Making Sense of the Magical, Mystical Melon

• Melons have been around for many millennia, and there are several varieties, both landraces, and new hybrids worldwide.
• Climacteric and non-climacteric ripening is seen in the various melon varieties.
• External inspection of sensory attributes like rind color or flavor development is not objective to decide ripeness or maturity.
• Recent applications of Near Infrared Spectroscopy to measure sugar, titrable acidity, and pH can improve the melon production.

The prolific melon varieties are evidence of the popularity of this ancient fruit. Applying modern technology, like Near Infrared (NIR) Spectroscopy, can fine-tune the production of these fruits to better meet consumer tastes. Commercial NIR spectrometers allow people to test melons non-destructively to accurately monitor internal fruit quality, which is impossible through manual external inspection of fruits.

Melon Varieties

Figure 1: Melons (Cucumis melo) come in many shapes, sizes, and colors. Manchali et al. 2021. (Image credits: https://doi.org/10.3390/plants10091755)

Melons, also called muskmelons or Cucumis melo, belong to the family Cucurbitaceae and have been famous for their taste and health benefits for millennia. The nutritious fruits are a rich source of vitamins A & C, minerals, fiber, and polyphenols with antioxidant and anti-inflammatory properties. Melons also have a high water content (about 90%), are low in calories (about 45 per NLEA serving), carbohydrates (1% daily value), and proteins (1% daily value).

What types of melons are there?

Though the most popular melon varieties are Honeydew and Cantaloupe, there are seven major melon cultivar groups:

1. Inodorus melons, also called Winter Melons, are large and smooth-skinned, with white flesh. Examples include honeydew, Persian melons, and casaba.

2. Cantalupensis melons, like North American cantaloupe, have a rough rind and sweet orange fruit flesh.

3. Reticulatus melons, such as muskmelons, have a netted rind.

4. Flexuosus melons can grow to be one meter long and have an acidic flavor.

5. Conomon melons, known as Asian pickling melons, are oval shaped with greenish, striped flesh.

6. Chito melons are a small “mango melon” with whitish flesh.

7. Dudaim melons are fragrant but inedible and often used as ornamental plants.

Crossbreeding between melon cultivars produces a wide array of attributes, including

• Size

• Shape

• Rind type

• External color

• Flesh color

• Texture

• Sweetness

Generally, the flesh is sweet or mild, with a possible musky flavor. The skin and flesh color can be whitish, green, yellow, or orange. There are scores of other melon varieties and landraces on the market, originating from various parts of the globe.

The post-harvest storage time will also depend on genetics. So, melon varieties need customized handling during storage and transportation.

Where did melons originate?

Though determining the specific origins of any commodity is often difficult and hotly debated, likely regions of melon’s origin include India, Central Asia, and Egypt.

The earliest melon seeds on record date back to 3000 B.C. and were found in Iran and China. Some consider melon to have travelled across Europe initially from Persia, modern-day Iran. Others think Asia, especially India, could be the place of origin because of its high level of melon diversity, from which it is thought to have spread to Africa. Egypt, Greece, and Italy also have records of melon cultivation from the third millennium B.C.

It is also possible that melons were domesticated twice independently, leading to multiple points of origin.

Based on the presence of wild and semi-wild melons and continental drift, scientists consider peninsular India and south-eastern Africa could be the place of origin for muskmelon. The current varieties of muskmelon are thought to originate from another species, C. metuliferus, in Africa.

The wide distribution of melons could also be due to natural dispersal by animals and humans.

Regardless of their origin, melons have evolved over time and currently have broad genetic, morphological, and biochemical diversity.

How are melons grown?

Melon is a crop that likes warm growing conditions, can’t withstand cold, and does well in tropical to sub-tropical regions.

Melons are annuals, meaning they grow and complete their entire lifecycle in a single year. Like all annuals, they are herbaceous and have little to no woody parts–even the stem is soft, flexible, and green.

