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Note: there are some answers to this question on Chemistry.SE, but I prefer a sceptical approach here.

I ask this question in connection with a post on Facebook (in Indonesia), which asked:

Hi everyone, I bought some mangoes today and the seller told me they had been ripened chemically with "karbit". I checked on Google and I saw that karbit was Calcium Carbide, a carcinogen that is banned in several countries. I tried in another shop and the mangoes there were "karbitan" as well. Finally, in a third shop in Pasar Semat Sari, the seller had both "karbitan" and "masak pohon" mangoes (tree ripened mangoes). This was the first time I heard of this and I wonder if this chemical ripening is really that common in Indonesia. Also, does anyone know if this “karbit” is as harmful as I read it was?

In my experience from talking to a banana seller in Indonesia nearly all their bananas are ripened with 'karbit', although sometimes not.

Indonesia has poor use of accurate terminology, so I was also doubtful whether 'karbit' is necessarily in fact calcium carbide.

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    Are you sure it is used as a way to ripen fruit and not something that is used to make the product that ripens fruit? According to wiki it is used as the source and not actually ripening fruit, en.wikipedia.org/wiki/Calcium_carbide
    – Joe W
    Jun 21 at 17:13
  • 4
    A source of acetylene, yes. 2H2O + CaC2 gives the desired gas, C2H2, plus Ca(OH2). Adding water to it doesn't make it a separate product
    – thelawnet
    Jun 21 at 17:33
  • 2
    Sure, but you make it sound like Calcium Carbide is used to ripen fruit but that is not the case as it is used in the production of the product that will ripen the fruit. There is a difference between the two chemicals in question.
    – Joe W
    Jun 21 at 17:40
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    I disagree, there is a big difference in using chemical A and using chemical B which is made from chemical A. It is possible that Chemical A can be harmful and Chemical B is not.
    – Joe W
    Jun 21 at 20:08
  • 7
    Complaining that calcium carbide is used to produce acetylene would be akin to complaining that chlorine is used to make table salt. Adding a single ingredient (water and sodium, respectively) drastically changes the properties of the substances in question. Jun 22 at 1:37

1 Answer 1

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To answer my last point first, I checked a local shopping site (which sells EVERYTHING), and 'karbit' or 'carbide' is indeed often not carbide at all.

Ethephon

Several sellers sold 'karbit' which was in fact ethephon, including the brands 'Orbyte', 'Ethrel' (by Bayer), etc. This is priced around $1.70/100 ml, and is 480g/litre of ethephon, applied by spraying. https://www.bayer.com/en/id/ethrel-480-sl In fact anything calling itself 'liquid karbit' is not carbide at all. https://www.tokopedia.com/search?st=product&q=karbit%20cair&srp_component_id=02.01.00.00&srp_page_id=&srp_page_title=&navsource=

According to Wikipedia: https://en.wikipedia.org/wiki/Ethephon

"The toxicity of ethephon is very low,1 and any ethephon used on the plant is converted very quickly to ethylene.6

The use of this chemical is allowed in the European Union."

A 2021 story from India claims

https://timesofindia.indiatimes.com/city/vijayawada/ethephon-replaces-carbide-as-fruit-ripening-agent/articleshow/82360064.cms

"Ethephon is registered as an insecticide", and is "equally dangerous to public health"

This first is an interesting, possibly misleading claim; it appears that Ethephon is marketed mostly as a plant growth regulator.

However

"Ethephon also inhibits the activity of plasma butyrylcholinesterase (BuChE) in humans, dogs, rats, and mice" https://pubs.acs.org/doi/abs/10.1021/tx000027w

Despite the fact that it may have such effects, and some insecticides are ChEIs, it does NOT appear to be accurate to describe it as an insecticide.

Further down the same article and in other sources, it appears that ethephon is NOT in fact registered as an insecticide, but may contain 'traces of insecticide' https://www.thehindu.com/news/cities/Hyderabad/why-permit-ethephon-usage-hc-asks-centre/article32469205.ece

This article includes the particularly pointless statement "The bench remarked that number of cancer patients in the country had gone up in the past three decades." which is rather irrelevant given recent use of Ethephon and lack of evidence for causation.

The basic mechanism for ethephon is the production of ethylene gas, the chemical C2H4, which naturally ripens fruit.

The Times of India article also claims a variety of wild claims

A variety of wild claims, transcribed below.

