WHEN AIZA WAS EIGHT, she discovered that eggs taste better in the dark.

"I couldn't sleep, so I snuck out of bed to get a snack. I didn't want to wake anyone up," the Filipino-Chinese college student explains. "Normally I'd eat at the most one egg. I must have eaten three!"

It turned out that Aiza had mixed up the two shakers her grandmother kept on the kitchen table: one for salt, and one for MSG, or monosodium glutamate.

To the Chinese, it's wei jing, or "essence of taste." To the Vietnamese, it's bot ngot, or "sweet powder." To countless others, it's Ajinomoto, after one of the substance's primary manufacturers, based in Japan. Since its introduction about 100 years ago, MSG has become a culinary staple in restaurants, noodle shops, and millions of homes throughout Asia. Generations have been suckled on MSG, a sugarcane-derived substance that enhances the taste of food by tricking your brain into thinking you're eating more savory meatiness than you really are.

Scientifically speaking, it's human nature to like MSG: people are biologically programmed to seek out MSG's active portion, otherwise known as free glutamate. Since the late 1960s, however, it's been accused of causing everything from headaches to flushing, tingling in the face and arms, to fatigue and chest pains. These symptoms even have a collective name, "Chinese Restaurant Syndrome," which has given Chinese food a bad rap and helped turned MSG into a radioactive substance, at least in health-conscious America. Is MSG really as bad as they say? And what’s the science behind these claims?

Let's take it from the top. Free glutamate is one of the amino acids that make up protein. It's a familiar substance—it occurs in natural abundance in certain foods, such as steak, shellfish, ripe tomatoes, dried mushrooms, cooked potatoes, wheat…and breast milk. Newborns are literally sucking it up from the getgo. Ounce for ounce, human milk contains ten times more free glutamate than cow's milk.

Digestion is one way to release glutamate from protein, but it's not the only one. When foods are fermented (soy sauce), cooked (canned fish), or aged (prosciutto), some of the proteins within get broken down, and the amount of free glutamate spikes up, way up—as much as 10 to 20 times more.

Taste buds on the tongue have molecular receptors that physically bind to free glutamate molecules and send a signal to the brain. The brain interprets these signals as the brothy, mouth-filling taste of happiness, which the Japanese refer to as umami, or "delicious."

Westerners are very familiar with four tastes: salty, sweet, bitter and sour, each with its own molecular taste receptors on the tongue. But umami—the fifth taste—is garnering a lot of attention from food-savvy folk.

"Take a fresh anchovy, pack it in salt, let it ferment for 9 months, and you get fish sauce. An umami bomb," says Bill Briwa, a chef instructor at the Culinary Institute of America in Napa, California.

A free glutamate bomb, too, we'd add.

Briwa explains that each of the other four tastes is rooted in survival. "Sweet means that the fruit you're eating is ripe. Sour, that the fruit is under-ripe or spoiled. Bitter taste can mean that something is poisonous, while salty means micronutrients or minerals. But umami…" he pauses for effect. "Umami means good quality."

As a side note, this makes the addition of MSG—and therefore umami by proxy—to instant ramen a no-brainer for ramen makers.

In 1908, Kikunae Ikeda, a researcher at Tokyo Imperial University, pinpointed MSG as the natural compound responsible for the tasty flavor of kombu in broth. Ikeda promptly coined the term umami, patented his discovery, and convinced the founder of Ajinomoto to mass-produce MSG crystals.

The rest, as they say, is history. In the last century since Ikeda first found MSG, the Asian love affair with the substance, produced from fermented sugar cane, has grown into a 1.4 billion dollar a year obsession. According to the business research firm SRI Consulting, some 3.5 billion pounds of MSG is consumed yearly worldwide—that's the equivalent weight of twice the population of Tokyo. About 82 percent of this is gobbled up in Asia alone.