Postharvest Biology and Technology 31 (2004) 287–294

Skin colour and pigment changes during ripeningof ‘Hass’ avocado fruit

Katy A. Cox, Tony K. McGhie, Anne White, Allan B. Woolf∗

The Horticulture and Food Research Institute of New Zealand Limited, Mt Albert Research Centre,Private Bag 92 169, Mt Albert, New Zealand

Received 6 March 2003; accepted 27 September 2003

Abstract

Skin of ‘Hass’ avocados (Persea americana Mill.) changes colour from green to purple/black as fruit ripen. This colour changeis important as an indicator of ripeness for both industry and consumers. We sought to characterise pigment changes duringripening of ‘Hass’ avocados. Skin colour and changes in chlorophyll and anthocyanin concentrations in relation to fruit firmnessat three ripening temperatures (15, 20 and 25◦C) were studied. As fruit ripened, skin colour changed from green, to purple, toblack, which was observed as a decrease inL, chroma and hue. Total anthocyanins in skin tissue increased during ripening, butthis increase was due almost entirely to a single anthocyanin; cyanidin 3-O-glucoside. Cyanidin 3-O-glucoside concentrationincreased after 3–6 days postharvest (depending on ripening temperature), and levels increased earlier, and were higher, withhigher ripening temperature. Chlorophyll a and b levels decreased until 4–5 days after harvest, but did not change significantlythereafter. No significant differences in chlorophyll concentration were observed among the three ripening temperatures. Fruitharvested directly from the tree (unripe) with skin darkening showed similar pigments changes to that during ripening (reducedchlorophyll and increased cyanidin 3-O-glucoside). Colour change in ‘Hass’ avocados from green to purple, then black, resultsfrom an initial decrease in chlorophyll content, followed by an increase in the levels of the anthocyanin, cyanidin 3-O-glucoside.© 2003 Elsevier B.V. All rights reserved.

Keywords: Persea americana; Anthocyanins; Cyanidin 3-O-glucoside; Chlorophyll; Ripening temperature; Fruit firmness

1. Introduction

During ripening, the skin of ‘Hass’ avocados(Persea americana Mill.) changes from green to pur-ple/black. Although skin colour change is importantfor industry and consumers since it is an indicationof fruit ripeness, little is known about the pigment

∗ Corresponding author. Tel.:+64-9-815-4200;fax: +64-9-815-4202.

E-mail address: awoolf@hortresearch.co.nz (A.B. Woolf).

changes responsible for these changes. In addition,late-season skin darkening can be observed on fruitbefore harvest. Although ripening time is reducedfor late-season fruit, the fruit may be dark but un-ripe. This colouration can confuse customers as thefruit appears to be ripe, but is not soft. Use of lowerripening temperatures, such as 15◦C, is an impor-tant postharvest tool for controlling rots (Hopkirket al., 1994). However, at these temperatures skincolouration is reduced such that a fruit can be ripei.e. soft, but show little skin darkening/purpling

0925-5214/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.postharvbio.2003.09.008

288 K.A. Cox et al. / Postharvest Biology and Technology 31 (2004) 287–294

(Woolf and White, unpublished data;Hofman et al.,2002).

Prabha et al. (1980)identified anthocyanins in theskin of an unnamed avocado cultivar (P. americana).Cyanidin 3-O-galactoside and cyanidin-3,5-digluco-side acylated withp-courmaric acid were synthe-sised upon ripening in a ratio of 9:1 (Prabha et al.,1980). Lancaster et al. (1997)reported varying levelsof anthocyanins, carotenoids, and chlorophyll, al-though neither the cultivar nor the stages of ripenesswere identified.Woolf and Laing (1996)found thatthe concentration of chlorophyll in the skin was thesame in unripe (3 days postharvest) and ripe ‘Hass’fruit.

