valuable pepper cultivar, Sweet Delilah, a team of horticultural scientists Hamed, Bartolo, and Jayanty, tested many packaging films and 1-methylcyclopropene (1-MCP) treatment. Sweet Delilah peppers have high concentrations of flavonoids, phenolics, and ascorbic acid; the variety has a good flavor but a short shelf life.
The horticultural scientists chose to test 1-MCP since it is an ethylene perception inhibitor that binds to ethylene receptors. By preventing ethylene from acting, 1-MCP delays the ripening gas’s effects, like senescence, softening, and color and nutritional content change.

The four packaging materials tested were

• polypropylene flat bag (P12F)
• laminated poly-nylon (30NV)
• coextruded vacuum pouch (30NVC), and
• polyethylene (P15G)

The horticultural scientists measured the effect of combinations of packaging and storage conditions on weight loss, firmness, bioactive compounds, antioxidant activities, ascorbic acid, and respiration rates.

The vegetables were harvested at two stages green/immature and red/mature. Three peppers were packed into the four test packaging films and sealed. Three replicates of each packaging film were used. They were stored for 21 days at 7.2°C and 90% relative humidity (RH) to test quality. The control consisted of unpackaged peppers kept on a tray for comparison.

Three to five peppers were freeze-dried and ground for further analysis by wet chemistry to test concentrations of bioactive compounds and ascorbic acid. The scientists then used DPPH and ABTS assays to record antioxidant activities.

To test treatment, 60 nl L-1 of 1-MCP gas was introduced into a closed container where red peppers. Green peppers and control red peppers were not treated with 1-MCP. After 21 days of storage, red peppers were kept for four days at 15.5°C and 75% RH to simulate retailing conditions.
Quality parameters of weight loss, firmness, respiration, and ethylene were measured before the experiment and then after 7, 14, and 21 days of cool storage, and four days in retail conditions.

Weight loss was calculated by weighing the peppers. The firmness of red and green pepper was measured with a texture analyzer. The F-900 Portable Ethylene Analyzer, manufactured by Felix Instruments – Applied Food Science was used to estimate respiration rate and ethylene production. The tool can take rapid, non-destructive, and precise ethylene measurements and can be additionally equipped to measure oxygen and carbon dioxide. The device was selected for its precise usefulness in measure all three ripening gasses. The color was recorded with a chromameter.

Changes in Quality Parameters

The packaging films and 1-MCP combinations didn’t have the same effect on all quality parameters, so there was no single “winning combination,” but other interesting findings followed. 

Weight Loss

Figure 1: “Percent weight loss in Sweet Delilah at green stage (A) red stage (B) at 7.2 °C and 90 % humidity; (MC: marketing conditions after 4 days),” Hamed et al. 2021. (Image credits: DOI: 10.25177/JFST.6.2.RA.10754)

Red and green packaged peppers suffered less weight loss than unpackaged control peppers. However, there were differences between the protection provided by the films.

The lowest weight loss in green peppers was in packages with P15G (0.70%), followed by P30NVC (0.72%), P30NC (0.89%), and P12F (1.06%). So, the difference between P15G and P12F is significant. Control unpacked green peppers lost 3.37% weight after 21 days.

Red peppers lost more weight than green peppers, even inside packages. Here P30NC performed the best with 1.82% weight loss, followed by P15G (2.03%), P30NVC (2.35%), and P12F (2.28%), while the control red peppers lost 4.91%.
Weight loss occurs due to respiration, water loss, and evaporation, all of which depend on temperature and relative humidity. Moreover, the weight loss in red peppers was also due to the effect of the 1-MCP treatment.

Firmness

Changes in firmness can occur due to water loss as a result of transpiration and ripening. The red pepper showed significantly more loss of firmness than green peppers. However, the ethylene inhibitor 1-MCP did help in reducing firmness losses in red peppers.

Green peppers’ firmness losses were the least in P15G (3.11%), followed by P12F (4.84%), P30NVC (4.84%), and P30NV (4.87%). Control green peppers lost 12.1% firmness in the same time and storage conditions.

Control red peppers lost as much as 25.07% firmness. The 1-MCP treated red peppers packaged in P30NVC (13.04%) did best, followed by P15G (16.6%), P30NV (16.8%), and P12F (16.9%). So, the 1-MCP treated red peppers successfully retained their firmness even after 25 days of storage.

Respiration Rate and Ethylene Production

Figure 2: “Respiration rate in Sweet Delilah green stage (A) and red stage (B) at 7.2°C and 90% humidity. MC: marketing conditions (After 4 days),” Hamed et al. 2021. (Image credits: DOI: 10.25177/JFST.6.2.RA.10754)

Limiting respiration rate and ethylene production is one of the main aims of storage and packaging for postharvest fresh produce.

