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Journal of Food Engineering 86 (2008) 25–29

Effects of controlled freezing-point storage at 0 �C on qualityof green bean as compared with cold and room-temperature storages

Li Guo a, Ying Ma a, Da-Wen Sun a,b,*, Peng Wang c

a College of Food Science and Engineering, Harbin Institute of Technology, 202 Haihe Road, Harbin 150090, Chinab Food Refrigeration and Computerised Food Technology, University College Dublin, National University of Ireland, Earlsfort Terrace, Dublin 2, Ireland

c College of Food Science and Technology, Northeast Agricultural University, 59 Mucai Street, Harbin 150030, China

Received 27 April 2007; received in revised form 25 August 2007; accepted 3 September 2007Available online 8 September 2007

Abstract

Green bean (Phaseolus vulgaris L.) was stored under controlled freezing-point condition (0 �C) as compared with room-temperature(25 �C) and cold (8 �C) storages for up to 18 days. Results showed that the peak rate of respiration was 109.2 CO2 mg/kg h after storageat 0 �C for 12 days, which was significantly lower than those with the other two storage temperatures. Weight loss increased significantlywith increasing storage temperature. Measurements on other parameters such as soluble solid and surface colour also indicated that con-trolled freezing-point storage at 0 �C could maintain better product quality.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Phaseolus vulgaris L.; Green bean; Controlled freezing-point; Shelf-life; Cold storage; Food quality

1. Introduction

Green bean (Phaseolus vulgaris L.) is grown extensivelyin all major continental areas, due to its high nutritionalquality and sensory properties. Green bean is normallyharvested seasonally, and stored at room-temperatureswith short shelf-life of about seven days, therefore extend-ing its shelf-life is an important issue. Research has indi-cated that for frozen storage at �18 �C, some qualityattributes (e.g. colour and flavour) of the stored green beanwere well retained, but nutritional parameters, such asascorbic acid and starch, degraded (Martins & Silva,2004). Other studies have been performed on the effectsof controlled atmosphere on storage quality of green bean.Sanchez-Mata, Camara, and Diez-Marques (2003a, 2003b)have proposed to extend shelf-life of green bean with high

0260-8774/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2007.09.005

* Corresponding author. Address: Food Refrigeration and Computer-ised Food Technology, University College Dublin, National University ofIreland, Earlsfort Terrace, Dublin 2, Ireland. Tel.: +353 1 7165528; fax:+353 1 4752119.

E-mail address: dawen.sun@ucd.ie (D.-W. Sun).URLs: www.ucd.ie/refrig, www.ucd.ie/sun (D.-W. Sun).

respiration rate and nutritive value by using controlledatmosphere. However, the controlled atmosphere tech-nique is generally not suitable for domestic uses.

On the other hand, research has confirmed that con-trolled freezing-point storage at non-freezing tempera-ture-zone between the freezing point of water and that ofan individual material can prolong the storage-life of freshfood and also provide good quality retention (Mizunoet al., 1990), as a result some home refrigerators are nowdesigned with 0 �C compartment to prolong food preserva-tion. In the controlled freezing-point storage, temperaturesis the most important factor. However, detailed informa-tion on the technique is not available. Therefore, in thisstudy, the effects of storage temperatures on quality ofgreen bean stored at 0 �C is studied.

2. Materials and methods

2.1. Materials

Green bean (Phaseolus vulgaris L.) with commercialmaturity according to shape, firmness and colour, was

26 L. Guo et al. / Journal of Food Engineering 86 (2008) 25–29

obtained from a local farm (Heilongjiang Academy ofAgricultural Science, Harbin, China). The beans werepicked randomly from 200 plants, and a total of 20 kggreen beans were packed in plastic boxes and transportedto the laboratory within 2 h after harvest and stored at rel-evant storage temperatures. During storage, respirationrate, weight loss, titrable acidity, reducing and total sugars,total soluble solids and skin colour were measured. All theexperiments (except for the respiration rate) were repeatedthree times and the average values are reported.

