PHYSIOLOGICAL STUDIES OF THE FRUITS OF THE PINEAPPLE
[ANANAS COMOSUS (L.) MERR.] WITH SPECIAL
REFERENCE TO PHYSIOLOGICAL
BREAKDOWN
ERSTON V. MILLER
Received July 17, 1950
Introduction
Physiological breakdown of the fruits of the pineapple [Ananas comosus
(L.) Merr.] is characterized by a darkening of the flesh of the fruitlets, beginning near their attachment to the core in a region an inch or two above the base of the fruit. The-disorder progresses upward and outward, subsequently involving all of the fruitlet except the ovary and a layer approximately one quarter inch deep below the skin. Only in advanced stages do the surface of the fruit and the core become involved. Hence in most fruits there may be no external symptoms of the disorder. Severe cases of physiological breakdown are frequently discovered in pineapples that appear normal and attractive from the exterior. The disorder is not a form of over-ripeness as is true of "internal breakdown" of apples.
One of the commonest causes of physiological breakdown is exposure of the fruits to relatively low temperatures. The disorder has been observed in Florida in the fruits that were induced to ripen during the winter months by forcing the bloom with acetylene gas during "off season." Physiological breakdown has been reported to occur under similar conditions in Australia
(3). Also pineapples harvested during the summer months may develop breakdown during storage or transit if holding temperatures are too low
(5, 8), the greener fruits being more subject to the disorder than the riper fruits. Critical temperatures appear to be in the range of 00 to 50 C.
Other conditions than low temperatures may initiate the mechanism causing physiological breakdown. The disorder has been observed in imported fruits that have not been subject to low temperatures during transit.
Likewise a somewhat similar if not identical disorder may be induced by heavily waxing mature fruits or by holding them in air-tight containers.
The investigations now being reported were undertaken for the purpose of learning more about the fundamental causes of physiological breakdown with the hope that information so obtained might lead to methods of prevention of this disorder. It was hoped too that the information might be useful in studying physiological disorders of other fruits and of vegetables in cold storage.
Materials and methods
These studies were begun in Florida in 1942 and were transferred to the
University of Pittsburgh in 1947. Up to and including 1948 three com66
MILLER: PHYSIOLOGICAL STUDIES OF PINEAPPLE
67
mercial varieties of pineapples were employed in these investigations-Red
Spanish, Cayenne, and Abachi. In 1949 Natal and Queen varieties were included. Thus each of the larger horticultural groups (Spanish, Cayenne, and Queen), as described by HuME and MILLER (4), were represented.
(Queen, Natal, and Abachi are listed by the above authors as belonging to the Queen group.)
Studies were first made of the chemistry and physiology of these varieties to determine if there were any varietal characteristics tha't might be associated with susceptibility to physiological breakdown. The work was subsequently expanded to include observations on physiological and chemical changes produced by subjecting the fruits to storage at low temperatures.
The pineapples were harvested in the "mature green" or "market ripe" stage of maturity and shipped by railway express from Florida to Pittsburgh. This stage is characterized by a uniformly green color, with the eyes or fruitlets well rounded out. It does not include the fruits with a purplish color, such as are often picked for shipments to greater distances.
It is realized that pineapples do not attain their maximum content of soluble solids unless permitted to reach the "turning" stage of maturity before picking. However, only fruits in the mature green stage are customarily picked for commercial shipments from Florida and these not only constitute the kinds of fruits subjected to conditions of storage and transport that are conducive to development of breakdown, but they also represent fruits that are typical of those that reach the ultimate consumer.
An entire crate of fruits was employed in each experiment. In instances in which the fruits were rather large, as in the case of Cayennes and Abachi from Miami, the number of fruits per crate was 16 and 18, respectively.
In all other lots there were between 25 and 36 fruits in each crate. To compensate for individual variation each fruit was sampled separately and individually analyzed for solids, acids, pH, and ascorbic acid. For peroxidase determinations the juice of four or five fruits was combined.
