Chitosan Coating Effect on Storability and Quality of Fresh Strawberries

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Chitosan Coating Effect on Storability and Quality

of Fresh Strawberries

AHMED EL GHAOUTH, JOSE PH AWL, RATHY PONNAMPALAM, and MARCEL BOULET

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ABSTRACT

The effect of chitosan coating (1.0 and 1.5% w/v) in controlling decay

of strawberries at 13C was investigated as compared to a fungicide,

iprodione (Rovralm). Chitosan coating significantly reduced decay of

berries (PsO.05) compared to the control. There was no significant

difference between chitosan and fungicide treatments up to 21 days

storage. Thereafter, Rovrala-t reated berries decayed at a higher rate

than chitosan-coated berries. Chitosan-coated berries stored at 4C

were firmer, higher in titratable acidity, and synthesized anthocyanin

at a slower rate than Rovrala-treated or nontreated berries. Chitosan

coating decreased espiration rate of the berries with a greater effect

at higher concentration.

INTRODUCTION

STRAWBERRY FRUIT (Fragatiu ananussu) is highly per-

ishable and its storage ife is often terminated by fungal infec-

tion causedby Botrytis cinereu and Rhizopus sp. (Maas, 1981).

The most prevalent method of maintaining quality and con-

trolling decay of st rawberries is by rapid cooling after harvest

and storage at low temperatures, typically 1C with high hu-

midity. Since effective control of temperature during transit

and storage of strawberries is difficult, other means of pres-

ervation have been sought. Because strawberries can tolerate

elevated CO, atmosphere, they are transported in pallet-bags

under high COZ (Bell, 1986). Although high CO* controls de-

cay (El Kazzaz et al., 1983), prolonged exposure to CO1 can

cause development of off-fl avors (Woodward and Topping,

1972).

Postharvest decay of strawberries can also be controlled by

application of fungicides (Jordan, 1973; Aharani and Barkai-

Golan, 1987). However, fungicides leave residues and the

number of fungicide-tolerant postharvest pathogens s growing

(Spotts and Cervantes, 1986). Thus, efforts have been made

to replace fungicides by natural products or to intensify natural

defensesof the tissue to control decay and prolong storage ife

(Adikaram et al., 1988; Boulet et al., 1989).

Recently, semi-permeable coatings have been advanced to

improve storability of perishable crops (Lowing and Cutts,

1982). Application of a blend of sucrose esters

of fatty acids and sodium carboxymethyl cellulose) to bananas

delayed ripening by modifying the internal atmospheres Banks,

1984). Delay of ripening was also reported in pears and apples

coated with c


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60-

Time ( days )

Fig. 1 Effect of chitosan coating, 1.0% (o), 1.5% (o), and RovraP

(iprodione) treatment @) on the decay control of strawberries

stored at 13C compa red to the control (water-treated) berries

e). Mean s of four replicates. Vertical bars represent LSD at 5%

level among treatments.

Quality attributes

The effect of chitosan coating and Rovrale on quality was assesse d

separatelywith non-inoculatedberries. They were dipped in Rovral@

suspension 100 ug/mL) or in chitosan solutions (1.0 a nd 1.5% w/v)

or in water control and stored at 4C as described. All treatments

contained0.1% Tween 80. Each treatment had 4 replicates, and each

replicate consistedof 70 berries. Quality of the berries was assesse d

eachweek. A sampleof 15-20 berries n total was randomly removed

from each reatment and analyzed or firmness, titratable acidity and

anthocyanincontent. To determine firmness, the berries were sliced

into halves and each half was punch tested. The penetration force

(Newton) of the flesh was measuredwith an Instron Universal Testing

Machine (Model 1101, Instron Corp., Canton, MA) using a 4 mm

flat plunger (Holt, 1970). Titratable acidity was expressedas mg of

citric acid/100mL. Acidity was determinedusing 10 g aliquot of puree

in 40 mL deionized water and titrating with 0.1 N NaOH to an end-

point of pH 8.1. Anthocyanins were extractedwith acidified ethanol

from a 2 g aliquot of a homogenateof 7 berries, according to t he

method of Fuleki an d Francis (1968). Anthocyanin content was ex-

pressedas mg anthocyanin/lOOg trawberry homogenate.The quality

evaluation data were subjected o Analysis of Variance (ANOVA).

Respiration

The respiration rate of strawberriesat 4C was determinedby plac-

ing them (120 g) in an air tight container (1.35-L) for 2 to 4 hr. Then

a 5 mL gassamplewas withdrawn with a gas ight hypodermic syringe

and analyzed or CO* using a gas chromatograph equipped with a

TCD detector and a Poropak N Column. Four replicatesof each reat-

ment were analyzed.

