- Email: [email protected]
Texture hysteresis of pistachio kernels on drying and rehydration
Journal of Food Engineering 166 (2015) 335–341
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Texture hysteresis of pistachio kernels on drying and rehydration Asgar Farahnaky ⇑,a,b, Elahe Kamali b a School of Biomedical Sciences, Graham Centre for Agricultural Innovation and ARC Industrial Transformation Training Centre for Functional Grains, Charles Sturt University, Wagga Wagga, NSW, Australia b Department of Food Science and Technology, School of Agriculture, Shiraz University, Shiraz, Iran
a r t i c l e
i n f o
Article history: Received 20 November 2014 Received in revised form 23 June 2015 Accepted 29 June 2015 Available online 2 July 2015 Keywords: Pistachio Drying Rehydration Texture Hysteresis
a b s t r a c t To prolong shell life, pistachio kernels are first dried and then in many cases they are rehydrated prior to or during food formulations. Texture of the rehydrated kernels is one of key organoleptic properties for consumer acceptability. In this research pistachio kernels were dried and then rehydrated to different moisture levels to compare the effect of these treatments on their textural properties. This was performed to examine texture differences between pistachio kernels of same water content but different drying, rehydration or thermal histories. Texture profile analysis data showed that hardness, gradient and compression energy of all pistachio samples increased with moisture reduction and significant differences were observed between the textural parameters of dried and rehydrated samples with same moisture level and lower values were obtained for rehydrated samples. It was evident from the results that thermal history of pistachio plays a key role in determining its textural and mechanical properties. Having pistachio kernels of same moisture and different drying and rehydration histories does not guarantee similar textural properties. These findings are clearly showing that texture deteriorations or changes can occur when fruits are first dried and then rehydrated to the initial moisture level. This can have practical implications in developing and designing foods with texture being their main sensory property. Ó 2015 Published by Elsevier Ltd.
1. Introduction Pistachio is mainly cultivated in Iran, USA, Turkey, Syria, Greece and Italy and its production and consumption constantly is increasing. In recent decades new pistachio producing countries (e.g. Australia) have emerged (FAOSTAT, 2008). In 2003, Iran produced 54.7% (275,000 tons) of world’s pistachio production and exported 184,946 tons (Razavi and Taghizadeh, 2007). This was the largest non-petroleum export product from Iran (Kouchakzadeh, 2013). Almost all of the harvested pistachio is dried, with shell or shelled, and used for color and flavor in confectionery and cookery or consumed directly (Gamli and Hayaoglu, 2007). Similar to other nuts, pistachio is a good source of vitamins, minerals, sterols, phenolic compounds and fatty acids (Brufau et al., 2006; Ryan et al., 2006; Venkatachalam and Sathe, 2006; Miraliakbari and Shahidi, 2008). Pistachio composition, especially unsaturated fatty acids, and also pistachio moisture at the time of harvest (about 40–50% (db) depends on date and location climate) make it sensitive to rancidity and molding (Brooke et al., ⇑ Corresponding author at: School of Biomedical Sciences, Graham Centre for Agricultural Innovation and ARC Industrial Transformation Training Centre for Functional Grains, Charles Sturt University, Wagga Wagga, NSW, Australia. E-mail address: [email protected] (A. Farahnaky). http://dx.doi.org/10.1016/j.jfoodeng.2015.06.036 0260-8774/Ó 2015 Published by Elsevier Ltd.
1997). Therefore proper drying of pistachio to 5–7% (db) moisture is very important for its final quality and shelf life extension. Drying is a process during which the majority of the water normally present in a food is removed (Erbay and Icier, 2009) resulting in a low moisture food (usually less than 10%) (Krokida and Marinos-Kouris, 2003). It is a common preservation method for vegetables and fruits to increase their shelf life by decreasing water activity, slowing down microbial growth, and reducing transportation cost because of reduction in production weight and volume (Prabhanjan et al., 1995). However the increase in temperature and removal of water can result in other reactions that can inversely change the materials properties (Maskan, 2001). Structure and original tissue destruction, shrinkage, reduction of porosity and textural changes are some of these possible alterations (Maskan, 2001; Krokida et al., 2000; Lewicki and Jakubczyk, 2004). Texture is one of the main attributes of dried/rehydrated foods that is used to optimize food products processing (Bourne, 2002). Food texture is defined as all the structural, mechanical and rheological properties of a material sensed by tactile, mechanical, hearing and visual receptors (Lawless and Heymann, 1998). It can be considered as an attribute to assess acceptability and quality of products. Hardness is one of textural properties of vegetables and fruits parameters that indicate their freshness and quality (Konopacka and Plocharski, 2004).
