Postharvest Biology and Technology 16 (1999) 37–41

Heat treatment prevents postharvest geotropic curvature ofasparagus spears (Asparagus officinalis L.)

Robert E. Paull *, Nancy Jung Chen

Department of Horticulture, College of Tropical Agriculture and Human Resources, Uni6ersity of Hawaii at Manoa,3190 Maile Way, Honolulu, HI 96822, USA

Received 7 May 1998; accepted 8 November 1998

Abstract

Negative geotropic curvature of fresh asparagus spears after harvest was prevented by a brief heat treatment. Theheat treatment was immersion in heated water at 47.5°C for 2–5 min and cooling to storage temperature as soon aspossible after heat treatment. The heat treatment can be varied from 45°C for longer than 5 min to 50°C for 2.5 minor less to avoid significant loss of spear appearance. The treatment temperature and time needed to be adjusted forspear diameter, small diameter spears required a shorter exposure time or lower temperature. © 1999 Elsevier ScienceB.V. All rights reserved.

Keywords: Quality; Breakdown; Cooling; Appearance; Moisture loss

1. Introduction

The appearance of asparagus spears is of sig-nificance to consumers. Fresh asparagus spearsare susceptible to natural negative geotropicgrowth curvature (Morris and Watada, 1960), andthe curvature or bending of the spears is lessappealing, being associated with a lack of fresh-ness and lowering of grade rating (Anon., 1986).The curvature has been reduced by maintaining

low temperatures during storage and shipping(Morris and Watada, 1960) or packaging andshipping the spears vertically, but these ap-proaches are not completely satisfactory due todifficulty of maintaining low temperaturethroughout marketing and uncertainty that thecontainers that hold the spears vertical will bestacked correctly.

Hara et al. (1997) reported that exposure of redginger inflorescence to hot water (49°C) for 12min reduced geotropic bending. This study wasundertake to determine if a heat treatment wascapable of reducing negative geotropism in as-paragus spears.

* Corresponding author. Tel.: +1-808-9567369; fax: +1-808-956-3542.

E-mail address: paull@hawaii.edu (R.E. Paull)

0925-5214/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved.

PII: S 0925 -5214 (98 )00101 -X

R.E. Paull, N. Jung Chen / Posthar6est Biology and Technology 16 (1999) 37–4138

2. Materials and methods

2.1. Plant material

Asparagus spears were obtained from a plant-ing in Kunia, Hawaii. The spears were cut atground level between 7 and 9:30 AM, placed incrushed ice and returned to the laboratory within3 h. Spears of the same diameter (5–20 mm) atthe base and length (18 cm) were used in thevarious tests unless otherwise indicated. Spears,after being sorted for size and length, were im-mersed in a heated water bath held at specifictemperatures from 42.5 to 50°C for up to 12.5min. After hot water treatment, spears were im-mediately cooled in an ice–water mixture for 10min, then laid horizontal in a plastic tray withmoist sheets of paper to maintain humidity. Thecovered plastic trays were held at 2, 10, or 22°C asindicated in the various tests. Spears ca. 4 and 10mm in diameter, were treated separately in heatedwater at 47.5°C for 2.5, 5, 7.5, and 10 min in onetest, then stored at 10°C for 7 days.

2.2. E6aluation

Spear curvature was measured with a protrac-tor after 7 days storage. In one series of tests,spear curvature was measured 1, 2, 4, and 6 daysafter heat treatment. Tip breakdown and overallspear appearance was subjectively evaluated on ascale of 1 unsaleable to 7 good. Crispness wassubjectively determined by breaking the spear inthe middle and ranking the break as; clean or nota clean break and flaccid. Results for crispnesswere expressed as a percentage of spears showinga clean break. Mass loss was determined byweighing spears before treatment and at finalevaluation. Spear length was measured with flex-ible ruler.

2.3. Statistical analysis

Each treatment consisted of four replicationswith five spears per replication. Results were ana-lyzed by analysis of variance and means comparedby Duncan–Waller multiple range test.

3. Results and discussion

Treating asparagus spears at 42.5°C for 10 minin heated water and stored at 10°C, reduced cur-vature of spears to 13° from 28° for untreatedspears, whereas treatment at 45, 47.5 and 50°Ccompletely prevented subsequent asparagus spearcurvature (Fig. 1). Overall spear appearance 7days after treatment was not affected until spearswere exposed to 45°C for 10 min, then declinedrapidly (Fig. 1). Loss of overall appearance alsowas rapid when spears were treated at 50°C formore than 2.5 min and less at 45°C for up to 7.5min (Fig. 2). Asparagus spears treated at 50°C for2.5 min and longer showed no curvature (Fig. 2).Spears treated at 45°C still showed slight curva-ture after 12.5 min exposure when evaluated afterseven days at 10°C.

After harvest, the rapid growth of the spear tipsand textural changes in the basal position of thespear occurs (Clore et al., 1976; King et al., 1993).Treating spears at 47.5°C for 4 min, minimizedcurvature with a slight loss in spear crispness, tipbreakdown and overall appearance (Table 1).Storage at 10°C led to significant curvature thatwas prevented by a 47.5°C, 4 min hot water

Fig. 1. The effect of exposure for 10 min in heated water onspear curvature (�) and overall spear appearance (+ ) after 7days storage at 10°C. Analysis of variance: temperature effecton appearance, Pr\F, 0.0001, msd=0.51; curvature, Pr\F,0.0001, msd=4.67.

