Tomatoes

TL;DR. Botanically a berry, legally a vegetable since an 1893 US Supreme Court tariff case, and the one food where processing beats fresh on nutrition: cooking and canning multiply the absorbable lycopene several-fold, so tinned tomatoes and tomato paste are better lycopene sources than the raw fruit. The modern supermarket tomato is bland for a known genetic reason, traced to a mutation selected for uniform ripening in the 1930s that also cut the fruit's sugar and carotenoid production. Never refrigerate an unripe tomato; the aroma genes shut down and do not fully recover.

1. What it is

Solanum lycopersicum, family Solanaceae, alongside potato, aubergine, pepper, and tobacco. Botanically a berry. Culinarily and legally a vegetable.

The legal question was settled in Nix v. Hedden (1893), when the US Supreme Court ruled tomatoes were vegetables for the purposes of the Tariff Act, because "in the common language of the people" they were served with dinner rather than as dessert. The court explicitly acknowledged the botany and ruled on usage, which is the correct way to resolve the question (Chapter 1).

Plant habit splits commercially in a way that matters:

• Determinate ("bush"): grows to a set size, sets all its fruit over a short window, and dies. This is what processing tomatoes are, because a once-over machine harvest requires it.
• Indeterminate ("vine"): keeps growing and fruiting until frost or the grower stops it. This is what glasshouse and garden tomatoes are.

2. Where it comes from

Origin: western South America (Peru, Ecuador, northern Chile), where the wild ancestor S. pimpinellifolium produces pea-sized fruit. Domestication happened in Mesoamerica, and the name comes from the Nahuatl tomatl. Spanish colonists took it to Europe in the sixteenth century.

Europe was suspicious for two hundred years. Tomatoes are nightshades, they resembled known poisonous plants, and wealthy diners eating acidic tomatoes off pewter plates leached lead from the pewter and got sick, which reinforced the reputation. Italy adopted them earliest and most enthusiastically, and the tomato only became central to Italian cooking in the eighteenth and nineteenth centuries, which is startlingly recent for something that now defines the cuisine.

Production: roughly 190 million tonnes a year, the largest vegetable crop in the world by tonnage. China produces about a third; India, Turkey, the United States, Egypt, and Italy follow. Roughly a quarter to a third of the crop is processed into paste, sauce, and tinned products, with California and Italy dominating processing.

3. How it is grown

Climate. Warm-season annual, frost-intolerant. Optimum 21 to 27 °C by day and 15 to 20 °C at night. Above about 32 °C pollen becomes non-viable and fruit set fails, which is a live problem in a warming climate. Below 10 °C growth stops. Fruit colour development is also temperature-sensitive: above roughly 30 °C, lycopene synthesis is inhibited while carotene continues, which is why tomatoes ripening in extreme heat go orange-yellow rather than red.

Soil. Adaptable, pH 6.0 to 6.8, well-drained. Consistent moisture is the key requirement: irregular watering causes blossom end rot, a black sunken patch at the base of the fruit which is a calcium transport failure, not a calcium deficiency in the soil. Calcium moves with water, and erratic watering means the fastest-growing tissue at the far end of the fruit misses out. Adding calcium to the soil rarely fixes it; consistent watering does. Erratic watering also causes splitting.

Field growing for processing: determinate varieties, drip-irrigated, machine-harvested in a single pass, delivered to a factory on a schedule set by the plant.

Protected cropping for fresh market: this is where most of the tomatoes in northern Europe come from. Dutch and Spanish glasshouses grow indeterminate vines hydroponically in rockwool or coir, trained up strings to 10 metres or more over a season, with computer-controlled nutrient solution, CO₂ enrichment to 800 to 1,000 ppm, supplementary lighting, and biological pest control (Chapter 4). Yields exceed 500 tonnes per hectare per year against 50 to 100 in the field.

Pollination in glasshouses requires bumblebees, specifically. Tomato flowers hold pollen in tubular anthers that release it only when vibrated at a particular frequency, which is buzz pollination. Honeybees cannot do it; bumblebees can. Before commercial bumblebee rearing began in Belgium in the late 1980s, glasshouse tomatoes were pollinated by workers touching each flower with an electric vibrating wand. The bumblebee industry transformed the economics of protected tomato growing worldwide and is now a substantial business in its own right.

Harvest. Climacteric. Field tomatoes for shipping are picked at "mature green" or "breaker" stage and ripened with ethylene at the destination. Vine-ripened and truss tomatoes are picked later and cost more. Processing tomatoes are picked fully ripe because they go straight to a factory.

The blandness problem, mechanistically. This is one of the best-documented cases of a breeding trade-off. In the 1930s breeders selected a mutation producing uniform ripening: fruit that turns from uniform light green to uniform red, rather than having a dark green shoulder. It made harvest scheduling and grading much easier. In 2012, Ann Powell and colleagues published in Science the finding that the gene involved, SlGLK2, controls chloroplast development in the fruit. Fruit with functioning GLK2 has more chloroplasts, photosynthesises more, and accumulates more sugar and more carotenoids. The uniform-ripening mutation knocked that out. Breeders traded flavour and nutrition for appearance without knowing what they were trading. Subsequent work has confirmed that restoring the gene increases sugars and carotenoids.