Poses risks to public health

  • Ethephon is registered as an insecticide and is mostly imported from China
  • Local companies repack Ethephon into sachets called “fruit ripening agent”
  • Ethephon is widely used to ripen mangos
  • Every sachet of the fruit ripening agent sold in the market contains 20% Ethephon
  • Some companies declare that the remaining 70% is Chinese talc, while there are doubts about the remaining 10%
  • Chinese talc is contaminated with asbestos fibre that can cause lung cancer

Dangers

  • Highly corrosive substance, can cause skin burns, oesophagus burns, damage to upper respiratory tract, irreversible eye damage
  • Research by the Food and Agriculture Organisation suggests that Ethephon can cause suppression of plasma cholinesterase, an enzyme associated with the nervous system

While I found a 20% sachet of 0.6g from China https://westchemicals.en.made-in-china.com/product/gwBTShKCSUry/China-0-6g-Bag-Fruit-Plant-Growth-Regulator-Mango-Banana-Ripener-Ethephon-20-Gr.html there are many other formulations

Speculating about whether talcum powder, which MIGHT contain asbestos, and which is probably not the best thing to liberally sprinkle on yourself every day after bathing due to a cumulative exposure risk, could somehow pose a risk to consumers of fruit which had been ripened in proximity to the same is rather ridiculous.

There is a rather detailed peer review of ethephon for use as spray on wheat and barley in the EU. https://efsa.onlinelibrary.wiley.com/doi/epdf/10.2903/j.efsa.2006.174r

Ethephon was found NOT to be carcinogenic, and an acceptable daily intake of 0.03mg/kg was set as 1/1000 of the NOAEL (no adverse effect level) in dogs. Ethephon, like almost any substance, has a lethal dose (LD50), which is 1.564g/kg (cf. table salt at 3g/kg). However it is better considered to be corrosive, a skin irritant, and not to be inhaled - it is unlikely anyone would consume such a quantity accidentally.

Checking on Youtube, I found that ethephon is used by banana sellers in Indonesia who mix it with water and either spray or brush it onto the fruit, before putting it an box for several days:

https://www.youtube.com/watch?v=VBuh97rbCEc

Since ethephon does not appear to be a dangerous chemical, and is applied to a banana with thick skin, then the credibility of claims that it is harmful, and claims about 'traces of pesticide' appear to be totally fanciful.

Other EU reports suggest that ethephon is not likely to be harmful e.g., when used to ripen Japanese persimmons https://www.efsa.europa.eu/en/efsajournal/pub/4747

Calcium carbide

The claims about the toxicity of ethephon from India seem to fanciful, and place the larger claims about 'carbide', also mostly from India in some doubt

However, I also found actual calcium carbide on sale in Indonesia and in use according to Youtube videos to ripen fruit.

enter image description here

This is 51% calcium oxide (lime), 41% calcium carbide, and slightly alarmingly 8% 'other matter'.

As far as calcium carbide goes, according to this 2015 story, a judge in Hyderabad called those who used it 'worse than terrorists' https://www.newindianexpress.com/states/telangana/2015/aug/23/Meeting-to-Curb-Carbide-Menace-803504.html

Calcium carbide is indeed explosive, as noted at Chemistry Stack Exchange. According to this, a box of mangoes exploded after it was deliberately set on fire: https://indianexpress.com/article/blogs/why-you-should-never-site-fire-to-a-box-of-mangoes/

However it is not clear the size of the explosion risk posed by carbide as it is used normally

There are a variety of poorly written articles from India claiming it is harmful:

https://www.currentscience.ac.in/Volumes/99/12/1664.pdf

"calcium carbide is extremely hazardous to the human body because it contains traces of arsenic and phosphorus"

A false statement in that calcium carbide is literally CaC2, and does not necessarily contain any arsenic or phosphorus, although of course impure calcium carbide might contain this.

However it appears that carbide does contain arsenic, in general, due to the way it is produced.

This article https://www.researchgate.net/publication/318509156_Method_for_detection_and_removal_of_arsenic_residues_in_calcium_carbide_ripened_mangoes_CHANDEL_et_al/link/5a9289b2a6fdccecff046a99/download

by Chandel et al claims 0 arsenic in untreated fruit, which I find rather suspect, as other sources would suggest 0.6-50 ppb arsenic in mango in Hainan, China from soil: https://www.researchgate.net/publication/259127041_Occurrence_of_arsenic_in_fruit_of_mango_plant_Mangifera_indica_L_and_its_relationship_to_soil_properties

Chandel et al show dipping in a carbide solution causes arsenic both on the skin and in the pulp, and likewise using carbide powder. The study found 85 ppb of arsenic in mango pulp market-purchased samples, and between 30 and 60 ppb dipping fruit in carbide solution or using carbide in a box. Although this might sound alarming, infant rice cereals have tended to have higher levels of arsenic than this: https://www.fda.gov/food/cfsan-constituent-updates/fda-makes-available-results-testing-infant-rice-cereal-inorganic-arsenic - 100ppb is the FDA's target

A second Indian study by Lakade et al https://doi.org/10.1080/19440049.2019.1605206

shows up to a maximum of 5 ppb in their naturally ripened samples, while the highest levels of arsenic were achieved by using 2% calcium carbide in a closed container, of 280 ppb.

It is stated that by dissolving the carbide in water, the arsine gas is released, which means less is transferred to fruit dipped in such a solution, compared to the 'calcium carbide' simply being placed in a box.