R.E. Paull, N. Jung Chen / Posthar6est Biology and Technology 16 (1999) 37–41 39

Fig. 2. The effect of 45°C (�) and 50°C (�) for varyingperiods of immersion in heated water on asparagus spearcurvature (o) and overall spear appearance (------) after 7 daysstorage at 10°C. Analysis of variance: appearance, Pr\F,temperature 0.0001, time 0.0001, temperature× time 0.0001;curvature, Pr\F, temperature 0.0234, time 0.0003, tempera-ture× time 0.0007.

1990). Spears stored at 2°C showed little curva-ture after 6 days (Fig. 3A), irrespective of whetherthe spears were treated with heated water or not.Exposure of spears to 1 day at 22°C after 5 daysat 2°C followed by one more day at 2°C beforeevaluation allowed significant curvature to occur(Table 1). Untreated spears curved 29° in 2 dayswhen stored at 10°C, curving a further 7° curva-ture over the next 4 days. Spears treated at 47.5°Cfor 5 min, then held at 10°C showed B5° curva-ture over the first 2 days after treatment (Fig. 3A)and showed little subsequent curvature. The hotwater treatment significantly reduced geotropiccurvature irrespective of the storage temperaturebut did not prevent spear length increase. Spearlength increase was greater after hot water treat-ment than in the untreated controls at 2, 10 and22°C (Fig. 3B).

Smaller diameter spears showed B5° curvatureafter a 2.5 min treatment with heated water at47.5°C, while 5 min was necessary for the largerdiameter spears (Fig. 4). Spears, irrespective ofdiameter, had B3° curvature with 5 min orlonger treatment in 47.5°C heated water. In addi-tion, larger diameter spears had slightly less lossin appearance than the smaller diameter spears.Drake and Lambert (1985) also found that largerdiameter spears lost less mass in storage thansmaller diameter spears.

The heat treatment, can also possibly be ap-plied with hot air, or by spray with a hot waterspray, though only hot water dipping was used in

treatment. Untreated spears showed significantlyless crispness than treated spears when stored at10°C, suggesting the heat treatment may reducethe rate of basal fiber formation.

Temperature of asparagus spears should be re-duced after harvest to maintain quality (Lipton,

Table 1Effect of a hot water treatment of 47.5°C for 4 min on asparagus spears curvature, mass loss, crispness, tip breakdown and overallappearance evaluated after 7 days at 10°C or 5 days at 2°C then 1 day at 22°C followed by another 1 day at 2°C

Overall appearanceMass loss (%)Curvature (°) Tip breakdownCrispness (%)

Stored 10°CControl +1.327.3 66 7 7

3.1 −1.5 72 6.8 5.847.5°C 4 min

Stored 2°—1 day 22°C100+0.7 79.8Control 7

2.347.5°C 4 min −2.2 85 6.8 5.5

Pr\F Pr\FAnalysis of variance Pr\F Pr\F Pr\F0.00010.23210.00110.02810.0061Storage condition

0.0001 1.0000 1.0000 0.0060 0.0001Heat treatment

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Fig. 3. The pattern of curvature (A) and length increase (B) ofspears treated with 47.5°C for 5 min in hot water (------) oruntreated (—) then held at 2, 10, and 22°C and evaluateddaily. Analysis of variance: curvature, Pr\F, time 0.0001,storage temperature 0.0001, heat 0.0001, storage tempera-ture×heat treatment 0.0001; length, Pr\F, time 0.0001, stor-age temperature 0.0001, heat 0.0001, storage temperature×heat 0.3444.

Fig. 4. The curvature (closed symbols) and appearance (opensymbols) of asparagus spears treated for different exposuretimes at 47.5°C in heated water on curvature held at 10°C for7 days. Asparagus spears of two different diameters weretreated, ca. 4 mm (—) and 10 mm (--------). Analysis ofvariance: curvature, Pr\F, diameter 0.7987, treatment time0.0001, diameter× treatment time 0.0001; appearance, Pr\F,diameter 0.4266, treatment time 0.0001, diameter× treatmenttime 0.0855.

Acknowledgements

This is College of Tropical Agriculture andHuman Resources, Journal Series No. 4355.

References

Anon., 1986. United States Standards for Grades of FreshAsparagus. Code of Federal Regulations: Agriculture, Vol7. Part 51.3720–51.3733.

Clore, W.J., Carter, G.H., Drake, S.R., 1976. Pre and post-harvest factors affecting textural quality of fresh aspara-gus. J. Am. Soc. Hort. Sci. 101, 576–578.

Drake, S.R., Lambert, E.F., 1985. Influence of calcium treat-ment, spear size and storage time on fresh and frozenasparagus quality. J. Food Qual. 8, 101–111.

Hara, A.H., Hata, T.Y., Hu, B.K-S., Tsang, M.M.C., 1997.Hot air induced thermo tolerance of red ginger flowers and

these studies. It is important that the means usedto apply the heat treatment be efficient in contact-ing all surfaces to be heated. After treatment, theshoots must be cooled by immersion in cold waterto the storage temperature, as soon as possibleafter the heat treatment, to minimize any loss ofappearance. Hernandez-Rivera et al. (1992) hasshown that a delay of 4 h from harvest beforecooling can lead to increase in shear force at thebase of the spear. The short heat treatments de-scribed above would be similar to a delay incooling. The heat treatments proposed, if adjustedfor spear diameter, could be used commercially tomaintain spear straightness during marketing withminimal loss of appearance.

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