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Scientia Horticulturae 134 (2012) 245–247

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Scientia Horticulturae

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Short communication

Effect of air temperature on rind colour development in pomegranates

F.J. Manera b, P. Leguaa, P. Melgarejo a, R. Martínez a,∗, J.J. Martínez a, Fca. Hernández a

a Plant Science andMicrobiology Department, Universitas Miguel Hernandez, Ctra Beniel 3.2, 03312Orihuela (Alicante), Spainb Physics and Computer Architecture Department,Universitas Miguel Hernandez, Ctra Beniel 3.2, 03312Orihuela (Alicante), Spain

a r t i c l e i n f o

 Article history:

Received 1 August2011Received in revised form 9 November 2011Accepted 17 November 2011

Keywords:

Colour coordinatesC * and h∗

abPomegranate rindTemperature

a b s t r a c t

The colour quality of many fresh and processed fruits may influence consumer acceptance. Pomegranateacceptability depends on a combination of quality attributes related to physical–chemical and mechanical

properties such as rind colour, sugar content, acidity, and flavour. This study was undertaken to inves-tigate any correlation between the rind colour of pomegranates and the environmental temperature, aswell as to study the evolution of colour parameters with the environmental temperature. The results con-firmed a strong correlation between the colorimetric coordinates (L*, a*, b*, C * and h∗ab), measured duringfruit development and maturation, and the maximum, mean and minimum temperatures. A high corre-lation coefficient of 0.9 indicated the significant effect of air temperature on rind colour development.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Spain produces about 40,000t peryear of pomegranates (Punica granatum L.). Several studies confirmed the excellent organolep-tic and nutritional properties of pomegranates (Al-Said et al.,2009; He et al., 2010; Ozgen et al., 2008; Shwartz et al., 2009).Moreover, the traditional importance of pomegranate fruit asa medicinal plant is currently the subject of renewed researchbecause of its anti-carcinogenic, anti-microbial and anti-viralprop-erties (Al-Maiman and Ahnad, 2002; Bell and Hawthorne, 2008;Kotwal, 2007; Reddy et al., 2007). Although knowledge about theimportance of pomegranate in human nutrition has tremendouslyincreased in recent years, the external colour of the fruit has notbeen studied in detail. Pomegranate maturity status is commonlyassessed based on rind and juice colour and acidity (Cristosto et al.,2000).

Pomegranate acceptability by consumers and processorsdepends basically on a combination of several quality attributesas rind colour, sugar content, acidity, and flavour (Al-Said et al.,

2009). Some researchers studied the correlation between rindcolour parameters (L*, a*, b*, C * and h∗ab) and acidity, total solublesolids,citricacidandanthocyanincontent( Dafny-Yalinetal.,2010).Yet no literature was available regarding the potential effect of airtemperature on pomegranate rind colour development. Therefore,the current study focused on investigating any possible correla-tion between pomegranate rind colour parameters and the air

∗ Corresponding author. Tel.: +34 966749644; fax: +34 966749693.E-mail address: rafa.font@umh.es (R. Martínez).

temperature. How those colour coordinates developed as air tem-perature fluctuated.

2. Materials and methods

 2.1. Plant material

The pomegranate cultivar chosen for this study was the ‘ME-15’accession (‘Mollar de Elche 15’). The cultivar was selected accord-ingto four main criteria:sweetness, soft-seeded,largefruitsize andhigh yields. The commercial variety ‘ME-15’ was selected from thepopulation cultivar ‘Mollar de Elche’ (ME), which is one of the mosthighly valued worldwide because of its outstanding flavour andhigh antioxidant, vitamin and mineral contents. The selected plantmaterial belonged to the main pomegranate gene bank of the EU,located at the experimental field station of Miguel Hernández Uni-versity in the province of Alicante, Spain (02◦0350E, 38◦0350N,and 25masl).

The orchard was established in 1992. Pomegranate trees weretrained to the vase-shaped system and planted at a spacing of 4 m×3 m. They were drip irrigated,and standard cultural practiceswere performed (pruning, thinning, fertilisation and pest controltreatments).

 2.2. Experimental design

Two pomegranate trees were randomly selected for fruitharvesting. Pomegranates were collected from alltree sides at mid-height (3 fruits per tree side); that is a total of 12 pomegranatesper tree. Six colour measurements per fruit were taken along the360◦ equatorial perimeter, given a total of 144 measurements

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246 F.J. Manera et al./ Scientia Horticulturae 134 (2012) 245–247 

0

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   J  u  n  e   2  n   d  w  e  e   k

