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Postharvest Biology and Technology 87 (2014) 140–143

Contents lists available at ScienceDirect

Postharvest Biology and Technology

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nterpreting textural changes in low temperature stored tomatoes

alash Biswas ∗, Andrew R. East, Errol W. Hewett, Julian A. Heyesentre for Postharvest and Refrigeration Research, Massey University, Private Bag 11222, Palmerston North 4474, New Zealand

r t i c l e i n f o

rticle history:eceived 24 June 2013eceived in revised form 28 August 2013ccepted 29 August 2013

eywords:

a b s t r a c t

Low temperature storage alters tomato textural properties, resulting in unusual changes in firmness,while ripening during cool storage can confound these chilling-induced textural changes. Inconsistentresults have been reported related to chilling-induced alteration in tomato texture. The effects of chillingon tomato texture were investigated using fruit stored at 2.5 or 6 ◦C (chilled) or 20 ◦C (non-chilled) for 27d before transfer to 20 ◦C. Given that many factors influence the firmness of chilling-injured tomato and

olanum lycopersicumolourissue integrityurgor lossecay severity

different measurement methods indicate different characteristics of tomato texture, the present studyemployed a range of textural measurement techniques in order to interpret chilling-induced texturalchanges in tomatoes during long term storage. Analysis of data from a range of textural methods indicatedthat storage at 6 ◦C mainly induced loss of turgor whereas 2.5 ◦C induced loss of tissue integrity alongwith turgor loss. Plotting textural changes against colour as an indicator of ripening allowed a clearer

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definition of chilling-indu

. Introduction

Low temperature storage often alters tomato textural propertiesesulting in unusual changes in firmness. Inconsistent results haveeen reported in describing chilling-induced tomato softening. Dif-erent research groups have observed variation in chilling effectsn firmness changes including increased softening (Efiuvwevwerend Thorne, 1988; Marangoni et al., 1995), inhibited softeningRugkong et al., 2010), or depending on the length of chilling expo-ure, both excessive softening and persistent firmness (Hobson,987; Rugkong et al., 2011).

Texture of tomatoes has been measured by acoustic stiffnessensor (Hertog et al., 2004), flat-plate compression (Giovannonit al., 1989) or by puncture (Holt, 1970) of whole fruit and eacheasurement technique is related to texture in a particular way

Bourne, 1994). Use of different texture measurement techniquesy researchers may potentially contribute to confounding resultsegarding chilling-induced alteration of tomato texture.

Differences in extent of chilling injury (CI) damage either by dif-erences in storage temperature or storage time may also influencehilling-induced textural alterations. Some studies (Almeida anduber, 2008) used fruit which showed slight CI (on the basis of

olour development), whereas others (Rugkong et al., 2011) usedruit with obvious ripening impairment and yellowing. In addi-ion, if fruit are examined early in the storage period, results will

∗ Corresponding author. Tel.: +64 220100641.E-mail addresses: P.Biswas@massey.ac.nz, biswaspalash1@gmail.com

P. Biswas).

925-5214/$ – see front matter © 2013 Published by Elsevier B.V.ttp://dx.doi.org/10.1016/j.postharvbio.2013.08.018

extural change.© 2013 Published by Elsevier B.V.

clearly be different from those examined after prolonged storage.Importantly, cultivar differences in different studies may furthercontribute to the inconsistent results.

Mechanisms of chilling-induced textural change can be con-founded with normal ripening-associated changes in softeningand not all studies have carefully separated the textural changesresulting from these two physiological processes. Therefore, itis important to differentiate the ripening-associated softeningfrom the changes that reflect chilling-induced textural alterations.Although determining the degree and mechanism of softening dueto these physiological processes can be difficult (Bourne, 1994;Almeida and Huber, 2008), the present study attempted to differ-entiate the softening trends resulted from these two physiologicalprocesses, with colour as an indicator of ripening.

Given that many factors influence the firmness of chilling-injured tomatoes and different measurement methods will indicatedifferent characteristics of tomato texture, the present studyemployed a range of textural measurement techniques in orderto interpret chilling-induced textural changes in tomatoes dur-ing long term storage. Fruit were chilled at mild and severe lowtemperatures to induce differences in the degree of chilling injury.The results may be able to suggest which techniques are better atdetecting chilling-induced textural change of tomatoes.

2. Materials and methods

2.1. Plant material and storage condition

Tomatoes (Solanum lycopersicum cv. Cedrico) were grown atMassey University Plant Growth Unit during the early summer

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Table 1Decay incidence (%) and severity (0–4 scale) of mature-green tomatoes after 10 d at20 ◦C preceded by 27 d treatment temperatures (n = 15).

