Medicinal Plant Extracts for Post-harvest Fresh Produce Preservation

• Medicinal plants have phytochemicals and antioxidants that could have applications for fresh produce preservation instead of chemicals.
• Research findings show that medicinal plant extracts can improve several physical and chemical quality attributes.
• Medicinal plant extracts reduce disease incidence and severity of pest attacks by direct action against causal agents or by strengthening the plant defense system.
• The effectiveness of treatments depends on the specific interaction of plant extracts, concentrations, species, application methods, and environmental factors.

Postharvest fresh produce suffers physiological and biochemical changes and spoilage during storage, leading to economic losses. Medicinal plant extracts could be alternatives to synthetic chemicals for postharvest protection and preservation of fresh produce quality. This article lists the major medicinal plant extracts’ applications for preserving fresh produce discussed by Nxumalo et al. 2021.

Fresh Produce Preservation to Prevent Spoilage

Horticultural crops are highly perishable, undergoing physiological and biochemical changes that lead to undesirable disorders and quality degradation. In Africa, postharvest losses of fruits and vegetables can be as high as 70%, and globally, quantitative food losses and wastes for fruits and vegetables alone are estimated to be around 40–50%.

Physical damage, poor handling, inadequate transportation and storage, poor packaging, and postharvest pathogens contribute to these losses. Fungal infections, including Rhizoctonia solani, Alternaria alternata, Colletotrichum gloeosporioid, Penicillium digitatum, and Botrytis cineria, cause postharvest losses during transit and storage.

Synthetic fungicides are currently the most effective method to control postharvest pathogens. Still, their use is facing increasing restrictions internationally due to health concerns and consumer demand for safe and cost-effective natural alternatives.

Natural Alternatives

Using medicinal plant extracts presents a potential alternative to synthetic chemicals, especially in the postharvest phase of horticultural crops.

The rich diversity of phytochemicals and antioxidants found in various parts of medicinal plants can contribute to the development of effective and sustainable strategies for crop protection. These bioactive compounds may possess antimicrobial, anti-inflammatory, and antioxidant properties that could help mitigate postharvest losses due to pests, diseases, and oxidative stress.

The historical use of medicinal plants in traditional medicine across different cultures suggests that they have a long-standing reputation for their therapeutic properties. However, conducting thorough phytochemical analysis, assessing biological activities, and evaluating safety parameters are crucial steps in bridging this gap and establishing the efficacy of medicinal plants in crop protection and preservation.

Researchers have gained scientific insights into the potential of medicinal plants as alternatives to synthetic pesticides and fungicides after conducting thorough phytochemical analysis, biological activities against pathogens and pests, and safety evaluation.

The many ways medicinal plant extracts can help preserve fresh produce are discussed below.

Medicinal Plant Extracts Improve Quality

The various medicinal plant extracts can directly improve fresh produce quality parameters like ethylene production, moisture content, weight, color, sweetness, taste, and nutritional value, increasing consumer acceptance.

Ethylene Production

Plant extract treatments create a modified atmosphere around the fruit, which reduces ethylene synthesis and slows ripening and decay. For example, Aloe vera gel (AVG) coating on nectarines stored at 0 ± 0.5°C and around 90% RH for six weeks significantly reduced ethylene production compared to control fruits. Control fruits had more ethylene (6 µmol kg−1 h−1) than AVG-coated fruits (2 µmol kg−1 h−1), which didn’t produce any ethylene up to 6 d of fruit ripening.

Lower ethylene levels in coated fruits were due to the increased respiration rate of control fruits during storage, which was 41% higher than AVG-coated fruits after eight days of ripening.

A mixture of Aloe vera and basil seed mucilage also reduced ethylene levels in apricot and cherry laurel fruits.

Moisture Loss

Moisture loss occurs due to metabolic activities like fruit respiration and transpiration.

Medicinal extract coatings created a modified atmosphere around fruits that slowed metabolism and decay rate. Lowering respiration and transpiration rates also helps fruits retain their moisture content better. AVG-coated nectarines had 65% less moisture loss than control nectarines.

Zingiber officinale (ginger), Aloe vera, and Allium sativum (garlic) mixed in edible coating reduced moisture loss in guavas after 15 days of storage in ambient conditions.

Coated fruits stored at a lower temperature suffer lesser moisture loss than those in ambient conditions, as in neem leaf extract-coated apples.

Loss in Weight

Medicinal plant extracts have positive effects in reducing weight loss and maintaining the quality of fruits during storage by reducing loss of moisture, ethylene production, and respiration rate.

• A coating mixture of ginger extract, gum arabic, sativum, and aloe coating reduced weight loss (22.36%) compared to the control (28%) in guava.
• Lemon verbena bio-extract reduced weight loss in strawberries stored at 4°C, with the lowest loss observed at 600 mg L−1.
• Aloe vera gel coating that lowered moisture loss also helped weight retention in nectarines after six weeks.

