Scientia Horticulturae, 34 (1988) 211-218 211 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Relationship of Rind Gloss and Groundspot Color to Flesh Quality of Watermelon Fruits during Maturation

KENNETH A. COREY' and DONALD V. SCHLIMME

Department o[ Horticulture, University o[ Maryland, College Park, MD 20742 (U.S.A.)

Scientific article No. A4623, Contribution No. 7619 of the Maryland Agricultural Experiment Station

(Accepted for publication 11 August 1987)

ABSTRACT

Corey, K.A. and Schlimme, D.V., 1988. Relationship of rind gloss and groundspot color to flesh quality of watermelon fruits during maturation. Scientia Hortic., 34:211-218.

Changes in groundspot color and rind gloss were measured on fruits of two watermelon cultivars during maturation and ripening to determine their suitability as non-destructive criteria of inter- nal flesh quality. Groundspot Hunter a values increased linearly for 'Charleston Gray' and 'Blue Belle' watermelons with advancing maturity, while soluble solids accumulation and Hunter a val- ues of the flesh had reached maxima. Rind gloss of 'Charleston Gray' fruits declined at a stage of ripeness when soluble solids and intensity of redness of the flesh had reached plateaus. Results suggest that objective measurements of groundspot color and rind gloss have potential value as criteria for the determination of ripeness of watermelon fruits.

Keywords: CitruUus lanatus Thunb.; fruit quality; fruit ripening; maturity indices; texture.

Abbreviation: DPA--days past anthesis.

INTRODUCTION

Determination of the appropriate stage of ripeness at which to harvest wa- termelons of optimum quality is difficult due to the subjective and variable nature of the commonly-used maturity indices. Mizuno and Pratt (1973) ob- served a gradual senescence of the tendril in closest proximity to fruit as the fruit ripened. However, the exact status of the tendril, e.g. green, wilted or brown, was highly variable, and a wilted tendril was not always associated with

1Present address: Department of Plant and Soil Sciences, University of Massachusetts, Amherst, MA 01003, U.S.A.

0304-4238/88/$03.50 © 1988 Elsevier Science Publishers B.V.

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maximum red color and soluble solids of the flesh. The color of the rind in contact with the soil surface (groundspot) of many watermelon cultivars changes from green-white to pale yellow upon ripening (Nip et al., 1968; So- teriadou, 1970). However, in recent years many cultivars have been released in both the ice-box (<4 .5 kg/fruit) and large-fruited (>4.5 kg/fruit) cate- gories that have a dark blue-green rind with a more distinctly colored yellow groundspot. Changes in groundspot color on fruit of such cultivars are more difficult to evaluate subjectively.

It has also been observed that a subtle decline in glossiness of the rind of 'Charleston Gray' fruits occurs with advancing ripeness. Structural and quan- titative changes in the cuticle may occur with ripening that could serve as the basis for a non-destructive technique for ripeness determinations.

The objective of the present study was to describe changes in internal and external physical and chemical attributes of fruit of two morphologically con- trasting watermelon cultivars. Specifically, the objective was to determine if changes occur in groundspot color and rind gloss during ripening which might provide suitable non-destructive criteria for selection of opt imum maturity fruit.

MATERIALS AND METHODS

F r u i t tagging a n d samp l ing . ~ Watermelon (Citrullus l a n a t u s Thunb., culti- vars 'Charleston Gray' and 'Blue Belle' ) seedlings were planted on 5 June 1984 at the University of Maryland Vegetable Research farm in Salisbury, MD. Plants were grown in accordance with standard recommendations for the mid- Atlantic region of the U.S.A. Within 1 day of anthesis, female blossoms were tagged with dated labels. Fruits were harvested on 12, 19, 23, 25, 27, 29, 31, 34 and 36 DPA for 'Blue Belle' and 12, 19, 29, 31 and 36 DPA for 'Charleston Gray'. Sample size varied from 4 to 6 fruits depending on the availability of uniform-size fruit at a specific chronological age.

A n a l y t i c a l m e a s u r e m e n t s . - - All analytical measurements were made within 2 days of harvest. Hunter L, a and b measurements were made on the surface of samples of background rind, groundspot rind and flesh taken from the seedless central region (heart) of fruit. This region has been previously shown to con- tain the highest quantity of soluble solids ( MacGillivray, 1947; Ng and Corey, 1984). Samples of background rind were taken from unblemished, uniformly colored regions approximately 180 ° opposite the groundspot. Two readings on each of 2 segments of background rind and flesh and 1 segment of groundspot rind were taken from each fruit. All segments were t r immed to approximately 0.5 cm thickness and were of sufficient area to provide complete coverage of the viewing port. Color measurements were made with a Pacific Scientific Spectrogard Color System using a 10 ° observer angle, I l luminant C, and 2.2-

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cm-diameter viewing port. The instrument was calibrated with the black and white standard plates provided. All measurements were made with spectral reflectance included (SPIN) (Anon., 1983). Measurements with spectral re- flectance excluded (SPEX) were also made on background rind segments. The difference between A L SPIN (L s t a nda r d -L sample) and zlL SPEX (L s t a n d a r d - L sample) was calculated to provide an indirect measurement of surface gloss (zlL SPIN - AL SPEX ).

