Effect of vacuum impregnation with calcium lactate on the osmotic dehydration kinetics and quality of osmodehydrated grapefruit

Journal of Food Engineering 90 (2009) 372–379

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Effect of vacuum impregnation with calcium lactate on the osmotic dehydration kinetics and quality of osmodehydrated grapefruit M.J. Moraga, G. Moraga, P.J. Fito, N. Martínez-Navarrete * Department of Food Technology – Institute of Food Engineering for Development, Universidad Politécnica de Valencia, Camino de Vera s/n, 46022, Valencia, Spain

a r t i c l e

i n f o

Article history: Received 31 January 2008 Received in revised form 25 June 2008 Accepted 10 July 2008 Available online 17 July 2008 Keywords: Vacuum impregnation Osmotic dehydration Pulsed vacuum osmotic dehydration Calcium lactate Respiration rate Microbial counts Mechanical properties

a b s t r a c t The effect of calcium lactate (2%) on osmotic dehydration kinetics and on the respiration rate, mechanical properties and shelf-life of fresh, vacuum impregnated (VI) and pulsed vacuum osmodehydrated (PVOD) grapefruit was evaluated. An isotonic solution was used for VI and a 55°Brix sucrose solution for PVOD treatments. Vacuum pulse was carried out at 50 mbar for 10 min and osmotic treatment was extended to 180 min. An increase from 5 to 8 days in the shelf-life of grapefruit was achieved due to sample dehydration and to 11 days if calcium is added to the osmotic solution, with no effect on the mechanical properties of the sample. This effect seems to be related with the decrease in the cellular respiration rate caused by dehydration and enhanced with the presence of this ion. Nevertheless, the water effective diffusion coefficient is reduced from 3.64  1011 to 1.80  1011 m2/s when calcium lactate was added during the osmotic treatment. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The fact that many fruits and vegetables possess antioxidant properties which are related to the prevention of degenerative diseases is widely recognized (Kaur and Kapoor, 2001). Grapefruit contains important micronutrients and phytochemical compounds, such as vitamin C, vitamin E, carotenoids, flavonoids such as hesperidin, narirutin and naringin, and other compounds with antioxidant capacity (Del Caro et al., 2004; Peiró et al., 2006) which are essential to remain healthy. Consumer demand for fresh ready-to-use products has led, over the last 20 years, to an increasing interest in minimally processed fruits and vegetables, as these products combine freshness and convenience (Kim et al., 1993). Minimally processed fruits and vegetables are products that contain living tissues, which have suffered minor changes from their fresh state. Unfortunately, common processes undergone by fresh fruit, such as peeling, cutting and slicing, accelerate product deterioration since they provoke cell disruption with the subsequent decompartmentation of enzymes and substrates, thus resulting in an enhanced rate of physiological reactions. Quality loss occurs due to enzymatic browning, firmness reduction, off-flavour development, a decrease in nutritional value, physiological changes and microbiological

* Corresponding author. Tel.: +34 96 387 70 07x73655; fax: +34 96 387 73 69. E-mail address: [email protected] (N. Martínez-Navarrete). 0260-8774/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2008.07.007

growth (Brecht, 1995; Watada et al., 1996; Pretel et al., 1998), all of which depend not only on the storage time and temperature but also on the packaging used (Gunes and Chang Lee, 1997). Osmotic dehydration (OD) is a process that may be used to increase the shelf-life of products, minimally decreasing the quality properties of the fresh fruit. In the last few years, the osmotic dehydration of fruits has become relevant as a technique to decrease water activity, thus increasing the product’s stability (RaoultWack, 1994). During the osmotic dehydration process, a cellular tissue is dipped in a highly concentrated solution in order to promote water loss in the cells of the fruit. Nevertheless, due to the open structure of the tissue in the intercellular spaces, there also takes place a diffusion of external solutes and a hydrodynamic gain of external solution (Chiralt and Talens, 2005). Vacuum application for a short period of time at the beginning of the osmotic process (pulsed vacuum osmotic dehydration, PVOD) has beneficial effects on process kinetics and fruit quality in many fruits and also helps to reduce energy costs (Fito and Chiralt, 2000). Fruit impregnation with osmotic solution occurs in this case, which implies that the gas is exchanged in the pores for the external fluid (Fito, 1994). In this sense, solutes with a physiological function such as calcium can be incorporated into the cellular structure of the fruit, which could affect osmotic dehydration kinetics. The kinetics of OD processes is usually evaluated in terms of water loss, weight loss and solid gain (Fito and Chiralt, 1997) and mainly depends on raw material characteristics (RaoultWack, 1994) and on operational conditions, such as solution