336
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341
It is well documented that thermal history of biomaterials affects their physicochemical properties. One of the phenomena occurring in biopolymers during storage at low moisture is known as physical ageing. Physical ageing is a well-known phenomenon in polymers and occurs when a polymer is rapidly cooled from the equilibrium rubber or liquid state into the non-equilibrium glassy state which is similar to drying process of many foods. The dry matter will gradually relax toward equilibrium, as it is stored in the dry state. The gradual relaxation of the polymer is time dependent and manifests itself as changes in its mechanical properties, such as density and hardness (Farahnaky et al., 2005). Rehydration is a common process prior to consumption of many dehydrated foods. Aiming at restoring fresh products properties, in this process dried products are placed in direct or indirect contact with water. During rehydration water is absorbed followed by structural swelling and leaching out of solutes at the end of rehydration process (Lee et al., 2006; Marin et al., 2006). Rehydration is also an indicator of structural and cellular disruption caused by drying of pistachio materials (Krokida et al., 1999; Krokida and Marinos-Kouris, 2003; Lewicki, 1998a,b; Sacilik and Elicin, 2006). Dislocation and rupture of cellular structure (irreversible changes), due to loss of totality and therefore the hyphae of dried products are collapsed and flattened, and hydrophilic attributes decreased, hence sufficient water cannot be absorbed and rehydration becomes incomplete (Krokida and Marinos-Kouris, 2003). The ultimate objective of rehydration process to obtain a product with textural properties similar or close to its original texture which will depend on processing and drying steps and conditions (Kompany et al., 1993; McMinn and Magee, 1997; Lewicki, 1998a,b; Sanjuan et al., 1999). Nowadays, most of pistachio is used as snack in its dry form (roasted or raw) moreover pistachio is used in a large number of dishes, starters, side dishes, salads, puddings and confectionaries where dry pistachio kernels or cuts are exposed to moderate to high water environments. For example in meat industry shelled pistachio kernels are rehydrated and mixed with sausage farsh prior to filling, and in traditional puddings (e.g. Shole-Zard in Iran), Turkish delight, pistachio cakes, pistachio brownies, pistachio ice cream, chicken and pistachio Korma and pistachio cheese, pistachio kernels or cuts are rehydrated prior to or during cooking or processing (Al-Moghazy et al., 2014; Gamli and Hayaoglu, 2007; López-Calleja et al., 2014). Therefore sensory properties of rehydrated pistachio kernels and cuts and in particular their texture is of great importance. Review of the literature showed no report on pistachio drying and especially rehydration of pistachio in relation to texture, hence the objective of this study was to determine the effect of drying and rehydration on pistachio texture and texture hysteresis. 2. Materials and methods 2.1. Sample preparation Fresh pistachio kernels of Kalle-Ghuchi variety with the moisture content of 61.33% (dry basis) were purchased from the local market in Shiraz, Iran. To avoid any moisture loss, the fresh pistachio kernels were packed and sealed in polyethylene bags and kept at 4 °C for further experiments. For all treatments the pistachio kernels were shelled and peeled manually and one sample from each cotyledon was cut with a sharp knife and a cylindrical shape sample cutter (7.5 mm diameter). 2.1.1. Moisture determination Moisture content of fresh pistachio was determined by oven drying at 105 °C until constant weight (AOAC, 2000). A three digits
(0.001) analytical balance (Sartorius Model: ENTRIS323-1S) was used for moisture determination. 2.1.2. Pistachio drying To study texture hysteresis of pistachio kernels, pistachio samples of different moisture contents were required. Air forced drying of pistachio cuts was performed using a cabinet dryer (Stal-Astra, Proctor Schwartz, USA). Drying was done for up to 300 min at 70 °C and flow rate of 2 m/s. The distance between the trays was about 10 cm. To have a homogenous moisture migration from top and bottom of each sample, the samples were placed on a metal mesh and then transferred to the drier. At the beginning of drying and after 5, 10, 25, 40, 65, 90, 160 and 300 min of drying nine pistachio samples were taken for further tests and their moisture contents were calculated gravimetrically, by taking into account their initial and final weights and initial moisture contents. The average standard deviation of moisture contents was about 3%. 2.1.3. Rehydration To obtain rehydrated pistachio cuts of known moisture, completely dried samples (as obtained at the end of drying in the previous section) were placed in closed boxes over a super saturation solution of NaCl until the moisture content of samples reached 4.38%, 8.31%, 15.14%, 21.53%, 30.31%, 35.22%, 40.59% and 61.33% (these moisture levels were chosen to match the moisture contents of the nine samples taken after drying) by frequent weighing of the samples and using the Eq. (1) (Hii et al., 2009):
X i ¼ ðM i M ds Þ=M ds 100
ð1Þ
where Xi is moisture content (dry basis, db), Mds and Mi are weights of dry solid and sample at time i, respectively. The average standard deviation of moisture contents was about 3%. 2.2. Textural analysis Pistachio texture was tested using a Texture Analyzer (Texture Analyzer, TAPlus, Stable Microsystems, Surrey, England) with a 30 kg load cell. Texture profile analysis (TPA) was performed using a cylindrical aluminum probe with diameter of 100 mm. Double compression test was done to 15% of sample initial height at a test speed of 1 mm/s and a waiting time of 10 s between the two compressions. Texture Exponent Lite software provided by the equipment’s manufacture was used to analyze force–time graphs and five textural parameters were derived from each TPA: hardness (peak force of first compression), gradient (slope of force– deformation), compression energy as area under first peak SSE until maximum (Farahnaky et al., 2012), springiness, the ratio of the time recorded for the second bite of compression until peak force to that of the first bite, cohesiveness as the ratio of the area under the second to that of the first compression (Bourne, 2002). Measurements of texture were carried out at room temperature (25 °C) and for each sample 6 replicates were performed. 2.3. Modeling of texture data The resulting textural parameters were fitted into three experimental models (Eq. (2) (linear), Eq. (3) (quadratic), and Eq. (4) (exponential)) to examine the suitability of these equations in predicting texture–moisture changes in pistachio at a wide range of moisture contents.
y ¼ ax þ b
ð2Þ
y ¼ ax2 þ bx þ c
ð3Þ
y ¼ b þ yo expx=a
ð4Þ
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341
where y refers to texture parameter in a defined moisture content, yo is initial texture parameter, x is moisture content and, a, b, c, are constant parameters. Goodness of the fits was calculated using R2 (determination coefficient), RMSE as root mean square error (Eq. (5)), SSE, sum square error (Eq. (6)), v2 the chi-square (Cox et al., 2012), (Eq. (7)) and E% the percent mean relative deviation modulus (Eq. (8)) between the experimental and the predicted values (Peleg, 1998; Deng et al., 2011).
