The Truth on Techniques and Traditions for Choosing Ripe Watermelons

Earlier this month, I published an article on harvesting watermelons.  I took you through the paces of choosing the perfect ripe watermelon through various outward indications. These signs I took for granted, assuming most of them to be tried and true.

The more I thought about it, however, the more I suspected at least one of those tricks to be mere myth or lore. Today, my son and I want to separate fact from fallacy by putting each of these methods to the test.  With six watermelons, we will explore six traditional indications for judging a good watermelon. 

Putting Watermelon Tests to the Test

These tests should work whether you get your melon straight off the vine, from a farmers’ market, or from the grocery store. The indications for the best watermelons that we’ll test are:

  1. Dark, dull watermelon
  2. Yellow ground spot / field spot, instead of the white bottom that indicates unripe melons
  3. Soft spots 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

We’re going to measure our watermelon ripeness scientifically with a refractometer. This instrument measures the percentage of sugar content in the melons. Stacy Lyn will then take a bite of each melon to give it the taste test. Granted, that’s a subjective measure, but she has 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/bloom-end orientation.  We tested each watermelon 8 times, with each twig being tested once on each side of the melon.

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 various criteria
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. Stacy Lyn wanted this to work, and it may some of the time, but in this 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.  Further, the pitch correlated with appearance, giving 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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