Calcium for Extending the Shelf Life of Fresh Whole and Minimally

7/31/2019 Calcium for Extending the Shelf Life of Fresh Whole and Minimally

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Dublin Institute of Technology

ARROW@DIT

Articles School of Food Science and Environmental Health

2007-01-01

Calcium for Extending the Shelf Life of FreshWhole and Minimally Processed Fruits and

VegetablesAna Belen Martin-DianaDublin Institute of Technology, [email protected]

Daniel RicoDublin Institute of Technology, [email protected]

Jesus Maria FriasDublin Institute of Technology, [email protected]

J. Barat

Universidad Politecnica de Camino de Vera

Gary HenehanDublin Institute of Technology, [email protected]

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Recommended CitationMartin-Diana, A., Rico, D., Frias, J., Barat, J., Henehan, G., Barry-Ryan, C.: Calcium for extending the shelf life of fresh whole andminimally processed fruits and vegetables: a review. Trends in Food Science & Technology, Volume 18, Issue 4, April 2007, Pages

210-218.
http://arrow.dit.ie/http://arrow.dit.ie/schfseharthttp://arrow.dit.ie/schfsehmailto:[email protected],%[email protected]:[email protected],%[email protected]://arrow.dit.ie/schfsehhttp://arrow.dit.ie/schfseharthttp://arrow.dit.ie/


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Effect of ozone and calcium lactatetreatments on browning and textureproperties of fresh-cut lettuce

D. Rico,A. B. Martin-Diana, J.M Frias, GTM Henehan andCatherine Barry-Ryan

School of Food Science and Environmental Health, Dublin Institute of Technology (DIT), Cathal Brugha Street, Dublin 1,Ireland

Abstract: The effects of three treatments, 1 mg L1 ozone at 1820 C, 15 g L1 calcium lactate (CLac) at 50 Cand a combination thereof, were compared on fresh-cut lettuce over 10 days of refrigerated storage. Respirationrate, browning and texture were examined as main quality indicators. The use of ozone produced a significantly(P < 0.05) higher oxygen decline than the use of CLac (from ay 3 to day 10). At the end of storage, CLac (alone ordcombined with ozone) samples had higher oxygen content ( 9%) than ozone samples ( 6%). Enzymatic activit ydecreased significantly (P < 0.05) in ozone samples. Polyphenol oxidase activity in fresh-cut lettuce treated withozone (alone or combined with CLac) showed lower values on day 1 (


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2

Figure 1. Flow diagram showing the procedure for the production of minimally processed fresh-cut lettuce.

Storage 10 days at 4

Dried, pooled, mixed and packaged

in bags (200 mm 320 mm) 35 OPP

Calcium lactate (15 g L-1) at 50

Remove water

Ozone (1 mg L-1)Calcium lactate (15 g L-1) at 50 Ozone (1 mg L-1)

Washing treatment (1 min)

Minimal processing (cutting lettuce)

Iceberg lettuce. Each batch was ~20kg (~50 heads of lettuce of ~400 g).Three independent trials were carried

out from Dec 2003 to May 2004

MATERIALS AND METHODSRaw materialIceberg lettuce (Lactuca sativa sp.) was grown inIreland. The product was purchased from a localsupermarket and stored at 4 C for 24 h until used.

Selection of washing treatmentsPrevious work showed that CLac was a promisingalternative to the industrial standard of 120 mg L 1free chlorine in washing treatments for fresh-cutlettuce.10 CLac treatment proved to be as effectiveas chlorine treatment and enhanced the nutritionalvalue of the final product. A later extensive studyof the effect of temperature and CLac concentration

Experimental designExperiments were conducted from December 2003to May 2004. All procedures were performed ina special food-processing room at 1820 C. Thetre tsfro s