Understanding the pigment changes during ‘Hass’ripening is important since we may be able to manip-ulate colouring in some way to better co-ordinate soft-ening and colour change. The purpose of this projectwas to identify individual anthocyanins and concentra-tions of anthocyanins and chlorophylls in the skin of‘Hass’ avocados as fruit ripened at 15, 20 and 25◦C.An initial examination of late-season colouration wasalso carried out.

2. Materials and methods

2.1. Summary of treatments

Late-season (high maturity) ‘Hass’ avocados (P.americana, Mill.) were ethylene treated (to reduceripening variability), then ripened at 15, 20 or 25◦C.Fifteen fruit were randomly selected daily at 20 and25◦C and every second day at 15◦C. The nine fruitof most similar colour were selected for further anal-ysis (three replicates of three fruit). Fruit firmnesswas measured and skin colour determined objectively(L × C × h◦) and subjectively (eye colour rating).Skin tissue was then sampled, and anthocyanin andchlorophyll concentrations determined by HPLCand spectrophotometry, respectively. Skin samplingceased once fruit were ‘ripe’.

2.2. Fruit

‘Hass’ avocados (200–250 g) were harvested froma commercial orchard in Whangarei, New Zealand inMarch (late-season). Fruit were collected from the or-

chard and ethylene treatment commenced 1 day afterharvest. Fruit maturity was determined by dry mattercontent. A longitudinal wedge was taken from eachof nine fruit and the skin and seed material removed.Three wedges, were shredded together in a food pro-cessor and approximately 25 g of the shredded fleshwas sub-sampled, weighed, oven dried to a constantweight (65◦C for 24 h) and reweighed, to obtain thedry matter content. Fruit had a dry matter contentof 41.5 ± 0.7% indicating a high level of horticul-tural maturity. Ethylene treatment (100�l l−1, 24 h)was carried out at 17◦C in 360 l tubs equipped withcirculating fans and ca. 400 g of hydrated lime main-tained CO2 at ≤ 0.2 kPa. Tubs were flushed after 12 h(minimum O2 of ca. 17 kPa), and ethylene re-injected.Following ethylene treatment, fruit were randomisedinto trays before placement into 15, 20 or 25◦C toripen.

2.3. Postharvest assessments

Fruit were ripened in the dark at 15, 20 and 25◦C,and were assessed for the following parameters (in thefollowing order).

2.4. Skin colour

Skin colour of nine fruit was rated by one trainedassessor using the following scale: 1, emerald green;2, forest green; 3, olive green; 4, purple; and 5,black. Colour variability around the fruit was av-eraged. These same nine fruit were then measuredobjectively using a Minolta Chroma Meter CR300(Minolta, Osaka-light source D65) by averaging threemeasurements taken around the fruit equator. Dataare expressed inL × C × h◦ units (McGuire, 1992).

2.5. Firmness assessments

Fruit firmness was measured using the Andersondigital Firmometer (Anderson Manufacturing, Tau-ranga, New Zealand; as described byWhite et al.,1999). The firmometer value derived (0–110) is thefirmometer reading (mm displacement) multiplied by10. As fruit soften, firmometer values increase. Fruitwere considered ‘ripe’ when an average Firmome-ter value of 100 was reached, at which point furthertissue sampling ceased.

K.A. Cox et al. / Postharvest Biology and Technology 31 (2004) 287–294 289

2.6. Skin tissue collection

Samples of avocado skin (periderm tissue) weretaken from three replicates each of three fruit andpooled for pigment analysis. Approximately 10 g oftissue was removed from around the equator, usingthe method outlined byWoolf and Laing (1996). Skintissue was frozen immediately in liquid nitrogen, andstored at−80◦C until analysed.

