Sept. 1, 2021
Aug. 17, 2021
Due to the rising preference for organic food, it is becoming necessary to replace chemical coating on fruits with natural alternatives. Recent studies from South Africa have found a potential replacement for avocados, which could substantially reduce postharvest losses. A small handheld ethylene measuring device played a key role in this exciting new research.
Avocado (Persea americana Mill) fruits are mostly exported and transported long distances.
Avocado is a climacteric fruit that can be harvested before it is mature and allowed to ripen later, while it is being transported or stored. However, avocado suffers heavy post-harvest losses due to its high-metabolic rate.
Ethylene is a gas produced by the fruit, which causes ripening and results in respiration. The rate of ethylene production is higher in avocados than other climacteric fruits, hence its ripening and possible storage time are shorter. At later stages, it leads to loss of moisture and up to 90% reduction in weight.
Since weight is often the criteria for trade, there are huge economic implications involved in longer storage. Additionally, synthetic wax coatings used to reduce moisture loss are not acceptable for fruits meant for the European Union (EU) market.
Fruit decay triggered by mechanical damage, chilling injury, and pest attack is another challenge. Several fungi take advantage of damaged tissue to infect and cause decay. Chemical fungicides that could control these fungal infections have been found to be carcinogenic, and traces left on the fruit can be toxic and pose a health risk for consumers.
Avocado producers have been trying natural alternatives to both these chemical additives.
Edible, natural coatings made of soluble substances make it easy to get an even full coating of fruits. Carboxymethylcellulose (CMC) is one such water-soluble derivative made from cellulose. It acts as a barrier to the movement of water vapor and gases to prevent the fruit from drying up.
A recent breakthrough from a research group in South Africa could provide a simple solution to the problem of two common fungal diseases, anthracnose and stem-end rot, that occur in postharvest avocados.
A group of scientists, Tesfay et al., from the Postharvest Laboratory of the Department of Horticulture at the University of KwaZulu-Natal published their findings of a new coating mix made from moringa (Moringa oleifera) leaves in 2017.
Figure 1: Moringa extract reduced ethylene production and improved appearance in Hass. Images B, D, and F show the damage to fungal hyphae caused by moringa extract, Tesfay, et al. 2017. (Image credits: https://doi.org/10.1016/j.scienta.2017.08.047)
Moringa fruit and leaves are common vegetables widely used in tropical countries that are known to be rich sources of valuable nutrients like vitamins, β-carotene, phenolics, fatty acids, flavonoids, and other bioactive compounds.
It was the chlorogenic acid, quercetin, and kaempferol in the moringa that interested the scientists. They decided to test the methanolic and ethanolic extracts from both the moringa leaves (MLE) and moringa seeds (MSE) in a mixture of 1% CMC.
Hass and Gem were the two avocado species chosen for the experiment and coated with 1% CMC +2% of MLE or MSE, and the fifth set of control fruits had no coatings. All fruits were kept in controlled cold storage at 5.5°C and 95% relative humidity for 21 days. They were later stored at ambient room temperatures of 21 ± 1 °C and 60% relative humidity.
Measurements of changes in fruit quality parameters, such as firmness, and gas analysis of respiration (or production of carbon dioxide) and ethylene amounts were conducted on days 0, 7, 14, 21, and 28.
The instrument used for gas analysis was the F-950 Three Gas Analyzer, produced by Felix Instruments Applied Food Science. This is a small tool that weighs less than one kilo and can be used in a wide range of temperatures. It is rapid and gives accurate measurements within 30 seconds, and has a PolarCept!™ water filter to remove non-ethylene hydrocarbons. The F-950 can measure 0-200 ppm of ethylene at a resolution of 0.1ppm.
The scientists also tested the antifungal efficacy of moringa extracts through pathogenicity tests. There were two kinds of tests. One test occurred in Petri dishes in the laboratory. This test was an in vitro test of the fungus located on moringa extracts in a potato dextrose agarose medium. The other test was an in vivo test on the moringa coated avocado fruits. The disease incidence and severity were evaluated for selected strains of fungus of Colletotrichum gloeosporioides, Alternaria alternata, and Lasiodioplodia theobromea.
The scientists found that the coatings with moringa extract reduced the production of ethylene in comparison to the control, and the difference became stronger with passing time; see Figure 1. Hence, it effectively proved that the effect of moringa is long lasting. Of the two moringa treatments, MLE showed the best results and was significantly superior in action to MSE.
The MLE and MSE coatings also slowed the rate of respiration and produced less carbon dioxide in treated fruits, and the fruits were firmer than in control. Once again, MLE coatings were more effective than MSE extracts. Moreover, there was a significant reduction in the rate of ripening due to the reduced ethylene production. Hence, MLE also improved shelf life.