M.J. Moraga et al. / Journal of Food Engineering 90 (2009) 372–379

373

Nomenclature A aw Deff E FF l M RQ RRi V xi Y

area under the force–deformation curve (N) water activity effective diffusivity (m2/s) slope of the linear part of the force–deformation curve at low deformation force at fracture point (N) semi-thickness of the sample (m) mass of the samples (g) respiration quotient respiration rate, expressed as i: O2 consumption or i: CO2 production (mli kg1 h1) volume of headspace (ml) in Eq. (3) mass fraction of the component i (w: water, s: soluble solid) in the sample (g i/g product) reduced driving force in the product liquid phase at time t of dehydration

concentration and temperature (Barat et al., 2001), exposure time (Escriche et al., 2000) or pressure (Barat et al., 2001; Fito and Pastor, 1994). One very serious problem with fresh-cut fruit products is that of tissue softening which can limit shelf-life. Fresh-cut fruit firmness is an important quality attribute that can be affected by cell-softening enzymes present in the fruit tissue (Varoquaux et al., 1990) and by a decrease in turgor due to water loss. The role calcium plays in increasing cell rigidity has been related to its ability to bind with mid lamella pectate (Rolle and Chism, 1987). In this way, the firmness of the flesh of fresh-cut fruit products may be improved if treated with calcium compounds. Dipping fresh-cut products in solutions of 0.5–1.0% calcium chloride is very effective at maintaining product firmness (Ponting et al., 1971, 1972). Calcium lactate has recently been shown to be as effective as the chloride form, without imparting a bitter flavor at higher concentrations (Luna-Guzmán and Barreto, 2000). On the other hand, a decrease in the respiration rate has been observed in samples treated with calcium. This effect may be related to the increase in the membrane rigidity which blocks the gas interchange (Saftner et al., 1999; Serrano et al., 2004), to a delay in the arrival of the senescence (Lester, 1996) or to the level of active water transport inhibition (Kinoshita et al., 1995). Microbial decay of fresh-cut fruit may occur much more rapidly than in vegetable products due to the higher levels of sugars found in most fruit. However, the acidity of fruit tissue usually helps to delay bacterial growth, but not the growth of yeasts and moulds. The predominant microorganisms associated with spoilage of fresh-cut vegetables are bacteria (e.g. Pseudomonads spp.), whereas the predominant microorganisms associated with the spoilage of fresh-cut fruit products are yeasts and moulds (Gorny et al., 1998). The objective of this work was to study the effect of osmotic dehydration and vacuum impregnation with calcium on the mechanical properties and on some aspects related to the shelf-life of the grapefruit, such as respiration rate and microbial counts. The effect of calcium on the osmotic dehydration kinetics was also studied. 2. Materials and methods 2.1. Raw material Grapefruits (Citrus paradisi), of the cultivar Star Ruby, were purchased in local markets in Valencia (Spain). Grapefruit pieces were

yi

concentration of the gas i in the head-space, i: CO2 or O2, (ml i/100 ml) zw mass fraction of water in the sample liquid phase (g water/g liquid fraction) DMi relative mass variation of the component i, w: water; s: solutes, (lost or gained g i/g fresh sample) DMT total mass variation of samples (g/g fresh sample) eF deformation at fracture point (N) Superscripts 0 fresh sample t time t of dehydration 1 equilibrium t0 time t of measurement t 00 initial time of measurement