2.7. Pigment analysis of avocado skin by HPLC

Sample preparation: Frozen skin tissue was groundto a powder under liquid nitrogen and ca. 0.5 g ex-tracted with 5 ml of 10% acetic acid/methanol (v/v) inthe dark for ca. 24 h at 4◦C. Following centrifugationat 3000×g for 10 min the supernatant was diluted 1:1with methanol:water:acetic acid (50:50:10, v/v/v) andanalysed by HPLC. Samples were prepared and ex-tracted under safe-green light to minimise anthocyanindegradation.

Anthocyanins were separated and analysed using anHPLC system similar to that described byMarkhamet al. (1997). The HPLC system was composed ofJASCO units (LG-980-02 ternary gradient controller,AS-950 autosampler, and a UV-975 UV/Vis detector).The chromatography column was a Merck LiChro-sphere 5 um PR-18e 4.0 × 300 mm, maintained at35◦C. Mobile phases were: (A) 1.5% H3PO4, and (B)acetic acid:acetonitrile: H3PO4:water (20:24:1.5:54.5,v/v/v/v). The solvent programme started with 20% B,increasing to 70% B at 25 min, then 90% B at 30 min.After a further 5 min the solvent composition wasreturned to 20% B ready for the next injection. Thesample injection volume was 100�l and detectionwas at 530 nm. Anthocyanin concentrations were cal-culated with reference to a standard curve preparedfrom authentic cyanidin 3-O-galactoside (Plantech,UK) using a Waters Millennium32 ChromatographyData Manager.

2.8. Chlorophyll extraction and measurement

Chlorophyll extraction and estimation was car-ried out using the method ofLancaster et al. (1997),with some modifications. Skin tissue was extractedusing 5 ml of cold acetone containing 1% w/v cal-cium carbonate to prevent degradation. The tissue

sample was then homogenised with an Ultra-turrex(Jancke and Kundel, Munich, Germany) for 30 s atmaximum speed. After clarification by centrifugation(15 000× g, 10 min) the absorbance was measured at645, 652, and 633 nm and the chlorophyll concentra-tion calculated using the equation of Machachlan andZalik (Holden, 1965).

2.9. Late-season skin colouration

A preliminary examination was made to determinethe anthocyanin and chlorophyll level in fruit whichshowed late-season colouration on the tree (i.e. whenunripe). Fruit with skin colour ratings of 2, 4 and 5(see above) were harvested at the same time as theabove experiment. One day after harvest, skin tissuewas sampled and anthocyanin and chlorophyll con-centrations measured as describe above.

2.10. Statistical analysis

One-way ANOVA and a General Linear model wasused to determine means and differences in pigmentconcentrations, using a Tukey’s comparison test.Pearsons correlation coefficients were calculated foreach of the various factors (Table 1) on individual an-thocyanin and chlorophyll concentrations. Statisticalanalysis was carried out using MINITAB (version 12).

3. Results

3.1. Skin colour changes in ripening fruit

Over 8–12 days, fruit softened and skin colourchanged from green to purple, then black (Fig. 1A).Visually, this equated to an approximately linearpattern for each temperature. Colour changes dur-ing ripening at 20 and 25◦C were similar, but fruitripened at 15◦C were significantly slower to colourand did not achieve a fully black colouration (rating5; Fig. 1A) even though the fruit would be consideredfully ripe (soft) by this time (Firmometer value >100;Fig. 4).

Hue angle, chroma and lightness all decreased withripening (Fig. 1B, C and D), and were correlatedwith visual colour ratings (Table 1). However, thepattern of colour change as measured by these three

290 K.A. Cox et al. / Postharvest Biology and Technology 31 (2004) 287–294

Table 1Pearson correlation coefficients between objective (Minolta chromameter values;L, C, h◦) and subjective (visual or “eye” rating) of colourmeasurements/firmness and total anthocyanin (total anth.) and cyanidin 3-O-glucoside (Cyan-3-glu.) concentrations found in the skin of‘Hass’ avocados ripened at 15, 20 and 25◦C after ethylene treatment (100�l l−1, 24 h)