RMSE ¼
SSE ¼
v2 ¼
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 Xn ðX exp X pre Þ2 i¼1 N
n 1X ðX exp X pre Þ2 N i¼1
Pn
E% ¼
X pre Þ2 NZ
i¼1 ðX exp
100 N
Pn
X pre j X exp
i¼1 jX exp
ð5Þ
ð6Þ
ð7Þ
ð8Þ
where Xexp is defined as the experimental texture value, Xpre is the texture value predicted by the model, N is the observations number and Z is the number of constants. 2.4. Stress relaxation test Stress relaxation tests of pistachio cuts were conducted using a Texture Analyzer by compressing each pistachio cut to 15% of its original height. After compression the force was recorded for 120 s during which the probe was still. The tests were performed at room temperature on 6 replicates for each sample. The force– time data obtained from stress relaxation tests were fitted into Maxwell model (Mohsenin, 1970; Sherman, 1970) with n = 3 represented by Eq. (9) to analyze the pistachio textural behavior under compress. Maxwell model consists of n components that each component is comprised of a dashpot (viscous) and spring (elastic) elements (Yildiz et al., 2013).
EðtÞ ¼ Ee þ
n X
Ei expðt=si Þ
337
both dried and rehydrated pistachios were lower at the high moisture level (61.33%, Fig. 1a) than those of low moisture (4.38%, Fig. 1b). For more detailed information, force–time curves were analyzed and main textural parameters (hardness, gradient and compression energy) of the dried and rehydrated samples at different moisture levels are presented in Fig. 2. The texture hardness of pistachio cuts increased with moisture decrease. This could be related to the fact that higher moisture contents will result in more swollen and softer structures as reported by Hii et al. (2012) who compared fresh and dehydrated cocoa beans and reported similar findings. Reduction of extra and intracellular spaces, canals and cavities, rapid loss of moisture content and case hardening have been referred to as the reasons of hardness increase with moisture decrease (Deng et al., 2014; Chang et al., 2011; Valencia-Pérez et al., 2008). This trend was also observed for rehydrated pistachio cuts in which the samples of higher moisture levels had lower hardness values. Cox et al. (2012) reported the texture softening of seaweed texture on rehydration at different temperatures and García-Segovia et al. (2011) have pointed out that mushroom fibers softening due to filling cavities with water is related to softer texture of rehydrated samples. The inverse relationship of texture hardness with moisture was in line with the findings of other researchers on broccoli, mushrooms and chestnuts (Sanjuán et al., 2001; Hernando et al., 2008; Moreira et al., 2008). Similar results can be drawn from compression energy and gradient values. One interesting and general phenomenon observed for textural parameters is that the studied textural parameters were greater for dried samples than those of rehydrated samples at same moisture level and all textural parameters showed similar trends (Fig. 2). Compression forces of all dried samples are greater than those of rehydrated samples with the same moisture level, for example the compression peak forces of dried and rehydrated samples at
ð9Þ
i¼1
where E(t) is the modulus decaying at time t, t is time of decay (s),
s(s) is the relaxation constant for ith Maxwell element, Ei is the decay modulus of each element, and Ee is the equilibrium or residual modulus (Cespi et al., 2007). The decay stress (Ei) is the indicator of materials elasticity (Khazaei and Mann, 2005) and Ee, the equilibrium modulus, represents the stiffness and strength of the components (Campus et al., 2010; Tang et al., 1998). 2.5. Statistical analysis The statistical analysis of the results was conducted by performing the analysis of variance to determine significant differences (P < 0.05) among the samples. Duncan test was then used for grouping of the samples using SPSS software (version 16). For stress relaxation test, the force-relaxation time data were fitted to Maxwell model by minimizing sum of squares using Solver (Excel, Microsoft Office 2007, Microsoft Ltd.). 3. Results and discussion 3.1. Textural measurements Force–time graphs of TPA test of dried and rehydrated pistachio cuts with moisture content (db) of 61.33% and 4.38% are presented in Fig. 1a and b, respectively. As expected, compression forces of
Fig. 1. Compression curves of dried and rehydrated pistachio kernels with moisture contents (db) of 61.33% (a) and 4.38% (b).
338
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341
polymers will come in closer contact leading to formation of stable hydrogen and other non-covalent interactions. Therefore during drying gradually and continuously more micro pores will become unavailable. On hydration however when water comes in contact with dried food, less available sites will become interacted with water molecules and more free water or water of high mobility will be present. Therefore on rehydration water first fills the open spaces and then the more external zones (Moreira et al., 2008). This will result in differences in physical response of dehydrated and rehydrated pistachio cuts to deformation force and manifests as texture hysteresis. Springiness and cohesiveness of dried and rehydrated pistachios at different moisture levels are presented in Table 1. Reducing moisture content from 61.33% to 4.38% decreased both springiness and cohesiveness of dried samples and rehydrated samples. The springiness and cohesiveness of high moisture pistachio cuts before drying was 0.913 and 0.804, respectively. This indicates that at high moisture pistachio texture is more elastic and its viscous component is minor, therefore texture destruction after 15% compression has been rather low and texture integrity remained high. Comparison of springiness and cohesiveness of dried and rehydrated pistachios shows reduction of these parameters in rehydrated samples as compared to their corresponding samples with same moisture level. For example at moisture level of 8.31% springiness of dried and rehydrated samples were 0.719 and 0.488 and cohesiveness was 0.452 and 0.337, respectively. This is revealing that by drying and rehydration of pistachio, not only texture hardness but also textural behavior (in terms of elastic or viscous domination), is subject to change. Capillary voids filled with moisture can be the cause of cohesiveness increase by moisture increase during rehydration (Kotwaliwale et al., 2007; Khin et al., 2007). When food biopolymers absorb water it appears that they become more elastic. The springiness of rehydrated cells can decrease due to reduction in ability of cells to recover their initial form (Khin et al., 2007). 3.2. Texture modeling
Fig. 2. The curves of textural parameters (compression energy (a), gradient (b) and hardness (c)) changes for experimental and predicted values (from exponential model) for both dried and rehydrated pistachio kernels. Bars show stdev for six replicates.
