Scientists are currently looking at manipulating flavor and aroma (the two are inextricably linked) by means of genetic engineering. This work may lead to better tasting and more nutritious produce and increased pest resistance in plants. It may even have a profound impact on the entire commercial agricultural industry.
Trying to alter or improve the flavor and aroma of fruits, vegetables and flowers has long been the realm of plant breeders. To begin a breeding program, one first must collect a diverse population of genetic plant material, then carefully select stud plants and make crosses with the singular goal of improving the flavor or aroma of a given fruit, flower or vegetable.
This type of breeding is called selective breeding. Selective breeding, or artificial selection, is the intentional breeding of a plant with desirable traits in an attempt to produce offspring with similar desirable characteristics or with improved traits. There are several obstacles to this approach. It consumes massive amounts of space and time to grow up a speculative cross and determine if it has been successful at achieving one’s goals. Also, plants only breed with other plants of the same familial order, making the resulting possibilities limited, and because we do not yet fully understand the mechanisms that are responsible for flavor and aroma, we have been stumbling around in the proverbial dark.
Before scientists can modify flavor, first they must understand the complex matter of what flavor is. “Human perception of ‘flavor’ involves integration of a massive amount of quantitative information from multiple sensory systems… Chemically, flavor is the total of a large set of primary and secondary metabolites that are measured by the taste and olfactory systems (Klee, 2010).”
Taste is the amalgamation of all of the sensory data from the five classes of taste receptors in the mouth: sweet, sour, salty, bitter and umami (savory). Quantifying flavor is a challenge by itself, but as anyone who has ever had a cold will tell you, flavor is inextricably linked to the sense of smell.
As mammals, humans rely greatly on the combination of senses (i.e., taste and smell) to form sensory experiences because our senses are not as developed as those of other mammals. Humans have 10 square centimeters of olfactory epithelium compared to the 169 square centimeters of olfactory epithelium in a German shepherd (which is why they are the preferred drug sniffing dog breed).
The flavor and aroma we experience from a given fruit is determined by complex mixtures of often hundreds of volatile compounds. A strawberry has over 300 compounds that contribute on multiple levels to make up the characteristic flavor we associate with a ripe strawberry (Honkanen & Hirvi, 1990).
A tomato has more than 400 aromatic volatiles that constitute its aroma and flavor, but only 15 to 20 in sufficient enough quantity to impact flavor. The volatiles are composed of the metabolites of several chemical groups that include acids, aldehydes, ketones, alcohols, esters, sulfur compounds, furans, phenols, terpenes, epoxides and lactones.
Although the individual concentration of these substances vary from tissue sample to tissue sample, their concentration makes up 10 to 100 ppm of a fruit’s fresh weight.
The compounds responsible for flavor are generally formed during the ripening stage of flower and fruit development when the metabolism of the plant changes and catabolism of high-molecular weight molecules such as proteins, polysaccharides and lipids degrade and are converted into volatile metabolites (Asaphaharoni & Efraimlewinsohn).
Catabolism can be thought of as destructive metabolism, or the breakdown of complex molecules in living organisms to form simpler ones, along with the release of energy. It is during this stage of ripening that flushing a plant’s growing medium (depriving the plant of nutrition) and forcing it to catabolize its stored metabolites can most impact the final flavor.
Prior investigations of fruit flavors focused on identifying compounds present in various fruit species (Honkanen & Hirvi, 1990). Along with the classification of flavor compounds, researchers often identified the substances that were responsible for the unique scent we attribute to a particular fruit (methoxyfuraneol for strawberries and isoamylacetate for bananas).
Current research on fruit flavor is focused on the genes that directly influence fruit flavor formation. Future success at manipulating fruit flavor hinges on the research being carried out today-gathering information about the genes and metabolic pathways that generate fruit flavors.
Other avenues of research include experiments that use genes isolated from plants other than fruits, such as the leaves and glandular trichomes of various herbs to modify flavor.
Bio-engineering fruit flavor may seem like a waste of time, but there is a growing consensus among consumers that in recent decades the overall flavor quality of produce has declined.
This decline has been attributed to breeders selecting for particular traits such as disease resistance, appearance, firmness, post-harvest shelf life and yield. This focus on fiscally beneficial traits has resulted in less expensive, year-round produce that frankly does not taste good.
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Genetically modifying flavor is not restricted to introducing “new flavors or enhancing existing ones but also includes the removal of undesirable metabolites that generate ‘off-flavors.’ Since most of the molecules that compose the flavor profiles of fruit may exhibit antifungal or antibacterial bioactivity, it is conceivable that manipulation of fruit flavor will not only influence the flavor profile of fruit but will also confer resistance to pests and pathogens.” (Asaphaharoni & Efraimlewinsohn)
The first genetically modified tomato called the Flavr-Savr (also known as CGN-89564-2) was approved for commercial production in 1994. Using genetic engineering, the naturally produced enzyme that generates an “off” flavor and mushy texture was turned off. The result was a vine-ripened tomato that could be shipped with minimal bruising and spoilage. Due to poor flavor and mounting costs, the crop was pulled from production in 1997.
The prevalent method currently employed to manipulate flavor is called transgenic genetic engineering. The transgenic approach refers to the modification of an organism by transferring a gene or genetic material from one organism to another. A gene is a segment of DNA that codes for the production of a protein, and those proteins determine particular traits. For example, consider the gene for flower color.
The arrangement of the nucleic acid compounds on a chromosome in one plant tells the flower cells to produce certain proteins that make the flower blue. On another plant, the nucleic acid compounds are arranged differently, instructing the plant to make pink. Some genes control regions of a chromosome. These regions are like a light switch or a thermostat.
They turn the gene on or off, or regulate the amount of protein produced. While cells carry identical DNA codes, different cells have different functions. For example, the gene that makes a flower pink is not needed in the root, so it is turned off in the root cells and turned on in the cells of the flower. (Spears, Klaenhammer, & Petters)
An advantage of transgenic genetic engineering is that precise alterations can be engineered into cultivars that are already proven commercially. Two of the most common genetically modified crops in production are cotton and corn that have been modified with the addition of a gene from the bacteria Bacillus thuringiensis.
The resultant crops are toxic to caterpillars but safe for humans. A major obstacle of utilizing the transgenic approach is that the present regulatory environment makes it very expensive to gain approval for genetically modified organisms. Additionally, even if approval is obtained for a genetically modified crop, there is a growing social movement that vehemently opposes genetically modified produce.
We recommend if you want a great “old-time” tasting tomato, go visit your local farmers market once the tomatoes hit the stand or you can pick up some organic heirloom seeds and grow them yourself!
The day might be coming; however, for better or worse, when commercial greenhouses will be packed with high yielding, disease-resistant flavorful genetically engineered tomatoes; if you choose to eat them that will be up to you. To stay apprised of Farm Bill legislation in your state, get involved with a local advocacy group, and always try your best to know your food.
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