Many of the posts on EatOrToss are written after interviews and email exchanges with scientists who study fruits, vegetables and other foods. I was thrilled when one of those scientists, Dr. Macarena Farcuh, an assistant professor at the University of Maryland, invited me to her lab to see that work in action.
Dr. Farcuh frequently works with apples, and the October day when I stopped by was no exception. Emily Johnson, a graduate student, and Christopher Eliker, an undergraduate, were busy evaluating samples from Maryland groves for things like color, firmness, starch and sugar concentrations. Farcuh’s lab considers those readings, along with upcoming weather, when recommending harvest timing to Maryland growers.
“When we give the recommendation to the growers we consider everything together,” she said. “Sometimes you don’t get to the desired firmness but you have the color, you have the sugars and then the weather is going to be terrible the next week. So you are like ‘OK, I think you should just go ahead of harvest instead of losing all your fruit if there’s a storm or something coming on, then you’ll have a lot of drop and fungi.’”
Here’s a virtual tour, starting just outside the Plant Sciences Building, which houses the Farcuh lab.
Chris, Emily and Dr. Farcuh work in a lab in this building.
Evaluating size and color
During the apple harvest season, Chris headed to orchards on Sundays to collect batches to test. On the day I visited, he was working with Cripps Pink apples from Eastern and Western Maryland. He started by measuring their diameters and evaluating their color. This apple, for example, has about 65 percent “blush” or red coloration.
Color evaluations can be done visually, or with a tool called a DA meter. “DA” stands for “difference of absorbance.” The tool shines a light onto an apple and then registers how the light is reflected back, which indicates how much chlorophyll (specifically chlorophyll-a) is in the skin, and therefore how green it still is. But color alone won’t guarantee the level of ripeness in a fruit; sometimes greener apples can still be perfectly ripe. The lab looks at other factors too.
Pressure test
While apples are known for being deliciously crisp, an apple we humans would enjoy eating is less firm than its younger self. As fruit matures and ripens, it softens; how far along that softening process is can be measured with a pressure test. Here’s Chris, prepping apples that test by slicing off a piece of skin.
Next, he punches the apple with a tool called a penetrometer to measure how crisp the flesh is.
Apples destined for less than two months of storage can be harvested with pressure readings of 13 to 15 pounds. Those harvested for long term storage are best harvested with a firmness reading of 15 pounds or more–the extra firmness will help them last longer. Dr. Farcuh said that late-season apples, like the Cripps Pink they were evaluating while I was there, tend to be harder and are often harvested around 20 pounds.
This apple came with a firmness reading of 22 pounds on one side and 18.7 pounds on the other side. For perspective, it takes a bit more than five pounds of pressure to crack open an egg.
Starch test
In ripening fruits, starches turn into sugars. An apple with too much starch will be underripe and not sweet enough. An apple in which most of the starches have been converted to sugars will be ready to eat, but without much remaining shelf life. To determine starch levels, scientists dip halved apples in iodine. Iodine ignores sugar molecules, but binds to starches, turning them blue-black.
Once the apples have spent enough time in the iodine solution, they can be evaluated against a chart. Different varieties have different starch patterns at different levels of maturity. These apples are coming in at a four or so on the starch-iodine index.
Any tested apples have to be thrown away and cannot be eaten, fed to animals or composted because iodine is poisonous. It’s so corrosive it has to be stored in a dark jar with a plastic lid–the iodine would disintegrate metal. Scientists always wear gloves when working with iodine.
Soluble solids and sugar content
Broadly speaking “soluble solids” refers to solids that can dissolve. The soluble solids you’re most likely to find in apples are sugars, pectins, organic acids and amino acids. And within an apple’s soluble solids, sugars dominate. So, often, when scientists like Dr. Farcuh are testing soluble solid content, what they’re really after is a sense of the sugar concentration.
To understand soluble solid concentration, they crush the juice out of bits of apple (with a simple tool like a garlic press) and place a small amount of juice in a tiny well on a calculator-sized device called a digital refractometer. The tool evaluates how light refracts off the juice, which signals the percentage of soluble solids in the juice. Soluble solid content tends to rise, leading to overall sweeter apples, during warmer years with more sunlight, as photosynthesis increases. Rainy growing seasons or trees that are overloaded with fruit can lead to lower soluble solid concentrations. Even within the same tree, fruit that get more sun exposure have higher soluble solid concentrations.
Sending out the results
For the three months of the local apple harvest season, Dr. Farcuh’s lab collaborates on a weekly email that assesses Maryland’s crops and provides input for harvest time. The report goes out to 300 subscribers in five states. Here’s the last report of the season, which includes data gathered during the tests featured above.
“Every year we get very positive feedback from growers throughout the Mid-Atlantic indicating how helpful and timely it is for them to get an idea of the maturity stage of their fruit and how it has become a tool that consistently and accurately helps them with their harvest decisions,” Dr. Farcuh told me.
For more on the apple maturity assessment process, check out Apple Maturity Indices, a video Dr. Farcuh, Emily and another student produced that recently won an award from the American Society for Horticultural Science. Dr. Farcuh’s write up How Can Growers Determine Apple Fruit Maturity and Optimal Harvest Dates? also provides more detail. To keep up with the Farcuh lab, follow its Facebook page and YouTube channel and visit the lab website. Dr. Farcuh has also partnered with another University of Maryland professor on Extensión en Español, a blog about agriculture, gardening and producing fruits and vegetables in Maryland.
Big thanks to the Farcuh Lab for showing me around!
