J. Sci. Food Agric. 1985,36, 352-358

The Effects of Harvest Maturity, Ripeness and Storage on Kiwifruit Aroma

Harry Young and Vivienne J . Paterson

Division of Horticulture and Processing, DSIR, Private Bag, Auckland, New Zealand

(Munuscript received 1 November 1984)

Quantitative gas chromatographic analysis of the volatile aroma components has been used as an objective measure of the effects of maturity at harvest, ripeness and storage on the aroma volatiles of kiwifruit. Increasing ripeness is associated with a rapid increase in the levels of aroma volatiles, especially esters, while increasing storage time prior to ripening is accompanied by a decrease in the amount of aroma volatiles.

Keywords: Kiwifruit; maturity; ripeness; storage; aroma volatiles; headspace collection; gas chromatography.

1. Introduction

Kiwifruit, Actinidia deliciosa (A. Chevalier) C. F. Liang et A. R. Ferguson var. deliciosa (syn. A . chinensis Planchon var. hispida C. F. Liang), although first developed as a commercial horticultural crop in New Zealand, are now grown in several other countries. In New Zealand export kiwifruit are normally harvested at minimum soluble solids levels of 6.2" Brix ("B). When harvested the fruit are very hard and may be stored for several months. Young et al.l have previously studied the volatile aroma components of kiwifruit and have identified ethyl butanoate, hexanal and trans-hex-2-enal as important contributors to the aroma.

Although there have been several studies on the effects of harvest maturity and storage on kiwifruit ,2-5 they have been limited to the physiology and the non-volatile chemical constituents. This study has attempted to determine objectively the effects of harvest maturity, storage period and ripeness on the levels of the aroma compounds.

2. Experimental 2.1. Fruit Kiwifruit of the major commercial cultivar 'Hayward', grown at the DSIR Research Orchard at Te Puke, were harvested between 29 April and 8 June 1983. Fruit were harvested at four different stages of maturity,6 at soluble solids levels of 5.7, 6.1, 8.0 and 10.8"B. Fruit were packed in standard export trays with plastic liners and held at 0°C in an ethylene-free cool-store. When required for analysis the fruit were removed from cool-storage, and immediately dipped in Ethrel 48 (Ivon Watkins-Dow Ltd) (lglitre-l) for 25s. The ethylene released by the Ethrel facilitates even ripening. The fruit were allowed to dry, repacked into the trays and kept at 22°C until they had reached the required firmness as measured by a hand-held Effegi penetrometer (7.9mm head).

Fruit from each batch were dipped at 2-day intervals so that under-ripe, ripe and over-ripe fruit were available for sampling on the same day. For freshly harvested fruit the time taken to reach the ripe stage was 6 days. Since kiwifruit slowly soften during cool-storage', stored fruit required less ripening at 22°C. When necessary they were removed from the 22°C room and

352

Effects of harvest maturity, ripeness and storage on kiwifruit aroma 353

stored at 1x"C to reduce the rate of softening so that the time between dipping and sampling was similar to that of freshly harvested fruit. After storage for 21 weeks some fruit were already too soft to be included in the experiment and were discarded.

2.2. Collection of volatile components Working quickly to minimise loss of volatile components, seven fruit were peeled and pressed through a stainless steel mesh (with 2 m m holes) supported on top of a stainless steel beaker cooled in ice/water. The pulp collected in the beaker was thoroughly mixed and a sub-sample of 0.5-1 .Og was weighed into a S0ml Quickfit (Jobling, England) test-tube fitted with gas inlet and outlet tubes. Distilled water (2ml) was added. The volatile components were collected in a 100 mg Chromosorb 105 (Johns-Manville, USA) trap' attached to the outlet tube, by flushing the headspace with oxygen-free nitrogen at 20mlminp' for 2 h . During this period the sample was stirred and maintained at 30°C. The volatiles from a duplicate sub-sample were collected in a similar manner.

