Short communication
Fumigation with nitric oxide to extend the postharvest life
of strawberries
R.B.H. Wills
a,*, V.V.V. Ku
a, Y.Y. Leshem
baCentre for Food Industry Research and De6elopment,Faculty of the Central Coast,Uni6ersity of Newcastle,P.O.Box127,
Ourimbah,NSW2258,Australia
bFaculty of Life Sciences,Bar-Ilan Uni6ersity,Ramat Gan52900,Israel Received 30 April 1999; received in revised form 26 July 1999; accepted 21 August 1999
Abstract
Strawberry (Fragaria ananassaDuch. cv. Pajaro) fruit were fumigated with nitric oxide immediately after harvest. Fumigation was performed in an anaerobic nitrogen atmosphere for up to 2 h at 20°C at nitric oxide concentrations from 1.0 to 4000ml l−1then held at 20 and 5°C in air containing 0.1ml l−1ethylene, a concentration prevalent in ambient air at fruit and vegetable markets. Treatment at both temperatures extended the postharvest life of strawberries, the most pronounced effect being obtained with nitric oxide in the concentration range 5 – 10 ml l−1 which produced \50% extension in shelf life. The possibility of commercial application is discussed. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Fragaria ananassa; Strawberries; Nitric oxide; Ethylene; Postharvest life
www.elsevier.com/locate/postharvbio
1. Introduction
The free radical gas nitric oxide (NO) has com-manded considerable attention in animal research in recent years (Feldman et al., 1993). NO exerts contrasting effects on many physiological and pathological processes in mammalian tissues and has found therapeutic use to alleviate aschemia, asthma and other pulmonary hypertension
com-plications. Nevertheless benefits are concentration dependent since high endogenous levels are asso-ciated with stroke, septic shock and migraines (Schmidt and Walter, 1994; Moncada et al., 1991). The existence and direct characterisation of endogenous NO in higher plants was reported only recently by Leshem and Haramaty (1996) who found that, on a molar basis, pea foliage emitted more NO than ethylene and that the ethylene precursor aminocyclopropane-1-car-boxylic acid (ACC) enhanced both NO and ethylene emission. It has been suggested that ethylene production in growing plants may be
* Corresponding author. Tel.:+61-2-43484140; fax:+ 61-2-43484148.
E-mail address:ftrbhw@cc.newcastle.edu.au (R.B.H. Wills)
regulated by NO and that under short term envi-ronmental stress such as water deficit, heat and salinity, NO emission may act as a natural stress-coping agent (Leshem and Haramaty, 1996). Hausladen and Stamler (1998) have reviewed the role that NO plays in antimicrobial function in plants.
NO has since been found to be ubiquitous in postharvest climacteric and nonclimacteric fruit, vegetables and flowers, with higher levels present in unripe than in ripe tissues (Leshem and Wills, 1998; Leshem et al., 1998). Since ethylene accu-mulation initiates ripening of climacteric produce and enhances senescence of nonclimacteric pro-duce, it was speculated that application of NO might retard ripening and senescence in posthar-vest tissues and preliminary surveys on a range of produce have supported this contention (Leshem et al., 1998). Strawberries are a high value fruit but marketing is limited by a short postharvest life. The postharvest life can, however, be ex-tended by minimising the concentration of ethylene in the atmosphere around fruit (El-Kaz-zaz et al., 1983; Wills and Kim, 1995). In this paper we report on the ability of exogenous appli-cation of NO at a range of concentrations and application times to extend the postharvest life of strawberries.
2. Materials and methods
Hydroponically grown strawberries (Fragaria ananassa Duch. cv. Pajaro) were obtained from a farm on the Central Coast of New South Wales, Australia, during the 1997 and 1998 growing sea-sons. Fruit were transported to the laboratory within 4 h of harvest and medium size berries without deterioration were selected for experimen-tation. Each experimental unit comprised 15 strawberries (about 250 g) which were placed in a sealed 4 l plastic container. The container was flushed with humidified nitrogen gas at 700 ml l−1 for about 1 h to displace all the oxygen in the container. It is necessary to maintain a near zero oxygen level when NO is present due to its rapid oxidation to nitrogen dioxide (NO2) with a half life of 5 – 12 s (Snyder, 1992). NO was then added
into the container by injection of an aliquot of cylinder gas (BOC, Sydney) containing 4 ml NO l−1nitrogen where concentrations of up to 500ml l−1 were required. For concentrations \500 ml l−1 the containers were flushed with gas directly from the NO cylinder at 1 l min−1 until the required concentration of NO in the container was achieved. The containers remained sealed for up to 2 h at 20°C after which the fruit was stored at 20 or 5°C in glass jars ventilated with humi-dified air containing ethylene at 0.1 ml l−1
. This concentration of ethylene was chosen to simulate concentrations encountered in commercial mar-keting of strawberries (Wills and Kim, 1995). Two control treatments were included in each experi-ment comprising an untreated unit and a unit held in a nitrogen atmosphere. The quality of straw-berries was assessed daily for fruit held at 20°C and every second day at 5°C. At each examina-tion berries with mould growth, rotting, softening or colour change were removed and the percent of berries in an acceptable condition was calculated. A linear regression of the percent acceptable berries against time was derived for each treat-ment. The postharvest life was calculated from the regression equations as the time when 80% of the berries remained acceptable for marketing. Data presented in Tables 2 – 4 are the results of trials carried out with three replicates each of 15 fruit while data in Table 1 are with five replicates at 20°C and four replicates at 5°C. The results were subjected to analysis of variance and calculation of the least significant difference (L.S.D.) at the P=0.05 level.