20 kg ( 50 heads of lettuce of 400 g each). The

atmen were conducted in parallel and preparedm the ame batch of product. Each batch was

first washing treatment consisted of 1 mg L1 ozone in

distilled water, the second treatment was a solution of15 g L1 calcium lactate (CLac) at 50 C and the thirdtreatment consisted offirst washing the samples in1 mg L1 ozone and then treating them with 15 g L 1CLac at 50 C. Three independent trials (165 bagsper batch, 55 bags per treatment) were conducted(Fig. 1). For each treatment: three bags were used forheadspace monitoring throughout the entire storageperiod; sensory analysis required two bags per panellist(14), i.e. 28 in total; also, on each of days 1, 3, 7 and10, two bags for texture, two bags for colour and twobags for enzymes were used after being opened andpooled. For texture and colour, more than 25 sampleswere analysed per treatment and day. For enzymes,

measurements were duplicated.

treatments alone or combined with calcium treatmentshave been attributed to the action of heat-activatedpectin methylesterase and/or to increased calciumdiffusion into tissues at higher temperatures.19,22

The main objective of this work was to determinethe effect of calcium lactate, an effective and safebiopreservative, combined with heat shock on fresh-cut lettuce and compare it with that of ozone,

a novel preservative washing solution. Ozone isa strong antimicrobial agent with high reactivityand penetrability and spontaneous decomposition toa non-toxic product.23,24 Several researchers haveshown that treatment with ozone appears to havea beneficial effect in extending the storage life ofbroccoli and seedless cucumber.23,25Although theantimicrobial capacity of ozone has been widelyreported, few studies on quality have been carriedout,26,27 and none have evaluated the potential forsynergistic effects of ozone combined with otherwashing treatments.

This study is focused on the respiration rate,appearance of brown discolouration and loss offirmness thorough monitoring of suitable qualitymarkers.


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on fresh-cut lettuce was investigated.11 The resultsshowed that 15 g L1 CLac at 50 C was the optimalcondition to reduce browning and maintain firmness(crispness).11,21,28 The second treatment selected wasozone as a novel treatment to preserve fresh-cutvegetables, since ozone has been reported to bean efficient reducer of browning23,29 owing to itshigh reactivity and penetrability and its spontaneous

decomposition to a non-toxic product. Followingguidelines and levels reported in the literature,1 mg L1 ozone treatment was used as optimalfor maintaining the shelf life in optimal and safeconditions.30

Quality markersVarious quality markers were analysed as indicators ofrespiration, browning and texture: headspace, colourmeasurement, browning-related enzymes, texture-related enzymes, Instron and sensory analysis.

Headspace analysis

A Gaspace analyser (Systech Instruments, Oxon,UK) was used to monitor levels of CO2 and O2during storage. Gas extractions were performed witha hypodermic needle, inserted through an adhesiveseptum previously fixed to the bags, at a flow rate of30 mL min1 for 10 s, monitoring with a sensitivity of0.001 (O2 ) and 0.1 (CO2 ). Accuracy of readings was:0.5% below 10% and 0.1% above 10% for O2 ;0.1% below 1%, 0.2% between 1 and 10% and2% above 10% for CO2 . Three bags per treatmentwere monitored for each experiment, and the bags forother analyses were measured separately. The trialswere also triplicated.

Processing (washing and packaging procedure)Lettuce samples were treated according to minimalprocessing procedures. Each lettuce was preparedaccording to accepted practices. The two outer leaveswere removed and the core was excised with a stainlesssteel knife. The lettuce was cut in half and each halfwas further cut into four pieces. Different sanitisedknifes were used during the process in order to avoidcross-contamination.

Washing treatments were performed by immersionof the fresh-cut lettuce in the ozone and CLactreatment solutions. Ozonated water was generatedby bubbling gaseous ozone into distilled water.Ozone gas mixture was produced using an activeoxygen generator machine (model HV-103, AnalyticalTechnology Inc., Hampshire, UK) connected to adissolved ozone monitor (model C15-64, AnalyticalTechnology Inc.). The gas was then pumped into thesystem using an aquatic air pump at a flow rate of2.5 L min1 . Ozonated water was held in a containeruntil it reached a concentration of 1 mg L1 at 4 C.The ozone concentration was measured in the range010 mg L1 with a sensitivity of 0.001 mg L1 above0.005 mg L1 . Repeatability was 0.01 mg L1 andlinearity was 0.5%. Calcium lactate (Sigma, St Louis,MO, USA) was diluted to 15 g L1 (pH 6.5) in distilledwate at 50 C.r

