STUDY ON PRESTABILIZATION OF PUMPKIN ( CUCURBITA MOSCHATA ) BY OSMOTIC DEHYDRATION IN QUATERNARY...

STUDY ON PRESTABILIZATION OF PUMPKIN(CUCURBITA MOSCHATA) BY OSMOTIC DEHYDRATION IN

QUATERNARY COMPLEX SOLUTION

A. DEROSSI1*, T. DE PILLI1, R. GIULIANI1, I. ORLANDO2,L. PALMIERI2 and C. SEVERINI1

1Department of Food ScienceUniversity of Foggia

via Napoli 25, 71100 Foggia, Italy

2Stazione Sperimentale per l’Industria delle Conserve Alimentari (SSICA) ofParma Angri (SA)-based

Via Nazionale 121, 84012, Italy

Accepted for Publication December 22, 2008

ABSTRACT

Pumpkin is a very appreciated vegetable for both unique sensory char-acteristics and significant content of substances with healthy effects. Very few“pumpkin-processed foods” with adequate shelf life are available on themarket because of poor knowledge relative to its behavior during industrialprocesses. The possibility to reduce water activity (Aw) and pH of pumpkinsamples by osmotic treatment in complex (quaternary) solution was studied byresponse-surface methodology (RSM). The reduction of pH and Aw until valuesrecognized as safe for spore germination of Clostridium botulinum andClostridium perfringens was respectively obtained without significant colordegradation. Moreover, the modulation of the independent variables, carriedout by using the best-fit equation, was aimed at prestabilizing pumpkin creams.

PRACTICAL APPLICATIONSjfpe_364 398..413

The poor knowledge concerning the behavior of pumpkins during indus-trial processing reduces their uses to produce a variety of “pumpkin food” withadequate shelf life. In this article, the possibility of applying an osmotictreatment in a complex (quaternary) solution in order to simultaneously obtaina reduction in water activity (Aw) and pH was presented. In addition, theutilization of model equation allows the simulation of osmotic dehydration,

* Corresponding author. TEL: +39-0881-598245; FAX: +39 0881-589222; EMAIL: [email protected]

Journal of Food Process Engineering 34 (2011) 398–413. All Rights Reserved.© 2010 Wiley Periodicals, Inc.DOI: 10.1111/j.1745-4530.2009.00364.x

398

giving information on the process conditions usable to reach Aw and pH valuesrecognized as risk threshold for spore germination of Clostridium botulinumand Clostridium perfringens.

INTRODUCTION

Pumpkins are rich in vitamin, pectin, mineral salts and several com-pounds beneficial for human health (Djutin 1991). Pumpkins are annual orperennial climbing or trailing herbs, comprising about 25 species, some ofwhich are economically important, such as Cucurbita maxima, Cucurbitamoschata and Cucurbita pepo (Mayor et al. 2007a).

The content of b-carotene ranges from 0.06 to 7.4 mg/100 g, that ofa-carotene ranges from 0 to 7.5 mg/100 g and that of lutein from 0 to 17 mg/100 g (Murkovic et al. 2002). Moreover, consumers highly appreciate thepumpkin’s soft taste and flavor. In Italy, pumpkins are widely tilled and usedto prepare several homemade seasonings. Nevertheless, pumpkins are usuallysold as raw material, and very few products with adequate shelf life can befound in the markets. Some authors (Ptitchkina et al. 1998) studied the effectsof pumpkin powder on functional properties (bread making) and organolepticacceptability of wheat bread. The authors showed that small additions ofpumpkin powder in dough produces bread with high loaf volume and a gooddistribution of porosity. Nevertheless, the poor knowledge concerning thebehavior of pumpkins during industrial treatments hinders their economicdevelopment and reduces their use in producing pumpkin food with adequateshelf life.

It is well known that the lowering in water activity (Aw) can reducechemical and physical degradative reactions as well as microbial growth(Labuza 1970), but often, in this way, foods can develop very different sensorycharacteristics from the fresh material (texture, taste, color, etc.).

Osmotic dehydration (OD) is widely used to reduce water content of foodby soaking it in hypertonic solution of sugars and/or salts. During OD, acountercurrent flow of water from biologic tissue to osmotic solution andsolute in the opposite side occurs with the result that foods lose weight andshrink. The driving force of OD is the osmotic pressure gradient betweenfood and solution, but several variables are able to affect the kinetic of masstransfer and food quality (Nsonzi and Ramaswamy 1998a; Barat et al. 1999;Salvatori and Alzamora 2000; Genina-Soto et al. 2001; van Nieuwenhuijzenet al. 2001).

