About two months ago, on a solo afternoon where I didn't feel like being alone, I lured my buddy Ted over for dinner with some lovely, high-end pork chops. Ted loves a good meal and is Homer Simpson-esque in his appreciation of well-prepared meat.

I was testing a new $399 sous vide machine called the Mellow, a sort of space-age aquarium with sharp lines, app control, and the ability to both keep food cold then cook it, all in the same tub of water—a modern spin on the set-it-and-forget-it slow cooker. I dropped the bagged chops in the water bath in the afternoon, and the Mellow cooled them until it was time for the cooking to begin two hours later. At that point, it heated the sous vide bath and had them ready for a quick sear right at dinner time. Ted didn't disappoint either, tearing into the perfect medium-rare chops with zeal.

Among the "busy people" throughlines of Mellow's homepage video, one of the stories shows a dad hopeless enough to be feeding his grade-schooler some sort of microwave meal, selling it with that "lips-kissing-fingertips" gesture, before graduating to a Mellow-made dinner for the smiling tyke.

The pork chops were my second success of the day. At lunchtime, I had used the Mellow with equally stellar results, making some cod that I cooked in the bag with a white miso marinade. Served with some rocket and a smashed roast potato, it was a luxurious lunch.

Following those meals, I looked back at the time-temperature chart that the Mellow's app creates and saves for each meal, and was surprised how gradually the temperature dropped in the cooling stage. The cod was only in the water for 41 minutes before it started heating up, but the chops were in there much longer. Though the charts for each had low resolution, the machine seemed surprisingly confident lounging around in what the USDA calls the "danger zone"—the temperatures between 40 and 140 degrees Fahrenheit where the warmth encourages bacterial growth in foods like fish, meat, and poultry.

Anybody who's ever earned a food-handler's permit is familiar with the danger zone. The idea is to keep food out of the zone, but if you do need to pass through it, do it quickly, keeping harmful bacteria and the toxins some of them produce at bay. To keep it simple, the USDA advises home cooks to never leave food out of refrigeration for more than two hours.

There's plenty of reason for caution; bacteria like Salmonella, E. coli, Campylobacter, Bacillus cereus, and Clostridium botulinum love to party in the danger zone.

Temp Work

A week later, I cooked some burgers. During my time spent testing the device, by which point it was already on the market, Mellow didn't provide actual recipes like some other prominent sous-vide manufacturers, instead instructing the user to simply "season your food with herbs of your preference and put it inside a food-safe bag." Since then, the company has added some recipes to its website, and the app has been updated to include presets for cooking additional varieties of meat. But at the time, users were left mostly in the dark.

Using my trusty ThermoWorks Smoke thermometer, I put one probe into a burger, sealing the bag as best I could around the cord, and another probe into the water bath in order to be able to monitor both temperatures.

Better meat quality and proper handling lowers the risk of sickness due to food-borne illness, but it's worth noting that ground meat of any stripe poses a particular risk—harmful bacteria tends to collect on the exterior of meat, and when the cut goes through the grinder, it is spread evenly throughout the interior. Those bacteria can make you sick or even kill you, so it's important to play it safe as you can with ground beef. While those risks are mitigated by cooking at a specific temperature for a specific amount of time which kills off bacteria, something like Clostridium botulinum produces the neurotoxin which gives you botulism, something that's not neutralized in the cooking process and that may possibly be present if you were cooking a recipe that included vegetables or legumes.

While some of the Mellow's literature suggests using cool water or jumpstarting the process with ice cubes, the "first time guidelines" say you can fill the Mellow's tub with room-temperature water, put your food in and walk away. Even that home-page video shows two bags of meat being dropped into 68-degree water, and the in-app quick-start guide explicitly says to start with "cool or room temperature" water. I followed suit, starting with water at 70 degrees Fahrenheit.

Thanks to the spreadsheet and chart made with the app on the ThermoWorks Smoke Gateway Wi-Fi accessory, I could follow what was happening inside the water bath, reading both water temperature and the temperature in the center of the burger.