61.33% moisture level were about 30 and 6 N (Fig. 1a and b), respectively. Differences in texture hardness, compression energy or gradient between a pistachio sample dried to a certain moisture level and a sample completely dried and then rehydrated to the same moisture level is appearing to be consistent and may be called ‘‘texture hysteresis’’. From Fig. 2 it might be concluded that texture hysteresis of pistachio is most observed in the moisture range of 15–30% where texture differences between the dried and rehydrated samples are greater. Texture hysteresis is referring to the fact that when a food is dried and then rehydrated to the initial moisture level, then texture difference will exist between the fresh and the rehydrated samples of same moisture. This is similar to moisture hysteresis when desorption and adsorption curves of food materials are drawn on isotherm graphs in which if the lines do not overlap, then moisture hysteresis is confirmed (Ansari et al., 2011). Generally, during drying water is removed from foods and as a result food components including polysaccharides and protein
Hardness, gradient and compression energy data were fitted into linear, quadratic and exponential models to quantify texture changes and differences between dried and rehydrated samples. R2, RMSE, SSE, v2 and E% were calculated from the experimental and predicted data to choose the best model with higher value of R2 and lower values for other parameters (Table 2). Values of R2 ranged from 0.14 to 0.995 and confirmed that the exponential model was the best for modeling pistachio texture data with the highest R2 and the lowest values for RMSE, SSE, v2 and E%. Fig. 2a –c shows the predicted (lines) and experimental data Table 1 Springiness and cohesiveness of dried and rehydrated pistachio kernels at different moisture levels. Moisture content (%)
0 4.38 8.31 15.14 21.53 30.31 35.22 40.59 61.33
Springiness
Cohesiveness
Dried pistachio
Rehydrated pistachio
Dried pistachio
Rehydrated pistachio
0.402a ± 0.07 0.599a ± 0.03 0.719a ± 0.06 0.746a ± 0.11 0.778a ± 0.06 0.805a ± 0.06 0.866a ± 0.04 0.861a ± 0.05 0.913a ± 0.01
0.402a ± 0.07 0.423b ± 0.03 0.488b ± 0.04 0.519b ± 0.01 0.642a ± 0.02 0.625b ± 0.06 0.728b ± 0.04 0.759a ± 0.07 0.804b ± 0.02
0.231a ± 0.02 0.376a ± 0.01 0.452a ± 0.06 0.648a ± 0.02 0.642a ± 0.01 0.695a ± 0.03 0.683a ± 0.08 0.712a ± 0.00 0.758a ± 0.02
0.231a ± 0.02 0.255b ± 0.01 0.337b ± 0.02 0.324b ± 0.01 0.392b ± 0.05 0.474b ± 0.02 0.531b ± 0.07 0.601b ± 0.06 0.640b ± 0.01
Different letters show significant differences between the dehydrated and rehydrated samples at same moisture content for each parameter (a < 0.05).
339
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341 Table 2 Statistical parameters of linear, quadratic and exponential models of textural parameters of dried and rehydrated pistachio kernels. Model
Parameter
Linear
R2* RMSE* SSE*
Dried pistachio Compression energy
Gradient
Hardness
Compression energy
Gradient
Hardness
0.901 0.208 0.043 0.065 0.075
0.824 0.545 0.293 0.440 0.091
0.953 0.172 0.030 0.045 0.035
0.624 1.594 2.550 3.825 1.774
0.703 0.986 0.968 1.452 0.752
0.721 0.725 0.523 0.784 0.443
0.140 2.653 7.062 10.594 1.345
0.502 0.972 0.951 1.427 0.089
0.186 1.414 2.004 3.007 1.083
0.443 1.069 1.143 1.714 0.144
0.317 1.387 1.905 2.858 0.513
0.431 1.125 1.257 1.886 0.475
0.927 0.261 0.069 0.103 0.197
0.878 1.172 1.375 2.063 0.259
0.936 0.240 0.060 0.090 0.084
0.995 0.091 0.008 0.012 0.037
0.891 0.674 0.462 0.693 0.212
0.991 0.165 0.027 0.041 0.076
v2* E%* Quadratic
R2 RMSE SSE
v2 E% Exponential
Rehydrated pistachio
R2 RMSE SSE
v2 E%
R2, is coefficient of determination and the higher the better (R2 = 1 is the highest). The standard error of estimate (SEE) and root mean square error (RMSE) were calculated to give indications of the precision of the estimation of the experiment, the lower these values the better. X2 (chi-square) is an indication of sum of errors taking into account the number observations and model constants and the lower the better. The mean relative percentage deviation modulus (E%), with a modulus value below 10% is indicative of a good fit for practical purposes. *
Table 3 Constant rate (a) and residual texture (N) (b) of textural parameters in exponential model of dried and rehydrated pistachio kernels. Modeling parameters
a b
Compression energy
Gradient
Hardness
Dried pistachio
Rehydrated pistachio
Dried pistachio
Rehydrated pistachio
Dried pistachio
Rehydrated pistachio
46.34a ± 2.24 6.14a ± 0.35
9.24b ± 0.26 0.91b ± 0.04
66.50a ± 5.62 7.33a ± 0.89
17.57b ± 0.97 2.29b ± 0.37
81.36a ± 8.39 10.06a ± 1.18
10.93b ± 1.24 1.89b ± 0.60
Different letters show significant differences between the samples in terms of each parameter in each row (a < 0.05).
