Fruit ripening happens at the tail end of fruit maturation process. It involves a series of enzyme-controlled activities that converts the hard-mealy fruit into a soft, juicy, flavorful and nutritious food. The fruit ripening process can be manipulated by use of both chemical and biological aids that hasten the fruit ripening process. Biological processes include the use of ripe fruits, cassia leaves, and albizia leaves while chemical aids include glycol, ethylene and calcium carbide. In Kenya, the demand for ripe fruits, especially bananas and mangoes, has risen over the recent past enticing the traders to look for alternative means of meeting the demand. To ensure a constant supply of ripe fruits during seasons of low production, unscrupulous traders use calcium carbide, a chemical compound that releases acetylene when hydrated. Acetylene gas is the chemical analogue of ethylene gas, which is naturally found in fruits and has been proven to accelerate the fruit ripening process. The use of this chemical in the artificial ripening of fruits has come under sharp criticism following its association with arsenic, which is a known carcinogen. Acetylene gas has also been associated with hypoxia and pneumoconiosis. Calcium carbide can also cause ulceration of mucus membranes and even induce serious dizziness. Despite being a banned chemical substance in foods, calcium carbide is still widely being used (illegally) in Kenya and other developing countries contrary to the legislation. This widespread use is attributed to the ease of obtaining it, relatively cheap cost, and slack implementation of the law.
Calcium carbide, climacteric fruits, pneumoconiosis
Fruits contribute a large portion of human diet. In fact, following the recent woes in the public health sector and an alarming spike in lifestyle diseases linked to diet, many people are turning to healthy options (WHO and FAO, 2017).
This means that fruits and vegetables are getting more attention than ever before. The increased attention to fruits has made fruit vendors to seek alternative means, unscrupulous even, of bridging the gap to ensure that they satisfy consumer demands.
What does the ripening process entail?
Ripening is a genetically controlled process that involves conversion of the fruit components into highly nutritious elements. For instance, the skin of mangoes and bananas change color from green to yellow and the starchy endocarp softens as the fiber is broken down to simple sugars(Gupta, 2017).
There are also flavor changes that make the banana or mango emit flavor compounds often associated with ripening fruits. There is always concentration of sugars, which mask the acid in these fruits making the fruit generally sweeter to taste.
If uncontrolled, the ripening process continues as the sugars are further broken down to very simple components, the fats decompose to release fatty acids and the proteins release their amino acids (Ibid). This would result in total rotting of the entire fruit.
The origins of accelerated fruit ripening process
Several biological and chemical methods have been proven to aid the fruit ripening process. For starters, biological methods have been used to aid fruit ripening for many centuries. Early Egyptians used fig leaves to accelerate the ripening process. Ancient Chinese put a ripe mango in a container full of green bananas somehow triggered and accelerated the ripening process.
The story was not different in Europe. European farmers learned that one rotten apple caused spoilage for the entire consignment hence the popular saying, “one rotten apple spoils the bunch.” In many parts of Africa and South America, cassia leaves and albizia leaves have been used for a very long time to accelerate the fruit ripening process.
Chemical ripening agents for accelerated fruit ripening
One problem with the biological methods is that they take longer time to achieve full effect. This is contrary to chemical methods that can produce results in as little as 24 hours. Chemical ripening methods have the advantages of portability and quick action, which make them preferable over the biological methods that are bulky and slow acting (Fattah and Ali, 2012). The chemicals are also easy to apply and one does not have to worry about storage space.
Some of the most common chemical ripening agents include glycol, ethylene, and calcium carbide. Ethylene is one of the safest chemical ripening agents that can be applied on fruits meant for the mass market. It is approved by the Food and Drug Administration (FDA) as a safe artificial ripening agent and is commercially available from many outlets (FDA, 2016). However, the chemical is quite expensive and many small-scale traders are not willing to spend a lot of money on installing ethylene ripening equipment (Hossain, Akhtar, and Anwar, 2015).
How much does calcium carbide cost?
Fruits are susceptible to spoilage and damage during transportation, especially when they are ripe. For this reason, they are harvested and transported to the destination market in raw form after which they are artificially ripened using calcium carbide.