   J  u  n  e   3  r   d  w  e  e   k

   J  u  n  e   4   t   h  w  e  e   k

   J  u   l  y   1  s   t  w  e  e   k

   J  u   l  y   2  n   d  w  e  e   k

   J  u   l  y   3  r   d  w  e  e   k

   J  u   l  y   4   t   h  w  e  e   k

   A  u  g   1  s   t  w  e  e   k

   A  u  g   2  n   d  w  e  e   k

   A  u  g   3  r   d  w  e  e   k

   A  u  g   4   t   h  w  e  e   k

   S  e  p   t   1  s   t  w  e  e   k

   S  e  p   t   2  n   d  w  e  e   k

   S  e  p   t   3  r   d  w  e  e   k

   S  e  p   t   4   t   h  w  e  e   k

   O  c   t   1  s   t  w  e  e   k

   O  c   t   2  n   d  w  e  e   k

   O  c   t   3  r   d  w  e  e   k

   V  a   l  u

  e  s  o   f   C   *  a  n   d

   h  u  e   *

  0

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35

40

   T   ª   (   º   C   )

C* hue* Tª max Tª min Tª mean

33.7ºC

27.2ºC

20.8ºC

Fig. 1. Chroma(C *) and hue angle (h*) evolution of pomegranate rind and temperature response curve forthe 2-year study.

(12 pomegranates/tree×6 measurements/pomegranate×2 trees).Rind colour evaluations were weekly assessed from phenologicalstage I (young fruit in the first half of June) to stage L (harvest in thethirdweekofOctober)(Melgarejoet al., 1996). Theexperimentwasconducted for two consecutive harvesting seasons (2008–2009).

The correlations among the colorimetric variables (L*, a*, b*,C * and h∗ab) and the minimum, maximum and mean temperatureswere studied along with the temperature response curve (Table 2).Every temperature used for establishing these potential correla-tions was the mean value of the temperatures referring to the 45days prior the day measurements were made (data not shown).The temperature response curve (TI) was calculated from anthesisat mid-May (phenological stage F) to full maturation in late October(phenological stage L) using a base temperature for pomegranate

of 9 ◦C (Melgarejo et al., 1996).The correlationamongthe colorimetric variablea*andthemax-imum, minimum and mean temperatures was established usingonly the data interval onset from a*>0(redcolour).Thesamecrite-rionwasfollowedforthe L* coordinate, coinciding with theintervalin which L* values were the highest (maximal brightness of fruitrind). For coordinate b*, the timing for assessment ran from thesecond half of June to the first week of August, coinciding with thefirst half of the cell enlargement phase of fruits.

 2.3. Determination of fruit colour 

Colour measurements were performed using a Minolta col-orimeter (CR-300, Minolta Ramsey, N.J., USA). Colour was assessedaccording to the Commission Internationale de l’ Éclairage (CIE)and expressed as L*, a*, b*, C ∗ab and h∗ab colour values. The coor-dinates L*, a* and b* indicate the lightness of the colour (L* = 0and L* = 100 represent black and white, respectively), its positionbetween green and red (negative and positive a* values indicategreenness and redness, respectively) and between blue and yel-low (negative and positive b* values point towards blueness andyellowness, respectively). The C ∗ab value (chroma,

√ a2 +b2) defines

colour saturation and the hue angle (h∗ab) colour shadiness (arctanb*/a*; where 0◦ = red-purple, 90◦ = yellow, 180◦ = bluish-green and

270◦ =blue). As suggested by McGuire (1992), the hue angle andchroma are accepted as the more intuitively and understandablecolour variables.

 2.4. Statistical analysis

Statistical analyses were performed using the SPSS 16.0 soft-ware package for Windows. Descriptive statistics were used toprocess and analyse all collected data. The Pearson correlation

 Table 1

Pomegranate rind colour coordinates and temperatures for the 2-year study.

Dates L* a* b* C * hue T max T min T mean TI

 June 2nd week 56.4 −19.8 42.046.5 115.3

27.1 14.5 20.8 383.0 June 3rd week 56.6 −20.8 42.8 47.6 115.9 28.0 15.4 21.7 487.5 June 4th week 56.9 −20.5 43.0 47.6 115.5 29.2 16.2 22.7 612.1 July 1st week 56.8 −20.1 43.7 48.1 114.7 30.4 17.2 23.8 731.1 July 2nd week 58.6 −19.3 44.3 48.3 113.6 29.8 18.0 24.6 860.7 July 3rd week 60.2 −18.4 44.6 48.3 112.4 32.3 18.8 25.5 984.8 July 4th week 61.5 −16.6 46.3 49.2 109.7 32.8 19.6 26.2 1118.5Aug 1st week 63.4 −14.9 48.1 48.6 107.9 33.5 20.1 26.8 1243.3Aug 2nd week 65.4 −14.9 48.5 50.6 107.2 33.7 20.7 27.2 1373.8Aug 3rd week 68.4 −12.2 49.5 51.3 103.8 33.6 20.9 27.3 1498.7Aug 4th week 70.4 −9.0 48.8 49.7 100.5 33.8 20.8 27.3 1628.8Sept 1st week 72.4 −6.0 49.5 50.5 96.9 33.5 20.8 27.2 1761.5Sept 2nd week 76.1 0.4 50.7 49.9 89.6 33.7 20.8 27.2 1946.3Sept 3rd week 74.1 5.5 49.8 49.7 83.7 33.2 20.4 26.8 2049.9Sept 4th week 72.0 9.6 47.7 49.3 78.8 32.5 19.7 26.1 2147.9Oct 1st week 69.3 12.3 45.8 47.5 75.0 30.9 18.7 24.8 2256.3Oct 2nd week 68.2 13.9 44.0 46.4 72.6 30.0 17.6 23.8 2367.8Oct 3rd week 65.8 12.7 42.4 44.4 73.5 28.3 16.4 22.9 2478.7

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 Table 2

Pearson’s correlationcoefficients (95%LSD)forcolourvariables andair temperaturesforthe 2-year study.

