How to Get Bananas Ripe Fast: Your Ultimate Guide to Perfect Yellow Goodness

Oh, the familiar kitchen dilemma! Just last week, my friend Sarah was practically tearing her hair out. She’d promised to bring her famous banana bread to a potluck that evening, a real crowd-pleaser, but there was one tiny, rather green problem: her bananas were stubbornly unripe. They looked like they’d just been plucked from the tree, firm and a pale, almost vibrant green. Panic was setting in. We’ve all been there, right? You’ve got a recipe that calls for ripe, spotty bananas – the kind that practically melt into a sweet, fragrant mash – but all you’ve got are these hard, unyielding emeralds. It’s a common frustration for home bakers and smoothie enthusiasts alike.

So, how do you get bananas ripe fast when time is of the essence? The quickest and most effective methods involve leveraging ethylene gas, either by trapping it around the fruit (like in a paper bag with other ripe produce) or by gently applying heat (such as in an oven or microwave for cooking purposes). While some methods are better for immediate baking needs, others offer a gentle nudge toward perfectly yellow, snackable fruit.

Let’s peel back the layers (pun absolutely intended!) and dive into the fascinating science and practical techniques behind speeding up nature’s course. Because nobody wants to wait an eternity for that perfect banana, whether it’s for a morning smoothie, a comforting loaf of banana bread, or just a delicious, naturally sweet snack.

Understanding Banana Ripening: The Science Behind the Softening

Before we dive into the “how-to,” it’s incredibly helpful to understand the “why.” What exactly is happening when a banana ripens, and how can we manipulate that process? It’s not just a magical transformation; it’s a finely tuned biological dance driven primarily by a fascinating plant hormone called ethylene gas.

The Role of Ethylene Gas

Bananas, like many other fruits classified as “climacteric fruits,” continue to ripen after being harvested. This ripening process is kick-started and accelerated by ethylene gas. Think of ethylene as the fruit’s internal alarm clock, signaling to itself and neighboring fruits that it’s time to mature. As a banana starts to ripen, it naturally produces more and more ethylene. This gas then triggers a cascade of enzymatic reactions within the fruit:

Starch Conversion: The most noticeable change is the breakdown of complex starches into simpler sugars. This is why green bananas are starchy and less sweet, while yellow bananas are delightfully sugary. Enzymes like amylase are the primary drivers here.
Cell Wall Softening: Pectin, a complex carbohydrate that acts as a structural “glue” in plant cell walls, breaks down. This is why a firm, green banana becomes soft and yielding as it ripens. Pectinase enzymes are at work.
Flavor and Aroma Development: Along with sweetness, a host of volatile compounds are produced, giving ripe bananas their characteristic flavor and inviting aroma.
Color Change: Chlorophyll, the green pigment, breaks down, revealing the yellow carotenoid pigments underneath. Eventually, as the banana over-ripens, melanization occurs, leading to those familiar brown spots.

Our goal in speeding up ripening, then, is largely about concentrating this ethylene gas or using external factors like heat to accelerate these enzymatic processes.

Temperature’s Impact

Temperature also plays a crucial role. Enzymes, being proteins, are highly sensitive to temperature. Generally, warmer temperatures (within a reasonable range) increase the rate of chemical reactions, including those involved in ripening. This is why you should never store unripe bananas in the refrigerator if you want them to ripen; the cold halts ethylene production and enzymatic activity, essentially putting the ripening process on hold, sometimes permanently affecting their ability to ripen properly even after being brought back to room temperature.

The Classic & Most Reliable: Paper Bag Power

When you want to gently but effectively hasten the ripening of bananas without fundamentally altering their texture or flavor for raw consumption, the paper bag method is your best bet. It’s an age-old trick, but remarkably effective because it directly addresses the science of ethylene.

How It Works: Trapping the “Ripe Now” Signal

A paper bag acts like a mini ripening chamber. As bananas naturally produce ethylene gas, the paper bag traps this gas around the fruit, concentrating it. This higher concentration of ethylene signals the banana to ripen faster. The beauty of a paper bag over plastic is that it still allows for some air circulation and moisture escape, preventing mold growth while maintaining a beneficial ethylene environment.