The peak force required to shear a 100-g sample of watermelon flesh chunks was measured with a shear press (Food Technology Corp. Texture Test Sys- tem), using a TR5 Texturerecorder and a CS-1 standard shear compression test cell and blades. A force-deformation curve was obtained when full scale on the texturerecorder was 45.3 kg, and a 20-s piston downstroke speed was used. Measurements of soluble solids and pH were made on juice recovered from samples run through the shear press. An Abbe-3L refractometer operated at a prism temperature of 20 °C was used to obtain ° Brix of the watermelon juice pressed through two layers of disposable tissues. The pH of the juice was measured with a Corning Model 7 pH meter using pH 4 standardization buffer.

In a second experiment, fruit were harvested at 3 different stages of ripeness; under ripe, optimum ripe and over ripe. Fruit were judged to fall into those ripeness categories based on subjective evaluations of tendril condition, groundspot color and surface gloss. Upon cutting the fruits in half longitudi- nally, a second judgement of the ripeness stage of each fruit was made based on color and textural appearance. Under-ripe fruits were classified as not showing full color development (i.e. pink). Over-ripe fruits displayed a pithy or porous condition of the flesh. Samples of flesh from the central seedless region of individual fruits were placed in beakers, sealed and held at - 18 ° C. Samples were later thawed, blended for 20 s in a Waring blendor, and filtered through qualitative-grade filter paper. A 10-ml sample of the clear filtrate was taken for determinations of pH and titratable acidity. Titratable acidity was measured by titration with NaOH to pH 8.2 endpoint and expressed as per- centage malic acid, the major organic acid present in watermelon flesh ( Pratt, 1971; Chisholm and Picha, 1986).

RESULTS AND DISCUSSION

I n t e r n a l characterist ics . ~ Changes in soluble solids, pH, texture and color of flesh samples following anthesis are shown in Fig. 1A-D. Soluble solids in- creased with maturation and ripening, reaching a plateau at approximately 23 DPA for 'Blue Belle' and 29 DPA for 'Charleston Gray' (Fig. 1A). The general shape of this trend is in agreement with the findings of Elmstrom and Davis (1981) and those of Brown and Summers (1985). In those studies, major dif- ferences among cultivars in total sugar accumulation with growth and devel-

214

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0 - 1 2 15 18 21 24 27 30 33 36"12 15 18 21 24 27 30 33 36 Days past anthesis Fig. 1. Changes in (A) soluble solids, (B) peak force, (C) pH and (D) Hunter a of the flesh of watermelon fruits from 2 cultivars during maturation and ripening. Vertical bars indicate 1 SE.

opment were not detected, but the relative concentrations of the sugars glucose, fructose and sucrose varied with cultivar.

The time-course change in pH did not parallel the change in soluble solids accumulation. Flesh pH did not increase until approximately 19 DPA (Fig. 1B), but thereafter increased at a relatively constant rate throughout the data collection period ( 36 DPA). This t rend indicates that harvesting watermelons when soluble solids content has reached a plateau may not optimize quality since the acid content of the flesh would not be sufficiently low or the soluble solids to acid ratio sufficiently high. The sugar to acid ratio has been demon- strated to be correlated with product acceptance in watermelon (Nip et al., 1968) and in other fruits (Scott and Walls, 1947). In the present study, as fruit matured and ripened there was a significant decline in the acid content of 'Blue Belle' flesh concomitant with a significant increase in soluble solids (Table I). Although significant changes in titratable acidity and pH were not detected for 'Charleston Gray', the trend was similar. The net result of these changes in the 2 flesh components is that the ratio of soluble solids to acids tends to increase upon ripening.

