Fruit Quality - How Do Fruit Get TheirFlavor?

In general, fruit quality is defined by three majorattributes: color/appearance, texture, and fruitflavor.

Photo: Dr. Macarena Farcuh, University of Maryland

Though growers are often paid based on the physicalcharacteristics of their fruit (such as color, size, and lack ofdefects, texture), it is essential that the flavor of the fruit isconsidered, as this is the main driver for repeated purchases byconsumers.

Why is Fruit Flavor Important?The flavor of the fruit is very closely linked with theconsumer's preference and continued consumption of anygiven fruit variety and should be strongly considered. Meetingconsumer expectations for flavor of specific fruit, not onlycauses consumers to buy more fruit, but it allows growers toincrease their profitability. Consumers are willing to pay ahigher price for distinctly flavorful varieties that are going toenhance their experience consuming fresh fruit.

There is nothing more disappointing for consumers thanfinding the perfect apple from the grocery store — a beautifulred color, no bruises, a firm texture — but when taking thefirst bite, the flavor is boring, flat, and is overall adisappointment. Once a consumer has purchased a fruit withlittle or poor flavor, they are inclined to not buy those fruitagain and/or move on to a different grower or a different fruitall together. Producing fruit with complex and delicious flavoris extremely important to the consumer's enjoyment of their

eating experience.

What is Fruit Flavor?Flavor is described as the interaction between taste and aroma.Taste relates to the ratios and intensities of non-volatilecompounds, specifically sugars, and acids. Sugars and acidsare detected by five classes of receptors in the tongue – sweet,sour, salty, bitter, and umami (protein taste, represented byglutamate). Volatile compounds, which create the aromas offruit, are detected by over 650 types of olfactory nerve endingsfound in the nose.

The sweetness of a fruit is influenced by the quantity andcomposition of sugars. Higher contents of sugar in the fruitincreases the sweetness of the fruit. Additionally, differentforms of sugar affect the sweetness of the fruit. In fruit such asapples, peaches, and plums, the main sugars present aresorbitol, sucrose, fructose, and glucose. Each of these sugarshave a different degree of sweetness. Fructose has 1.7 timesthe sweetness of sucrose, while glucose and sorbitol only have0.8 and 0.6, respectively. For example, if one apple variety hashigher contents of fructose and another variety has higherglucose, the former will taste sweeter.

The acidity of a fruit is influenced by the content andcomposition of organic acids, and the amount of each type ofacid found in each fruit. For example, the dominant acid inapples, peaches, and plums is malic acid.

The balance between the sweetness and acidity of fruit basedon the quantity and composition of their sugars and acids isimportant for developing a complex and interesting taste thatwill enhance fruit flavor.

Another key component of flavor is aroma. Fruit aroma isinfluenced by the quantity and composition of volatilecompounds. The volatiles that are well-known to affect fruitflavor include esters (fruity aroma), alcohols (fruity or earthyaroma), aldehydes (slightly grassy and bitter aroma), lactones(peach-like aroma), and terpenoids (scented oils aroma).Studies have shown that the flavor intensity of a fruit can becorrelated with the quantity and composition of volatilespresent. For example, strawberries that presented higher levelsof certain key volatiles were perceived as sweeter and highlypreferred by consumers, as compared to other strawberryvarieties lacking these volatiles.

Page 2 Fruit Quality - How Do Fruit Get Their Flavor?

How is Fruit Flavor Measured?Fruit flavor can be measured instrumentally as well as throughthe use of sensory science. Sensory science is amulti-disciplinary field that uses scientific measurements tointerpret the human response to the senses of sight, smell,taste, touch, and hearing. This form of science is able to linkthe product to the person in a direct way.

Instrumentally, sugar content of the fruit is determined bymeasuring the soluble solid content using a portablerefractometer. A refractometer measures how light is bent as itpasses through a sample, which is correlated to a specificpercentage of sugar in the fruit, and thus is related to fruitsweetness. Titration measures the content of the dominant acidpresent in the fruit and can also be measured and calculatedusing portable acidity/pH meters.

Aroma volatiles are challenging to measure with portableinstruments. They are quantified by using GasChromatography – Mass Spectroscopy (GC-MS), a lab-basedtechnique which helps to separate and identify compounds intheir gaseous forms based on their masses.

To correlate these instrumental measurements to consumerperception, two major methods of sensory evaluation are used:consumer testing and descriptive analysis. Consumer testingincludes subjective data about the preferences of a large groupof untrained tasters (usually more than 100 panelists), whiledescriptive analysis includes questionnaires for a panel of 8-12trained tasters who are able to rate specific attributes related tofruit quality.

Portable sugar (refractometer) and acidity meter (left); GasChromatography-Mass Spectroscopy (GC-MS) equipment(center); samples for sensory evaluation (right). Photo: Dr.Macarena Farcuh, University of Maryland

What Factors Affect FlavorDevelopment?

GeneticsA key factor for determining fruit flavor lays in the geneticbackground of the variety that was chosen. When determiningwhich variety to establish, it is important to choose varietiesthat were bred for flavor as one of its priorities. Additionally,the choice of rootstock, for fruit such as apples, can influencefruit flavor potential. Studies have shown that particularrootstocks can affect the levels of organic acids and sugarcontent found in the same variety of apple.

Environmental factorsEnvironmental factors also play a hugely important role inaffecting the flavor of the fruit and strongly interact with thegenetic background. Although often times these factors cannotbe controlled, they must be taken into account when aiming toimprove the flavor of the fruit that are being produced. Themajor environmental factors affecting fruit flavor developmentinclude temperature, relative humidity, and sunlight during thegrowing season.