ket ( 200 g vegetable product L1 ) by immersion inEach treatment was carried out in a different bas-

the corresponding washing solution for 1 min withagitation and subsequently drying for 5 min in anautomatic salad spinner. To minimise product hetero-

geneity, processed vegetables were pooled, mixed andsubsequently packaged in bags (200 mm 320 mm)of 35 m oriented polypropylene (OPP, Amcor Flex-ibles Europe, Bristol, UK). The permeability ofthdi

13 000 mL m2 day1 atm 1 respectively at 5 C.

e film was defined by oxygen (O2 ) and carbonoxide (CO2 ) transmission rates of 12 000 and

Each package contained 100 g of product. Thepackages were chilled in a blast freezer at 0 C for2 min before being heat sealed under atmosphericconditions.

Browning-related enzymes: peroxidase (POD, EC1.11.1.7) and polyphenol oxidase (PPO, EC 1.10.3.1)Both enzymes were assayed in homogenates that wereprepared as follows. Lettuce (10 g) was placed ina homogeniser (Polytron model PT 3000, Ontario,Canada) in a 1:2 (w/v) ratio with 0.5 mol L1phosphate buffer, pH 6.5, containing 50 g L1polyvinylpyrrolidone. Homogenisation was carried outtwice at 4 C and 5500 rpm for 1 min each time, witha break of 3 min between homogenisations in order toavoid excess heating of the sample. The homogenatewas then centrifuged at 12 720 g for 30 min at 4 Cand filtered through one layer of crepe bandage. Theresulting crude extract was used without further purifi-cation. All extracts were stored at 4 C in the dark andused immediately.

POD activity was assayed spectrophotometrically.The reaction mixture consisted of 0.2 mL of extractand 2.7 mL of 0.05 mol L1 phosphate buffer, pH 6.5,containing 100 L of hydrogen peroxide (1 mL L1 )as oxidant and 200 L of p-phenylendiamine ashydrogen donor. The oxidation of p-phenylendiaminewas monitored at 485 nm and 25 C. One unit ofenzyme activity was defined as an increment of 0.1 inabsorbance per minute.

PPO activity was assayed spectrophotometrically bya modified method based on Refs 31 and 32. The

reaction mixture contained 0.1 mL of crude extractand 2.9 mL of substrate solution (0.020 mol L1catechol as substrate in 0.05 mol L1 phosphatebuffer, pH 6.5). The rate of catechol oxidation wasfollowed at 400 nm for 2 min at 25 C. One unit ofenzyme activity was defined as an increment of 0.1 inabsorbance per minute. Three independent trials werecarried out. All enzymatic measurements were madein duplicate.

Pectin methylesterase (PME, EC 3.1.1.11)PME activity was measured using the methoddescribed by Kimball.33 Briefly, 10 g of sample was

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diluted in an extraction solution (0.2 mol L1 sodiumphosphate buffer, pH 7.5, containing 1 mol L1sodium chloride and 10 mmol L1 dithiothreitol) andhomogenised at 4 C for 2 min at 5500 rpm. Themacerate was incubated at 4 C for 30 min withagitation and then centrifuged at 12 500 g for 30 minat 4 C. A 1 mL aliquot of this extract was mixedwith 40 mL of substrate solution (1 g L1 pectin). The

solution was adjusted to pH 7 with 1 mol L1 NaOHand then readjusted to pH 7.5 with 0.05 mol L1NaOH. After the pH had reached 7.5, 0.2 mL of25 mmol L1 NaOH was added. The time required toreturn to pH 7.5 was recorded. Activity was quantifiedas carboxyl groups formed by the hydrolysis of methylesters of pectin and was measured by titration using apH electrode to monitor the production of H+ . PMEactivity units were calculated using the formula33

(parallel, oblique or perpendicular orientation) withrespect to the shear cell blades directly affects themeasurement.34 For this reason, photosynthetic tissuewith perpendicular orientation was selected for theexperiments. Values were expressed as (maximum load- minimum load)/maximum load, which was defined asthe crispiness coefficient (CC).21 Three independenttrials were carried out. More than 25 samples were

analysed per treatment and day.