The fruits were prepared for analysis by peeling, coring and macerating in a Waring Blendor. The juice was filtered through two thicknesses of cheese cloth. Determinations for total soluble solids, acids, pH, and ascorbic acid were made immediately. Samples of juice were placed in a deepfreeze cabinet (- 200 C) and held for subsequent enzyme determinations.
Total soluble solids in the juice were determined by means of a specific gravity hydrometer.
In determining total acids, aliquots of the filtered juice were titrated with tenth-normal sodium hydroxide, using phenolphthalein as an indicator.
The results were expressed as citric acid.
A Beckman and a Leeds and Northrup pH meter were used for making pH determinations. Each instrument employs a calomel half-cell and a glass electrode.
Ascorbic acid was determined by combining equal portions of juice and
8%o acetic acid and titrating with 2,6-dichlorobenzenone-indophenol.
68
PLANT PHYSIOLOGY
Peroxidase activity was determined after a considerable amount of exploratory work in an attempt to develop a colorimetric method which was both rapid and accurate. The method finally employed was as follows:
One ml. of pineapple juice was made up to 100 milliliters with distilled water. A reaction mixture was prepared which consisted of 1 ml. of each of the following: (a) the diluted juice; (b) distilled water; (c) buffer solution (pH 5.2); (d) hydrogen peroxide solution (25 x 10b molar); and (e) catechol solution (1%). This mixture was held at 250 C and shaken intermittently for 30 minutes. At the end of this time the reaction came to completion by developing a brown color characteristic of oxidized catechol.
The solutions were translucent and the depth of color was measured by means of a Klett-Summerson colorimeter using a blue filter (transmitting at approximately 4200 A).
It was found that the reaction could be carried out satisfactorily in Van
Slyke Cullen tubes, using aeration instead of shaking. This method required 5 ml. of each reagent. In either case the reaction was carried to completion so that the final color did not change, fade, nor darken even after the lapse of 24 hours. A blank determination was always made using distilled water instead of the juice. The enzymatic nature of this process was indicated by the fact that boiled juice gave a negative test for peroxidase.
Results
The results obtained in the studies during 1948 appear in table I. The three varieties, Red Spanish, Cayenne, and Abachi, do not show great differ-
TABLE I
COMPARISON OF CERTAIN JUICE CONSTITUENTS OF THREE VARIETIES OF
PINEAPPLES IN THE "MATURE GREEN" OR "MARKET RIPE" STAGE
OF MATURITY. SEASON OF 1948.
Red Spanish
Cayenne
Abachi.
Source
Pompano, Florida
Miami, Florida
W. Boynton, Florida
Total soluble solids Total acids Ascorbic acid
%
Variety
Gm. per
100 ml.
0.92
0.98
0.71
Mg. per
1Io ml.
17.8
19.7
51.9
10.3
9.1
9.5
ences in regard to total soluble solids and total acids. The solids ranged from 9.1% to 10.3% and the acids were in the range of 0.71 to 0.98 gram per 100 ml. of juice. The Abachi pineapples during this season contained slightly less acid than the other two varieties but this is apparently not a varietal difference because this relationship was not observed in tests made during the following year.
The most striking difference between varieties was to be found in the ascorbic acid content. The Red Spanish, with 17.8 and the Cayenne with
19.7 mg. per 100 ml. of juice, were quite similar, while the Abachi pineapple
69
MILLER: PHYSIOLOGICAL STUDIES OF PINEAPPLE
contained an average of 51.9 mg. per 100 ml. of juice. Although individuals of the Abachi variety showed considerable variation in ascorbic acid, the difference between this and other varieties was highly significant.
During the season of 1949 the number of lots of pineapples was increased. The list included Cayennes, two lots of Abachi, and one of Queen and Natal, all from Florida. Toward the close of this season a shipment of Cayennes from Mexico and Red Spanish from Puerto Rico were received.