Unsurprisingly, the highest respiration rates were observed in control samples in both green and red peppers. In packaged green and red peppers, the respiration rate slowed down over time, see Figure 2.

Green peppers in P12F films had the lowest respiration rate after 21 days of storage. Red peppers treated with 1-MCP had the lowest respiration rate when packed in P30NVC, and were significantly lower than the respiration in untreated red peppers in P12 and P15G films.

Ethylene production was lowered in all packaged peppers and was the least (9.25 ppm kg-1
h-1) in red peppers treated with 1-MCP. The highest levels of ethylene of 13.6 ppm kg-1
h-1 were recorded in control peppers, at the end of the experiment, see Figure 3.

Figure 3: “Ethylene levels in Sweet Delilah at the red stage at 7.2°C and 90% humidity. Data are the mean of six replicates with standard deviation. MC: marketing conditions (After 4 days),” Hamed et al. 2021. (Image credits: DOI: 10.25177/JFST.6.2.RA.10754)

Color Changes and Pepper Quality

Peppers owe their ripe colors of yellow, orange, and red to flavonoids and biochemical synthesis. Thus, color is not only an indicator of maturity but also of internal nutritional quality.

The chroma values of green peppers decreased over time and those of red peppers increased. The packaged green peppers retained their color better than the control.

There was a color variation between control green peppers and packaged ones. Though the best results were in green pepper packaged in P30NVC (chroma values 20.31), there were no significant differences compared to peppers in other films, see Figure 3. 

Red peppers treated with 1-MCP saw delayed ripening and color changes during storage. However, the best color was in treated red peppers in P30NVC. Conversely, untreated peppers in P12F had the poorest results.

Figure 4: “Chroma values in Sweet Delilah at green stage (A) and red stage (B) at 7.2°C and 90% humidity. MC: marketing conditions (After 4 days),” Hamed et al. 2021. (Image credits: DOI: 10.25177/JFST.6.2.RA.10754)

Biochemical Changes

Control samples showed the greatest loss in phenolic content, flavonoids, and ascorbic acid.

Though total phenols were similar at the start of the experiment, at the end, the various films produced differences. Green peppers lost the least amount of phenols in P30NV (4.93%) and the most in controls (11.98%). Similarly, red peppers treated with ethylene inhibitor 1-MCP also did best in P30NVC.

P30NVC also resulted in the lowest levels of flavonoid loss in green peppers (3.79%), followed by P30NV, P12F, and P15G. In comparison, controls lost 7.77% of flavonoids.

Ascorbic acid, or vitamin C, is found in the skin of ripening fruits. The maximum decrease in vitamin C levels due to storage occurred in controls. Over time ascorbic acid levels fall even in packaged peppers, but the loss was least in peppers in 30NVC films.

Antioxidant Activities

Storage was not shown to change antioxidant activities in green peppers. Both assays showed no difference between control and packaged green peppers.

Red peppers had higher DPPH activity than green peppers due to the higher total phenolics and ascorbic acid content compared with green peppers. Control had 5.86% less ABTS activity after storage whereas red peppers in P15 suffered only a 1.9% loss. So, the 1-MCP treatment was again beneficial.

Treatment was a success

The films P15G and P12 F produced lower levels of respiration, ethylene, and weight loss. P30NVC was good at maintaining total phenols, total flavonoids, and ascorbic acid, while P12G film was effective in retaining antioxidant activities.

In contrast to these mixed results, the treatment of red peppers with ethylene inhibitor 1-MCP produced good overall results and extended shelf life by delaying weight loss and color change, while maintaining firmness. 1-MCP also maintains the nutritional value of red peppers by reducing the loss of phenols, flavonoids, and ascorbic acid. Thus, scientists recommended 1-MCP treatment for Sweet Delilah peppers. Since all packages gave better results than leaving red and green peppers open, the choice of packaging material will depend on the quality parameter that people want to enhance.

In studies like these, the ability to quickly and accurately measure ethylene, CO2, and O2 is vital. To learn more about the line of Felix Instruments devices used in this study, click here.

—

Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Read the original peer-reviewed paper here:
Hamed, M., Bartolo, M., & Jayanty, S.S. (2021). Studying the Influence of Storage Conditions, 1-MCP, and Packaging Films on Quality of Sweet Dalilah Green and Red Stage Peppers (Capsicum annuum L.). Journal of Food Science & Technology 6(2): 356-370.