A good practice before establishing a new orchard is toconduct small variety trials to evaluate which varieties arecapable of developing flavor under the specific environmentalconditions. As there is strong interaction between varieties andtheir growing environment, a variety that is successful in onearea may not be as successful in another region.

Preharvest Factors, Orchard Management,and Cultural PracticesFruit flavor can be affected by different orchard managementpractices, such as planting density, tree structure, irrigationregime, light manipulation, crop load, nutrition, and pestcontrol methods.

Practices that increase the amount of sunlight reaching thefruit, such as pruning or the use of reflective groundcovers,have been shown to increase flavor development as well ascolor.

Crop load management is another important factor affectingfruit flavor development. In a study done on apples, trees withlower crop loads were found to have fruit with increasedflavor development. This was mainly due to higher levels ofaroma volatiles and sugars in the apple fruit with lower cropload. Many times, trying to aim for the highest yield will playagainst flavor development.

Irrigation management can also have an effect on fruit flavor.It is important to maintain balanced irrigation levels in theorchards. Excess irrigation will decrease the overall flavorproduced during the fruit growth. Irrigating in intervals toavoid inducing stress on the tree is good practice and willpositively impact fruit flavor development.

Concerning nutrition, excessive levels of nitrogen willdecrease the "fruity" aromas that are developed in the fruit,while increasing the "green" and "grassy" aromas. Manystudies have shown that moderate nitrogen supply for the treeincreased fruit flavor, but an excessive amount actuallydeteriorates fruit flavor.

Page 3 Fruit Quality - How Do Fruit Get Their Flavor?

Fruit Maturity at HarvestHarvesting practices and the maturity of the fruit at harvest isan often-forgotten key factor in maintaining fruit flavor. Fruitproduced for wholesale distribution typically tends to beharvested before fully ripe in order to ensure that the fruit canbe transported easily without being damaged. Unfortunately,in this case the fruit's flavor is likely not completelydeveloped. On the other hand, if fruit are harvestedover-mature, there is an increase in the "fermented" flavor,which is disliked by consumers. Thus, in the Mid-Atlanticwhen harvest is delayed waiting for red skin colordevelopment in apples, fruit flavor is directly affected.Harvesting over-mature apples is harmful for flavordevelopment, overall fruit quality and fruit storage capacity.This directly impacts profitability.

Harvesting fruit at the correct time is a 'balancing act' betweengreat flavor, overall quality, and shelf-life capacity. Favoringone of these factors over others may affect the marketability ofthe fruit, so it is of key importance to harvest fruit at thecorrect time.

Postharvest FactorsOnce the fruit is harvested, flavor can be maintained or lost,which is why improving postharvest factors is critical toensure flavor maintenance. These factors are mainly a balanceof storage temperature, relative humidity levels, oxygen (O

2 )

and carbon dioxide (CO 2 ) balance, and ethylene levels.

The combination of storage temperature and length of storagecan cause loss of flavor and development of physiologicaldisorders that will affect fruit marketability. Storagetemperatures vary with each fruit and each variety to ensurethe best possible maintenance of flavors. Practices such asfruit conditioning, e.g. storing fruit at 68°F for 24-48 hours orat 50°F for 7 days before cold storage, are used in peaches and'Honeycrisp' apples, respectively, to avoid the development ofphysiological disorders that will induce flavor loss.

In terms of relative humidity, the target is to achieve rangesbetween 85-95%. If the humidity levels are too low, the watercontent of the fruit can decrease which leads to fruitshriveling, weight loss, and consequent changes in flavorprofiles. In humidity levels over 95%, condensation on thefruit may lead to mold growth.

The balance between O 2 and CO

2 concentration during

storage can also directly affect fruit flavor. Storing fruit with alow O

2 supply can cause the fruit to begin undergoing

anaerobic respiration which leads to a quick break down ininternal sugars, essential to the overall flavor of the fruit. Ahigh level of CO

2 negatively influences the respiration of the

fruit in storage, affecting the acidity levels of the fruit. Lowerlevels of O

2 and high levels of CO

2 can lead to fermentation,

which results in the formation of an 'off-flavor' which isdisliked by consumers. Controlling the atmosphere of applesin storage allows the fruit to maintain their flavor and notdevelop these unwanted "off-flavors".

Ethylene, a gaseous plant hormone that enhances fruit ripeningand can be produced by the fruit itself, plays an important rolein fruit flavor development during postharvest. High ethylenelevels during storage enhance the development of aromavolatiles, but make fruit become soft and last less in storage.Low ethylene levels during storage, often achieved by usingcommercial products such as SmartFresh (1-MCP), whichblocks the perception of ethylene by the fruit, maintains highfruit firmness, but inhibit the production of aroma volatiles.Therefore, maintaining adequate ethylene levels duringstorage is a key factor for maintaining fruit flavor.

SummaryAs discussed throughout this article, taking the flavor of thefruit into account in every step of the production process isessential. Although it may be complex and difficult toevaluate, fruit flavor is one of the main factors that keepsconsumers enjoying eating fresh fruit and pursuing repeatedpurchases. Therefore, a great fruit flavor developmentconsequently increases the marketability and subsequentprofitability of fresh fruit.

AuthorsMacarena Farcuh, Ph.D.Assistant Professor and Extension SpecialistUniversity of Marylandmfarcuh@umd.edu

Erin McCaugheyCandidate for B.S. in ChemistryUniversity of Marylandemccaugh@terpmail.umd.edu

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