What’s In a Tomato
What a strange state of affairs that we know the precise chemical makeup of distant stars yet we don't really know what's in our food most of the time. What makes a meal, and what makes it healthy, or unhealthy? I have decided to break down one of my favorite foods and try to look at what it's really made of, and why I enjoy it. Besides personal preference this is a good specimen to study because it is currently the world's most popular fruit, with over 145 million tons produced annually, is used in a wide variety of dishes, and as of last year has had its genome sequenced. Behold, Solanum Lycopersicum, the tomato.
Of course there is no "real" or essential tomato. Thousands of cultivars are produced in differing climates and conditions. Tomberry tomatoes are only 1/2000 the size of massive beefsteak tomatoes, though most are somewhere in the middle, around 5cm in diameter. Some variants are grown specifically for canning. Tomatoes are acidic, making them easy to preserve in a can, and canned tomatoes are sealed at the peak of freshness. This gives them a higher antioxidant content and preserves the vitamins and minerals, making them even healthier than their fresh counterparts most of the time.1
Tomatoes possess one of the most powerful known antioxidants, lycopene. Lycopene is a type of Carotene, substances which transmit light energy in photosynthesis. With the chemical formula C40H56, tomatoes produce lycopene from 8 instances of isoprene (C5H8), which is itself made from a series of chemical reactions called the MEP pathway in the chloroplast. Its long, straight structure lowers the required energy for electrons to transition to higher energy states. This makes lycopene effective at absorbing higher frequency, and thus higher energy, wavelengths of light, only reflecting photons with longer wavelengths, i.e. red light. This also gives lycopene, and the tomato, its characteristic red color. As an effective antioxidant, a substance that quenches highly reactive ions of oxygen, there is evidence lycopene has a wide variety of positive health benefits, including a reduced risk of cancer.
Like most fruits and vegetables, tomatoes are almost entirely water by weight. Here is an image showing the approximate contents of a typical tomato.2
The sweetness and most of the calories in a tomato come from the glucose and fructose, which add up to about 18 calories. The protein adds another 7 calories. The fiber is almost entirely insoluble, which accelerates the movement of food through the digestive system. It is not digested itself, but it does feed bacteria in the colon, which produce a variety of healthy products from it like short chain fatty acids. The "ash", pretend it's red, contains the lycopene, beta carotene (Vitamin A precursor), the minerals Phosphorous, Sodium, Potassium, Calcium, and Magnesium, and the trace elements, Iron, Copper, Zinc, and Manganese. There are also trace amounts of substances called "flavonoids", chemicals used in plant metabolism like quercetin and rutin. Despite extensive study, these flavonoids in tomatoes have been found to have no physiological role in humans.3,4 Flavonoids are a type of "phytochemical", non-essential chemicals produced by plants that may one day be found to have an effect on human health. Lycopene is generally accepted as an effective antioxidant but currently there is very little evidence others have positive roles. Some may even be harmful.5
I purchased this tomato at a farmers' market in San Francisco. It cost me 60 cents, which is about half the price of a supermarket tomato, and contains 25 calories. That's 2.4 cents / calorie. By comparison, a double cheeseburger from McDonald's costs 99 cents and contains 440 calories6, which is 0.225 cents / calorie, more than an order of magnitude more cost effective. It would cost me $130/day to live on supermarket tomatoes, $65/day to live on farmers' market tomatoes, and $6/day to live on cheeseburgers. It's no wonder the poor eat poorly.
The tomato genome was sequenced in 2012. It was found that the 'domesticated' tomato differs from a wild tomato by only 0.6% of its genome, and has about 900 million bases. By comparison, the human genome has about 3.2 billion bases. The tomato genome is unique in that relatively little of its genome consists of repeated sequences of information, which is common in plants. The researchers also found evidence that tomatoes, potatoes, and grapes likely share a common ancestor, and that the tomato has undergone two whole-genome triplications.7
I have experimentally verified that tomatoes are delicious. To test, I purchased two additional tomatoes from a supermarket, one grown in a greenhouse, one outside. Both were about twice as expensive as the farmers' market tomato, though nearly identical in mass. Both are softer and have a stronger scent than the farmers' market tomato. I blindfolded myself and took two bites out of each, picked at random, with a minute break in between after taking notes. The outside tomato was the sweetest, and softest. The greenhouse tomato was juicier, had slightly firmer flesh, and the strongest scent once I bit in to it. To my surprise the tomato from the farmers' market was relatively dry, firm, and bland, my least favorite. The greenhouse tomato was my preferred cultivar, and curiously appears to have noticeably more seeds inside. This sample size is small so if you do this experiment yourself please post your findings in the comments.
Flavor depends on a combination of the sensations of taste and smell. Volatile chemicals are released when food is chewed and react with olfactory receptor neurons, while nonvolatile chemicals react with special skin cells, taste buds, on the tongue. There is evidence the majority of flavor humans experience comes from smell. Azondanlou et al. found that of its 400 identifiable volatile chemicals, only about 30 contribute to the tomato's flavor. The taste comes from the glucose and fructose, identified as sweet, and citric acid, identified as sour. As onmnivores, humans enjoy the taste of sweets, which signify the consumption of carbohydrates, the body's primary source of energy. Carnivores, like cats, cannot taste sweets at all.
We are not the only ones vying for the nutrients in food. These tomatoes were likely already teeming with bacteria when I bought them. Don't worry though, our saliva and stomach acids make short work of them. Unless food appears or smells spoiled our defense mechanisms should have no problem rendering the food harmless. After a couple of days however the bacteria, thriving on the water and carbohydrates, reduced my tomatoes to mush and "spoiled" them. Scientists around the world are working to improve our tomatoes through genetic engineering, making them larger, tastier, more robust, more nutritious, and perhaps even more resistant to pathogens and spoilage.
When I was a child I kept a tomato garden with my father. It was a lot of work keeping the pests away, keeping them in sunlight without choking their stems, and watering them daily. The final product ended up being much smaller and blander than those bought at the supermarket, though of course that was not the point. Tomatoes, like transistors, are much better when produced at scale. Perhaps one day I will show my son how people used to produce food, but I'd much rather teach him to code.