What we call flavor or savor is a joint effect of taste and odor in which the latter predominates.  There are only four tastes of importance, acid, alkaline, bitter and sweet.  The acid, or sour taste, is the perception of hydrogen atoms charged with positive electricity.  The alkaline, or soapy taste, is the perception of hydroxyl radicles charged with negative electricity.  The bitter and sweet tastes and all the odors depend upon the chemical constitution of the compound, but the laws of the relation have not yet been worked out.  Since these sense organs, the taste and smell buds, are sunk in the moist mucous membrane they can only be touched by substances soluble in water, and to reach the sense of smell they must also be volatile so as to be diffused in the air inhaled by the nose.  The “taste” of food is mostly due to the volatile odors of it that creep up the back-stairs into the olfactory chamber.

A chemist given an unknown substance would have to make an elementary analysis and some tedious tests to determine whether it contained methyl or ethyl groups, whether it was an aldehyde or an ester, whether the carbon atoms were singly or doubly linked and whether it was an open chain or closed.  But let him get a whiff of it and he can give instantly a pretty shrewd guess as to these points.  His nose knows.

Although the chemist does not yet know enough to tell for certain from looking at the structural formula what sort of odor the compound would have or whether it would have any, yet we can divide odoriferous substances into classes according to their constitution.  What are commonly known as “fruity” odors belong mostly to what the chemist calls the fatty or aliphatic series.  For instance, we may have in a ripe fruit an alcohol (say ethyl or common alcohol) and an acid (say acetic or vinegar) and a combination of these, the ester or organic salt (in this case ethyl acetate), which is more odorous than either of its components.  These esters of the fatty acids give the characteristic savor to many of our favorite fruits, candies and beverages.  The pear flavor, amyl acetate, is made from acetic acid and amyl alcohol—­though amyl alcohol (fusel oil) has a detestable smell.  Pineapple is ethyl butyrate—­but the acid part of it (butyric acid) is what gives Limburger cheese its aroma.  These essential oils are easily made in the laboratory, but cannot be extracted from the fruit for separate use.

If the carbon chain contains one or more double linkages we get the “flowery” perfumes.  For instance, here is the symbol of geraniol, the chief ingredient of otto of roses: 

  (CH_{3})_{2}C = CHCH_{2}CH_{2}C(CH_{3})_{2} = CHCH_{2}OH

The rose would smell as sweet under another name, but it may be questioned whether it would stand being called by the name of dimethyl-2-6-octadiene-2-6-ol-8.  Geraniol by oxidation goes into the aldehyde, citral, which occurs in lemons, oranges and verbena flowers.  Another compound of this group, linalool, is found in lavender, bergamot and many flowers.