surface caramelize to a deep rich brown and the flavor intensifies. This browning of sugars and
amino acids is called the Maillard reaction, after the French chemist who first identified it.
Onions may also change color when cooked. Onions get their creamy color from antho-
xanthins, pale-yellow pigments that turn brown if they combine with metal ions. (That’s
why onions discolor if you cook them in an aluminum or iron pot or slice them with a car-
bon-steel knife.) Red onions contain anthocyanin pigments that turn redder in acid (lemon
juice, vinegar) and bluish in a basic (alkaline) solution. And the chlorophyll molecules that
make the tops of green onions green are sensitive to acids. When heated, chlorophyll reacts
with acids in the vegetable or in the cooking water to produce pheophytin, which is brown.
The pheophytin makes green onion tops olive-drab. To keep green onions green, you have to
reduce the interaction between the chlorophyll and the acids. You can do this by leaving the
top off the pot so that the acids float off into the air or by steaming the onions in little or no
water or by cooking them so quickly that there is no time for the reaction to occur.
How Other Kinds of Processing Affect This Food
Drying. Drying onions into flakes removes the moisture and concentrate the nutrients.
Ounce for ounce, dried onions have approximately nine times the vitamin C, eight times the
thiamin, ten times the riboflavin, nine times the niacin, five times the iron, and eleven times
as much potassium as fresh onions.
Medical Uses and/or Benefits
Adverse Effects Associated with This Food
Halitosis. The onion’s sulfur compounds can leave a penetrating odor on your breath unless
you brush after eating. Fresh onions are smellier than cooked ones, since cooking breaks
down the sulfur compounds.
Food/Drug Interactions
Anticoagulants Green Onions are rich in vitamin K, the blood-clotting vitamin produced
naturally by bacteria in the intestines. Consuming large quantities of this food may reduce
the effectiveness of anticoagulants (blood thinners) such as warfarin (Coumadin). One
chopped, raw green onion (top and bulbs) contains 207 mcg vitamin K, more than three
times the RDA for a healthy adult.
Onions