Due to their long, thin, and weak stems, melons cannot grow upright, making the plants either “creepers” or “climbers.” Creepers remain on the ground, and the vines creep outward, covering the ground like pumpkins. Climbers have additional structures like tendrils, which they use to latch onto external support and manage to grow vertically, see Figure 2.

Botanically a berry, melons are monoecious, supporting male and female flowers on the same plant, and can weigh between 700g and 4kg.

Growing Season

The length of the growing season depends on genetics, and there are three groups- early, mid-season, and late varieties.

• Early melons need 85 days to grow and reach maturity. These include varieties of the hybrid honeydew ‘Bodacious’ or heirlooms like muskmelon ‘Minnesota Midget’ and cantaloupe ‘Noir des Carmes,’ to name a few.

• Mid-season melons need 85-95 days and include varieties like ‘Banana,’ a cantaloupe.

• Late varieties, such as Honeydew Orange and Crenshaw–a cross between casaba and cantaloupe–need 100 days or more to reach maturity. Santa Claus melons have a long storage life and are often eaten in late December for winter holidays.

Figure 2: “Melon cultivation under polyhouse conditions,” Manchali, et al. 2021. (Image credits: https://doi.org/10.3390/plants10091755)

Melons are grown in the open and in greenhouses, see Figure 2. Grafting improves the vigor of the plants and produces higher yields. Even fruit attributes can be enhanced- firmness is increased, extending the shelf-life of melons.

How do melons ripen?

After flower set and pollination, the fruit initially develops to grow in size. In the last stage, ripening changes the fruit composition, appearance, and texture to make it edible. Ripening makes fruits softer, sweeter, and changes their color.

Melons are unusual in that they show a wide range of ripening strategies. Some varieties such as cantalupensis, are climacteric, wherein ethylene is involved in the ripening process. Others, like those in the inodorous group, are non-climacteric, meaning no additional ripening will occur post-harvest.

Ethylene, the phytohormone, triggers ripening in climacteric fruits and is accompanied by a rise in respiration rate. Climacteric fruits accumulate starch. During ripening, starch is converted to sugar, accompanied by the synthesis of organic acids and volatiles that provide flavor. During this time, the cell wall degrades, softening the mesocarp to change the texture of fruits. At the same time, chlorophyll degrades, and flavonoids and carotenoids accumulate, changing skin and flesh color.

Unlike other climacteric fruits, in melon, the accumulation of pigments, sugar, and flesh softening is independent of ethylene. Therefore, the species is interesting for scientists who consider this could be an alternative ripening pathway in climacteric fruits, where ethylene-dependent and ethylene-independent pathways exist.

In non-climacteric melons, the ripening process occurs with the help of different hormones, but not ethylene.

Climacteric melons develop more aromatic compounds and orange flesh, but their shelf life is lower due to ethylene-induced softening. Non-climacteric melons, on the other hand, have greenish flesh, less aromatic content, and longer shelf-life.

There is a competition for assimilates as fruits develop. The fruits are larger, sweeter, firmer, and have heavier seeds in plants with a reduced fruit load.

Melons can last ten days without cooling in tropical conditions, but cut fruits deteriorate faster. Cut fruits can stay good for four days when stored in a controlled atmosphere.

Judging Melon Ripeness

Historically, ripeness judgement of the climacteric cantalupensis group was made by assessing the development of a musky odor. Ripeness in non-climacteric honeydew, on the other hand was traditionally identified in the field by a shift toward yellow in color profile.

Unfortunately, trying to judge internal quality traits based on external changes can be subjective, inconsistent, and tedious.

Given the complex ripening processes and the differences in ripe melon appearance in the different varieties, there is a need for convenient, objective measurement of internal quality based on biochemistry to monitor quality and ripeness.

New Methods

A 2010 study compared non-destructive techniques for internal quality assessment of melons. The methods evaluated were acoustic technology, NIR spectroscopy, dynamic technology, X-ray and computed tomography, and electrical and magnetic technology.