Treatments (◦C) Decay incidence (%) Decay severity (0–4 scale)

2.5 100 2.20a6 25 0.93b

P. Biswas et al. / Postharvest Biolo

sing standard glasshouse production practices (Biswas et al.,012). Fruit were harvested at the mature-green stage and trans-erred immediately to the laboratory and left overnight (20 ◦C).ruit of uniform size and free from defects were visually sortednd calyces were carefully removed. A total of 135 fruit were ran-omly grouped into three replicate batches (5 fruit each replicate)f 15 fruit for each treatment. The fruit were placed inside polyvinylhloride (PVC) tubes in a flow-through system supplying humidi-ed air (around 95% RH) at a total flow of 200 mL min−1 and heldt chilled (2.5 or 6 ◦C) or non-chilled (20 ◦C) conditions for 27 d.ecay severity of 15 fruit per treatment were evaluated after a 10

post-chilling period at 20 ◦C, according to the method describedy Biswas et al. (2012).

.2. Fruit firmness

Firmness of 15 fruit on each occasion for each treatment wasetermined on days 0, 13 and 27 d of storage and after a 10 d post-hilling period at 20 ◦C.

.2.1. Acoustic stiffness sensorNon-invasive acoustic response was determined by an acous-

ic stiffness sensor (Aweta BV, Nootdorp, The Netherlands). Aomato was placed on a cushioning material and was impactedrom beneath by a solid plastic rod (Tick length = 20 mm). A small

icrophone then captured the mechanical vibration (Microphoneain = 70). A stiffness index (S, Hz2 kg2/3) was produced by theevice based on the resonant frequency (f) of the first ellipticalode and the mass (m) of the tomato (Hertog et al., 2004). Stiff-

ess was measured as the average of three equatorial readings onhe fruit surface.

.2.2. Flat-plate compressionCompression force readings were taken at three different equa-

orial locations on each fruit by recording the maximum forceequired to compress the fruit for 2 mm using a flat cylindricalrobe (50.85 mm diameter) mounted on a TA-XT plus texture anal-ser (Stable Micro Systems Ltd, UK). The compression test was runith a flat plate using a pre-test speed of 5 mm s−1, a test speed of

mm s−1, and a trigger force of 1 N (following Hertog et al., 2004).

.2.3. Whole fruit punctureFor this destructive firmness measurement, a flat end cylindrical

robe (3.7 mm diameter) mounted on a TA-XT plus texture analyserStable Micro Systems Ltd., UK) was used. The test was run using

pre-test and a test speed both of 10 mm s−1, a trigger force of.1 N, and allowing the probe to travel 15 mm deep into the tissuet the equatorial region of the fruit, measuring the maximum forcencountered (following Hertog et al., 2004).

.2.4. Puncture from inside of a pericarp discSince the puncture from outside of whole fruit is largely influ-

nced by skin toughness, a puncture from inside of an excisedericarp disc was performed to ensure measurement of pericarprmness alone. For this measurement technique, equatorial discsere cut with a 15 mm diameter cork borer after the whole fruituncture had been completed and locular tissue was carefullyemoved. Pericarp discs were placed exocarp down on a steel plate.

flat end cylindrical probe (3.7 mm diameter) was used to punc-ure the disc tissue to a depth of 4 mm using same test run protocols described for puncture of whole fruit. From each fruit, two discsere made and each disc was punctured at two places.

20 86 1.07b

Mean separation within columns by LSD at ̨ = 0.05.

2.3. Statistical analysis

Analysis of variance (ANOVA) was calculated to analyse theeffect of treatments using the General Linear Model procedure ofSAS (SAS Institute, version 9.2, Cary, NC). The means were comparedby the Fisher’s LSD test at a significance level of 0.05.

3. Results and discussion

3.1. Decay incidence and severity

All fruit previously stored at 2.5 ◦C developed decay by the end ofpost-chilling transfer to 20 ◦C (100% incidence) typical of Alternariainfection, particularly around the stem scar, whereas 25% fruit haddecay incidence after 6 ◦C storage (Table 1). As is typical, all fruithad no decay symptoms on removal from chilling temperatures,but decay set in after that time. More than 80% of fruit stored at20 ◦C had white patches of decay symptoms scattered on the sur-face of the fruit and these fruit were mostly infected with ‘ripe rot’organisms, Phytophthora and Rhizopus.

3.2. Fruit firmness

Textural measurement values from all measures decreased pro-gressively in all treatments, with fruit at 6 ◦C firmer than fruit at2.5 ◦C (data not shown). Fruit stored at constant 20 ◦C showed anexponential decrease in firmness. Since both chilling and ripeningcan contribute to fruit softening during postharvest storage, firm-ness data were plotted against colour (as an indicator of ripening) toinvestigate the relationship between changes of these two qualityindices as a function of storage temperatures.