points. Constant parameters of exponential model, a as the texture changes rate constant and b (residual texture) were analyzed statistically and presented in Table 3. The constants rate of changes, a, obtained from hardness, gradient and compression energy were greater for dried samples as compared with rehydrated ones. This confirms the superior texture changes in rehydrated samples over the moisture range, for example a parameters from hardness values were 81.36 and 10.93 for dried and rehydrated pistachio cuts, respectively. Also residual texture parameter (b) of rehydrated samples (10.06 N) was significantly different from that of dried samples (1.89 N) as obtained for hardness data. 3.3. Stress relaxation test Stress relaxation graphs and modeling parameters are presented in Fig. 3 and Table 4. Ee, as an indication of residual modulus Table 4 Parameters of Maxwell model for dried and rehydrated pistachio kernels at different moisture levels. Moisture content (%)
Dried pistachio
Ee (N)
Fig. 3. Stress relaxation tests of dried and rehydrated pistachio kernels with moisture contents (db) of 61.33% (a) and 0% (b). Dots are experimental results and lines are modeled (Maxwell, n = 3) data.
0 4.38 8.31 15.14 21.53 30.31 35.22 40.59 61.33
a
23.08 21.00a 20.90a 15.42a 16.80a 12.02a 12.69a 5.40a 4.48a
Rehydrated pistachio
R2
s1
Ei a
1.90 2.39a 2.37a 1.63a 1.44a 1.24a 1.15a 0.86a 0.80a
a
30.23 25.25a 23.56a 21.38a 18.80a 17.40a 12.07a 12.41a 10.00a
0.94 0.95 0.88 0.87 0.91 0.93 0.94 0.94 0.92
Ee 23.08 6.39b 6.95b 5.24b 4.33b 2.05b 2.34b 1.92b 1.49b
R2
s2
Ei a
a
1.90 1.56b 1.77b 1.05b 0.74b 0.52b 0.34b 0.41b 0.23b
a
30.23 23.51a 22.82a 18.71b 16.15a 17.22a 14.52a 13.13a 7.04b
0.94 0.94 0.91 0.92 0.87 0.97 0.87 0.86 0.93
Different letters show significant differences between the dried and rehydrated samples in terms of each parameter in each row (a < 0.05).
340
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341
or texture at the end of stress relaxation, indicates that significant texture differences were found between dried and rehydrated samples for all moisture levels. For example residual texture was 21.00 and 6.39 N for dried and rehydrated pistachios at 4.38% moisture, i.e. the rehydrated samples had a weaker texture than the dried samples. Ei as the decay modulus of each element was greater for all dried samples than rehydrated samples with same moisture. The relaxation constant (s) showed significant differences at 61.33% moisture. Ei and Ee values confirmed that the tested materials had a solid-like structure (Bhattacharya, 2010).
4. Conclusion Double compression and stress relaxation tests were applied to dried and rehydrated samples to determine the mechanical behavior of samples and the results were modeled into linear, quadratic and logarithmic models and logarithmic was found as the best model. Hardness, gradient and compression energy of all pistachio samples increased with moisture reduction but significant differences between textural parameters for two processes were observed and the lower values for rehydrated samples were recorded at different moisture levels. This indicates that under a constant stress rehydrated pistachio kernels deform to a greater extent as compared to their corresponding dry samples at similar moisture levels. The extent of texture hysteresis of pistachio varied at different moisture levels and was most observed in the moisture range of 15–30%, therefore water mobility can be involved. It is evident from the results that thermal history of pistachio plays a key role in determining its textural and mechanical properties. Having pistachio kernels of same moisture content does not mean that textural properties will be the same even when other parameters such as variety are kept the same. In food formulations the delicate texture of pistachio kernel perceived by the consumers will depend not only on its moisture level but also on its thermal history during drying, storage and processing. Therefore, dried pistachio kernels will behave differently in high moisture food formulations as compared to fresh kernels. The outcome of this research is clearly showing that significant texture deteriorations or changes can occur when fruits are first dried and then rehydrated to the same moisture level and to gain structural characteristics similar to fresh fruits only moisture adjustment is not enough. Further research is suggested to evaluate the effect of thermal history and dehydration, rehydration processes and storage on textural properties of other food materials.
