The Science of Tomato Flavors

Don’t judge a tomato by its appearance. You may salivate when thinking about the luscious large red tomatoes you just purchased in your grocery store, only to find out that they are extremely bland and lack flavor once you actually bite into them after preparing the salad you had been looking forward to all day. You are not alone. Many consumers complain about the growing blandness of fruits. Up until a few decades ago, it was rather challenging to understand the scientific basis of fruit flavors. Recent biochemical and molecular studies of fruits now provide a window into fruit flavors and allow us to understand the rise of blandness.

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)

Note: An earlier version of this article was first published on the 3Quarksdaily blog.

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Climate Change: Heatwaves and Poverty in Pakistan

In the summer of 2010, over 20 million people were affected by the summer floods in Pakistan. Millions lost access to shelter and clean water, and became dependent on aid in the form of food, drinking water, tents, clothes and medical supplies in order to survive this humanitarian disaster. It is estimated that at least $1.5 billion to $2 billion were provided as aid by governments, NGOs, charity organizations and private individuals from all around the world, and helped contain the devastating impact on the people of Pakistan. These floods crippled a flailing country that continues to grapple with problems of widespread corruption, illiteracy and poverty.

 

The 2011 World Disaster Report (PDF) states:

In the summer of 2010, giant floods devastated parts of Pakistan, affecting more than 20 million people. The flooding started on 22 July in the province of Balochistan, next reaching Khyber Pakhtunkhwa and then flowing down to Punjab, the Pakistan ‘breadbasket’. The floods eventually reached Sindh, where planned evacuations by the government of Pakistan saved millions of people.

However, severe damage to habitat and infrastructure could not be avoided and, by 14 August, the World Bank estimated that crops worth US$ 1 billion had been destroyed, threatening to halve the country’s growth (Batty and Shah, 2010). The floods submerged some 7 million hectares (17 million acres) of Pakistan’s most fertile croplands – in a country where farming is key to the economy. The waters also killed more than 200,000 head of livestock and swept away large quantities of stored commodities that usually fed millions of people throughout the year.

The 2010 floods were among the worst that Pakistan has experienced in recent decades. Sadly, the country is prone to recurrent flooding which means that in any given year, Pakistani farmers hope and pray that the floods will not be as bad as those in 2010. It would be natural to assume that recurring flood disasters force Pakistani farmers to give up farming and migrate to the cities in order to make ends meet. But a recent study published in the journal Nature Climate Change by Valerie Mueller at the International Food Policy Research Institute has identified the actual driver of migration among rural Pakistanis: Heat.

Mueller and colleagues analyzed the migration and weather patterns in rural Pakistan from 1991-2012 and found that flooding had a modest to insignificant effect on migration whereas extreme heat was clearly associated with migration. The researchers found that bouts of heat wiped out a third of the income derived through farming! In Pakistan, the average monthly rural household income is 20,000 rupees (roughly $200), which is barely enough to feed a typical household consisting of 6 or 7 people. It is no wonder that when heat stress reduces crop yields and this low income drops by one third, farming becomes untenable and rural Pakistanis are forced to migrate and find alternate means to feed their family. Mueller and colleagues also identified the group that was most likely to migrate: rural farmers who did not own the land they were farming. Not owning the land makes them more mobile, but compared to the land-owners, these farmers are far more vulnerable in terms of economic stability and food security when a heat wave hits. Migration may be the last resort for their continued survival.

It is predicted that the frequency and intensity of heat waves will increase during the next century. Research studies have determined that global warming is the major cause of heat waves, and  an important recent study by Diego Miralles and colleagues published in Nature Geoscience has identified a key mechanism which leads to the formation of “mega heat waves”. Dry soil and higher temperatures work as part of a vicious cycle, reinforcing each other. The researchers found that drying soil is a critical component.. During daytime, high temperatures dry out the soil. The dry soil traps the heat, thus creating layers of high temperatures even at night, when there is no sunlight. On the subsequent day, the new heat generated by sunlight is added on to the “trapped heat” by the dry soil, which creates an escalating feedback loop with progressively drying soil that becomes devastatingly effective at trapping heat. The result is a massive heat-wave which can wipe out crops, lead to water scarcity and also causes thousands of deaths.

The study by Mueller and colleagues provides important information on how climate change is having real-world effects on humans today. Climate change is a global problem, affecting humans all around the world, but its most severe and immediate impact will likely be borne by people in the developing world who are most vulnerable in terms of their food security.  There is an obvious need to limit carbon emissions and thus curtail the progression of climate change. This necessary long-term approach to climate change has to be complemented by more immediate measures that help people cope with the detrimental effects of climate change by, for example, exploring ways to grow crops that are more heat resilient, and ensuring the food security of those who are acutely threatened by climate change.

As Mueller and colleagues point out, the floods in Pakistan have attracted significant international relief efforts whereas increasing temperatures and heat stress are not commonly perceived as existential threats, even though they can be just as devastating. Gradual increases in temperatures and heat waves are more insidious and less likely to be perceived as threats, whereas powerful images of floods destroying homes and personal narratives of flood survivors clearly identify floods as humanitarian disasters. The impacts of heat stress and climate change, on the other hand, are not so easily conveyed. Climate change is a complex scientific issue, relying on mathematical models and intrinsic uncertainties associated with these models. As climate change progresses, weather patterns will become even more erratic, thus making it even more challenging to offer specific predictions.

Climate change research and the translation of this research into pragmatic precautionary measures also face an uphill battle because of the powerful influence of the climate change denial lobby. Climate change deniers take advantage of the scientific complexity of climate change, and attempt to paralyze humankind in terms of climate change action by exaggerating the scientific uncertainties. In fact, there is a clear scientific consensus among climate scientists that human-caused climate change is very real and is already destroying lives and ecosystems around the world.

Helping farmers adapt to climate change will require more than financial aid.  It is important to communicate the impact of climate change and offer specific advice for how farmers may have to change their traditional agricultural practices. A recent commentary in Nature by Tom Macmillan and Tim Benton highlighted the importance of engaging farmers in agricultural and climate change research. Macmillan and Benton pointed out that at least 10 million farmers have taken part in farmer field schools across Asia, Africa and Latin America since 1989 which have helped them gain knowledge and accordingly adapt their practices.

Pakistan will hopefully soon engage in a much-needed land reform in order to solve the social injustice and food insecurity that plagues the country. Five percent of large landholders in Pakistan own 64% of the total farmland, whereas 65% small farmers own only 15% of the land. About 67% of rural households own no land. Women own only 3% of the land despite sharing in 70% of agricultural activities!  The land reform will be just a first step in rectifying social injustice in Pakistan. Involving Pakistani farmers – men and women alike – in research and education about innovative agricultural practices in the face of climate change will help ensure their long-term survival.

Mueller V, Gray C, & Kosec K (2014). Heat Stress Increases Long-term Human Migration in Rural Pakistan. Nature climate change, 4, 182-185 PMID: 25132865

Miralles, D., Teuling, A., van Heerwaarden, C., & Vilà-Guerau de Arellano, J. (2014). Mega-heatwave temperatures due to combined soil desiccation and atmospheric heat accumulation Nature Geoscience, 7 (5), 345-349 DOI: 10.1038/ngeo2141

Note: An earlier version of this article was first published on the 3Quarksdaily Blog.