Calcium carbide elicits similar effects in fruits as ethylene would only that it is cheaper than ethylene. This makes it a prime candidate for use in fruit ripening. In Kenya, the vice is engraved in the thriving fruit markets of Nairobi and other major markets around the country. According to media reports, the chemical is readily available from backstreet shops and costs about KSh. 50 for about 50 grams sachet(Koros, 2014).
Dangers of using calcium carbide in fruit ripening
The use of calcium carbide has been banned in many countries around the world (Asif, 2012). This follows study findings that have shown the negative effects of the use of the chemical on health. Calcium carbide has been found to contain traces of arsenic, a known carcinogenic compound. Its use predisposes the consumer to the risk of cancer, which is a serious disease of global health concern.
In Kenya alone, cancer was responsible for 29,000 deaths in 2012, which proves just how serious the scourge of cancer has become (Korir et al., 2015). The cancer problem continues to rise due to the numerous predisposing factors that beset most Kenyans.
Additionally, calcium carbide acts by releasing acetylene gas, which acts in a similar manner as ethylene. Acetylene is an industrial welding gas that has been proven to have a negative effect on mental development. It deprives the brain of oxygen leading to hypoxia.
Exposure to high levels of acetylene in the fruit ripening room can lead to mentally degenerative conditions like Alzheimer’s disease (Siddiqui and Dhua, 2010). The condition can be fatal in small children.
Other negative effects of the use of calcium carbide include ulceration when the chemical comes into contact with the mucus membranes of the body or wet skin. Pregnant women can experience dizziness and people with compromised immunity are vulnerable to the effects of this chemicals.
Climacteric and non-climacteric fruits
According to their ripening characteristics, fruits can be classified as either climacteric or non-climacteric. Climacteric fruits enter the climacteric phase after harvesting. They respond to ethylene and continue to produce ethylene in increasingly larger quantities as the ripening continues, (Amarakoon, Illeperuma, and Sarananda, 1999).
Such fruits are often harvested while still green so that they can be ripened near the consumption point. This is done to avoid damage to the fruits because ripe fruits are soft and susceptible to spoilage while on transit. Examples of climacteric fruits include mangoes, plums, apples, apricots, and guavas (Ibid).
On the other hand, non-climacteric fruits respond only slowly to ethylene treatment after harvesting. They do not show marked ripening after the fruit has been harvested from the tree. Such fruits are easy to handle post-harvest and do not experience marked damage during transportation. Examples include lemons, strawberry, oranges, litchi, and pomegranate.
Fruit development and ripening process
In a young seeded fruit, the seeds produce cytokinins that promote cell division leading to thickening of the fruit wall. They then start producing gibberellic acid to promote rapid thickening of the fruit.
The fruit development is terminated when the fruit reaches full size and the plant produces abscicic acid, which makes the seed dormant and deters it from sprouting while still in the fresh fruit. Seedless fruits are treated with dilute solution of gibberellic acid to promote thickening during fruit development (Mattoo and Pech, 2014).
Once the fruit reaches maturity, fruit ripening process begins. It is a complex process that is controlled by the genes and mediated by the enzymes in the fruit itself. Beginning with a hard, green, mealy, sour fruit, the ripening enzymes begin to effect changes on the fruit. The series of enzymes involved in the fruit ripening process take cue from a burst of ethylene produced at the onset of fruit senescence (Ibid).
The role of ethylene in fruit ripening
Ethylene is a natural gas produced by rapidly growing regions of a plant. Wounded fruits also produce ethylene, which explains why putting an injured fruit together with wholesome fruits will lead to faster ripening of the entire consignment (Pech et al., 2013). It is synthesized from the amino acid methionine in the presence adenosine methionine synthetase, a process that requires energy.
This converts methionine into an intermediate, adenosyl methionine, which further requires amino cyclopropane synthetase to produce amino Cyclopropane carboxylic acid (ACC). ACC is then oxidized in the presence of ACC oxidase enzyme to produce ethylene (Mattoo and Pech, 2014). Once produced, ethylene “switches on” the genes that control the sequencing of enzymes involved in the ripening process.
Changes observed during fruit ripening
Fruit ripening is almost always associated with color change. Chlorophyll is broken down and, in some cases, new pigments are formed. This causes color change from green to the rich variety of colors that characterize fruits. Key enzymes involved in the production of pigmentation are phenyl alanine ammonia lyase and flavone synthase that aid in the synthesis of anthocyanin (Deguchi et al., 2013).