References Al-Moghazy, M., Boveri, S., Pulvirenti, A., 2014. Microbiological safety in pistachios and pistachio containing products. Food Control 36, 88–93. AOAC, 2000. Association of Official Analytical Chemists, Official Methods of Analysis. AOAC, Washington DC. Ansari, S., Farahnaky, A., Majzoobi, M., 2011. Modeling the effect of glucose syrup on the moisture sorption isotherm of figs. Food Biophys. 6, 377–389. Bhattacharya, S., 2010. Stress relaxation behaviour of moth bean flour dough: product characteristics and suitability of model. J. Food Eng. 97, 539–546. Bourne, M.C., 2002. Food Texture and Viscosity,, .. Concept and Measurement, second ed. Academic Press, New York. Brooke, D.B., Bakker, F.W., Hall, C.W., 1997. Drying and Storage of Grains and Oilseeds. AVI Publishing, Van Nostrand Reinhold, New York, USA. Brufau, G., Boatella, J., Rafecas, M., 2006. Nuts: source of energy and macronutrients. Br. J. Nutr. 96, S24–S28. Campus, M., Addis, M.F., Cappuccinelli, R., Porcu, M.C., Pretti, L., Tedde, V., Secchi, N., Stara, G., Roggio, T., 2010. Stress relaxation behaviour and structural changes of muscle tissues from Gilthead Sea Bream (Sparus aurata L.) following high pressure treatment. J. Food Eng. 96, 192–198. Cespi, M., Bonacucina, G., Misici-Falzi, M., Golzi, R., Boltri, L., Palmieri, G.F., 2007. Stress relaxation test for the characterization of the viscoelasticity of pellets. Eur. J. Pharm. Biopharm. 67, 476–484. Chang, H.J., Xu, X.L., Li, C.B., Huang, M., Liu, D.Y., Zhou, G.H., 2011. A comparison of heat-induced changes of intramuscular connective tissue and collagen of beef
semitendinosus muscle during water bath and microwave heating. J. Food Process Eng 34, 2233–2250. Cox, S., Gupta, S., Abu-Ghannam, N., 2012. Effect of different rehydration temperatures on the moisture, content of phenolic compounds, antioxidant capacity and textural properties of edible Irish brown seaweed. LWT – Food Sci. Technol. 47, 300–307. Deng, Y., Wu, J., Su, S.Q., Liu, Z.D., Ren, L., Zhang, Y.L., 2011. Effect of far-infra red assisted heat pump drying on water status and moisture sorption isotherm of squid (Illex illecebrosus) fillets. Drying Technol. 29, 1580–1586. Deng, Y., Luo, Y., Wanga, Y., Yue, J., Liu, Z., Zhong, Y., Zhao, Y., Yang, H., 2014. Drying-induced protein and microstructure damages of squid fillets affected moisture distribution and rehydration ability during rehydration. J. Food Eng. 123, 23–31. Erbay, Z., Icier, F., 2009. Optimization of hot air drying of olive leaves using response surface methodology. J. Food Eng. 91 (4), 533–541. FAOSTAT, 2008. FAOSTAT Database Results, 2005.. Farahnaky, A., Badii, F., Farhat, I.A., Mitchell, J.R., Hill, S.E., 2005. Enthalpy relaxation of bovine serum albumin and implications for its storage in the glassy state. Biopolymers 78, 69–77. Farahnaky, A., Azizi, R., Gavahian, M., 2012. Accelerated texture softening of some root vegetables by ohmic heating. J. Food Eng. 113, 275–280. Gamli, O.F., Hayaoglu, I., 2007. The effect of the different packaging and storage conditions on the quality of pistachio nut paste. J. Food Eng. 78, 443–448. García-Segovia, P., Andrés-Bello, A., Martínez-Monzó, J., 2011. Rehydration of air dried Shiitake mushroom (Lentinus edodes) caps: comparison of conventional and vacuum water immersion processes. LWT – Food Sci. Technol. 44, 480–488. Hernando, I., Sanjuán, N., Pérez-Munuera, I., Mulet, A., 2008. Rehydration of freezedried and convective dried Boletus edulis mushrooms: effect on some quality parameters. J. Food Sci. 73, 356–362. Hii, C.L., Lawb, C.L., Clokea, M., Suzannahb, S., 2009. Thin layer drying kinetics of cocoa and dried product quality. Biosyst. Eng. 102 (2), 153–161. Hii, C.L., Law, C.L., Suzannah, S., 2012. Drying kinetics of the individual layer of cocoa beans during heat pump drying. J. Food Eng. 108, 276–282. Khazaei, J., Mann, D.D., 2005. Effect of moisture content and number of loadings on force relaxation behavior of chickpea kernels. Int. Aerophys. 19, 305–313. Khin, M.M., Zhou, W., Yeo, S.Y., 2007. Mass transfer in the osmotic dried pistachio of coated apple cubes by using maltodextrin as the coating material and their textural properties. J. Food Eng. 81, 514–522. Kompany, E., Benchimol, J., Allaf, K., Ainseba, B., Bouvier, J.M., 1993. Carrot dried pistachio for instant rehydration: dried pistachio kinetics and modeling. Drying Technol. 11 (3), 451–470. Konopacka, D., Plocharski, W.J., 2004. Effect of storage conditions on the relationship between apple firmness and texture acceptability. Postharvest Biol. Technol. 32 (2), 205–211. Kotwaliwale, N., Bakane, P., Verma, A., 2007. Changes in textural and optical properties of oyster mushroom during hot air drying. J. Food Eng. 78, 1207– 1211. Kouchakzadeh, A., 2013. The effect of acoustic and solar energy on drying process of pistachios. Energy Convers. Manag. 67, 351–356. Krokida, M.K., Kiranoudis, C.T., Maroulis, Z.B., 1999. Viscoelastic behavior of dehydrated products during rehydration. J. Food Eng. 40, 269–277. Krokida, M.K., Karathanos, V.T., Maroulis, Z.B., 2000. Compression analysis of dehydrated agricultural products. Drying Technol. 18, 395–408. Krokida, M.K., Marinos-Kouris, D., 2003. Rehydration kinetics of dehydrated products. J. Food Eng. 57, 1–7. Lawless, H.T., Heymann, H.H., 1998. Sensory Evaluation of Food: Principles and Practices. Chapman and Hall, New York, NY. Lee, K.T., Farid, M., Nguang, S.K., 2006. The mathematical modelling of the rehydration characteristics of fruits. J. Food Eng. 72, 16–23. Lewicki, P.P., 1998a. Effect of pre-drying treatment, drying and rehydration on plant tissue properties: a review. Int. J. Food Prop. 1, 1–22. Lewicki, P.P., 1998b. Some remarks on rehydration of dried foods. J. Food Eng. 36 (1), 81–87. Lewicki, P.P., Jakubczyk, E., 2004. Effect of hot air temperature on mechanical properties of dried apples. J. Food Eng. 64, 307–314. López-Calleja, I.M., de la Cruz, S., González, I., García, T., Martín, R., 2014. Survey of undeclared allergenic pistachio (Pistacia vera) in commercial foods by hydrolysis probe real-time PCR. Food Control 39, 49–55. Marin, B.E., Lemus, M.R., Flores, M.V., 2006. La rehidratación de alimentos deshidratados. Rev Chilena de Nutrición 33 (3), 527–538. Maskan, M., 2001. Drying shrinkage and rehydration characteristics of kiwi fruits during microwave drying. J. Food Eng. 48, 177–182. McMinn, W.A.M., Magee, T.R.A., 1997. Quality and physical structure of dehydrated starch based system. Drying Technol. 15 (6–8), 1961–1971. Miraliakbari, H., Shahidi, F., 2008. Lipid class compositions, tocopherols and sterols of tree nut oils extracted with different solvents. J. Food Lipids 15, 81–96. Mohsenin, N.N., 1970. Physical Properties of Plant and Animal Materials. Gordon and Breach Scientific Publishers, New York. Moreira, R., Chenlo, F., Chaguri, L., Fernandes, C., 2008. Water absorption, texture and color kinetics of air-dried chestnuts during rehydration. J. Food Eng. 86, 584–594. Peleg, M., 1998. An empirical model for description of moisture sorption curves. J. Food Sci. 53, 1216–1218. Prabhanjan, D.G., Ramaswamy, H.S., Raghavan, G.S.V., 1995. Microwave assisted convective air drying of thin layer carrots. J. Food Eng. 25, 283–293.