Step-by-Step Guide to Paper Bag Ripening

Choose Your Bananas: Start with firm, green or slightly yellow-green bananas. The greener they are, the longer it will take, but this method will still work.
Grab a Paper Bag (No Plastic!): A standard brown paper grocery bag works perfectly. Avoid plastic bags, as they trap too much moisture and can encourage mold, leading to spoilage rather than ripening.
Add Ethylene-Producing Friends (Optional, but Highly Recommended): This is where you can supercharge the process. Place one or two other ripe fruits inside the bag with your bananas. The best choices are:
- A very ripe banana: The riper it is, the more ethylene it’s producing, which will encourage its green counterparts.
- An apple: Ripe apples are excellent ethylene producers.
- A tomato: Also a good source of ethylene.
- An avocado: Another climacteric fruit that releases ethylene as it ripens.
These “friends” will significantly boost the ethylene concentration in the bag, making the process much faster.
Seal It Up (Loosely): Fold the top of the paper bag over a couple of times. You want to trap the gas but still allow a little breathing room. Don’t seal it airtight.
Find a Warm Spot: Place the sealed bag in a warm spot in your kitchen. A sunny countertop (but not direct, scorching sun that might cook them) or on top of the refrigerator (where it’s often slightly warmer) can help. Remember, warmth accelerates enzymatic activity.
Monitor Regularly: Check your bananas every 12-24 hours. Depending on their initial ripeness and whether you added other fruits, they could be perfectly yellow in as little as one day or take up to three days.

Tips for Success & What to Avoid

Don’t Overcrowd: While adding other fruits helps, don’t stuff the bag so full that air circulation is completely cut off.
Watch for Brown Spots: As soon as you see the desired yellow color and perhaps a few brown spots, take them out of the bag to prevent them from becoming overripe too quickly.
Use for Any Purpose: This method yields bananas perfect for snacking, slicing, smoothies, or baking. The flavor and texture will be ideal for whatever you have in mind.

The Oven Method: When You Need Ripe Bananas Right Now

Okay, let’s say Sarah’s potluck is in an hour, and she *still* needs those bananas for her famous bread. The paper bag method is great, but it requires a bit more foresight. For truly instant gratification, especially when your goal is baking or mashing, the oven method is a game-changer. But heed my warning: these bananas are not for eating raw!

How It Works: Heat Accelerates Everything

Heat dramatically speeds up the enzymatic processes responsible for ripening. In the oven, the heat rapidly breaks down starches into sugars and softens the cell walls, mimicking the natural ripening process but at an accelerated pace. The skins will blacken, but the fruit inside will transform into a soft, sweet pulp ideal for recipes.

Crucial Caveats: Not for Eating Raw!

I cannot stress this enough: bananas ripened in the oven are primarily for baking, mashing, or cooking. While they become sweet, their texture will be very soft, almost custard-like, and their fresh “banana” flavor will be altered, becoming more caramelized and less vibrant than a naturally ripened banana. Don’t expect to slice these onto your cereal or enjoy them as a standalone snack.

Step-by-Step Oven Ripening

Preheat Your Oven: Set your oven to a relatively low temperature, typically between 250°F and 300°F (120°C to 150°C). Lower heat for longer is generally better to avoid scorching the fruit.
Prepare Your Baking Sheet: Line a baking sheet with parchment paper or aluminum foil. This will catch any sticky drips as the bananas bake.
Bake Unpeeled Bananas: Place your whole, unpeeled green or yellow-green bananas directly on the prepared baking sheet. Leave them as they are; no need to prick them or peel them.
Watch for Blackening: Bake for 15-30 minutes, or until the banana skins are completely black and shiny. The exact time will depend on the oven temperature, the initial ripeness of your bananas, and their size. You might even see a little sticky syrup oozing out – that’s okay!
Cool Them Down: Once the skins are uniformly black, remove the bananas from the oven. Let them cool completely on the baking sheet. They will be very hot inside, and attempting to peel them while hot can be messy and dangerous. Cooling also allows the sugars to set a bit, enhancing their sweetness.

Best Uses for Oven-Ripened Bananas

Banana Bread & Muffins: This is arguably their primary purpose. The intense sweetness and soft texture are perfect for baked goods.
Smoothies: If you’re blending them, the texture isn’t as critical.
Banana Ice Cream (“Nice Cream”): Mash and freeze them for a quick, healthy dessert.