The results obtained during 1949 are shown in table II.
The Natal variety differed from the others in having 16.2% solids. As would be expected, the juice of this variety was very sweet. Both Natal and Queen resemble the Abachi variety in external appearance and in possessing a yellow flesh, although the Abachi has a more delicate texture than the other two. Soluble solids in all the other varieties ranged from 9.55%o to 13.18%. These values, together with the differences in acids, would
TABLE II
COMPARISON OF CERTAIN JUICE CONSTITUENTS OF FIVE VARIETIES
OF PINEAPPLES IN THE "MATURF. GREEN" OR "MARKET
RIPE" STAGE OF MATURITY. SEASON OF 1949.
Variety
Cayenne
Abachi
Abachi
Queen
Natal
Red
Spanish
Source
Miami, Fla.
Miami, Fla.
Ft. Pierce, Fla.
Ft. Pierce, Fla.
Miami, Fla.
constitute differences in dessert quality but it is doubtful if they constitute significant varietal differences in all cases because there are so many factors that affect the per cent. of total soluble solids in pineapples.
The Abachi pineapples from Miami, Florida, contained higher acid (1.03 gm. per 100 ml.) than the same variety grown in Ft. Pierce and was higher than all other lots. The other Florida lots of fruits were fairly similar in acid content, ranging from 0.86 to 0.96 gm. per 100 ml. It should be noted that pH varied considerably but did not appear correlated with total acids.
As in the previous season, the ascorbic acid showed the greatest variation. Both lots of Abachi pineapples were high in ascorbic acid. Those from Ft. Pierce contained 54.8 mg. per 100 ml. and those from Miami contained 43.1 mg. per 100 ml. In all of the studies thus far conducted the
Abachi pineapple has contained a greater content of ascorbic acid than any of the other varieties. It is apparent therefore that this is a varietal characteristic. The Queen variety was next highest with 35.8 mg. per 100 ml. The
70
PLANT PHYSIOLOGY
Cayennes from Miami contained 15 mg. ascorbic acid per 100 ml. of juice, being only slightly lower than those analyzed during the previous season.
One of the imported lots of fruits (Red Spanish) was included in this table primarily for the purpose of reporting the peroxidase activity, although solids, acids and ascorbic acid were also included. These data were obtained from the analyses of 12 fruits.
Highest peroxidase activity was shown by Red Spanish pineapples.
This variety showed an activity of 260 units on the colorimeter while the other varieties ranged from 27 to 92. Similar results had been obtained in preliminary studies during the previous season. Furthermore a number of random samples of Red Spanish pineapples selected from the local market also showed peroxidase activity higher than any other varieties tested. Red
Spanish fruits obtained from the market had been produced in Cuba. It therefore appears that high peroxidase activity is a characteristic of the Red
Spanish variety of pineapple irrespective of the area where grown.
During the season of 1949 the experiments were extended to include observations of the effect of low temperature on the physiology of fruits.
Upon arrival at the laboratory the fruits in each crate were divided into two lots, each being matched in regard to size and degree of maturity. One lot was held in the laboratory at room temperature (approximately 250 C) for
48 hours and the other was held at 50 C during the same time. (This holding period was selected because it had been found during the work in
Florida that physiological breakdown could be produced in green fruits by this treatment.) At the end of the storage period the fruits were held at room temperature until they were fully ripe. The pineapples were analyzed individually as described previously. These results appear in table III.
TABLE III
EFFECT OF LOW TEMPERATURE ON CERTAIN JUICE CONSTITUENTS
OF FIVE VARIETIES OF PINEAPPLES
Variety
Storage at the lower temperature produced no significant effects on solids, acids, or ascorbic acid. There were several instances in which the total solids or the acids showed slight differences between the two lots of fruits but these differences were not significant because of the large variation between individual fruits. Likewise the ascorbic acid appears to have increased as a result of the low temperature but here again the results were not statistically significant.