Sensory analysisSensory analysis of lettuce was carried out over 10 daysof storage by a panel of 14 untrained members withan age range of 2540 years. Fresh appearance ofsamples was scored on a hedonic scale from 0 (poorfresh appearance) to 5 (excellent fresh appearance).The sensory panel was selected from members ofthe department and the analysis was carried out inthe sensory evaluation laboratory. Data analysis wasperformed using CompusenseFive software (release4.4, Ontario, Canada). Three independent trials werecarried out.

PME = (25 mmol L1 NaOH) (X mL extracted)

(0.2 mL NaOH) 106 /(1 mL sample)

(10 g sample) time (min) (1)

Three independent trials were carried out. Allenzymatic measurements were made in duplicate.

Statistical analysisAnalysis of variance (multifactor and one-wayANOVA) was used to find differences between treat-ments, storage and their interaction for each of thevariables studied. Means were compared by the leastsignificant difference (LSD) test at a significance levelof P = 0.05 using Statgraphics software (version 2.1,Statistical Graphics Co., Rockville, MD, USA). Threeindependent trials were carried out.

Colour measurementColour was quantified using a colorimeter (Hunter-Lab, Reston, UK). A lettuce piece was placed directlyon the colorimeter sensor (3.5 cm diameter) and mea-sured; 2030 measurements were taken per treatment.The instrument was calibrated with white tile (L =93.97, a = 0.88, b 1.21) and green tile (L =

=

56.23, a = 21.85, b = 8.31) standards under D65luminosity conditions. Total colour change E =[(Lf Li )2 + (af ai )2 + (bf bi )2 ]1/2 was analysed,where Li = initial luminosity, Lf= final luminosity,af= a value at final time, ai = a val e at initial time,ubf= b value at final time and bi = b value at initialtime. Three independent trials were carried out. Morethan 25 samples were analysed per treatment and day.

RESULTS AND DISCUSSIONHeadspace analysisChanges in headspace gas composition (oxygen andcarbon dioxide) of fresh-cut lettuce were measuredover 10 days. The oxygen content in the packagesdecreased during storage, while the carbon dioxidecontent increased, as expected (Fig. 2). Changes inoxygen and carbon dioxide concentration were moredramatic from day 1 to day 3 than from day 3 to day10 for all treatments.

Oxygen decreased from its initial atmosphericconcentration (21%) to values around 15% after 1 day

of storage (Fig. 2(a)). This rapid decrease reflected ahigh respiration rate, presumably caused by the stressproduced by minimal processing.35 On the other hand,carbon dioxide increased significantly (P < 0.05) inall samples over the entire storage period (Fig. 2(b)),with the highest increases being observed from day 0to day 2.

Washing treatments significantly affected (P


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respiration rates.38 The higher respiration rate insamples treated with ozone could be associated withtissue and photosynthetic apparatus being damaged,which might alter the metabolic state.

ozone-treated samples. The fresh-cut lettuce washedwith the combined treatment had intermediate values,which indicates that the stress produced by the use ofozone was partially reduced by the use of CLac.

The results indicate that CLac caused a reductionin the respiration of lettuce (Fig. 2(a)), which isin agreement with the findings of other authorswho suggested that CLac reduces the respiration of

minimally processed fresh vegetables.36,37 Since therate of respiration of a vegetable is related to itssenescence state,38 a lower consumption of oxygenmight be due to a lower stress response in lettuce.The lower respiration rate in CLac-treated samplescould be explained by a possible post-woundingprotective effect of CLac at 50 C.21,28 Calcium seemsto maintain cell wall structure by interacting withthe pectic acid present in cell walls to form calciumpectate, thereby maintaining water activity and thusreducing the stress usually associated with higher

Browning-related enzymes (polyphenol oxidase

and peroxidase)After tissue wounding, many enzymes are released atthe wound site. Two of them, PPO and POD, areinvolved in tissue discolouration (e.g. russet spotting,rusty brown discolouration, etc.), contributing toa reduction in quality. Previous results showedsignificant differences in PPO and POD activity infresh-cut lettuce depending on the type of tissueexamined (photosynthetic or vascular).28 For thisreason, the present study was carried out entirely withphotosynthetic tissue to minimise variability betweenexperiments.