Another study from Malaysia found that higher levels of carbide were used to ripen mangoes compared to bananas, but both resulted in arsenic levels far below maximum FDA levels of 1000 ppb. https://mjas.analis.com.my/mjas/v25_n2/pdf/Nurasmat_25_2_9.pdf

The research article Artificial ripening on banana (Musa Spp.) samples: Analyzing ripening agents and change in nutritional parameters by Md. Nazibul Islam et al analysed two samples of calcium carbide powder, finding 160ppm Arsenic and 80ppm Phosphorus in one, and 0 Arsenic, and 120 ppm Phosphorus in the second.

While arsenic is clearly highly toxic, the study TOXIC LEVELS OF ARSENIC AND PHOSPHOROUS FOUND IN SOME COMMONLY CONSUMED FRUITS SOLD IN THE MARKET IN BENIN CITY notes that there is a tolerable upper intake of phosphorus of 4g/day. While I have not verified this, clearly phosphorus is not toxic in the way that arsenic is. In fact, phosphorus is a required element for human health, at 700mg/day RDA, or 1250mg day for adolescents (https://ods.od.nih.gov/factsheets/Phosphorus-HealthProfessional/). It does not seem plausible that phosphorus impurities from calcium carbide could be injurious to human health given this fact, and the much higher doses (cf. arsenic) required for toxicity.

The study noted that fruits purchased in Benin contained 24 ppb arsenic for mango pulp, 66 ppb for banana flesh, 23 ppb for plantain, and 97 for avocado, compared to zero for the sample they artificially ripened using carbide (perhaps their carbide was arsenic-free?).

Nutrition and flavour

In general both acetylene (from CaC2 (carbide) + 2H2O (water) -> C2H2 (acetylene) + Ca(OH)2 (slaked lime), and ethylene (C2H4) will ripen fruit. Ethylene is the signal for ripening in nature. Climacteric fruits can ripen after harvesting using ethylene, while non-climacteric fruits must be harvested when ripe: https://extension.umd.edu/resource/ethylene-and-regulation-fruit-ripening

Studies have shown better taste from naturally ripened bananas, but better appearance from artificially ripening. https://doi.org/10.17660/ActaHortic.1985.158.47

The study by Khan et al5 shows substantial variance in Vitamin C levels between an unripe banana (2100ppm), a naturally ripened banana (400ppm), one treated with ethephon (270ppm), and one with carbide (850ppm). Sugar levels were much more constant, at around 10% for the unripe banana, and 21-22% for the naturally and artificially ripened versions.

Conclusion

The article by Khan et al claims 'In carbide samples, arsenic content was found as high as 160 ppm which is much higher than the permissible limit set by US FDA (0.5–2 ppm)', however, this does not seem to be correct; there is a limit of 10 micrograms per cubic metre of air. (https://www.atsdr.cdc.gov/csem/arsenic/standards.html) It is not quite clear what exposure there would be from mixing impure calcium carbide with water, and using it to treat bananas and other fruit. However it seems that there could well be an occupational risk to the workers as a result of this exposure, albeit that arsenic-free carbide appears to exist.

As far as consumers go, while the data show that carbide use tends to result in increased arsenic, on the skin and pulp of fruit, arsenic is present in food anyway, and none of the data here really suggest that the uses for ripening fruit in developing countries result in dangerous levels of arsenic.

EU law provides for a limit of inorganic arsenic of 100ppb in infant rice, and between 200 and 300ppb otherwise, though rice can contain up to 2000ppb.

Although artificially ripened fruit will taste, feel, appear and have somewhat different vitamin profiles from naturally ripened, it is not true to say that these fruits have no vitamins or other similar fanciful claims.

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    "ethylene gas, the chemical C2H6" - should be C2H4. (Not enough characters for me to suggest an edit, alas.) Jun 22 at 0:13
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    If anyone is looking into the note about ethephon potentially containing traces of insecticide, my understanding is that this actually stems from it potentially being used in spraying equipment that also sprays insecticide, or stored in tanks that have held insecticide prior. Therefore the quantities (provided adequate cleaning) should be incredibly small.
    – James D
    Jun 22 at 22:20
  • @JamesD I'm not convinced that in the contexts referenced in the Indian articles they are spraying; my answer says 'ethephon' is sprayed, however for banana Bayer say 'dyeing' (I'm not quite clear what this means). According to Bayer it should be spread on apple and coffee fruit directly; however for bananas I don't think they would use a knapsack sprayer; rather you would cut the fruit off, and ripen later, in which case such a sprayer would not be very useful (a small spray bottle, presumably not used for anything else would make sense)
    – thelawnet
    Jun 24 at 13:56
  • Wow, very thorough analysis with a boatload of sources. Have my upvote, good sir/madam
    – automaton
    Jun 24 at 16:48
  • The problem with phosphorous in this context is that it is reduced to in calcium phosphide during calcium carbide production, which forms (mono)phosphane (aka phosphine) with water. That in turn is a toxic and mutagenic gas. The main concern would be of for anyone around when the acetylene is produced to induce ripening. Jun 24 at 20:27

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