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Water sorption isotherms and phase transitions in kiwifruit
Journal of Food Engineering 72 (2006) 147–156 www.elsevier.com/locate/jfoodeng
Water sorption isotherms and phase transitions in kiwifruit G. Moraga, N. Martı´nez-Navarrete, A. Chiralt
*
Department of Food Technology, Universidad Polite´cnica de Valencia, P.O. Box 22012, 46071 Valencia, Spain Received 9 August 2004; accepted 10 November 2004 Available online 24 December 2004
Abstract Adsorption and desorption isotherms were determined in entire and homogenized kiwifruit tissue. Fresh samples (desorption process) and freeze-dried samples (adsorption process), were conditioned at various water activities (0–0.675) at 30 °C and, at equilibrium, had attained different water contents. In each sample, glass transition was analysed by differential scanning calorimetry (DSC). BET and GAB models were fitted to sorption data and the Gordon and Taylor equation was used to model the water plasticization effect. Results showed that the different pretreatments applied did not imply differences in those relationships. The mean values for the parameters of the fitted models were: w0 = 0.057 g water/g dry product and C = 7.9 (BET model); w0 = 0.046 g water/g dry product, C = 10.6 and K = 1.20 (GAB model); k = 4.88 and Tg(as) = 40.3 °C (Gordon and Taylor model). The state diagram of the kiwifruit liquid phase was obtained including the characteristic glass transition temperature of the maximally cryoconcentrated matrix (m.c.m.) T 0g ¼ 52:0 0:4 °C, the melting temperature of ice crystals surrounding the m.c.m. T 0m ¼ 40:4 0:4 °C, and the amount of non-freezable water content W 0g ¼ 0:186 g water/g m.c.m. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: State diagram; Adsorption; Desorption; Pretreatments; Glass transition
1. Introduction Kiwifruits have a very short shelf-life because of softening and vitamin loss during storage, even when refrigerated (Agar, Massantini, Hess-Pierce, & Kader, 1999; OÕConnor-Shaw, Roberts, Ford, & Nottingham, 1994). The use of preservation processes such as freezing or drying is common to extend the product shelf-life. Freezing combines the effects of low temperature, which slows the rate of the deteriorative reactions and microbial growth, and the cryoconcentration effect of the fruit liquid phase, associated to ice crystals formation, and the subsequent water activity (aw) reduction. However, due to the high freezable water content of kiwifruit, freezing implies important losses in product quality (Cano, Fuster, & Marın, 1993a; Cano, Marın, & De *
Corresponding author. Fax: +34 963877369. E-mail address: [email protected] (A. Chiralt).
0260-8774/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.11.031
Ancos, 1993b). Dehydration treatments applied before freezing have been reported as a tool in kiwifruit cryopreservation, mainly due to the reduction of freezable water content (Chiralt et al., 2001; Robbers, Singh, & Cunha, 1997). Partial or total dehydration of the fruit has also been widely used. Air dehydrated kiwifruit products have an extended shelf-life due to the water content removal, but the use of elevated drying temperatures implies a substantial degradation in quality attributes (Maskan, 2001). Drying of plant tissues implies great structural changes and shrinkage. The active points for water binding after these changes are modified and some of these become inaccessible to water molecules during the dampening process, thus affecting sorption behaviour (Maskan & Go¨gu¨s, 1998; Palipane & Driscoll, 1992). An advantage of freeze-drying is that is carried out at low temperatures and the quality of freeze dried products is very high in comparison with that of the products
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dehydrated using other techniques (Ratti, 2001). Moreover, due to the direct removal of water vapour from ice crystals, the freeze dried product show an interconnected porous structure which can be rehydrated very effectively (Ratti, 2001). To optimise freezing or drying preservation processes and the quality of the final product, it is very useful to analyse the water sorption isotherms and phase transitions which occur in the product. The water sorption isotherm (relationship between water content and water activity) is an important tool, especially in low moisture foods. It can be applied in order to optimise the drying or rehydration conditions and determine the stability of the product during storage. Changes in the relative humidity of the atmosphere in contact with the dried food imply the evolution of its aw value and changes in water content, according to sorption isotherms, that, in turn can induce phase transitions in some phases of the food. Sample pretreatments (dehydration conditions or sample homogenisation) cause changes in the tissue structure and composition and some authors not only found different water sorption behaviour between whole and homogenised plant tissue, but also differences between water adsorption and desorption processes (Moraga, Martı´nez-Navarrete, & Chiralt, 2004). Water and soluble solids such as sugars are the main fruit components. During fruit processing or storage, phase transitions such as liquid–gas or liquid–solid changes can occur in the water of the aqueous phase. In processes such as freezing, concentration, air-drying, freeze-drying, spray-drying, backing, extrusion, etc., with a time short enough for the removal of water or cooling, the formation of an amorphous state which is a non-equilibrium state is usual (Roos, 1995). When the glass transition temperature (Tg) is reached by increasing temperature, amorphous materials may change from a solid glassy state to a liquid-like rubbery one increasing the molecular mobility. The importance of the Tg of amorphous food materials for processing and storage stability has been recognized and emphasized by Levine and Slade (Slade & Levine, 1991). Above the glass transition temperature, various timedependent structural transformations may occur in amorphous foods. Structural collapse of dehydrated structures, similar to stickiness and caking of food powders are related to a drastic decrease in the viscosity above the Tg (Levine & Slade, 1988; Roos & Karel, 1991a; Slade & Levine, 1991). The increase in the molecular mobility above the Tg may allow for the crystallization of amorphous compounds, especially in food products that contain low molecular weight sugars such as fruits. On the other hand, crispy foods such as breakfast cereals, extruded snacks, and other crispy cereal foods are often amorphous and lose the crispy texture due to thermal or water plasticization (Martı´nezNavarrete, Moraga, Talens, & Chiralt, 2004; Ross,
Roininen, Jouppila, & Tuorila, 1998). In this sense, the determination of the critical water activity or critical water content at storage temperature for the glass transition is important to optimize storage stability and quality of foods. In the case of frozen foods, the characteristic glass transition temperature of the maximally cryoconcentrated matrix (T 0g ), is extremely important in cryopreservation, and governs ice recrystallization rates and stability during food storage (Levine & Slade, 1988; Roos, 1995). State diagrams where transition temperatures are plotted against water content of the product, at constant pressure, are important tools for establishing proper processing and storage conditions of frozen and dehydrated foods. State diagrams have been reported for grape (Roos, 1987; Sa´ & Sereno, 1994), strawberry (Moraga et al., 2004; Roos, 1987; Sa´ & Sereno, 1994), apple (Bai, Rahman, Perera, Smith, & Melton, 2001; Sa´, Figueiredo, & Sereno, 1999; Sereno, Sa´, & Figueiredo, 1998) pineapple (Telis & Sobral, 2001) mango (Ayala, Walter, Martı´nez-Monzo´, Fito, & Chiralt, 2002) and persimmon (Sobral, Telis, Habitante, & Sereno, 2001), but no data were found for kiwifruit. The aim of this work was to obtain the state diagram of the kiwifruit liquid phase and the water sorption isotherms (adsorption and desorption) in order to optimise freezing, drying or rehydration processes and the stability of the final product during storage. To study the effect of different sample pretreatments, experiments were carried out on fresh and freeze dried samples, for both entire and homogenized kiwifruit tissue.
2. Materials and methods 2.1. Material, pretreatments and analysis Kiwifruit (var. Hayward) was used in this study. Fresh fruit was washed, peeled and conditioned to obtain the different kinds of samples. For sorption experiments, kiwifruit was submitted to different treatments before moisture conditioning. Sliced quarters (1 cm thick) were used as entire tissue (ET) and homogenized tissue (HT) (Ultraturrax T25 at 8000 rpm for 3 min) of kiwifruit. These samples were submitted to adsorption (A) and desorption (D) experiments, thus giving four different kinds of samples: ET-A, ET-D, HT-A, HT-D. Samples were freeze dried (frozen at 40 °C and freeze dried in a Telstar Lioalfa-6 Lyophyliser at 102 Pa) for the adsorption processes, and fresh samples were used for the desorption process. In that case, sample moisture conditioning was carried out by applying vacuum to accelerate the process and avoid microbial growth. For moisture conditioning in the samples (2 g), these were placed at 30 °C in hermetic chambers containing saturated salt solutions with different aw
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(LiCl: 0.112, CH3COOK: 0.225, MgCl2: 0.320, K2CO3: 0.432, Mg(NO3)2: 0.500 and CuCl2: 0.675 Greenspan, 1977). The sample weights were controlled till a constant value (Dm < ±0.0005 g) was reached, where the equilibrium was assumed to be reached (Spiess & Wolf, 1983). In each equilibrated sample, moisture content was analyzed (AOAC 20.013) and calorimetric analysis was carried out in order to analyze glass transition temperature (Tg) in each sample by differential scanning calorimetry. Equilibrated samples (10 mg) were placed into DSC pans (P/N SSC000C008 de Seiko Instruments) not hermetically sealed and analyzed using a DSC 220CUSSC5200 (Seiko instruments Inc.). Heating rate was 5 °C/min and temperature range varied between 100 and 100 °C, depending on sample moisture content. The mid point of the glass transition was considered as the characteristic temperature of the transition. The water content of each sample after the thermal analysis was confirmed by drying the samples in the pans (previously holed) in a vacuum oven at 60 °C ± 1 °C under pressure <100 mmHg until constant weight. To analyse the characteristic glass transition temperature of the maximally cryoconcentrated matrix (m.c.m.) (T 0g ) and the melting temperature of ice crystals surrounding the m.c.m. (T 0m ), fresh kiwifruit was placed into a pan, sealed and analyzed using the same DSC equipment. In this case, freezing rate was 2 °C/min and samples were cooled from room temperature to 35 °C, maintained at 35 °C for 30 min (annealing) and then cooled to 100 °C (at 10 °C/min), thus ensuring the maximum ice crystallization. Afterwards, the heating curve till 40 °C was registered at 5 °C/min heating rate. Other kiwifruit samples were dehydrated till reach different levels of freezable water content in order to obtain the ice melting curve of the kiwi liquid phase. In this sense, homogenized kiwifruit (Ultraturrax T25 at 8000 rpm for 3 min) was partially dehydrated using a conventional microwave obtaining different water content samples. Moisture content (AOAC 20.013) and the initial freezing temperature (Tm) were analyzed in fresh and partially dehydrated samples. The initial freezing temperature or ice crystallization (Tm) was obtained using a PolyScience Refrigerated Circulator 9101 at 15 °C. Samples (50 ml) were placed into a test tube, immersed in the cold refrigerated bath with a thermocouple and subjected to permanent stirring. The temperature of each sample was recorded every 2 s, thus obtaining the cooling curve of the product and the initial freezing point. All experiments and analyses were carried out in triplicate.
