Fruit Storage Research Laboratory

KIRYAT SHMONA 10200 ISRAEL

STORAGE OF AVOCADO FRUITS IN MODIFIED ATMOSPHERE PACKAGES

Fruit Laboratory: O. Nerya, A. Gizis, A. Zvilling, N. Ezov, R. Fogel-Sagee, A. Sharabi-Nov, R. Ben-Arie

Avocado-Gal: Y. Mizrahi Shaham, Ministry of Agriculture: M. Noi
 Introduction

Preliminary observations and personal communication (Ami Kenan – Zemach packing house), it appeared that avocado fruits could maintain their external and internal quality for more than 3 weeks, if they were packed in Bio-Fresh bags, manufactured by impregnating polyethylene with an ethylene absorbent at Grofit Plastics, Israel . These findings are supported by previous work, which demonstrated the efficacy of controlled atmosphere storage for avocado (Reznitski and Ziv, 1989; Meir et al., 1995) and a recent report that ethylene is conducive to gray discoloration of avocado pulp.

The objective of the present study was to enable the storage of the Fuerte and Hass avocado cvs. for at least 5 weeks after harvest and to obtain firm, healthy fruit without internal discoloration, which would ripen satisfactorily after shelf-life.
 Two trials were conducted, one with each cv., at the beginning of March, at the Avocado-Gal packing house.

Fuerte – Size 14 fruits were sorted, waxed, dried and packed on March 5, 2000, one day after harvest from 3 replicate orchards: Snir, Dan and Mahanaim. The packing treatments were as follows:

1. Control – bare fruit.
2. Bio-Fresh bag, folded.
3. Bio-Fresh bag, tied.

The fruit was stored at 6^oC for 32 or 39 days. The average fruit temperature in the unlinedboxes was 6.0 ±0.4^oC and in the lined boxes – 6.3 ± 0.1^oC. After each storage period, 4 boxes of fruit per treatment from each orchard were transferred to shelf-life conditions at 20^oC for 5 days. The fruit was removed from half of the bags prior to transfer to shelf-life. The remaining bags were transferred to 20^oC for 5 days, without being opened and held for another 5 days, after being opened. In all cases, the composition of the atmosphere within the bags, including ethylene, was assayed prior to opening.

Upon removal from cold storage the quality of the fruit in the opened bags was evaluated and 3 fruits were cut longitudinally, to assess their internal condition. Fruit assessment included: weight loss, decay, marketable fruit, hand firmness. After shelf-life all fruit were cut to assess discoloration.

Hass – Size 18 fruits were sorted, waxed, dried and packed on March 6, 2000, one day after harvest from 2 replicate orchards: Amiad and Daphna. The packing treatments were as follows:

1. Control – bare fruit.
2. Bio-Fresh bag, folded.
3. Bio-Fresh bag, tied.
4. LDPE bag, folded
5. LDPE bag, tied.

The fruit was stacked on pallets and stored in the commercial store of the packing house at 6^oC. Sample boxes (1-5/treatment/orchard) were transferred to the Fruit Lab after 6 and 7 weeks. Upon removal from storage the atmosphere in the bags was assayed, the bags wereopened and the fruit examines as described above, prior to transfer to shelf-life. In addition, fruit color was evaluated as green, turning or brown.
 

 
 Results

Fuerte

The composition of the atmosphere within the folded and tied bags is described in Fig. 1. O₂ levels, which were above 15% in folded bags during storage and declined to below 10% during shelf-life. CO₂ rose to 3% during storage and exceeded 5% during shelf-life. In the tied bags O₂ was generally between 5-7 during storage % and 3-4% during shelf-life, with CO₂ increasing to 6% and 7-8%, respectively. Ethylene accumulation in the bags, during both storage and shelf-life, was below the level of detection (10 ppb).

Figure 1. O₂ and CO₂ levels in folded and tied Bio-Fresh bags after storage at 6^oC 5 days’ shelf-life at 20^oC.
 Weight loss

Weight loss of fruit in the Bio-Fresh bags was significantly reduced, compared to the control (Table 1), irrespective of the method of closure. Moreover, there was no increase in weight loss of the bagged fruit during the last week of storage, in comparison with anaccelerated rate in the un-bagged fruit.

Table 1: Percentage of weight loss of Fuerte avocados stored at 6^oC (initial weight - 287g).