OzoneOzone + CLacCLac

(a) Ozone(b)Ozone + CLacCLacf

21 21 Headspacegas (carbondioxide %)Heads ace

gas gen%)

p(oxy18 18

e15 15

dcd12 12

9

6

3

00 1 2478

Storage (days)9 10

d

c

cd cdbc

cd

c9bc bc B

bc AB Bc c ABbcA

cbb

bc ccccb 6cab a A

b b bb 3

b

b b

0a0

9 10478Storage (days)

1 2

Figure 2. Changes in headspace gas composition ((a) oxygen and (b) carbon dioxide) in packages of fresh-cut lettuce stored at 4 C for 10 daysand treated with (1) ozone (1 mg L1 ), (2) calcium lactate (CLac, 15 g L1 at 50 C) or (3) ozone combined with calcium lactate (ozone + CLac ).Points designated on any curve by the same letter are not significantly different (P > 0.05). Lowercase letters are used for comparisons duringstorage and uppercase letters for treatment comparisons. Three independent trials were carried out in triplicate.

5000d

BPolyphenoloxidase (units

g-1)

c

4600 cc c

c4200 AB

bcb

b3800

a b A

a3400 OzoneOzone + CLacCLac

30001 3 7

Storage (days)10

Figure 3. Changes in polyphenol oxidase activity in fresh-cut lettuce stored at 4 C for 10 days and treated with (1) ozone (1 mg L1 ), (2) calciumlactate (CLac, 15 g L1 at 50 C) or (3) ozone combined with calcium lactate (ozone + CLac). Points designated on any curve by the same letter a ernot significantly different (P > 0.05). Lowercase letters are used for comparisons during storage and uppercase letters for treatment comparisons. Three independent trials were carried out in duplicate.

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Figure 3 shows the PPO activity profile duringstorage of fresh-cut lettuce. Samples treated withozone and ozone combined with CLac showed sig-nificantly lower (P < 0.05) PPO enzymatic activitiesthan samples treated with CLac alone. This decreasein PPO activity could be caused by the high oxidationpotential of ozone compared with CLac. AlthoughCLac has been described as an inhibitor of PPO

synthesis28 during storage, ozone showed a higher effi-cacy in the reduction of these enzymes. Saftner et al.39considered ozone to be an efficient organic matter oxi-dant that could reduce the microbial content, givingrise to a lower respiration rate, and decrease enzymesynthesis.40 Intermediate PPO values in the samplestreated with the combined method can be attributedto the opposing effects of the increase in stress (dou-ble washing process), which might increase enzymeactivity, and the ozone, which decreases it.

The oxidative effect of ozone might also haveproduced lower POD activity (Fig. 4). Althoughsignificantly (P < 0.05) lower values were observedwith the use of ozone treatments, either aloneor combined, compared with CLac alone, thesesignificant differences were only observed at thebeginning of storage (days 13). From day 7, noeffect of the treatment was observed.

Texture-related enzymes (pectin methylesterase)PME activity increased significantly during storage forall treatments, although showing fluctuations withinthe storage period (Fig. 5). This irregular behaviourmay be due to a wounding response and/or to changesin the solubility of the enzyme during storage.41 Suchbehaviour has been observed for certain browning-related enzymes.42 Other authors have attributedthis variability to intrinsic factors and to pre- and

e1000

Peroxidase(units g-1)800 c A

c ABd

cbc

b600 b c B

a400 bab

200 OzoneOzone + CLac

CLac01 3 7

Storage (days)10

Figure 4. Changes in peroxidase activity in fresh-cut lettuce stored at 4 C for 10 days and treated with (1) ozone (1 mg L1 ), (2) calcium lactate(CLac, 15 g L1 at 50 C) or (3) ozone combined with calcium lactate (ozone + CLac). Points designated on any curve by the same letter are not significantly different (P > 0.05). Lowercase letters are sed for comparisons during storage and uppercase letters for treatment comparisons.uThree independent trials were carried out in duplicate.

