The Truth about Watermelon Ripeness Tests

It’s watermelon harvest season again! We take our summer melons seriously in the South and try to pick them at their peak. Nobody wants to cut into an unripe watermelon or, on the other hand, a mushy overripe watermelon. Have you ever wondered about that watermelon ripeness test handed down by your grandmother? Does it really work, or is it just a bit of lore?

hand holding up watermelon slice with seeds against white background

The thud, rap, and taps on the watermelon’s belly…the thump test, the sniff test, and the straw test… are any of these tips and tricks really true, and do they really tell you how a melon tastes? Can they help you avoid mealy fruit and find that perfect watermelon juiciness?

In another post I give five tried-and-true steps to pick a ripe, sweet watermelon. We actually got scientific and put some of these methods to the test — and had a lot of fun in the process! This post steps through our experiments and their outcomes.

A while back, my son Forrest and I decided to separate fact from fallacy by putting each of six methods to the test. With six watermelons, we explored six traditional indications for judging a good watermelon. Here are the results.

Putting Watermelon Ripeness Tests to the Test

More than one watermelon ripeness test turned out to be reliable. These tests should work whether you get your melon straight off the vine from your own garden, from a farmers’ market, or from the grocery store. The indications for the best watermelons that we’ll test are:

  1. Dark, dull watermelon skin or rind
  2. Yellow ground spot / field spot, instead of the white bottom that indicates unripe melons
  3. Soft spots on underside of the watermelon means overripe
  4. Knock producing deep pitch = best ripe melon; hollow sound = unripe melons
  5. Straw will spin when placed on ripe watermelon
  6. Heavier is always better

Using Science and Our Senses

So how serious were we about these tests? We actually measured our watermelon ripeness scientifically with a refractometer. This instrument measures the percentage of sugar content in the melons. I then took a bite of each melon to give it the taste test. Granted, that’s a subjective measure, but I have a pretty good palate. This is really the true measure we are after anyway: the tastiest melon, right?

First, we used our senses to determine appearance, bottom color (yellow spot), firmness, and pitch. We could have used scientific instruments for each test, but if the results are too close for us to distinguish by our senses, then they are no good to us in the field or in the kitchen.

We found that we needed the other melons to determine qualities such as pitch and color. An instrument would not be able to help us judge the relative ripeness of a watermelon.

The Straw Test for Ripe Watermelon: Myth or Fact?

The straw test was surely the most interesting and fun to try. Many watermelon aficionados continue to trust this unique method after many decades of use.

A November 1945 issue of Popular Science describes the method of the straw test. A picker places “a broom straw horizontally across a watermelon. If the melon is ripe, the straw will turn to a 45-degree angle; if it is green, it will not move.” The subsequent January issue proposes the straw test works by the electrical force present in the watermelons.

For the straw test, we obtained 3 broom straws from 2 different brooms, and a “green” Johnson grass twig harvested just before the test. In a room with minimal wind, we tested the theory by placing the straw in the center, perpendicular to the stem / blossom end orientation. We tested each melon 8 times, with each twig being tested once on each side of the watermelon.

Choosing a Watermelon by Weight

Nearly every farmer you talk to will tell you to “pick a heavy one” when choosing watermelons. I interpret “heavy” as heavy for its size, and not heavy compared to the rest, or else I will almost always come home with the largest watermelon!

To test this criteria, we went further than the average picker and measured the melons by the weight as compared to volume. We found the weight on a kitchen scale and found the volume by dipping the melons in a 5-gallon bucket and finding how much water they displaced.

Watch the results in this video or read the results below. Very interesting outcome.

Results of Our Ripe Watermelon Tests

We determined the ripeness by the aid of a refractometer and our taste buds. A refractometer uses the fact that water refracts (bends) light differently with different concentrations of sugar to determine the percent sugar, in degrees Brix, in the watermelon juice.

When it comes down to peak ripeness, however, a lot can be said for taste. Tasting allows us to gather and compare all the aspects of a ripe watermelon; sweetness, acid, texture, and moisture. The percent sugar and taste correlated well in the results.

The graph below tells it all.

Chart of watermelon ripeness and flavor as measured by eight different criteria for six watermelons
Our chart of watermelon ripeness and flavor as measured by various criteria.

The appearance and color generally proved true to traditional standards, but a couple of the watermelons, like #3, broke the rule and still tasted decent.

The soft melon, #4, tasted the best of the 6 but possessed an old, grainy texture.

The “thump test,” or pitch, generally proved true.

We saw no pattern in the straw test. Honestly, I really wanted this to work, and it may some of the time, but in our experiment, it was not conclusive.

There was somewhat of a pattern in “heaviness,” or density, but watermelons like the heavy but inferior #2 (lacked acid), and the light but delightful #4 challenged the pattern.

Conclusions: What Are the Best Ways to Pick a Ripe Watermelon?

I believe, after looking at the results, that pitch matters the most when picking the best ripe watermelon. All deep sounding watermelons tasted great. A higher pitched sound was a sign of an unripe melon. Further, the pitch correlated so closely with appearance tests that there was really no need to even look at the melons.

However, by pitch alone, you might end up with an overly ripe watermelon. So if it is soft, pass it up. Nonetheless, if you’re choosing one method, I believe the pitch test to be your best bet. It has proven predictable enough to convert us: from now on, we will be “thumpers” when it comes to picking a ripe watermelon!

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