Strawberries represent one example of non-climacteric fruit, since they cannot further ripen after picking. Photo by Deborah Jeanne Sergeant

by Deborah Jeanne Sergeant

Growing fruit successfully relies upon harvesting good quality fruit. MidAtlantic Women In Agriculture presented “What is Fruit Quality?” as a recent webinar, featuring Dr. Macarena Farcuh, assistant professor and Extension specialist with the Department of Plant Sciences and Landscape Architecture, University of Maryland.

“What is fruit quality?” Farcuh asked. “Fruit quality has to do with color. It is a very important component of quality. Then we have texture.” A melon should have softer flesh than a crisp apple, for example.

“It will vary according to the type of fruit,” Farcuh said. “Then we have taste. Taste is composed by sweetness, given by the contents and composition of sugars within the fruit and the acid content – the balance between acid and sugars.”

The color can indicate these aspects of flavor. “Color will change as fruit ripens,” Farcuh noted. This happens as the picked fruit’s chlorophyll or green color degrades. An accumulation of pigment occurs, including non-photosynthetic pigments of anthocyanins (red and purple) and carotenoids (yellow and orange).

Farmers and researchers can use a handheld colorimeter to accurately measure fruit color. The device’s pulsed xenon lamp equipped with a photocell can detect amounts of light passing through a sample. Farcuh said a colorimeter offers “a value that we can compare on a quantitative scale” instead of a subjective observation of color.

Texture represents another facet of fruit quality. “We all know that the softening of fruits is crucial for handleability and storage of fruits but also for consumers,” Farcuh said. “We want a peach that is soft and juicy. We don’t want one that is firm with no juice. How could we enjoy that? The softening is related to three main aspects: cell wall modifications, turgor pressure and skin/cuticle composition. Throughout ripening, the cell wall is degrading and losing its tightness. It allows fruit to soften.”

The trigger pressure relates to the water balance within the fruit. The fruit’s skin composition also matters, as some can have cracks or exhibit structures that aid in maintaining a firmer or softer texture. A texture analyzer can help determine the amount of pressure needed to puncture the flesh of the peeled fruit.

“If we are talking about apples, peaches and plums, we use a puncture probe and a pressure probe to press the fruit,” Farcuh said. The measurement of compression mimics that of human chewing. “We can derive a series of texture-related parameters that will give us values and characterize our texture of our fruit,” Farcuh added. A penetrometer can help measure these parameters.

Taste, a vital measure of fruit quality, is comprised of the balance between sugars and organic acids. “Specific fruits have characteristic acids present in higher concentrations,” Farcuh said. The acid is higher in fruits like citrus fruits, for example.

The taste of fruit is measured by its starch content. “Apples are pretty particular,” Farcuh said. “They have a lot of starch. It’s an important carbohydrate that’s a component of fruits. When they have high levels of starch, we perceive they’re low in sugar.” To measure the amount of starch, a starch iodine test is used.

Aroma volatiles represent another measure of fruit quality. “Production of volatile organic compounds are detected by over 650 types of olfactory nerve endings in our nose,” Farcuh said. “We can perceive different volatiles at different intensities.” Volatiles affecting fruit flavor include esters for fruity aromas; alcohols, offering earthy, more fermented aromas; aldehydes for grassy and bitter aromas; and lactones for peach-like aromas.

Gas chromatography coupled to mass spectrometry can measure fruit aroma. “This technique helps separate and identify compounds in their gaseous forms based on their masses,” Farcuh said.

While scientific measurements are objective and important, consumer perception plays an important part in determining fruit quality. Farcuh said to correlate instrumental measurements to consumer perception, two major methods of sensory evaluation are used. These are consumer testing and descriptive analysis. Consumer testing uses a large group of untrained consumers, usually more than 100, to record their perceptions of fruit sample’s quality. Descriptive analysis uses a small panel of eight to 12 trained testers who rate attributes of fruit quality.

“The idea is to correlate with the instruments’ output with what we as humans perceive,” Farcuh said. “It’s a very subjective evaluation.”

The untrained testers may answer specific questions on a scale of 0 to 5 for softness, sweetness and other quality measures. The descriptive testers complete a few weeks of training before they test fruit.

Fruit is classified as climacteric versus non-climacteric. Climacteric fruits ripen even after harvest, enabling farmers to pick them green. They include apples, pears, peaches, bananas and kiwis. Non-climacteric fruits include oranges, cherries, pineapple, berries and peppers. “If non-climacteric fruits are not harvested at the right moment, their quality will be affected,” Farcuh said.

After harvest, fruit respirates and produces ethylene, which affects their quality. Ethylene is a gas hormone plants and fruits produce naturally which stimulates their own biosynthesis and affects ripening. An increase in respiration and ethlyne production allows fruits to ripen after harvest. But if it ripens too quickly, farmers cannot get their fruit to market before it degrades too much in quality.

“If we have more ethylene produced in the fruit, it will be more prone to bruises and disease and it’s going to be wasted sooner,” Farcuh said. Using a gas chromatograph can measure ethylene concentration in fruit. Controlling ethylene can reduce fruit loss and waste.

“Once ethylene production is triggered, it will not stop,” Farcuh said. “It starts the process of being less attractive. Generally, if you think about fruit operations, they’re harvesting huge amounts of fruits. It has to be transported and stored so we need a way to control ethylene production.”

One option is 1-methycycloropane, which blocks ethylene perception by fruit. “This is a very important tool because the ability to control ethylene production, and thus fruit ripening, is a powerful tool to reduce food waste and food loss enormously,” Farcuh said.