Sodium bicarbonate reduces postharvest decay development on melons

Postharvest Biology and Technology ELSEVIER

Postharvest Biology and Technology 10 (1997) 201-206

Sodium bicarbonate reduces postharvest development on melons Y. Aharoni d Dqmrtmeni

b Department

‘, E. Fallik +*, A. Cope1 ‘, M. Gil d, S. Grinberg

decay

“, J.D. Klein ’

c!fPostharoest

Science of’ Fresh Produce, Agricultural Research Organiration, The Vokani C’entrr. Bet - Dagan 50250, Israel of Fit,11 Crops, Agricultural Research Organization. The Vokani Center, Bet -Dagun 50250. Israel

Accepted 3 January 1997

Abstract Sodium bicarbonate (SBC) inhibited in vitro mycelial growth of A. ulternata, Fusurium spp. and R. stolonijkr. SBC action was fungistatic rather than fungicidal. Coating commercially harvested ‘Galia’ and ‘Ein-Dor’ melons with wax containing 2’s) SBC reduced decay incidence after storage and shelf life simulation by four to seven-fold, to a commercially acceptable level of 6&7X, compared to untreated or waxed-treated controls. This treatment also maintained the fresh and blemish-free appearance of the fruit at harvest. Higher concentrations of SBC (3%) were phytotoxic and significantly reduced general fruit appearance. A trial shipment by sea transport to Europe demonstrated that 2% SBC incorporated into a wax coating maintained the marketability of ‘Galia’ melon fruits compared to that of untreated fruit. SBC can be an alternative biocide to the fungicide imazalil, thus eliminating unwanted residues on melon fruits. 0 1997 Elsevier Science B.V. Keywords:

Cwumis

melo

var.

reticulatus;

Disease

control;

NaHCO,;

Allyl-1-2,4-Chlorophenyl-2-imidazol-l-

ylethylether

1. Introduction Both ‘Galia’ and ‘Ein-Dor’ melon (Cucumis melo var. reticulatus) cultivars are susceptible to the decay-causing fungi Alternaria alternata (Fr.) Keissler and Fusarium spp. , while ‘Ein-Dor’ is * Corresponding author. Fax: + 972 3 9683622/9604428; e-mail: [email protected]

also very susceptible to Rhizopus stolonifer (Ehr. Ex. Fr.) Lind (Barkai-Golan, 1981). Until recently, melons have been treated in the packhouse with a wax containing 2000 pi/l imazalil to control decay development during prolonged storage (Aharoni et al., 1992). However, this treatment leaves a residue of 3-5 mg/kg of imazalil on the fruit, an amount that exceeds the 0.5 mg/kg maximum permissible level (Anonymous, 1990).

0925-5214’97/$17.00 Q 1997 Elsevier Science B.V. All rights reserved. PI1 SO925-5214(97)01412-9

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Y. Aharoni et al. /Postharvest

Biology and Technology 10 (1997) 201-206

Alternative substances which are safe, effective and economical are needed to control post harvest diseases. Bicarbonate salts are widely used in the food industry at levels of up to 2% for leavening, pH-control, taste and texture development (Lindsay, 1985) and also have broad-spectrum antimicrobial activity (Miyasaki et al., 1986; Corral et al., 1988). The potential of bicarbonate salts for the control of post harvest pathogens has been demonstrated in citrus (Arimoto et al., 1977), carrot (Punja and Gaye, 1993), and bell pepper (Fallik et al., 1997). The objective of the present work was to determine the effectiveness of sodium bicarbonate in controlling fungal decay on two melon cultivars during prolonged storage and shelf life.

2. Materials and methods 2.1. In vitro tests

Sodium bicarbonate (SBC NaHCO,, Sigma) was used in all treatments. Salt solutions were filtered through a 0.45 pm millipore filter before adding them at different concentrations to autoclaved potato dextrose agar (PDA) (Difco), after the PDA had cooled to 55°C. The effect of SBC on mycelial growth of A. alternata, Fusarium spp. and R. stolonifer was tested by placing a 4 mm diameter disc from the periphery of a 10 day old culture in the center of a 9 cm Petri dish containing PDA with appropriate SBC concentrations. The cultures were incubated at 20°C for 7 days. Growth was determined as the average increase in colony diameter following incubation. The experiment was conducted three times with three replicate plates for each fungus. Since the pH of the SBC-amended medium was 8.3, control plates were adjusted to pH 8.3 with 5N NaOH. Plugs of mycelial mat from SBC-amended plates were transferred to PDA plates without SBC to determine whether the effect of SBC was fungistatic or fungicidal. Plates were incubated for 7 days at 20°C and colony diameter was measured.

