There's probably no one alive who knows the reason, 100 percent, for absolute certain.

Why did Bob Gibson come up with the idea of using mayonnaise with vinegar in a barbecue sauce for smoked chicken?

Chris Lilly, executive chef and vice-president of Big Bob Gibson Bar-B-Q, the iconic Decatur barbecue restaurant Gibson founded, has an interesting, logical theory. Although, "I can only speculate based on my conversations with his kids, back when I first joined the family," says Lilly, who is married to Gibson's great granddaughter Amy. "They're all since passed away."

A 6-foot-4, 300-pound railway worker, in the mid-1920s "Big" Bob Gibson began cooking in the backyard of his two-story, four-bedroom Danville Road home before he ever thought of opening a restaurant. It was just a weekend thing, for friends and family. Big Bob cooked whole chickens and whole pork shoulders in his hand-dug pit and served them from an oak-plank table nailed to a sycamore tree.

The scene at a backyard barbecue at Big Bob Gibson's Decatur residence in the mid-1920s. (Courtesy photo)

"He definitely had an affinity for vinegar based sauces," Lilly says. "His pork sauce is what I'd describe as sort of an Eastern-style North Carolina barbecue sauce. It's what you call a 'sop mop,' where he basted shoulders during the latter part of the cooking process. And I think that carried over for his chicken when he developed that sauce.

"His chickens sometimes stayed on his pit for three hours. When you're pulling a pit of chickens you've got to have some way to keep those chickens moist. Keep them from drying out. And I think that's where the mayonnaise came from. The fat in the mayonnaise basically used as a buffer against the chicken drying out once it came off the pit."

Mayo was an ingredient on many other country cooking staples, like potato salad and deviled eggs. But using it in a barbecue sauce was a radical idea. And an effective one. Tangy. Peppery. A flavor accentuator rather than dominator, melding with smoky chicken for a sum greater than its whole. It's a relatively simple sauce, in addition to the mayonnaise and vinegar other ingredients include lemon juice, salt and black pepper.

"People describe it as a mayonnaise sauce, but I would describe it as a vinegar sauce with mayonnaise in it, more than anything," Lilly says, who's been with Big Bob Gibson Bar-B-Q since 1991.

When Gibson opened his first restaurant in Decatur in 1925, the white sauce was there from day one. Big Bob moved locations frequently in the early years, in and just outside of Decatur, the white sauce always in tow, before establishing more permanent roots on Decatur's 6th Avenue in 1952. Big Bob Gibson Bar-B-Q's current locations include 1715 6th Ave. S.E. and 2520 Danville Road S.W., the latter just a couple miles away from the backyard where Big Bob's barbecue was born.

Of course, his white sauce's reach has expanded significantly. In addition to other takes on Gibson's original from barbecue restaurants in Decatur and across the state, "Alabama white sauce" has made its way onto menus at places like Chicago's eclectic The Publican and Napa, Calif. burger joint Gott's Roadside. (Lilly is particularly fond of The Publican's use of the sauce on chargrilled carrots: "It was absolutely phenomenal.") Then there are visitors from Germany, Sweden and other overseas countries whose barbecue pilgrimages to the American South wouldn't be complete without a stop at Big Bob Gibson Bar-B-Q, stuffing sauce bottles into their luggage before the flight home. Big Bob Gibson's has also shipped bottles of white sauce overseas, to countries including Australia and all across Europe.

Big Bob Gibson Bar-B-Q's Original White Sauce. (Courtesy photo)

Big Bob Gibson Bar-B-Q began bottling their Original White Sauce in the mid-90s. It's now available in grocery stores in Alabama, Tennessee, Mississippi, Georgia and Florida, as well as specialty shops across the country. "One problem we had when we first started bottling the sauce is the separation," Lilly says. "The oil would start separating and you'd get clear on the bottom and white on top so it took us a little time for us to get the recipe right for bottling so the stuff wouldn't separate."

In their restaurants, Big Bob Gibson Bar-B-Q uses the sauce as fast as they can make it. Each smoked chicken that comes off the pit in the pit room gets dipped in a vat of white barbecue sauce. "It gives it a good coating and it keeps the chicken from drying out while we hold it on the service line," Lilly, oversees the Danville Road location, says. "We don't do another sauce application until it gets to the table out front where the customer can add more white sauce if they want to."

Over at the 6th Avenue location, which Gibson's grandson and Lilly's father-in-law Don McLemore oversees, cooks make about 15 gallons of white sauce each day.

"We go through that every day here," McLemore says. "We make it in gallon jugs because it's easier for us to use that way." McLemore has been eating the white sauce on chicken since he was 10 or so. "At that time, the white sauce was so different from anything else," McLemore says. He also enjoys the white sauce on Big Bob Gibson Bar-B-Q smoked turkey and as a salad dressing. Little kids at the restaurant like to dip their Golden Flake potato chips into white sauce. "A lot of customers put it on the pork - now I personally myself don't care for it on pork but everything else I love it on," McLemore says. "You either love it or your hate it and the majority of folks love it."

Big Bob Gibson Bar-B-Q's smoked chicken with Original White Sauce. (Courtesy photo)

McLemore remembers his grandfather Big Bob as a man who loved to fish and hunt. And drink whiskey. "He was married three times," McLemore says. "He was a character but he loved life and lived a good life. He was a big guy, but he was a very friendly guy too, didn't meet a stranger, talked a lot." Gibson reportedly didn't try too hard to keep his white sauce recipe clandestine either, and cooks who worked for him are likely at least partially responsible for the sauce making its way elsewhere.

Over the years, there have been occasional assertions the white sauce actually did not originate with Big Bob.

"Folks from here in Decatur, 'Oh, my grandfather he told your grandfather how to do that,'" McLemore says. "I've been told that by many different people. But they can't tell you what's in it, so they don't know, they're just talking, trying to act like they told my grandfather how to do it."

Much like Gibson's inspiration for his white sauce, no one is exactly sure what happened to that old oak table the big man first served his barbecue from, behind his faded-gray home. Lilly would love to have a piece of the table or the sycamore tree it was nailed to. He does have a piece of wood siding from Gibson's first restaurant with the word "barbecue" in faded letters across it. It's now in Lilly's garage and will eventually hang in the current Danville Road restaurant.

Chris Lilly says if the restaurant was going to place one product in a time capsule to represent Big Bob Gibson's in the future it would "absolutley be a bottle of white barbecue sauce." They've made mark other ways too. In Memphis this May, Big Bob Gibson Bar-B-Q was named the World Championship Barbecue Cooking Contest's Grand Champion for the fifth time, following wins in 2000, 2003, 2011 and 2014.

When told of Lilly's theory about the white sauce's origins, to keep chickens moist on the pit, McLemore says, "That could very well be it. Hey, I would give somebody a thousand dollars if they could tell me the truth about how he started that. But I don't know of anybody that knows. My mother didn't even know how he came up with that idea." (Gibson had five children, all of whom eventually went into the barbecue business.)

One thing McLemore does know for certain is that Big Bob Gibson would be "totally amazed" his white barbecue sauce ended up in Chicago and California restaurants, in bottles and in European home kitchens: "I sure never in his wildest dreams would he even think about something like that. I wish I could have a patent on it where nobody else could use it but you can't do that (at this point). But actually in another way though it's quite an honor for my grandfather, that he started something like that and it's really grown."