All varieties can contain sulphur depending on the specific refining process used, but unsulphured products (lighter in color and smoother in flavor) are available. The lighter the molasses, the sweeter it is.
Here are the different varieties of molasses:
Blackstrap molasses: The syrup remaining after the third extraction of sugar from sugar cane. Blackstrap (derived in part from the Dutch stroop, meaning syrup) refers to the color of the molasses, which is extremely dark. It has a very strong, somewhat bittersweet flavor with a heady aroma. This variety is best used in recipes rather than as a straight sweetener such as pancake syrup. It contains many of the nutrients left behind by refined sugar crystals. By measure, it is 55% sucrose, the least sweet of the varieties.
Light molasses: Syrup remaining after the first processing of the sugar. It is generally unsulphured and is the lightest as well as sweetest variety. It is often used as a syrup for pancakes and waffles or stirred into hot cereals such as oatmeal. 65% sucrose.
Medium or Dark molasses: Remains after the second processing of the sugar. It is not as strong as blackstrap. About 60% sucrose.
Treacle: True treacle dates from Victorian times. A pale, refined molasses, it is notably sweeter and has a much more mellow flavor than molasses.
Nowadays, treacle is a blend of molasses and refinery syrup. It ranges in color from light gold to nearly black. British treacle can be substituted for molasses in most recipes, but much less frequently will molasses work as a replacement for treacle. If you do substitute molasses for treacle, use the lightest, unsulphured molasses you can find.
Sorghum molasses: Technically, this is not molasses. It comes from the sorghum plant, a cereal grain which is grown specifically for molasses rather than refined sugar. It is also referred to as unsulphured, West Indies or Barbados molasses. The syrup is made from the juice of the stalk which is cooked and clarified. The result is smooth with a clear amber color, free of sediment or graininess. Although it contains no sulphur, sorghum molasses generally does contain a preservative which is added to lengthen its short shelf life. When substituting for other sweeteners, use 1/2 to 3/4 of the sweetener amount called for in the recipe. Since it can ferment, sorghum molasses should be kept refrigerated unless you go through it fairly quickly.
65% to 70% sucrose.