RESULTS & DISCUSSION

Antifungal effect of chitosan coating and RovraP

Decay of strawberries was reduced significantly (P~0.05)

when inoculated-berrieswere either coated with chitosan or

dipped in Rovral@ Fig. 1). The early signs of m old develop-

ment in the strawberries, regardlessof the treatment, appeared

after 8 day s storage at 13C. After 21 day s at 13C, the per-

centages f decayed-berriesn c hitosan-coated 1.0 a nd 1.5%

w/v) and Rovral@-treated ere 11, 10 and 13% respectively,

while in t he control it w as 52%. Ther e was no significant

difference betweenchitosan and Rovral@reatment n control-

ling decay before 21 d storage. Thereafter, however, the Ro-

vral@-treated berries decayed at a slightly higher rate than the

chitosan-coated erries suggesting he active ingredient of the

fungicide could have been inactivated by the host. Further-

more, Rovral@-treated erries showed symptoms of phytotox-

6

2

1

0

7

14

21 26

35

TIME (days)

Fig. a-Textur e (A) and Titratable acidity (B) of control lb), Ro-

vral@-treated P), l.O%, (0) and 1 .5% (0) chitosan-co ated straw-

berries stored at 4C. Vertical bars represent SE values.

icity characteriz ed y formation of water-soakedareas.At the

end of storage 29 days), decayedberries n control was about

82% a nd in Rovrala treatment 33%. In contrast, the level of

decay in 1.0 and 1.5% chitosan-coatedberries was 22 and

19%, respectively. There was no addedbenefit to decay con-

trol by increasingconcentrationof chitosan rom 1.0 to 1.5%.

Chitosan has he capacity to inhibit growth of several ungi,

to i nduce chitinase, and to elicit phytoalexins n the host tis-

sues. Thus, the control of decay n strawberries could be at-

tributed to either the fungistatic property of chitosan per se or

to its ability to induce defenseenzyme s i.e. chitinase and p-

1,3-glucanase)and phytoalexins in plants or a combination.

Whatever the mode of action, chit osan proved more effective

than Rovral@ n controlling decay of strawberries at 13C.

Effect on quality attributes

Chitosan coating had a beneficial effect on flesh firmness,

titratable acidity and retarding synthesis of anthocyanin of

strawberries stored at 4C (Fig. 2 and 3). Those coated with

chitosan (1.0 and 1.5%), after 31 d were firmer and higher in

titratable acidity than the control or Rovral@-treated erries

(Fig. 2). Increasing chitosan concentration did not result in

any increase n retention of firmness or modif y titratable acid-

ity. Rovral@reatment was effective in controlling decay , but

did not i mprove firmness or acidity as compared o the control.

Chitosan coating delayed rate of ripening a s indicated by

anthocyanincontent (Fig. 3). Th e total anthocyani ncontent of

chitosan-coatedberries was the least amon g the treatments,

after 31 days storage. In addition, t he anthocyanin content of

Rovral-treated berries was greater than that of the control.

Volume 56, No. 6, 1991-JOURNAL OF FOOD SCIENCE-1619


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CHITOSAN COATING OF STRAWBERRIES. , . .

HITOSAN COATING OF STRAWBERRIES. , . .

35 -5

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g 2525

5 20-0-

E

s 15-5-

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5 lo-lo-

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E 5--

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v I

5

lb 1; 2b 2; 3b 3; 4'0 4;

Days in storage

Fig. 3-Anthocyanin content of control @), RovraP-treated @),

1.0% (0) and 1.5% (e) chitosan-coatedstrawberries stored at 4C.

Vertical bars represent SE values.

13

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AZ

. 11

y

ii g

V

c

.o

5

5

t

7

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8

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2 4 6 6 10 12 14 16

Days in storage

Fig. 4-CO, production of control @), 1.0% (0) and 1.5% (a) chi-

tosan-coated strawberries stored at 4C. Vertical bars represent

SE values.

This suggested hat RovraP treatment may have stimulated the

ripening process. On the contrary, chitosan-coated berries syn-

thesized anthocyanin at a slower rate, and coating neither af-

fected appearance nor caused apparent phytotoxicity.

Furthermore, chitosan-coated berries developed the full red

color after 35 days storage. Retention of firmness, higher ti-

tratable acidity and slower rates of anthocyanin production in

coated berries demonstrated that chitosan coating slowed down

metabolism and prolonged the storage life. Coating fruits with

semi-permeable film has generally been shown to retard rip-

ening by modifying the endogenous CO*, O2 and ethylene

levels of fruits (Lowings and Cutts, 1982). However, speci fic

instances of interference with normal fruit ripening have been

reported with

Chitosan Coating Effect on Storability and Quality of Fresh Strawberries

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