THE LEMON FRUIT

COMPOSITION AND PHYSIOLOGY

Specific Gravity
AN IMPORTANT FACTOR in determining the quality of a fruit is its specific gravity. The specific gravity of a fruit or its juice may increase, as in the orange, or it may remain practically the same, as shown for the juice in the lemon (table 3), from the time the fruit is very small until it is mature. Where there is an increase in specific gravity the increase may be due largely to a single substance, for example, sugar in the orange. On the other hand, there may be an increase of a single substance as the fruit grows to maturity, such as citric acid in the lemon, but there may be a commensurate decrease in other substances so that the specific gravity remains practically constant. The following are the results of investigations that have been made to determine the specific gravity of whole lemon fruits and of lemon juice.

Whole fruit.—Young (1915) picked and tested ten lots of lemons at intervals during the period from January 14 to March ll and found their specific gravity to range from 0.862 to 0.890 (av., 0.876). In general, the longer the fruits remained on the trees the lower their specific gravity. On January 13 he picked a large amount of fruit and placed it in storage. Nine samples of the stored fruit were tested at four-day intervals between January 14 and February 15. In these samples the specific gravity steadily increased up to the time of the last determination. The range was from 0.890 on January 14 to 0.968 on February 15, with an average of 0.938. In the case of frozen fruit, he found that there was a noticeable decrease in the specific gravity. This would be expected, due to the loss of water without a corresponding loss in volume. On the other hand, in spite of the loss of water, there was no marked change in the specific gravity of the pulp juice of frozen fruits. Any tendency toward an increase in specific gravity of the juice, due to the loss of water, was apparently overcome by the decrease in total soluble solids, as a result of respiration. (For data and a discussion of the losses of sugars and acids in the pulp juice of frozen lemons, see "Sugars," p. 64, and "Organic Acids," p. 74.)

The data presented from Young (1915) were later published as a portion of a University of California bulletin under the joint authorship of Thomas, Young and Smith (1919).

Data similar to those presented by Young on stored unfrozen and frozen lemons were published by Bailey and Wilson (1916). The frozen lemons were picked early on the morning following the freeze and stored at 45° to 50° F so that they would thaw slowly. They found that unfrozen lemons placed in storage and tested at intervals between January 21 and May 2 (16 determinations) had an average specific gravity of 0.943. Similar determinations showed that stored frozen lemons over the same period had an average specific gravity of 0.896 while frozen lemons which were picked from the trees at the regular testing intervals had an average specific gravity of only 0.740. The results show that the frozen fruit which thawed slowly in storage maintained a higher specific gravity than the frozen fruit that thawed more rapidly in the field.

Chace, Wilson and Church (1921) determined the specific gravity of mature Eureka lemons from seven areas and of mature Lisbon lemons from five areas in southern California. Samples were collected and tested once a month for 10 to 13 months in each area, entailing a total of 203 determinations which showed average specific gravities as follows: By applying statistical methods to their data these investigators found that the odds were 78 to 1 that the specific gravity of the Eureka lemons was significantly higher than that of the Lisbon lemons. They also report that the specific gravities were highest in midsummer, lowest during the winter, and, in general, lower in lemons with thick peels than in those with thin peels.

Turrell and Slack (1948) have recently determined the specific gravities of whole freshly picked lemon fruits grown in different areas in southern California. Those grown in the interior had a specific gravity of 0.911, those in the coastal area 0.953, and those in the intermediate area 0.934. The results suggest an actual difference in the specific gravities of the fruits grown in the different areas. These authors point out, however, that although the results are suggestive they do not prove to be statistically significant.

Juice.—As would be expected, the specific gravity of lemon juice is greater than that of the whole fruit. Bailey and Wilson (1916) stored lemons in January. At intervals between then and May they determined the specific gravity of the juice of 16 samples of the stored lemons. The values ranged from 1.031 to 1.046, with an average of 1.042.

The values immediately following this paragraph show the results of three recent determinations of the specific gravity of lemon juice. Sinclair and Eny (1945) determined the specific gravity of the juice of freshly picked mature lemons and that of the juice of lemons that had been kept in storage from one to three months. Bartholomew and Sinclair (unpublished) made similar determinations on the juice of the stem- and stylar-end halves of 15 lots of commercially mature lemons from 12 areas in southern California. Each lot of 50 lemons was taken directly from the trees, or from the packinghouse floor on the day they were picked. All fruits were green to silver in color. Sinclair and Eny (unpublished) also determined the specific gravity of the juice of lemons of different ages from the time they were about 2 cm in diameter until they were mature. That the fruits of a given size in these last tests were all of the same age was known because all fruits concerned were measured and tagged when they were very small.