2.2. In vivo tests ‘Galia’ melons (Cucumis melo var. reticulatus) were picked in the autumn of 1994 and the spring of 1995 at Ein-Tamar in the Arava desert region of Israel. ‘Ein-Dot-’ melons were picked in the summers of 1993 and 1994 at Kibutz Yifat in the Yizrael Valley. Six melons each were packed in cartons, and transferred to Bet-Dagan where they were randomly divided into groups of five cartons (30 melons) per treatment. The fruits were removed from the cartons and hand-sprayed until completely coated with Zivdar wax (SafePack, Raanana), designed specifically for melons. The wax contained 1, 2 or 3% SBC (w/v), or 2000 pi/l (active ingredient) imazalil (imazalil sulphate 50%, Allyl- 1-2,4-Chlorophenyl-2-imidazol- lJanssen, ylethylether). After spraying and drying at ambient temperature, the melons were repacked in the cartons and stored for 14 days at 3°C and 4 additional days of shelf life at 20°C (corresponding to sea freight delivery from Israel to Europe and retail marketing). The relative humidity at both temperatures was 85%. Fruit quality was evaluated after storage and shelf life according to the following parameters: General appearance: This was evaluated visually, with regard to the freshness of the fruit, decay and skin blemishes, on a scale of l-5, with 1, poor; 3, good; and 5, excellent quality. Fruit with a rating of less than 2.5 was considered unfit for marketing. Fruit firmness: This was measured with a Chatillon penetrometer equipped with a 6 mm plunger (John Chatillon and Sons, NY 11415), which penetrated through the peel into the pulp. Five fruits were tested each time, on opposite sides on each fruit. Total soluble solids (TSS): This was measured in the same five fruits tested for firmness, by removing a segment of flesh and allowing its juice to drop onto an Atago digital refractometer (Atago, 3210 Huncho, Tokyo 173). Decay incidence: Fruit was considered decayed once fungal mycelia appeared on the peel. Results are shown as percentage of fruit with decay.

Y. Aharoni

2.3. Trial shipment

et al. / Postharcest

Biology

to Europe by seu transport

A trial shipment to Rotterdam, Holland was conducted in May, 1995 in order to confirm the in vivo laboratory results. ‘Galia’ melons were harvested in the Arava valley and treated in a commercial packing house with 2% SBC incorporated into wax. Untreated melons served as controls. Six melons were packed after treatment in a shipping carton and cartons were palletised on six pallets (each contained 130 cartons per pallet). Each pallet contained 20 untreated and treated cartons that were palletised randomly. Fruit were maintained at 7°C during transport from Israel to Rotterdam. Temperatures were recorded with two Delta Trak temperature recorders (Delta Trak, US) in the middle of each pallet. After 13 days storage (from the time of packing until storage in Rotterdam), melons in 20 treated and untreated cartons were randomly evaluated for quality, while another 20 treated and untreated cartons were transferred to 20°C for 3 days of marketing simulation. Fruit were then evaluated for quality as follows: general appearance was evaluated as previously described; firmness was evaluated by hand on a scale of l-4: 1, very soft: 2, soft; 3, firm; and 4, very firm. Decay incidence was recorded once fungal mycelia appeared on the peel. Results are shown as percentage of fruit with decay. Results were analyzed using Duncan’s multiple range test at P = 0.05. The in vivo results presented are the means of six experiments with ‘Galia’ melons (autumn and spring) and six with ‘Ein-Dor’ melons (summer), since similar trends were observed during different growing seasons. Angular transformation was used for analysis of decay incidence.

3. Results

and Trchnolog~

10 (1997)

Fusarium spp. (Fig. 1). The effective concentrations of SBC that inhibited 50% of the mycelial growth (EC,,) of Rhizopus, Alternaria and Fusurium were 0.3, 0.85 and 1.35%, respectively (Fig. 1). Mycelial growth of Rhizopus was completely inhibited at a concentration of 0.5%. while growth of Alternariu and Fusarium was not completely inhibited even at 3”/0 SBC (Fig. 1). The action of SBC was fungistatic rather than fungicidal in inhibiting Altcrnariu. Fuwrium and Rhizopus. Mycelial plugs, when transferred from SBCamended to unamended PDA, grew similarly to plugs of unamended controls (Table 1). 3.2. In riro Decay incidence after 14 days storage was relatively high (lo”/,) in untreated ‘Galia’ melons. Waxing the fruit reduced decay incidence by 40%, while 1% SBC reduced it by 50% (Table 2). Treatments of 2 or 3”/0 SBC and imazalil eliminated decay. However, 3% SBC caused severe blemishes to the peel on about l/3 of the melons, resulting in a general appearance rating of 2.0 (Table 2). The untreated and wax only ‘Ein-Dor’ melons had less decay than ‘Galia’, and no decay was found on melons treated with 1, 2 and 3 SBC or imazalil after 14 days of storage (Table 2). All fruit was marketable according to general appearance, except fruit that was treated with 3% SBC (Table 2).

