a propionate ester group. Banana-plant leaves also fluoresce in the same way. Green (under-ripe) bananas do not fluoresce. That paper suggested that this fluorescence could be put to use "for optical in vivo monitoring of ripening and over-ripening of bananas and other fruit". Storage and transport Bananas must be transported over long distances from the tropics to world markets. To obtain maximum shelf life, harvest comes before the fruit is mature. The fruit requires careful handling, rapid transport to ports, cooling, and refrigerated shipping. The goal is to prevent the bananas from producing their natural ripening agent, ethylene. This technology allows storage and transport for 3–4 weeks at . On arrival, bananas are held at about and treated with a low concentration of ethylene. After a few days, the fruit begins to ripen and is distributed for final sale. Ripe bananas can be held for a few days at home. If bananas are too green, they can be put in a brown paper bag with an apple or tomato overnight to speed up the ripening process. Carbon dioxide (which bananas produce) and ethylene absorbents extend fruit life even at high temperatures. This effect can be exploited by packing banana in a polyethylene bag and including an ethylene absorbent, e.g., potassium permanganate, on an inert carrier. The bag is then sealed with a band or string. This treatment has been shown to more than double lifespans up to 3–4 weeks without the need for refrigeration. Sustainability The excessive use of fertilizers often left in abandoned plantations contributes greatly to eutrophication in local streams and lakes, and harms aquatic life after algal blooms deprive fish of oxygen. It has been theorized that destruction of 60% of coral reefs along the coasts of Costa Rica is partially from sediments from banana plantations. Another issue is the deforestation associated with expanding banana production. As monocultures rapidly deplete soil nutrients plantations expand to areas with rich soils and cut down forests, which also affects soil erosion and degradation, and increases frequency of flooding. The World Wildlife Fund (WWF) stated that banana production produced more waste than any other agricultural sector, mostly from discarded banana plants, bags used to cover the bananas, strings to tie them, and containers for transport. Voluntary sustainability standards such as Rainforest Alliance and Fairtrade are increasingly being used to address some of these issues. Bananas production certified by such sustainability standards