THE BANANA-A H
VON
CHALLENGE TO CHEMICAL INVESTIGATION
LOESECKE. CHEMICAL RESEARCH LABORATORY, UNITEDFRUITCO.,BOSTON, MA~SACHUSETT~
One hundred and twenty-six years have elapsed since what was probably the first importation of bananas into this country. At that time thirty bunches were brought from Cuba to New York. In 1929 the total importations were sixty-five million bunches. In past years the biochemist, physiological chemist, and physical chemist have made concerted efforts for a thorough study of this fruit.
Tannins It has been known for some time that an unripe banana is characterized by its high starch content, its astringency, and lack of sweetness. The ripe fruit, on the other hand, has little starch and is not noticeably astringent. I t is claimed that the astringency is due to tannins in the unripe fruit which are in the soluble form. As the fruit ripens the tannins are said to become insoluble or "fixed," as it were, in sort of a "vegetable leather," and hence cannot be tasted in ripe fruit. Analyses of the tannin content of the banana may be found in the literature, but just what is meant by "tannins" is not always clear. Yoshimural followed the tannin change during the ripening process and concluded the tannin remained unchanged and was independent of the process of maturation. The occasional gastrointestinal disturbances following the ingestion of unripe fruit have been linked with the tannin present in the frnit. The evidence is unconvincing and a more thorough investigation of the tannin change in the banana with special emphasis upon the kinds of tannins present a t diierent stages of ripeness would be a valuable contribution to the chemistry of this frnit.
Pectin The hardness of the green banana & due to the fact that the cells are cemented together by an insoluble substance known as protopectin. As the fruit ripens the protopectin is changed by enzymes into pectin and finally into various semi-acid and acid bodies. Pectin will swell in water to form a jelly, provided the concentration of sugar is correct and the pH is a t the optimum point. Neither protopectin nor pectic acid will form a jelly. As the protopectin is changed into pectin the fruit becomes soft in texture, because the cell walls are no longer cemented together by protopectin. During ripening the total pectin (protopectin plus pectin) remains practically constant, but the pectin increases until the fruit becomes overripe, a t which time a good part of the pectin has been changed into acid and semi-acid bodies. This is the explanation usually given to account for the softening of the fruit during maturation. In reality it is 'Yoshimura, K.,Z. Nahr. Gcnussm.. 21, 486 (1911). 1637
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not as simple as this, for there are occasional increases in the protopectin content that cannot be readily explained. The total pectin content of the banana is not great (about one per cent of the fresh pulp), but what relation this substance bears to the digestibility of the fruit is unknown a t the present time. Is banana pectin attacked by the gastric juice? Is it affected by the intestinal enzymes? Or does it merely act as roughage in the same manner as cellulose, to be
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Days after cutting. CHANGES IN THE COLOR AND SUGAR CONTENT DURING TEE LIFEOI. THE BANANA This diagram shows the changes in color of the peel and the sugar content of the pulp during the life of the banana. The curve for the sugar was derived from over five thousand analyses of the pulp of bananas. 1
2 3 4
5
subjected to bacterial decomposition in the large intestine? These are questions for the biochemist and physiological chemist to answer.
Starch and Sugars During ripening the starch in the bananas is converted into sugar (dextrose, levulose, maltose, and sucrose) by means of enzymes. B a i l e ~in ,~ his biochemical studies of the banana carried out eighteen years ago, was Bailey, J. Am. C h m . Soc.. 34, 1706 (1912); J. Bid. Chem., 11, 42 (1912).