The Microwave Method: A Quick Fix (with Limitations)

If you’re in an even bigger hurry than the oven allows, or only have one or two bananas, the microwave can be a decent last-resort option. It’s fast, but again, comes with significant limitations regarding texture and taste for raw consumption.

How It Works: Gentle Heat, Fast Softening

The microwave uses electromagnetic radiation to heat the water molecules within the banana, causing it to soften rapidly. This process accelerates the breakdown of cell walls, making the banana mushier and sweeter, but it doesn’t really replicate the full enzymatic ripening process that develops complex flavors.

Limitations: Compromised Texture & Flavor

Microwaved bananas will become soft and sweet enough for mashing, but their texture can be a bit rubbery or slightly dehydrated. The flavor also tends to be less complex and fresh compared to naturally or even oven-ripened bananas. Use this method only when mashing is the ultimate goal.

Step-by-Step Microwave Ripening

Prick the Bananas: Take your unpeeled bananas and prick them all over with a fork or knife. This prevents them from exploding in the microwave as steam builds up.
Microwave in Short Bursts: Place the pricked bananas on a microwave-safe plate. Microwave on high for 30-second intervals.
Check and Continue: After each interval, check the banana. You’ll notice the skin starting to blacken and the banana softening. Continue microwaving in 30-second bursts until the banana is soft to the touch and the skin is mostly black. This usually takes 1-2 minutes per banana, depending on its size and initial ripeness.
Cool Before Peeling: Let the bananas cool for a few minutes before peeling, as they will be very hot.

Best for Mashing, Not Slicing

Again, this method is almost exclusively for situations where you need mashed banana for baking or a very thick smoothie base. Do not expect to get a pleasant, sliceable, or raw-eating banana from the microwave.

The Air Fryer Approach: A Modern Twist on Heat Ripening

With air fryers becoming kitchen staples, it’s worth noting their capability in banana ripening. It functions similarly to the oven method, but often with greater speed and energy efficiency due to the circulating hot air.

How It Works: Convection Heat on Overdrive

An air fryer is essentially a small convection oven. The rapid circulation of hot air cooks and softens the banana quickly and evenly. Like the traditional oven method, it accelerates the breakdown of starches and cell walls, yielding a sweet, soft banana perfect for baking.

Step-by-Step Air Fryer Ripening

Preheat Air Fryer: Preheat your air fryer to around 300-350°F (150-175°C).
Place Unpeeled Bananas: Put your whole, unpeeled green or yellow-green bananas directly into the air fryer basket. Don’t overcrowd the basket; ensure good air circulation around each banana.
Cook and Monitor: Air fry for 7-10 minutes, checking them periodically. You’ll see the skins rapidly turning black. They might even start to split slightly.
Cool Completely: Once the skins are entirely black and the bananas feel soft when gently squeezed, carefully remove them from the air fryer basket. Let them cool down completely on a plate or wire rack before peeling and using.

Best For… You Guessed It: Baking!

Just like the oven method, air-fried bananas are best reserved for baking and mashing. They develop a delicious caramelized sweetness, making them an excellent choice for banana bread, muffins, or even a quick dessert when mashed with a little cinnamon and a dollop of whipped cream.

Burying Them in Rice: An Old-School Ethylene Trap

This is another lesser-known, yet effective, method that taps into the ethylene gas principle, much like the paper bag, but with an added benefit.

How It Works: Ethylene Trap Plus Moisture Absorption

Burying bananas in a container of uncooked rice serves a dual purpose. Firstly, the rice acts as a very efficient medium for trapping the ethylene gas produced by the ripening banana, concentrating it around the fruit. Secondly, the rice can also absorb some excess moisture, which can be beneficial in certain humid environments to prevent mold, though this is a secondary effect to the ethylene trapping.

Step-by-Step Rice Ripening

Find a Container: Choose a container large enough to completely submerge your bananas in rice.
Submerge in Rice: Place your green bananas at the bottom of the container. Then, pour uncooked rice (any common variety like white or brown rice will work) over them, ensuring the bananas are completely covered.
Seal and Wait: Put a lid on the container or cover it tightly with plastic wrap to help trap the ethylene gas.
Check Periodically: This method can take anywhere from 12 hours to 2-3 days, depending on the initial ripeness of the bananas. Check them every 12-24 hours until they reach your desired level of ripeness.