An additional component of watermelon flesh quality is texture or crispness of the flesh. Puncture force measurements made with a Magness-Taylor fruit tester ( Nip et al., 1968) and compression tests made with an Instron Universal Testing Machine (Peleg et al., 1976) have been reported for watermelon flesh tissue. The texture-testing system used in this study measures a combination of compression and shear forces exerted on a sample. The blades within the tester encounter resistance upon initial contact with the sample surface. This is indicated by a slight upward deflection from the baseline on the chart re-

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TABLE I

Changes in soluble solids, pH and titratable acidity of the flesh of 2 watermelon cultivars upon ripening 1

Cultivar Stage of Soluble pH Titratable Soluble solids/ ripeness solids acidity titratable

( % ) ( % malic acid) acidity

Charleston Gray

Blue Belle

Under 7.3 e 4.9 a 0.095 a 80.6 b Optimum 10.6 a 5.3 a 0.097 a 113.1 a Over 9.7 b 5.4 a 0.080 a 121.5 a

Under 8.0 b 4.8 c 0.125 a 63.7 b Optimum 10.8 a 5.4 b 0.068 b 162.7 a Over 9.9 a 5.9 a 0.058 b 175.7 a

1Mean separation within columns is for each cultivar separately using LSD (0.05).

cord ing (Fig. 2) . C o m p r e s s i o n forces t h e n cause a g radua l increase unt i l such t ime t h a t the b lades begin to shea r t h r o u g h the s amp le a n d a sha rp increase in force occurs. T h e r e a f t e r , c o m p r e s s i o n a n d shea r forces t a k e p lace c o n c u r r e n t l y a n d a p e a k force or p e a k s a m p l e res i s t ance to c o m p r e s s i o n a n d shea r is r eached fol lowed by a s t eady decl ine to the basel ine . R e p r e s e n t a t i v e e x a m p l e s of curves d e m o n s t r a t i n g these p rocesses for i m m a t u r e a n d ripe s amp le s of f lesh f rom b o t h w a t e r m e l o n cu l t iva rs are shown in Fig. 2. T h e p e a k force requi red to com- p r e s s / s h e a r s amp le s of w a t e r m e l o n f lesh dec reased l inear ly for b o t h cu l t ivars (Fig. 1C ). T h e r e were no s ign i f ican t d i f fe rences be tween cu l t ivars in the slope a n d in t e r cep t of the l inear regress ion equa t ions ( P > 0.05 ). T h e decl ine in re- s i s t ance to c o m p r e s s i o n a n d shear forces of w a t e r m e l o n f lesh wi th a d v a n c i n g

30

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B Charleston Gray Ripe

• Charleston Gray Immature

c Blue Belle Immature

20 15 10

D Blue Belle Ripe

O 5 20 "i5 10 5

Piston down-stroke travel time in seconds

Fig. 2. Representative curves showing changes in compression and shear forces during the course of texture measurements on watermelon flesh from 2 cultivars at the under-ripe and ripe stages.

216

~- - 3

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12 I I I I I I I ~'~1 15 18 21 24 27 30 33 36

Days past anthesis

Fig. 3. Changes in (A) Hunter a of the groundspot and (B) rind gloss on fruit of 2 watermelon cultivars during maturation and ripening. Vertical bars indicate 1 SE.

ripeness indicates that texture may be an important quality attribute to be considered along with sugars, acids and color. However, optimization of tex- tural quality would require further tests involving sensory evaluations made by a consumer taste panel.

Maximum red color development of the flesh, as indicated by changes in the Hunter a value, occurred at approximately 30 DPA for both cultivars (Fig. 1D). Although not statistically significant, the average Hunter a values for 'Blue Belle' were higher than those for 'Charleston Gray'. This suggests that there is genetic variation in the flesh content of the pigment lycopene, as doc- umented previously by Tomes et al. (1963). The plateau in flesh color occurred slightly later in growth and development than the accumulation of soluble sol- ids, but the increase in pH continued after the plateau in flesh color.

E x t e r n a l c h a r a c t e r i s t i c s . - - Changes in the color of the groundspot with growth and development were best characterized by the Hunter a values (Fig. 3A). The slightly negative a values represent a small amount of green coloration in the groundspot, presumably due to the presence of chlorophyll. The Hunter a values became less negative with advancing maturity for both cultivars, indi- cating a decrease in chlorophyll concentration in the groundspot region. Sta- tistical comparison of the regression lines revealed that there was a significant (P < 0,05 ) difference between the y-intercepts of the 2 lines, but no significant

217

(P > 0.05 ) difference in the slopes. The large proportion of variation explained by the linear regression of groundspot Hunter a value and chronological age (DPA) for both cultivars suggests that groundspot color may serve as a useful predictor of internal flesh quality. However, predictive models will need to be developed for specific cultivars based on a larger number of observations. Fur- thermore, selection of a specific groundspot color reading as a harvest criterion will necessitate relating such readings to soluble solids/titratable acidity of the flesh. Titratable acidity measurements were not made in the growth and de- velopment portion of the present study.

