Vol. V · The Molecules of Food Plate 2 of 11sugars, glucose, fructose, sucrose, lactose, maltose, sweetness, solubility, reducing sugarsMF

Sugars.

Table sugar is one sugar among several. Fructose is sweeter, lactose barely sweet, glucose browns faster — and they behave differently in every recipe.

Sugars are small carbohydrates. Glucose and fructose are single sugars; sucrose (table sugar) is one of each joined; lactose, milk sugar, is glucose joined to galactose; maltose, from the breakdown of starch, is two glucoses. They taste differently sweet — fructose about one and a half times as sweet as sucrose, lactose a fraction — and they dissolve to very different extents.

They also differ in chemistry the cook can see. Glucose, fructose, lactose and maltose are reducing sugars: they have a reactive end that joins the Maillard reaction readily (VIII.1), so a dough with honey or milk browns sooner than one with only table sugar, whose reactive ends are locked together. The workshop mixes sugars in water and computes how sweet the mixture tastes.

Below: the sweetness of a solution of the sugars you add, in sucrose equivalents, and how sweet each sugar is per gram.

What you will learn
Workshop
Relative sweetness per gram against sucrose = 1 (approximate values from sensory studies: fructose 1.5, glucose 0.7, maltose 0.4, lactose 0.3; sweetness depends on concentration and temperature). Sucrose equivalent of the mixture = Σ grams × relative sweetness.

Sweetness, solubility, browning

Ten grams of sucrose in 100 mL tastes like 10.0 g of sucrose, by definition; the same weight of fructose like 15.0 g and of lactose like 3.0 g. That is why honey and fruit (rich in fructose) taste sweeter than their sugar content suggests, and why milk, with nearly 5 % lactose, hardly tastes sweet. Solubility differs as much: about 200 g of sucrose dissolves in 100 g of water at room temperature, but only about 20 g of lactose — which is why lactose crystallises as gritty sandiness in old ice cream and condensed milk.

For browning, reducing sugars matter. A cake made with honey, or a bread brushed with milk, browns faster and darker than one sweetened only with sucrose; glucose syrup and fructose give darker crusts and caramels at lower temperatures (VII.8). Cooks use this both ways: a little honey or milk powder for colour, all sucrose for a pale meringue.

At the stove

Dissolve a teaspoon of sugar in a glass of water and a teaspoon of honey in another; taste both. Then brush two slices of bread, one with milk and one with water, and toast them side by side. The milk-brushed one browns first: lactose and milk proteins join the Maillard reaction.

Sucrose was refined from sugar cane in India by about the fifth century; the word "sugar" comes, through Arabic and Persian, from the Sanskrit śarkarā, "grit".

Terms

Monosaccharide
A single sugar unit such as glucose or fructose.
Disaccharide
Two sugar units joined: sucrose, lactose, maltose.
Reducing sugar
A sugar with a reactive end that takes part readily in browning.
Relative sweetness
Sweetness compared with sucrose at the same concentration.
Sugars differ in sweetness, solubility and browning; reducing sugars such as glucose and lactose brown faster than sucrose.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.