It showed the water temperature starting at 70 degrees Fahrenheit and, over the course of four hours, going down to 41 degrees. In the bag, the burger, which had already spent some time outside the fridge as I mixed in the herbs of my preference, warmed from 51 to 63 degrees Fahrenheit in the first half hour, then followed the water temperature down to 41. In short, it spent four hours in the danger zone—two hours longer than the USDA advises—before the cooking process began.

Risky Business

I had visions of Far Side-style cartoons of nefarious bacteria sporting burglar masks and, worried that I'd made E. coli burgers, I threw the patties in the trash and ordered takeout. To make sure this wasn't a fluke, I duplicated the burger cooking process overnight, this time simply tracking the water temperature, which took just as long to cool down to 40 degrees Fahrenheit.

Next, I called Stephanie Smith, the statewide consumer food safety specialist for Washington State University Extension, and described the cooling process.

"Wow! That's not good. Not good at all. Some bacteria can grow really fast. As few as 10 pathogenic E. coli can make you sick or end up killing you," she said. "The rule for consumers is that if it's been out of the fridge for two hours, it should go in the trash."

I called Mellow and talked about Smith’s concerns with a company spokesperson, marketing manager Tyler Cuddihey. He told me Mellow's creators "take food safety extremely seriously and respect everyone's opinion" and "could assure (me) it's perfectly safe." The company's PR rep followed it up by sending a food-safety statement (read it here) which referenced scientific studies and FDA guidelines. Their email also included a statement from Mellow's executive chef and food science lead, Filipe Leonor, pertaining to the unit’s safety.

I ran these responses past Smith. The first thing she pointed to was the use of a study in Mellow's food safety statement led by D.A. Ratkowsky. The study, called the "Model for Bacterial Culture Growth Rate Throughout the Entire Biokinetic Temperature Range" was published in The Journal of Bacteriology in 1983, the year Return of The Jedi was released in theaters.

"The model provided by Mellow in its food safety statement cannot be applied to all microorganisms that cause food-borne illness," said Smith. “Ratkowsky’s scientific study, while helpful, is no longer all-encompassing. It’s questionable for them to use the FDA guidelines for a consumer-based product; for consumer usage—the USDA guidelines should be used.”

Linda Harris, a food safety researcher and chair of the department of food science and technology at UC Davis, picked up on this point. She plugged the four-hour descent from 70 to 40 degrees into ComBase, a database used by scientists that uses models to show how microorganisms survive, grow, and die under a variety of food-related conditions. Looking at Salmonella and Clostridium botulinum, the model showed they wouldn't multiply in that period.

"Under these parameters, the opportunity for microorganisms to multiply is limited, if everything were working properly," Harris said. She also warned that you could push it, with something like lower-quality food, a bug of the machine, or poor food handling, to where you might see bacterial growth. "It’s amazing what different consumers will do with the same recipe."

I thought here of that guy in the video who was trying to feed his kid a microwave dinner and was now making the leap to sous vide. Would he get it right?

Harris then took issue with Mellow's food safety statement, saying it could be reworked to present the data more clearly. "It's not made well," she said. Harris also disagreed with Mellow's emailed statement, referring to a line from Mellow’s chef/scientist Filipe Leonor’s email that read, "In the example of a mechanically tenderized, minced, or processed meat such as a burger, our cooking algorithm is slightly different to allow full pasteurization using time/temperatures ratios."

Using the time and temperature data points collected when I was cooking my burger, Harris ran a different ComBase model and found that "reductions were predicted to be between 99 to 99.9 percent (or between "2 and 3 log") for both Salmonella and E. coli.”

Hey, almost 100 percent is good, right?

Nope.

"It makes a difference in microbiology,” Harris says. "Full pasteurization is 99.999 percent reduction. Minimum. By my calculations, they did not make full pasteurization."

The "99 to 99.9 percent" data for my burger means a 100 to 1,000-fold reduction for Salmonella and E. coli, yet a target 100,000-fold reduction (99.999 percent) is commonly used by the food industry to describe pasteurization. Furthermore, Harris mentioned that "the meat and poultry industry target reductions of Salmonella are closer to 10 million-fold."