2.3. Gas chromatography A Hewlett Packard 5790A gas chromatograph fitted with a flame ionisation detector was used. The injector and detector were maintained at 180 and 200°C respectively. The column used was a 40mxO.S mm i.d. support coated open tubular (SCOT) Carbowax 20M glass capillary column (Scientific Glass Engineering, Australia). The carrier-gas was hydrogen at 180 mm spl . The temperature programme used was isothermal at 40°C for 15 min, 2°C min-' for 10 min, then 4°C min-' to 180°C and holding.

An internal standard of lp l of octan-1-01 solution ( lpgpl - ' in pentane) was injected on to each trap before gas chromatographic (g.c.) analysis. The desorption method described by Young7 was used to transfer the collected volatiles from the traps on to the g.c. column. (The injector of the 5790AGC had to be modified with a heated extension in order to accommodate the stainless steel traps). Peak areas were measured using a Hewlett Packard 3390A integrator and were adjusted for actual sample weight and the internal standard area. Peak identification was confirmed using retention time data and by gas chromatographyimass spectrometry' (g.c./m.s.). A detector response factor for each compound was determined using authentic samples. For some of the minor components the detector response of a related compound was used (Table 1).

3. Results and discussion

Flesh firmness was used as a measure of the degree of ripeness. The term ripe was used to describe fruit softened to 0.5-0.6 kilogram-force (kgf). This was considered to be a good eating ripeness. Under-ripe fruit (0.8-1.1 kgf) needed approximately 2 more days at 22°C to reach the ripe stage. Over-ripe fruit (0.3-0.4 kgf) had been kept a t 22°C for 2 further days after becoming ripe.

A typical gas chromatogram of the volatiles from freshly harvested kiwifruit at the over-ripe stage is shown in Figure 1. The peak numbering refers to the compounds in Table 1. The levels of the various components after different lengths of storage are given in Tables 2-4. In addition to those compounds found in the previous study,' limonene and several additional esters have been identified. These esters were methyl propanoate, methyl 2-methylpropanoate, ethyl propanoate, ethyl 2-methylpropanoate, methyl pentanoate, propyl butanoate. ethyl pentanoate, methyl hexanoate, ethyl hexanoate and ethyl benzoate. With the exception of ethyl 2- methylpropanoate these compounds were either not measurable or were barely measurable in the under-ripe and ripe fruit.

The aroma of under-ripe kiwifruit had a green note but on softening, the aroma attained a fruity character. Very over-ripe fruit had a strong aroma which many people dislike. The present results show that with increasing ripeness there was a rapid increase in the total amount of volatiles, and a change in the relative levels of the components (Figure 2). The levels of

354 H. Young and V. J. Paterson

I 21

1 1 I I I I 10 20 30 40 50 0

Time (min)

Figure 1. Gas chromatogram of freshly harvested, over-ripe kiwifruit aroma volatiles. See Table 1 for the identity of the peaks and experimental for g.c. conditions.

Table 1. Identity of aroma volatiles found in kiwifruit

1 Ethyl acetate 2 Methyl propanoateimethyl 2-methylpropanoate" 3 Ethanol 4 Ethyl propanoate" 5 Ethyl 2-methylpropanoate" 6 Methyl butanoate 7 Pent-1-en-3-one 8 Ethyl butanoate 9 Ilexanal

10 Methyl pentanoate" 11 Propyl butanoate 12 Ethyl pentanoate 13 Pent-2-enal 14 Pent-1-en-3-01 15 Limonene" 16 Methyl hexanoate 17 cia-Hex-2-enal" 18 trans-Hex-2-enal 19 Ethyl hexanoate 20 Pentan-1-01 21 Hexan-1-01 22 cu-Hex-3-en-] -01 23 trans-Hex-3-en-1-01' 24 trans-Hex-2-en-1-01 25 Methyl benzoate 26 Ethyl benzoate

"The detector response of a related compound was used for these compounds

aldehydes, especially trans-hex-2-enal which gave the fruit a green character, decreased while the ester levels, which provided the fruity aroma, increased with increasing ripeness. In the under-ripe fruit there were only small amounts of esters present but these levels increased dramatically with ripening. As a typical example, the total esters of freshly harvested fruit picked at 8.O"B (unless indicated otherwise the "B refers to value measured at harvest) went from 33