Usually, OD is used to produce intermediate-moisture foods such ascandied fruit, semidried fruit salad and salted fish and meat (Marouzè et al.2001; Rastogi et al. 2002; Pan et al. 2003; Corzo and Bracho 2004; Chiralt

OSMOTIZED PUMPKIN IN COMPLEX SOLUTION 399

and Talens 2005; Mujaffar and Sankat 2005). Also, OD is performed aspretreatment before air-drying (Nieto et al. 1998), freezing (Maestrelli et al.2001) and vacuum-drying (Dixon and Jen 1977). Moreover, OD can mini-mize color loss (Nsonzi and Ramaswamy 1998b) as well as nutrient loss(Shi et al. 1999). Hence, the improvement of nutritional and organolepticquality as well as the partial microbial stability of pumpkin could beachieved by applying a suitable OD (Castilho et al. 2007). Nevertheless, fewpapers concerning the effects of OD on pumpkins are available on literature(Mayor et al. 2006, 2007a,b; Kowalska et al. 2008). Mayor et al. (2007a)studied the kinetics of OD of pumpkins in water–sucrose–NaCl solutionusing a concentration range of solute ranging from 45 to 58% for sucroseand from 0 to 11.25% for NaCl. They showed that the water diffusion coef-ficient increased with NaCl concentration. Castilho et al. (2007) studied thekinetic of OD on pumpkin slices in binary water–sucrose solution at 40, 50and 60% (w/w). They showed that both water and sucrose effective diffusioncoefficients were independent from osmotic concentration when 50 and 60%were used probably because of two opposite effects: first, the decrease ofdiffusivity as sucrose concentration increases (Henrion 1964); second, theincrease of sucrose concentration produces a damage tissue, which makesdiffusion easier.

Food pH reduction by acidifying-blanching, by dipping in acid solutionor by direct addiction of organic acids is among the most important prestabi-lization process that industries use to produce canned food. According to theFood and Drug Administration (FDA 2002), in order to avoid the germinationof Clostridium botulinum spores, a pH value below 4.6 is required. Unfortu-nately, in these cases, the acidification process can produce an unacceptablesour taste or color degradation. So, based on hurdle technology (Leistner andRodel 1976), a prestabilization treatment by a simultaneous pH and Aw reduc-tion could preserve the sensory characteristics of pumpkins, reducing theintensity of the two treatments. In a previous article, we studied the possibilityto prestabilize “pumpkin creams” by direct addition of organic acids andhumectants (Derossi et al. 2005). Severini et al. (2008) showed that a preser-vation of the natural color of pesto was achieved when Aw and pH values werereduced at the same time.

In this work, the response-surface methodology (RSM) was used to studythe effects of five process variables on pH and Aw reduction of pumpkinsduring OD treatment with two different aims: (1) to reach the Aw and pHreference values to control the growth and sporulation of C. perfringens andC. botulinum; and (2) to improve the knowledge of pumpkin behavior duringOD in complex (quaternary) solution. In addition, the use of RMS could beconveniently used to control the process. Also, the potential change in color ofpumpkin was evaluated.

400 A. DEROSSI ET AL.

MATERIALS AND METHODS

Raw Material

Two pumpkins (Cucurbita moschata) were purchased from a localmarket (Foggia, Italy) and kept in refrigerated condition (6C) until the experi-ments were performed.

Experimental Design

A central composite design (CCD) with five variables (NaCl, sucrose,lactic acid concentrations (w/w), treatment time and temperature of solution)and five levels (the values of each variable) was developed. Lactic acid con-centrations ranging between 0.10 and 0.16% (w/w) were chosen according tothe results of previous experiments performed on pumpkin cream (Derossiet al. 2005), as well as the type and concentrations of osmotic agents (NaCland sucrose) (Severini et al. 2008), with the aim to reach the minimum docu-mented value of C. perfringens spore germination calculated in model systems(Colantoni and Magri 1997). A total of 30 experiments were obtained on thebasis of the following equation:

n n n ntot c *= + + = + + =0 16 4 10 30 (1)

where n0 = 2k-1, k is the number of variables, nc is the number of central pointsand n* is the number of star points (Box et al. 1978). All experimental con-ditions are shown in Table 1.