Correlation Ripening temperature

15◦C 20◦C 25◦C

R2 P R2 P R2 P

Eye × L −0.986 0.000 −0.951 0.001 −0.991 0.001Eye × C −0.950 0.001 −0.941 0.002 −0.946 0.015Eye × h◦ −0.716 0.070 −0.839 0.018 −0.897 0.039Total anth.× eye 0.894 0.003 0.592 0.122 0.857 0.029Total anth.× L −0.907 0.002 −0.557 0.151 −0.860 0.028Total anth.× C −0.859 0.391 −0.353 0.391 −0.731 0.099Total anth.× h◦ −0.804 0.016 −0.433 0.284 −0.747 0.088Cyan-3-glu.× eye 0.915 0.003 0.848 0.008 0.957 0.003Cyan-3-glu.× L −0.925 0.001 −0.880 0.004 −0.970 0.001Cyan-3-glu.× C −0.870 0.001 −0.743 0.035 −0.897 0.015Cyan-3-glu.× h◦ −0.768 0.026 −0.795 0.018 −0.900 0.015Total anth.× Cyan-3-glu. 0.992 0.000 0.875 0.002 0.954 0.001

0 2 4 6 8 10 122

4

6

8

10

12

14

16

18

20

22

24

26

Bas

ic ti

nt o

f col

our

Bec

omin

gle

ss in

tens

e

Viv

idne

ss (

Chr

oma;

C)

Days from harvest

0 2 4 6 8 10 1222

24

26

28

30

32

34

36

Bec

omin

g da

rker

Ripened at 15 oC

Ripened at 20 oC

Ripened at 25 oC

Ave

rage

ligh

tnes

s (L

)

Days from harvest

0 2 4 6 8 10 1250

60

70

80

90

100

110

120

130

Blue; ho=270

Green; ho=180

Yellow; ho=90

Red; ho=0

Ave

rage

Hue

ang

le (

ho )

0 2 4 6 8 10 121.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0Black

Purple

Olive

Forestgreen

Emeraldgreen

Ave

rage

ski

n co

lour

rate

d by

eye

(A) (B)

(C) (D)

Fig. 1. Objective (L, C, h◦) and subjective (rating by eye) measurements of skin colour changes of ‘Hass’ avocados fruit during ripeningat 15, 20 and 25◦C.

K.A. Cox et al. / Postharvest Biology and Technology 31 (2004) 287–294 291

parameters was somewhat different. Between 1 and3 days of harvest, hue angle changed little, lightnessmoderately, while chroma changed more quickly andwas significantly higher for fruit ripened at 15◦C.After 3–4 days, all colour parameters declined,and 15◦C ripened fruit declined slower than 20 or25◦C.

3.2. Anthocyanin and chlorophyll concentrations inripening fruit

Typical HPLC chromatogram traces obtained forthe analysis of anthocyanins in green, olive andblack skin tissue are shown inFig. 2. There is alarge increase in anthocyanin concentration as av-ocado skin matures from green though the olive to

(A)

(B)

0 10 20 30

Retention time (minutes)

(C)

Abs

Abs

Abs

Fig. 2. HPLC trace of anthocyanins of ‘Hass’ avocado skintissue: (A) unripe (green); (B) partially ripe (olive); (C) ripe(purple-black). The arrows indicate the peak of most significantchange (identified as cyanidin 3-O-glucoside).

black stage and this increase is almost entirely dueto the increase of a single anthocyanin identified ascyanidin 3-O-glucoside. The anthocyanin concentra-tion of green skin was low and composed of severalunidentified anthocyanins although one of these wasidentified as cyanidin 3-O-galactoside as the retentiontime corresponded with an authentic standard. Theother anthocyanin peaks in green avocado skin werenot identified, but as they eluted before the standardcyanidin 3-O-galactoside that may be delphinidin-or cyanidin-based anthocyanins with higher levels ofglycoslyation (Goiffon et al., 1991).

