update: New York Times X, 2021: The Big Tuna Sandwich Mystery:

For about $500, his lab could conduct a PCR test — which rapidly makes millions or billions of copies of a specific DNA sample — and try to tell me whether this substance included one of five different tuna species.

There is some news about a report from Canada's CBC about fast-food chicken items containing as much as 50% soy protein, based on DNA testing. I won't do the report the service of linking to it directly, but you can search for the news "What's in your chicken sandwich? DNA test shows Subway sandwiches could contain just 50% chicken."

Isn't quantifying ratios of various DNA sequences fairly difficult, requiring rigorous sample preparation? Could results vary dramatically by varying cooking times, and the independent processing of the animal protein and the vegetable protein? Is there an reason to expect isolated DNA ratios to be a good indicator of protein ratios?

I'm also curious, is there a better way to at least try to do this study - perhaps search for key sequences of amino acids within the proteins to at least try to distinguish animal from vegetable proteins quantitatively?

edit: Here is a newer, more reliable, independent news item not written by the agency who paid for the testing.

The NPR news article DNA Tests Find Subway Chicken Only 50 Percent Meat, Canadian News Program Reports says:

The tests were conducted by a DNA researcher at Trent University's Wildlife Forensic DNA Laboratory, for a CBC Marketplace episode dedicated to testing fast-food chicken dishes, and have not been independently confirmed.

Later in the same article a strong statement by one of the companies refuting the results has also been provided.

  • $\begingroup$ Looking at the article, which didn't provide a link to anything published, they used several control sources (McDonald's, Wendy's, etc.) so assuming their method was truely the same in each case there is reason to suspect it contains significantly less chicken. "Just 50%" might be a stretch, though. It does seem like a good place to start at least, although the lack of actual publication and the rush to the press seems pretty unethical. $\endgroup$
    – jzx
    Mar 1, 2017 at 10:19
  • $\begingroup$ @jzx the history of the food - the chicken and any soy could be very different. Quantitative DNA analysis requires very careful and controlled sample preparation. Cooking temperature and time, chemical environment, pH, all these things are totally uncontrolled. Even if they pay lip service to the word "control" the samples are totally uncontrolled in a variety of factors that could impact final DNA concentration, in a way that differs between soy and chicken, for each tested product. $\endgroup$
    – uhoh
    Mar 1, 2017 at 10:41
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    $\begingroup$ I'm not saying your question is bad, in fact it's quite interesting. I also agree that methodology of any "study" should be published. But, the way you phrase the question makes it sound like you already have your mind made up on the answer before asking it. Anyways, my previous comment is not particularly relevant, so I have deleted it. Good day to you, sir. $\endgroup$
    – canadianer
    Mar 1, 2017 at 16:48
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    $\begingroup$ Importantly, if you look at the lab that did the testing, most of their work seems to involve identification, rather than quantification. Without more insight into their methods I can only say that there is a huge difference between determining whether a steak came from a moose or beef and determining that a hamburger is some % moose... $\endgroup$
    – Bryan Krause
    Mar 2, 2017 at 0:28
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    $\begingroup$ s3.documentcloud.org/documents/3477610/DNA-Report-Chicken.pdf $\endgroup$
    – canadianer
    Mar 2, 2017 at 1:57

2 Answers 2


In reference to the update in the question three years later, I'll include additional quotes from the same New York Times article.

“No amplifiable tuna DNA was present in the sample and so we obtained no amplification products from the DNA,” the email read. “Therefore, we cannot identify the species.”

The spokesman from the lab offered a bit of analysis. “There’s two conclusions,” he said. “One, it’s so heavily processed that whatever we could pull out, we couldn’t make an identification. Or we got some and there’s just nothing there that’s tuna.” (Subway declined to comment on the lab results.)

With all testing, there are major caveats to consider. Once tuna has been cooked, its DNA becomes denatured — meaning that the fish’s characteristic properties have likely been destroyed, making it difficult, if not impossible, to identify.

While this question falls somewhat between biology, chemistry, and food science, the use of quantitative DNA analysis as a proxy for estimation of relative protein fractions from two different species (in this case plant vs mammal) is probably on-topic here considering it is biologists who have invented, use, and interpret results of DNA amplification and sequencing analysis, and can best speak to their correct use and potential problems when used outside their accepted protocols.

I am not a trained biologist myself, so I can not state with authority that starting with a food product whose plant and animal components may have started from different places on Earth, been processed differently, then combined and further processed involving major excursions in heat (cooking) and in pH deviate substantially from accepted protocols for DNA quantitative analysis, but I have a strong hunch that this is probably so.

As @BryanKrause has noted in a comment, the laboratory in question's primary expertise is in animal identification - does a carcass or piece of meat or fish come from the species claimed or not, and this is a laudable service. But using relative fractions of DNA within a single sample of prepared food as proxy for relative contribution of protein is far outside that kind of service, and as far as I know, is not an accepted practice anywhere.

