Understanding tomato taste and flavour

Tomatoes (Solanum lycopersicum) is the world’s most valuable fruit and essential greenhouse crop. They deliver vital nutrients to our diets and serve as a pivotal model for plant biology research. Unfortunately, modern tomato varieties often lack the flavour that original or traditional cultivars boast. During the domestication and enhancement process, breeders prioritized traits like yield, disease resistance, fruit size, and visual appeal. Sadly, this focus often ignored the taste and nutritional quality of the fruits. The perfect tomato was in the eye of the beholder, not the taste. In terms of quality, everyone judged the book by its cover and not content. It is easy to distinguish between a good looking tomato and a rotten one. Taste is not so easy, everyone has different tastes and it is difficult to cater for each and everyone.

Breeders have given some attention to sugar and acid levels, understanding their crucial role in taste. However, studies indicate no strong link between fruit size and sugar content. More so, changes in cultivated tomatoes’ metabolite profiles over time have mainly led to a decline in flavour. But as living standards improve globally, consumers are showing a growing preference for tasty, high-quality tomatoes and are willing to pay more for superior quality.

This shift has spurred a new direction in tomato breeding, pivoting from yield-focused to flavour-focused. The breeding now aims to produce tomatoes that not only yield well but also taste great. This renewed interest in tomato flavour is sparking more scientific research into the genetics that create tasty tomatoes.

Tomato flavour is a complex mix of taste metabolites and volatile compounds, including sugars, acids, amino acids, vitamin C, and various volatiles. These elements contribute to basic tastes like sweetness, sourness, bitterness, and umami, and influence aromas that can either enhance or detract from the overall flavour. Research has identified over 400 volatiles in tomato fruits, yet only a handful, such as aldehydes, esters, alcohols, ketones, lactones, and terpenoids, are significant flavour influencers. There are more than 400 compounds that influence fruit taste which makes researchers work so complex and difficult to achieve quick results.

Historically, flavour research often isolated single compounds, which limited understanding of the complete flavour profile and the interactions between taste and aroma. Today, we recognize flavour as a complex trait influenced by various factors, both intrinsic like food shape and extrinsic like emotional states. Flavour perception is truly multimodal, involving not just taste but also texture and appearance, all of which combine to create the full flavour experience.

It is incredible to think that as recent as 2008 the term ‘flavoromics’ was coined by Reineccius at the 235th American Chemical Society (ACS) meeting in 2008 ¹⁾Reineccius G. Flavor deterioration during food storage. In: Leif H S, Jens R and Mogens L A (eds), Chemical Deterioration and Physical Instability of Food and Beverages. Elsevier, 2010, 95–112.

It is a comprehensive analysis and description of all flavor-related metabolites based on metabolomics²⁾Reineccius G. Flavor deterioration during food storage. In: Leif H S, Jens R and Mogens L A (eds), Chemical Deterioration and Physical Instability of Food and Beverages. Elsevier, 2010, 95–112.³⁾Andujar-Ortiz I, Peppard TL, Reineccius G. Flavoromics for determining markers of  cooked and fermented flavor in straw-berry juices. Abstr Pap Am Chem Soc. 2014;1191:293–312.⁴⁾Blank I, Wüst M, Yeretzian C. Expression of multidisciplinary flavor science: research highlights from the 12th Weurman symposium. J Agric Food Chem. 2009;57:9857–9.. The principles are now applied to various plant species, such as coffee, grape, strawberry, canola, blueberry, and citrus. Good news for tomato growers is that most of the research is targeted at the tomato plant.

The influence of sugars and acids in tomato taste

The main organic acids in tomato fruit that play a role in taste are:

• Malic acid
• Citric acid
• Ascorbic acid
• Tartaric acid and
• Glutamic acid

Malic acid and citric acid are the predominant organic compounds affecting fruit flavour and palatability. Citric acid is always present while malic acid is only found in unripe tomato fruit.

As the fruit ripens total sugar content increases by 4%, but the glucose content decreases and fructose content increases. The fructose content of a ripe tomato will be higher than the glucose content. It is not only the total sugar content that increases during fruit ripening but also the organic acid content ⁵⁾Agius C, von Tucher S, Poppenberger B et al. Quantification of sugars and organic acids in tomato fruits. MethodsX. 2018;5: 537–50..

Volatiles

The perception of sweetness in tomatoes, influenced by specific volatile chemicals, is a fascinating example of how flavour is more than just the direct response to basic taste stimuli. I think volatiles and its influence on taste fall under ‘fluffy’ science.

Certain volatile chemicals can make tomatoes taste sweeter than their actual sugar content would suggest. Some volatile compounds in tomatoes interact synergistically with the taste receptors on the tongue that detect sweetness. This interaction can amplify the sweet taste sensation without the actual increase in sugar content. The perceived sweetness is enhanced and influenced by certain esters and terpenoids. Most people think you taste your food on your tongue, but that is wrong. There are only 7 things your tongue can taste, the rest is of the “taste” is actually smelled. So volatile compounds are primarily responsible for the aroma of foods (smell), and the olfactory cues can greatly influence taste perceptions. When you smell something that your brain associates with sweetness, such as certain fruity or floral volatile compounds, it can predispose you to perceive the taste as sweeter. The overall experience of eating a tomato can also be influenced by visual (psychological) and textural (physical) cues, along with personal preferences and previous experiences with similar tastes. A tomato that smells particularly rich and pleasant can create a positive predisposition in the mind, enhancing the overall perception of sweetness and flavour quality. Volatile compounds can also interact with acids and other sugars in tomatoes to create a more complex flavour profile that enhances sweetness perception. This complexity can mask or balance other flavours like sourness or bitterness, making the sweet taste more pronounced.

The lost bitterness taste in tomatoes

Very few tomatoes have a bitter taste these days. It can still be found in some open field older varieties where climate can force chemical reactions in the plant to produce more of these chemicals. Bitterness is an undesirable trait in tomatoes, not in beer, that is why breeders have a negative selection towards the bitterness gene. Interestingly Naringin is one of the important metabolites responsible for the bitter taste of citrus⁶⁾Jung UJ, Kim SR. Effects of naringin, a flavanone glycoside in grapefruits and citrus fruits, on the nigrostriatal dopaminergic projection in the adult brain. Neural Regen Res. 2014;9:1514–7 which is also found in the peel of tomatoes⁷⁾Muir SR, Collins GJ, Robinson S et al. Overexpression of petu- nia chalcone isomerase in tomato results in fruit containing increased levels of flavonols. Nat Biotechnol. 2001;19:470–4.. This is unfortunate because naringin can reduce inflammation, protect you from various diseases, and promote heart health⁸⁾Dong J, Chen Y, Yang F et al. Naringin exerts therapeutic effects on mice colitis: a study based on transcriptomics combined with functional experiments. Front Pharmacol. 2021;12:729414.. But in order to to have an effect, you have to eat really bitter tomatoes. Sorry.

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