The analysis provided by vis-NIR spectroscopy was judged to be optimal, especially for on-line estimation of internal quality. Advancements in NIRS technology in the 12 years since this study make it an even greater asset to stakeholders throughout the supply chain.

Key Melon Quality Traits

The most common quality parameters studied in melons are sugars, pH, titratable acidity, internal defect, and maturity.

• Sugars or soluble solids content (SSC) is the parameter that is applied most widely in quality assessment and expressed as ºBrix. Varieties are differentiated based on their ºBrix. The lowest acceptable ºBrix in melons is 9%. Prior to the advent of NIR tools, ºBrix was measured by destructive refractometers.

• Titratable acidity measures the organic acid content of the fruits- citric, ascorbic, malic, and succinic acid levels. Titratable acids were earlier measured in the laboratory. As fruits ripen, the acidity decreases, but some acidity is required to ensure the fruits meet consumer taste preferences. Sugars and acidity, in tandem, are necessary to produce the flavor consumers like most.

• Volatiles that give melon flavor were earlier quantified by gas chromatography.

• Defects like “water-soaking” that occur in cantaloupe can also be detected by NIR-spectroscopy. Water soaking is a physiological disorder occurring in the final stages of fruit development and is not connected to ethylene. The cell walls are altered, and more water is moved into these parts of the fruits. The cause of water-soaking is not definite. It could be due to calcium deficiency or competition for resources in plants with many fruits. Traditionally, it was possible to detect only by cutting the fruit open.

Figure 3: “A Felix Instruments Melon Quality Meter scanning a cantaloupe for ºBrix value.”

The new F-751 Melon Quality Meter is a tool that can conveniently and accurately estimate ºBrix without any destruction of fruit. Due to ºBrix’s common use as a ripeness and maturity metric, it can help growers fix harvest dates and make optimal, data-driven decisions. Growers can upload information on fruit quality and use Felix Instruments FruitMaps application to get a spatial view of the maturity of their crops.

The tool has advanced chemometrics and models to analyze several varieties of melon, including the popular Cantaloupe and Honeydew.

Monitoring ripeness is also helpful for suppliers, packers of cut fruits, and distributors to keep track of melons’ health, discard spoilt melons and segregate fruits based on their stage of ripeness. Retailers can evaluate the quality of incoming fruits quickly based on NIR spectroscopy.

Monitoring quality with NIR spectroscopy at various stages of the supply chain helps stakeholders supply melons with a high-quality flavor profile. Reducing the risk of rejection due to bad quality can also reduce food loss and increase ROI.

Where are melons produced?

Continuing high demand for melons means growers and suppliers need robust tools to keep up with quality assessment.

Total world production of melons has been rising steadily from 25.7 M metric tons in 2015. In the year 2020, melon growers around the world produced an all-time high of 28.467 million (M) metric tons of melons.

China was the largest melon producer, growing 13.838 M metric tons from 385,756 hectares. Turkey was the second largest producer, with 1.7 M metric tons from 76,129 hectares, and India was third, with 1.3 M metric tons from 59,000 hectares. Other major melon producers include Iran, Kazakhstan, Guatemala, Brazil, Morocco, France, Egypt, and Spain.

As of 2021, China was still the largest producer of muskmelons, growing 13.8M metric tons of the fruit, however, very little of this production was designated for export. The largest muskmelon exporter in 2021 was Spain, with exports worth approximately $370M.

The US is a significant producer of muskmelon, cantaloupes, and honeydew, but since it is the largest consumer of these fruits, it is also a top importer. In 2021, the U.S. imported muskmelons worth $ 345M. In the U.S., melons are grown in the southern states, such as Arizona, California, Texas, Florida, and Georgia. They harvest melons starting from April through December.

Warm and dry weather due to climate change is playing havoc with production quantities in some parts of the world, like Europe, driving up prices by as much as 30%. Since melon demand is stable, using new technology can improve food security.

—

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

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