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HOUSTON -- During the early space shuttle days, NASA sent its astronauts into the heavens with fresh bread packed into a special food locker so they could make sandwiches.

But on one mission, a payload specialist from Mexico joined the shuttle crew, and he packed tortillas in the fresh food locker. Shortly thereafter, high above Earth, the rest of the crew saw the benefits of rolling food up in the tortillas.

NASA would never be the same.

As part of CNET Road Trip 2014, I've come to this southeast Texas city of 2.16 million people to visit Johnson Space Center, NASA's main astronaut training operation. Yesterday, in part one of my JSC coverage, I wrote about the Neutral Buoyancy Lab, where astronauts go 40 feet underwater to practice tasks they'll eventually perform on and outside the International Space Station. Today, I look at NASA's Space Food Systems Laboratory.

Houston, we have a (tortilla) problem

With NASA's astronauts now convinced that using tortillas was preferable to making sandwiches with normal bread, the space agency set about trying to ensure a steady supply. There was just one problem, according to Vickie Kloeris, the manager of the International Space Station Food System: there were no suitable tortillas to be found near Kennedy Space Center in Florida, the Space Shuttle launch site.

It wasn't, of course, that no one in the area sold tortillas, it was that none that could be found met NASA's microbiological needs -- essentially, that the food could survive in space. That was especially true when Shuttle missions started lasting multiple weeks, not days. NASA had plenty of history producing acceptable "extended shelf-life bread products" that flew aboard the Shuttles in anti-mold anaerobic packaging designed to provide muffins and flatbreads for the military.

But it had no experience with tortillas, so it had to start making them, Kloeris said. And it did, successfully producing them for four-month Shuttle missions. When it came time to pack food for missions to the International Space Station (ISS) that could last much longer, though, it was out of its league.

Thankfully, Taco Bell came to the rescue. As Kloeris remembered it, the food giant began producing a commercial tortilla pack that was microbiologically designed to last nine months. "So we got out of the tortilla (making) business," she said.

Won't float away

Due to severe constraints on available square-footage on the ISS, NASA packs some of its food and drinks dry, and astronauts must add liquid to make things edible. And given the law of partial pressure, Food Lab contractor manager Kimberly Glaus-Late explained, that liquid will try to leak out, which requires many of the packages to have a built-in septum-adapter assembly, essentially a one-way valve that allows liquid to be introduced but not to flow back out.

But other dishes, like beef stew, are considered tastier in a thermo-stabilized state, Glaus-Late said. That means they're packed with their water, and are ready to eat once the package is cut open.

You might think that astronauts trying to eat in the zero-gravity environment of the ISS would constantly be struggling to keep their beef stew from floating away. In fact, though, wet food in microgravity will either stick to itself or to its package, so there's little danger of it escaping.

Things like these are just part of NASA astronauts' food training. According to Glaus-Late, astronauts go through about four different training sessions, much of which is devoted to allowing them to taste different foods and determine their food preferences.

Although the vast majority of the food available on the ISS comes from a standard set of 200 food and beverage items -- with much of it sent to the space station long before new astronauts arrive -- each astronaut gets to pick a small set of "bonus containers" of special food. By small, NASA means nine total off-menu items for a six-month mission.

How each astronaut chooses their bonus items is up to them, but it may have something to do with their desire for specific snacks, or their knowledge that there are limited numbers of standard items they enjoy. For example, Glaus-Late said, there may only be three packages of M&Ms available the entire crew for an eight-day period, so one crew member may want more of the chocolate treat.

All told, NASA provides eight different categories of food to its astronauts: meats and fish; rehydratable meat; vegetables; soups; breakfast; beverages; and now, for the first time, condiments. According to Glaus-Late, the food lab has recently developed a new way of sending up small plastic bottles filled with things like ketchup, mustard, and even sriracha. All of this goes toward helping overcome monotony. As Kloeris put it, "Variety is key. The more variety the better during six months on the" ISS.

'Grandma's canning process'

For the most part, NASA isn't doing much food development these days. Most of its food-related efforts are going into production.

Kloeris said that most of the food is made in a lab at Texas A&M University, and NASA does the freeze-drying at Johnson Space Center, using a machine called a retort to vacuum seal the servings. "Basically," Kloeris said, the retort system is "grandma's canning process, automated."