Effects of harvest maturity, ripeness and storage on kiwifruit arnma 355

Table 2. Levels of aroma volatiles in freshly harvested kiwifruit'

Maturity ("Brix)

Keyb

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

~

UR

36.5 3.9

11.4 7.9

15.6 45.5

141.2 75.9

708.1 tr tr tr tr 17.9

7.1 100.3

4705.2 tr 21.0 19.3 0.6

tr 52.2 17.9 1.8

-

R

57.1 9.2

13.1 10.4 11.1

446.1 69.8

290.7 322.1

tr tr tr tr 11.6 tr 6.8

72.6 2385.9

3.2 0.9

20.6 tr tr 51.8

112.9 13.0

~

OR UR

877.0 51.4 15.9 5.7 47.0 7.8 55.8 4.1 tr 9.8

3072.7 149.9 30.7 67.6

5656.3 56.9 195.5 406.8 tr tr tr tr tr tr tr tr 10.0 10.4 tr 1.2 15.7 5.2

42.1 69.4 1240.9 2435.1

54.4 tr 0.8 0.7

17.2 19.6 0.9 0.9 3.5 2.4

35.6 44.9 164.5 36.1 99.1 1.6

R

292.9 14.5 26.1 12.9 6.1

1048.1 37.8

711.6 240.9

tr tr tr tr 8.6

tr 6.4

40.3 1385.7

7.1 tr 21.5

1.5 2.0

46.4 145.6 33.3

~~

OR

540.2 13.0 42.2 36.2 tr

2824.1 39.8

4159.9 212.7

tr tr tr tr

~

8.4 2.2

16.9 36.4

1239.3 43.4 tr 14.0 0.7

tr 52.1

143.1 94.6

UR

108.2 6.0

10.9 11.0 tr

127.9 43.7 64.2

153.7 41.9 tr tr tr 7.8

4.3 58.6

1434.1 tr tr 20.4

0.9 tr 49.2 11.3 3.2

-

10.8

R OR

258.3 12.4 16.3 19.2 2.8

1831.6 37.1

1240.1 83.4 32.4 tr tr tr 5.7

9.4 32.9

997.2 14.6 tr 21.1

0.8 tr 38.2 82.9 21.6

-

445.0 25.1 38.1 43.2 tr

6230.7 31.0

5799.1 114.6 44.6 tr tr tr 5.8

23.1 33.0

924.4 21.9 tr 14.8 tr tr 22.0

311.2 119.7

-

"Values for 5.7"B fruit are not available owing to instrumental problems. bKey refers to that in Table 1. Levels are given inpg per 100g.f.w. (tr=below integrator threshold but detectable on the

chromatogram, -=not detectable) and are the average of two duplicate sub-samples. Variations between duplicates were less than 5%.

UR = under-ripe; R =ripe; OR =over-ripe.

(under-ripe) to 787pg (over-ripe) per lOOg fresh weight (g.f.w.). This change took place over a period of 4 days at 22°C. The levels of some of the minor esters increased from barely measurable (less than 10pg per 100g.f.w.) to over 50pg per 100g.f.w. Ethyl 2-methylpropanoate was the exception. It decreased with increasing ripeness in fruit harvested at 6.1 and 8.O"B. With the freshly harvested 10.8"B fruit, only very small amounts were found, even in the under-ripe fruit (Table 2). In fruit stored for 9 weeks, only trace amounts were found in 5.7"B fruit but in the more mature fruit the decreasing trend with ripening was again apparent (Table 3). Fruit stored for 21 weeks also showed a decreasing trend (Table 4). It is interesting to note that ethyl 2-methylpropanoate and methyl 2-methylpropanoate (which was not resolved from methyl propanoate) were the only branched-chain compounds identified.

With the freshly harvested fruit, the levels of the esters at a given firmness, especially the more abundant methyl and ethyl butanoate, were greater in the late-pick fruit than in early-pick fruit. Methyl butanoate was the major ester in ripe fruit but when over-ripe the ratio of ethyl butanoate to methyl butanoate increased, such that it was the dominant ester in 6.1 and 8.O"B fruit.