9’ oi, 0.0

I,

0.5

1.0

Concentration

3. I. In vitro tests

3.0

2.0

.,.I

(X)

Fig. I. Inhibition of mycelial mat growth of Alrernuricl trltcrspp. and Rhizopus stolonijer at different sodium bicarbonate concentrations after 7 days incubation at 20°C (bars = k SE.). nata, Fusurium

In general, Rhizopus stolonfir was more sensitive to SBC than were Alternaria alternatu and

203

201~ 206

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Y. Aharoni et al. /Postharvest

Biology and Technology 10 (1997) 201-206

Table 1 Colony diameter (mm) of Alternaria alternata, Fusarium spp. and Rhizopus stolonijer before and after transfer from PDA amended with SBC to unamended PDA. Mycelial plugs were incubated for 7 days at 20°C on each medium Colony diameter (mm) A. alternata

Initial SBC concentration 0 0.5 3.0

(%)

Fusarium spp.

R. stolonifer

Before

After

Before

After

Before

After

82 & 0.6 12 + 0.9

81 k 0.8

83 k 0.8

82 * 0.3

88 f 0.2 4 + 0.2

88 * 0.5 87 If: 0.6

82 + 0.5

21 i 0.3

83 + 0.6

Untreated ‘Galia’ and ‘Ein-Dot-’ melons, as well as those treated with wax only or with 1% SBC, had a high rate of decay incidence after storage and marketing simulation (Table 2). Treatments with imazalil or 2 or 3% SBC markedly reduced decay incidence. Melons treated with imazalil or 2% SBC had a high score for general appearance after storage and marketing simulation (Table 2). Untreated, wax-only and 3% SBC fruits were not suitable for marketing after storage and marketing simulation. Both ‘Galia’ and ‘Ein-Dor’ melons were very firm at the beginning of the experiments. Firmness during storage decreased from 72 to about 60 N in the ‘Galia’ and from 82 to 72 N in the ‘EinDor’ melons. Firmness decreased further during shelf life by 42% (42 N) of the initial in ‘Galia’ and 32% (56 N) in the ‘Ein-Dor’ melons. However, the melons of the two cultivars were still relatively firm. Different treatments had no significant effect on firmness. TSS of the ‘Galia’ and ‘Ein-Dor’ melons were about 12% at the beginning of the experiment. No significant change was found after storage and shelf life, and the various treatments did not affect TSS. 3.3. Trial shipment After 13 days storage no decay was found on untreated or treated fruit (Table 3). However, untreated fruit were less firm, with a significantly

lower rating for general appearance (Table 3). After an additional 3 days at 20°C treated fruit had significantly better quality as evaluated by general appearance, decay incidence and firmness, compared to untreated fruit (Table 3).

4. Discussion Sodium bicarbonate at a concentration of 2% (w/v) has potential for controlling Rhizopus, AZternaria and Fusarium decay on ‘Galia’ and ‘EinDor’ melons during prolonged storage and shelf-life, while maintaining fruit quality. In vitro exposure to SBC reduced mycelial growth of R. stolonifer, A. alternata and Fusarium spp. , the main storage pathogens of these two cultivars. The direct and indirect effects of bicarbonate salts on microorganisms have previously been noted (Punja and Grogan, 1982; Montville and Goldstein, 1989; Depasquale and Montville, 1990). The inhibitory effect of bicarbonate salts on A. alternata, a decay-causing fungus on sweet bell pepper (Capsicum annuum) as well as on melons, was probably due to the reduction in fungal cell turgor pressure which resulted in collapse and shrinkage of hyphae and spores, and consequent inability of fungi to sporulate (Fallik et al., 1997). A prestorage spray treatment of melons with wax containing 2% SBC reduced fruit decay to a

205

Y. Aharoni et (11./Postharvest Biology and Technology 10 (1997) 201-206 Table 2 Quality of ‘Galia’ and ‘Ein-Dor’ melons, treated with wax containing I4 days at 3°C and an additional 4 days at 20°C Treatment

General

appearance”

I, 2 or 3% (w/v) SBC or imazalil

Decay

(I -~5)

incidence

(2000 pl!l a.i.J, stored

(‘XI) ‘Em-Dor’

‘Galia’

‘Galia’

‘Ein-Dor‘

4.5

4.5

Untreated Wax-only

2.5 c 3.0 b

SBC I’!& SBC 2’%1 SBC 3’Y Imazali;’

3.0 4.0 2.0 4.0

b a d a

4.0 4.0 4.0 4.0 2.0 4.0

a a a a b a

IO a 6b 5b oc oc oc

1.0 I.0 I.0 3.0 2.0 4.0

d d d b c a

1.0 1.0 2.0 3.0 2.0 4.0

d d c b c a

44 a 38 ab 34 b

for

Before storage Untreated Storage

0

0

at 3°C

Marketing

simulation

SBC I’?’0 SBC 2% SBC 3% Imazalil appearance:

I, poor;

2, good;

IC

4d 2e

Treatment

General

Before storage Untreated storage

Untreated 2’1:) SBC

t wax simulation

Untreated 2% SBC t wax

‘Firmness:

a b b c c

I

d

appearance”

(1 5)

Decay

incidence

(‘XI)

Holland.

