Changes in baseball chemistry could contribute to rise in home runs

A surge in home runs during Major League Baseball (MLB) games since 2015 has prompted fans, players, and researchers alike to question whether the balls have been deliberately altered, or “juiced.” Now, chemists have cracked them open to see whether changes to the balls’ cores have helped them fly farther (ACS Omega 2019, DOI: 10.1021/acsomega.9b00405).

Home run rates increased by over 60% between June 2014 and June 2019—with a steady rise beginning after the 2015 MLB All-Star Game when teams typically replenish their ball supply for the rest of the season. Professional baseball leagues have made changes to the game before, like lowering the pitching mound, to increase home runs and boost entertainment and revenue. However, MLB and its commissioner have denied making changes to the design of the balls.

In 2018, MLB convened a 10-member scientific committee to investigate what could be behind the surge in home runs. The committee found evidence that the balls produced for the 2017 season by MLB’s sole manufacturer, Rawlings, had reduced drag compared to prior years. In December 2019, four members of the original committee released a preliminary follow-up report, which maintained there was no evidence that newer balls were deliberately altered, and instead the surge in home runs was likely due to manufacturing variability.

Back in 2018, as the committee was composing its initial report, the television show ESPN Sport Science was commissioning research of its own. The physics of the game and the biomechanics of the players had been well studied, but the chemical composition of the ball had received relatively little attention. So ESPN asked scientists at the University of Southern California’s Keck School of Medicine to look inside the balls using computed tomography scans. The results were published on FiveThirtyEight.com, and the peer-reviewed results appear in the new ACS Omega paper.

The core or “pill” of a baseball is composed of three layers: two rubber layers and an innermost cork layer. The researchers scanned four baseballs used in MLB games between 2014 and the 2015 All-Star Game, and found that the balls’ cores were 56.7% denser than four balls used subsequently until the 2017 season.

To look further into this decrease in density, Soumitra Basu’s group at Kent State University, led by former graduate student Nathan Beals, turned to trusted chemical techniques like scanning electron microscopy, energy-dispersive X-ray spectroscopy, and thermogravimetric analysis. They determined that the newer baseballs were more porous than the older ones and contained about 10% less silicon.

Basu and Beals specialize in nanotechnology for therapeutic development, but they were excited to apply their knowledge of chemistry to baseballs. Basu explains that a less dense, more porous pill could make the ball bouncier and increase its speed off the bat—although there are so many factors at play that pill density alone likely does not account for the rise in home runs.

Meredith J. Wills, a data scientist working in ball- and player-tracking at SportsMEDIA Technology who was not affiliated with the study, agrees the decrease in density could affect bounciness, but additional experiments are needed to match the chemistry to the physics.

“What’s becoming increasingly clear,” Wills says, “is that even subtle changes seem to have a lot of importance.” The more researchers understand about these changes, the easier it will be to predict the effects of future manufacturing decisions. In fact, she adds, the 2019 season also experienced a spike in home runs, likely due to another small change in baseball production.

According to the December MLB report, the balls used during the 2019 season—in circulation after those analyzed in the ACS Omega study—had a lower seam height, which decreased drag and increased home runs. By comparison, the 2019 postseason balls appeared to have increased drag, causing a mysterious decline in home runs that the committee could not explain because the ball sample size was too small.

Although the MLB report does not touch on the chemistry of the baseball core, Beals says studies like these underscore the many complex dynamics that can affect how the ball behaves.

So was the ball juiced between the 2015 All-Star Game and the 2017 season? “It’s not our place to suggest such things,” Basu says. “We did the science, and here are the results.”

Beals hopes their results will compel others to continue to probe the ball’s chemistry, and perhaps even inform game technology. After he completes his postdoc studying pancreatic cancer, he may return to baseball research, though it’s unlikely. “I’m definitely out of the game for now,” Beals says.