In the old days, NASA fed its astronauts plenty of military-grade MREs, or meals ready to eat. But over time, the agency determined that the MREs were geared toward young servicemembers who needed a lot of salt in their diet. Astronauts, however, found the meals were too high in salt and fat, so in 1998, NASA began developing its own thermo-stabilized products Today, Kloeris said, NASA produces 65 different thermo-stabilized meals, all of which would be unfit for public consumption by U.S. Food and Drug Administration standards, since they are officially considered "experimental foods."

Despite moving the astronauts away from military MREs, NASA flight surgeons began recognizing an alarming trend around 2009 or 2010, Kloeris said. By that time, there had been astronauts aboard the ISS continuously since 2000, and the surgeons began noticing that some of the returning crew members were suffering from a permanent loss of visual acuity, she said, that was pinned on increased intercranial pressure -- a pressure on the optic nerve.

Daniel Terdiman/CNET

The doctors were not certain what was causing the problem, Kloeris said, but decided to order further reductions in sodium in the standard astronaut diet as a precaution. Cutting back on salt is almost always a good thing, but for NASA, that meant most of the commercial products it was buying went "out the door," Kloeris said, since they usually had high salt content. "So we had to start making all our own freeze-dried products," she said.

Since then, the food lab has reformulated about 90 of its 200 products, effectively reduced the salt content in the astronauts' diet by 43 percent, she said, earning the lab's Space Food System Sodium Reformulation Team" a special NASA Group Achievement Award.

Unfortunately, it's too early to tell whether the dietary change has had an impact on the intercranial pressure problem, particularly because so much of the food stored on the ISS has been there since well before the reformulation project. Only just now, Kloeris said, are crew members beginning to eat food that was produced entirely after that project commenced.

Fresh food

Another factor that limits the astronauts' food experience is the frequency of cargo launches, since NASA and its partners can only bring food to the ISS so often. But Kloeris said there's a hope that as cargo launches increase over time, NASA may be able to send more food up -- meaning more fresh food for the astronauts.

Already, she said, SpaceX's cargo launches have allowed NASA to send up things like apples and citrus fruits that can last a while without refrigeration. When SpaceX does a launch, there's a 5-day lead time for fresh food. Another NASA commercial cargo partner, Orbital, has a 12-day lead time, meaning less fresh food. By comparison, Space Shuttle missions used to have a 2-day lead time. At five days or less, astronauts can eat (for a short period) things like grapes and avocados. "They really like getting the fresh food," Kloeris said. "That's what they miss. When they get it, it's pretty much gone right away."

Biggest challenges

Though it may be many years before NASA ever sends manned missions to Mars, the space agency is already thinking through many of the factors that will affect such missions.

Food, of course, is one of them.

For one thing, Kloeris said, the agency has to contend with the fact that any Mars missions are all but certain to last a minimum of three years. That means food bound for the Red Planet has to have a 5-year shelf life.

Already, though, Kloeris and her team are researching food sciences for Mars missions, she said. The biggest challenges are coming up with quality food that can withstand the chemical changes that take place over several years in space. Vitamin C is a big stumbling block, she said, as it degrades quickly.

As well, they need to figure out how to pack enough variety to keep the astronauts happy.

Still, many of the thermo-stabilization processes NASA has already perfected are likely to work as well for Mars as they do for the ISS, she said. And there are new procedures, such as high-pressure processing, coming out of the military that kill bacteria while using less heat and which could help with Mars missions.

In the meantime, in a world in which almost everyone you meet has some sort of special food needs, be it no gluten, or no red meat, or no dairy, Kloeris said NASA has been spared dealing with that particular issue. "It hasn't happened, because they have to be so healthy to be astronauts," she said. "We haven't really run into astronauts with dietary restrictions."

That said, Kloeris allowed that perhaps her department's biggest challenge would be "if a vegan wants to fly to the ISS. It is going to be hard for them to get all their nutrition needs met."

Added Kloeris, "Hopefully, I'll be retired by then."

Keep an eye out for more behind-the-scenes stories and photo galleries as I travel throughout Texas, Oklahoma, Arkansas, Tennessee, and Kansas during this year's Road Trip. I'll seek out most interesting technology, military, aviation, architecture, and other destinations our country has to offer. From U.S. Air Force basic training to NASA's Johnson Space Center and FedEx's massive package-sorting hub, and much more, Road Trip 2014 will take you along with me.