149
treatment, the equations fitted to each individual series and those fitted to different groups of series were statistically compared through the values of statistics E (Eq. (1)) which was compared with tabulated F-Snedecor as a function of the values of DFDR and SFDRi (Eq. (1)), at 95% significance level (Moraga et al., 2004) P ðRSSg ni¼1 RSSiÞ=DFDR Pn Pn ð1Þ E¼ i¼1 RSSi= i¼1 FDRi where RSSg: residual square sum of the function fitted to a group of series. RSSi: residual square sum of the function fitted to an individual series. DFDR: difference between freedom degrees of the residuals of the function fitted to a group of series (FDRg) and the sum of freedom degrees of the residuals of the individual fittings of the series involved in the group (SFDRi). FDRg: freedom degrees of the residuals of the function fitted to a group of series. FDRi: freedom degrees of the residuals of the function fitted to an individual series. 2.3. Models fitted to experimental data In order to predict water sorption behaviour of samples, the BET (Brunauer, Emmett, & Teller, 1938) model and the GAB (Guggenheim, Anderson and de Boer) (Van den Berg & Bruin, 1981) model were used (Table 1, Eqs. (2) and (3), respectively). To predict the plasticization effect of water, experimental Tg (midpoint) xw (g water/g product) data were fitted by the Gordon and Taylor model (1952) (Table 1, Eq. (4)). The relationship between Tg and aw was predicted by a linear regression developed by Roos (1987) (Table 1, Eq. (5)) and another equation proposed by Khalloufi, El-Maslouhi, and Ratti (2000) (Table 1, Eq. (6)). For the prediction of the kiwifruit melting curve, the initial freezing temperatures (Tm) of fresh and partially dehydrated samples including T 0m were fitted using an empirical model (Table 1, Eq. (7)) based on the method of Tchigeov (Fikiin, 1998). Mass fraction of ice (xI) in samples as function of the temperature was calculated using Eq. (8) (Table 1), deduced from mass balances in the melting curve.
3. Results and discussion 2.2. Statistical comparison among the different experimental series in terms of the fitted models
3.1. Water sorption behaviour
In order to evaluate the differences in sample behaviour (sorption or plasticization) as a function of the pre-
The water sorption data for the four samples studied at 30 °C are plotted in Fig. 1. They show the amount of
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Table 1 Models used for fitting the experimental data Model
Expression
BET (1938)
w0 C aw ð1 aw Þ ð1 þ ðC 1Þ aw Þ
we ¼
w0 C K aw ð3Þ ð1 K aw Þ ð1 þ ðC 1Þ K aw Þ
we: water content (g water/g solids) aw: water activity w0: monolayer value (g water/g solids) C: constant related to monolayer sorption heat K: constant related to multilayer sorption heat
Tg ¼
ð1 xw Þ T gðasÞ þ k xw T gðwÞ ð1 xw Þ þ k xw
xw: mass fraction of water (g water/g product) Tg: glass transition temperature (°C) Tg(w): glass transition temperature for amorphous water (°C) Tg(as): glass transition temperature for anhydrous solids (°C) k: constant model
Gordon and Taylor (1952)
T g ¼ Aaw þ B
Khalloufi et al. (2000)
we(g water/ g dry matter)
0.25
Aa2w þ Baw þ C aa2w þ baw þ 1
ð6Þ
Tm ¼
A B 1 þ lnð2x wÞ
ð7Þ
xI ðT Þ ¼
x0w xccs w ðT Þ 1 xccs w ðT Þ
ð8Þ
0.15
0.10
0.05
0.00 0
0.1
0.2
0.3
ð4Þ
ð5Þ
ET-D HT-D ET-A HT-A
0.20
ð2Þ
Tg ¼
Tchigeov (Fikiin, 1998)
Mass balance used to obtain the mass fraction of ice
we: water content (g water/g solids) aw: water activity w0: monolayer value (g water/g solids) C: sorption energy constant
we ¼
GAB (1981)
Roos (1987)
Nomenclature
0.4
aw
0.5
0.6
0.7
0.8
Fig. 1. Water sorption isotherms of kiwifruit at 30 °C for different treatments. Experimental points and fitted BET (- - -) and GAB (––) models.
Tg: glass transition temperature (°C) aw: water activity A and B: constants Tg: glass transition temperature (°C) aw: water activity A, B, C, a and b are calculated from parameters obtained with Eqs. (3) and (4) as: A = Tg(as)K2(1 C) B = K[Tg(as)(C 2) + Cw0Tg(w)k] C = Tg(as) a = K2(1 C) b = K[C 2 + Cw0k] Tm: melting temperature (°C) xw: mass fraction of water (g water/g product) A and B: constants xI: mass fraction of ice (g ice/g product) x0w : initial moisture content (g water/g product) xccs w : moisture content of the cryoconcentrated solution (g water/g product)
water adsorbed (we) as a function of water activity (aw) at a constant temperature. The maximum water content value obtained at equilibrium with the highest aw (0.675) was 19.3 g water/100 g product. For the prediction of water sorption behaviour in food materials, several empirical and theoretical sorption models are available (Chirife & Iglesias, 1978; Van den Berg & Bruin, 1981). Experimental points from each series were fitted by two well-known sorption models: the BET (Brunauer et al., 1938) and the GAB (Guggenheim, Anderson and de Boer) (Van den Berg & Bruin, 1981) models that are given by Eqs. (2) and (3), respectively. The typical sigmoid curves of food sorption isotherms predicted by both BET and GAB models are shown together with experimental data in Fig. 1. Only experimental data with aw < 0.50 were fitted to the BET model because from that value, the model hypothesis fails and the equation is not able to predict sorption behaviour (Labuza, 1968). Table 2 shows the
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Table 2 Parameters of BET (aw < 0.5) and GAB models fitted to experimental data for samples submitted to different treatments (R2: determination coefficient) Treatment
HT-A ET-A HT-D ET-D All samples
BET
GAB
w0
C
R2
w0
C
K
R2
0.061 0.058 0.059 0.053 0.057
8.4 7.0 7.4 8.9 7.9
0.996 0.976 0.927 0.930 0.929
0.050 0.047 0.050 0.042 0.046
11.5 8.7 8.7 13.3 10.6
1.18 1.20 1.17 1.23 1.20
0.999 0.943 0.782 0.876 0.855
w0: monolayer moisture content (g water/g dry solids); C: Guggenheim constant related to monolayer sorption heat; K: constant related to multilayer sorption heat.