With increasing maturity at the time of harvest the levels of the aldehydes in under-ripe and ripe fruit, namely hexanal and cis- and trans-hex-2-ena1, decreased. In fact the hex-2-enal level of under-ripe fruit picked at 10.8"B was similar to that of over-ripe early-picked fruit. In over-ripe fruit the levels of aldehydes were similar, regardless of when the fruit was picked.

Table 3. Levels of aroma volatiles found in kiwifruit stored for 9 weeks

Maturity (“Brix)

5.7 6.1 ~

10.8

Key UR R OR UR R OR UR R OR UR R OR

1 18.7 2 2.2 3 9.3 4 1.8 5 tr 6 31.8 7 46.8 8 2.8 9 175.5

10 tr 11 tr 12 tr 13 tr 14 10.4 15 16 0.1 17 63.7 18 1641.4 19 tr 20 21 18.3 22 I .0 23 0.1 24 39.5 25 14.6 26

-

-

-

245.7 17.8 10.7 4.2

671.2 50.4

103.5 208.9

17.6 tr tr t r

-

9.6

5.9 53.5

1762.4 2.8

23.7 0.6 6.5

43.9 193.5 60.9

-

-

2328.8 83.1 3.7 10.5

149.2 16.7 24.1 tr tr 8.4

544.2 54.2 25.4 70.1

1859.3 11.2 123.8 265.8

t r tr tr tr tr tr tr 5.5 12.8 1.2 tr 4.8 tr

36.1 71.7 1139.9 2018.2

67.2 3.5 2.1

16.6 33.8 2.4 2.2 0.1 10.5

41.9 88.7 97.5 6.7

107.4 2.0

-

-

376.5 7.8

33.1 13.0 1.8

1019.0 26.7

501.5 110.0

tr tr tr tr 5.6

tr 3.7

34.2 711.2

6.8 tr 17.3 1.8 1.4

32.4 84.4 17.6

1031.6 22.0 34.9 3.1 52.0 1.7 37.1 2.5 t r 1.2

2296.8 24.6 37.9 29.9

3116.0 2.1 131.3 131.4

tr t r tr tr tr tr tr 7.1 5.0

t r 9.7 0.1

37.1 24.7 1064.6 827.7

34.0 tr tr 0.9 16.1 15.8 0.8 1.1 1.3 1.1

23.8 41.0 139.9 3.0 92.6 6.3

-

-

189.9 30.2

7.3 tr

2331.3 45.3

229.0 257.3

0.2 tr tr tr 10.2 0.2

16.0 66.2

1459.8 1.2

tr 28.7

1.2 2.1

49.6 98.8 6.2

10.3

686.1 5.6 8.7 17.2 1.8 8.3 27.1 1.8 5.9 19.4 1.9 7.3 tr 4.8 tr

3075.7 17.6 707.2 47.9 20.3 20.8

1255.2 tr 160.1 181.8 114.5 88.6

tr t r tr tr tr t r tr tr tr tr

tr tr

2.4 tr

6.6 2.9 3.6

10.9 - 8.4 33.0 18.2 22.1

895.9 693.3 441.3 7.5 - 0.9

tr tr 1.1 17.0 8.8 24.7 0.8 0.9 1.3 1.3 1.3 0.7

25.9 23.7 31.5 115.0 7.0 19.9 31.7 - 4.2

-

191.5 10.0 8.7

15.1 t i

4663.5 11.7

1730.3 12.2 5.8

tr tr tr 3.3

14.2 11.14

523.0 8.5

13.3 tr tr 17.4

170.6 36.4

-

-

See Table 2 for details.

