Banana ripening

We’re all familiar with the high perishability of bananas. Once ripe, you have maybe a week to eat them before everything goes black and horrible. Like most fruit you can slow this process down by cool storage, but as a tropical species, if you go below about 13°C they suffer from chilling injury. So putting them into the average domestic refrigerator set round 4°C will quickly cause skin and flesh blackening. What can be done if you prefer to eat them fresh rather than bypass the problem by drying, smoothies, cooking etc? Freezing doesn’t work very well as cell structure is disrupted, producing a less-than-appealing gooey mess when thawed. Large commercial operations can use a few other techniques (eg modified atmosphere storage) but these are not doable at the domestic level. Bananas are climacteric, and ripening is triggered by the production of the gaseous plant hormone ethylene. Indeed they produce so much of this compound during this stage that they can be used to speed up ripening of other climacteric fruit when stored together. This process can be slowed down by using hormone antagonists, but again, not easily or often practiced by the home-grower.

If you purchase them you can side-step the problem by only ever buying what can be eaten in a few days, but this won’t allow you to take advantage of a great bargain and buy many more. And if you grow your own, you can sometimes get many hands in rapid succession and you lose control of the 'steady as she goes' approach. Well, a recent study in the journal Fruits, The International Journal of Tropical and Subtropical Horticulture (Ozdemir (2016) 71, 115-122) reported that the intensity of light influences the speed of harvested fruit ripening, and this could help. The following is a summary:

The effect of continuous light treatment (24 μmol m⁻² s⁻¹) on the ripening of banana fruit (Musa spp., AAA group, Cavendish subgroup cv. Grand Nain) during postharvest handling was investigated. The changes in physiological and physical parameters related to banana ripening such as the respiration rate, ethylene production rate, weight, colour, texture and sugar content were analysed during storage. Light treatment accelerated the ripening of bananas, which was characterised by a shortened pre-climacteric period. Light-treated bananas reached a respiratory climacteric peak 8 days earlier than bananas stored in the dark. Similar delays were also observed in the onset of climacteric ethylene rise, colour change, texture loss and soluble sugar accumulation between bananas stored under continuous light and darkness. Light treatment led to excessive weight loss in bananas. At the climacteric peak, respiration and ethylene production rates were significantly higher in light-treated bananas than bananas stored in the dark. Also, significantly higher amounts of soluble sugars were accumulated in light-treated bananas than in bananas stored in the dark. It is concluded that light treatment can be used as a method for accelerating ripening of green bananas, especially during retail display, in order to provide consumers with fruits at their best eating quality.

Commentary:

• This effect of light on banana fruit is not unexpected as they’re living organs and in their normal on-plant environment they receive sunlight, respire, photosynthesise etc, and some such receptivity and metabolic activity continues after harvest until total senescence.
• Light intensity was controlled by using a high-pressure sodium lamp as environmental light is variable. The level used was about half that of a 36W fluorescent light.
• The author was mainly focussed on the effect of light on accelerating ripening, but the reverse phenomenon ie dark storage slowing ripening, is useful knowledge for us.
• Green mature bananas were used. Although not stated, it is assumed they hadn’t been treated with ethylene (as is standard practice in the industry) as fruit behaviour was followed for up to four weeks at room temperature (20 - 22°C); treated bananas don’t last this long. If you buy your bananas they will inevitably have been gassed, whereas your own will not (unless you use eg ethephon). My suspicion is that once gassed and primed, the extent of slowing by dark storage would be compromised. A similar phenomenon exists with other fruit eg avocado – if picked while still hard-mature, they can be cold-stored for several weeks, but once they become soft-ripe then storage reduces to only a few days.
• If you have excess fruit, can’t do 13°C storage and whether gassed or not, this study suggests dark storage at room temperature might give you another week or so before they go off.