obtained parameters: the monolayer moisture content w0, and the sorption energy constant, C. The monolayer moisture content was similar in all series, the values ranging between 0.053 and 0.061 g water/g dry solids. These values were lower than 0.1 g water/g sample, which is the maximum value reported for food materials (Tsami, Marinos-Kouris, & Maroulis, 1990). Despite the theoretical limitations of the BET adsorption analysis, the monolayer value was found to be a reasonable guide with respect to various aspects of interest in dried foods, often considered as the optimal water content for stability of low-moisture foods (Iglesias & Chirife, 1982; Karel, 1973; Labuza, 1980). The other BET constant, C, related to the sorption energy allows us to classify sorption isotherms according to BrunauerÕs classification (Brunauer, Deming, Deming, & Teller, 1940). As the C value was higher than 2, the four obtained sorption isotherms can be classified as type II, like other fruits such as strawberry (Moraga et al., 2004; Roos, 1993), apple (Martı´nez-Monzo´, 1998) and blackberry (Maskan & Go¨gu¨s, 1998). The effect of solute–solvent interactions from aw > 0.50 limits the use of the BET equation to fit the data properly. In this sense, the GAB equation (Table 1, Eq. (3)) has been recommended (Wolf, Spiess, & Jung, 1985) as the fundamental equation for the characterisation of the water sorption of food materials in the water activity range 0.1–0.9. Table 2 shows the three GAB parameters obtained. The GAB model introduced a second well-differentiated sorption stage for water molecules and an additional energy constant, K. The obtained K values were, in all series, near to 1. One of the three GAB constants is, as in the BET equation, the monolayer capacity. In all sorption series the monolayer moisture content value obtained by the GAB model was slightly lower than that obtained by the BET model (values from 0.042 to 0.050 g water/g dry solids). The third parameter, C, is also an energy constant as the BET constant, but with slightly different physical meanings (Timmermann, Chirife, & Iglesias, 2001). Differences in the sorption behaviour of samples as a function of the pretreatment, were evaluated comparing the equations fitted to each individual series and the equation fitted to all data series through the value of sta-
tistic E compared with tabulated F-Snedecor as a function of the values of DFDR and SFDRi (Eq. (1)), at 95% significance level. As could be expected from the similar equilibrium data points in all series, no significant differences among samples were obtained from the statistical analysis, the value of statistic E (Eq. (1)) being lower than the tabulated F-Snedecor (a < 0.05). Adsorption and desorption processes did not exhibit the phenomenon of hysteresis as has been observed in other fruits such as raisins, currants, figs, prunes, apricots (Tsami et al., 1990) and strawberry (Moraga et al., 2004), although fruit sorption behaviour of muscatel and aledo grapes did not show hysteresis (Va´zquez, Chenlo, Moreira, & Carballo, 1999). In kiwifruit the different pretreatments applied did not imply statistical differences in water sorption behaviour. No significant differences were obtained for homogenized and entire tissue, as has been previously observed in adsorption curves of freeze-dried blueberries (Lim, Tang, & He, 1995) and strawberries (Moraga et al., 2004). Significantly different desorption behaviour was found when comparing whole and pureed strawberries; the entire tissue showing a greater water binding capacity than the homogenized one in almost all the studied aw range. The results obtained suggested that all water sorption data may be fitted to obtain the BET and GAB parameters of kiwifruit (Table 2, Fig. 1). The monolayer moisture content, that can be considered as the optimal water content for the stability of the low-moisture kiwifruit product was 5.4 g water/100 g sample. The aw calculated in equilibrium with the monolayer value was 0.260. It is a security value beyond which deteriorative reactions may be accelerated in the product and gives us the maximum relative humidity of storage air at 30 °C (26%) to assure kiwifruit stability. 3.2. Water plasticization behaviour Thermograms obtained from DSC analyses in ET-D kiwifruit samples equilibrated at different water activity levels are shown in Fig. 2. Similar curves were obtained for the other samples submitted to different pretreatments. DSC curves were similar to those determined
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0.1 mW aw=0 Endo DSC
aw=0.112 aw=0.225 aw=0.320 aw=0.432 aw=0.500 aw=0.675 Tg -100
-80
-60
-40
-20
0
20
40
60
80
100
T (ºC) Fig. 2. Glass transition analysed by DSC in ET-D kiwifruit samples at different water activity levels.
by others authors in other fruits in the same domain of aw (Roos, 1987; Sa´ & Sereno, 1994). The thermograms obtained showed the typical second-order transition, the glass transition of the amorphous materials formed during each pre-treatment in kiwifruit, that produce a step change in the heat flow due to changes in heat capacity at the temperature of phase transition. The main soluble solids ( 13%) of kiwifruit are sugars that are in an amorphous state. The transition occurs over a temperature range of 10–30 °C and in Fig. 3 the midpoint temperature in each pretreated samples is shown. The amorphous state of sugars is a thermodynamically metastable state and there is always a driving force towards the equilibrium, the crystalline state. Molecules of amorphous solids in the glassy state are not able to change their spatial arrangement to the highly ordered crystalline equilibrium state. As the temperature is in-
65
ET-D HT-D ET-A HT-A
Tg (ºC)
25
-15
-55
-95 0.2
0.15
0.1
0.05
0
xw (g water/ g sample) Fig. 3. Relationship between Tg (midpoint) and the moisture content of kiwifruit submitted to different treatments. Experimental points and both Gordon and Taylor models fitted: with one parameter (- - -) and with two parameters (––).
creased to above glass transition the molecular mobility increases, which often allows for the crystallization of sugars in low-moisture foods (Roos, 1995). All kiwifruit samples with aw P 0.112 were in a rubbery state, with Tg values lower than 30 °C. Nevertheless, no crystallized sugars seem to be present in these samples. Despite no so high temperatures to observe the melting of sugars were reached in the DSC assays, this can be deduced from sorption isotherms. If a crystallization of amorphous sugars occurs from a determined aw value, a loss of adsorbed water in these samples will occur and the sorption isotherm will show the typical discontinuity associated with this phenomenon (Iglesias, Chirife, & Buera, 1997; Roos, 1993). In no case kiwifruit sorption isotherms (Fig. 1) showed this behavior, thus indicating the kinetic stability of amorphous compounds in kiwifruit in the moisture range considered. As expected from the moisture levels reached, no endotherms associated to ice crystal melting were observed, implying the absence of freezable water content in equilibrated kiwifruit samples with aw 6 0.675. Several authors reported that water crystallization in fruits becomes relevant from aw values greater than 0.80 (Roos, 1995; Sa´ & Sereno, 1994; Telis & Sobral, 2001; Welti-Chanes et al., 1999). Water soluble compounds are the main components in the solid fraction of the fruits, the ratio of soluble to insoluble ones being in the order of 10–1. So, the observed glass transition must be attributable to these solids being homogeneously distributed in the initially liquid phase of the fruit. Values of Tg obtained for the different samples as a function of moisture content are shown in Fig. 3, where the water plasticization effect can be observed, causing a dramatic decrease of the glass transition temperature at relatively low moisture gains, such as has been observed for similar products (Roos, 1995; Roos & Karel, 1991b; Slade & Levine, 1991). The Gordon and Taylor model (1952) (Table 1, Eq. (4)) has proved to be a reliable predictor of glass transition temperatures of sugars at various water contents and has been used in several fruit samples. Food materials such as fruits can be considered as binary mixtures of solids and water, which allows us to predict water plasticization using the Gordon and Taylor equation. The model can be reorganized into the form of a straight line and fitted to experimental Tg (midpoint) xw (g water/g product) data considering 135 °C to be Tg value of pure water (Tg(w)) (Roos, 1995). The equation can be used with one parameter (k), using the experimental Tg of the anhydrous solids of the sample analysed by DSC (Tg(as)), or with two parameters (k and Tg(as)) if experimental (Tg(as)) value is not known. In Table 3, the parameters obtained from the two possible model fittings are shown. A statistical comparison did not show differences in the plasticization behaviour as a function of the sample pretreatment the value
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153
Table 3 Parameters of Gordon and Taylor models fitted to experimental data for samples submitted to different treatments Treatment
2 parameters (1)
Tg(as)(1)
k
71.2 65.0 66.3 71.8 68.6
8.03 7.16 7.42 8.45 7.76
SE
Tg(as)(2)
k(2)
2.86 2.39 2.07 2.41 1.21
41.5 40.6 41.1 39.1 40.3
4.98 4.84 4.89 4.90 4.88
R2
0.985 0.971 0.932 0.943 0.955 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 0 2 Tg(as)(1): experimental glass transition of anhydrous solids; k(1): Gordon and Taylor constant model; SE: standard error (SE ¼ i ðY i Y i Þ =N ), where Yi = experimental value; Y 0i ¼ predicted value; N = number of data points; Tg(as)(2) and k(2): Gordon and Taylor constants model; R2: determination coefficient.
of statistic E (Eq. (1)) being lower than the tabulated F-Snedecor (a < 0.05). This behaviour seems to indicate that no significant differences in composition (mean molecular weight of the solutes) of the fruit liquid phase occur due to pretreatments since these would affect the studied Tg relationship (Roos, 1995). In this sense, the Gordon and Taylor parameters obtained from all experimental data can be used (Table 3). Fig. 3 shows experimental data and both curves predicted by Gordon and Taylor model. When the model was fitted considering the experimental Tg(as) value (68.6 °C), the k parameter optimized from the model (7.76) was enough to predict the glass transition temperatures of kiwifruit samples with a water content below 0.1 g water/g product. For the Tg prediction of kiwifruit samples with a higher water content the model with two parameters (k and Tg(as)) fitted better than the one parameter model, the values of the parameters obtained being k = 4.88 and Tg(as) = 40.3 °C. This k value was similar to those reported for other fruits such strawberry, blueberry, blackberry (Khalloufi et al., 2000; Moraga et al., 2004) and sugar solutions (Roos, 1995). These parameters were useful for predicting the glass transition temperatures of all the kiwifruit samples studied except the anhydrous ones, the value of the parameter Tg(as) being 25–30° lower than the experimental data. 3.3. Glass transition temperature, water activity and water content relationships Another relationship of interest in the prediction of the physical state in food materials is given in Fig. 4. It shows a plot of Tg data against aw that gave a linear relationship applied to all kiwifruit samples with aw > 0, such as has been described in other products having an aw of 0.1–0.8 (Martı´nez-Navarrete et al., 2004; Martı´nez-Navarrete, Martı´nez-Monzo´, Pedro, & Chiralt, 1998; Moraga et al., 2004; Roos, 1987, 1995) (Table 1, Eq. (5)). No significant differences among samples were observed, so all the experimental series with aw > 0 were fitted together (Tg (°C) = 125.7aw + 35.46, R2 = 0.923). This linear equation is useful because it allows for a rapid and fairly reliable method for locating the Tg of
ET-D HT-D ET-A HT-A
75
40
Tg (ºC)
HT-A ET-A HT-D ET-D
1 parameter
5
-30
-65
-100 0
0.2
0.4
0.6
0.8
aw Fig. 4. Tg–aw relationship for kiwifruit samples submitted to different treatments. Experimental points and fitted Roos (––) and Khalloufi (- - -) models.
