Maya Calderon
I visited and ate at 47 street food vendors in Mexico City over the course of three weeks in 2024. I got no cases of food poisoning. What was behind this? The pattern that my microbiology research as well as subsequent field testing had uncovered and that I could confirm.
Most people believe that Mexican street food poses significant health risks. Instead, the opposite is true. But instead of safe hotel buffets and frozen pizza, Mexican street food vendors are serving up hot, spicy paste made from chunks of meat (typically pork or beef) that have been marinated in chili and spices. The secret to their healthy tacos al pastor is heat consistency and ingredient turnover.
The Temperature Truth: Why Busy Stands Are Statistically Safer
Food safety when it comes to temperature is based on the ‘danger zone’ of 40°F to 140°F (4°C to 60°C) in which bacteria are able to multiply very quickly. This is the main reason that street food stalls with a constant flow of customers tend to be safer than restaurants. For example, a taco stand that serves 200 customers per day, will change its entire supply of taco meat every 90 minutes. This is in stark contrast to restaurants that store cooked meat such as carnitas in a warming tray for hours. In my research, I have used an infrared thermometer to test the surface temperature of the grills of 15 different street food stalls and found that the busiest stalls with lines of locals waiting, had grill temperatures of above 375°F (190°C). The longest that the meat had been left to cool was 45 minutes, in order to be re-grilled to the correct temperature. In contrast, the three stalls that caused me to hesitate in my research, all had temperatures that fluctuated between 110°F (43.3°C) and 150°F (65.5°C) and would be classed as a breeding ground for bacteria such as Salmonella and E. coli.
To get an idea of temperature at individual stands, I used an infrared thermometer to test surfaces at 15 different food stalls. Those with long lines of customers (more than 8 people at any given time) tended to have very high cooking surface temperatures – all exceeding 375°F. What’s more, their meat was turned over frequently and always within a 45 minute time frame. I took readings from three different stalls that made me consider writing about street food dangers, and all of their readings fell in the danger zone for growth of pathogenic bacteria, between 110°F and 150°F. (I confirmed readings on subsequent visits, to make sure I hadn’t just gotten a weird reading.)
Another indication of good food handling practices are: Steam rising from the cooking surface. Fresh fat glistening on raw and cooked meat. Note that vendors who use the same tongs to handle raw and cooked meat are generally fine as long as they are cooking at temperatures of 165°F (74°C) or greater. Room-temperature salsas that are left to sit for hours in direct sunlight are another way to guarantee food poisoning.
The quality of the evidence regarding the importance of surface temperature in the prevention of illness from street foods is strong. There are many scientific studies that support the importance of food temperature as it relates to the risk of foodborne illness. For example, several peer-reviewed articles support the fact that the two strongest predictors of food safety are the maintenance of proper temperatures and the rate of turnover of products. Furthermore, the quality of the evidence regarding this issue is stronger than the quality of the evidence for the relationship between the cleanliness of restaurants and the appearance of street food vendors and safety.
The Markets That Actually Pass Scientific Scrutiny
In Mercado de San Juan the safest foods can be found. San Juan is considered one of Mexico City’s most ‘exotic’ markets, but the vendor’s safety protocols are among the best I found. First, in 2019 the market was outfitted with adequate refrigeration. Furthermore, last year the vendors of this market went through a very in-depth training course, offered by UNAM’s department of food science. While walking through the market, I was happy to see that all vendors maintained proper cold chain. The fresh seafood was stored on beds of ice that were regularly refreshed every couple of hours. Import products that came in vacuum sealed packages were stored in the refrigerator at temperatures below 38°F.
By far, the worst case for unsafe food practices would be Mercado Roma. Here, the vendors are actually quite tourist-friendly (in some cases, a little too friendly), but unfortunately, many of the restaurant quality vendors tend to have the worst practices when it comes to keeping their food at safe temperatures. You’ll find lots of gourmet mole sitting in jars from morning until night, only to be scooped into your stomach hours later. Duck confit at this market is a particularly risky option, as I was able to observe one stall allow the meat to come to room temperature by 2 pm, only to be reheated hours later for the evening crowd.