Treatment	32 days’ storage	39 days’ storage
Control	3.9A	5.1a
Bio-Fresh, folded	1.2b	1.2b
Bio-Fresh, tied	0.9b	0.9b

Fruit quality at removal from storage

After 32 days' storage, fruit from all treatments was firm and had a good appearance, i.e. was marketable. However, only about 30% of the fruit from the control and the folded bags did not have pulp discoloration (Table 2). In the tied bags discoloration was significantly reduced in both extent and severity. After an additional week’s storage, only fruit in the tied bags remained firm and marketable, but there was a slight increase in the extent of discoloration.

After removal of the bags and transfer to shelf-life, fruit from the tied bags softened, but also decayed to a considerable extent, even though this was less than in the control fruit. Most of the fruit, in all treatments, had pulp discoloration, but it was much less severe in fruit that had been stored in the tied bags.

In bags that remained tied during 5 days’ shelf-life, the CO₂ level rose to 8% and O₂ decreased to 3% (Fig. 1), without any ethylene accumulation. The fruit did not soften or decay under these conditions, but pulp discoloration increased, especially after removal of the fruit from the bags, when the fruit also softened rapidly. Fruit that remained in the folded bags during shelf-life softened even more and the development of decay and discolorationwas more extensive than in the un-bagged control fruit (data not shown).

Table 2. Quality of Fuerte avocado fruits at removal from storage at 6^oC and after 5 days’ shelf-life at 20^oC, without bags.

Storage duration: days at  6oC 20oC		Treatment	Firmness*	Decay (%)	Marketable (%)	Healthy pulp(%)	Discoloration index **
32	0	Control	3.37b	0	100a	30.6b	1.46a
		Bio-Fresh, folded	3.48b	0	100a	30.6b	1.54a
		Bio-Fresh, tied	4.00a	0	100a	52.8a	1.18b
32	5	Control	1.33b	84.1a	15.9a	11.1a	2.56a
		Bio-Fresh, folded	1.33b	83.2a	1.5a	10.9a	2.50a
		Bio-Fresh, tied	2.61a	34.2b	25.3a	11.2a	1.62b
39	0	Control	2.05b	8.8a	38.6b	7.7	2.33
		Bio-Fresh, folded	2.05b	0.0b	29.8b	--	--
		Bio-Fresh, tied	4.00a	0.6b	95.4a	5.6	1.61
39	4	Control	1..04b	60.4a	33.5b	3.5a	3.12a
		Bio-Fresh, folded	1.13ab	70.1a	38.0b	4.8a	3.05a
		Bio-Fresh, tied	1.31a	4.6b	95.8a	6.7a	2.05

• *Firmness index: 4= firm fruit, 1= soft fruit
• **Discoloration index: 1=slight (, 25%), 2=moderate (25-50%), 4=severe (>50% of the cut surface area).
• a-b Different letters indicate significant differences between numbers within the same column, at p<0.05.

Hass

[The results for this part of the trial are incomplete due to the loss of replicates from the commercial storage facility and the mistaken inclusion of fruits from another cv. in some replicates.]

During 2 months’ storage the level of CO₂ inside tied Bio-Fresh bags increased to 6-7% with a concomitant decrease in the O₂ level to 7-9% and a very slight accumulation of ethylene (Table 3). In comparison, the CO₂ in tied PE bags was lower and the levels of O₂ and ethylene were higher. These differences in the composition of the atmosphere surrounding the fruit evidently were the cause of the main difference in fruit quality, as expressed in the extent and severity of pulp discoloration (Table 4), the principal factor in quality deterioration during 7 weeks’ storage. Apart from this, the fruit remained firm and was apparently marketable.

After 8 weeks’ storage, the un-bagged fruit (control) was mostly soft or softeningupon removal from storage, whereas the bagged fruit was still firm. From its external appearance, all the fruit was still marketable, but its internal condition indicated pulp discoloration¸ which was only very slight in fruit from the tied Bio-Fresh bags. Following 6 days’ shelf life fruit in all treatments was soft with severe pulp discoloration and about half of it was not fit fconsumption.

Table 3: composition of the atmosphere inside Hass pakages, upon removal from storage at6^oC .