Ripening temperature had a highly significant lin-ear and non-linear (P < 0.01) effect on anthocyaninconcentrations (total, and other) found in the skin.Higher ripening temperatures resulted in higher cyani-din 3-O-glucoside levels. For total anthocyanin andcyanidin 3-O-glucoside concentrations, there was aninteraction (P < 0.05) between days from harvest andripening temperature. The correlation between cyani-din 3-O-glucoside and the four colour measurementswas significantly higher than that for total anthocyanin(Table 1).

An initial decline in chlorophyll a and b concen-tration occurred during ripening, but after 4–5 daysof ripening at all temperatures, chlorophyll levels didnot change significantly (Fig. 3C). During this pe-riod, the ration of chlorophyll a and b was 2:1. Therewere no significant differences in the concentrationof chlorophyll a and b between the three ripeningtemperatures.

3.3. Ripening/softening over time

The softening curves of fruit ripened at 15, 20and 25 ◦C, as determined by the Anderson Firmome-ter, all followed a sigmoidal pattern (Fig. 4), andtook 12, 8 and 7 days respectively, to reach eatingripeness.

3.4. Late-season skin colouration

The examination of late-season skin colouration(i.e. colouration when fruit is unripe) showed that to-tal anthocyanin and cyanidin 3-O-glucoside increasedwith increasing skin darkening (as rated by eye)(Table 2). Chlorophyll concentration showed somedecrease.

292 K.A. Cox et al. / Postharvest Biology and Technology 31 (2004) 287–294

0 2 4 6 8 10 120

50

100

150

200

250

300

350

400 Ripened at 15oC

Ripened at 20oC

Ripened at 25oC

(B)

Ave

rage

Cya

nidi

n-3-

gluc

osid

e

(mg

kg-1)

0 2 4 6 8 10 120

50

100

150

200

250

300

350

400

450

(A)

Ripened at 15 oC

Ripened at 20 oC

Ripened at 25 oCA

vera

ge to

tal a

ntho

cyan

in

conc

entr

atio

n (m

g kg

-1)

0 2 4 6 8 10 12

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

(C)

Chlorophyll a: 15oC

Chlorophyll b: 15oC

Chlorophyll a: 20oC

Chlorophyll b: 15oC

Chlorophyll a: 25oC

Chlorophyll b: 25oC

Ave

rage

chl

orop

hyll

conc

entr

atio

n (m

g g-1

)

Days from harvest

Fig. 3. (A) Total anthocyanin, (B) cyanidin 3-O-glucoside and (C) chlorophyll a and b concentrations in the skin of ‘Hass’ avocado fruitduring ripening at 15, 20 and 25 ◦C.

4. Discussion

In ‘Hass’ avocados, we found a single anthocyanin,cyanidin 3-O-glucoside responsible for the increasein the total anthocyanin concentration. Cyanidin3-O-galactoside was found at harvest and during

ripening, as previously found by Prabha et al. (1980),but in ‘Hass’ did not increase as skin colour changed.The increase in cyanidin 3-O-glucoside was highlycorrelated with the changes in colour as measured byboth eye rating, and chromameter. Higher ripeningtemperatures resulted in earlier and greater levels of

K.A. Cox et al. / Postharvest Biology and Technology 31 (2004) 287–294 293

0 2 4 6 8 10 12

20

40

60

80

100

120

(mm

dis

plac

emen

t x 1

0)

Ripened at 15 oC

Ripened at 20 oC

Ripened at 25 oC

Ave

rage

Firm

omet

er v

alue

Days from harvest

Fig. 4. Softening of ‘Hass’ avocados fruit during ripening at 15, 20 and 25 ◦C as measured by the Anderson digital Firmometer.

cyanidin 3-O-glucoside, and these patterns may bereflected in the eye rating, and hue angle, Chromaand lightness data (Fig. 1A and B).