selected on the basis of a similar degree of ripeness (ratio °Brix/ acidity = 5–7) and apparent fruit quality (color and firmness). They were stored in refrigerated chambers at 10 °C and at 85–90% relative humidity until they were used (less than 24 h). Before treatments, fruit pieces were manually washed and peeled and cut, perpendicularly to the fruit axis, into 1 cm thick half slices. Only the four central slices were used for treatments. Fig. 1 shows the process flow diagram explaining the different studies carried out, as detailed below. 2.2. Sample treatments 2.2.1. Pulsed vacuum osmotic dehydration (PVOD) Grapefruit samples were submitted to osmotic dehydration processes in a temperature-controlled water bath at 30 °C (J.P. Selecta S.A., Precisterm S-141, Barcelona, Spain). Sucrose (food grade commercial sugar) mixed with heated (30 °C) distilled water until total dissolution, was used to prepare a 55°Brix osmotic solution (ratio solution/fruit 10:1). Calcium lactate 5-hydrated (Number CAS 5743-47-5, Panreac, Barcelona, Spain) was added to osmotic solution (0% and 2% (w/w) calcium lactate). A pressure of 50 mbar was applied to the system for the first 10 min of the osmotic process, afterwards restoring the atmospheric pressure for 10 min more in order to promote the previous sample impregnation with the osmotic solution. The impregnated grapefruit samples were immersed in a plastic tank filled with the osmotic solution and a plastic screen was placed on the basket to keep the slices totally immersed and separate from the stirrer working at 250 rpm (Heidolph Instruments, RZR 2102 control, Schwabach, Germany), used to homogenize the syrup continuously during the operation. For kinetic studies, samples were kept immersed in the osmotic solution for different times (t = 0, 15, 30, 45, 60, 90, 180, 300, 480 and 600 min) after which they were withdrawn from the solution and analyzed as described below (Section 2.3). Moreover, PVOD samples dehydrated for 180 min were analyzed as to the different aspects commented on in Section 2.4. 2.2.2. Vacuum impregnation (VI) In order to separate the effect of the vacuum pulse and the osmotic dehydration in the study, the same calcium levels (0% and 2% (w/w) of calcium lactate) were added to an isotonic solution (18°Brix, aw = 0.987 0.003) and vacuum impregnation of grapefruit samples was performed in the same conditions (10 min at 50 mbar and then atmospheric pressure was restored and maintained for 10 min more).

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FRESH-CUT grapefruit

KINETIC STUDY ΔM,xw, xs

Deff

EFFECT OF TREATMENT

PVOD grapefruit 55ºBrix sucrose solution 0, 2% calcium lactate 10 min 50 mbar + 0-600 min 180 min 1013 mbar 1013 mbar

xs xw aw ΔM [Ca+2] Mechanical properties RRO2 RRCO2

VI grapefruit 18ºBrix sucrose solution 0, 2% calcium lactate 10 min 50 mbar + 10 min 1013mbar

EFFECT OF STORAGE (0-15days) Microbial growth RRO2 RRCO2

Fig. 1. Process flow diagram explaining the procedure of the study. PVOD: pulsed vacuum osmodehydrated grapefruit; VI, vacuum impregnated grapefruit; DM, weight loss; xs, soluble solids mass fraction; xw, water mass fraction; aw, water activity; Deff, effective diffusivity; [Ca+2], calcium concentration; RR, respiration rate.

2.3. Kinetic model PVOD grapefruit samples immersed in the osmotic solution with 0% and 2% calcium lactate (PVOD and PVODCa) for different times were analyzed in triplicate as to weight, soluble solid content and moisture content. Soluble solid content was determined with a 20 °C thermostated refractometer Atago Co., ABBE 3T, Tokyo, Japan. For moisture content, samples were dried in a vacuum oven at 60 °C till constant weight was reached (AOAC method 934.06, 2000). All these analyses were carried out on samples previously homogenized to 8000 rpm (IKAÒ, Ultra-turrax T25, Staufen, Germany). Experimental data were employed to calculate effective diffusivity of the water and solutes transferred during the osmotic treatments. Transport phenomena due to pseudodiffusional mechanisms were calculated from a simplified solution of Fick’s law. Due to the shape and dimensions of the slices, the major mass transfer may be assumed to occur through the cut surface of the slice and the sample may be considered as an infinite plane sheet. For this geometry, and considering the experiment as a short timeconsuming process, Eq. (1) (Crank, 1975) and Eq. (2) were used to estimate Deff.

