Also known as Naturlose®
Tagatose is hard to classify--it's really a sugar, but it's not one that humans can digest. I've chosen to put it with the polyols. Tagatose is a monosaccharide that occurs naturally in small quantities in some dairy products.
Tagatose has a clean sweet taste. It is about 90% as sweet as sucrose on a weight basis.
Tagatose, like the polyols, is only partly absorbed by the body, and not converted to energy very efficiently. Its caloric value depends on several factors, as discussed in my essay "Polyols and Calories." In the USA, tagatose provides 1.5 calories per gram for labeling purposes. In the European Union, it is listed at 2.4 calories per gram.
Tagatose solubility in water is about 0.6 g per mL. It is less hygroscopic (absorbing moisture from the air) than fructose. Unlike the polyols, tagatose can participate in browning reactions--this is an important quality in baking applications.
Tagatose occurs naturally (at low levels) in some yogurts, since it is a metabolic intermediate of lactobacilli. It has GRAS status in the USA. The Joint FAO/WHO Expert Committee on Food Additives (JECFA) has approved tagatose as a "novel food ingredient", with no limitation on its usage. Unabsorbed tagatose can cause some digestive system unhappiness, including gas, rumbling sounds (borborygmus), and diarrhea. You can read about this in my essay, "Polyols--Digestive Issues."
Tagatose is chemically similar to fructose.