kiwifruit stored under determined relative humidity conditions, for aw 6 0.675. Differences between the intercept in this equation and the experimental Tg of the anhydrous solids agree with the actual sigmoid relationship between Tg and aw in the complete aw range described for other products (Martı´nez-Navarrete et al., 1998, 2004; Roos, 1995) and that must also be assumed for kiwifruit. Another equation used to fit the relationship between Tg and aw data has been proposed by Khalloufi et al. (2000) (Table 1, Eq. (6)). The expression, that is valid from aw = 0, combines the Tg of pure compounds of the product and data on the equilibrium moisture sorption by five parameters calculated using GAB and Gordon and Taylor parameters. The value of the parameters obtained in kiwifruit samples were 557.1, 30.4, 40.3, 13.8 and 13.2 respectively for A, B, C, a and b. Although, the model follows the typical sigmoid relationship described between Tg and aw, the Tg of the anhydrous solids predicted by the model is still lower than the experimental value (Fig. 4). In Fig. 5, a combination of water sorption (GAB model) and plasticization (Gordon and Taylor equation) behaviour of kiwifruit samples was plotted. This
G. Moraga et al. / Journal of Food Engineering 72 (2006) 147–156 0.50
30
0.45
10
0.40 0.35 0.30
-30
0.25 -50 0.20 -70
0.15
-90
45 15
-110
0.05 CWC 0.00
-130 0
CWA
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
aw
Fig. 5. Relationship between temperature and water activity (- - -) and between water content and water activity (––). CWA: critical water activity; CWC: critical water content.
diagram allows us to obtain the critical water content (CWC) and the critical water activity (CWA) at which the glass transition occurs, at a determined storage temperature of the product (Roos, 1995). However, the temperature dependence of the water activity must be taken into account for predicting all modeling and interpretating the sorption data. If the product is commercialized at 30 °C, the critical values for water activity and water content that takes the product from glassy to rubbery state are: CWA = 0.031 and CWC = 1.4 g water/ 100 g sample (Fig. 5). Above these values, dehydrated kiwifruit become sticky and the crystallization of amorphous compounds could take place. The CWA and CWC values are lower than the monolayer moisture content and the water activity of the sample at equilibrium, which indicates that the wo is not a water content that assures quality preservation during storage of dried kiwi fruit, as has been reported by Moraga et al. (2004) and Roos (1993) for strawberry.
-15 Tm’ = -40.4 -45 Tg’ = -52.0
-75
0.10
-105 Wg’ = 0.186 -135 1
0.8
0.6
0.4
0.2
0
xw (g water/ g sample) Fig. 6. State diagram of kiwifruit (T 0g : glass transition temperature of maximally cryoconcentrated fruit liquid phase; T 0m : melting temperature of ice crystals surrounding the maximally cryoconcentrated fruit liquid phase; W 0g ¼ non freezable water content). x: experimental Tm points.
-2 -3 -4 Endo DSC (mW)
T (ºC)
-10
75
T (ºC)
50
x w (g water/ g sample)
154
-5 -6
∆H (mJ/mg)
-7 -8 -9 Tg’ -10 -70
Tm’ -50
-30
-10
10
30
T (ºC) Fig. 7. DSC thermogram showing the characteristic T 0g and T 0m analysed in fresh kiwifruit.
3.4. State diagram of the kiwifruit liquid phase To establish proper processing and storage conditions for kiwifruit samples with freezable water content, the complete state diagram of the liquid phase of kiwifruit was obtained by analyzing the ice melting curve, including T 0m value, and T 0g value and (Fig. 6). These transition temperatures of the maximally cryoconcentrated kiwifruit liquid phase were characterized from the thermogram obtained by DSC in fresh kiwifruit after the maximum ice crystallization was reached (Fig. 7). The onset temperature of ice melting endotherm, characteristic of kiwifruit, was T 0m ¼ 40:4 0:4 °C and the characteristic glass transition temperature of the maximally cryoconcentrated matrix (m.c.m.) T 0g ¼ 52:0 0:4 °C. This value was very close to the Tg values of samples with aw = 0.675. The amount of non-freezable water content in kiwifruit was W 0g ¼ 0:186 g water/g m.c.m., calcu-
lated using the T 0g value and the Gordon and Taylor parameters. The initial freezing temperatures (Tm) of fresh and partially dehydrated samples including T 0m were fitted using an empirical model for the prediction of the melting curve in all the composition range (Table 1, Eq. (7)). The equation, based on the method of Tchigeov (Fikiin, 1998), was reorganized into the form of a straight line and fitted to experimental Tm (°C) xw (g water/g product) data, thus obtaining the two empirical parameters a and b. The values of parameters were A = 7.26 and B = 0.73 (R2 = 0.995). The initial freezing temperature in fresh kiwifruit was 1.8 °C. While temperature decreases, ice formation increases and the residual liquid phase becomes more cryoconcentrated thus increasing the viscosity. Ice formation can continue until the maximally cryoconcen-
G. Moraga et al. / Journal of Food Engineering 72 (2006) 147–156
trated liquid phase, that contains the non-freezable water content of kiwifruit (W 0g , Fig. 6), is reached. Such freeze-concentrated materials may be considered as composed of ice and solutes that are plasticized by the unfrozen water (Roos, 1995). In order to avoid ice recrystallization during the frozen storage of the product, it is important to reach temperatures below Tg that are usually lower than used by commercial freezers. The cryostabilization technology emphasizes that T 0g governs ice recrystallization rates and the stability of the frozen food (Levine & Slade, 1988). In kiwifruit, the T 0g , related to the solute molecular weight, is 52 °C. To improve kiwifruit quality after thawing, storage temperature might be maintained below this value. The state diagram can also be applied when evaluating amounts of ice in foods at different temperatures or water content of initial product, which is extremely important in quality preservation of frozen fruits. Fig. 8 shows the mass fraction of ice (xI) in frozen kiwifruit as a function of temperature for fresh sample (83% moisture content) and partially dehydrated (70% moisture content). Eq. (8), deduced from mass balances at each temperature in the diagram has been used to estimate these curves from melting curve data. The ice formation begins at 1.8 °C in fresh kiwifruit and at 5 °C approximately 55.5 g ice/100 g product is already formed. From that temperature the ice formation continuous but more slowly till T 0m (40.4 °C) at which temperature, the amount of ice formed is maximum (81.7 g ice/100 g product). This implies that 98% of the water present in the product will be frozen and 2% will remain in the cryoconcentrated solution. If the frozen fruit is stored at 18 °C, the amount of ice formed at equilibrium is 77.2 g ice/100 g product (93 g ice/100 g water) whereas only 56 g ice/100 g product (80 g ice/100 g water) is formed if the sample is previously dehydrated till 70% moisture.
0.9 0.8
0.6 0.5 0.4 0.3
x I (g ice/ g sample)
0.7
0.2 0.1 0 -40
-35
-30
-25
-20
-15
-10
-5
0
T (ºC)
Fig. 8. Mass fraction of ice (xI) in frozen kiwifruit as a function of temperature. Fresh kiwifruit, 83% moisture content (––) and partially dehydrated kiwifruit, 70% moisture content (- - -).
155
4. Conclusions Similar water sorption behaviour and water plasticization was observed for entire and homogenized kiwifruit tissue, where no hysteresis was detected for adsorption and desorption curves. So, water–fruit interactions are only slightly sensitive to structural changes which occur during drying or homogenising the tissue in the studied conditions. At 30 °C, the critical values for water activity and water content related to glass transition are CWA = 0.031 and CWC = 1.4 g water/ 100 g sample, respectively. Above these values dehydrated kiwifruit can become sticky and the crystallization of amorphous compounds could take place. Samples with a water content of over 19% (W 0g ) will have freezable water content. To improve frozen kiwifruit quality after thawing and avoid ice recrystallization, storage temperature must be maintained below 52 °C (T 0g ). Reducing the water content to 70% prior to freezing will decrease the amount of ice at 18 °C from 77% to 56%, which may considerably improve the quality preservation during frozen storage.
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Roos, Y. H. (1993). Water activity and physical state effects on amorphous food stability. Journal of Food Processing and Preservation, 16, 433–447. Roos, Y. H. (1995). Phase transitions in food. San Diego, CA: Academic Press. Roos, Y. H., & Karel, M. (1991a). Applying state diagrams to food processing and development. Food Technology, 66–71. Roos, Y. H., & Karel, M. (1991b). Plasticizing effect of water on thermal behaviour and crystallization of amorphous food models. Journal of Food Science, 56, 38–43. Ross, Y. H., Roininen, K., Jouppila, K., & Tuorila, H. (1998). Glass transition and water plasticization effects on crispness of a snack food extrudate. International Journal of Food Properties, 1(2), 163–180. Sa´, M. M., Figueiredo, A. M., & Sereno, A. M. (1999). Glass transitions and state diagrams for fresh and processed apple. Thermochimica Acta, 329, 31–38. Sa´, M. M., & Sereno, A. M. (1994). Glass transitions and state diagrams for typical natural fruits and vegetables. Thermochimica Acta, 246, 285–297. Sereno, A. M., Sa´, M. M., & Figueiredo, A. M. (1998). Glass transition and state diagrams for freeze-dried and osmotically dehydrated apple. In C. B. Akritidis, D. Marinos-Kouris, & G. D. Saravacos (Eds.), Proceedings of the 11th International Drying Symposium (pp. 1214–1220). Thessaloniki: Ziti Editions. Slade, L., & Levine, H. (1991). Beyond water activity: Recent advances based on an alternative approach to the assessment of food quality and safety. Critical reviews in Food Science and Nutrition, 30(2/3), 115–360. Sobral, P. J. A., Telis, V. R. N., Habitante, A. M. Q. B., & Sereno, A. (2001). Phase diagram for freeze-dried persimmon. Thermochimica Acta, 376, 83–89. Spiess, W. E. L., & Wolf, W. R. (1983). The results of the COST 90 Project on water activity. In R. Jowitt, F. Escher, B. Hallstro¨m, H. F. Th. Meffert, W. E. L. Spiess, & G. Vos (Eds.), Physical properties of foods (pp. 65–91). London and New York: Applied Science Publishers. Telis, V. R. N., & Sobral, P. J. A. (2001). Glass transitions and state diagram for freeze-dried pineapple. Lebensmittel Wissenschaft and Technologie, 34, 199–205. Timmermann, E. O., Chirife, J., & Iglesias, H. A. (2001). Water sorption isotherms of food stuffs: BET or GAB parameters? Journal of Food Engineering, 48, 19–31. Tsami, E., Marinos-Kouris, D., & Maroulis, Z. B. (1990). Water sorption isotherms of raisins, Currants, Figs, Prunes and Apricots. Journal of Food Science, 55, 1594–1597, 1625. Van den Berg, C., & Bruin, S. (1981). Water activity and its estimation in food systems: Theoretical aspects. In L. B. Rockland & G. F. Stewart (Eds.), Water activity: Influences on food quality (pp. 1–43). New York: Academic Press. Va´zquez, G., Chenlo, F., Moreira, L., & Carballo, L. (1999). Desorption isotherms of muscatel and aledo grapes, and the influence of pretreatments on muscatel isotherms. Journal of Food Engineering, 39, 409–414. Welti-Chanes, J., Guerrero, J. A., Barcenas, M. E., Aguilera, J. M., Vergara, F., & Barbosa-Ca´novas, G. V. (1999). Glass transition temperature (Tg) and water activity (aw) of dehydrated apple products. Journal of Food Process Engineering, 22, 91–101. Wolf, W., Spiess, W. E. L., & Jung, G. (1985). Sorption isotherms and water activity of food materials. New York: Elsevier Sciences Publishing Co.
