SUITABiliTY OF VARIOUS INDICESFOR ASSESSING RIPENESS IN MANGO

Abstract

Pulp penetration pressure (Pp),degree of shoulder development, degree of skin colouration,degree of pulp colouration and total soluble solids content were quantified from the timefruit became mature until two weeks after the general harvest date. Pp was considered tobe superior as an index for assessing ripeness for the reasons that it showeda strictly decreas-ing trend and is an objective measure. Correlations between Pp and each of the remainingindices were visibly poor. On the basis of these correlations, none of the remaining indiceswere considered to be adequate substitutes for Pp, assuming that Pp accurately reflectsripeness in terms of the time required for fruit to become eat-ripe. In directly comparingPp with degree of pulp colouration, the currently adopted index, on the strength of theirrelationship with the time required for the stage of eat-ripeness to be reached, the relation-ship for Pp was strongest. Since the relationship with Pp was strong in absolute terms, itwas concluded that Pp is a good index, per se, for assessing ripeness in mango.

Maturation refers to the stage when afruit becomes a fully grown organ aboutto enter the ripening phase of its exis-tence (Leopold and Kriedemann, 1975).In South Africa, mangoes are harvestedafter maturation and once processes as-sociated with ripening have been initiat-ed. Ripeness for sea export is currentlybased on the degree of transition in pulpcolouration from white to yellow or yel-low/orange. Fruit are deemed suitablefor export by sea when this transition isone-third complete or "at 33%." The ex-tent of the transition is rated visually af-ter cutting the fruit through transverse-ly.

Bezuidenhout (1988)reported that incor-rect maturity was the most significant·factor for fruit rejections during the1986/87 export season. Rejections weredue to fruit being either under-ripe orover-ripe. Kirsten (1991) reported thatone of the major problems associatedwith South African mangoes exported toEurope during the 1990/91 export sea-son, was that of "immaturity" coupledwith the incapacity of fruit to ripen. DuPreez (pers comm) concluded from ob-servations made of mangoes arriving atRungis market, Paris, in March, 1991,that South African fruit arriving by seawere often "immature," requiring up to14 days to ripen at the temperatures pre-vailing in Paris at the time. Furthermore,flavour was unsatisfactory on ripening,the fruit having an abnormally low totalsoluble solids content (TSS). Poor eat-ing quality has previously been asso-ciated with picking too early in the sea-son (Medlicott et.al., 1987; Medlicottet.al., 1990a).

It would appear that the suitability of thepresent method of assessing ripenessof mangoes for sea export is question-able. The following reasons might be

recognized for the presently adopted in-dex being inadequate:

* The correlation between degree ofpulp colouration (Pc) and the time re-quired for the stage of eat-ripeness tobe reached (Tr) is poor.

* Differences exist between cultivarswith respect to Tr from the time fruitshow 33% pulp colouration.

* The method of assessing Pc gives riseto a high degree of error due to it be-ing largely subjective.

An "ideal" index for assessing ripenessof mango for picking and storage mightbe recognized as being one that satis-fies the following:* Change of the index, in terms of a

reduction or increase with time, com-mences prior to the optimum time ofpicking.

* The change during the period prior tothe optimum time of picking is largeenough to be detected by the methodof measurement.

* The method of measurement is ac-curate and simple to perform.

* The correlation between the degree ofchange and Ti is good irrespective ofthe conditions of storage and ripeningto which a consignment of fruit is ex-posed.

* Cultivar differences with regard to theforegoing correlation do not exist orare small enough to be disregarded.

* Measurement does not result in de-struction of the fruit.

* The change of the index with time iseither strictly increasing or decreasingwithout there being high rates ofchange at any stage.

Research results indicate that pre-storage ripeness has a strong bearingon the suitability of the storage temper-ature (Medlicott et ai, 1987, 1990a,1990b). Furthermore, differences in thisrelationship with respect to cultivar are

Merensky TechnologicalServices, P.O. Box 14,

Duivelskloof,0835

also apparent (Oosthuyse, 1990).The op-timum stage of ripeness to commencecold-transit storage for the temperatureregimes locally under consideration forsea export, have still to be determinedfor the mango cultivars exported fromSouth Africa. For such determinationsto be meaningful, the identification ofan index that reliably indicates stage ofripeness is essential.

Since the optimum stage of ripeness topick and commence cold-storage hasstill to be ascertained for the variouscultivars exported locally, it might be ad-ded, in reconsidering the requirementsof an ideal index, that measurablechange of the index commence from thetime of maturation.