Osmotic dehydration treatments (OD)

Pumpkins previously washed and peeled were manually cut into cubesamples. The dimension of samples was approximately 1.5 ¥ 1.5 ¥ 1.5 cm.Ten cubes with a weight of 6 � 0.1 g were randomly chosen from all samplesand used for each experiment. Osmotic solutions were prepared, adding dif-ferent concentrations of commercial NaCl (Italkali SPA, Palermo, Italy), com-mercial sucrose (Inagra SRL, San Maurizio, RE, Italy) and lactic acid (CarloErba Reagents SPA, Milano, Italy) according to the CCD reported in Table 1.Osmotic solutions were heated at different temperatures ranging between 25and 65C by using a thermostatic bath (Ecoline, Lauda, Germany). The osmotictreatments were performed in static condition (without agitation) and with asolution–product mass relation of 4:1. Taking into account the number andweight of pumpkin cubes (above defined) we used a volume of ~240 mL ofsolution for each experiment. At the end of process, the samples were washedfor a few seconds with tap water and gently blotted with tissue paper in orderto remove the adhering osmotic solution. All treatments were carried out intriplicate.


Aw Measurements

After each experiment, pumpkin samples were homogenized through anautomatic blender (Waring Commercial, Torrington, CT) until a homogeneouscream was obtained. Aw was measured by a hygrometer AQUALAB CX-2(Decagon, Device Inc., Pullman, WA) previously calibrated by using sixsaturated salt solutions (NaCl, K2CO3, K2SO4, LiCl, MgCl2, BaCl2 (Bariumchloride)). All measurements were carried out in triplicate.

Determination of pH

pH measurements were performed on pumpkin cream obtained asdescribed in previous section. A pH-meter mod. micropH (Crison, Instru-

TABLE 1.Experimental conditions of osmotic dehydration process developed

on the basis of Central Composite Design

Run NaCl(% w/w)

Sucrose(% w/w)

Lactic Acid(% w/w)

Time(min)

Temperature(C)

1 17 25 0.12 45 552 17 25 0.12 75 353 17 25 0.16 45 354 17 25 0.16 75 555 17 35 0.12 45 356 17 35 0.12 75 557 17 35 0.16 45 558 17 35 0.16 75 359 23 25 0.12 45 35

10 23 25 0.12 75 5511 23 25 0.16 45 5512 23 25 0.16 75 3513 23 35 0.12 45 5514 23 35 0.12 75 3515 23 35 0.16 45 3516 23 35 0.16 75 5517 14 30 0.14 60 4518 26 30 0.14 60 4519 20 20 0.14 60 4520 20 40 0.14 60 4521 20 30 0.10 60 4522 20 30 0.18 60 4523 20 30 0.14 30 4524 20 30 0.14 90 4525 20 30 0.14 60 2526 20 30 0.14 60 6527 20 30 0.14 60 4528 20 30 0.14 60 4529 20 30 0.14 60 4530 20 30 0.14 60 45


ments, Allen, Spain) previously calibrated with buffer solutions (pH 4.00 andpH 7.01) was used, and three pH measurements were carried out for eachsample.

Determination of Colorimetric Parameters

Color assessments were performed with a Minolta Chroma MeterCR-300 (Osaka, Japan) by using Commission Internationale d’Eclairage(CIE) L*a*b* space color. The instrument was calibrated with a white stan-dard tile (Y = 92.8; x = 0.3132; y = 0.3192). The experimental of L*a*b* spacecolor were expressed in terms of hue angle or tonality, defined as arctan[(b*/a*)], and Chroma or saturation index was defined as [a*2 + b*2]1/2 (Little1975) because these parameters give more information related to the spatialcolor distribution (Little 1975; Ihl et al. 1994).

Statistical Analysis

Aw, pH and color indexes were submitted to statistical analysis by anonlinear regression (Standard Least Square Fitting) to fit a second-ordermathematical model using the following polynomial equation:

y B B B Bi i j j ij i j= + + +02Σ Σ Σx x x x (2)

where y is the modeled dependent variable (Aw and pH); B0 is a constant value;xi, xj and xixj are the independent variables; and Bi, Bj and Bij are the regressioncoefficient values of the model. This model allows to evaluate the effects ofthe linear (xi), quadratic (xi

2 ) and combined (xixj) terms of the independentvariables to be assessed on the independent ones. The goodness of fit ofmathematical model was evaluated by means of relative deviation (E%), stan-dard error (SE) and P level. E% was obtained as follows:

E% *= −∑100 Npred

i

N

expexp

exp

exp

Also, in order to describe both the individual and interactive effects of theindependent variables on pH, Aw and color values, and iso-response surfaceswere developed. All data were analyzed by using Statsoft ver. 6.0 software(StatSoft Inc., Tulsa, OK).

