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  1. That always has been a disputable question - unripe and overripe fruits - is it dangerous to eat? My mother never allowed me to eat such fruits, but she couldn't explain why not. In fact, unripe fruits for me are much more tasty, though still I try to avoid eating them. Perhaps there is nothing to be afraid of and it is just my childhood's mum warning traits playing on me? 
  2. As with any living organism, the life cycle of fruit has continual enzyme activity. Once full ripeness has been reached, as the fruit begins to ferment, the sugars are converted into alcohol and carbon dioxide by the activity of enzymes, resulting in decay. This is why overripe fruit is soft and discoloured and has a strong odour. Many of the fungal pathogens responsible for this decay are present in the unripe fruit but do not become active until the fruit is fully ripened.[1] It is estimated that as much as 30% of all harvested fruit is discarded because of decay that occurs after harvesting.[1]  There are two main types of fruit ripening processes. In climacteric fruits such as apples, peaches, bananas and tomatoes, the ripening process is associated with increased ethylene production and increased cellular respiration.[2] [3] This is why these fruits continue to ripen after harvesting – especially if placed in a bag, which allows for the accumulation of ethylene and promotes further ripening.[4] Non-climacteric fruits such as grapes, citrus fruits and strawberries do not ripen further after harvesting.[2] An exception is the avocado, which does not commence the ripening process until after harvest.[5]  Both types of fruits undergo many changes during their development, including alterations in colour, firmness and texture.[2] In blackberries, for example, the levels of anthocyanin (a colour pigment that also acts as an antioxidant) have been shown to change during ripening.[6] In the Marion variety, anthocyanin increased from 74.7 to 317mg/100g fresh weight and in Evergreen blackberries from 69.9 to 164mg/100g during the complete ripening process. Levels of phenolic compounds (another group of antioxidants) dropped, but only very slightly, during the same period.[6] Other changes include alterations in enzyme activity, such as those seen in pineapples and papayas during ripening, reaching their maximum in the ripe fruits.[7] Papain (enzyme from papaya) and bromelain (enzyme from pineapple) are known for their beneficial antibacterial properties and found useful applications in the pharmaceutical and food industries.[7] [8] The ratio of starches and sugars also changes during the ripening process, with starch being converted to sugar and thus increasing fruit sweetness.[2] This means that ripe (and especially overripe) fruits have a higher glycaemic index, and their intake quickly raises blood sugar, increasing the risks of diabetes, especially when fruits are eaten in excessive amounts.[9]  For reasons of taste, both underripe and overripe fruits are not regularly consumed. Underripe food can often have a bitter, acidic taste, while overripe fruit can have an off-putting smell, taste and/or texture. Consumption of fruit in either state can cause gastric upsets and have an impact on gut flora. The Jamaican fruit ackee, for example, can cause vomiting and even death if eaten before it is ripe.[10] Bananas, however, have health benefits when eaten either under or overripe. One animal study concluded that the consumption of unripe bananas could aid colonic fermentation (helping to avoid inflammations) and have an impact on glycaemic control by improving insulin sensitivity, which is especially beneficial for individuals with diabetes.[11] Overripe bananas also have many health benefits, including neutralising gastric acidity and lining the stomach, thus reducing irritation from gastric ulcers. Some sources claim that underripe bananas have also been shown to promote healing and aid both constipation and diarrhoea; however, there is not enough consistency between different sources.