Pectinmeth rase (m O-min-1 g-1)

ylesteol CO6800 d

5800 c c Bc

b b Ab4800

bA

3800 b b aba

2800

1800Ozone

800 Ozone + CLacCLac

01 7

Storage (days)103

Figure 5. Changes in pectin methylesterase activity in fresh-cut lettuce stored at 4 C for 10 days and treated with (1) ozone (1 mg L1 ), (2) calciumlactate (CLac, 15 g L1 at 50 C) or (3) ozone combined with calcium lactate (ozone + CLac). Points designated on any curve by the same letter a ernot significantly different (P > 0.05). Lowercase letters are used for comparisons during storage and uppercase letters for treatment comparisons. Three independent trials were carried out in duplicate.

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a correlation. Fresh-cut lettuce treated with ozoneand CLac combined had a similar reduction in PPO(throughout the entire storage period) and POD(during the first day of storage) activities to samplestreated with ozone.

postharvest factors, which can affect enzyme activity,vitamin content, etc.43

Fresh-cut lettuce treated with CLac had significantly(P < 0.05) higher PME activity values than samplestreated with ozone or the combined method (Fig. 5).Previous studies have shown that CLac combined withheat shock produces an activation of PME.11,19,22 Thebeneficial effects on texture of heat treatments and

CLac solution have been explained by the activationof PME.19 PME is responsible for cleaving themethoxyl groups from methylated pectic substances,generating free pectic acids44 that contain newlyavailable carboxyl groups.

This activation of PME caused by CLac and heatshock was inhibited by the treatment with ozone.Samples treated with ozone in combination with CLacshowed the same levels of PME activity as samplestreated with ozone alone. This suggests a strongerinhibitory effect on this enzyme (also on PPO andPOD) caused by ozone treatment than the activationproduced by CLac treatment.

Instron analysis (crispiness coefficient)The variation in CC was analysed during storage(10 days) for all samples (Fig. 7). During storage asignificant (P < 0.05) reduction in CC was observedin all cases, which was associated with a loss ofturgor and fresh-like textural properties. Significantdifferences between treatments were found. Samplestreated with ozone showed the lowest values of CC,while samples treated with CLac had the highestvalues. This finding might be partly associated withPME activation (Fig. 6). The use of heat shock (50 C)and the addition of CLac can make tissues firmerby binding to the pectin carboxyl groups that aregenerated through the action of PME.45 Samplessubjected to the double treatment had similar values

to samples treated with ozone alone.

Colour analysisTotal colour differences ( E) were analysed usingCIE L a b parameters for fresh-cut lettuce. Changesin colour during storage (P < 0.05) were observed(Fig. 6). Samples subjected to the double treatmentshowed greater changes in colour than samplessubjected to individual treatments. However, nodifferences were found between samples treated withCLac and samples treated with ozone.

This greater change in colour with the doubletreatment could be associated with the damage andstress produced by the double manipulation (physicalbrowning) rather than being caused by enzymaticbrowning (Fig. 3). Although colour variation canbe associated with enzymatic browning by phenolicoxidation over time,45 49 our results did not show

Sensory analysisFresh appearance is the main attribute that consumersuse to evaluate the quality of vegetables and fruits,since people buy with their eyes.50,51Appearance,browning and texture are key aspects used in sensoryanalysis to evaluate the general quality of a product.This is especially true for lettuce, where browning andlack offirmness are critical factors in perceived loss ofquality.52

Sensory analysis was used to assess the quality offresh-cut Iceberg lettuce over 10 days. Mean scoresfor sensory fresh appearance are given in Table 1.Fresh-cut lettuce showed significantly lower scoresfor appearance at the end (10 days) of storage thanfrom day 1 to day 7. The average values on day 7

e12 B

Total colourdiffe ce

(

E)

ren10d

bc

8 Ab

b Ac

6ab

4ab

a2 Ozone

Ozone + CLacCLac

01-3 7-103-7

Storage (days)

