When a tomato is a just tomato

posted in: Science | 4
Image of tomatoes by Rachel Andrew via Flickr. Text added by Anastasia Bodnar.

Another paper comparing organic and conventional tomatoes (sort of) was published in PLOS One on 20 February 2013: The impact of organic farming on quality of tomatoes is associated to increased oxidative stress during fruit development.

In contrast to so many other studies that aim to see if there is a difference between conventional and organic produce, these researchers used the same cultivar: Débora, a typical tomato variety used in Brazil. We know that different varieties can vary a lot in things like mineral and antioxidant content. So, removing this variability from the experiment helps a lot.

Unfortunately, they didn’t do every thing in a way that would present the most  objective results, in my humble opinion.

Tomato nutrient content is based on density

Tomatoes, like many other fruits, can vary in water content based on the environment in which they were grown. As the authors of this paper say: “Mass and size were about 40% higher in fruits from conventional growing systems than in fruits from organic farming. Such differences could originate either from differences in nitrogen availability or from limitations to growth imposed by the more stressing conditions prevailing in organic farming.” In other words, if the plants are more stressed out (as they often are in organic farming, at least in theory due to higher pest pressure and less applied nutrients) then they will grow smaller, more dense fruits. Perhaps the title of the paper should be “The impact of organic farming on density of tomatoes is associated with increased oxidative stress during fruit development”.

The researchers “correct” for the difference in mass, which means that tomatoes that are less dense will look like they are more nutritious. Why does this matter? Well, are you going to eat a serving of tomato based on mass, or are you going to eat a whole or half of a tomato? Sometimes a tomato is just a tomato, not a health food supplement.

The researchers don’t seem to think so, though, concluding: “Our work clearly demonstrates that tomato fruits from organic farming have indeed a smaller size and mass than fruits from conventional growing systems, but also a substantially better quality in terms of concentrations in soluble solids and phytochemicals such as vitamin C and total phenolic compounds.” Except, we know the “quality” depends on the density!

If you re-correct the values, it turns out that there isn’t much difference between organic and conventional tomatoes, and as many studies have shown previously, some nutrients are higher in organic while some are higher in conventional. Conclusion: choose organic if that is what you prefer, but nutritional differences are not a good reason. For nutrition, simply eating more fruits and veggies is the way to go!

The values below were not corrected. As expected, the organic tomatoes were a little bit smaller and weighed a lot less. I’ve bolded the higher of each pair.

Item Units Organic Conventional
Size centimeters 4.20 5.46
Weight grams 75.15 124.93
Citric acid % of total acid 0.36 0.28
Total chlorophyll soil plant analysis development 40.18 40.29

These values were corrected, some by the fresh weight (which means the less dense aka more water filled conventional tomatoes will have lower values) and some by protein. The ones that were corrected by fresh weight I simply multiplied by the weight per tomato*. The protein values weren’t reported in the paper for some reason, but protein was also based on fresh weight, so I uncorrected those values by the fresh weight too (it’s not perfect, but should give us a decent estimate). Site note: along with the missing protein values, something called “thiobarbituric acid reactive substances” was described in the methods but the values were not reported.

Item Units Corrected Uncorrected
Organic Conventional Organic Conventional
Soluble Solids °Brix (concentration of sucrose w/w) 6.00 3.83 0.45 0.48
Total phenolic content mg galic acid equivalents / kg FW 556.50 232.50 41.82 29.05
Anthocyanins mg / kg FW 3.60 9.90 0.27 1.24
Yellow flavonoids mg / kg FW 43.70 25.70 3.28 3.21
Vitamin C mg / kg FW 264.70 170.90 19.89 21.35
Lipid peroxidation nnmol MDA / g FW 19.24 8.06 1.45 1.01
Total antioxidants umol Trolox / g FW 128.34 136.28 9.64 17.03
Phenylalanine ammonia-lyase µmol trans-cinnamic acid / mg P 11.43 4.79 0.86 0.60
Ascorbate peroxidase µmol H2O2 / min * mg P 1.01 0.98 0.08 0.12
Catalase µmol H2O2 / min * mg P 5.07 16.46 0.38 2.06
Superoxide dismutase UA / mg P 121.76 22.70 9.15 2.84
Cilantro Peach Tomato Salad by Margaret in Minnesota via Flickr.