That is not the only cause. Picking green, refrigerating, and selecting for firmness and shelf life all contribute. But it is a specific, identified, genetic cause, and it is rare to be able to say that about a food quality complaint.

4. What is inside it

Per 100 g:

The lycopene column is the story of this chapter. Tomato paste has more than ten times the lycopene of raw tomato per 100 g, and it is also more absorbable, for two reasons: heat breaks down the cell walls that trap it, and heat converts lycopene from the trans form found in raw fruit to cis isomers, which are better absorbed.

Adding oil multiplies it again. A study feeding tomato sauce with and without oil found substantially higher plasma lycopene with oil, because lycopene is fat-soluble and needs a lipid phase to enter micelles (Chapter 10).

The result: a plate of pasta with tomato sauce cooked in olive oil delivers far more usable lycopene than a raw tomato salad. Mediterranean cooking arrived at this by taste centuries before anyone measured it.

Other components: potassium (substantial, and enormous in concentrates), vitamin C (degraded by cooking, which is the one thing raw tomato does better), folate, and, in the skin and flesh, flavonoids including naringenin chalcone and rutin. Glutamate is present at high levels, which is why tomatoes taste savoury and are used as an umami base worldwide; cooking and drying concentrate it further, which is why tomato paste, sun-dried tomatoes, and ketchup all deliver so much flavour per gram.

5. What it does in your body

Lycopene is a carotenoid without provitamin A activity, and it accumulates in the prostate, skin, liver, and adrenal glands. It is one of the more efficient singlet-oxygen quenchers among dietary carotenoids in laboratory conditions, though as Chapter 17 argues, direct antioxidant action is probably not the main mechanism for dietary carotenoids in the body.

Skin. There is reasonable trial evidence that regular tomato paste consumption (around 40 g a day for 10 to 12 weeks) modestly increases resistance to UV-induced erythema, in the region of 20 to 30 percent. It is not sunscreen and it is a real, measured effect.

Blood pressure and lipids. Meta-analyses of lycopene and tomato product intervention trials find small reductions in blood pressure and LDL, with substantial heterogeneity.

Acidity. Tomatoes are acidic (pH around 4.0 to 4.6) and are a common reflux trigger. This is direct chemical irritation and relaxation of the lower oesophageal sphincter, not an allergy.

Nightshades and inflammation. The claim that tomatoes and other nightshades cause or worsen arthritis is widespread and not supported by evidence. Tomato contains tiny amounts of tomatine, a glycoalkaloid far less toxic than potato's solanine and largely bound in the gut. A small number of people report symptom improvement on nightshade elimination, which is worth testing individually and is not a general finding.

6. What the evidence actually shows

Established: cooking and processing increase lycopene bioavailability substantially. Tomato products are the dominant dietary lycopene source in most populations. Blossom end rot is a calcium transport problem, and the flavour loss has an identified genetic cause.

Strong: higher tomato and lycopene intake is associated with lower prostate cancer risk in pooled observational analyses, with the association strongest for cooked tomato products. The 2007 FDA review concluded the evidence for lycopene and cancer was limited rather than convincing, and subsequent meta-analyses continue to find a modest inverse association. Consistent, observational, not proven.

Mixed: lycopene supplements for prostate cancer prevention, which have not reproduced the food association; blood pressure and lipid effects, which are small.

Thin: tomato for skin ageing beyond the modest UV erythema finding, and any claim about tomato juice detoxifying anything.

7. Who should eat more, who should be careful

Good for: almost everyone. Tinned tomatoes are one of the cheapest, most useful, and most nutritious items in a store cupboard, and they are one of the few processed foods that outperform their fresh equivalent.

Be careful if:

• You have reflux (GORD). Tomatoes and tomato products are among the most consistent triggers.
• You have chronic kidney disease. Tomato paste and sun-dried tomatoes are extraordinarily high in potassium (over 1,000 and over 3,000 mg per 100 g respectively), and they are among the first foods restricted.
• You are watching sodium. Tinned tomatoes vary; passata and chopped tomatoes are usually low or no salt, tomato juice and many sauces are not. Check the label.
• You have oral allergy syndrome to grass or latex; tomato cross-reactivity is documented, usually mild.
• You have interstitial cystitis or recurrent mouth ulcers, where acidic foods commonly aggravate symptoms.

Ages. Fine from around 6 months, though the acidity often causes a harmless perioral rash. Whole cherry tomatoes are a choking hazard for under-4s: cut them lengthways in quarters, for the same reason as grapes.