2.2. Storage conditions

Three storage conditions were used, i.e., room-tempera-ture storage, cold storage and controlled freezing-pointstorage. For room storage, the room-temperature was setat 25 ± 2 �C and controlled by an air-conditioner (MSH-J11UV, Mitsubishi, Japan), the green bean samples wereplaced on a table in the room. The tests were performedfor up to 12 days as after that the beans had deterioratedto such an extent that the tests could not be performed.For cold storage, the temperature of the refrigerator(KK28E76, Siemens, Germany) was set at 8 ± 2 �C, thegreen bean was placed on the glass board in it. For con-trolled freezing-point storage (Fig. 1), special arrangementwas made with two plastic buckets used. The green beanwas placed in an inner bucket, which was then placed insidean outer bucket, and ice–water mixture was filled betweenthe walls of the two buckets so that the temperature in theinner bucket was precisely controlled at 0 �C. A digitalthermometer (ZDR-11, Zheda Electric Equipment Co.,China) was used to monitor the temperature. In this study,all the samples were stored in low air flow (air rate<0.15 m/s), and relative humidity of storage was 80–90%at 8 �C and 0 �C, while 30–40% at 25 �C.

2.3. Respiration rate

Green bean (1000 g) was placed in a closed glass con-tainer, separated with 20 ml 0.4 M sodium hydroxide onthe bottom. The container was placed at the same storage

Fig. 1. Experimental arrangement for green bean stored at precisetemperature of 0 �C.

temperature. After 1 h, the samples were taken away, thesodium hydroxide was mixed with 5 ml saturated BaCl2and 1% phenolphthalein (as the indicator). The mixturewas then titrated by 0.2 M oxalic acid until pink colour dis-appeared, based on which, the carbon dioxide was calcu-lated by a conversion table (Zhang et al., 2003). Themeasurement was calibrated according to the ChineseNational Standard GB/T 601-2002 (Anon., 2002) beforeexperiment.

2.4. Reducing sugar and total sugar

Samples of 10 green beans were homogenized in a blen-der (HR1724, Philips, Netherlands) for 1 min. The slurrywas heated in a water bath set at 50 �C for 20 min, filteredusing a filter paper, the filtrate was used for reducing sugarsanalysis. The same slurry was dissolved in 6 mol/l HClsolution, and heated by a boiling water bath for 30 min, fil-tered using a filter paper, the filtrate was used for total sug-ars analysis. The filtrate was neutralised with 6 mol/lNaOH. The sugars were then reacted with 3,5-dinitrosali-cylic acid solutions to form a red compound and theabsorbance were measured using a spectrophotometer(UV-1700, Shimadzu, Japan) at 540 nm against a reagentblank, the standard curve was obtained by using standardglucose solutions.

2.5. Total soluble solids and titrable acidity

Hundred grams of diced green beans with 100 g dis-tilled water were homogenized, the homogenate was fil-tered through a filter paper and the filtrate wascentrifuged at 2000 rpm for 1 min using a desktop centri-fuge (LDZ5-2, Beijing Medical Centrifuges Ltd., China).Total soluble solid contents were measured by using ahand refractonmeter (WAY, Shenguang ElectronicsLtd., Shanghai, China).

To measure the titrable acidity, 10 ml homogenate wasmixed with 0.3 ml 1% phenolphthalein, the mixture wastitrated with 0.1 M sodium hydroxide to permanent pinkcolour (pH 8.1). The data were interpolated correspondingto pH 8.1, then calculated as citric acid (mg)/sample (100 g)(Rocha, Brochado, Kirby, & Morais, 1995).

2.6. Skin colour

Samples of 10 green beans were used for measuring skincolour. Surface colour of each individual green bean wasmeasured by a colorimeter (8C-80C, Kangguang Instru-ments Ltd., Shanghai, China) and expressed as �a* valueand hue value.

2.7. Statistical analysis

Variance analysis and Duncan multiple-range test wereperformed by the procedure of SAS 8.0 (SAS Institute,Cary, NC).