Table 4. Levels of aroma volatiles found in kiwifruit stored for 21 weeks

Maturity (“Brix)

5.7 6.1 ~

10.8

Key UR R OR UR R OR UR R OR UR R OR

1 45.0 356.9 2 tr tr 3 128.9 59.6 4 3.5 11.5 5 5.1 0.7 6 - 68.5 7 29.9 18.2 8 8.0 368.2 9 135.6 81.1

- - i n 11 tr tr 12 tr tr 13 tr tr 14 3.1 2.8 15 16 17 35.7 21.6 18 748.7 535.0 19 - 5.6 20 tr tr 21 6.5 6.6 22 tr tr 23 tr 0.3 24 14.1 16.3 25 4.9 5.3 26 0.9 3.3

- - - -

See Table 2 for details

1124.6 96.3 tr 4.4

133.0 41.4 15.7 2.6 1.5 0.3

251.6 60.5 26.9 20.9

1433.1 37.0 75.6 100.5 tr tr tr tr tr tr t r 2.1 2.6

tr 1.5 1.7

17.4 22.6 471.8 529.4 27.8 -

0.6 tr 5.8 5.8 0.7 0.1 0.4 Q.3

14.7 10.7 3.1 3.9 7.3 0.5

-

-

143.8 6.0

42.4 4.6 tr

107.3 24.5 62.9 95.5

tr tr tr tr 3.1 tr 2.6

17.8 499.2

0.5 0.4 6.7 0.3 0.6

11.3 5.1 1.1

522.4 2.3

50.1 12.8 tr

488.4 14.8

964.6 89.2 tr tr tr tr 2.6

3.2 15.1

408.2 16.1 0.8 7.7 0.7 0.6

12.5 9.9 7.6

-

h.5 5.1 8.5 tr 8.5

25.5 51.7 3.3

211.7

tr tr tr 7.3

-

- -

62.6 1239.9

tr 15.9 2.4 tr

43.2 7.7

-

-

195.1 7.9

21.4 14.0 2.9

163.2 27.8

129.5 197.6

tr tr tr 5.0

5.0 42.0

931.8 tr 0.6

16.8 1.5 0.7

33.3 18.2 6.7

-

-

494.0 8.6 232.5 43.1 3.0 5.8 43.4 7.4 9.8 19.4 2.9 10.7 tr 4.5 tr

1712.3 5.4 740.0 34.3 16.9 16.7

703.3 4.3 410.4 209.6 81.2 99.0

t i tr tr tr tr t r tr tr tr tr tr tr 2.7 2.7 2.1

tr 9.2 - 4.5

32.2 22.9 17.8 983.1 477.8 488.8

8.2 - 3.0 1.7 0.5 tr

18.4 8.5 13.5 2.1 0.6 1.0 1.8 tr 0.9

31.5 20.9 20.4 41.6 tr 8.4 5.2 - 5.4

- -

56.4 21.8 32.6 12.8

4776.5 15.7

1980.5 109.3

5.0 tr tr tr

-

2.5

12.5 17.8

449.6

tr 11.9 0.7

16.9 53.8 27.2

-

10.3

1 .n

Effects of harvest maturity, ripeness and storage on kiwifruit aroma

150-

120-

90 -

60 -

30 -

357

M a t u r i t y ('B) 6. I 0.0 10.8

Figure 2. Levels of aroma volatiles of freshly harvested kiwifruit at increasing harvest maturities, and different stages of ripeness. UR=under-ripe; R=ripe; OR=over-ripe. B=Total volatiles; m=total esters; m=total aldehydes.

5 0

40

N

0 x 30

I

e w 20

- I .

L. '3 0

10

r

Matur i ty ('8) 5.7 6. I 8.0 10.8

Figure 3. Levels of aroma volatiles of ripe kiwifruit at different harvest maturities and after increasing storage time. O=total volatiles; @=total esters; B=total aldehydes.

358 H. Young and V. J. Paterson

Stored fruit were sampled at 9 and 21 weeks. Longer than 18 weeks is considered as long term storage.6 Fruit harvested at different maturities showed different trends in the levels of volatiles, after storage. With 5.7 and 6.1"B fruit the total volatiles decreased with increasing storage time. The 8.O"B fruit showed a slight increase in total volatiles after 9 weeks but decreased to less than half after 21 weeks. There was a large drop in total volatiles in the 10.8"B fruit after 9 and 21 weeks storage (Figure 3).