[12] [13] By eating fruits, especially free from pesticides, you may reduce the risks of many illnesses thanks to their high content of vitamins, minerals and enzymes. But to avoid gastric upsets and other adverse effects associated with eating under or overripe fruit, it is advisable to choose fruit at its optimum stage of ripeness. This can sometimes be difficult to decipher, especially when purchasing packaged fruits from the supermarket, but taste and smell are good indications of fruit ripeness as well as their weight, since ripe fruits are heavier due to higher juice content. You may also complete some fruit ripening process by putting it into a bag (e.g., bananas or avocados). Knowing the fruits you eat, you will take the full benefits put into it by nature.   [1] Alkan, N., & Fortes, A. (2015). Insights into molecular and metabolic events associated with fruit response to post-harvest fungal pathogens. Frontiers In Plant Science, 6. http://dx.doi.org/10.3389/fpls.2015.00889  [2] Cherian, S., Figueroa, C., & Nair, H. (2014). ‘Movers and shakers’ in the regulation of fruit ripening: a cross-dissection of climacteric versus non-climacteric fruit. Journal Of Experimental Botany, 65(17), 4705-4722. http://dx.doi.org/10.1093/jxb/eru280 [3] Theologis, A. (1992). One rotten apple spoils the whole bushel: The role of ethylene in fruit ripening. Cell, 70(2), 181-184. http://dx.doi.org/10.1016/0092-8674(92)90093-r [4] Yahia, E. (2009). Modified and controlled atmospheres for the storage, transportation, and packaging of horticultural commodities. Boca Raton: CRC Press/Taylor & Francis. [5] Lu, Q., Zhang, Y., Wang, Y., Wang, D., Lee, R., & Gao, K. et al. (2009). California Hass Avocado: Profiling of Carotenoids, Tocopherol, Fatty Acid, and Fat Content during Maturation and from Different Growing Areas. Journal Of Agricultural And Food Chemistry, 57(21), 10408-10413. http://dx.doi.org/10.1021/jf901839h [6] Siriwoharn, T., Wrolstad, R., Finn, C., & Pereira, C. (2004). Influence of Cultivar, Maturity, and Sampling on Blackberry (RubusL. Hybrids) Anthocyanins, Polyphenolics, and Antioxidant Properties. Journal Of Agricultural And Food Chemistry, 52(26), 8021-8030. http://dx.doi.org/10.1021/jf048619y [7] Paull, R. (1993). Pineapple and papaya. Biochemistry Of Fruit Ripening, 291-323. http://dx.doi.org/10.1007/978-94-011-1584-1_10 [8] dos Anjos, M., da Silva, A., de Pascoli, I., Mikcha, J., Machinski, M., Peralta, R., & de Abreu Filho, B. (2016). Antibacterial activity of papain and bromelain on Alicyclobacillus spp. International Journal Of Food Microbiology, 216, 121-126. http://dx.doi.org/10.1016/j.ijfoodmicro.2015.10.007 [9] Carbohydrates and Blood Sugar. (2017). The Nutrition Source. Retrieved 18 September 2017, from https://www.hsph.harvard.edu/nutritionsource/carbohydrates/carbohydrates-and-blood-sugar/ [10] Joskow, R., Belson, M., Vesper, H., Backer, L., & Rubin, C. (2006). Ackee Fruit Poisoning: An Outbreak Investigation in Haiti 2000–2001, and Review of the Literature. Clinical Toxicology, 44(3), 267-273. http://dx.doi.org/10.1080/15563650600584410 [11] Dan, M., Cardenette, G., Sardá, F., Giuntini, E., Bello-Pérez, L., & Carpinelli,  . et al. (2015). Colonic Fermentation of Unavailable Carbohydrates from Unripe Banana and its Influence over Glycemic Control. Plant Foods For Human Nutrition, 70(3), 297-303. http://dx.doi.org/10.1007/s11130-015-0493-6 [12] Mitsou, E., Kougia, E., Nomikos, T., Yannakoulia, M., Mountzouris, K., & Kyriacou, A. (2011). Effect of banana consumption on faecal microbiota: A randomised, controlled trial. Anaerobe, 17(6), 384-387. http://dx.doi.org/10.1016/j.anaerobe.2011.03.018 [13] ORHAN, İ. (2001). Biological activities of Musa species. Ankara Universitesi Eczacilik Fakultesi Dergisi, 039-050. http://dx.doi.org/10.1501/eczfak_0000000354