Firmness

Natural extracts and coatings can be viable alternatives to synthetic fungicides for preserving fruit firmness. One mechanism that suggests this is achieved is by reducing disease susceptibility. Treatments with neem, aloe vera, thyme oil, and chitosan have demonstrated notable effects on fruit firmness.

• Neem leaf extracts treated with ‘Starking Delicious’ Apples retain more firmness than controls after 180 days in cold storage.
• Aloe vera gel-coated nectarines were 30% firmer than control fruits. Interaction between treatment and ripening time significantly affected fruit firmness.
• Thyme oil and chitosan treatment in avocados showed more firmness and was more effective than chemical treatments like prochloraz fungicide.

Color Development

The effect of medicinal treatments on the color development of fruits can be different.

Plant extracts influence fresh produce color development and retention by delaying ripening by creating a semi-permeable atmosphere on the fruit surface, modifying the fruit’s internal O2 and CO2 concentrations.

• Thus, thyme oil and chitosan treatment helped avocados retain greenness, while uncoated fruits lost their hue.
• However, Aloe vera gel treatment did not influence the color of cherry laurel fruits (Prunus laurocerasus L.), similar to uncoated ones. However, modified atmosphere packaging (MAP) showed better color retention. The best color development of red was seen when AVG and MAP were combined.
• AVG and marigold flower extract seem to have different effects on the color development of various fruits. Coatings for fresh produce stored at 14°C (55% RH) for 45 days produced the best skin color retention in apples, followed by tomatoes, but papaya and chikoo showed slightly dull colors. Bananas show 50% brown color development. The internal pulp color was good (100%) in apples and tomatoes, 75% in papaya, 50% in chikoos, and 25% in bananas.

Information on the specific interaction of plant extracts and species is necessary for potential applications of these treatments for maintaining or enhancing the color quality of fruits during storage.

Total Soluble Solids

Plant extract treatments improved sweetness or total soluble solids (TSS) in climacteric fruits due to the hydrolysis of starch into sugars during ripening. Storage was an essential factor that influenced results.

Effects of wild-sage, bitter leaf, and lemon verbena extracts on stored fruits’ TSS have been tested.

• Wild-sage extract and bitter leaf increased TSS values in mango fruits stored at ambient temperatures over a 16-day storage period. The TSS values were high- 17.2, 17.47, and 17.93° Brix for wild-sage, wild-sage, and bitter leaf extract, respectively.
• In the case of lemon verbena extract for the strawberry fruits stored at 4°C, various concentrations of the extract and storage time interacted to influence TSS values. The highest TSS was obtained with an extract of E600 mg L−1 concentration after 8 and 12 days of storage. The TSS values increased from 2.60 to 5.00 during the 12 days of storage.
• Garlic extract and gum arabic initially resulted in lower guava sugars than controls. However, after 15 days of storage, the sugar levels were higher in treated fruits.

Titrable Acidity Content

Plant extract treatments retain fresh produce’s titrable acidity (TA) by slowing the fruits’ metabolism, inhibiting ripening senescence, and converting starch to sugars. Uncoated controls that experience higher metabolism show a faster loss of TA. The TA levels, especially in ratio to TSS, are vital in deciding taste. Consumers prefer a balance to too much TSS.

Aloe vera-treated nectarine (29%) had significantly higher TA than control (0.45%) once ripe. The ratio of TSS: TA was more elevated in controls since treated fruits since the acids underwent oxidation and had more TSS than treated fruits.

Antioxidant Content

Treatments involving natural extracts and coatings can positively influence the antioxidant content of fruits, potentially enhancing their shelf life and nutritional quality during storage. Increased antioxidants also improve fruit defense against diseases. There are several explanations for how plant extracts enhance antioxidant content in fruits. One of them states that plant extracts inhibit ethylene production that delays flavonoids, total phenolic content, and anthocyanin accumulation, so they are not degraded during storage.

• Treatment with lemon verbena bio-extract increased total flavonoid and phenolic content during storage at 4°C with 90–95% RH for 12 days in strawberries. Lemon verbena essential oil and cold storage improved raspberry fruits’ total flavonoid content (see Figure 1). Lemon verbena bio-extract prevented the activity of enzymes that decompose anthocyanins in litchi fruits.
• Guava fruits coated with garlic extract and gum Arabic showed a significant increase in total flavonoid content compared to the control after 15 days of storage at room temperature.
• However, Aloe vera gel coating and cold storage for 42 days maintained lower levels of total antioxidants than uncoated fruit, possibly due to increased enzyme activity of ascorbic acid oxidase, cytochrome oxidase, and peroxidase.