RESULTS AND DISCUSSION

Figure 1 shows the effects of independent variables on Aw values. It isclear that the magnitude of interactive term of time–temperature on Aw was


higher than sodium chloride concentration (-13.19 and -4.35, respectively);furthermore, the E% values equal to 2.23 and P level of <0.001 stated thatthe polynomial equation well fitted the experimental data. On the basis ofthese results, it seems that our experimental concentration of sucrose wasunable to decrease Aw of samples. These results seem not to be in accor-dance with literature because other scientists showed that sucrose concen-tration significantly affects water loss and Aw of pumpkin (Castilho et al.2007). Besides, several works showed the effect of sucrose concentration onwater mass exchange of several vegetables during OD (Rastogi and Ragha-varao 1994; Rastogi et al. 1997; Passo Tsamo et al. 2005). The reduction ofAw of samples during OD is caused by (1) the water flowing toward osmoticsolutions (i.e., dehydration process); and (2) the binding of osmotic agents(i.e., sugars or salts) with residual amount of water into vegetable tissues.So, it would be possible to explain our results taking into account the fol-lowing considerations: (1) sucrose has a higher molecular weight (342.30)than sodium chloride (58.44), and this big difference strongly affects themolecular concentration (molarity [M]) of osmotic solution. In particular,according to CCD used in this work, the molar concentration of solutionsranged from 0.58 to 1.16 M and from 2.39 to 4.44 M for sucrose and salt,respectively. Additionally, sucrose needs a long time to flow inside vege-table tissue because of its big size compared with sodium chloride ions.

FIG. 1. STANDARDIZED EFFECTS OF INDEPENDENT VARIABLES ON Aw VALUES


In fact, some researchers showed that several hours are necessary to reachwater mass exchange equilibrium when sucrose solution was used (Rastogiet al. 1997). Because we performed osmotic treatment for a time rangingfrom 30 to 90 min, it is reasonable to suppose that only few molecules ofsucrose would travel inside the pumpkin tissue as compared with sodiumchloride ions.

Also, it is well known that sodium chloride can reduce Aw more thansucrose because of the capability of Na+ and Cl- ions to bind water by strongelectrostatic bind; instead, sucrose interacts with water by weak hydrogenbond.

Figures 2 and 3 show the trend of independent variables on Aw values.According to the Pareto diagram, it is possible to observe the synergistic effectof time and temperature on Aw values (Fig. 2); after 30 min of soaking andincreasing temperature from 25 to 65C, Aw values decreased from 0.95 to

0.952

0.96

0.94

0.92

0.9

0.88

0.86

0.84

0.82

25.00

30.00

35.00

40.00

45.00

50.00

55.00

60.00

65.00 90

80

70

60

50

40

30

0.95 0.94 0.93 0.92 0.91 0.9 0.89 0.88 0.87 0.86 0.85 0.84

Wa

ter

activ

ity (

Aw

)

Temperature (°C)tim

e (min)

FIG. 2. TREND OF WATER ACTIVITY VALUES OF PUMPKIN SAMPLES ASA FUNCTION OF TEMPERATURE AND TREATMENT TIME DURING OSMOTIC

DEHYDRATION TREATMENTS


0.938. The same reduction was observed, applying an osmotic treatment at25C and increasing time from 30 to 90 min (Fig. 2). Also, the minimum Aw

value (0.84) was obtained when maximum temperature and treatment timewere used (65C and 90 min). Figure 3 shows the effect of NaCl concentrationand temperature on Aw values. In particular, this figure was obtained, keepingtime treatment as constant at the center of the CCD (60 min). Aw decreasedfrom about 0.958 to 0.92 using a salt concentration of 14% and increasingtemperature of solution between 25 and 65C. On the other hand, keepingconstant temperature at 25C and increasing salt concentration, the reduction ofAw value was only of 0.01 unit (from about 0.96–0.95). Instead, if the sametreatments were performed at 65C, Aw values decreased from 0.92 to 0.88. So,by using RSM, it is possible to define different combinations of the studiedvariables to reach Aw values below 0.935 that represent the threshold for spore

0.96 0.95 0.94 0.93 0.92 0.91 0.9 0.89 0.88

0.96

0.94

0.92

0.9

0.88

0.86

14.00

16.00

18.00

20.00

22.00

24.00

26.0060.0065.00

55.00

50.00

45.00

40.00

35.00

30.00

25.00

Wa

ter

activ

ity (

Aw

)

Temperature (°C)

Nacl (% w/w)