The values for pH were lower in the low temperature lots than in those held at room temperature. This was true for all varieties although statistical significance was shown only in the Abachi and Natal varieties. Thus the pH of the juice of the Abachi pineapples from Miami was 3.68 in those stored at room temperature and 3.46 in those held at 50 C for 48 hours. In the Abachi fruits from Ft. Pierce the pH was 3.62 at room temperature and
3.49 in those held at 50 C. The pH of the Natal fruits held at room temperature and in cold storage was 3.81 and 3.60, respectively. The results for total acids were not as consistent as were those for pH.
Comparison of normal fruits and tissues with those showing breakdown
As mentioned previously, two imported lots of pineapples were received during August of the 1949 season. These arrived after completion of the experiments with Florida-grown fruits and after commercial shipments from
Florida had practically ceased. The imported fruits consisted of a crate of
Red Spanish from Puerto Rico and a crate of Cayennes from Mexico. Onehalf of the Red Spanish and all of the Cayennes had physiological breakdown upon arrival. Advantage was taken of this situation to observe the difference between sound and disordered fruits and also between sound and disordered flesh of the same fruits.
In table IV will be found the results of the analyses of the Red Spanish pineapples from Puerto Rico. Each fruit was analyzed separately as was done previously, there being nine sound fruits and 13 showing breakdown.
No significant differences between the two lots were shown in regard to
TABLE IV
COMPARISON OF JUICE CONSTITUENTS OF NORMAL RED SPANISH PINEAPPLES
WITH THOSE SHOWING PHYSIOLOGICAL BREAKDOWN
Total soluble
solids
Total acids
Gm. per 100 ml.
.....
%
9.73
0.58
3.90
20.9
......
9.15
0.59
3.98
3.4
Description of fruits
Normal fruits*
Fruits with**
pH
Ascorbic acid
Mg. per 100 ml.
physiological breakdown *
Average of analyses of nine individual fruits.
**Average of analyses of 13 individual fruits.
PLANT PHYSIOLOGY
72
solids, acids or pH. The ascorbic acid, on the other hand, showed a lower value in the disordered fruits. For example, normal pineapples contained
20.9 mg. per 100 ml. of juice as compared with only 3.4 mg. per 100 ml. in those with physiological breakdown.
The results of comparisons between normal and disordered flesh of two varieties of pineapple are shown in table V. Care was exerted to take the samples of each kind of flesh from comparable areas, that is, the same distance from crown to base, and in most cases analyses were made on opposite halves. This precaution is necessary because there appears to be a decrease in acids and an increase in solids in the flesh of a pineapple when progressing from crown to base.
TABLE V
COMPARISON OF THE JUICE CONSTITUENTS OF NORMAL AND DISORDERED
TISSUE OF PINEAPPLE FRUITS
Fruit number 912
925
934
805
806
807
Description of tissues
Normal
Disordered
Normal
Disordered
Normal
Disordered
Normal
Disordered
Normal
Disordered
Normal
Disordered
In general the results in table V are similar to those in table IV. There was a slightly greater variation in total soluble solids. The decrease in ascorbic acid in the disordered flesh was more pronounced in Cayennes than in Red Spanish. In the former, the differences between sound and disordered fruits ranged from 5.1 to 7.6 mg. whereas in the Red Spanish the differences ranged from 1.6 to 6.4 mg. It is possible that physiological breakdown was further advanced in Cayennes than in the Red Spanish, since all of the Cayennes in the shipment showed physiological breakdown while only half of the Red Spanish fruits were affected.