Effectiveness and Considerations

The rice method is generally quite effective, often comparable to the paper bag method in speed. It’s a good alternative if you don’t have a paper bag or other ripe fruits readily available. The bananas will ripen naturally, making them suitable for raw consumption or baking.

The “Fruit Bowl Synergy”: Leveraging Nature’s Ripe Friends

This is perhaps the simplest and most passive approach, excellent if you’re not in a desperate rush but want to give your bananas a gentle nudge.

How It Works: Passive Ethylene Sharing

Simply placing your unripe bananas in a fruit bowl alongside other ripe, ethylene-producing fruits allows for a natural exchange of the ripening gas. The ethylene from the ripe fruits encourages the green bananas to begin their own ripening process.

Best “Ripe Friends” for Your Bananas

Any of the following will contribute ethylene to the atmosphere of your fruit bowl:

Ripe apples
Tomatoes
Avocados
Pears
Kiwi

Considerations

Speed: This method is slower than trapping the gas in a bag. Expect an extra day or two compared to standalone ripening.
Open Air: Because it’s an open environment, the ethylene gas dissipates more readily, making it less concentrated and therefore slower.
Keep an Eye Out: Be mindful that while the bananas are ripening, your other ripe fruits might be progressing towards over-ripeness faster too!

Sunlight Exposure: A Gentle Warming Method

While not a rapid-fire method, using sunlight can certainly contribute to faster ripening by providing gentle warmth.

Mechanism: Gentle Warmth, Not Direct Cooking

Placing bananas in a warm, sunny spot (like a windowsill) increases their ambient temperature. As we discussed, warmth boosts enzymatic activity, thus speeding up the ripening process. However, direct, intense sunlight can also cause the skins to blacken prematurely without fully ripening the flesh, or even cook them if it’s too hot, so moderation is key.

Limitations

Not the Fastest: This is a slower method, perhaps shaving a day off natural ripening, but not delivering instant results like the oven.
Uneven Ripening: The side exposed to the sun might ripen faster than the shaded side.
Risk of Overheating: In very hot climates or strong direct sun, the bananas can essentially “bake” on the outside, leading to mushy spots or premature blackening without even ripening.

Factors Influencing Ripening Speed

Understanding these variables can help you better control the ripening process, regardless of the method you choose.

Temperature: This is perhaps the most significant factor. Warmer temperatures (around 68-72°F or 20-22°C) are ideal for natural ripening. Colder temperatures (like in a refrigerator) will halt ripening, while excessive heat (like an oven) will force it.
Humidity: While too much humidity can lead to mold, a moderate level is beneficial. Extremely dry air can sometimes cause bananas to shrivel before fully ripening. The paper bag method helps maintain a good microclimate.
Initial Ripeness: A banana that’s already slightly yellow will ripen much faster than one that’s dark green. The greener the banana, the longer it will take, even with accelerated methods.
Presence of Other Ripe Fruits: As explored with the paper bag and fruit bowl methods, ethylene-producing fruits are powerful catalysts.
Airflow: While trapping ethylene is key, some airflow is also important. Completely airtight containers can lead to anaerobic conditions that produce off-flavors or encourage mold. Paper bags strike a good balance.

When Not to Rush It: The Art of Natural Ripening

While we’ve focused heavily on speeding things up, it’s vital to acknowledge that sometimes, patience truly is a virtue. For optimal flavor and texture, especially when consuming bananas raw, natural ripening is often superior.

The Benefits of Natural Ripening for Raw Consumption

Bananas that ripen naturally at room temperature develop a more complex, nuanced sweetness and a perfectly creamy, yet firm, texture that’s difficult to replicate with accelerated methods. The full spectrum of aromatic compounds has time to develop, resulting in that quintessential banana flavor we all love. For slicing onto cereal, adding to fruit salads, or just peeling and eating, naturally ripened bananas are usually the gold standard.

How to Store Bananas for Natural, Optimal Ripening

Room Temperature: Always store unripe bananas at room temperature (around 68-72°F or 20-22°C) on your countertop or in a fruit bowl.
Away from Direct Sunlight (usually): While some gentle warmth can help, intense direct sun can lead to uneven ripening or premature blackening.
Consider Separation: If you want to slow down ripening (once they hit your desired yellow stage), you can separate them from other ripe fruits or even separate the bunch from each other. The stem of a banana bunch is a major site of ethylene release. You can even wrap the stems in plastic wrap to slow ethylene escape.
Avoid Refrigeration (initially): As mentioned, never put green bananas in the fridge. Once they’re fully ripe and yellow, however, you *can* refrigerate them to extend their shelf life by a few days. The skins will turn black, but the flesh inside will remain ripe and firm for longer.