FIG. 3. TREND OF WATER ACTIVITY VALUES OF PUMPKIN SAMPLES ASA FUNCTION OF NaCl CONCENTRATION AND TEMPERATURE DURING OSMOTIC

DEHYDRATION TREATMENTS


germination of C. perfringens (Colantoni and Magri 1997). For instance, anAw value of <0.930 can be obtained by using the following operative condi-tions: 14% salt concentration; temperature of 60C and soaking time of 1 h(Fig. 3). Concerning the color changing of pumpkins, statistical analysisshowed that no variables significantly affected hue angle and chroma values(P > 0.05). Figure 4 shows the color values of raw material and osmotizedsamples; only small differences in terms of chroma values were observedbetween the samples.

Figure 5 shows the results of RSM on pH values of samples. The inter-active terms of acid-time and acid-temperature significantly affected pHvalues; however, they were slightly different and, as expected, both negativelycorrelated with pH values (-4.14 and -4.32, respectively). Also, in this case,E% value of 0.68 and a P-level of <0.001 stated the good fitting of experi-mental data.

Figure 6 shows the trend of pH values of pumpkin samples as a func-tion of temperature and time when lactic acid, sodium chloride and sucroseconcentration were kept constant at the center of CCD. When the osmosis

osmotized samples raw material

2030

4050

6070

8090

100

Chroma

0

50

100

150

200

250

300

Hu

e an

gle

FIG. 4. HUE ANGLE AND CHROMA VALUES OF PUMPKIN SAMPLES AFTER OSMOTICDEHYDRATION IN DIFFERENT OPERATIVE CONDITIONS


was performed at 25C and time increased from 30 to 90 min, pH valuesdecreased by 0.4 (from 4.9 to 4.5). The same reduction was obtained after30 min of treatment and after an increase of temperature from 25 to 65C.Instead, when the maximum values of both independent variables wereapplied, a pH value of 4.1 was reached. At the same time, it is interesting toevaluate the effect of the same operative conditions on pH and Aw values.For instance, performing a treatment at 60C for 70 min could be possible toobtain a pH value of ~4.3, sufficiently low to inhibit the C. botulinum sporegermination (FDA 2002) (Fig. 6). The same operative conditions bring to anAw value low enough (~0.90) to prevent the growth of C. perfringens(Fig. 2). By using the best-fit equations, it was also possible to obtain thetrend of Aw and pH as a function of time treatment, keeping the other inde-pendent variables as constant. In particular, Fig. 7 shows the trend of Aw andpH values of pumpkin samples as a function of time when the followingexperimental conditions were used: 0.14% lactic acid, 14% NaCl and twodifferent temperatures (45 and 65C). At 45C, a pH and Aw below 4.5 and0.935, respectively, could be reached after 60 min of treatment. Instead,when the osmosis was performed at 65C, the same results could be obtainedafter 41 min of soaking time. Therefore, several combinations of indepen-dent variables can be identified by interpolation to obtain pumpkin sampleswith desirable pH and Aw values.

FIG. 5. STANDARDIZED EFFECTS OF INDEPENDENT VARIABLES ON pH VALUES


CONCLUSIONS

OD of pumpkin in quaternary solution allowed to reach Aw andpH values low enough to control the Clostridium sporulation. In ourexperimental conditions, sucrose was not significant to reduce Aw valuesprobably for its physical and chemical characteristics. Having the treatmentperformed at 60C for 70 min, a pH of 4.3 was obtained. This value wassufficiently low to inhibit the C. botulinum spore germination; moreover,in the same conditions, an Aw value low enough to prevent the growth ofC. perfringens was reached. Also, slight color differences were measuredbetween raw and osmotized material. To sum up, these results can beused to define different conditions usable to reach safe Aw and pHvalues from the point of view of microbial growth (C. perfringens andC. botulinum).

4.9 4.8 4.7 4.6 4.5 4.4 4.3 4.2 4.1

5.2

5.0

4.8

4.6

4.4

4.2

4.0

3.8

3.6

40

50

60

70

80

90 6560

5550

4540

3530

2530

PH

Temperature (°C)

time (min)

FIG. 6. TREND OF pH VALUES OF PUMPKIN SAMPLES AS A FUNCTION OF TIME ANDTEMPERATURE OF OSMOTICALLY TREATED PUMPKIN SAMPLES


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STUDY ON PRESTABILIZATION OF PUMPKIN ( CUCURBITA MOSCHATA ) BY OSMOTIC DEHYDRATION IN QUATERNARY...

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