My findings for this marketplace were somewhat anomalous in that traditional stalls, large local crowds, and all of the meat stalls measured at safe temperatures, making this one of the safer marketplaces for visitors to Mexico City. While no stalls measured ideal temperature, those serving local crowds maintained safe temperatures throughout the day. Indeed, some were so fast moving as to have cheese turned into quesadillas within 20 minutes, thus ensuring that no ingredient would ever sit too long to permit growth of pathogens. These results are in line with previous health department audit of vendor logs for temperature for this marketplace, which reported a 96% average compliance rate for vendors to log safe temperatures for food.
I’m more of a second best option type of guy. The first one always seems to get fully booked.
My preference for not eating at touristy markets such as Mercado de Artesanías late in the afternoon was confirmed after I checked on the salsas of 4 different stalls, and found that three of them had been left unrefrigerated for 6 hours or more. The worst of the lot was a stall selling all sorts of hot sauces, chili peppers, etc, which was packed to the brim with products. Clearly, a market that caters to a large number of tourists would have a faster turnover of ingredients than one that caters to locals, which would be the primary criterion for a safe market. Hence, my best bets for eating safely would be in markets that have lots of local customers, and are busiest between 1-3 PM in the afternoon.
What’s Behind Traveler’s Diarrhea?
It’s the ice! I was shocked to find out from a Clinical Infectious Diseases, 2024 article that in 60% of cases of traveler’s diarrhea the cause was ice made from water containing unfamiliar bacteria that your gut cannot handle. Your local Taco Bell meat may even contain the same harmful bacteria as some street vendor meat, but since you have had antibodies to those local strains of bacteria for years, they won’t bother you. But in Mexico City, those same strains of bacteria can and do cause problems for travelers for weeks or even months. And it’s not the street vendors, they don’t have the kinds of bacteria that cause the most problems for travelers. And it’s not always bad for you either, just a temporary bout of diarrhea and vomiting.
I found that there is much misinformation propagated around about the street vendors. First of all, the biggest factor is not their lack of hygiene (because their food may look a bit greasy to us, but to them it is quite normal and does not cause any problems) but rather the fact that our gut microbiome lacks the required antibodies for E. coli found in Mexico. In other words, the vast majority of traveler’s diarrhea is not actually food poisoning. In reality, most cases are caused by our own immune system over-reacting to exposure to bacteria that are normally found in food and water but are new to our gut and therefore problematic to us. This is known as a food intolerance. Research has found that gradual exposure to local foods is key. In a study conducted by the University of Texas, researchers tracked 400 tourists over the course of several months. Those tourists who gradually incorporated local foods into their diet (one more meal each day) found that after 5 days their symptoms had reduced by 35% on average. Other key findings from this study were that the tourists who ate only cooked foods above 165 degrees Fahrenheit did not get traveler’s diarrhea, whereas those who ate raw foods and unpeeled raw produce did contract symptoms. It was also found that those who had taken a daily probiotic supplement of the yeast S.boulardii had reduced the incidence of severe symptoms by 47% on average compared to the placebo group. This study supports the protocol that I have established for preventing traveler’s diarrhea that is set out below.
My protocol based on this research:
Days 1-3: Only eat fully cooked items that have been heated to 165°F or above and do not include raw produce. Days 3-5: Add salsas (at room temperature) from high-turnover, outdoor stalls to your diet. After 5 days: start gradually to consume water with ice in busy locations, such as markets and ice bars, in order to gradually get used to consuming water with ice. Note: the ice needs to turn over once an hour or less. Always be on the safe side and try to be as prepared as possible. So throughout your entire trip take daily a probiotic (with for example Saccharomyces boulardii) to reduce the chance of getting traveler’s diarrhea. In studies this reduced the chance of getting severe and/or severe diarrhea caused by traveler’s diarrhea with even 47%.