When firmness was assessed by the acoustic stiffness sensor,both chilled (2.5 or 6 ◦C) and non-chilled (20 ◦C) fruit softened, butsoftening followed different trajectories relative to colour change(Fig. 1A). If we consider softening to be coordinated with colourchange at 20 ◦C in a manner that represents “normal” ripening,then at 2.5 or 6 ◦C, stiffness loss decouples from the colour changeexpected from coordinated normal ripening. During the post-storage period, stiffness loss of tomatoes previously stored at 20 ◦Ccan be attributed to ripening as all fruit turned red, whereas lossof stiffness with little colour change in fruit previously stored at 6or 2.5 ◦C can be interpreted as primarily a chilling-induced effect.Excessive softening in mature-green tomatoes after 2–3 weeks ofstorage at 5 or 7 ◦C has been reported previously (Marangoni et al.,1995; Efiuvwevwere and Thorne, 1988). Acoustic responses arelargely a measure of mechanical stiffness of tissue which is basedon both the cell wall mechanical strength and the tension underwhich the tissue is held by turgor (Hertog et al., 2004). It is possi-ble that this chilling-induced softening in fruit stored at 6 or 2.5 ◦Cwas primarily due to a progressive loss in turgor as chilling-inducedtextural damage increased.

Compression analysis indicated a similar trend to acoustic

stiffness measurement; however, the deviation of fruit softeningbetween chilled (6 or 2.5 ◦C) and non-chilled (20 ◦C) fruit was lessdramatic (Fig. 1B). Firmness values derived from compression testsare a combination of many fruit properties, such as the strength and

142 P. Biswas et al. / Postharvest Biology and Technology 87 (2014) 140–143

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Fig. 1. Relationship between colour changes (hue) and firmness of mature-green tomatoes held at 2.5 (�), 6 (�) or 20 ◦C (�). Firmness was measured by acoustic stiffnesssensor (A), compression force (B), puncture force (C) and puncture from inside of a pericarp disc (D). Dashed lines with open symbols indicate post-chilling transfer of fruitt ◦ ard err

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o 20 C preceded by 27 d treatment temperatures. Error bars represent the standeader is referred to the web version of this article.)

oughness of the skin, integrity of pericarp, number of locule walls,ize of fruit and overall turgor (Holt, 1970). Results reported heregree with others (Jackman et al., 1990; Marangoni et al., 1995) thatrmness values derived from flat-plate compression tests may note sensitive enough to reveal differences in tomato fruit firmnessharacteristic of slight CI.

When fruit firmness was measured using a destructive punc-ure from outside of the whole fruit, slope of softening againstolour changes for chilled and non-chilled fruit stored under differ-nt treatment temperatures followed a similar path (Fig. 1C). When

probe punctures through the epidermis, the primary force mea-ured is skin strength, with additional contributions from cell walligidity, cell packing and cell–cell adhesion (Tieman and Handa,989). The similar slope of softening for chilled and non-chilledruit in this study suggest that skin strength may not be markedlyffected by CI.

When puncture force was applied from inside a pericarp disc,ifferences in firmness loss plotted against colour were foundnly in the extreme chill treatment, i.e. 2.5 ◦C (Fig. 1D). Thisesult suggests that puncture from inside is a more sensitiveest than puncture from outside of epidermis for determininghilling-associated textural changes in tomatoes and since thisest primarily relates to changes in cell-to-cell adhesion and cellall strength, these attributes changed the most in the most

hilled fruit (e.g. 2.5 ◦C – stored fruit). It is possible that storaget 6 ◦C caused only ‘minor’ changes in texture such as loss of tur-

or (detected here by acoustic stiffness) whereas storage at 2.5 ◦Cesulted in ‘severe’ chilling damage and loss of tissue integrityshown by dramatic change in internal puncture resistance). Extentf CI damage measured by decay severity also suggested that

ror (n = 15). (For interpretation of the references to color in this figure legend, the

storage at 2.5 ◦C induced higher chilling damage than 6 ◦C storage(Table 1).

4. Conclusion

A chilling-induced alteration in textural properties was previ-ously reported as a CI symptom. In this study, both chilled andnon-chilled fruit induced softening progressively, however, with asignificantly different trajectory compared to ripening-associatedcolour change. It was deduced that softening in fruit stored at 2.5 or6 ◦C was chilling-induced whereas for the fruit stored at 20 ◦C it wasripening-related. This technique of plotting firmness data againstcolour makes it possible to differentiate chilling-induced softeningfrom changes associated with ripening. In addition, it appears thatacoustic sensor and puncture of pericarp tissue from inside are use-ful techniques to describe the chilling-induced textural changes intomato.

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