Figure 6. Total colour change (CIE L a b parameters) in fresh-cut lettuce stored at 4 C for 10 days and treated with (1) ozone (1 mg L1 ),(2) calcium lactate (CLac, 15 g L1 at 50 C) or (3) ozone combined with calcium lactate (ozone + CLac). Points designated on any curve by thesame letter are not significantly different (P > 0.05). Lowercase letters are used for comparisons during storage and uppercase letters for treatment comparisons. Three independent trials were carried out; >25 samples per trial were analysed.

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16d d

Crispinesscoefficient(CC

c

b)12 bb bB

AA

ab

b b8

a

4OzoneOzone + CLacCLac

01 3 7

Storage (days)10

Figure 7. Effect of washing treatment on crispiness coefficient (CC). Measurement of CC of fresh-cut lettuce stored at 4 C for 10 days and reatedtwith (1) ozone (1 mg L1 ), (2) calcium lactate (CLac, 15 g L1 at 50 C) or (3) ozone combined with calcium lactate (ozone + CLac). Pointsdesignated on any curve by the same letter are not signi ficantly different (P > 0.05). Lowercase letters are used for comparisons during storage and uppercase letters for treatment comparisons. Three independent trials were carried out; >25 samples per trial were analysed.

Table 1. Sensory evaluation of fresh appearance rating (scale 15) of fresh-cut lettuce stored at 4 C for 10 days and treated with (1) ozone(1 mg L1 ), (2) calcium lactate (CLac, 15 g L1 at 50 C) or (3) ozone combined with calcium lactate (ozone + CLac)

Treatment Day 1

3.45 0.50b3.80 0.23b3.65 0.56b

Day 3 Day 7 Day 10

3.55 0.80b3.51 0.12b3.71 0.37b

OzoneOzon + CLaceCLac

3.85 0.60b3.68 0.20b3.91 0.27

2.29 0.30a2.43 0.23a2.60 0.20a

Each value represents mean standard deviation for fresh appearance. Values followed by different letters in the same row indicate signi ficantdifferences during storage. No significant differences were observed between treatments. Three independent trials were carried out; 14 panellists ineach trial scored the samples. Ratings: 0 = poor fresh appearance; 3 = acceptable fresh appearance; 5 = excellent fresh appearance.

were considered acceptable (score above 3), showing

that all treatments maintained good quality until theend of storage. Differences between treatments werenot observed. At the end of storage, lower freshappearance values were observed in samples washedwith ozone and ozone combined with CLac. However,the differences were not significant. The resultsobtained in the sensory analysis did not correlatewith the instrumental results, which might be dueto the high variability of the product and/or the limiteddiscriminative ability of human perception.

suitable treatment, i.e. safety of use, no by-product

production, dairy industry waste reutilisation, calciumenrichment of the product and a possibly more lastingeffect, as ozone rapidly disappears. For these reasonsthe authors consider CLac (used at 50 C) a moresuitable treatment than ozone in order to extend thequality of fresh-cut lettuce.

ACKNOWLEDGEMENTSThe authors would like to acknowledge the finan-cial support of Technological Sector Research(20022006). Thanks are also extended to AmcorFlexibles Europe (Bristol, UK) for kindly providing

the film bags.CONCLUSIONSThe use of an ozone washing treatment to extendthe quality of fresh-cut lettuce was compared with atreatment using CLac at 50 C and a combinationof both, with unequal results. A reduction inenzyme activity was produced by ozone treatments,resulting in a potentially beneficial effect of reducedenzymatic browning, although this potential reductionin browning was not revealed by colour analysis.However, this enzyme inactivation showed a negativeeffect, as the reduction in activity of the texture-relatedenzyme PME was correlated with a lower crispinesscoefficient. Depending on further studies to evaluatethe effect on the microbial load of these two treatments

(and the combination of both), some advantages ofCLac compared with ozone make the former a more

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Calcium for Extending the Shelf Life of Fresh Whole and Minimally

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