3. Results and discussion
The strawberries were fumigated with NO at 1 and 5ml l−1
for 2 h then stored in air containing 0.1 ml l−1
exten-sion was due primarily to delayed onset of rotting and fruit softening. Since the postharvest life be-tween fruit held in air and fumigated with nitro-gen was similar, the extension in postharvest life was due to the presence of NO with no contribu-tion from a modified atmosphere effect arising from low oxygen.
The effect of a larger range of NO concentra-tions from 1 to 4000 ml l−1
was subsequently examined. The beneficial effect of fumigation with 1 and 5 ml l−1 NO was confirmed with a 60% increase in postharvest life being achieved with 5 ml l−1 NO at both 20 and 5°C (PB0.01) (Table 2). However, at higher NO concentrations the extension in postharvest life was not as great. Strawberries fumigated with 500 ml l−1 NO
showed no significant increase in postharvest life when stored at 20°C while the increase at 5°C was about 30%.. The factor limiting postharvest life at 500 ml l−1 was development of browning around the calyx. Fruit fumigated with 4000 ml l−1 NO had virtually no postharvest life with the berries rapidly developing a dull colour and severe black-ening around the calyx soon after fumigation.
Since a concentration of 5ml l−1 NO appeared optimal to extend the postharvest life of strawber-ries in both studies, the effect of fumigation with NO at 2.5 – 15ml l−1was examined. For strawber-ries held at both 20 and 5°C an NO concentration of 5 – 10 ml l−1 significantly extended postharvest life while fumigation with 2.5 and 15 ml l−1 did not (Table 3). The maximum extension in
Table 1
Postharvest life of strawberries fumigated with NO at 1 and 5ml l−1then stored at 20 and 5°C in air containing 0.1ml l−1ethylene and extension in postharvest life of NO-treated over air-stored fruit
Storage temperature (°C) Postharvest life (days) L.S.D. (P=0.05) NO (ml l−1)
Nitrogen Air
1 5
0.87
20 1.6 2.0 2.5 3.7
1.44
5 4.5 4.5 6.9 6.2
Postharvest life extension (%)
20 25 56 131
0 53 38
5
Table 2
Postharvest life of strawberries fumigated with NO at 1–4000ml l−1then stored at 20 and 5°C in air containing 0.1ml l−1ethylene and extension in postharvest life of NO-treated over air-stored fruit
Storage temperature (°C) Postharvest life (days) L.S.D. (P=0.05) Air Nitrogen NO (ml l−1)
1 5 50 500 4000
2.3 2.0 3.5 3.9
20 3.2 2.7 0.1 1.08
6.1 2.66
5 6.4 7.7 9.6 7.7 8.1 0.1
Storage life extension (%)
52
– 39 17 –
20 70
Table 3
Postharvest life of strawberries fumigated with NO at 2.5–15ml l−1then stored at 20 and 5°C in air containing 0.1ml l−1ethylene and extension in postharvest life of NO-treated over air-stored fruit
Postharvest life (days)
Storage temperature (°C) L.S.D. (P=0.05)
Air NO (ml l−1)
2.5 5 7.5 10 15
20 1.0 1.0 1.5 1.7 1.6 1.2 0.41
5 4.9 6.1 8.1 6.6 6.4 5.6 1.53
Storage life extension (%)
20C 0 50 70 60 20
5 24 65 35 31 14
Table 4
Postharvest life of strawberries fumigated with NO at 5ml l−1for varying times then stored at 20 an 5°C in air containing 0.1ml l−1ethylene and extension in postharvest life of NO-treated over air-stored fruit
Storage temperature (°C) Postharvest life (days) when fumigated with 5ml l−1NO for (h) L.S.D. (P=0.05)
0.5 1.0 1.5
0 2.0
20 1.3 1.4 1.5 1.7 2.1 0.32
5.8 6.5 6.4 6.9
5 4.7 1.22
Storage life extension (%)
8 15 31 62
20
23 38
5 36 47
postharvest life was about 70% at 20°C and 60% at 5°C. Regression analysis of the data generated a significant quadratic equation of y=0.87+ 0.16x−0.009x2; r2= +0.87 (PB0.01) at 20°C and y=5.2+0.48x−0.030x2
; r2
=0.57 (PB
0.08) at 5°C; where y=postharvest life and x= NO concentration. The equations indicate the maximum postharvest life is achieved at a fumiga-tion concentrafumiga-tion between 5 and 10 ml l−1 NO.
The effect of fumigation time on extension in postharvest life was assessed using NO at 5ml l−1 and subsequent holding of fruit at 20°C. Posthar-vest life was extended as fumigation time in-creased (Table 4); hence maximum postharvest life in this experiment was achieved with 2 h fumigation.
Application of NO at 5 – 10ml l−1
to strawber-ries for 2 h after harvest may have commercial
potential as the treatment consistently extended postharvest life \50% over fruit held in air. at
The mechanism by which NO exerts its effect is presumed to be by inhibiting ethylene action. It is of interest that NO has a similar effect to that obtained with low concentrations of the ethylene antagonist, 1-methylcyclopropene (1-mcp). Ku et al. (1999) found that the fumigation of strawber-ries with 1-mcp at 5 – 15 nl l−1 for 2 h extended the postharvest life but higher concentrations de-creased postharvest life. It is possible there is some similarity in the mode of action of NO and 1 – mcp. The concentration dependent nature of the effect of NO on strawberries is consistent with the beneficial/adverse effects found for NO in animal systems (Moncada et al., 1991).
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