Package type	Storage duration (days)	O2 (%)	CO2 (%)	Ethylene (ppb)	No. of replicates
Bio-Fresh, folded	48	18.2	2.4	--	10
	56	18.3	2.5	ND	4
Bio-Fresh, tied	48	6.9	7.2	--	9
	56	9.0	6.0	50	5
LDPE, tied	48	14.4	5.2	--	2
	56	15.5	5.0	1500	4

ND = not detectable

Table 4. Quality of Hass avocado fruits at removal from storage at 6^oC and after shelf-life at 20^oC, without bags.

Storage duration: days at 6oC 20oC		Treatment	Firmness*	Decay (%)	Marketable (%)	Healthy pulp(%)	Discoloration index **
48	0	Control	3.58a	1.86a	100a	5.0ab	2.13a
		Bio-Fresh, folded	3.72a	1.90a	98.9a	4.0ab	2.31a
		Bio-Fresh, tied	3.89a	1.48a	99.4a	26.7a	1.15b
		LDPE tied	4.00a	1.69a	100a	0b	1.90ab
48	4	Control	1.65A	2.62A	100A	0A	3.81A
		Bio-Fresh, folded	1.45A	2.53A	99.2A	2.3A	2.53B
		Bio-Fresh, tied	1.62A	2.51A	89.1A	8.8A	1.53C
		LDPE tied	1.56A	2.50A	100A	0A	2.62B
56	0	Control	2.00b	2.06a	100a	0b	4.00a
		Bio-Fresh, folded	3.89a	2.10a	100a	0b	1.85bc
		Bio-Fresh, tied	4.00a	1.70a	100a	11.5a	1.32c
		LDPE folded	3.96a	2.01a	100a	0b	2.27b
		LDPE tied	4.00a	1.63a	100a	0b	2.24b
56	6	Control	1.30A	2.78A	60.8A	0A	4.00A
		Bio-Fresh, folded	1.39A	2.72A	49.5A	0A	3.69A
		Bio-Fresh, tied	1.59A	2.58A	61.5A	0A	1.99b
		LDPE folded	1.33A	2.41A	26.1A	0A	3.05AB
		LDPE tied	1.37A	2.31A	32.2A	0A	2.21B

• *Firmness index: 4= firm fruit, 1= soft fruit
• ** Peel color index: 1=green, 2=green+brown, 3=brown
• **Discoloration index: 1=slight (, 25%), 2=moderate (25-50%), 4=severe (>50% of the cut surface area).
• a-b, A-B: Different letters indicate significant differences between numbers within the same column, at p<0.05.

Summary

There appears to be a potential for extending the storage duration of the avocado cvs. Fuerte and Hass to 5 and 6 weeks, respectively, by MAP in Bio-Fresh bags. Albeit, the quality of the Fuerte fruit in this trial left something to be desired after 5 days’ shelf-life following 32 days’ storage at 6^oC, due to decay and pulp discoloration, the fruit used in the trial was the last of the season, known to be of inferior storage quality. MAP storage improved fruit quality in comparison with the control, and it is to be anticipated that earlier harvested fruit would be of superior quality under similar storage conditions. This assumption will be examined during the next season.

Fruit of the Hass cv. maintained satisfactory external appearance during 7 weeks’ regular storage and subsequent shelf-life. However, internal discoloration was considerable upon removal from storage. In Bio-Fresh tied bags pulp discoloration was reduced in extent and severity, but not sufficiently. From the rapid rate of deterioration that occurred during an additional week in storage, it can be deduced that fruit quality in the tied Bio-Fresh bags would have been satisfactory after only 5-6 weeks’ storage. Accumulation of 5% CO₂ in the tied LDPE bags inhibited fruit softening, but not pulp discoloration. The latter was evidently reduced as a result of both ethylene absorption and CO₂ accumulation, as in folded bags, although there was no ethylene CO₂ reached only 2.5% and discoloration occurred. These findings are supported by those of Meir et al. (1995), who showed that in controlled atmosphere storage of avocado, 8% CO₂ prevented discoloration at different O levels. Although they did not measure ethylene levels, their experiment was conducted in a flow-through system, whereby ethylene would not have accumulated. Possibly, this was the reason that they achieved good quality fruit after 7 weeks’ storage.

From the point of view of commercial application, it is important to emphasize the necessity to remove the fruit from the bags upon transfer from low to high temperature. In the above trial, fruit quality was assessed only after 5 days at 20^oC in the tied bags, when severe damage was observed. In next year’s trials it will be necessary to evaluate fruit quality after shorter terms of shelf-life, in order to determine when the fruit should be removed from the bags.