Water sorption isotherms and phase transitions in kiwifruit G. Moraga, N. Martı´nez-Navarrete, A. Chiralt
*
Department of Food Technology, Universidad Polite´cnica de Valencia, P.O. Box 22012, 46071 Valencia, Spain Received 9 August 2004; accepted 10 November 2004 Available online 24 December 2004
Abstract Adsorption and desorption isotherms were determined in entire and homogenized kiwifruit tissue. Fresh samples (desorption process) and freeze-dried samples (adsorption process), were conditioned at various water activities (0–0.675) at 30 °C and, at equilibrium, had attained different water contents. In each sample, glass transition was analysed by differential scanning calorimetry (DSC). BET and GAB models were fitted to sorption data and the Gordon and Taylor equation was used to model the water plasticization effect. Results showed that the different pretreatments applied did not imply differences in those relationships. The mean values for the parameters of the fitted models were: w0 = 0.057 g water/g dry product and C = 7.9 (BET model); w0 = 0.046 g water/g dry product, C = 10.6 and K = 1.20 (GAB model); k = 4.88 and Tg(as) = 40.3 °C (Gordon and Taylor model). The state diagram of the kiwifruit liquid phase was obtained including the characteristic glass transition temperature of the maximally cryoconcentrated matrix (m.c.m.) T 0g ¼ 52:0 0:4 °C, the melting temperature of ice crystals surrounding the m.c.m. T 0m ¼ 40:4 0:4 °C, and the amount of non-freezable water content W 0g ¼ 0:186 g water/g m.c.m. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: State diagram; Adsorption; Desorption; Pretreatments; Glass transition
1. Introduction Kiwifruits have a very short shelf-life because of softening and vitamin loss during storage, even when refrigerated (Agar, Massantini, Hess-Pierce, & Kader, 1999; OÕConnor-Shaw, Roberts, Ford, & Nottingham, 1994). The use of preservation processes such as freezing or drying is common to extend the product shelf-life. Freezing combines the effects of low temperature, which slows the rate of the deteriorative reactions and microbial growth, and the cryoconcentration effect of the fruit liquid phase, associated to ice crystals formation, and the subsequent water activity (aw) reduction. However, due to the high freezable water content of kiwifruit, freezing implies important losses in product quality (Cano, Fuster, & Marın, 1993a; Cano, Marın, & De *
Corresponding author. Fax: +34 963877369. E-mail address: [email protected] (A. Chiralt).
0260-8774/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.11.031
Ancos, 1993b). Dehydration treatments applied before freezing have been reported as a tool in kiwifruit cryopreservation, mainly due to the reduction of freezable water content (Chiralt et al., 2001; Robbers, Singh, & Cunha, 1997). Partial or total dehydration of the fruit has also been widely used. Air dehydrated kiwifruit products have an extended shelf-life due to the water content removal, but the use of elevated drying temperatures implies a substantial degradation in quality attributes (Maskan, 2001). Drying of plant tissues implies great structural changes and shrinkage. The active points for water binding after these changes are modified and some of these become inaccessible to water molecules during the dampening process, thus affecting sorption behaviour (Maskan & Go¨gu¨s, 1998; Palipane & Driscoll, 1992). An advantage of freeze-drying is that is carried out at low temperatures and the quality of freeze dried products is very high in comparison with that of the products
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dehydrated using other techniques (Ratti, 2001). Moreover, due to the direct removal of water vapour from ice crystals, the freeze dried product show an interconnected porous structure which can be rehydrated very effectively (Ratti, 2001). To optimise freezing or drying preservation processes and the quality of the final product, it is very useful to analyse the water sorption isotherms and phase transitions which occur in the product. The water sorption isotherm (relationship between water content and water activity) is an important tool, especially in low moisture foods. It can be applied in order to optimise the drying or rehydration conditions and determine the stability of the product during storage. Changes in the relative humidity of the atmosphere in contact with the dried food imply the evolution of its aw value and changes in water content, according to sorption isotherms, that, in turn can induce phase transitions in some phases of the food. Sample pretreatments (dehydration conditions or sample homogenisation) cause changes in the tissue structure and composition and some authors not only found different water sorption behaviour between whole and homogenised plant tissue, but also differences between water adsorption and desorption processes (Moraga, Martı´nez-Navarrete, & Chiralt, 2004). Water and soluble solids such as sugars are the main fruit components. During fruit processing or storage, phase transitions such as liquid–gas or liquid–solid changes can occur in the water of the aqueous phase. In processes such as freezing, concentration, air-drying, freeze-drying, spray-drying, backing, extrusion, etc., with a time short enough for the removal of water or cooling, the formation of an amorphous state which is a non-equilibrium state is usual (Roos, 1995). When the glass transition temperature (Tg) is reached by increasing temperature, amorphous materials may change from a solid glassy state to a liquid-like rubbery one increasing the molecular mobility. The importance of the Tg of amorphous food materials for processing and storage stability has been recognized and emphasized by Levine and Slade (Slade & Levine, 1991). Above the glass transition temperature, various timedependent structural transformations may occur in amorphous foods. Structural collapse of dehydrated structures, similar to stickiness and caking of food powders are related to a drastic decrease in the viscosity above the Tg (Levine & Slade, 1988; Roos & Karel, 1991a; Slade & Levine, 1991). The increase in the molecular mobility above the Tg may allow for the crystallization of amorphous compounds, especially in food products that contain low molecular weight sugars such as fruits. On the other hand, crispy foods such as breakfast cereals, extruded snacks, and other crispy cereal foods are often amorphous and lose the crispy texture due to thermal or water plasticization (Martı´nezNavarrete, Moraga, Talens, & Chiralt, 2004; Ross,
Roininen, Jouppila, & Tuorila, 1998). In this sense, the determination of the critical water activity or critical water content at storage temperature for the glass transition is important to optimize storage stability and quality of foods. In the case of frozen foods, the characteristic glass transition temperature of the maximally cryoconcentrated matrix (T 0g ), is extremely important in cryopreservation, and governs ice recrystallization rates and stability during food storage (Levine & Slade, 1988; Roos, 1995). State diagrams where transition temperatures are plotted against water content of the product, at constant pressure, are important tools for establishing proper processing and storage conditions of frozen and dehydrated foods. State diagrams have been reported for grape (Roos, 1987; Sa´ & Sereno, 1994), strawberry (Moraga et al., 2004; Roos, 1987; Sa´ & Sereno, 1994), apple (Bai, Rahman, Perera, Smith, & Melton, 2001; Sa´, Figueiredo, & Sereno, 1999; Sereno, Sa´, & Figueiredo, 1998) pineapple (Telis & Sobral, 2001) mango (Ayala, Walter, Martı´nez-Monzo´, Fito, & Chiralt, 2002) and persimmon (Sobral, Telis, Habitante, & Sereno, 2001), but no data were found for kiwifruit. The aim of this work was to obtain the state diagram of the kiwifruit liquid phase and the water sorption isotherms (adsorption and desorption) in order to optimise freezing, drying or rehydration processes and the stability of the final product during storage. To study the effect of different sample pretreatments, experiments were carried out on fresh and freeze dried samples, for both entire and homogenized kiwifruit tissue.