It was initially thought that an increase in Hunter b values of the groundspot would occur, indicative of the degree of yellow coloration, since it is commonly believed that the groundspot of ripe fruit exhibit a slight yellow coloration and unripe fruit do not (Pantastico, 1975). In the present study, we were unable to detect changes in Hunter b values with advancing maturity (data not shown). A possible explanation for this observation is that the visual observation of increased yellowness is due to an unmasking of xanthophyll and/or carotenoid pigments resulting from a decline in chlorophyll content. Research on changes in the relative concentrations of pigments in the groundspot rind will be nec- essary to test this hypothesis.

Changes in rind gloss were calculated from measurements of Hunter L val- ues with and without the inclusion of the specular component of reflectance. The exclusion of the specular component of reflectance results in measure- ments of only diffuse or scattered reflected light. Therefore, the increase in L value upon inclusion of the specular reflected light serves as an estimate of sheen or gloss. Light tends to be scattered (diffuse reflectance) after striking an uneven surface such as a piece of watermelon rind, resulting in the impres- sion of a dull, flat finish. There was a trend toward increasing rind gloss (AL S P I N - AL SPEX) of 'Blue Belle' fruits with increasing age (Fig. 3B ). In con- trast, rind gloss of 'Charleston Gray' fruits remained nearly constant until about 29 DPA and then decreased as the fruit ripened. Ripe fruits of 'Blue Belle' (34 and 36 DPA) were glossier than those of 'Charleston Gray', indi- cating that there may be differences in quantity and structure of the surface waxes of watermelon cultivars. The decline in gloss of 'Charleston Gray' fruits, although not dramatic, could provide a useful index of determining ripeness, since it occurred at a stage when soluble solids and red color had reached a plateau (Fig. 1A and D). Portable instruments for measurement of gloss are currently available. Direct measurements of light reflected from the surface of the fruit as they relate to internal flesh quality of different cultivars will be needed to test the value of rind gloss as a harvest criterion. An examination of quantitative and structural changes in the epicuticular waxes on the rind sur- face will also be necessary to explain observed changes in gloss which are ap- parently cultivar-dependent. Refinement of techniques for measurement of gloss and additional work on the relationship of changes in rind gloss to flesh

218

qual i ty a t t r ibu tes of wa te rme lons could lead to a non-des t ruc t ive technique for select ion of h igh-qua l i ty f ru i t s in ha rves t ing and grading operat ions .

REFERENCES

Anonymous, 1983. Instruction Manual 4, Spectroguard Color System. Pacific Scientific, Gardner/Neotech Instrument Division, Silver Spring, MD.

Brown, A.C., Jr. and Summers, W.L., 1985. Carbohydrate accumulation and color development in watermelon. J. Am. Soc. Hortic. Sci., 110: 683-687.

Chisholm, D.N. and Picha, D.H., 1986. Distribution of sugars and organic acids within ripe wa- termelon fruit. Hortscience, 21: 501-503.

Elmstrom, G.W. and Davis, P.L., 1981. Sugars in developing and mature fruits of several water- melon cultivars. J. Am. Soc. Hortic. Sci., 106: 330-333.

MacGillivray, J.M., 1947. Soluble solids content of different regions of watermelons. Plant Phys- iol., 22: 637-640.

Mizuno, S. and Pratt, H.K., 1973. Relations of respiration and ethylene production to maturity in the watermelon. J. Am. Soc. Hortic. Sci., 98: 614-617.

Ng, T.J. and Corey, K.A., 1984. Melon variety trials. University of Maryland, College Park, MD, HR 69-85.

Nip, W.K., Burns, E.E. and Paterson, D.R., 1968. Physical, chemical and organoleptic attributes of 'Charleston Gray' watermelons at different stages of maturity. Proc. Am. Soc. Hortic. Sci., 93: 547-551.

Pantastico, Er. B., 1975. Postharvest Physiology, Handling, and Utilization of Tropical and Sub- tropical Fruits and Vegetables. Avi, Westport, CT, 560 pp.

Peleg, M., Gomez, B.L. and Malevski, Y., 1976. Compressive failure patterns of some juicy fruits. J. Food Sci., 41: 1320-1324.

Pratt, H.K., 1971. Melons. In: A.C. Hulme (Editor), The Biochemistry of Fruits and their Prod- ucts. Vol. 1. Academic Press, London, p. 219.

Scott, L.E. and Walls, E.P., 1947. Ascorbic acid content and sugar-acid ratios of fresh fruit and processed juice of tomato varieties. Proc. Am. Soc. Hortic. Sci., 50: 269-272.

Soteriadou, A., 1970. Proceedings of a Conference on Tropical and Subtropical Fruits, Tropical Products Institute, London, p. 207.

Tomes, M.L., Johnson, K.W. and Hess, M., 1963. The carotene pigment content of certain red fleshed watermelons. Proc. Am. Soc. Hortic. Sci., 82: 460-464.