L. Guo et al. / Journal of Food Engineering 86 (2008) 25–29 27

3. Results and discussion

3.1. Respiration rate

The respiration rate of green beans increased to thepeak, after an initial decrease for two days storage. Thetime to reach the respiration peaks were affected by thestorage temperatures, i.e., 187.9 CO2 mg/kg h at 25 �Cafter storage for five days, 171.6 CO2 mg/kg h at 8 �Cafter 11 days storage, and 109.2 CO2 mg/kg h at 0 �Cafter storage for 12 days (Fig. 2). Similar data were alsoobtained in experiments repeated in different seasons(Guo, 2004). Green beans stored at 25 �C increased inCO2 production by enlarging seed followed by a rapiddecrease in CO2 evolution by the pods (Watada & Mor-ris, 1967). The current result agrees with the respirationpattern of snap beans stored at 20 �C with respirationrate in the range of 100–240 CO2 mg/kg h depending onmaturity (Watada & Morris, 1967), and similar to those(170–234 CO2 mg/kg h at 15–20 �C, respectively) reportedby Gross, Wang, and Saltveit (2004). Fig. 2 also indi-cated that the occurrence of the peak respirationdepended on the storage temperatures. Duan et al.(2001) investigated the storage of snap bean and alsofound the following respiration peaks depending on stor-age time and temperature: 183 CO2 mg/kg h at 25 �Cafter storage for four days, 195 CO2 mg/kg h at 12 �Cafter eight days storage, 193 CO2 mg/kg h at 10 �C after11 days storage, and 187 CO2 mg/kg h at 7 �C after stor-age for 15 days. The green bean stored at 0 �C presenteda lower respiration rate with slow metabolism, which issimilar to the long bean with 80 CO2 mg/kg h stored at0 �C (Zong, Cantwell, Morris, & Rubatzky, 1992). Thisis due to at low temperatures, the enzyme activities ofpolyphenol oxidase, ascorbate oxidase and glycolic oxi-dase are lowered, leading to the decrease in respirationrates. Similar patterns of low respiration rates wereobserved in sweet cherry and kiwi fruit stored at 0 �C(Alique, Zamorano, Martinez, & Alonso, 2005; Manolop-oulou & Papadopoulou, 1998).

0

40

80

120

160

200

0 3 6 9 12 15 18

Storage period (days)

Res

pira

tion

rat

e (C

O2

mg

/ kg

h)

25 ºC 8 ºC 0 ºC

Fig. 2. Influence of storage temperature on respiration rate of green bean.

3.2. Weight loss

Generally speaking, low temperature and high humiditynot only causes fruits and vegetable to reduce respirationrate, but also transpiration rate, both of which result inweight loss. The respiration rate of stored green bean wasnegatively correlated with storage temperature signifi-cantly, and the loss was 2.0% after storage at 0 �C, as com-pared with 23.9% at 8 �C after storage for 18 days. Hightemperature and low humidity induced over 44.3% loss at25 �C for the same storage period (Fig. 3).

3.3. Changes in titrable acidity

In general, the titrable acidity of green beans descended(P < 0.05) in the early stage of storage, especially at 25 �C,while the acids depleted less (P > 0.05) at 8 �C and 0 �Cafter six days (Fig. 4). The loss of ascorbic acid withstorage time could be probably enhanced by the activityof ascorbate oxidase, which is dependent on the pH ofthe vegetable and storage temperature (Sanchez-Mataet al., 2003b; Giannakourou & Taoukis, 2003). The acidsretained by low temperature storage were available for pro-longing shelf-life and retaining nutritional values of greenbeans.

3.4. Changes in sugars

The degree of sugar consumption for respiration alsoaffected the shelf-life and storage quality of fruits and veg-etable. The reducing and total sugars of green beandecreased (P < 0.05) at 25 �C storage (Fig. 5), due to higherrespiration rate and formation of polysaccharide, whichagreed with the study of Martinez et al. (1995). Therefore,high temperature renders beans susceptible to hardeningphenomenon (Shiga, Lajolo, & Filisetti, 2004). Low tem-perature storage induced starch conversion to reducingsugars for preventing green beans from chilling injury, sothe starch content of beans decreased quite rapidly underthis condition (Martins, Lopes, & Silva, 2005; Nouriana,Ramaswamy, & Kushalappa, 2003). Sanchez-Mata et al.(2003a) reported the total sugar of ranging from 2.07 to

0

5

10

15

20

25

30

35

40

45

50

0 3 6 9 12 15 18Storage period (days)

Wei

ght l

oss

(%)

25 ºC 8 ºC 0 ºC

Fig. 3. Influence of storage temperature on weight loss of green bean.

