More Information

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 lighttreated 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.

Frozen banana smoothies

If you give your banana plants lots of tlc they can be extremely productive, and depending on how many you’ve got you might have several hands ripening up at any one time – faster than you can eat them all fresh. Unfortunately, we all know that ripe fruit has a very short storage time of only about a week at room temperature. This can be extended for maybe another week if stored no lower than 13°C as below this they suffer chilling injury, typically first seen as skin blackening. But you can go far beyond these storage time frames through use of two simple techniques.

The first involves drying slices, which come out beautifully with enhanced flavour. Provided you get the moisture level down sufficiently you can store them just chewy rather than brittle for many months without deterioration. Also these homemade ones are far healthier than the dried banana chips you see in supermarkets, where they start off with a healthy fruit with basically zero saturated fat, and then after their standard frying they can finish up at around 30% fat.

The second is to make frozen banana smoothies. For optimal frozen storage time, fresh fruit and vegetables are best blanched before freezing; this denatures enzymes that would otherwise cause more rapid deterioration given their action is only slowed down and not stopped with freezing. Fruit are generally more acid than vegetables (some more so than others) so often you can get away without blanching if you don’t try and keep them for more than a couple of months or so. Start by peeling your bananas, maybe cut them in two, and then store in the freezer at -20°C. Like many fruit and vegetables that have been frozen, banana pieces finish up being terribly mushy and not nice at all when thawed, as the integrity of cell walls that give them their normal smooth texture when fresh will have been disrupted by ice crystals. This is solved by taking these frozen pieces and putting them through a slow-revving fruit blender than can be set to collect both fibre and pulp. There are many suitable devices available commercially, one being the US-designed and manufactured Champion. These are expensive brand new but often you can get second-hand ones at a fraction of the cost. The attraction of properly made machines is their longevity/reliability and availability of spare parts if ever needed.

Everyone has different palates and there are a few people amongst us who don’t like bananas; the rest of us think they’re a really good fruit. Evidence of the scale of consumption in Australia was when cyclone Larry devastated the main supplies in north Queensland some years back and unbelievably this had a negative impact on our national GDP. Smoothies come out with the consistency of very thick yoghurt and taste just like ice-cream without the nutritional negatives of added fat, sugar etc. You can also enhance their appeal and flavour by freezing some blueberries and sprinkling these in at the time of processing, giving a striped appearance in the final product. However the straight ‘vanilla’ form is good enough to not really need any enhancements and gets the thumbs up from just about everyone who’s tried them.