8. When and how to eat it

• Cook them with oil. This is the practical headline of the chapter. Sauce, roasted, tinned, or as paste, with olive oil, delivers several times the usable lycopene of a raw slice.
• Eat raw tomatoes too, for the vitamin C that cooking destroys. Both, not either.
• Keep the skin and seeds. The skin holds a disproportionate share of the lycopene and flavonoids; the jelly around the seeds holds most of the glutamate and therefore most of the savoury flavour. Recipes that instruct you to seed and peel are optimising for texture at a real cost.
• Salt them ahead. Salting sliced tomatoes 10 to 15 minutes before serving draws out water, concentrates flavour, and improves texture.
• Roast or reduce for depth. Slow cooking drives Maillard and caramelisation reactions that build the deep savoury character of a long-cooked sauce.
• Sun-dried and paste are flavour concentrates, effectively seasoning rather than vegetables.
• Do not cook acidic tomato sauce for long periods in bare cast iron or aluminium; acid strips seasoning and leaches metal. Enamelled or stainless is better.

9. Choosing, storing, and ripening

Choosing.

• Smell the stem end. A good tomato is fragrant; the volatile compounds responsible are exactly what supermarket tomatoes lack. No smell, no flavour, reliably.
• Heavy for its size, with taut, unwrinkled skin and slight give.
• Deep uniform colour with no green shoulder for red types.
• Buy in season and locally where possible. This is one of the produce items where the difference is enormous and obvious.

Storing. This is one of the most consequential storage facts in the whole book.

Never refrigerate an unripe tomato. Work at the University of Florida led by Denise Tieman showed that storing tomatoes at 5 °C for as little as a week substantially reduced the expression of the genes producing volatile aroma compounds, and that the loss was only partially reversible on rewarming. The tomato loses its smell and therefore most of its flavour, permanently. This is a measured genetic effect, not folklore.

• Counter, stem side down, out of direct sunlight. Stem down reduces moisture loss and bruising through the scar.
• Refrigerate only fully ripe tomatoes, and only for a day or two, and bring them back to room temperature before eating.
• Tomatoes emit ethylene, so keep them away from leafy greens.
• Tinned tomatoes are shelf-stable for years. Once opened, transfer out of the tin to a container.

10. The varieties worth knowing

Heirlooms deserve a fair assessment. They frequently taste better, partly because they lack the uniform-ripening mutation and partly because they are grown and sold locally and picked ripe. They are also lower-yielding, disease-susceptible, and do not ship. Neither category is simply superior; the trade is between flavour and logistics, and modern flavour-focused breeding programmes are actively working to get both.

11. Myths and confusions

• "Tomatoes are a fruit, so people who call them vegetables are wrong." Both classifications are internally consistent and answer different questions.
• "Nightshades cause inflammation and arthritis." No good evidence in the general population.
• "Sun-dried tomatoes are a health food." They are concentrated tomato, which means concentrated potassium and lycopene and, in oil-packed versions, a lot of oil and salt.
• "Tinned food is less nutritious than fresh." For tomatoes, the opposite is true for lycopene, and the vitamin C loss is the only real cost.
• "BPA in tomato tins is a serious risk because tomatoes are acidic." Acidity does increase migration from can linings, which is why most manufacturers moved to BPA-free linings. See Chapter 93 for what is actually known.
• "You should always peel and deseed tomatoes." You lose lycopene with the skin and umami with the seed jelly.

12. The bottom line

• Tomatoes are the world's largest vegetable crop by tonnage, botanically berries, and legally vegetables since 1893.
• Cooked and processed beats raw for lycopene by a factor of ten or more, and adding oil multiplies absorption again. Tinned tomatoes and tomato paste are nutritional bargains.
• The blandness of supermarket tomatoes has an identified genetic cause: a uniform-ripening mutation selected in the 1930s that also disabled the fruit's ability to make sugar and carotenoids.
• Never refrigerate an unripe tomato. Cold shuts down the aroma genes and the loss is not fully reversible.
• Keep the skin and the seed jelly: one holds the lycopene, the other holds the savouriness.
• Blossom end rot is a watering problem, not a calcium deficiency.

Sources and notes

Composition figures are from USDA FoodData Central; production figures are approximate FAOSTAT values for the early 2020s. Domestication follows Blanca et al.'s genomic work; Nix v. Hedden, 149 U.S. 304 (1893), is the tariff case. Glasshouse production, hydroponics, CO2 enrichment, and yields follow Wageningen University and Dutch horticulture reporting. Buzz pollination and the commercial bumblebee industry follow Velthuis and van Doorn, Apidologie, 2006. Blossom end rot as a calcium transport failure follows Ho and White's work. The uniform ripening mutation and SlGLK2 is Powell et al., Science, 2012. Refrigeration and aroma gene expression is Zhang et al., PNAS, 2016, from the University of Florida group led by Klee. Lycopene bioavailability from processed tomato follows Gartner et al., American Journal of Clinical Nutrition, 1997, and Fielding et al. on oil co-consumption. Prostate cancer associations follow Giovannucci's pooled analyses; the FDA's 2007 qualified health claim review concluded the evidence was limited. Tomato paste and UV erythema protection follows Stahl et al.'s trials. Potassium content of concentrates matters in CKD per KDIGO guidance.

Open questions. Whether lycopene itself, rather than the whole tomato, does anything is unresolved: supplement trials have not reproduced the food associations. How much of modern tomato blandness is the SlGLK2 mutation versus picking green and refrigerating is not cleanly separable.

👉 Next: pears, pomegranates, and the temperate others.