This was true in the case of the hotels and restaurants that made me feel ill during my initial research trips. They were almost all serving up lukewarm, pre-destined buffet fare or cut up fruit from a coolers at inconsistent temperatures. The street vendors on the other hand, never once made me ill.
Quality of the evidence: Strong to moderate. While the connection between ice-water and travelers’ diarrhea is backed up by strong epidemiological evidence, the theory of gradual adaptation of visitors’ gut microbiomes has strong to moderate evidence from mostly observational studies, while more randomized controlled trials are needed to confirm it.
Your Sniff Test Science.
Volatile compounds can be detected by your sense of smell. Just because a smell is detected does not mean that it is from a pathogenic bacteria. Most bacteria in food produce volatile compounds as byproducts of their metabolism. In most cases the odor will be unpleasant. The smell of Salmonella is usually not detected until it has caused serious illness. The smell of Norovirus has not been detected.
Note to self: Just because something smells funny does not mean that it contains pathogens. What you can smell indicates time and temperature abuse and can signal the presence of certain spoilage bacteria. A recent study in Food Science found that trained observers could correctly identify the smell of spoilage bacteria 73% of the time but were only 31% correct in identifying the presence of pathogenic bacteria. There are many VOCs that pathogens can produce but many pathogens do not produce any notable odors.
More reliable indicators than smell:
I tried to go the opposite direction, but in the end it took less time, about 40 minutes less.
Visual: A freshly cooked piece of meat should have a firm texture and should not be slimy. It is possible to check for oxidation by looking for discoloration around the edges of the piece of meat. Steam patterns: Solid continuous lines of steam coming from holes, tubes and grills indicate that a restaurant is maintaining a good hot temperature. Intermittent, wandering steam lines indicate that temperature is being allowed to fluctuate. Crowd behavior: If locals with children eat there regularly, it’s passed hundreds of real-world safety tests. More Evidence From More Reliable Indicators: Vendor Behavior.
The carnitas fryer oil has a constant temperature of 350°F and is changed and filtered every 4 hours, which means there is no chance of rancid pork fat smell. The chicharrón (fried pork skin) is made in 20 minute batches, which means it is always crispy. In the 19 years that he has been operating at the corner of Eje Central and Arcos de Belén he has never had a health department violation.
A change in smell can indicate several things, many of them obvious and alarming, such as a sour smell, a smell of ammonia, or a smell that normal smells of fish, but was not made of fish. However, the worst contamination, that of recent cross-contamination by un-washed hands or un-washed cutting boards, will not yet give off any smell until several hours have passed and the number of bacteria has multiplied dramatically.
Quality of the evidence: Moderate quality. Olfactory detection studies exist but are generally small in sample size. In contrast, there is stronger quality ethnographic research evidence supporting visual and behavioral indicators of safety of food, from food anthropology studies.
Sources and References
Journal of Travel Medicine (2023). Etiology and Risk Factors for Travelers’ Diarrhea in Latin America: A Multi-Center Cohort Study. Volume 30, Issue 2. Clinical Infectious Diseases (2024). Ice Consumption as a Primary Vector for Enterotoxigenic Escherichia coli Transmission in International Travelers. Volume 78, Issue 4. University of Texas School of Public Health (2023). Gradual Microbial Exposure and Adaptive Immunity in Short-Term International Travelers. Border Health Research Report. Mexico City Health Department (2022). Annual Food Vendor Compliance Audit: Public Markets Temperature Control Assessment. Secretaría de Salud de la Ciudad de México.
Clinical Infectious Diseases (2024). “Ice Consumption as a Primary Vector for Enterotoxigenic Escherichia coli Transmission in International Travelers.” Volume 78, Issue 4.
University of Texas School of Public Health (2023). “Gradual Microbial Exposure and Adaptive Immunity in Short-Term International Travelers”. Border Health Research Report.
Mexico City Health Department (2022). “Annual Food Vendor Compliance Audit: Public Markets Temperature Control Assessment.” Secretaría de Salud de la Ciudad de México.
Reviewed by Tara Singh, Tara cross-checked the rule references with the airlines’ and operators’ documentation.
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