What Makes Flavor in Fruit?

Justus G. Kirchner.

Without their aromatic flavors, fruits are insipid pulps. If the volatile oils are removed from citrus juices, for example, there remains a juice having only a sweet taste and an acid taste, which come from the sugars and the acids. The characteristic fruity flavor is gone. The juice has no further appeal.

We speak of the taste of a fruit, but actually we are referring to its flavor. Taste is one component of flavor. The other is odor. We sometimes hold our noses when we swallow an unpleasant-tasting medicine; actually we are keeping the olfactory organs from being brought into use.

Taste includes four qualities, singly or in combination : Sweetness, sourness, saltiness, and bitterness. Hotness, as in pepper, and coolness, as in menthol, are sometimes also classified as taste sensations.

A sweet taste is best detected by the tip of the tongue, a salty one by the forward edges, a sour one by the back edges, and a bitter one by the back. This variation explains why we do not taste many bitter substances until after we have swallowed them.

We usually associate a sweet taste with sugars, although there are compounds having no relation to sugars that are also sweet sometimes even sweeter than sugars. An example is saccharin, which is almost 500 times sweeter than sugar.

Sweetness is generally attributed to the molecule as a whole, but in some instances, notably in certain inorganic compounds, it is associated with ionic stimuli. Beryllium salts are sweet, for example, and this sweetness is associated with the beryllium part of the molecule, which in solution takes on a positive charge and becomes an ion.

Salty and sour tastes are associated with salts and acids, respectively. Sodium chloride (common table salt) has the taste generally associated with salt. Not all chemical salts have this taste, however; many are intensely bitter.

The acid taste may be detected in most fruits. It ranges from the mild banana to the sour lemon. Tartness depends on the amount of acid and on the kinds and amounts of other substances present. For example, two solutions that contain exactly the same amounts of acid, as measured by chemical standards, but with different amounts of sugar, will differ in their sour taste. In other words, the sugar does not destroy the acid but merely tends to mask the sour taste.

ODOR IS MUCH MORE COMPLICATED than taste. It cannot be broken down easily into classifications. Sweet and bitter and salty and sour are distinct classes of taste, but there are no corresponding classes of odors.

Numerous attempts have been made to set up a classification system for odors. The most recent is that of E. C. Crocker and L. F. Henderson, of the Arthur D. Little Laboratories. They recognize four fundamental odors: Fragrant, or sweet; acid, or sour; burnt; and caprylic, or goaty. Each is subdivided into eight divisions, based on intensity. To facilitate classification, a set of eight standard chemicals has been set up for each of the four fundamental odors. Every odor can then be classified by a four-digit number, the size of each digit depending on the intensity of the particular fundamental odor. For example, toluene would be coded by the number 2424 and citral would-be 6645.

The aromatic part of fruit flavors arises from the presence of minute quantities of volatile chemical constituents. These are compounds that travel through the air and affect the olfactory organs and give rise to a flow of saliva and related activity. They impart to the fruit its characteristic flavor. The aromatic constituents are extremely sensitive to physical and chemical treatment, and they may be lost or changed as a result of processing.

One of the biggest problems in processing fruits is to preserve the natural fresh flavor and prevent the development of unpleasant off-flavors. Processing techniques have been developed mostly from the standpoint of engineering rather than of the effects on the flavor of the product, primarily because little has been known about the chemistry of the flavoring constituents. This is not surprising, because the isolation and identification of the flavoring constituents are not simple tasks. For one thing, the flavoring components are present in such small quantities that tons of fruit are required to yield a few grams of the aromatic flavoring material. Because the flavor substances of fruits are complex mixtures, separation into their constituent parts results in the collection of only small fractions of a gram for study.

THE CANNING OF CITRUS JUICES presents a particular problem in the preservation of natural flavor, in both the freshly canned and the stored product. During pasteurization, a cooked flavor often develops; during storage, a separate and distinct off-flavor arises, the result of time and temperature of storage.

The development of off-flavors is also different in each fruit. It is most intense in the lemon and a little less intense in orange and grapefruit. The cooked flavor of canned citrus juice is preferred by some to the fresh juice flavor, but the storage off-flavor is unpleasant to everyone.

The solution to the problem of eliminating or preventing off-flavors in canned citrus juices lies in a knowledge of the chemistry of the flavoring constituents in the fresh and processed products.