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unable to detect maltose in the fruit, but recent work in this laboratory (unpublished data) established its presence. The quantitative determination of the different sugars in the banana, followed along as the fruit ripens under controlled conditions, awaits the chemist. If levulose is the predominating sugar in the ripe fruit this may explain, in part, why diabetics are better able to tolerate the carbohydrate of the banana than carbohydrates of other fruits and vegetables. Furthermore, the laxative effect of ripe bananas may be due to levulose, for Folin and Berglund3 have shown administration of levulose is accompanied by diarrhea. The treatment of citrus fruits with ethylene and other gases of the same series has been carried out with success by Denny, Harvey, and others. The results with bananas have not always been so striking. If the fruit is going to ripen it will do so about as quickly with ethylene as it will without. However, in the case of "dormant fruit" (the meaning of which we do not know with any degree of certainty) ethylene hastens ripening. Just why this is so we do not know. Does this gas stimulate the enzymes? Perhaps, but Rea OR STARCH GRAINS IN THE and ~ i l l i ~ ihave ~ 4 shown PXOTOMICROGRAPH PULFOF AN UNRIPEBANANA that corn and wheat starch As t h e f r u i t ripens these starch grains are seen can be converted to reducing to disappear almost completely. sugars without the presence of an enzyme, the conversion taking place by merely bubbling ethylene into the starch suspension. More recently Nord and Franke6 have shown that in the case of tobacco leaves ethylene increases the permeability of the cells and acts as a protector. A ripe banana contains as high as 18 per cent reducing sugars (see diagram on page 1538) which may be fermented under suitable conditions to alcohol. The alcohol obtained may, in turn, be oxidized t o form acetic Wolin and Berglund, J . Bid. Chem., 51, 213 (1922). Rea and Millinix, J. Am. Chem. Soc., 49, 2116 (1927). Word and Franke, J. Bid. Chem., 79, 27 (1928). 4
1540
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acid or vinegar. Banana vinegar is light yellow in color and possesses an agreeable aroma and taste comparing favorably with cider vinegar.=
Latex When the peel of a green banana is punctured, a sticky, milky fluid oozes from the wound. Under the microscope this liquid is seen to consist of numerous spherical bodies ranging in diameter from 3.5 to 31.5 p . These particles can be made to coalesce, under suitable conditions. formine a n exceedingly t a c k y gum. Esch7 claimed this substance was India rnbher and later Griebels also spoke of it as rubber. More recent investigations have shown the material is not ~ b b e r , but a body very closely resembling c h i ~ l e ,con~ taining 44.4% acetonesoluble resins, 34.4% ether extract (hydrocarbons), 13.6% albans, 15.4% fluavil, 2.4% protein, and 0.4% ash. Attempts to vulcanize the gum were not successful. Because of its low melting point, difficulty of extraction and instability, it could not be readily used as a substiLONGITUDINAL SCCTIONO F A BANANA STALK tute for chicle. At a the fibro vascular bundles areshown, imThe peel of the banana portant in the preparation of paper; b indicates the also contains pectin rangparenchyma. or pith. ing in amounts slightly more than in the pulp. This pectin can be extracted by dilute acids (preferably organic acids) and the extract either concentrated or dried to obtain a commercial pectin.1° von Loesecke, Ind. Eng. Ckem., 21, 175 (1929). Gummi Markl., 4, 83 (1915). 8 Griebel, Z. Nahr. Genursm.. 48, 221 (1923). 'van Loesecke, Rubber Age, 22, 129 (1927). lovon Loesecke, Fruit Produds 3. & Am. Vinegar Ind., 8, 11 (1929) 0
' Esch,
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Color Changes in the Peel As the banana ripens the peel of the fruit changes in color from green to yellow. (See diagram on page 1538.) The green color of the peel is due to chlorophyll (a b) which ranges from 102.9 to 51.7 milligrams per kilogram of fresb peel in the unripe fruit a t discharge from the boat a t an American port, and decreases as the fruit ripens." The yellow color of the peel of the ripe fruit is due to xanthophylls and carotin and these pigments have been found to remain approximately constant throughout ripening. In other words, a green banana contains just as much yellow pigment as a ripe banana, the color being masked by the chlorophylls in the case of the unripe fruit. When the chlorophylls break down, the yellow color of the xanthophylls and carotin becomes apparent.