1  Yt ¼ 2

Yt ¼

 1=2 Deff t

ztw  z1 w z0w  z1 w

l

2

p

ð1Þ

ð2Þ

In Eq. (2), for the equilibrium situation, the same composition of the osmotic solution was assumed for the product liquid phase and so 1/2 zw allows 1 ¼ 0:45. The slope of the linear fitting of (1  Yt) vs. t us to obtain Deff.

Calcium quantification was carried out using an ion chromatograph (Methrom Ion Analysis, Herisau, Switzerland), using a universal standard column (Metrosep C2-150, 4.0  150 mm) along with an eluent composed of tartaric acid (4.0 mmol/l) and dipicolinic acid (0.75 mmol/l), equipped with electronic detectors. In all cases, fruit samples were previously homogenized and centrifuged (J.P. Selecta S.A., Medifriger-BL, Barcelona, Spain) for 10 min at 12,000 rpm, to take 1 ml of supernatant. Measurements were carried out in duplicate. The mechanical properties of samples were analyzed at 25 °C by means of a puncture test using a Universal Texture Analyzer (Stable Micro Systems, TA.XT2, Surrey, England). A cylindrical 10 mm diameter punch was used, considering a relative deformation of 95% and a deformation rate of 0.2 mm s1. Six replicates were performed in each sample. A closed or static system was chosen to measure the respiration rate. Samples (150 g) were placed in 884 ml hermetic glass containers provided with a septum and stored in a temperature-controlled chamber (J.P. Selecta S.A., Hot-Cold M, Barcelona, Spain) at 10 °C for 6 days. Two replicates were performed for different samples. Volume samples of air from the headspace were withdrawn, at different times, with a needle connected to a gas analyzer. A head-space-gas analyzer, (PBI Dansensor A/S, CheckMate 9900, Ringsted, Denmark), was used to determine the O2 and CO2 contents inside the hermetic glass containers. Gas sampling was carried out every 30 or 60 min during the first 2 h and every 60 or 90 min until completing 8 h of measurement. After this, the containers were opened to renew the ambient air of the headspace. The respiration rate, expressed as O2 consumption rate and CO2 production rate, was calculated from the previously developed Eq. (3). 0 yti

¼

t0 yi 0

"

# ðM 0 =1000Þ 0 t  100RRi V

ð3Þ

2.4. Analysis Fresh-cut, PVOD (treated for 180 min) and VI samples, with and without 2% calcium lactate added to the solution (F, PVODCa, PVOD, VICa and VI, respectively), were analyzed in order to determine calcium concentration, mechanical properties and respiration rate. Respiration rate was also controlled for six storage days at 10 °C. Microbiological counts were analyzed in fresh-cut, PVODCa and PVOD grapefruit samples stored at 10 °C for 17 days. Water and soluble solids content were determined as described in Section 2.3. For aw a dew point hygrometer (Decagon, AquaLab CX-2, Washington, USA) was used.

In this equation, V was calculated from the volume of the glass and the volume of samples obtained from its mass and density. The ‘+’ sign is used to calculate CO2 generation and the ‘’ sign to calculate O2 consumption. Respiration rate values were referred to fresh-cut sample mass to make comparisons possible. The ratio of CO2 produced to O2 consumed, known as the respiratory quotient, was calculated. Samples (stored in PET packages at 10 °C) were analyzed as to total microbial count and yeasts and moulds in duplicate using Plate Count Agar (Scharlab, Barcelona, Spain) 48–72 h at 30 °C and Sabouraud Chloramphenicol Agar (Scharlab, Barcelona, Spain)

M.J. Moraga et al. / Journal of Food Engineering 90 (2009) 372–379

3–5 days at 30 °C, respectively. Sample dilutions were prepared and, after the incubation time, Petri dishes with a number of colonies between 30 and 300 for total count and between 0 and 30 for moulds and yeast were considered. Results were expressed as colony forming units (cfu) per gram of sample. Analyses of variance (ANOVA) were applied to evaluate differences among treatments, using StatgraphicsÒ Plus 5.1 software.