With ripe fruit the total aldehyde levels decreased with increasing storage time, with the exception of 8.O"B fruit which showed a slightly increased level after 9 weeks of storage. After 21 weeks, however, this also decreased to a level lower than that in freshly harvested fruit. The total esters, which consisted mainly of methyl and ethyl butanoate, increased after 9 weeks storage for 6.1 and 8.0"B fruit but dropped to well below that of the freshly harvested fruit after 21 weeks storage. Total esters in 10.8"B fruit dropped to less than a third of those in freshly harvested fruit after the first 9 weeks (Figure 3). After 9 weeks storage ethyl butanoate was the major ester in over-ripe fruit harvested at lower maturity levels (5.7 and 6.1"B). At the end of 21 weeks storage ethyl butanoate was the major ester found in ripe 5.7"B fruit as well as over-ripe 5.7 and 6.1"B fruit. It would appear that increasing ripeness and storage time favour higher levels of the ethyl ester, but increasing harvest maturity favours higher levels of the methyl ester.

The levels of the two aromatic esters identified, methyl and ethyl benzoate, showed similar trends to those of other esters, decreasing after 21 weeks to less than 20% of the levels in freshly harvested fruit (Tables 2 4 ) .

Pratt and Reid3 found that the ethylene-induced respiratory response of harvested kiwifruit decreased with increasing storage time. They suggested that the fruit may be undergoing a slow ripening process during storage. This hypothesis is supported by the fact that the softening process during storage is accompanied by an increase in soluble solids level^.^ If aroma compounds were released during this slow ripening process they would be lost and this could explain the lower levels of aroma compounds found in the stored fruit. Further studies would be required to determine whether the softening process during storage is the same as that triggered by Ethrel at 22°C.

In conclusion, kiwifruit show a dramatic increase in the levels of aroma volatiles over a short period of ripening. Fruit which were immature when harvested, not only had lower levels of aroma volatiles, they also had a very high aldehyde to ester ratio giving these fruit an intense green note. While kiwifruit can be easily stored for extended periods without showing any visual deterioration, the levels of the aroma volatiles in ripened fruit were considerably reduced, compared with those of the ripened freshly harvested fruit. The loss of aroma volatiles was more pronounced in fruit picked at low and high soluble solids levels. The best fruit for short-term storage would be those picked at 8.O"B, as these fruit show the highest levels of aroma volatiles after 9 weeks. The 10.8"B fruit, even after short term storage, did not maintain the high levels found in freshly harvested fruit.

References 1.

2.

3.

4.

Young, H.: Paterson, V. J.; Burns. D. J . W. Volatile aroma constituents of kiwifruit. J . Sci. Food Agric. 1983. 34, 81-85. Reid, M. S.: Heatherbell, D. A,: Pratt, H. K. Seasonal pattern in chemical composition of thc fruit of Actinidia chinemis. J. A m . SOC. Hort. Sci. 1982, 107, 316319. Pratt, H. K.; Reid, M.S. Chinese gooseberry: seasonal patterns in fruit growth and maturation. ripening, respiration and the role of ethylene. J . Scr. Food Agric. 1974, 25, 747-757. Wright, H. B.; Heatherbell, D. A . A study of respiratory trends and some associated physio-chemical changes of Chinese gooseberry fruit Actinidia chinensis (Yang-tao) during the later stages of development. N.Z. J . Agric. Res. 1967, 10, 405-414.

5. Harris, C. M.; Covey, H . M.; Harvey, J . M. Effect of harvest date, storage period, and ripening time on the quality of Chinese gooseberries. USDA, Market Res. R e p . 1972, no. 940.

6. Harman, J . E.; Hopkirk. G. Kiwifruit testing for maturity. Horticulture Produce and Practice (HPP213 Is[ revise). Ministry of Agriculture and Fisheries. Wellington, New Zealand. 1082.

7. Young, H . Direct desorption of traps for capillary column gas chromatography. J . Chromatogr. 1981, 214, 197-201.