California Valencia orange juice is the only citrus juice that has been investigated to any great extent. J. A. Hall and C. P. Wilson, of the California Fruit Growers Exchange, crushed whole oranges, removed the peel oil, and recovered 182 grams of juice oil from more than 10,000 gallons of juice. Even that amount was not enough to identify all the flavoring components. As might be expected from the character of the metabolic products of plant tissue, the Valencia orange juice contained small quantities of ethyl alcohol and, in addition, the esters of terpene alcohols, along with the usual aldehyde compounds. But the major part of the alcohol fraction was an unidentified compound, whose physical constants closely resembled those of the terpene alcohol, linalool.

BECAUSE COMMERCIALLY processed orange juice contains small amounts of peel oil from the juicing operation, it might be possible that this material has a role in the development of off-flavors. In contrast to the oil from the juice sacs, orange peel oil has been thoroughly investigated. It is 90 to 95 percent d-limonene. The remaining part contains aldehydes and terpene alcohols.

Many investigators have believed that the volatile oils in citrus juices account for the development of off-flavors in the canned product. Their experiments showed that canned juices with an oil content of less than 0.007 percent developed no off-flavors after 18 months in storage, while those with an oil content of 0.01 percent or more readily developed off-flavors.

Other investigators have supported the theory of the deterioration of the essential oil, believing that the unsaturated hydrocarbons or alcohols are responsible and that the d-limonene fraction of the oil also has a part in causing off-flavors. On the other hand, workers in the Department's laboratory in Winter Haven, Fla., concluded that the off -flavor might be due partly to an oxidation of the fatty material in the juice. The addition of antioxidants to the canned products, however, did not appear to have any stabilizing effect.

LITTLE IS KNOWN about the chemistry of the flavoring constituents of the other citrus fruits. The navel orange presents an interesting problem, because its juice turns extremely bitter shortly after it is extracted. For that reason, the housewife does not prepare navel orange juice in the evening and hold it overnight in the refrigerator for breakfast the next morning.

The peel, section membranes, and center core of this orange contain a compound that is extracted into the acid juice during the reaming process and turns bitter from contact with the acid in a few hours. The bitter principle is called limonin, and its presence prevents the use of navel orange juice for commercial processing. Somehow the presence of the bitter principle is closely related to the maturity of the fruit. It is more prevalent in immature oranges and decreases as the fruit ripens. It is also more intense in some growing seasons than in others.

Grapefruit is something of an anomaly. Some persons believe that grapefruit juice owes its bitterness to the presence of quinine. Instead, it is the bitter glycoside, naringin, in the peel, section membranes, and center core. Naringin differs from limonin (which also is present in the peel, section membrane, and center core) in that it is bitter in its natural form and imparts a bitter flavor to the juice if too much Pressure is used in the reaming process, Particularly if the fruit pulp is pressed to obtain a higher yield of juice. Naringin is also associated with the maturity of the fruit. It is more intense at the beginning of the season and gradually drops toward the end.

Little work has been done on lemon juice. L. Francesconi, however, has tentatively identified an alcohol and an acid, other than citric acid, in the juice.

The Department's research laboratory in Pasadena has started a study of the chemistry of the flavoring constituents of fresh, canned, and stored citrus juices. The aim is to determine the constituents present in fresh juices and what happens to them after being heated in cans and stored.

Attempts have been made to return the volatile flavoring materials removed in the low-temperature concentration of citrus juices to the product to provide a full-flavored concentrate. As the process is now carried out, the juice is concentrated to a 5 to 1 ratio meaning that 4 parts of water would have to be added to 1 part of concentrate to obtain the consistency of the original juice. To the concentrate is then added 1 part of fresh juice, giving a 4 to 1 concentrate as it is sold in the stores. That means that 1 part of citrus juice is furnishing the volatile flavoring materials for 5 parts of concentrate, a considerable dilution of the original flavor. If that flavor dilution could be prevented by capturing the oils lost during the concentration, the housewife would enjoy a more flavorful product. The processor in turn wants to do so because of the cost factor; by stopping at the 4-to-1 ratio of concentration he would save one-fifth of the cost of the present concentrating method.

The volatile flavoring material as recovered from the concentration of citrus juices is extremely unstable, and, until ways are found to stabilize it, the fortification of the flavor of the concentrate will be impossible. The Pasadena laboratory is investigating this problem in connection with its related study on basic flavoring constituents.

Often the identification of all the flavoring constituents in a fruit is not the work of one man or one group, but rather the cumulative efforts of several laboratories over a period of years. Research on the flavoring components in apples is a good example.