DM T ¼

DM i ¼

Mt  M0 M0 M t xt  M 0 x0i Mi

375

ð4Þ

ð5Þ

3.2. Composition of fresh and treated samples 3. Results and discussion 3.1. Calcium effect on water transport Water transport throughout the cell’s tissue structure occurs by different mechanisms, passive (diffusion, osmotic, bulk flow, etc.) and active (protein channels) (Agre et al., 1998). The effective diffusivity of water estimated during an osmotic process, calculated as described in Eq. (1), is a kinetic coefficient that includes all the transport mechanisms. Therefore, its value must allow for the evaluation of the impact of any factor, such as the presence of ion calcium, in the general water transport through the cellular tissue. Fig. 2 shows the mass balance, water loss and solute gain plotted against the gravimetrical loss of mass, validating the experimental data. Eqs. (4) and (5) were used to calculate them. The effective diffusivity of water estimated in our two experimental conditions, osmotic treatment with and without 2% w/w of lactate calcium added in the osmotic solution, was 1.80  1011 and 3.64  1011 m2/s (R2 0.939 and 0.856), respectively. The values of Deff in samples without calcium coincide with those reported by other authors working with other fruits under similar conditions (Barrera et al., 2004; Giraldo et al., 2003). The effect of calcium on decreasing Deff has also been previously described (Barrera et al., 2004). From these values, it is possible to conclude that Ca2+ plays a role in intracellular osmotic stress defence by restraining the water transport through plasmalemma membrane. This water transport reduction caused by the increase in calcium concentration could be explained by taking into account the described effect of this cation in the regulation of some protein channels like Protein PM228A (aquaporin) involved in active water transport which occurs with adenosine triphosphate (ATP) consumption (Lynch et al., 1989; Tyerman et al., 1999; Knight and Knight, 1999).

y= 0,9614x

PVOD

PVODCa

R2= 0,9854

3.3. Respiration rate

y= 0,9914x R2= 0,9939

ΔMt (g/g)

0 -0,15

-0,1

Table 1 shows the mass fraction of water, soluble solids and ion calcium analyzed in the different samples under consideration. As can be observed, VI treatment slightly affects the final composition of samples which will be related with the relatively small quantity of the solution impregnated in these samples and its isotonic character. The effect of osmotic treatment is reflected in both the water and soluble solid content of PVOD samples and, in agreement with the results presented before, the effect of calcium on the final water content of dehydrated samples can be observed. The calcium concentration in fresh grapefruit samples was similar to that published by other authors studying the same fruit variety (Peiró et al., 2006). As the composition of the three kinds of samples (F, VI and PVOD) was different, the mass fraction of ion calcium referred to the corresponding fresh sample mass was calculated, taking into account the weight of the sample before and after PVOD and VI processes. These values and the changes of calcium during the treatments are also shown in Table 1. In samples without added calcium, it is possible to observe a loss of this ion (negative values in the last column) mainly due to the loss of the native liquid phase of the fruit during vacuum pulse. Similar losses were obtained for VI and PVOD samples, thus showing the importance of the hydrodynamic mechanisms promoted by pressure in the transport of this mineral. The slightly greater value in PVOD samples may be due to the additional flow of native calcium, together with the water, to the osmotic solution during the osmotic process. In samples with calcium added in the external solution, the ion transport inside the fruit, caused by both the vacuum pulse and the calcium gradient between the fruit and the external solution, is confirmed (positive values in the last column). As VI samples were not in contact with the external solution as long as PVOD ones, the amount of calcium gained in the first case was lower. During this time, the calcium can penetrate inside the fruit and cross the cellular membranes by different active and passive mechanisms.