Ars Technica is a popular technical news source that generally does a excellent job of summarizing scientific topics when they do cover them, in my opinion at least. In an update to the article Subway releases data after scientists weigh in on 50% chicken test [Updated] they summarize released information from two independent laboratories using an "ELISA methodology designed for food products to quantify soy in the chicken." ELISA is a quantitative method that can be used to measure the presence of various proteins directly, and does not use DNA as a proxy.

Update, March 6: Subway has released the lab reports from both of its independent tests. Both Maxxam Analytics in Ontario, Canada and Elisa Technologies, Inc. in Florida used enzyme-linked immunosorbent assays (ELISA) designed for food products to quantify soy in the chicken. ELISA's are a standard type of assay that generally detect and quantify substances based on binding by an antibody. In the assay, antibody binding kicks off a detectable chemical reaction, commonly resulting in a color change. In the case of Elisa Technologies, the lab used an antibody that binds to soy flour proteins and the lab used known concentrations of those soy proteins for comparison to determine the quantity of soy protein in Subway's chicken samples.

In all samples, Elisa detected 3 parts-per-million or less of soy proteins, which is well below one percent of the chicken. Maxxam detected 5.3 ppm of soy protein in the chicken, which is still well below one percent. (emphasis added)

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    $\begingroup$ The testing by subway only raises more questions since they, too, did not release their methodology. Also, what does ppm mean in this context? $\endgroup$
    – canadianer
    Mar 15, 2017 at 3:26
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    $\begingroup$ @canadianer these are very good points, thanks! I am not sure how much has been released in toto, this is the best source I could find so far. It is possible a more complete report has been released elsewhere. I'll keep looking. $\endgroup$
    – uhoh
    Mar 15, 2017 at 3:35
  • $\begingroup$ I'm going to tentatively accept my own answer in this case. Please flag me if there's a change or new information that I've missed, or just leave an improved answer! $\endgroup$
    – uhoh
    Mar 26, 2017 at 11:26

I now read the article. They don't include/show a standard curve so take it with a grain of salt.

It's possible to do Carbon Isotope analysis to distinguish plant vs. animal carbon from an isolated protein fraction.

Besides that you could also do quantitative mass-spectrometry and look for animal vs. plant peptides.

If you want both cell number, protein abundance of known targets there's the fancy Mass Cytometry that could do this.

That being said, IMHO: DNA-sequencing is probably fine and definitely robust. If you set-up a standard curve (mix soy and chicken at known ratios and DNA sequence it) then that's probably the easiest/cheapest way to do this. Without a standard curve testing this though you can't draw a definitive conclusion.

Also from the NPR article; analysis from other fast-food sources of meat gave lower Soy DNA ratios with Subway being an outlier. That's an important distinction to make. Subway subsequently claiming that it's 100% chicken appears more misleading after previously saying there was soy as a stabilizer.

  • $\begingroup$ Can you provide some evidence to support the "probably fine"? The mix is processed at high temperature with several added chemicals, none of which are directly related to standard laboratory procedures for handling and preserving DNA samples for quantitative analysis. Can you also support that a DNA ratio can serve as a quantitative proxy for a protein ratio? I appreciate your answer, but right now as written, it's looks like an unsupported opinion. $\endgroup$
    – uhoh
    Mar 1, 2017 at 22:24
  • $\begingroup$ Temperature and chemical treatment wouldn't matter since all the DNA is being treated the same. Maybe if it's heavily degraded the values are low you get into a non-linear problem. One way or another there isn't enough information to say what they did is correct or not, so yeah it is partly based on opinion, the methods I outline are ways to definitively test it. My opinion is also that fast-food places will serve junk if it's cheap, subway chicken in particular barely resembles a chicken breast. $\endgroup$
    – Artem
    Mar 1, 2017 at 22:43
  • $\begingroup$ That's not necessarily correct. Animal and plant DNA start out in very different types of cells, with different ratio of protein to DNA. They are processed separately for quite a while before being added together. The chicken will probably be sterilized fairly quickly (compared to soy) due to the risk of salmonella or other dangerous organisms. $\endgroup$
    – uhoh
    Mar 1, 2017 at 22:48
  • $\begingroup$ Sure, if you want to find flaws in this study they are there I'm not denying it. Is that difference enough to explain the 1.5 order of magnitude discrepancy (1% vs. 50%) between what the company vs. researchers are saying? I'm skeptical. $\endgroup$
    – Artem
    Mar 1, 2017 at 22:52
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    $\begingroup$ I could also imagine different outcomes depending on the standardization procedure. For example, if the products from soy are mostly protein isolates, different procedures might leave more or less DNA intact. If Subway's process left 50x more DNA than the preparation used in the standards, that could also lead to an overestimate. I think overall it is going to be hard to interpret these results without a full publication-style report. $\endgroup$
    – Bryan Krause
    Mar 2, 2017 at 0:23

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