Troubleshooting Common Banana Ripening Issues

Even with the best intentions, banana ripening can sometimes throw us a curveball.

Why Aren’t My Bananas Ripening?

Too Cold: The most common culprit! If your bananas were stored in a cool pantry or refrigerator at some point, their ripening process might have been stalled or permanently impaired.
Not Enough Ethylene: If they’re sitting all alone on a cold counter, there simply isn’t enough concentrated ethylene gas to kickstart the process efficiently. Try the paper bag method with a ripe apple.
Very Green to Start: Some bananas are harvested incredibly green and might take an extra long time, no matter what. Patience (or a heat-based method) is key.

My Bananas Went from Green to Mush! What Happened?

Over-Acceleration: You might have overdone it with the heat (oven/microwave) or left them in a very hot, sealed environment for too long.
Over-Ripe Ethylene Source: If you put a super-duper ripe, nearly black banana in a bag with green ones and left it too long, the sheer volume of ethylene could have pushed them past ripe straight to mush.
Pre-existing Damage: Sometimes bananas get bruised during transport, and these spots can break down rapidly, leading to mushiness before the rest of the banana ripens.

Can I Ripen Peeled Bananas?

Technically, you *can* peel a green banana and try to ripen it, but it’s generally a bad idea for eating whole. The exposure to air will cause rapid oxidation (browning) and drying out. While you might be able to get it softer for mashing by putting it in a bag or microwaving it briefly, the flavor and texture will be severely compromised. It’s always best to ripen bananas in their natural protective peel.

Expert Tips for Banana Enthusiasts

As someone who practically lives on bananas, I’ve picked up a few extra nuggets of wisdom that might help you on your quest for banana perfection:

Strategic Shopping: When grocery shopping, always buy bananas at different stages of ripeness. Get a few green ones for later in the week, some yellow ones for immediate snacking, and maybe a spotty bunch for that weekend baking project. This ensures a continuous supply of perfectly ripe bananas.
Separate to Slow, Keep Together to Speed: If you have a bunch of bananas and want them to last longer, separate them from each other and store them apart. Each banana produces its own ethylene, and grouping them amplifies the effect. Conversely, keeping them bunched together will encourage them to ripen faster. Wrapping the crown/stem of the bunch in plastic wrap or foil can also slightly slow down the release of ethylene from that primary source.
Different Varieties, Different Speeds: While the common Cavendish banana is what most of us buy, remember that other varieties (like plantains, red bananas, or manzano bananas) ripen at different rates and have different ideal ripeness stages. Know your banana!
Freezing for Future Use: Once your bananas hit peak ripeness (especially if they start getting too many brown spots for raw eating), peel them, chop them into chunks, and freeze them in an airtight container or freezer bag. Frozen ripe bananas are phenomenal for smoothies, “nice cream,” or thawing and using in baked goods. It’s a great way to prevent waste!

Frequently Asked Questions About Ripening Bananas

Let’s address some of the most common questions people have when trying to get their bananas just right.

How long does it typically take for bananas to ripen naturally?

Under normal room temperature conditions (around 68-72°F or 20-22°C), green bananas usually take about 3 to 7 days to ripen fully. This timeframe can vary based on their initial ripeness when purchased, the specific variety, and the ambient temperature of your kitchen. If they start out very green and firm, expect them to lean towards the longer end of that spectrum. Factors like being kept in a bunch or near other ripe fruits can slightly reduce this natural ripening time.

Can I ripen bananas in the refrigerator?

No, you absolutely should not put unripe (green) bananas in the refrigerator if your goal is to ripen them. Refrigeration halts the production of ethylene gas and significantly slows down the enzymatic processes responsible for ripening. Once a green banana has been exposed to cold temperatures, it often struggles to ripen properly even if you bring it back to room temperature; it may simply turn black and mushy without developing the characteristic sweetness and flavor of a ripe banana. However, once bananas are fully ripe (yellow with some brown spots), you can place them in the refrigerator to extend their shelf life by a few extra days. The skin will turn black, but the flesh inside will remain good for consumption.