0

-0,05 -0,02

-0,06 -0,08

ΔMwΔ+Ms (g/g)

-0,04

-0,1 -0,12 -0,14 Fig. 2. Mass balance: water loss (DMw) and solute gain (DMs) plotted against loss of mass (DMt).

In order to analyze any effect of the treatments on the cellular metabolism, the respiration rate of the different grapefruit samples was measured. Dehydration treatment provokes a decrease in the cell metabolisms, including respiration (Lewicki et al., 2001; Tovar et al., 2001). The different degree of cellular alteration caused by the treatments will have an impact on the measured respiration rate which involves the response of all the cells in the sample. Cell viability is maintained if the concentration of the components in the dehydrated tissue cell is close to that of the fresh one. Loss of cell functionality in PVOD samples will occur and so a reduction in the respiration rate could be expected. Nevertheless, altered viable cells could increase their respiration rate as a response to the osmotic stress. The impact of the alteration of the tissue structure (collapse of external cells, filling of intercellular space with external solution, etc.) on gas transport properties in the tissue will also affect the respiration behaviour (Castelló et al., 2006). Fig. 3 shows the O2 consumption, CO2 generation and RQ of all the samples under consideration and their evolution throughout the storage time. As can be observed, dehydrated samples showed a significant decrease (p < 0.05) in both O2 consumption and CO2

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Table 1 Mean values ± standard deviation of mass fraction of water (xw), soluble solids (xs) and ion calcium analyzed in the fresh-cut, vacuum impregnated and pulsed vacuum osmodehydrated grapefruit with and without calcium (VICa, VI, PVODCa and PVOD, respectively) Treatment

xw

xs

mg Ca2+/100 g sample

mg Ca2+/100 g fresh-cut sampleb

DCa2+c

a

0.8955 ± 0.0002 0.8868 ± 0.0005 0.8905 ± 0.0007 0.7320 ± 0.0014 0.7661 ± 0.0005

0.089 ± 0.002 0.1091 ± 0.0007 0.10670 ± 0.00008 0.24 ± 0.03 0.205 ± 0.002

17.0 ± 0.9 11.6 ± 0.5 56.15 ± 0.6 15.13 ± 0.11 88.9 ± 0.6

17.0 ± 0.9 11.7 ± 0.5 52.3 ± 0.6 12.33 ± 0.09 74.4 ± 0.5

– 5.2 ± 0.5 35.5 ± 0.6 5.81 ± 0.09 56.3 ± 0.5

Fresh-cut VI VICa PVOD PVODCa

Mean value of the two batches used for VI and PVOD treatments. Calcium content of different samples referred to the corresponding fresh sample, calculated as explained in the text. DCa2+ = (mg Ca2+/100 g fresh-cut sample) in treated samples  (mg Ca2+/100 g fresh-cut sample) in the corresponding batch of fresh sample.

8

8

7

7

6

2

5 4 3

2

2

2

RR O (mLO /kg h)

c

RR CO (mLCO /kg h)

a b

2 1

6 5 4 3 2 1 0

0 0

1

2

3

4

5

6

7

0

1

2

3

4

5

6

7

Storage time (days)

Storage time (days) 3.5 3 2.5

RQ

2 1.5 1 0.5 0 0

1

2

3

4

5

6

7

Storage time (days) Fig. 3. Respiration rates in terms of O2 consumption and CO2 generation and respiration quotient (RQ) of grapefruit (fresh-cut (––), vacuum impregnated (-d-), vacuum impregnated with calcium (-s-), pulsed vacuum osmodehydrated (j) and pulsed vacuum osmodehydrated with calcium (h)) during six storage days.