Colour coordinate T max T min T mean TI

L* 0.9826 0.9662 0.9839 −0.9674a* −0.9633 −0.9767 −0.9699 0.9682b* 0.9736 0.9643 0.9619 0.9903C * 0.9815 0.9849 0.9723 −0.9677

h∗ab

0.9534 0.9374 0.9472 −0.9682

coefficients (95% LSD) were determined among the colorimetricvariables and the air temperature.

3. Results and discussion

 3.1. Evolution of the colour parameter L*

The coordinate L* increased constantly from phenological stageI (young fruit), reaching a maximum during the second week of September (76.1) when top temperatures were recorded (Fig. 1).Afterwards, the L* values dropped constantly until harvest in lateOctober (Table 1) as previously reported by Shwartz et al. (2009).

 3.2. Evolution of the colour coordinate a*

The colorimetric coordinate a* showed negative values (green-ish) until the second week of September, when rind colour turnedfrom green to red(positivevalues) andcoinciding with toptemper-atures records and the maximum value of L* (Table 1). From thenonwards, while coordinate a* gradually increased (redness), the L*values constantly decreased; the highest values of a* were reachedbetween the first and second week of October (Table 1). Duringthis period the green colour of pomegranate rind was increasinglyreplaced by the red one. Similar results were found for ‘Mollar’pomegranate accessions (Gil et al., 1995) and for two Israelian cul-tivars (Shwartz et al., 2009).

 3.3. Evolution of the colorimetric coordinate b*

The colorimetric coordinate b* exhibited high and positive val-ues during the periods of fruit development and ripening (Table 1).The b* values of pomegranate rind significantly fell from the sec-ondweekofSeptemberonward,indicatingthatbluepigmentswerereplacing the yellow colour during fruit maturation. The results forcoordinate b* completelyagreedwith those found by Shwartz et al.(2009).

 3.4. Correlation between pomegranate rind colour and air 

temperature

Table 2 showed the Pearson correlation coefficients (95% LSD)for the colorimetric variables (L*, a*, b*, C * and h∗ab) and the airtemperature. The colour coordinatesof pomegranate rind were sig-nificantly correlated with the air temperature for the 2-year study.Values of r > 0.9 indicated that temperature definitively influencedpomegranate rind colour evolution.

 3.5. Evolution of the saturation index (C*) and hue angle (h∗ab)

Fig. 1 showed the evolution of the saturation index C * (chroma)and the hue angle (h∗ab), which indicates the quadrant of the colourdiagram and the angle with respect to the axis of coordinate a* onwhere the values laid. The C * values were always positive and didnot exactly indicate the point of rind colour evolution when data

were recorded. Fig. 1 also showed how C * steadily increased until

approximately the secondweek of September.Values of h∗ab greaterthan 90◦ were observed from phenological stage I (115.3◦) to thefirst week of September (96.9◦), when the coordinate a* becamepositive (Table 1).

Pomegranate rind revealed greener tones at the beginning of fruit development. The Chroma index and the hue angle togethershowed exactly when pomegranate rind colour turned from greento red, which was corroborated by the evolution of coordinate a*.The Chroma index evolution of pomegranate rind remained fairlysteady during fruit development and ripening since there was atrade-off among the values of coordinates a* and b*. The graphicalrepresentation of the hue angle showed a turning point during thesecond week of September, when values dropped below 90◦ andtemperature records started dropping, coinciding with the onsetof pomegranate rind colour change. As weeks go by, the hue anglegradually dipped to values of 77◦, 71◦ and 68◦, increasing rind red-ness as this pomegranate cultivar properly matures.

4. Conclusion

The results clearly confirm that the evaluated colorimetricparameters of pomegranate rind were highly correlated with theairtemperature duringfruit development andripening. All correla-tion coefficients were higherthan 0.9 (Table2), whichindicated thesignificant contribution of air temperature on rind colour develop-ment in pomegranates.

Between the first and second week of September pomegranaterind colour turned from green to red (a*≈0) with temperaturerecords of 34◦C, 27 ◦C and 21 ◦C (the maximum, mean and min-imum values, respectively). From then onwards to fruit ripening(first half of October), redness gradually increased (a* >0) while b*and L* progressively dropped.

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