Mung bean

Morphology

Mung bean (Vigna radiata) is a plant species of Fabaceae and is also known as green gram.^[10] It is sometimes confused with black gram (Vigna mungo) for their similar morphology, though they are two different species.^[11] The green gram is an annual vine with yellow flowers and fuzzy brown pods. There are three subgroups of Vigna radiata, including one cultivated (Vigna radiata subsp. radiata) and two wild ones (Vigna radiata subsp. sublobata and Vigna radiata subsp. glabra). It has a height of about 15–125 cm (5.9–49.2 in).^[12]

Mung bean has a well-developed root system. The lateral roots are many and slender, with root nodules grown.^[13] Stems are much branched, sometimes twining at the tips. Young stems are purple or green, and mature stems are grayish-yellow or brown. They can be divided into erect cespitose, semi-trailing and trailing types.^[13] Wild types tend to be prostrate while cultivated types are more erect.^[12]

Leaves are ovoid or broad-ovoid, cotyledons die after emergence, and ternate leaves are produced on two single leaves. The leaves are 6–12 cm long and 5–10 cm wide. Racemes with yellow flowers are borne in the axils and tips of the leaves, with 10–25 flowers per pedicel, self-pollinated. The fruits are elongated cylindrical or flat cylindrical pods, usually 30–50 per plant. The pods are 5–10 cm long and 0.4–0.6 cm wide and contain 12–14 septum-separated seeds, which can be either cylindrical or spherical in shape, and green, yellow, brown, or blue in color.^[13] Seed colors and presence or absence of a rough layer are used to distinguish different types of mung bean.^[12]

Growth stages

Germination is typically within 4–5 days, but the actual rate varies according to the amount of moisture introduced during the germination stage.^[14] It is epigeal, with the stem and cotyledons emerging from the seedbed.^[15]

After germination, the seed splits, and a soft, whitish root grows. Mung bean sprouts are harvested during this stage. If not harvested, it develops a root system, then a green stem which contains two leaves and shoots up from the soil. After that, seed pods begin to form on its branches, with 10–15 seeds contained in each pod.^[14]

The maturation can take up to 60 days. Once matured, it can reach up to 30 inches (76 cm) tall, with multiple branches with seed pods. Most of the seed pods become darker, while some remain green.^[14]

Nitrogen fixation and cover crop

As a legume plant, mung bean is in symbiotic association with Rhizobia which enables it to fix atmospheric nitrogen (58–109 kg per ha mung bean). It can provide large amounts of biomass (7.16 t biomass/ha) and nitrogen to the soil (ranging from 30 to 251 kg/ha).^[11] The nitrogen fixation ability not only enables it to meet its own nitrogen requirement, but also benefits the succeeding crops. It can be used as a cover crop before or after cereal crops in rotation, which makes a good green manure.^[11]

Varieties

The mung bean varieties now are mainly targeted in resistance to pests and diseases, particularly the bean weevil and mung bean yellow mosaic virus (MYMV). For now, the main varieties include Samrat, IPM2-3, SML 668 and Meha in India; Crystal, Jade-AU, Celera-AU，Satin II，Regur in Australia; Zhonglv No. 1, Zhonglv No. 2, Jilv No. 2, Jilv No. 7, Weilv No. 4, Jihong 9218, Jihong 8937, Bao 876-16, Bao 8824-17 in China. Also, with the help of the World Vegetable Center, the traits of mung bean have been considerably improved.^{[17][18][19][20]}

'Summer Moong' is a short-duration mung bean pulse crop grown in northern India. Due to its short duration, it can fit well in-between of many cropping systems. It is mainly cultivated in East and Southeast Asia and the Indian subcontinent. It is considered to be the hardiest of all pulse crops and requires a hot climate for germination and growth.

Climate and soil requirements

Mung bean is a warm-season and frost-intolerant plant. Mung bean is suitable for being planted in temperate, sub-tropical and tropical regions. The most suitable temperature for mung bean's germination and growth is 15–18 °C (59–64 °F). Mung bean has high adaptability to various soil types, while the best pH of the soil is between 6.2 and 7.2. Mung bean is a short-day plant and long days will delay its flowering and podding.^[21][22]

Harvest

The yield potential of mung bean is around 2.5 to 3.0 t/ha, however, usually due to the resistance to environmental stress and improper management, the average productivity for mung bean is only 0.5 t/ha. Due to the indeterminate flowering habit of mung bean, when facing proper environmental conditions, there can be both flowers and pods in one mung bean plant, which makes it difficult to harvest it. The perfect harvesting stage is when 90% of the pods' colour in one yield has been black. Mung beans can use a harvester for harvesting. It is important to set up the header in case of over-threshing.^[23][24]

Transportation and storage condition

The perfect moisture of grain for transportation is 13%. Before storage, the cleaning and grading process must be done. The ideal storage condition should keep the mung bean's moisture at exactly 12%.^[23][24]