generation when compared to fresh-cut fruit and VI samples. The reduction in the respiration rate caused by PVOD treatments, compared to that of the fresh-cut samples, was maintained throughout storage time, so that no significant differences (p > 0.05) were observed during the storage period. In this period, dehydrated samples treated with calcium presented significantly (p < 0.05) lower RRCO2 values. VI samples showed a significant increase (p < 0.05) in the respiration rate when compared to fresh-cut samples, more marked in RRCO2 , which can be related to the stress provoked by the vacuum pulse. When calcium lactate was added to the isotonic solution a significant (p < 0.05) decrease in O2 consumption and CO2 generation was observed. The reducing effect of calcium on the respiration rate of O2 and CO2 of VI samples was maintained throughout storage time. After the 4th day of storage, fresh-cut and impregnated samples significantly increased (p < 0.05) the res-

piration metabolism, although the values of the PVOD samples remained low. The effect of calcium on the respiration rate of VI and PVOD samples could be due to the increase in the ATP concentration (Kinoshita et al., 1995). This increase in the cytoplasmic ATP may be related to the blockage of aquaporins by Ca2+, as has been commented on before, and it remains available for other biochemical routes. On the other hand, the reduction of the amount of oxygen available in tissue, provoked by the filling of the fruit pores with the external solution, seems to contribute to the development of an ethanolic fermentation in VI and PVOD samples, as can be deduced from the respiratory quotient of over 1. However, dehydrated samples treated with calcium did not show fermentative routes after treatment (RQ  1). This may be explained by its higher ATP avail-

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M.J. Moraga et al. / Journal of Food Engineering 90 (2009) 372–379

6 5

F(N)

4 3 FF

2

A E

1

εF

0 0

0.1

0.2

0.3

0.4

0.5

ε

0.6

0.7

0.8

0.9

1

Fig. 4. Example of force–deformation curve obtained from the puncture test performed on fresh grapefruit sample (FF and eF, force and deformation at fracture point; A, area under the curve; E, slope of the linear part at low deformation).

ability and its lower energy requirements, which supposes no development of anaerobic metabolic routes. During the storage time, whereas the PVOD samples maintained the RQ values close to 1, those of the VI samples were higher than 1, showing the beneficial effect of dehydration treatments on the reduction of cell metabolism. 3.4. Mechanical properties In order to study the influence of the treatment on the mechanical behaviour of samples, the force–deformation curves were ob-

35

F

VI

PVOD

VI

PVOD (c)

8 7

25 (a, b) (a, b)

15

(a, b)

6

(a, b)

E(N)

E(N)

F

9

(c)

30

20

tained from the performed puncture test. From these curves, the slope of the linear range at low deformation, the force and deformation at fracture point and the area under the curves were analyzed. Fig. 4 shows, as an example, the curve obtained for a fresh-cut sample and the different parameters evaluated. The linear part of the curve prior to the fracture point is related to the product resistance to deformation and through FF and eF parameters the firmness of the samples can be quantified. A is related to the energy needed to deform the samples till 95%. As can be observed in Fig. 5, when compared to fresh-cut fruit, VI treatments showed no significant differences (p > 0.05) in any of the parameters analyzed. In this sense, the deformation–relaxation phenomena of the solid matrix which occurred during the vacuum pulse (Fito and Chiralt, 1997) and the change in air and liquid volume fraction did not imply significant changes in the structure of the grapefruit tissue, which can be due to the elastic nature of the tissue. Moreover, in contrast to the results obtained by other authors working with apple tissue (Anino et al., 2006), no significant difference was found on the mechanical properties of VI grapefruit samples treated with 2% added calcium lactate. PVOD treatments increased significantly (p < 0.05) all the mechanical parameters analyzed in the grapefruit. In other fruits, such as mango (Torres et al., 2006), strawberry or kiwifruit (Chiralt and Talens, 2005), a decrease in the force–deformation curves related to the loss of cell turgor, an alteration of middle lamella, an alteration of cell wall resistance, the establishment of water and solute concentration profiles and changes in sample size and shape, have been described during osmotic treatments. In this case, the structure of this kind of tissue is completely different to the

(a)

5

(a)

(a, b)

(a)