Harris suggested following the USDA guidelines to cook for 46 minutes at 134 degrees Fahrenheit, or 121 minutes at 130 degrees for full pasteurization.

I also called Marianne Gravely, senior technical information specialist at the USDA, and described my experience with the machine. She said that the Mellow's cooling process "took too long to get down to a cold enough temperature. We would tell a consumer that it's too long in the danger zone ... We don’t have jurisdiction over what they sell, but if it’s for home use, it shouldn’t put the consumer in the situation where they’re handling their food in an unsafe way."

I immediately felt better about pitching those burgers and scheduled another call with the Mellow team, including the CEO, the cofounder, and two of the company's food scientists. One minute before our call began, Mellow's team emailed me a revised food safety statement. This one was seven pages long and on letterhead, instead of the three-pager they'd sent three days prior. The new one featured more charts and diagrams, including one of the USDA's danger zone thermometer which was labeled as "Figure 1 . Fallacious 'Danger Zone' graph: 40°F–140°F." As Harris later pointed out when I showed it to her, Mellow's statement cherry-picked a mishmash of consumer, restaurant, and food processing recommendations—standards that don’t necessarily apply to the home cook—then just rolled them into one document. (Read Mellow's revised food safety statement.)

I started on the call with the Mellow team by asking for comment on USDA specialist Gravely's assessment that the cooling process kept foods in the danger zone too long. Mellow CEO Gary Itenson added, “The cooling times of Mellow complies with the FDA and USDA."

Here, Mellow cofounder Catarina Violante steered me toward some hard-to-decipher text in the new safety document that reads: "Mellow cooling system performance has all the above prerequisites into account; it reaches 40°F and prior to it reaches 50°F in safe timings - "food shall have an initial temperature of 5°C (41°F) or less when removed from temperature control and the food temperature may not exceed 21°C (70°F) within a maximum time period of 6 hours" [sic].

It's not clear why this section of the document seems to be working in reverse cooking order, but I think they're saying that the food cools quickly enough in the Mellow. They then refer to the 1983 Ratkowsky document which, if I'm making the connection correctly, encourages the use of just-from-the fridge food and ignores that it might need some preparation. The Mellow team also told me to look at section 4)a) of their new safety document, which reads, "Even though it may take a few hours to reach the 40°F mark, it reaches 70°F and consequently 50°F quickly enough to maintain the ingredient safe until it reaches 40°F and below."

"The FDA is telling us that safety is a combination of time and temperature,” Violante said.

I reminded them here that I was asking about the USDA specialist's statement. Violante then called the USDA guidelines "very general."

"People who cook sous vide are more aware of what's safe and what's not," she said, sending my mind back to the microwave dinner guy.

When I asked about the Ewok-era Ratkowsky data on bacterial culture growth rates, Itenson referred me to table A-2, an FDA spreadsheet about pathogen growth and toxin formation copied into their just-minted food-safety document. Only later did I note that table A-2, the only table of its kind in Mellow's document, was only for "controlling pathogen growth and toxin formation in fish and fishery products," which struck me as odd, because we’d been talking about hamburgers; fish never came up in our conversation. As Smith later put it, “What they provide in their food-safety documents are an incomplete snapshot.”

Later, when presented with an email that included the ComBase models used by Harris, Mellow CEO Itenson provided what their PR firm’s representative referred to as the “final statement from Mellow.” Itenson reminded me that although Mellow is intended for in-home use where food would be immediately consumed after it was cooked, the product is "designed and manufactured to meet the same food safety guidelines followed by top chefs."

It’s hard to hold a conversation with company representatives who pivot on almost every question, particularly when those pivots come when they’re asked about concerns voiced by three food safety specialists. I like it when Tom Cruise flies through the danger zone, but I don’t want my kitchen gear encouraging risks in it. There’s no way I can recommend the Mellow.