After harvest there was an initial decrease in chloro-phyll content of the skin, but little change later inripening. While in some fruit colour change is due to“unmasking” of existing pigments (Wills et al., 1989),our results suggest that for ‘Hass’ avocados, the ini-tial changes in colour from “emerald green” to darkergreen (stages 1–2; < 3 days postharvest), may be dueto chlorophyll degradation. Colour changes after thisinitial phase appear to be primarily due to synthe-sis of cyanidin 3-O-glucoside, although carotenoids orxanthopylls might play some role since, for example,Lancaster et al. (1997) measured carotenoids in avo-cado skin.

In this work we applied a range of ripening temper-atures with the aim that fruit ripened at 15 ◦C wouldnot darken in colour significantly, even when fully ripe(as we have observed in previous work; White and

Table 2Skin colour (eye rating) and concentration of total anthocyanin, cyanidin 3-O-glucoside and chlorophyll content in the skin of ‘Hass’avocado fruit taken directly from the tree (i.e. unripe). Ratings 4 and 5 are exhibiting late-season skin colouration. Mean values ± standarderror of the mean (S.E.M.) are presented

Skin colour(rating)

Total anthocyanin(mg kg−1)

Cyanidin 3-O-glucoside(mg kg−1)

Chlorophyll

a b Total

2 150 ± 3 10 ± 1 0.43 ± 0.01 0.16 ± 0.00 0.63 ± 0.024 573 ± 63 371 ± 46 0.31 ± 0.01 0.14 ± 0.01 0.50 ± 0.025 524 ± 85 321 ± 64 0.36 ± 0.00 0.16 ± 0.01 0.57 ± 0.02

Woolf, unpublished data). Although the skin did notturn black (stage 5), darkening/purpling (stage 4) hadoccurred by the time the fruit were eating soft. Theextent of dark colouration observed, which was signif-icantly higher than we have observed in other work,was most likely due to use of very late-season, highmaturity fruit.

Our examination of late-season skin colourationsuggests that it may be a result of the same pig-ment changes that are observed as fruit ripen, viz.increased levels of anthocyanin, and a reduction inchlorophyll content. Anthocyanin synthesis has beenobserved in response to environmental stresses. In‘McIntosh’ apples, a certain level of energy from thelight was required to initiate anthocyanin synthesis(Lancaster, 1992). This trend has also been shownin other apple cultivars (Siegelman and Hendricks,1958). Temperature and time were also found to havean influence on anthocyanin production (Siegelmanand Hendricks, 1958). These factors may also have an

294 K.A. Cox et al. / Postharvest Biology and Technology 31 (2004) 287–294

effect on ‘Hass’ avocados. Green fruit at harvest mayhave experienced diminished levels of one or all ofthese factors. For example, fruit positioned within theinner canopy is hidden from direct sunlight and thusexperiences lower temperatures (Woolf et al., 1999),and this may result in fruit which are less stressed,hence producing less anthocyanins.

It is interesting to note that growing environmentappears to play a role in skin colouration also since itappears that fruit grown in California generally doesnot change to a “purple” stage, but rather turns directlyto a black colour (Arpaia, pers. com.). This is not gen-erally observed for Chilean and New Zealand-grown‘Hass’ fruit in the Californian markets.

We conclude that that the colour change duringripening, and late-season colouration of ‘Hass’ avoca-dos is due to an initial reduction in chlorophyll concen-tration, followed by an increase in synthesis of cyani-din 3-O-glucoside. These pigment changes are highlycorrelated with skin colour change, which is also in-fluenced by the ripening temperature.

Acknowledgements

We wish to thank project supervisors from MasseyUniversity, Drs. Debra Dawson and Lynn McIntyre.Dr. William Laing also assisted with chlorophyll anal-ysis. Also to Dr. Peter Hofman for contributions toconcept development. This work was funded by theFoundation for Research Science and Technology.

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