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Esters A green banana has a cucumber-like odor, but as the fruit ripens the characteristic banana aroma develops. This is due to the formation of esters, but the mechanism of their formation is still obscure. The odorous constituents of the ripe fruit are present in very small quantities, amounting to about 0.0033 per cent of the weight of the pulp (unpublished data). The chief ester is amyl acetate (the so-called "banana oil" of commerce used in certain paints and lacquers) but valerates of ethyl and methyl alcohol are also known to be present. The odorous constituents of the banana are extremely unstable and are destroyed even when the greatest of care is taken to recover them unchanged. So far as the writer is aware no one has ever succeeded in commercially extracting the esters of the banana so that the extract would be comparable with the odor of the fresb fruit. Such an extract, if successfully prepared, would be of great commercial importance as a flavoring material for ice cream, desserts, and tonics. The buying public is being educated more and more to demand natural products and to scorn all that is artificial. Protein and Fat The banana contains but little protein (from 0.8 to 1.3%, depending upon the variety of the fruit), but there are no data available in the literature concerning the extraction of the protein per se. The biochemist and physiological chemist are now tackling this problem and the investigation will not only throw more light on the nutritive value of the fruit itself, but will also add to our knowledge of the chemistry of proteins. Nor has the fat of the banana been investigated. It would be interesting, for instance, to learn whether bananas contain ergosterol, the provitamin D. If the fruit does contain this sterol, banana flour could be irradiated and become an important source of vitamin D. 1' von Loesecke, J. Am. Ckem. Soc., 51, 2434 (1929).
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1930
Nutritive Value The nutritive value of the banana has been the subject of many researches and these researches have definitely shown the fruit to be permissible to people of all ages. Gastrointestinal disturbances following the ingestion of the fruit are due to either eating unripe fruit or to some allergy certain individuals may have for bananas. Many cannot eat eggs or strawberries or tomatoes because of some allergy, yet these products have never been condemned in the same manner as bananas. Researches of Eddy and others have shown the banana to be an excellent source of vitamins A and C; a good source of vitamin B (i. e., B, the factor necessary to prevent beriberi in man, and G , the factor necessary to prevent pellagra), deficient in D and not lacking in vitamin E. Its potency in vitamin C, the availability and the relative cheapness of the fruit make it a competitor with tomato and orange juice for infant feeding on vitamin basis alone. Banana Flour Numerous attempts have been made, and are still being made, to prepare a suitable banana flour or powder. It is comparatively simple to prepare a flour, but i t is quite another matter to develop a wide market for it. Flour is prepared from either green or ripe fruit, the latter being the more difficult to obtain because of the hygroscopic nature of the sugars in the ripe fruit. Green banana flour has a characteristic taste varying between "shorts" and violets. Its use in baking is limited because it contains no gluten and also because i t darkens when heated, imparting an unsightly appearance to the baked goods. Ripe flour has a sweet taste and may occasionally suggest the aroma of the fruit from which i t was obtained. The most promising use for this product seems to be in the field of therapeutics where its properties should be of value in certain dietaries, and in instances where fresh fruit is not readily available. There are undoubtedly other uses for the flour as yet unconceived. Opportunity awaits the chemist who can prepare banana flour cheaply and find an unlimited market for his product after he has prepared it. Analyses of Banana Flow
Water Protein
Fats Ash Starch Sucrose Reducing sugars Crude fiber
Green Banana Flour 12.08% 4.85 0.61 3.09 69.35 2.28 0.95 0.73
Ripe Bawna Flour 2.59% 4.09 1.91 3.05 29.87 33.25 15.62 ..,
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Banana Stalks h ' e stalk of the fruit, to which the hands of bananas are attached, contains about 93 per cent water and 1 per cent ash. The ash is very rich in potash, varying from 53 to 63 per cent potassium oxide, depending upon the variety of fruit. During the World War and the period immediately following the conflict, when potash shipments from Germany were curtailed, numerous attempts were made to recover this potash from the stalk^.'^ At the present market price of potash it would require about one ton of the stalks to obtain $3.00 worth of potash. Banana stalks have also been suggested as the basis for a dyeL3and as the raw material for the preparation of paper,l4.l5hut the difficulty of collecting the material, in this country at least, would seem to preclude any great use for it. lZ
Ellis, 3. Soc. Chem Ind., 35, 456 (1916).
.
Billings and Christie, Ind. Eng. Chem.,
9. 153 (1917). , la
U. S. Pat. 1,243,042, Oct. 16, 1917. WochbLd. Pafierfabr., Sondemummer, 57, 75 (1926) Brit. Pat. 293,219, Aug. 15, 1927.
" Reifegerste, l5
