Why Does Watermelon Sometimes Turn to Mush Inside?

The half watermelon I bought at the fruit shop

I walked past a fruit shop in the evening, and they had several half watermelons out for sale. I thought they'd been cut fresh that day. I'd bought from this shop quite a few times before, so I didn't think much of it and took one home.

When I got it home, the rind felt very soft. The flesh was soft as well. It looked like the melon had been in a fridge and then left out for a while. In the photo, the flesh looks wet and glassy with pale patches. I took one bite, and the taste hadn't changed. But I didn't know if it was okay to eat, and I kept thinking about getting an upset stomach.

Why does watermelon go mushy?

Watermelon is already very watery, so it doesn't take much for the texture to go. As a watermelon gets older, natural enzymes break down the pectin that holds the cell walls together. The cells lose their shape and let out their juice, so the flesh goes soft and wet. ScienceInsights explains this, and it says an overripe melon that is only mealy is usually safe if there's no sour smell, slime or mould.

Cutting speeds this up. A recent study on fresh-cut watermelon looked at what it calls water soaking, where the flesh gets wet and translucent during storage. It found that genes for cell wall breakdown, ethylene and stress responses turned up as the cut fruit aged. That fits what I saw in my photo, though I can't say for sure that's what happened to mine.

Temperature matters too. The National Watermelon Promotion Board says a whole watermelon kept at 0°C for two days can develop an off flavour, pitting and colour loss. It also says freezing breaks down the rind and makes the flesh mealy and mushy. A USDA-funded watermelon information sheet adds that watermelons can go mushy near fruit that gives off ethylene gas. So a melon that has been chilled and then left out could soften quickly. I don't know what happened to mine, and a soft rind on a cut half tells me it had been sitting for a while.

Is a mushy watermelon safe to eat?

If a whole melon is a bit mushy, doesn't smell sour, isn't slimy and has no mould, it's usually a quality problem and not a safety one. A cut half from a shop is different, because a cut melon is a higher-risk food.

The NSW Food Authority classes cut melons as potentially hazardous foods and says there have been several outbreaks in Australia linked to melons. Its survey of shops in Sydney also found that most greengrocers don't display cut melon under temperature control, and that supermarkets don't always refrigerate cut watermelon. The same survey found the quality of the samples was very good overall, so this isn't a reason to panic. Queensland Health's guidance for rockmelon says pre-cut melon should be kept at 5°C or below, and that cut melons shouldn't be displayed for more than two hours without temperature control. That page is about rockmelon, but it's the closest Queensland guidance I could find.

The FDA has advice that fits my story exactly. When you buy pre-cut produce, such as half a watermelon, choose only ones that are refrigerated or on ice. It also says that illness from harmful bacteria usually starts within 1 to 3 days, but can start as early as 20 minutes or as late as 6 weeks after eating. So the fact that my bite tasted normal doesn't tell me much. Bacteria usually don't change how food tastes or smells.

Can you just cut off the outside layer of overnight watermelon?

Everyone in the family says "just cut off the top layer," and it sounds reasonable. The trouble is that it only removes what you can see. Bacteria on the rind can be carried into the flesh by the knife when the melon is cut, according to a Wisconsin food safety fact sheet. I couldn't find a source that says how deep bacteria go, so I wouldn't rely on trimming to make a doubtful melon safe.

The NSW survey is also careful about this. It found that fruit kept for the next day was generally trimmed and re-wrapped before it went back on display, and it says re-trimming should be discouraged, especially if the fruit is at room temperature. I don't know if that happened with the half I bought, and I'm not saying the shop did anything wrong. It's just a reason not to assume a half watermelon was cut fresh that day.

How long a cut melon lasts depends on how it was kept. For a melon that was kept cold from the moment it was cut, the general guidance is 3 to 5 days in the fridge. One rule I've seen for leftovers is 24 hours, which is a lot more careful. For a melon I got home with an unknown history, the 24-hour rule is closer to how I'd treat it. Whether it's safe depends on the whole storage story, and a knife can't fix that.

Three things that decide if leftover watermelon is safe

Was it refrigerated straight after cutting?

Cut watermelon shouldn't sit out for more than two hours, and less than that on a hot day. The USDA-funded sheet I mentioned says to discard cut watermelon after 2 hours in the temperature danger zone. So don't cut a melon, leave it in the living room for a few hours and then remember it at bedtime. The best way is to cut only what you'll eat and put the rest in the fridge straight away. Fresh is always better than leftover.

Has it been kept away from other food in the fridge?

A fridge has raw meat and raw vegetables in it, and their juices and bacteria can end up on a melon that's just sitting there open. The FDA says to keep raw meat, poultry and seafood on the lower shelves so the juices can't drip onto produce, and to keep produce that's eaten raw away from them. What I'd do is put the melon in its own container, or wrap it in fresh cling film, and keep it on an upper shelf or in the fruit area. Listeria is the one to think about here. FSANZ says cut melon is one of the ready-to-eat foods that can support the growth of Listeria, and that can happen slowly even in the fridge.

What does the flesh look and smell like?

If any of these show up, throw it out and don't feel bad about it. A sour or fermented smell is one. A sticky or slimy surface is another. So is a lot of liquid leaking out, or flesh that has gone soft and collapsed. Honestly, that last one was close to what I had, which is part of why I'm not comfortable with it. If you're wondering about the fermented smell, I wrote about it in my post on getting drunk off overripe fruit.

Who should skip leftover watermelon

Older people, young children, pregnant women and anyone with a sensitive stomach should avoid leftover cut watermelon if they can. Queensland Health says Listeria and Salmonella can cause very serious illness in pregnant women, babies, the elderly and people with reduced immunity. The FDA lists young children, older adults, pregnant people and people with weakened immune systems as the groups with the most risk. For those people, I'd say a piece of melon isn't worth the gamble.

Buying pre-cut watermelon

If you want to avoid this altogether, the simplest thing is to buy a whole watermelon and cut it yourself. Scrub the rind with a clean brush under running water first, since the FDA says that stops bacteria on the outside being carried in by the knife. If you do buy a half, look at whether it's in a fridge case or on ice. It's fine to ask when it was cut. I also have a post on whether melon will ripen after it's been cut that goes through how to store cut melon, and one on what fruit is in season if you want to know when watermelon is at its best.

Does your family manage to finish a watermelon on the same day?

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