This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Sugar is found in fruits and vegetables. Beets, carrots, onions, peas, and other vegetables contain appreciable amounts. In vegetables like peas and corn the sugar is deposited as the insoluble carbohydrate, starch, in the mature seed. Immature peas and corn deteriorate rapidly in sugar content after they are gathered. Kertesz reports that starch increases and sugar decreases in stored fresh peas, but the sugar loss is not due to the transformation of sugar to starch. It is suggested that the apparent increase of starch content is because of loss of other constituents; and, whereas the loss of the sugar is not satisfactorily clarified, it may be consumed in respiration. Losses in sugar content can be prevented by inactivation of the respiratory enzymes by blanching, cooking, or storing near freezing temperatures.
The vegetables with a fairly high sugar content usually have a sweeter flavor if steamed or if just enough water is used to cook them and it is evaporated to dryness at the end of the cooking period. Even mild-flavored onions may be sweeter and better flavored if the water is evaporated than when an excess is left. But there seem to be some exceptions to most rules, and the choice of method of cooking is often a matter of judgment or circumstance. More could be written about flavor, but it has been mentioned so often in connection with different constituents of fruits and vegetables that a great deal would be repetition.
A combination of sugar, acids, and aromatic substances gives flavors that render fruits and vegetables palatable and attractive food products.
Aromatic compounds. This classification is not given as a plant chemistry one, but as a group to include all substances that may give characteristic odor and thus flavor to foods.
Many of the aromatic compounds are esters, like amyl acetate, which gives a characteristic odor to pears and is called "pear oil." The odor of pineapple is due to methylbutyrate, which is designated "pineapple-oil." Isoamyl isovalerate produces the characteristic odor of apples.
Thatcher includes all the substances that give characteristic odor to plants under the term "essential oils and resins." Some of these substances are terpenes, alcohols derived from terpenes, the phenols, and sulfuretted oils. Oils of lemon, peppermint, cinnamon, clove, lavender, and others are classified as essential oils.
Allyl isosulfocyanide, oil of mustard, and allyl sulfide, oil of garlic, constitute the best known of the oils containing sulfur.
Tannins impart an astringent, bitter flavor to some foods.