Watermelons get pricier — here's how to pick a good one

"Ordinary Koreans can't even afford to buy a watermelon."

That claim, mocking Korean purchasing power, spread widely on Chinese SNS under the label "watermelon theory." A video of a Chinese man walking through Seoul's Myeong-dong carrying a whole watermelon — while Koreans supposedly looked on with envy — racked up high view counts.

It is absurd misinformation, but a glance at current watermelon prices makes it hard to laugh off entirely. Like the so-called "golden apple" phenomenon that drove up apple prices, watermelons are no longer a fruit most people can buy without a second thought.

According to the Ministry of Statistics' consumer price index for June, watermelon prices rose 10.9% from the same month a year earlier — well above the 3.2% average increase for agricultural, livestock and fishery products. As of Thursday, the Korea Agricultural Marketing Information Service (KAMIS) put the average retail price of a whole watermelon at 23,116 won ($16).

Given the higher prices, knowing what to look for when buying a watermelon pays off. The most common method is to tap it. The saying "a hollow thud means it's delicious" has a scientific basis.

According to the Rural Development Administration, the resonance a watermelon produces when tapped can indicate how ripe it is. The vibration varies depending on the moisture content, flesh density and any hollow spaces inside, which is why the sound differs. Farmers and sorting facilities routinely tap watermelons to assess their condition.

A properly ripe watermelon has firm flesh evenly filled with moisture, so vibrations travel uniformly through it — producing a clear, hollow thud when tapped.

An underripe watermelon lacks sufficient flesh density and gives off a sharp, metallic ring. An overripe one fails to transmit vibrations properly, producing a dull, muffled thump.

You can also feel the vibration with your hands. Support the watermelon with one hand and tap the center lightly with the other. If the vibration travels all the way to the bottom, the fruit is usually ripe throughout. Little to no vibration is a sign it may be overripe.

Sound and vibration alone, however, cannot tell you how sweet a watermelon is. For sweetness, check the size of the navel — the small, circular indentation on the bottom of the fruit. A smaller navel generally indicates higher sugar content.

There was also a long-held belief that a fresh watermelon should have a long, T-shaped stem — the dried tendril connecting it to the vine. In 2015, however, the Ministry of Agriculture, Food and Rural Affairs revised the retail grading and handling practices that had used a long T-shaped stem as a freshness standard. The ministry determined there was no scientific basis for the claim and that maintaining long stems added unnecessary sorting and handling costs. Today, watermelons with stems trimmed to within 3 centimeters are the norm.

The Rural Development Administration's freshness standard for watermelon stems focuses not on length or shape but on whether the stem has changed color or dried out. Watermelons lose moisture from the stem first. A stem that remains green or greenish-light brown indicates freshness. Avoid watermelons with stems that are completely dried, brown or blackened.

The stripes on the rind are another indicator. Look for dark stripes that are sharp and evenly distributed, and a glossy skin. Faint or pale stripes may be a sign the watermelon is underripe.

Heft matters too. When comparing watermelons of the same size, choose the heavier one — a weightier watermelon usually means the flesh and moisture are packed in tightly.

With single-person households on the rise and storage convenience in demand, many retailers now sell watermelons pre-cut. For sliced watermelon, cracked or split flesh on the cut surface may be a sign it is no longer fresh.