by Jalees Rehman
In a recent article, the scientists Harry Klee and Denise Tieman at the University of Florida summarize some of the most important recent research on the molecular biology of fruit flavors, with a special emphasis on tomatoes. Our perception of "flavor" primarily relies on two senses – taste and smell. Taste is perceived by taste receptors in our mouth, primarily located on the tongue and discriminates between sweet, sour, salty, bitter and savory. The sensation of smell (also referred to as "olfaction"), on the other hand, has a much broader catalog of perceptions. There are at least 400 different olfactory receptors present in the olfactory epithelium – the cells in the nasal passages which perceive smells – and the combined activation of various receptors can allow humans to distinguish up to 1 trillion smells. These receptors are activated by so-called volatile organic compounds or volatiles, a term which refers to organic molecules that are vaporize in the mouth when we are chewing the food and enter our nasal passages to activate the olfactory epithelium. The tremendous diversity of the olfactory receptors thus allows us to perceive a wide range of flavors. Anybody who eats food while having a cold and a stuffy nose will notice how bland food has become, even though the taste receptors on the tongue remain fully functional.
When it comes to tomato flavors, research has shown that consumers clearly prefer "sweetness". One obvious determinant of sweetness is the presence of sugars such as glucose or fructose in tomatoes which are sensed by the taste receptors in the mouth. But it turns out that several volatiles are critical for the perception of "sweetness" even though they are not sugars but instead activate the smell receptors in the olfactory epithelium. 6-Methyl-5-hepten-2-one, 1-Nitro-2-phenylethane, Benzaldehyde and 2-Phenylethanol are examples of volatiles that enhance the positive flavor perceived by consumers, whereas volatiles such as Eugenol and Isobutyl acetate are perceived to contribute negatively towards flavor. Interestingly, the same volatiles can have no effect or even the opposite effect on flavor perception when present in other fruits. Therefore, it appears that for each fruit, the sweetness flavor is created by the basic taste receptors which sense sugar levels as well as a symphony of smell sensations activated by a unique pattern of volatiles. But just like instruments play defined yet interacting roles in an orchestra, the effect of volatiles on flavor depends on the presence of other volatiles.
This complexity of flavor perception explains why it is so difficult to define flavor. The story becomes even more complicated because individuals have different thresholds for olfactory receptor activation. Furthermore, even the volatiles linked with a positive flavor perception – either by enhancing flavor intensity or letting the consumer sense a greater "sweetness" then actually present based on sugar levels – may have varying effects when they reach higher levels. Thus, it is very difficult to breed the ideal tomato that will satisfy all consumers. But why is there this growing sense that fruits such as tomatoes are becoming blander? Have we simply not tried enough tomato cultivars? A cultivar is a plant variety that has been bred over time to create specific characteristics, and one could surmise that with hundreds or even thousands of tomato cultivars available, each of us might identify a distinct cultivar that we find most flavorful. The volatiles are generated by metabolic enzymes encoded by genes and differences between the flavor of distinct cultivars is likely a reflection of differences in gene expression for the enzymes that regulate sugar metabolism or volatiles generation.
The problem, according to Klee and Tieman, is that the customers of tomato breeders are tomato growers and not the consumers who garnish their salads or create tomato-based masalas. The goal of growers is to maximize shelf-life, appearance, disease-resistance, yield and uniformity. Breeders focus on genetically manipulating tomato strains to maximize these characteristics. The expression GMO (genetically modified organism) describes the use of modern genetic technology to modify individual genes in crops and often provokes a litany of attacks and criticisms by anti-GMO activists who fear potential risks of such genetic interventions. However, the genetic breeding and manipulation of cultivars has been occurring for centuries or even millennia using traditional low tech methods but these do not seem to provoke much criticism by anti-GMO activists. Even though there is a theoretical risk that modern genetic engineering tools could pose a health risk, there is no scientific evidence that this is actually the case. Instead, one could argue that targeted genetic intervention may be more precise using modern technologies than the low-tech genetic breeding manipulations that have led to the creation of numerous cultivars, many of whom carry the "organic, non-GMO" label.
Klee and Tieman argue that consumers prefer flavor, variety and nutrition instead of the traditional goals of growers. The genetic and biochemical analysis of tomato cultivars now offers us a unique insight into the molecular components of flavor and nutrition. Scientists can now analyze each cultivar that has been generated over the past centuries using the low-tech genetic manipulation of selective breeding and inform consumers as to their flavor footprint. Alternatively, one could also use modern genetic tools such as genome editing and specifically modify flavor components while maintaining disease-resistance and high nutritional value of crops such as tomatoes. The key to making informed, rational decisions is to provide consumers comprehensive information based on scientific evidence as to the nutritional value and flavor of fruits, as well as the actual risks of genetically modifying crops using traditional low tech methods such as selective breeding and grafting or newer methods which involve genome editing.
Reference
Klee, H. J & Denise M. Tieman (2018). The genetics of fruit flavour preferences. Nature Reviews Genetics, (published online March 2018)