The results of these three sets of determinations were as follows: These results show that the specific gravity of the juice of lemons remains reasonably constant regardless of age, effects of storage, and the half of the fruit (stem or stylar) from which it comes. As would be expected, there was some variation in different lots of a given group. The stored fruits showed the greatest difference between individual lots, a difference of 0.0099 in specific gravity which would mean a change of 2.40 per cent in soluble solids. The averages of the freshly picked fruit and the stored fruit, however, showed a difference equal to only 0.20 per cent soluble solids. The differences between the averages of the stem and stylar halves of the fruit and those between the young fruit and the more mature fruit were equal to only 0.05 and 0.26 per cent soluble solids, respectively.

Color in Peel
The lemon fruit owes its color principally to two groups of fat-soluble green and yellow plastid pigments, the chlorophylls and carotenoids. There is some evidence that under certain conditions the flavonoids also may contribute at least a small amount of yellow color to the lemon peel from which the chlorophyll has disappeared. For example, Weatherby and Chang (1943) found 1.66 mgs of quercetin equivalent (flavones and their derivatives) per gram of fresh lemon peel. The values for oranges and grapefruit were noticeably lower—0.076 mg and 0.036 mg, respectively (see also Wilson, Weatherby and Bock, 1942). It should be remembered, however, that although such substances as quercetin are yellow in pure crystalline form they usually combine with sugars to form glycosides (Braverman, 1949) which are colorless or almost colorless.

As long as any portion of the fruit remains green in color, it, like the green leaves, continues to manufacture carbohydrates, some of which are stored in the plastids in the form of starch grains. When the fruit is very young, the plastids, even in the innermost tissues, contain chlorophyll and starch. As the fruit enlarges, the chlorophyll and starch disappear progressively out¬ward until the entire fruit becomes yellow.

Dufrenoy (1929) found that the normal change in color of the fruit from green to yellow is caused by the simultaneous loss of chlorophyll and the digestion and disappearance of the starch from the plastids. As the chlorophyll and starch disappear, fat bodies form in the parts of the plastids formerly occupied by the starch grains. The fat-soluble carotenoids are then absorbed from other portions of the plastids by these fat bodies, thus giving the fruit its natural yellow color. This process can be hastened artificially by exposing the green-colored fruit to ethylene gas.

Miller and Winston (1939) found that in limes, lemons, and grapefruits, a portion of the carotenoids tends to disappear while the chlorophyll is disappearing. For example, they found 9.15 parts per million total carotenoids in the peel of mature green Villafranca lemons, but only 3.35 ppm in similar lemons that had remained on the trees until they were yellow. This may at least partially explain why the mature color of these fruits is lighter than the color of mature oranges and tangerines, in which the carotenoids increase in amount as the chlorophyll disappears.

Carotenoid-bearing plastids being present in all parts of the fruit, they are present in the extracted juice. Swift (1946) found that reamed juice contained 50 per cent more carotenoids than pressed juice.

The chlorophylls (a and b) are all-important in the process of forming carbohydrates from water and carbon dioxide. While the carotenoids (carotene and xanthophyll), which are responsible for the yellow color in the lemon fruits, may serve as a source for vitamin A, their physiological functions are not definitely known. They are closely associated with the chlorophylls, however, and it is probable that they too play an important part in the manu¬facture of carbohydrates. The greater the ratio of xanthophyll to carotene in the juice or peel of citrus fruits the deeper orange their color. For the most recent general discussion of carotenoids, see Karrer and Jucker, 1950.
A green-colored lemon that turns "silver" (intermediate be¬tween green and yellow) during cool weather may sometimes become slightly greener as it continues to grow during the warmer months that follow, but it never regains its original green color. Limited observation has indicated also that silver lemons may become slightly greener following the application of zinc spray as a corrective for mottle-leaf. Unlike the orange, however, there appears to be very little tendency for the lemon fruit to turn green again after it once becomes yellow, regardless of how long it remains on the tree.

Lipids
Miller (1938) defines lipids as the esters of fatty acids and alcohols and their hydrolytic products which are solu¬ble in a fat solvent, and he states that "the lipids are said to be the most abundant, the most widely distributed throughout the plant, and apparently the most important from a biological standpoint, of the fatty substances that occur in plants." They are probably present in some form in every living cell. Since the lipids are especially abundant in rapidly growing cells, it is thought that they play an important part in the activities of protoplasm. The findings of different investigators have led to the suggestions that the lipids may be concerned with vital processes such as absorp¬tion, secretion, respiration, and transpiration.

Elbert T. Bartholomew and Walton B. Sinclair: THE LEMON FRUIT Its Composition and Product. University of California Press. Barkeley and Los Angeles. 1951