The in vitro tests showed that, of the three pathogens, the Colletotrichum gloeosporioides infection was the most severe and Alternaria alternata the least. It also showed that ethanolic extracts were more effective than methanolic tests in controlling them.
Electron microscopy analysis showed that all the moringa treatments cause damage to the hyphae of all fungi; see Figure 1. However, the in vitro laboratory tests showed that moringa extracts were effective in controlling C. gloeosporioides and A. alternata, but not Lasiodioplodia theobromea.
In vivo tests on the fruits showed that CMC+ ethanolic MLE coating could reduce both the incidence and severity of Colletotrichum spp. and Alternaria spp. MLE inhibited the occurrence of C. gloeosporioides and A. alternata by 43.6% and 42.9%, respectively, compared to untreated controls.
As a result of this experiment, the scientists recommend the use of this new combination of 1% CMC + 2% MLE as a potential multipurpose organic postharvest coating for avocados to improve quality, increase shelf-life, and control anthracnose and stem-end rot.
They recommended the new coating for further industry testing before being used commercially.
The same research group decided to see if they could further improve postharvest conditions for avocados to extend the storage time, improve fruit quality, and reduce decay due to postharvest pathogens.
This time they focused exclusively on ethylene control.
Since the ripening process is controlled by ethylene, reducing its level in the environment can control the speed of ripening to increase shelf-life. This would also prevent fruit softening and internal discoloration.
Figure 2: How ozone helps fruit quality preservation. (Image credits: Fresh Plaza.com, https://www.freshplaza.com/article/9100583/ozone-reduces-the-risk-of-foodborne-pathogens-in-overseas-shipments/)
Ozone is well known for degrading ethylene gas by oxidizing it. The gas has been declared safe and is also used by organic growers. Moreover, it has been proven to enhance storage conditions by killing pathogens; therefore, it is included in Controlled Atmosphere storage where gas levels are strictly monitored; see Figure 2.
The scientists decided to test ozone in combination with moringa leaf extract coatings.
They used the avocado variety Gem, which was coated with 1% CMC + moringa leaf extract, and compared it to the control with no coating. Ozone (0.25 ppm) was applied for 12 hours on days 0, 7, 14, and 21 for all fruits. Different combinations of ozone applications were tested.
Some fruits were exposed to ozone on two days: the 7th and 14th day, 7th and 21st day, or 14th and 21st day.
The three-day treatments tested were: 0, 7th, and 14th day; 0, 7th, and 21st day; and 7th, 14th, and 21st day.
Data was collected for all fruits for the quality parameters of firmness, sugar content, electrical conductivity (EC), volatile compound levels, and fruit weight loss. At the same time, gas analysis was also conducted with the F-950 Three Gas Analyzer.
Data was collected at the start and then on days 7, 14, 21, and 28.
There was also an ozone regulator and sensor to monitor and control ozone levels.
The scientists found that ethylene formation remained the same up to day 14 for all treatments of ozone and control, but later there was a sharp increase in ethylene production in control up to 11 mg/kg fruit, while ozone exposed fruits released only 4 mg/kg and 6 mg/kg fruit for three-day and two-day ozone exposure, respectively. There were no significant differences among the different date combinations in the three- and two-day ozone exposure sets. The presence of coatings had little effect.
Fruit loss patterns closely followed the pattern of ethylene production. There was only 2% fruit loss in three-day ozone treatments and 3% loss in two-day ozone-exposed fruits; whereas, controls lost about 7%. Again, MLE coating did not have much effect.
Firmness dropped in all fruits after 21 days, but there was a significant difference between ozone exposure and controls. Also, coated fruits exhibited more firmness than uncoated fruits.
Respiration rates were lower in ozone exposed fruits than controls, especially on the 21st
day, with fruits that were coated showing less respiration at all times. Similarly, EC was also less in coated and ozone exposed fruits.
Due to coating, the production of sugars, mainly D-mannoheptulose, improved consistently at all levels of ozone exposure.
Thus, using ozone to control ethylene was successful, especially in combination with moringa leaf extract.
Reducing mass loss, and improving shelf life, quality, and appearance of avocados can greatly increase returns on investment in avocado production. Additionally, these changes can result in less food loss and resultant carbon emissions. The F-950 Three Gas Analyzer’s gas analysis is precise enough to be used for data collection in scientific experiments, but it was designed for use in the field and throughout the supply chain. So, regardless of what standards take effect, stakeholders at all levels can use the F-950 with the assurance that they have state-of-art control over their postharvest environment.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
Feature image courtesy of Carlos Sillero
Kubheka, S., Tesfay, S., Mditshwa, A., & Magwaza, L. (2020). Evaluating the Efficacy of Edible Coatings Incorporated with Moringa Leaf Extract on Postharvest of ‘Maluma’ Avocado Fruit Quality and Its Biofungicidal Effect. (2020). HortScience 55(4):410–415. https://doi.org/10.21273/HORTSCI14391-19
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