Fruit

Firmness

5

0

4

2.7 b 4.0 a

0 0

2.9 b 3.4 a

1.9 b 3.0 a

20 a 3b

2.2 b 2.1 a

quality

was

(I m4)h

at 7°C

After marketing

“General

23 IX I6 6 4

5, excellent,

Table 3 The quality of ‘Galia’ melons as tested in a trial shipment exported by sea transport to Rotterdam, evaluated I3 days after storage at 7°C (from harvest) and after an additional 3 days at 20°C

After

a b c c c c

at 20°C

Untreated Wax-only

“General

3.5 I.2 0 0 0 0

at 20°C

1, poor; 2, good; 5, excellent, I, very soft; 2, soft; 3, firm; 4, very firm

appearance:

commercially acceptable level during storage and marketing simulation. However, lo/ SBC did not control decay development, while a higher SBC concentration (3%) caused phytotoxicity which led to a lesser general appearance. Dipping sweet bell peppers (C. annuum) in 3% potassium bicarbonate, led to increased weight loss, decreased firmness and further decay (Fallik et al., 1997).

These observations may indicate that the phytotoxic effect of bicarbonate salts arises from changes in the composition of the cuticular wax, which otherwise protects against water loss. A trial shipment of ‘Galia’ melons treated with 2% SBC incorporated in the wax validated the in vivo laboratory results. Postharvest decay development was significantly reduced while the keep-

206

Y. Aharoni et al. /Postharvest

Biology and Technology

ing quality of the fruits was maintained. Bicarbonate salts have broad spectrum antimicrobial properties and are generally recognised as safe (GRAS) compounds which do not require expensive testing and validation by regulatory agencies. They are therefore very promising candidates for postharvest decay control, especially in fresh commodities to which the application of synthetic fungicides is banned.

Acknowledgements

Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 1909-E, 1996 Series.

References Aharoni, Y., Copel, A., Davidson, H. and Barkai-Golan, R., 1992. Fungicide application in water and in wax for decay control in ‘Galia’ melons. N.Z.J. Crop Hortic. Sci., 20: 177-179. Anonymous, 1990. Maximum residue limits for imazalil. Plant Protection Division, Beerse, Belgium. Arimoto, Y., Hommd, Y. and Misato, T., 1977. The effect of sodium hydrogencarbonate on the occurrence of citrus storage diseases. J. Pesticide Sci., 2: 1633 167.

10 (1997) 201-206

Barkai-Golan, R., 1981. An annotated check-list of fungi causing postharvest diseases of fruits and vegetables in Israel. Spec. Public. Agric. Res. Org., Bet Dagan, Israel. No. 194. p. 36 Corral, L.G., Post, L.S. and Montville, T.J., 1988. Antimicrobial activity of sodium bicarbonate. J. Food Sci., 53: 981-982. Depasquale, D.A. and Montville, T.J., 1990. Mechanism by which ammonium bicarbonate and ammonium sulfate inhibit mycotoxigenic fungi. Appl. Environ. Microbial., 56: 3711-3717. Fallik, E., Grinberg, S. and Ziv, O., 1997. Potassium bicarbonate reduces postharvest decay development on bell pepper fruits. J. Hort. Sci., 72: 35-41. Lindsay, R.C., 1985. In: O.R. Fennema (Editor), Food Chemistry. Marcel Decker, NY, p. 642. Miyasaki, K.T., Genco, R.J. and Wilson, M.E., 1986. Antimicrobial properties of hydrogen peroxide and sodium bicarbonate individually and in combination against selected oral gram-negative facultative bacteria. J. Dental Res., 65: 1142-1148. Montville, T.J. and Goldstein, P.K., 1989. Sodium bicarbonate inhibition of aflatoxigenesis in corn. J. Food Prot., 52: 45-48. Punja, Z.K. and Gaye, M.M., 1993. Influence of postharvest handling practices and dip treatments on development of black root rot on fresh market carrots. Plant Dis., 77: 9899995. Punja, Z.K. and Grogan, R.G., 1982. Effects of inorganic salts, carbonate-bicarbonate anions, ammonia, and the modifying influence of pH on sclerotial germination of Sclerotium rolfii. Phytopathology, 72~6355639.