Figure 1: “Lemon verbena (Lippia citrodora) essential oil effects on antioxidant capacity and phytochemical content of raspberry (Rubus ulmifolius subsp. sanctus),” Ishkeh et al. (2019). (Image credits: https://doi.org/10.1016/j.scienta.2018.12.040)

Physiological Disorders

Plant extracts can also prevent chilling injury by maintaining cell wall integrity and increasing antioxidants. Skin browning was reduced since the coatings barrier led to less oxidation and loss of phenolic acids.

Combining gum arabic, oleic acid, and cinnamon oil that forms an edible coating prevents chilling injury and skin browning in guavas. It’s essential to note that the effectiveness of such coatings may depend on various factors, including concentrations, application methods, and the specific characteristics of the fruit being treated.

Plant Extracts Against Diseases

Plant extracts with antimicrobial compounds reduce disease incidence, acting directly against pathogens and strengthening plants’ defense against microbes. The best remedies were identified by comparing the effects of one or more plant extracts in various fruits.

1. Ruta chalepensis (fringed rue), 50% showed the most significant reduction in the severity of soft-rot in mango, compared to Eucalyptus globulus (eucalyptus), Vernonia amygdalina (bitter leaf) and Datura stramonium (jimsonweed).
2. Aloe vera alone or with thymol (99.5%) reduces fungal infection by stolonifer, B. cinerea, or P. digitatum in nectarine.
3. Phyllanthus niruri (gulf leaf-flower) in edible coating followed by turmeric treatment control postharvest anthracnose in dragon fruits even after 28 days.
4. Chitosan or Aloe vera gel alone or mixed with thyme oil reduced anthracnose severity in avocados in ambient temperatures of 20°C.
5. Azadirachta indica (neem) and 4% Citrus sinensis (citric) extract were the most efficient treatments against anthracnose in berries stored at ambient temperatures, compared to garlic, Copaifera langsdorfii (diesel tree), Cinnamomum zeylanicum (cinnamon) or Eugenia caryophyllata (clove) extracts.
6. Shallot (Allium cepa × Allium sativum) leaf extracts as dips for bananas controlled crown-rot disease by 86% compared to controls after 12 days of storage at room temperature and humidity.

Plant Extracts Against Pests

Natural pesticides can improve food production since 50% of food is lost to pest and insect attacks. The additional benefit is preventing insecticide resistance since plant extracts contain several active ingredients, which do not persist long in the environment. Some successful plant extract remedies are listed below.

1. Lippia javanica (fever tea) leaf powder extract and Solanum delagoense (bitter apple) ripe fruit pulp were excellent at controlling aphids and tomato spider mites. Fever tea and bitter apple reduced mite numbers by 86% and 75% and aphids by 83% and 75%, respectively.
2. Jimsonweed, bitter apple, and Bobgunnia madagascariensis (snake bean plant) were all effective against ladybird beetles’ attack on radishes, spinach, cabbage, and tomatoes, based on a lethal dose at 50%. Since jimsonweed and bitter apple extracts are safer, they are recommended over snake bean plant extract for pest control and use in integrated pest management programs.
3. Tephrosia vogelii (fish bean) kills more green bean aphid (Aphis fabae) than candida. The fish bean plant has active components that have insecticidal, acaricidal, and ichthyotoxic effects since they are ovicidal, anti-feedant, and a stomach poison for insects.

Natural Pesticide Action Is Specific

There is no doubt that medicinal plant extracts can be a safe alternative to chemicals to enhance fruit quality, fight diseases and pests, and improve fruit and vegetable production. However, the above review shows that generalizing effects is impossible. So, research is needed to find plant extracts suitable for each species and a particular objective.

Scientists could greatly benefit from non-destructive, rapid, and precise data collection and analysis of fresh produce external and internal qualities on the move in the field or laboratories. Devices like the F-750 Produce Quality Meter from Felix Instruments Applied Food Science that use near-infrared spectroscopy to estimate color, TSS, TA, and dry matter for all fresh produce can be an asset during research. The company also supplies devices customized for mangos, avocados, kiwifruits, and melon in its F-751 series. This research can contribute to sustainable horticultural crop protection and preservation.

Sources

Ishkeh, S. R., Asghari, M., Shirzad, H., Alirezalu, A., & Ghasemi, G. (2019). Lemon verbena (Lippia citrodora) essential oil effects on antioxidant capacity and phytochemical content of raspberry (Rubus ulmifolius subsp. sanctus). Scientia horticulturae, 248, 297-304.

Khaliq, G., Ramzan, M., & Baloch, A.H. (2019). Effect of Aloe vera gel coating enriched with Fagonia indica plant extract on physicochemical and antioxidant activity of sapodilla fruit during postharvest storage. Food Chem., 286, 346–353

Nxumalo, K.A., Aremu, A.O., & Fawole, O.A. (2021). Potentials of Medicinal Plant Extracts as an Alternative to Synthetic Chemicals in Postharvest Protection and Preservation of Horticultural Crops: A Review. Sustainability,13, 5897. https://doi.org/10.3390/su13115897