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341 Razavi, S.M.A., Taghizadeh, M., 2007. The specific heat of pistachio nuts as affected by moisture content, temperature, and variety. J. Food Eng. 79, 158–167. Ryan, E., Galvin, K., O’Connor, T.P., Maguire, R., O’Brien, N.M., 2006. Fatty acid profile, tocopherol, squalene and phytosterol content of Brazil, pecan, pine, pistachio and cashew nuts. Int. J. Food Sci. Nutr. 57, 219–228. Sacilik, K., Elicin, A.K., 2006. The thin layer drying characteristics of organic apple slices. J. Food Eng. 73 (3), 281–289. Sanjuan, N., Simal, S., Bon, J., Mulet, A., 1999. Modelling of broccoli stems rehydration process. J. Food Eng. 42 (1), 27–31. Sanjuán, N., Cárcel, J.A., Clemente, G., Mulet, A., 2001. Modelling of the rehydration process of broccoli florets. Eur. Food Res. Technol. 212, 449–453. Sherman, P., 1970. Industrial Rheology. Academic Press, New York.
341
Tang, J., Tung, M., Zeng, A.Y., 1998. Characterization of gellan gels using stress relaxation. J. Food Eng. 38, 279–295. Valencia-Pérez, A.Z., Garcia-Morales, M.H., Cardenas-Lopez, J.L., Herrera-Urbina, J.R., Rouzaud-Sandez, O., Ezquerra-Brauer, J.M., 2008. Effect of thermal process on connective tissue from jumbo squid (Dosidicus gigas) mantle. Food Chem. 107, 1371–1378. Venkatachalam, M., Sathe, S.S., 2006. Chemical composition of selected edible nut seeds. J. Agric. Food Chem. 54, 4705–4714. Yildiz, Ö., Yurt, B., Basßtürk, A., Toker, Ö.S., Yilmaz, M.T., Karaman, S., Dag˘lıog˘lu, O., 2013. Pasting properties, texture profile and stress–relaxation behavior of wheat starch/dietary fiber systems. Food Res. Int. 53, 278–290.
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Texture hysteresis of pistachio kernels on drying and rehydration Asgar Farahnaky ⇑,a,b, Elahe Kamali b a School of Biomedical Sciences, Graham Centre for Agricultural Innovation and ARC Industrial Transformation Training Centre for Functional Grains, Charles Sturt University, Wagga Wagga, NSW, Australia b Department of Food Science and Technology, School of Agriculture, Shiraz University, Shiraz, Iran
a r t i c l e
i n f o
Article history: Received 20 November 2014 Received in revised form 23 June 2015 Accepted 29 June 2015 Available online 2 July 2015 Keywords: Pistachio Drying Rehydration Texture Hysteresis
a b s t r a c t To prolong shell life, pistachio kernels are first dried and then in many cases they are rehydrated prior to or during food formulations. Texture of the rehydrated kernels is one of key organoleptic properties for consumer acceptability. In this research pistachio kernels were dried and then rehydrated to different moisture levels to compare the effect of these treatments on their textural properties. This was performed to examine texture differences between pistachio kernels of same water content but different drying, rehydration or thermal histories. Texture profile analysis data showed that hardness, gradient and compression energy of all pistachio samples increased with moisture reduction and significant differences were observed between the textural parameters of dried and rehydrated samples with same moisture level and lower values were obtained for rehydrated samples. It was evident from the results that thermal history of pistachio plays a key role in determining its textural and mechanical properties. Having pistachio kernels of same moisture and different drying and rehydration histories does not guarantee similar textural properties. These findings are clearly showing that texture deteriorations or changes can occur when fruits are first dried and then rehydrated to the initial moisture level. This can have practical implications in developing and designing foods with texture being their main sensory property. Ó 2015 Published by Elsevier Ltd.