Pests, diseases and abiotic stress

Most of the mung bean cultivars have a yield potential of 1.8–2.5 tons/ha. However, the actual average productivity of mung bean hovers around 0.5–0.7 t/ha. Several factors constrain its yield, including biotic stresses (pests and diseases) and abiotic stresses.^[25] Stresses not only decrease productivity but also affect the physical quality of seeds, making them unusable or unfit for human consumption. All the stresses collectively can lead to significant yield losses of up to 10–100%.^[25]

Market

Mung bean plants have a long history of being consumed by humans. The main consumed parts are the seeds and sprouts. The mature seeds provide an invaluable source of digestible protein for humans in places where meat is lacking or where people are mostly vegetarian.^[41] Mung bean has a large market in Asia (India, Southeast Asia and East Asia) and is also consumed in Southern Europe and in the Southern US.^[11] Mung bean protein is considered safe as a novel food (NF) pursuant to Regulation (EU) 2015/2283.^[42] The consumption of mung bean varies depending on the geographic region. For instance, in India, mung bean is used in sweets, snacks and savoury items.^[43] In other parts of Asia, it is used in cakes, sprouts, noodles and soups. In Europe and America, it is mainly used as fresh bean sprouts. The consumption of mung beans as such in the US is in the order of 22–29 g/capita per year,^[44] while the consumption in some areas of Asia can be as high as 2 kg/capita per year.^[45]

Mung bean is considered an alternative crop in many regions, which is generally preferable to sign a contract for the growing process before planting. In the US, the average price of mung bean is around $0.20 per pound. This is double the price of soybeans. The difference in production costs for mung bean and soybean is due to post-harvest cleaning and/or transportation. Overall, mung bean is considered to have market potential for its drought tolerance, and it is a food crop and not a feed crop, which can help buffer the economic risk from variability in commodity crop prices for farmers.^[44]

Nutritional value

The mung bean is recognized for its high nutritive value. A mung bean contains about 55–65% carbohydrate (equal to 630 g/kg dry weight) and are rich in protein, vitamins and minerals.^[48] It is composed of about 20–50% protein of total dry weight, among which globulin (60%) and albumin (25%) are the primary storage proteins (see table). The mung bean is considered to be a substantive source of dietary proteins. The proteolytic cleavage of these proteins is even higher during sprouting. Mung bean carbohydrates are easily digestible, which causes less flatulence in humans compared to other forms of legumes. Both seeds and sprouts of the mung bean produce lower calories compared to other cereals, which makes it a more attractive bean to obese and diabetic individuals.^[10]

Culinary

Whole cooked mung beans are generally prepared from dried beans by boiling until they are soft. Mung beans are light yellow in colour when their skins are removed.^[2] Mung bean paste can be made by hulling, cooking, and pulverizing the beans to a dry paste.^[2]

Bean sprouts

Mung beans are germinated by leaving them in water for four hours of daytime light and spending the rest of the day in the dark. Mung bean sprouts can be grown under artificial light for four hours over the period of a week. They are usually simply called "bean sprouts". However, when bean sprouts are called for in recipes, it generally refers to mung bean or soybean sprouts.

Mung bean sprouts are stir-fried as a Chinese vegetable accompaniment to a meal, usually with garlic, ginger, spring onions, or pieces of salted dried fish to add flavour. Uncooked bean sprouts are used in filling for Vietnamese spring rolls, as well as a garnish for phở. They are a major ingredient in a variety of Malaysian and Peranakan cuisine, including char kway teow, hokkien mee, mee rebus, and pasembor.

In Korea, slightly cooked mung bean sprouts, called sukjunamul (Korean: 숙주나물), are often served as a side dish. They are blanched (placed into boiling water for less than a minute), immediately cooled in cold water, and mixed with sesame oil, garlic, salt, and often other ingredients.

In the Philippines, mung bean sprouts are called togue and are most commonly used in lumpia rolls called lumpiang togue.^[55][56]

In India, mung bean sprouts are cooked with green chili, garlic, and other spices.

In Indonesia the food are often used as fillings like tahu isi (stuffed tofu) and complementary ingredient in many dishes such as rawon and soto.

In Japan, the sprouts are called moyashi.

Starch

Mung bean starch, which is extracted from ground mung beans, is used to make transparent cellophane noodles (also known as bean thread noodles, bean threads, glass noodles, fensi (粉絲), tung hoon (冬粉), miến, bún tàu, or bún tào). Cellophane noodles become soft and slippery when they are soaked in hot water. A variation of cellophane noodles, called mung bean sheets or green bean sheets, are also available.

In Korea, a jelly called nokdumuk (Korean: 녹두묵; also called cheongpomuk, 청포묵) is made from mung bean starch; a similar jelly, colored yellow with the addition of gardenia coloring, is called hwangpomuk (황포묵).

In northern China, mung bean jelly is called liangfen (涼粉; 'chilled bean jelly'), which is a very popular food during summer. The Hokkiens add sugar to mung bean jelly to make it a dessert called Lio̍k-tāu hún-kóe (綠豆粉粿; 'mung bean flour cake').

Plant-based protein

Mung beans are increasingly used in plant-based meat and egg alternatives such as Beyond Meat and Eat Just's Just Egg.^[57]