Oxidizing enzymes
GREEN (2) states that the principal enzymes in plants that cause oxidation of ascorbic acid either directly or indirectly are ascorbic acid oxidase, peroxidase, and polyphenol oxidase. For the past two years in this labora-
MILLER: PHYSIOLOGICAL STUDIES OF PINEAPPLE
73
tory pineapple juice has been tested for the above enzymes using the method of GERHARDT and EZELL (1) as modified by JIMENEZ (6). In all these tests except one the ascorbic acid substrate was oxidized by the boiled juice and not by the fresh juice, no matter whether the test was made for ascorbic oxidase, peroxidase or polyphenolase. Many tests were made and the reagents were varied so as to cover wide ranges of pH and various concentrations of H202, all with the same results. Boiled juice was added to the fresh juice to ascertain if an activator were present in boiled pineapple juice as was shown for apple juice by Gerhardt and Ezell. The addition of boiled juice to the fresh juice did not change the results.
During the latter part of November a crate of Natal pineapples arrived from Sebring, Florida. All of these fruits had physiological breakdown.
Tests for oxidizing enzymes made with these fruits gave positive results for the first time. Crystalline ascorbic acid was used as a substrate as in the previous tests. The oxidizing power of the juice alone was tested as well as juice plus H202 and juice plus both H202 and catechol. The juice alone and the juice plus H202 and catechol oxidized the ascorbic acid, giving results well above those obtained with boiled juice. This suggests that both ascorbic acid oxidase and peroxidase may be present in the juice. However, these results indicate only trends because there were not enough samples available to afford opportunity for statistical analyses. Furthermore, until additional studies can be made, one cannot be certain whether these values in the enzyme studies were the result of using fresh juice in place of frozen samples, whether these enzymes are more active in winter-ripened fruits than normally ripened ones, or whether some other. factors were involved.
Discussion
In considering varietal differences attention should be called to the relatively high content of ascorbic acid in the Abachi pineapple. During the eight years in which these investigations have been conducted the Abachi variety of pineapple has always been found to be higher in ascorbic acid than other varieties under observation. In the results obtained during 1949 the Abachi pineapples averaged 54.8 mg. of ascorbic acid per 100 ml. of juice. A number of individual fruits in the lot were found to range between
60 and 70 milligrams of ascorbic acid per 100 ml. of juice. It must be recalled too that all pineapples employed in these studies had been harvested in the mature green stage of maturity. The author has observed that pineapples harvested in the "turning" stage of maturity contain more ascorbic acid than those picked mature green. On the whole then, the Abachi pineapple can be placed in the same category as citrus fruits as a source of vitamin C. Most of the literature reporting vitamin C content of pineapples apparently refers to other varieties than the Abachi, judging by the relatively low amounts usually reported.
Many of the commercial growers of pineapples consider members of the
Queen group (Queen, Natal, Abachi) to be more susceptible than other types to physiological breakdown. The writer is inclined to concur in this
74
PLANT PHYSIOLOGY
belief. Although breakdown may be rather easily induced experimentally in all types of fruits the disorder appears to occur under normal conditions of handling much more frequently in the Queen type.
In the earlier stages of these studies it appeared as if high ascorbic acid content were associated with susceptibility to physiological breakdown.
Thus, two members of the Queen group (Abachi and Queen) are relatively rich in ascorbic acid. Had the Natals likewise contained large quantities of vitamin C then a direct relationship between ascorbic acid and breakdown would be suggested.
It has been reported (7) that fruits relatively rich in ascorbic acid are the ones most likely to develop non-enzymatic browning, such as occurs in processed fruits. This type of browning is accompanied by decrease in ascorbic acid content of the product. It has been shown in the present investigation that physiological breakdown in pineapples is accompanied by destruction of ascorbic acid but it has not been definitely established whether the process is enzymatic or non-enzymatic.
It is possible that changes in soluble tannins enter into the development of physiological breakdown. It is known for example that many fruits become less astringent as they ripen because the tannins are rendered insoluble. Tannins are characterized by the presence of phenol groups. It seems that blackening of pineapples during onset of physiological breakdown might be the result of oxidation of these tanins. This is strengthened by the universal observation that green fruits are much more susceptible to breakdown than are ripe fruits.