2. Materials and methods 2.1. Material, pretreatments and analysis Kiwifruit (var. Hayward) was used in this study. Fresh fruit was washed, peeled and conditioned to obtain the different kinds of samples. For sorption experiments, kiwifruit was submitted to different treatments before moisture conditioning. Sliced quarters (1 cm thick) were used as entire tissue (ET) and homogenized tissue (HT) (Ultraturrax T25 at 8000 rpm for 3 min) of kiwifruit. These samples were submitted to adsorption (A) and desorption (D) experiments, thus giving four different kinds of samples: ET-A, ET-D, HT-A, HT-D. Samples were freeze dried (frozen at 40 °C and freeze dried in a Telstar Lioalfa-6 Lyophyliser at 102 Pa) for the adsorption processes, and fresh samples were used for the desorption process. In that case, sample moisture conditioning was carried out by applying vacuum to accelerate the process and avoid microbial growth. For moisture conditioning in the samples (2 g), these were placed at 30 °C in hermetic chambers containing saturated salt solutions with different aw
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(LiCl: 0.112, CH3COOK: 0.225, MgCl2: 0.320, K2CO3: 0.432, Mg(NO3)2: 0.500 and CuCl2: 0.675 Greenspan, 1977). The sample weights were controlled till a constant value (Dm < ±0.0005 g) was reached, where the equilibrium was assumed to be reached (Spiess & Wolf, 1983). In each equilibrated sample, moisture content was analyzed (AOAC 20.013) and calorimetric analysis was carried out in order to analyze glass transition temperature (Tg) in each sample by differential scanning calorimetry. Equilibrated samples (10 mg) were placed into DSC pans (P/N SSC000C008 de Seiko Instruments) not hermetically sealed and analyzed using a DSC 220CUSSC5200 (Seiko instruments Inc.). Heating rate was 5 °C/min and temperature range varied between 100 and 100 °C, depending on sample moisture content. The mid point of the glass transition was considered as the characteristic temperature of the transition. The water content of each sample after the thermal analysis was confirmed by drying the samples in the pans (previously holed) in a vacuum oven at 60 °C ± 1 °C under pressure <100 mmHg until constant weight. To analyse the characteristic glass transition temperature of the maximally cryoconcentrated matrix (m.c.m.) (T 0g ) and the melting temperature of ice crystals surrounding the m.c.m. (T 0m ), fresh kiwifruit was placed into a pan, sealed and analyzed using the same DSC equipment. In this case, freezing rate was 2 °C/min and samples were cooled from room temperature to 35 °C, maintained at 35 °C for 30 min (annealing) and then cooled to 100 °C (at 10 °C/min), thus ensuring the maximum ice crystallization. Afterwards, the heating curve till 40 °C was registered at 5 °C/min heating rate. Other kiwifruit samples were dehydrated till reach different levels of freezable water content in order to obtain the ice melting curve of the kiwi liquid phase. In this sense, homogenized kiwifruit (Ultraturrax T25 at 8000 rpm for 3 min) was partially dehydrated using a conventional microwave obtaining different water content samples. Moisture content (AOAC 20.013) and the initial freezing temperature (Tm) were analyzed in fresh and partially dehydrated samples. The initial freezing temperature or ice crystallization (Tm) was obtained using a PolyScience Refrigerated Circulator 9101 at 15 °C. Samples (50 ml) were placed into a test tube, immersed in the cold refrigerated bath with a thermocouple and subjected to permanent stirring. The temperature of each sample was recorded every 2 s, thus obtaining the cooling curve of the product and the initial freezing point. All experiments and analyses were carried out in triplicate.
149
treatment, the equations fitted to each individual series and those fitted to different groups of series were statistically compared through the values of statistics E (Eq. (1)) which was compared with tabulated F-Snedecor as a function of the values of DFDR and SFDRi (Eq. (1)), at 95% significance level (Moraga et al., 2004) P ðRSSg ni¼1 RSSiÞ=DFDR Pn Pn ð1Þ E¼ i¼1 RSSi= i¼1 FDRi where RSSg: residual square sum of the function fitted to a group of series. RSSi: residual square sum of the function fitted to an individual series. DFDR: difference between freedom degrees of the residuals of the function fitted to a group of series (FDRg) and the sum of freedom degrees of the residuals of the individual fittings of the series involved in the group (SFDRi). FDRg: freedom degrees of the residuals of the function fitted to a group of series. FDRi: freedom degrees of the residuals of the function fitted to an individual series. 2.3. Models fitted to experimental data In order to predict water sorption behaviour of samples, the BET (Brunauer, Emmett, & Teller, 1938) model and the GAB (Guggenheim, Anderson and de Boer) (Van den Berg & Bruin, 1981) model were used (Table 1, Eqs. (2) and (3), respectively). To predict the plasticization effect of water, experimental Tg (midpoint) xw (g water/g product) data were fitted by the Gordon and Taylor model (1952) (Table 1, Eq. (4)). The relationship between Tg and aw was predicted by a linear regression developed by Roos (1987) (Table 1, Eq. (5)) and another equation proposed by Khalloufi, El-Maslouhi, and Ratti (2000) (Table 1, Eq. (6)). For the prediction of the kiwifruit melting curve, the initial freezing temperatures (Tm) of fresh and partially dehydrated samples including T 0m were fitted using an empirical model (Table 1, Eq. (7)) based on the method of Tchigeov (Fikiin, 1998). Mass fraction of ice (xI) in samples as function of the temperature was calculated using Eq. (8) (Table 1), deduced from mass balances in the melting curve.
3. Results and discussion 2.2. Statistical comparison among the different experimental series in terms of the fitted models
3.1. Water sorption behaviour
In order to evaluate the differences in sample behaviour (sorption or plasticization) as a function of the pre-
The water sorption data for the four samples studied at 30 °C are plotted in Fig. 1. They show the amount of
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Table 1 Models used for fitting the experimental data Model
Expression
BET (1938)
w0 C aw ð1 aw Þ ð1 þ ðC 1Þ aw Þ
we ¼
w0 C K aw ð3Þ ð1 K aw Þ ð1 þ ðC 1Þ K aw Þ
we: water content (g water/g solids) aw: water activity w0: monolayer value (g water/g solids) C: constant related to monolayer sorption heat K: constant related to multilayer sorption heat
Tg ¼
ð1 xw Þ T gðasÞ þ k xw T gðwÞ ð1 xw Þ þ k xw
xw: mass fraction of water (g water/g product) Tg: glass transition temperature (°C) Tg(w): glass transition temperature for amorphous water (°C) Tg(as): glass transition temperature for anhydrous solids (°C) k: constant model
Gordon and Taylor (1952)
T g ¼ Aaw þ B
Khalloufi et al. (2000)
we(g water/ g dry matter)
0.25
Aa2w þ Baw þ C aa2w þ baw þ 1
ð6Þ
Tm ¼
A B 1 þ lnð2x wÞ
ð7Þ
xI ðT Þ ¼
x0w xccs w ðT Þ 1 xccs w ðT Þ
ð8Þ
0.15
0.10
0.05
0.00 0
0.1
0.2
0.3
ð4Þ
ð5Þ
ET-D HT-D ET-A HT-A
0.20
ð2Þ
Tg ¼
Tchigeov (Fikiin, 1998)
Mass balance used to obtain the mass fraction of ice
we: water content (g water/g solids) aw: water activity w0: monolayer value (g water/g solids) C: sorption energy constant
we ¼
GAB (1981)
Roos (1987)
Nomenclature
0.4
aw
0.5
0.6
0.7
0.8
Fig. 1. Water sorption isotherms of kiwifruit at 30 °C for different treatments. Experimental points and fitted BET (- - -) and GAB (––) models.
Tg: glass transition temperature (°C) aw: water activity A and B: constants Tg: glass transition temperature (°C) aw: water activity A, B, C, a and b are calculated from parameters obtained with Eqs. (3) and (4) as: A = Tg(as)K2(1 C) B = K[Tg(as)(C 2) + Cw0Tg(w)k] C = Tg(as) a = K2(1 C) b = K[C 2 + Cw0k] Tm: melting temperature (°C) xw: mass fraction of water (g water/g product) A and B: constants xI: mass fraction of ice (g ice/g product) x0w : initial moisture content (g water/g product) xccs w : moisture content of the cryoconcentrated solution (g water/g product)
water adsorbed (we) as a function of water activity (aw) at a constant temperature. The maximum water content value obtained at equilibrium with the highest aw (0.675) was 19.3 g water/100 g product. For the prediction of water sorption behaviour in food materials, several empirical and theoretical sorption models are available (Chirife & Iglesias, 1978; Van den Berg & Bruin, 1981). Experimental points from each series were fitted by two well-known sorption models: the BET (Brunauer et al., 1938) and the GAB (Guggenheim, Anderson and de Boer) (Van den Berg & Bruin, 1981) models that are given by Eqs. (2) and (3), respectively. The typical sigmoid curves of food sorption isotherms predicted by both BET and GAB models are shown together with experimental data in Fig. 1. Only experimental data with aw < 0.50 were fitted to the BET model because from that value, the model hypothesis fails and the equation is not able to predict sorption behaviour (Labuza, 1968). Table 2 shows the
G. Moraga et al. / Journal of Food Engineering 72 (2006) 147–156
151
Table 2 Parameters of BET (aw < 0.5) and GAB models fitted to experimental data for samples submitted to different treatments (R2: determination coefficient) Treatment
HT-A ET-A HT-D ET-D All samples
BET
GAB
w0
C
R2
w0
C
K
R2
0.061 0.058 0.059 0.053 0.057
8.4 7.0 7.4 8.9 7.9
0.996 0.976 0.927 0.930 0.929
0.050 0.047 0.050 0.042 0.046
11.5 8.7 8.7 13.3 10.6
1.18 1.20 1.17 1.23 1.20
0.999 0.943 0.782 0.876 0.855
w0: monolayer moisture content (g water/g dry solids); C: Guggenheim constant related to monolayer sorption heat; K: constant related to multilayer sorption heat.
