Lactose and Sugars

All lactose is sugar, but not all sugar is lactose. And that's just the start of the confusion.

When most people talk about "sugar" they're referring to sucrose, the sugar found in sugar cane and sugar beets, among other sources. Sucrose is so common that if you see just plain "sugar" in an ingredients list on a food package, it means sucrose and nothing else. Any other type of sugar has to be mentioned specifically by name, either chemical name or common name. Honey is a type of sugar, and so is high fructose corn syrup, and molasses, and a host of other sweeteners.

Lactose is a special case, because it's the one sugar that so many people have trouble digesting. Those of us who are lactose intolerant lack the enzyme called lactase which breaks the lactose down into the simpler sugars called glucose and galactose.

And that leads to an interesting question, answered by Robert L. Wolke, professor emeritus of chemistry at the University of Pittsburgh whose latest book is "What Einstein Told His Cook 2, the Sequel: Further Adventures in Kitchen Science" (W.W. Norton, 2005), in a column in the Washington Post online.

I am a public health nutritionist and often need to recommend dairy alternatives or lactose-reduced products for clients who do not digest lactose well.
Here is my question: Why are the sugar levels of milk, yogurt and lactose-free milk all the same? It seems like the yogurt should have lower levels due to the consumption of milk sugar by the culture bacteria. And lactose-free milk should certainly have lower levels because of whatever they do to take the lactose out. However, the labels of all three products read about the same in terms of sugar.

It's puzzling, but true.

According to the U.S. Department of Agriculture's National Nutrient Database for Standard Reference, whole milk contains an average of 5.25 percent lactose by weight, while unflavored whole-milk yogurt contains 4.66 percent. Presumably, then, the culture bacteria have been allowed to consume only 0.59 percent of the milk's lactose before being forcibly restrained by cooling or killed by heat. By that time, the fermentation bacteria will have converted enough lactose into lactic acid to curdle the milk to the desired consistency.

The real head-scratcher, though, is lactose-free milk. If they've taken the lactose out of it, how come it still contains about 5 percent sugar -- the same as in whole milk?
The answer is that lactose-free milk is made not by removing the lactose but by adding the enzyme lactase. Just as sucrose is made of two simple sugars bound together, lactose is made of the two simple sugars glucose and galactose, bound together. The lactase enzyme splits the lactose into its two components, which are digestible by lactose- intolerant people.

But glucose and galactose are still sugars, and the FDA requires the aggregate amount of all sugars to be listed in the Nutrition Facts chart.

Good question, good answer, and a good rest of column as well. Take time to read it all.