What you see: Areas of brown, dry-looking apple flesh; likely also some seed-shaped holes.
What it is: Carbon dioxide injury.
Eat or toss? This is not an apparent food safety risk. However, even the normal-looking portions of apple flesh may not taste good. If you’re determined, trim around the brown and give it a sample. But it’s likely a “toss.”
Apples can turn brown inside if exposed to too much carbon dioxide
One great feature of apples is their incredibly long shelf life. Those apples you often see at spring farmers markets are usually from the previous fall. And that’s OK!
But to maintain their crisp flesh and signature flavors they have to be harvested at the right time and stored properly. The temperature, humidity and even balance of carbon dioxide and oxygen in the air can impact how well they do in storage.
While there are a number of issues that could cause an apple to brown on the inside, in this case it looks like something in the air wasn’t quite right.
Because, while in storage, apples are still alive and breathing. Their cells take in oxygen and expel carbon dioxide through those tiny dots you may have noticed. This process produces energy and keeps their cells humming.
This “breathing” is called respiration and does not require sunlight. It’s entirely separate from photosynthesis, in which plants take in carbon dioxide and expel oxygen with the help of energy from the sun.
So, imagine crates of apples wintering in a cool, ventilated storage room, which typically looks like the image below.
The apples are all sucking up oxygen and expelling carbon dioxide. Sometimes too much carbon dioxide builds up, crowding out the oxygen the apples crave.
Cells have a workaround: fermentation, which can make energy without oxygen. But that last resort isn’t ideal for the apple’s interior. The cells weaken, their contents leak, mix and turn brown.
Fermentation also produces alcohol, which you might taste in an apple like this. Another fermentation byproduct? More carbon dioxide! In this case, the little almond-shaped holes are pockets of carbon dioxide.
Harvest timing and storage conditions can cause carbon dioxide injury in apples
Macarena Farcuh, a horticulture professor at the University of Maryland, told me that an apple’s susceptibility to carbon dioxide injury also depends on when it is harvested.
If an apple is harvested too late, it’s more likely to breathe faster and thus need to resort to fermentation faster.
Another path to brown spots and holes? Poorly calibrated oxygen and carbon dioxide ratios.
In some warehouses, people control the balance of gases in the air. Reducing the amount of oxygen can slow the rate at which produce respires, which can stretch their shelf life. But, too little oxygen can also injure cells and cause the apples to resort to fermentation.
And, before we go, here’s one more photo of apples in storage!
- Dr. Macarena Farcuh, PhD. Assistant professor in horticulture. College of Agriculture and Natural Resources. University of Maryland.
- Physiological Disorders. Washington State University Extension. Tree fruit postharvest export education.
- Dealing with carbon dioxide sensitivity. Good Fruit Grower. Melissa Hansen. February 15, 2012.