4 3

10

2 5

1

0

0 Fresh-cut

0%Ca

2%Ca

Fresh-cut

Treatments F

9

VI

F

PVOD

7 5 4

(a, b)

EF

F

F (N)

6 (a, b)

3

(a)

(a)

0%Ca

2%Ca

2 1 0 Fresh-cut

Treatments

2%Ca

Treatments

(c)

8

0%Ca

0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

PVOD

VI (c)

(c)

(b) (a, b) (a)

Fresh-cut

0%Ca

2%Ca

Treatments

Fig. 5. Mean value of mechanical parameters obtained for fresh-cut grapefruit (F) and grapefruit submitted to vacuum impregnation (VI) and pulsed vacuum osmotic dehydration (PVOD) treatments. FF and eF, force and deformation at fracture point; A, area under the curve; E, slope of the linear part at low deformation; (a–c) indicate homogeneous groups established by the ANOVA (p < 0.05).

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M.J. Moraga et al. / Journal of Food Engineering 90 (2009) 372–379

b 9

9

8

8

7

7

6

6

5

5

log cfu/g

log cfu/g

a

4 3

4 3

2

2

1

1

0

0 0

2

4

6

8

10

12

14

16

18

0

2

4

time (days)

6

8

10

12

14

16

18

time (days)

Fig. 6. (a) Total microbial counts and (b) yeasts and moulds, in both fresh-cut grapefruit (––) and grapefruit submitted to pulsed vacuum osmotic dehydration with 0% LCa (j) and 2% LCa added (h) during storage time (10 °C).

above mentioned fruits. The grapefruit pulp is formed by segments with long cells containing the juice and part of it is lixiviated during the cutting process of the samples. This fact can be responsible for the different mechanical response of this fruit compared to the parenchymatic tissue of other kinds of fruits, such as apple. The mechanical behaviour of dehydrated samples was not significantly affected by calcium concentration.

Acknowledgements

3.5. Microbial growth

References

Fig. 6 shows both total microbial counts and yeasts and moulds in fresh-cut grapefruit and PVOD grapefruit during storage time. The limit of microbiological growth established to determine the shelf-life of each sample was established as one of the most restrictive found in foods (Pascual and Calderón, 2000): total microbial counts 104 colony forming units per gram (cfu/g) and yeasts and moulds 102 cfu/g. In all cases, the limit of 102 cfu/g of yeasts and moulds was reached quicker than the limit for total counts, so the first one was used to establish the shelf-life of samples. Due to the low pH of citrus fruits, most of the microbial alterations are due to the yeasts, more than 90% according to Cardona et al. (1992), and the rest are mainly due to moulds. In this sense, the fresh-cut fruit showed considerable contamination after 5 storage days, coinciding with the increase in the respiration rate observed from 4th storage day (Fig. 3). The dehydrated sample showed, as expected, a longer shelf-life, up to 8 days. The treatment with calcium lactate seems to imply an improvement of microbiological fruit quality, maintained for up to 11 days. Similar behaviour has been observed by Luna-Guzmán and Barreto (2000) who studied the reduction of microbial growth by using calcium lactate to extend the shelf-life of fresh-cut cantaloupe. This can be related to the reduction of the cellular metabolism caused by the increase in the intracellular ATP concentration due to the effect of calcium. 4. Conclusions Calcium can be incorporated into the cellular structure of the grapefruit by means of vacuum pulsed osmotic dehydration. The dehydration of samples implies a decrease in the cellular respiration rate and, consequently, an increase in the shelf-life of the processed fruit, both of which are more marked if calcium is added to the sample. The decrease of water effective diffusivity due to the presence of calcium points to the interaction of this ion with the aquaporins of the plasmalemma membrane, thus increasing the ATP concentration inside the cell. This increase will justify the observed decrease in the respiration metabolism.

The authors wish to thank the Ministerio de Educación y Ciencia and the Fondo Europeo de Desarrollo Regional (FEDER) for the financial support given throughout the Projects AGL2002-01793 and AGL 2005-05994.

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