obtained parameters: the monolayer moisture content w0, and the sorption energy constant, C. The monolayer moisture content was similar in all series, the values ranging between 0.053 and 0.061 g water/g dry solids. These values were lower than 0.1 g water/g sample, which is the maximum value reported for food materials (Tsami, Marinos-Kouris, & Maroulis, 1990). Despite the theoretical limitations of the BET adsorption analysis, the monolayer value was found to be a reasonable guide with respect to various aspects of interest in dried foods, often considered as the optimal water content for stability of low-moisture foods (Iglesias & Chirife, 1982; Karel, 1973; Labuza, 1980). The other BET constant, C, related to the sorption energy allows us to classify sorption isotherms according to BrunauerÕs classification (Brunauer, Deming, Deming, & Teller, 1940). As the C value was higher than 2, the four obtained sorption isotherms can be classified as type II, like other fruits such as strawberry (Moraga et al., 2004; Roos, 1993), apple (Martı´nez-Monzo´, 1998) and blackberry (Maskan & Go¨gu¨s, 1998). The effect of solute–solvent interactions from aw > 0.50 limits the use of the BET equation to fit the data properly. In this sense, the GAB equation (Table 1, Eq. (3)) has been recommended (Wolf, Spiess, & Jung, 1985) as the fundamental equation for the characterisation of the water sorption of food materials in the water activity range 0.1–0.9. Table 2 shows the three GAB parameters obtained. The GAB model introduced a second well-differentiated sorption stage for water molecules and an additional energy constant, K. The obtained K values were, in all series, near to 1. One of the three GAB constants is, as in the BET equation, the monolayer capacity. In all sorption series the monolayer moisture content value obtained by the GAB model was slightly lower than that obtained by the BET model (values from 0.042 to 0.050 g water/g dry solids). The third parameter, C, is also an energy constant as the BET constant, but with slightly different physical meanings (Timmermann, Chirife, & Iglesias, 2001). Differences in the sorption behaviour of samples as a function of the pretreatment, were evaluated comparing the equations fitted to each individual series and the equation fitted to all data series through the value of sta-
tistic E compared with tabulated F-Snedecor as a function of the values of DFDR and SFDRi (Eq. (1)), at 95% significance level. As could be expected from the similar equilibrium data points in all series, no significant differences among samples were obtained from the statistical analysis, the value of statistic E (Eq. (1)) being lower than the tabulated F-Snedecor (a < 0.05). Adsorption and desorption processes did not exhibit the phenomenon of hysteresis as has been observed in other fruits such as raisins, currants, figs, prunes, apricots (Tsami et al., 1990) and strawberry (Moraga et al., 2004), although fruit sorption behaviour of muscatel and aledo grapes did not show hysteresis (Va´zquez, Chenlo, Moreira, & Carballo, 1999). In kiwifruit the different pretreatments applied did not imply statistical differences in water sorption behaviour. No significant differences were obtained for homogenized and entire tissue, as has been previously observed in adsorption curves of freeze-dried blueberries (Lim, Tang, & He, 1995) and strawberries (Moraga et al., 2004). Significantly different desorption behaviour was found when comparing whole and pureed strawberries; the entire tissue showing a greater water binding capacity than the homogenized one in almost all the studied aw range. The results obtained suggested that all water sorption data may be fitted to obtain the BET and GAB parameters of kiwifruit (Table 2, Fig. 1). The monolayer moisture content, that can be considered as the optimal water content for the stability of the low-moisture kiwifruit product was 5.4 g water/100 g sample. The aw calculated in equilibrium with the monolayer value was 0.260. It is a security value beyond which deteriorative reactions may be accelerated in the product and gives us the maximum relative humidity of storage air at 30 °C (26%) to assure kiwifruit stability. 3.2. Water plasticization behaviour Thermograms obtained from DSC analyses in ET-D kiwifruit samples equilibrated at different water activity levels are shown in Fig. 2. Similar curves were obtained for the other samples submitted to different pretreatments. DSC curves were similar to those determined
152
G. Moraga et al. / Journal of Food Engineering 72 (2006) 147–156
0.1 mW aw=0 Endo DSC
aw=0.112 aw=0.225 aw=0.320 aw=0.432 aw=0.500 aw=0.675 Tg -100
-80
-60
-40
-20
0
20
40
60
80
100
T (ºC) Fig. 2. Glass transition analysed by DSC in ET-D kiwifruit samples at different water activity levels.
by others authors in other fruits in the same domain of aw (Roos, 1987; Sa´ & Sereno, 1994). The thermograms obtained showed the typical second-order transition, the glass transition of the amorphous materials formed during each pre-treatment in kiwifruit, that produce a step change in the heat flow due to changes in heat capacity at the temperature of phase transition. The main soluble solids ( 13%) of kiwifruit are sugars that are in an amorphous state. The transition occurs over a temperature range of 10–30 °C and in Fig. 3 the midpoint temperature in each pretreated samples is shown. The amorphous state of sugars is a thermodynamically metastable state and there is always a driving force towards the equilibrium, the crystalline state. Molecules of amorphous solids in the glassy state are not able to change their spatial arrangement to the highly ordered crystalline equilibrium state. As the temperature is in-
65
ET-D HT-D ET-A HT-A
Tg (ºC)
25
-15
-55
-95 0.2
0.15
0.1
0.05
0
xw (g water/ g sample) Fig. 3. Relationship between Tg (midpoint) and the moisture content of kiwifruit submitted to different treatments. Experimental points and both Gordon and Taylor models fitted: with one parameter (- - -) and with two parameters (––).
creased to above glass transition the molecular mobility increases, which often allows for the crystallization of sugars in low-moisture foods (Roos, 1995). All kiwifruit samples with aw P 0.112 were in a rubbery state, with Tg values lower than 30 °C. Nevertheless, no crystallized sugars seem to be present in these samples. Despite no so high temperatures to observe the melting of sugars were reached in the DSC assays, this can be deduced from sorption isotherms. If a crystallization of amorphous sugars occurs from a determined aw value, a loss of adsorbed water in these samples will occur and the sorption isotherm will show the typical discontinuity associated with this phenomenon (Iglesias, Chirife, & Buera, 1997; Roos, 1993). In no case kiwifruit sorption isotherms (Fig. 1) showed this behavior, thus indicating the kinetic stability of amorphous compounds in kiwifruit in the moisture range considered. As expected from the moisture levels reached, no endotherms associated to ice crystal melting were observed, implying the absence of freezable water content in equilibrated kiwifruit samples with aw 6 0.675. Several authors reported that water crystallization in fruits becomes relevant from aw values greater than 0.80 (Roos, 1995; Sa´ & Sereno, 1994; Telis & Sobral, 2001; Welti-Chanes et al., 1999). Water soluble compounds are the main components in the solid fraction of the fruits, the ratio of soluble to insoluble ones being in the order of 10–1. So, the observed glass transition must be attributable to these solids being homogeneously distributed in the initially liquid phase of the fruit. Values of Tg obtained for the different samples as a function of moisture content are shown in Fig. 3, where the water plasticization effect can be observed, causing a dramatic decrease of the glass transition temperature at relatively low moisture gains, such as has been observed for similar products (Roos, 1995; Roos & Karel, 1991b; Slade & Levine, 1991). The Gordon and Taylor model (1952) (Table 1, Eq. (4)) has proved to be a reliable predictor of glass transition temperatures of sugars at various water contents and has been used in several fruit samples. Food materials such as fruits can be considered as binary mixtures of solids and water, which allows us to predict water plasticization using the Gordon and Taylor equation. The model can be reorganized into the form of a straight line and fitted to experimental Tg (midpoint) xw (g water/g product) data considering 135 °C to be Tg value of pure water (Tg(w)) (Roos, 1995). The equation can be used with one parameter (k), using the experimental Tg of the anhydrous solids of the sample analysed by DSC (Tg(as)), or with two parameters (k and Tg(as)) if experimental (Tg(as)) value is not known. In Table 3, the parameters obtained from the two possible model fittings are shown. A statistical comparison did not show differences in the plasticization behaviour as a function of the sample pretreatment the value
G. Moraga et al. / Journal of Food Engineering 72 (2006) 147–156
153
Table 3 Parameters of Gordon and Taylor models fitted to experimental data for samples submitted to different treatments Treatment
2 parameters (1)
Tg(as)(1)
k
71.2 65.0 66.3 71.8 68.6
8.03 7.16 7.42 8.45 7.76
SE
Tg(as)(2)
k(2)
2.86 2.39 2.07 2.41 1.21
41.5 40.6 41.1 39.1 40.3
4.98 4.84 4.89 4.90 4.88
R2
0.985 0.971 0.932 0.943 0.955 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 0 2 Tg(as)(1): experimental glass transition of anhydrous solids; k(1): Gordon and Taylor constant model; SE: standard error (SE ¼ i ðY i Y i Þ =N ), where Yi = experimental value; Y 0i ¼ predicted value; N = number of data points; Tg(as)(2) and k(2): Gordon and Taylor constants model; R2: determination coefficient.