0

10

20

30

40

50

0 3 6 9 12 15 18

Storage period (days)

Titr

able

aci

dity

(m

g/10

0g b

ean)

25 ºC 8 ºC 0 ºC

Fig. 4. Influences of storage temperature on titrable acidity of green bean.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 3 6 9 12 15 18Storage period (days)

Red

ucin

g su

gar

(g/1

00g

bean

)

25 ºC 8 ºC 0 ºC

1.0

2.0

3.0

4.0

0 3 6 9 12 15 18Storage period (days)

Tot

al s

ugar

(g/

100g

bea

n)

25 ºC 8 ºC 0 ºC

Fig. 5. Influence of storage temperature on reducing sugar (a) and totalsugar (b) of green bean.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 3 6 9 12 15 18Storage period (days)

Tot

al s

olub

le s

olid

s (g

/100

g be

an) 25 ºC 8 ºC 0 ºC

Fig. 6. Influence of storage temperature on total soluble solids of greenbean.

4

5

6

7

8

9

10

0 3 6 9 12 15 18Storage period (days)

Skin

col

or (

-a*)

25 ºC 8 ºC 0 ºC

104

108

112

116

120

124

0 3 6 9 12 15 18Storage period (days)

hue

25 ºC 8 ºC 0 ºC

Fig. 7. Influence of storage temperature on skin colour of green beans.

28 L. Guo et al. / Journal of Food Engineering 86 (2008) 25–29

2.53 (g/100 g bean) for green beans stored for up to 18 daysat 8 �C, while Sanchez-Mata, Camara-Hurtado, and Diez-Marques (2002) indicated the total sugar in the range of1.76–2.92 (g/100 g bean) for freeze-dried green beans, bothdetermined by HPLC. These reported data are lower thanthe current result, probably due to different measurementtechnique used. Schwald, Chan, Breuil, and Saddler(1988) reported the possibility of different values of reduc-ing sugars obtained by colorimetric methods as comparedwith HPLC technique.

3.5. Changes in total soluble solids

The total soluble solids decreased (P < 0.05) in the earlystage of storage for respiration consumption, then metabo-lism of the beans modulated to new condition with increas-ing physiological supplements from beans, and increase intotal soluble solids of beans may also attribute to loss ofmoisture (Nanda, Rao, & Krishnamurthy, 2001) (Fig. 6).

3.6. Changes in skin colour

In the CIELAB space, �a* value shows green colourand +b* yellow colour. The �a* value and hue value of

L. Guo et al. / Journal of Food Engineering 86 (2008) 25–29 29

green beans stored at 25 �C decreased rapidly (P < 0.05)after six days, which indicated the change of the colourfrom green to olivine colour, finally to yellow. The greenbean stored at 0 �C kept green as indicated by the littlechanges (P > 0.05) in its �a* value and hue value(Fig. 7). At lower storage temperatures, skin colour is sta-bilised probably by the formation of metal–chlorophyllcompounds (Martins & Silva, 2002).

4. Conclusions

Experiments were conducted to store green beans forup to 18 days under three conditions, i.e., controlledfreezing-point storage at 0 �C, cold storage at 8 �C androom-temperature storage at 25 �C. Compared with theother two storage conditions, controlled freezing-pointstorage at 0 �C showed preferable higher physiological(respiration rate, acids and sugar) and commercial (solu-ble solid, weight and colour) qualities. Therefore, thegreen beans stored at 0 �C should be recommended forprolonging the shelf-life of the product. Research onthe application of the controlled freezing-point storagetechnique should be extended to cover more fruit andvegetable varieties.

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