For the purpose of appraising Pp,degree of skin colouration (Sc), Pc,degree of shoulder development (Sh)and TSS individually as indicators of thestage of ripeness in mango, andpresenting a case for the use of Pp asan index, the following were ascertainedin the present study:

1) The changes with time of Pp, Pc,TSS, Sh, and Sc from the time of ma-turation.

2) Correlations between Pp and each ofthe remaining indices mentionedabove.

3) The relationship between Pp and Tr,and between Pc and Tr, for pickedfru it.

MATERIALS AND METHODS

Two experiments were performed, thefirst to ascertain "1" and "2" above (Ex-periment I), and the second to ascertain"3" above (Experiment II).

Experiment I

During the first week of July, 1990, 10healthy Sensation mango trees having

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Fig 1 Fruit growth and the concurrent changes in total solublesolids content (TSS), pulp penetration pressure (Pp) anddegree of shoulder development (Sh) from 6 weeks beforethe general harvest date (GDH) until two weeks after thisdate (cv Sensation).

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Fig 3 Scatter plot of degree of shoulder development (Sh)versuspulp penetration pressure (Pp) (cv Sensation).

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Fig 7 Relationship between pulp penetration pressure (Pp) and thetime required for the fruit to become eat-ripe (Tr) (cv Kent).

Fig 8 Relationship between degree of pulp colouration (Pc) andthe time required for the fruit to become eat·ripe (Tr) (cvKent).

tionship was stronger than the latter, asindicated by these percentages, sug-gests that Pp is a better index for as-sessing ripeness than Pc. Furthermore,the high value of R2 for the former rela-tionship signifies that Pp is a good in-dex, per se, for assessing ripeness inmango.

Conclusions

The foregoing presents a strong casefor the use of Pp an index for assess-ing ripeness in mango. It is noteworthy,however, that the deficiencies of the re-maining indices considered, may havelargely been a consequence of themethods used to quantify them.

In considering the criteria stipulated foran ideal index, Ppwas indicated to satis-fy the following:

• Measurable change commences fromthe time fruit reach maturity.

• The method of quantification is ac-curate and simple to perform.

• The change with time is either strict-ly increasing or decreasing withoutthere being high rates of change atany stage.

The criterion that the method of quan-tification be non-destructive was notmet.

The question of whether the correlationbetween Pp and Tr is good irrespectiveof the conditions of storage and ripen-ing to which a consignment of fruit isexposed, and of whether cu Itivar differ-ences exist with regard to this correla-tion, were not addressed in this study.

The ultimate test of Ppas a index for as-sessing degree of ripeness lies in itscommercial application. The salientpoint in favour of Pp as an index, is thatit indicates degree of softening in an ob-jective manner,considering softening tobe the principal change associated withripening in mango.

BEZUIDENHOUT, J J, 1988. Indices to deter-mine the maturity of mangoes for picking.SA Mango Growers' Assoc. Yearbook 8,14-16.

KIRSTEN, J, 1991. SA moet mango-sokkiesoptrek. Landbouweekb/ad, Aug 23, pp22-25.

TANK BIRDDIE SPUITMENSE

AGRICULTURAL & FORESTRY EQUIPMENT (PlY) LTOLAND80U & 80SBOU TOERUSTING (EDMS) BPK

LEOPOLD,A C AND P E KRIEDEMANN, 1975.Plant Growth and Development. McGraw-Hill, New York.

MEDLICOn, A P, J M M SIGRIST, S B REY-NOLDS, AND A K THOMPSON, 1987. Ef-fect of ethylene and acetylene on mangofruit ripening. Ann App/ Bio/111, 439-444.

MEDLICOn, A P, M N'DIAYE, AND J M MSIGRIST, 1990a. Harvest maturity and con-centration and exposure time to acetyleneinfluence initiation of ripening in man-goes. J Arner Soc Hort Sci 115, 426-430.

MEDLICOn, A P, J M M SIGRIST, AND 0 SY,1990b.Ripening of mangoes following lowtemperature storage. J Arner Soc Hort Sci115, 430-434.

OOSTHUYSE, S A, 1990. Effect of loweringthe temperature of the cold-storage re-gime on the quality of export mangoes af-ter four weeks of storage. SA MangoGrowers' Assoc Yearbook 10, 38-56.

ACKNOWLEDGEMENTSThanks are due to Dr DL Milne for proof-reading, and Gustaf van Veijeren formaking tres available for experimentalwork.