Storage of pineapples. at 5° C for 48 hours resulted in several instances in a lower pH value than in the fruits held continuously at room temperature. It is not known whether this represents a specific physiological effect of low temperatures or whether it merely indicated that ripening was retarded. The percentages of soluble solids and acids of the two lots indicate that they were generally comparable in regard to maturity.
These studies are being continued with greater attention being given to oxidizing and other enzymes.
Summary
A study of the juice constituents of several varieties of pineapples indicated that although soluble solids, pH, and total acids show considerable variation, the varieties show no specific differences unless it might be the
Natal, which is usually high in solids.
Ascorbic acid on the other hand is higher in the Abachi and Queen varieties than in Red Spanish, Natal, and Cayenne.
Red Spanish pineapples showed higher peroxidase activity than all of the other varieties employed in these studies.
The only significant differences between constituents of normal fruits and those showing physiological breakdown was the very low percentage of ascorbic acid in the latter.
MILLER: PHYSIOLOGICAL STUDIES OF PINEAPPLE
75
Subjecting pineapples to 50 C for 48 hours resulted in a significantly lowered pH in the juice of three lots of pineapples.
Tests were made for ascorbic acid oxidase, peroxidase, and phenolase using a modification of the method of Gerhardt and Ezell. Boiled juice oxidized more ascorbic acid than did fresh juice except in the case of Natal pineapples which had ripened in the early winter. The latter were "off season" fruits produced by forcing the bloom with acetylene.
The author is indebted to Miss Harriet Treelisky and Miss Eva E.
Schall for assistance with the chemical analyses of the pineapples.
Appreciation is expressed also to the following importers and growers for their cooperation in making the pineapples available for study. The importers were Mr. L. P. Garcia, Tropical Fruits Company, P. 0. Box 621,
Laredo, Texas; Mr. Frank Maniello, Maniello Brothers and Mayersahn,
324 Washington Street, New York, New York.
The shippers were Miami Pineapple Growers Association, P. 0. Box 668,
Miami, Florida; Mr. F. G. Palen, Box 111, Pompano, Florida; Mr. P. K.
Platts and Sons, P. 0. Box 265, Ft. Pierce, Florida; Mr. D. S. Radebaugh,
Pineapple Plantations, 413 South Commerce Street, Sebring, Florida; Mr.
Oscar Winchester, Box 124, Boynton Beach, West Boynton, Florida.
DEPARTMENT OF BioLOGicAL SCIENCES
UNIrERSITY OF PITTSBURGH
PITTSBURGH, PENNSYLVANIA
1.
2.
3.
4.
5.
6.
7.
8.
LITERATURE CITED
EZELL, BOYCE D. and GERHARDT, FISK. The use of ascorbic acid (crystalline vitamin C) as a substrate in oxidase measurements. Jour.
Agric. Res. 60(2): 89-100. 1940.
GREEN, DAVID E. Mechanisms of biological oxidations. Cambridge
Press. 1941.
GROSZMANN, H. M. Pineapple culture in Queensland. Dept. of Agric. and Stock Pamphlet No. 124. 1948.
HUME, H. H. and MILLER, H. K. Pineapple culture. II. Varieties.
Florida Agric. Expt. Sta. Bull. 70: 37-62. 1904.
IVANOFF, S. S. Non-parasitic internal breakdown of pineapple fruits.
Texas Agric. Expt. Sta. Substation No. 19, Winter Haven, Texas.
(Mimeographed.) August 20, 1945.
JIMENEZ, MIGUEL A. A study of oxidizing enzymes of guava. Food
Res. 12(4): 300-310. 1947.
MRAK, E. M. and STEWART, GEORGE F. Advances in Food Research.
Vol. I. Academic Press Inc., New York. 1948.
ROSE, DEAN H., BROOKS, CHARLES, BRATLEY, C. O., and WINSTON, J. R.
Market Diseases of Fruits and Vegetables. Citrus and other subtropical fruits. U. S. Dept. Agric. Misc. Pub. No. 498. 1943.