of statistic E (Eq. (1)) being lower than the tabulated F-Snedecor (a < 0.05). This behaviour seems to indicate that no significant differences in composition (mean molecular weight of the solutes) of the fruit liquid phase occur due to pretreatments since these would affect the studied Tg relationship (Roos, 1995). In this sense, the Gordon and Taylor parameters obtained from all experimental data can be used (Table 3). Fig. 3 shows experimental data and both curves predicted by Gordon and Taylor model. When the model was fitted considering the experimental Tg(as) value (68.6 °C), the k parameter optimized from the model (7.76) was enough to predict the glass transition temperatures of kiwifruit samples with a water content below 0.1 g water/g product. For the Tg prediction of kiwifruit samples with a higher water content the model with two parameters (k and Tg(as)) fitted better than the one parameter model, the values of the parameters obtained being k = 4.88 and Tg(as) = 40.3 °C. This k value was similar to those reported for other fruits such strawberry, blueberry, blackberry (Khalloufi et al., 2000; Moraga et al., 2004) and sugar solutions (Roos, 1995). These parameters were useful for predicting the glass transition temperatures of all the kiwifruit samples studied except the anhydrous ones, the value of the parameter Tg(as) being 25–30° lower than the experimental data. 3.3. Glass transition temperature, water activity and water content relationships Another relationship of interest in the prediction of the physical state in food materials is given in Fig. 4. It shows a plot of Tg data against aw that gave a linear relationship applied to all kiwifruit samples with aw > 0, such as has been described in other products having an aw of 0.1–0.8 (Martı´nez-Navarrete et al., 2004; Martı´nez-Navarrete, Martı´nez-Monzo´, Pedro, & Chiralt, 1998; Moraga et al., 2004; Roos, 1987, 1995) (Table 1, Eq. (5)). No significant differences among samples were observed, so all the experimental series with aw > 0 were fitted together (Tg (°C) = 125.7aw + 35.46, R2 = 0.923). This linear equation is useful because it allows for a rapid and fairly reliable method for locating the Tg of
ET-D HT-D ET-A HT-A
75
40
Tg (ºC)
HT-A ET-A HT-D ET-D
1 parameter
5
-30
-65
-100 0
0.2
0.4
0.6
0.8
aw Fig. 4. Tg–aw relationship for kiwifruit samples submitted to different treatments. Experimental points and fitted Roos (––) and Khalloufi (- - -) models.
kiwifruit stored under determined relative humidity conditions, for aw 6 0.675. Differences between the intercept in this equation and the experimental Tg of the anhydrous solids agree with the actual sigmoid relationship between Tg and aw in the complete aw range described for other products (Martı´nez-Navarrete et al., 1998, 2004; Roos, 1995) and that must also be assumed for kiwifruit. Another equation used to fit the relationship between Tg and aw data has been proposed by Khalloufi et al. (2000) (Table 1, Eq. (6)). The expression, that is valid from aw = 0, combines the Tg of pure compounds of the product and data on the equilibrium moisture sorption by five parameters calculated using GAB and Gordon and Taylor parameters. The value of the parameters obtained in kiwifruit samples were 557.1, 30.4, 40.3, 13.8 and 13.2 respectively for A, B, C, a and b. Although, the model follows the typical sigmoid relationship described between Tg and aw, the Tg of the anhydrous solids predicted by the model is still lower than the experimental value (Fig. 4). In Fig. 5, a combination of water sorption (GAB model) and plasticization (Gordon and Taylor equation) behaviour of kiwifruit samples was plotted. This
G. Moraga et al. / Journal of Food Engineering 72 (2006) 147–156 0.50
30
0.45
10
0.40 0.35 0.30
-30
0.25 -50 0.20 -70
0.15
-90
45 15
-110
0.05 CWC 0.00
-130 0
CWA
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
aw
Fig. 5. Relationship between temperature and water activity (- - -) and between water content and water activity (––). CWA: critical water activity; CWC: critical water content.
diagram allows us to obtain the critical water content (CWC) and the critical water activity (CWA) at which the glass transition occurs, at a determined storage temperature of the product (Roos, 1995). However, the temperature dependence of the water activity must be taken into account for predicting all modeling and interpretating the sorption data. If the product is commercialized at 30 °C, the critical values for water activity and water content that takes the product from glassy to rubbery state are: CWA = 0.031 and CWC = 1.4 g water/ 100 g sample (Fig. 5). Above these values, dehydrated kiwifruit become sticky and the crystallization of amorphous compounds could take place. The CWA and CWC values are lower than the monolayer moisture content and the water activity of the sample at equilibrium, which indicates that the wo is not a water content that assures quality preservation during storage of dried kiwi fruit, as has been reported by Moraga et al. (2004) and Roos (1993) for strawberry.
-15 Tm’ = -40.4 -45 Tg’ = -52.0
-75
0.10
-105 Wg’ = 0.186 -135 1
0.8
0.6
0.4
0.2
0
xw (g water/ g sample) Fig. 6. State diagram of kiwifruit (T 0g : glass transition temperature of maximally cryoconcentrated fruit liquid phase; T 0m : melting temperature of ice crystals surrounding the maximally cryoconcentrated fruit liquid phase; W 0g ¼ non freezable water content). x: experimental Tm points.
-2 -3 -4 Endo DSC (mW)
T (ºC)
-10
75
T (ºC)
50
x w (g water/ g sample)
154
-5 -6
∆H (mJ/mg)
-7 -8 -9 Tg’ -10 -70
Tm’ -50
-30
-10
10
30
T (ºC) Fig. 7. DSC thermogram showing the characteristic T 0g and T 0m analysed in fresh kiwifruit.
3.4. State diagram of the kiwifruit liquid phase To establish proper processing and storage conditions for kiwifruit samples with freezable water content, the complete state diagram of the liquid phase of kiwifruit was obtained by analyzing the ice melting curve, including T 0m value, and T 0g value and (Fig. 6). These transition temperatures of the maximally cryoconcentrated kiwifruit liquid phase were characterized from the thermogram obtained by DSC in fresh kiwifruit after the maximum ice crystallization was reached (Fig. 7). The onset temperature of ice melting endotherm, characteristic of kiwifruit, was T 0m ¼ 40:4 0:4 °C and the characteristic glass transition temperature of the maximally cryoconcentrated matrix (m.c.m.) T 0g ¼ 52:0 0:4 °C. This value was very close to the Tg values of samples with aw = 0.675. The amount of non-freezable water content in kiwifruit was W 0g ¼ 0:186 g water/g m.c.m., calcu-
lated using the T 0g value and the Gordon and Taylor parameters. The initial freezing temperatures (Tm) of fresh and partially dehydrated samples including T 0m were fitted using an empirical model for the prediction of the melting curve in all the composition range (Table 1, Eq. (7)). The equation, based on the method of Tchigeov (Fikiin, 1998), was reorganized into the form of a straight line and fitted to experimental Tm (°C) xw (g water/g product) data, thus obtaining the two empirical parameters a and b. The values of parameters were A = 7.26 and B = 0.73 (R2 = 0.995). The initial freezing temperature in fresh kiwifruit was 1.8 °C. While temperature decreases, ice formation increases and the residual liquid phase becomes more cryoconcentrated thus increasing the viscosity. Ice formation can continue until the maximally cryoconcen-
G. Moraga et al. / Journal of Food Engineering 72 (2006) 147–156
trated liquid phase, that contains the non-freezable water content of kiwifruit (W 0g , Fig. 6), is reached. Such freeze-concentrated materials may be considered as composed of ice and solutes that are plasticized by the unfrozen water (Roos, 1995). In order to avoid ice recrystallization during the frozen storage of the product, it is important to reach temperatures below Tg that are usually lower than used by commercial freezers. The cryostabilization technology emphasizes that T 0g governs ice recrystallization rates and the stability of the frozen food (Levine & Slade, 1988). In kiwifruit, the T 0g , related to the solute molecular weight, is 52 °C. To improve kiwifruit quality after thawing, storage temperature might be maintained below this value. The state diagram can also be applied when evaluating amounts of ice in foods at different temperatures or water content of initial product, which is extremely important in quality preservation of frozen fruits. Fig. 8 shows the mass fraction of ice (xI) in frozen kiwifruit as a function of temperature for fresh sample (83% moisture content) and partially dehydrated (70% moisture content). Eq. (8), deduced from mass balances at each temperature in the diagram has been used to estimate these curves from melting curve data. The ice formation begins at 1.8 °C in fresh kiwifruit and at 5 °C approximately 55.5 g ice/100 g product is already formed. From that temperature the ice formation continuous but more slowly till T 0m (40.4 °C) at which temperature, the amount of ice formed is maximum (81.7 g ice/100 g product). This implies that 98% of the water present in the product will be frozen and 2% will remain in the cryoconcentrated solution. If the frozen fruit is stored at 18 °C, the amount of ice formed at equilibrium is 77.2 g ice/100 g product (93 g ice/100 g water) whereas only 56 g ice/100 g product (80 g ice/100 g water) is formed if the sample is previously dehydrated till 70% moisture.
0.9 0.8
0.6 0.5 0.4 0.3
x I (g ice/ g sample)
0.7
0.2 0.1 0 -40
-35
-30
-25
-20
-15
-10
-5
0
T (ºC)
Fig. 8. Mass fraction of ice (xI) in frozen kiwifruit as a function of temperature. Fresh kiwifruit, 83% moisture content (––) and partially dehydrated kiwifruit, 70% moisture content (- - -).
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4. Conclusions Similar water sorption behaviour and water plasticization was observed for entire and homogenized kiwifruit tissue, where no hysteresis was detected for adsorption and desorption curves. So, water–fruit interactions are only slightly sensitive to structural changes which occur during drying or homogenising the tissue in the studied conditions. At 30 °C, the critical values for water activity and water content related to glass transition are CWA = 0.031 and CWC = 1.4 g water/ 100 g sample, respectively. Above these values dehydrated kiwifruit can become sticky and the crystallization of amorphous compounds could take place. Samples with a water content of over 19% (W 0g ) will have freezable water content. To improve frozen kiwifruit quality after thawing and avoid ice recrystallization, storage temperature must be maintained below 52 °C (T 0g ). Reducing the water content to 70% prior to freezing will decrease the amount of ice at 18 °C from 77% to 56%, which may considerably improve the quality preservation during frozen storage.
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