When bread gets browned on both sides because of exposure to heat, it becomes toast. Having a separate English word indicates the perception that a transformation has taken place between a slice of bread and a warm piece of toast. This implies that toast is somehow different from bread. Toasting bread certainly changes its appearance and crunchiness, but it actually has little effect on its nutritional value.
What Happens When You Toast Bread
As the bread gets heated, some of the moisture evaporates, so the slice of bread will weigh a little less after it has been toasted. The heat also produces a chemical reaction on the surface of the bread that causes it to turn brown. This reaction, named for Louis Camille Maillard, the French scientist who discovered it in the early 20th century, is called the Maillard reaction.
Significance of the Maillard Reaction
Heating the bread causes a series of biochemical reactions between carbohydrates or sugars and amino acids, which are smaller units of proteins, on the surface of the bread. This changes the appearance of the bread, browning the surface of the toast, and causes a change in the flavor, because the chemical reaction creates new molecules on the surface of the bread through the Maillard reaction.
Toasting Does Not Cut Calories
Unfortunately, toasting bread as part of a diet to cut carbohydrates will not be effective. The Maillard reaction does not significantly reduce the calories or carbohydrates in the slice of bread. The browning only takes place on the surface where the bread is placed next to the heating elements inside a toaster.
According to online nutrition information, a slice of untoasted wheat bread has 65 kcal while a toasted slice has 64.9 kcal. That is only a slight difference. Both the untoasted and toasted slice of bread have 11.8g carbohydrate, 2.3g protein and 1g total fat.
A Word about Acrylamide
Apart from considering the slight nutritional change made in toast by the number of calories, you should be aware of a byproduct from the biochemical reactions that occur in toasted bread. The reaction between carbohydrates and amino acids produces a compound called acrylamide. Acrylamide is a chemical which, taken in high doses, has been linked to cancer, according to a 2009 article by New York University professor Marion Nestle, published on the website for "The Atlantic Monthly" magazine.
Recommendations for Toast
In addition to the usual nutritional advice to eat whole wheat rather than white, according to MayoClinic.com, a person should avoid eating very burnt areas of toast, and to aim for eating bread that is toasted no more than a light brown. This helps to minimize the amount of exposure to acrylamide. Although further research continues to be done about the link between burnt toast and cancer in women due to acrylamide, a healthy dose of caution might be warranted.
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