As you can see, the number of values where organic is higher than conventional changes from 8:3 to 5:5, and many of the differences shrink. We also have to consider that we are not fruit flies being forced to eat only tomatoes. Tomatoes are just one part of a healthy diet, and we get nutrients from many sources. Antioxidants and flavnoids of various types are found in all fruits and veggies, so add some variety to that tomato salad to get variety in nutrients! How about some yellow peaches and red onion? Then add cilantro, basil, or mint! Yum!

What about yield?

While there are a lot of different things to consider when it come to agriculture (taste is a big one!), I’m always interested to know how efficient a system is with regard to land use. Using this tomato study, can we find out the yield of various nutrients per acre?

In this particular study, the  rows 1 m apart with 0.4 m between plants for both organic and conventional fields. It’s great that they were consistent, but unfortunately they do not tell us how many fruits each plant produced, so we can’t determine yield from the data. Happily, we can use data that the USDA collected about tomato yields.

The USDA collected organic tomato yield data during the last Census of Agriculture in the 2008 Organic Production Survey. The Economic Research Service collects conventional tomato yield data every year. Here’s how the tomatoes stack up (yield in tons):

  • Conventional (fresh) 14.79
  • Conventional (processing) 41.5
  • Conventional (total) 34.51
  • Organic (certified only) 23.05
  • Organic (certified and exempt) 22.32
  • Organic (exempt only) 1.81

Some of these values are a little weird… I’m guessing that the processing yields are so much higher because the tomatoes can be a bit uglier than those sold as fresh tomatoes. It’s frustrating that the organic tomatoes aren’t similarly separated out because we can expect a similar difference between fresh and processed in organic. Farms that are exempt for certification make less than $5,000 per year, possibly because their yields are so low. And what is up with the bizarrely high yield for conventional total in 1964? Anyway…

In 2008, the total conventional yield is over 10 tons per acre higher than certified organic yield. To put it another way, you’d need 1.5 organic acres to grow the same amount of tomatoes. Note: I used total conventional here because the organic numbers also include tomatoes for fresh and processing uses.

See Today’s Organic, Yesterday’s Yields by Steve Savage for a great summary of the USDA yield data.

Now, let’s look at the nutrients per acre.

Item Organic Conventional
Soluble Solids 10.39 16.51
Total phenolic content 963.97 1002.39
Anthocyanins 6.24 42.68
Yellow flavonoids 75.70 110.80
Vitamin C 458.52 736.81
Lipid peroxidation 33.33 34.75
Total antioxidants 222.31 587.55
Phenylalanine ammonia-lyase 19.80 20.65
Ascorbate peroxidase 1.75 4.23
Catalase 8.78 70.96
Superoxide dismutase 210.91 97.87

With such a small difference in nutrients per tomato, does it make sense to sacrifice so many nutrients per acre? Instead, let’s see some research in agronomic practices to see what can be done to boost nutrients while keeping yields high. A bonus: integrating key organic methods into conventional agriculture could have a positive impact on sustainability, and since there are far more conventional farms than organic, that translates to a lot of positive change. Another option could be choosing tomato varieties that accumulate more nutrients while maintaining high yields. How about some purple tomatoes?

Citation:  Oliveira A.B., Moura C.F.H., Gomes-Filho E., Marco C.A., Urban L., Miranda M.R.A. & El-Shemy H.A. (2013). The Impact of Organic Farming on Quality of Tomatoes Is Associated to Increased Oxidative Stress during Fruit Development, PLoS ONE, 8 (2) e56354. DOI:

There seem to be two articles about this particular paper already in Science Seeker: Stress Makes Organic Fruits Healthier Than Conventional and Organic Tomatoes Contain More Vitamin C Than Conventionally Grown. So, I’m late to the party but hopefully this post will be useful because I take a more skeptical and in depth view than these folks did.

Want to redo the analysis yourself? Here’s my Excel file.

* Yes, I ignored the metric units when I uncorrected. Otherwise, some of the values became very large and hard to compare. Plus it doesn’t matter if you are comparing 5 and 6 or 5000 and 6000 since the ratio is the same.

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Anastasia is Policy Director of Biology Fortified, Inc. and the Co-Executive Editor of the Biofortified Blog. She has a PhD in genetics with a minor in sustainable agriculture from Iowa State University. Her favorite produce is artichokes!