In addition to color change, acids are broken down to near neutral PH, which reduces the tart in the fruit. Amylases take cue and digest starch into simple sugars, which increases the sweetness of the fruit and promotes the juiciness of the fruit by enhancing osmosis.
Pectinases and cellulases degrade the pectin and cellulose in the fruit making them slide past each other. This increases the softness of the fruit. In the very last stage, volatile compounds are formed, which increases the aroma of the ripened fruit as they volatilize (Pech et al., 2013).
Since these processes are mediated by enzymes, the ripening process can be controlled by manipulating the enzyme activity. Subjecting the ripening fruit to low temperatures below the optimal conditions for enzyme activity arrests the ripening process by inactivating the enzymes (Ahmad et al., 2001). Therefore, by putting the fruit in a refrigerator, one can prolong the shelf life of the fruit. It is possible to keep the fruit at a certain level of ripeness under refrigeration.
Calcium carbide and fruit ripening
Calcium carbide is a chemical compound that is industrially produced to be used in the metal industry and in fertilizer production. It is made by heating coke under very high temperatures above 2000°C in the presence of lime (Asif, 2012). The reaction takes place as illustrated by the chemical equation below.
CaO + 3 C → CaC 2 + CO
The roasting process produces calcium carbide and carbon monoxide. Pure calcium carbide is a colorless, odorless, crystal compound. According to material safety data sheet, calcium carbide has not been classified as a hazardous material when swallowed or when it comes into contact with the skin. However, it has been proven to cause drowsiness and dizziness. Chronic exposure may lead to development of pneumoconiosis (CHEMWATCH, 2011).
Despite there being little evidence to show the potency of calcium carbide as a chemical hazard on its own, the real danger lies on the impurities and products of the chemical. Calcium carbide that is found in the local market has a purity of 85 percent (Asif, 2012).
The other proportion contains the impurities that make the chemical very potent. According to Kjuus, Andersen, and Langard (1986), technical grade calcium carbide contains traces of arsenic, which is a known carcinogenic compound. In their study, they found that exposure to this chemical among mine workers led to higher incidences of cancer among the subject group.
In other studies, acetylene gas produced by calcium carbide, which is a welding gas used in the metal industry has been found to have a negative effect on mental health (Hossain, Akhtar, and Anwar, 2015; Siddiqui and Dhua, 2010). In the presence of moisture, calcium carbide produces acetylene gas and calcium hydroxide. The reaction is illustrated by the equation below.
CaC2 + 2 H2O → C2H2 + Ca(OH)2
Artificial ripening by calcium carbide
The acetylene gas produced through hydration of calcium carbide is a chemical analog of ethylene gas. It imitates the properties of ethylene gas and is able to produce similar effects in fruits. Very small doses of the gas are capable of triggering the sequence that leads to fruit ripening. Studies by Amarakoon, Illeperuma, and Sarananda (1999) shows that only five grams of calcium carbide is required to elicit ripening in 100 kilograms of mangoes.
The traders usually wrap the required amount of hydrated calcium carbide in a piece of paper and drop the wrapped sample in the ripening box. Once the reaction starts, acetylene gas is produced, which triggers fruit ripening process to begin. The gas is so potent that its effects are evident in just under 24 hours of exposure (Sogo-Temi, Idowu, and Idowu, 2014).
In other cases, however, traders sprinkle the ground calcium carbide on the fruit to be ripened to increase exposure to the ripening gas and speed up the process (Hossain, Akhtar, and Anwar, 2015). In these rare cases, the traders use extremely high doses of the chemical to effect ripening of the fruit so that they can move their product into the merket quicker. Here is where the danger really comes into play.
The chemical is never washed off the fruits once they are ripe. The fruit reaches the consumer when it is heavily laden with calcium carbide from the ripening room (Gupta, 2017). Given the challenges of obtaining clean water that is rampant in many developing countries, there is high likelihood that many consumers eat their fruits as they come from the market stalls.