What’s the best method for ripening bananas for banana bread?

For banana bread and other baked goods, the best methods are those that rapidly convert starches to sugars and soften the fruit, even if the texture isn’t ideal for raw eating. The oven method is hands-down the quickest and most effective way to get bananas ready for baking almost instantly. Baking them at 250-300°F (120-150°C) until the skins are black yields a wonderfully sweet, soft, and slightly caramelized pulp perfect for mashing. The air fryer method is a close second for speed and results. If you have a little more time (1-3 days), the paper bag method with an apple or a ripe banana will also produce beautifully ripe bananas that are excellent for baking, with a more naturally developed flavor.

Do all fruits produce ethylene gas?

No, not all fruits produce ethylene gas or respond to it in the same way. Fruits are generally categorized into two groups: climacteric and non-climacteric. Climacteric fruits, like bananas, apples, avocados, tomatoes, peaches, and pears, continue to ripen after being harvested and produce a burst of ethylene gas that signals this ripening. Non-climacteric fruits, such as citrus fruits (oranges, lemons), grapes, pineapples, and berries, do not ripen significantly after being picked and produce very little ethylene. They will not get sweeter or softer once off the plant, though they might become overripe and rot.

Is there a way to slow down banana ripening?

Yes, there are several ways to slow down banana ripening once they reach your desired stage or if you simply want your green bananas to last longer. The most common method is to store fully ripe bananas in the refrigerator; the cold temperature inhibits further ripening, though the skins will turn black. Another trick is to separate the bananas from the bunch, as the crown is a significant source of ethylene. You can also try wrapping the stems of the banana bunch in plastic wrap or aluminum foil. Lastly, keep bananas away from other ripe fruits in your fruit bowl, as their ethylene emissions will accelerate the ripening of your bananas.

Can I eat unripe green bananas? Are they healthy?

You can certainly eat unripe green bananas, but they have a very different taste and texture than ripe ones. They are much firmer, less sweet, and have a starchy, sometimes slightly bitter or astringent taste. Nutritionally, green bananas are high in resistant starch, which acts as a prebiotic, feeding beneficial gut bacteria and potentially aiding in blood sugar control. As a banana ripens, much of this resistant starch converts to simple sugars. So, while they are healthy, their taste and texture might not be appealing to everyone for raw consumption. They are often cooked or used in specific culinary applications (like plantains) rather than eaten raw.

What about ripening bananas in water? Does that work?

No, ripening bananas in water is generally not an effective or recommended method. While some might suggest submerging bananas in warm water to soften them, this primarily just heats them and can make them mushy without truly accelerating the complex chemical ripening processes that convert starch to sugar and develop flavor. Furthermore, leaving bananas submerged in water for extended periods can lead to waterlogging, spoilage, and mold growth rather than proper ripening. Stick to methods that leverage ethylene gas or controlled heat for best results.

Does peeling a banana make it ripen faster?

No, peeling a banana does not make it ripen faster in a desirable way. In fact, peeling a banana exposes its flesh to oxygen, which causes rapid oxidation and browning. While the exposed flesh might soften somewhat due to dehydration, it won’t develop the proper sweetness or flavor that comes from the natural ripening process. The peel acts as a protective layer, allowing the internal processes to occur optimally. If you want to speed up ripening, it’s always best to keep the banana in its skin and use one of the ethylene-trapping or heat-based methods.

What’s the difference between artificially ripened and naturally ripened bananas?

Naturally ripened bananas undergo a gradual process where their own ethylene production triggers the conversion of starches to sugars, softening of cell walls, and development of complex flavors and aromas. This results in a sweet, creamy banana with a distinct taste. Artificially ripened bananas, particularly those destined for commercial sale, are often harvested green and then exposed to a controlled burst of external ethylene gas (often in ripening chambers) to initiate the ripening process quickly. While this makes them turn yellow and soften, the rapid acceleration can sometimes lead to bananas that are yellow on the outside but still starchy or less sweet on the inside, or that lack the full depth of flavor of a naturally ripened fruit. However, for most home cooks, the methods discussed here (like the paper bag) simply augment the natural ethylene production, providing a result closer to natural ripening.

How to get bananas ripe fast