Health risks associated with calcium carbide
Studies have linked the use of calcium carbide to the risk of getting cancer. The rampant use of this chemical is deeply concerning given the recent upsurge of cancer incidences in Kenya and even globally. According to research by Korir et al., (2015), there has been a steady increase in the incidences of cancer in Kenya, with cervical and breast cancer accountig for 44 percent of the cancer-related deaths in Kenya.
Mentally degrading diseases and conditions such as Alzheimers and other pulmonary diseases like pneumoconiosis have also been reported to be on the rise. Predisposing conditions such as chronic exposure to calcium carbide and its products is the least that is required in the fight against the spiraling incidences of lifestyle diseases.
Ingestion of the chemical can lead to ulceration of the gastrointestinal tract. When it comes into contact with the skin, burns may occur and severe eye irritations may occur if the chemical comes into contact with the eyes. If inhaled, it may lead to irritation of the respiratory tract and trigger serious conditions like pneumoconiosis. The condition can be very fatal in small children and people with compromised immunity (Gupta, 2017; Asif, 2012).
Identifying fruits ripened with calcium carbide
Identification of calcium carbide ripened fruits is key to avoiding contamination by reducing exposure. Precautionary measures should be put in place to enure that consumers can at least tell apart naturally ripened fruits from the chemically ripened ones. Ability to effect this can help reduce the chemical load on the fruits prior to consumption and minimize the negative health effects.
The characteristics of naturally ripened fruits is tabulated agaist calcium carbide ripened fruits below. The table has been developed from the data obtained from the work of Asif (2012).
Natural ripened fruit
Carbide ripened fruit
Patches of chlorophyll on the skin – especially mangoes.
Bright and uniformly colored skin of both mangoes and bananas.
Dark spots on ripe bananas
No spots on bananas.
Mangoes produce a lot of juice.
Mangoes feel drier and produce less juice.
Better keeping quality.
Poor keeping quality.
Fruit is harder and may not be ripe at the center.
Legislation over the use of calcium carbide
Calcium carbide has gained popularity among fruit traders in many parts of the world. This is partly due to the effectiveness of the chemical in accelerating fruit ripening, its relative low cost, and it is easily available from many back-street shops.
Even though the chemical is banned in many countries, it is still widely used illegally (Islam, Mursalat, and Khan, 2016). In Kenya for instance, there is a legislation that prohibits the use of unapproved chemical additives in foods meant for human and animal consumption. The legislation is provided under the act of parliament, CAP 254, amendment of 2013 (KEBS, 2014).
Despite the clear provision of the law, there are media reports of widespread use of the chemical in the country by fruit vendors (Mungai, 2018; Koros, 2014). The chemical is so easy and cheap to obtain that unscrupulous traders find it tempting due to its proven effectiveness in ripening of the fruits. However, studies have shown that the chemical is a potential carcinogen due to the traces of arsenic (Kjuus, Andersen, and Langard, 1986).
The acetylene gas produced by calcium carbide has also been shown to have negative effect on the brain. It deprives the brain of oxygen leading to hypoxia and degenerative conditions like Alzheimers diseases. In severe cases, death can occur if the victim is exposed to sufficiently high doses of the gas.
Calcium carbide, though a cheap and faster way of ripening fruits, pose more harm than good. The chemical compound contains trace elements that have been linked to cancer. Exposing Kenyans to these predisposing conditions through the fruits they buy from the market will only exacerbate the already worsening situation of cancer-related deaths.
This is exacerbated by the slack implementation of government regulations by the concerned authorities. It is therefore imperative that each individual should take responsibility for their own health and safety.
1. Consumers should always wash fruits before consumption. This can help reduce the traces of the ripening chemicals remaining on the fruits before consumption.
2. Organic fruit ripening methods should be promoted. For centuries, people have used natural ethylene producers like albizia leaves to hasten fruit ripening. It is high time that these methods are optimized for industrial adoption.
3. Rapid and affordable tests for identifying and confirming contaminated fruits should be formulated. This will help in rapid confirmatory screening of the fruits to test their suitability for human consumption.
4. Finally, the government, through its regulatory agencies should enhance the enforcement of food safety regulations, especially those concerning the use of calcium carbide in fruit ripening. This will help reduce the rampant use of this chemical and reduce risks associated with the use of calcium carbide among the Kenyan population.
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