1. Introduction Pistachio is mainly cultivated in Iran, USA, Turkey, Syria, Greece and Italy and its production and consumption constantly is increasing. In recent decades new pistachio producing countries (e.g. Australia) have emerged (FAOSTAT, 2008). In 2003, Iran produced 54.7% (275,000 tons) of world’s pistachio production and exported 184,946 tons (Razavi and Taghizadeh, 2007). This was the largest non-petroleum export product from Iran (Kouchakzadeh, 2013). Almost all of the harvested pistachio is dried, with shell or shelled, and used for color and flavor in confectionery and cookery or consumed directly (Gamli and Hayaoglu, 2007). Similar to other nuts, pistachio is a good source of vitamins, minerals, sterols, phenolic compounds and fatty acids (Brufau et al., 2006; Ryan et al., 2006; Venkatachalam and Sathe, 2006; Miraliakbari and Shahidi, 2008). Pistachio composition, especially unsaturated fatty acids, and also pistachio moisture at the time of harvest (about 40–50% (db) depends on date and location climate) make it sensitive to rancidity and molding (Brooke et al., ⇑ Corresponding author at: School of Biomedical Sciences, Graham Centre for Agricultural Innovation and ARC Industrial Transformation Training Centre for Functional Grains, Charles Sturt University, Wagga Wagga, NSW, Australia. E-mail address: [email protected] (A. Farahnaky). http://dx.doi.org/10.1016/j.jfoodeng.2015.06.036 0260-8774/Ó 2015 Published by Elsevier Ltd.
1997). Therefore proper drying of pistachio to 5–7% (db) moisture is very important for its final quality and shelf life extension. Drying is a process during which the majority of the water normally present in a food is removed (Erbay and Icier, 2009) resulting in a low moisture food (usually less than 10%) (Krokida and Marinos-Kouris, 2003). It is a common preservation method for vegetables and fruits to increase their shelf life by decreasing water activity, slowing down microbial growth, and reducing transportation cost because of reduction in production weight and volume (Prabhanjan et al., 1995). However the increase in temperature and removal of water can result in other reactions that can inversely change the materials properties (Maskan, 2001). Structure and original tissue destruction, shrinkage, reduction of porosity and textural changes are some of these possible alterations (Maskan, 2001; Krokida et al., 2000; Lewicki and Jakubczyk, 2004). Texture is one of the main attributes of dried/rehydrated foods that is used to optimize food products processing (Bourne, 2002). Food texture is defined as all the structural, mechanical and rheological properties of a material sensed by tactile, mechanical, hearing and visual receptors (Lawless and Heymann, 1998). It can be considered as an attribute to assess acceptability and quality of products. Hardness is one of textural properties of vegetables and fruits parameters that indicate their freshness and quality (Konopacka and Plocharski, 2004).
336
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341
It is well documented that thermal history of biomaterials affects their physicochemical properties. One of the phenomena occurring in biopolymers during storage at low moisture is known as physical ageing. Physical ageing is a well-known phenomenon in polymers and occurs when a polymer is rapidly cooled from the equilibrium rubber or liquid state into the non-equilibrium glassy state which is similar to drying process of many foods. The dry matter will gradually relax toward equilibrium, as it is stored in the dry state. The gradual relaxation of the polymer is time dependent and manifests itself as changes in its mechanical properties, such as density and hardness (Farahnaky et al., 2005). Rehydration is a common process prior to consumption of many dehydrated foods. Aiming at restoring fresh products properties, in this process dried products are placed in direct or indirect contact with water. During rehydration water is absorbed followed by structural swelling and leaching out of solutes at the end of rehydration process (Lee et al., 2006; Marin et al., 2006). Rehydration is also an indicator of structural and cellular disruption caused by drying of pistachio materials (Krokida et al., 1999; Krokida and Marinos-Kouris, 2003; Lewicki, 1998a,b; Sacilik and Elicin, 2006). Dislocation and rupture of cellular structure (irreversible changes), due to loss of totality and therefore the hyphae of dried products are collapsed and flattened, and hydrophilic attributes decreased, hence sufficient water cannot be absorbed and rehydration becomes incomplete (Krokida and Marinos-Kouris, 2003). The ultimate objective of rehydration process to obtain a product with textural properties similar or close to its original texture which will depend on processing and drying steps and conditions (Kompany et al., 1993; McMinn and Magee, 1997; Lewicki, 1998a,b; Sanjuan et al., 1999). Nowadays, most of pistachio is used as snack in its dry form (roasted or raw) moreover pistachio is used in a large number of dishes, starters, side dishes, salads, puddings and confectionaries where dry pistachio kernels or cuts are exposed to moderate to high water environments. For example in meat industry shelled pistachio kernels are rehydrated and mixed with sausage farsh prior to filling, and in traditional puddings (e.g. Shole-Zard in Iran), Turkish delight, pistachio cakes, pistachio brownies, pistachio ice cream, chicken and pistachio Korma and pistachio cheese, pistachio kernels or cuts are rehydrated prior to or during cooking or processing (Al-Moghazy et al., 2014; Gamli and Hayaoglu, 2007; López-Calleja et al., 2014). Therefore sensory properties of rehydrated pistachio kernels and cuts and in particular their texture is of great importance. Review of the literature showed no report on pistachio drying and especially rehydration of pistachio in relation to texture, hence the objective of this study was to determine the effect of drying and rehydration on pistachio texture and texture hysteresis. 2. Materials and methods 2.1. Sample preparation Fresh pistachio kernels of Kalle-Ghuchi variety with the moisture content of 61.33% (dry basis) were purchased from the local market in Shiraz, Iran. To avoid any moisture loss, the fresh pistachio kernels were packed and sealed in polyethylene bags and kept at 4 °C for further experiments. For all treatments the pistachio kernels were shelled and peeled manually and one sample from each cotyledon was cut with a sharp knife and a cylindrical shape sample cutter (7.5 mm diameter). 2.1.1. Moisture determination Moisture content of fresh pistachio was determined by oven drying at 105 °C until constant weight (AOAC, 2000). A three digits
(0.001) analytical balance (Sartorius Model: ENTRIS323-1S) was used for moisture determination. 2.1.2. Pistachio drying To study texture hysteresis of pistachio kernels, pistachio samples of different moisture contents were required. Air forced drying of pistachio cuts was performed using a cabinet dryer (Stal-Astra, Proctor Schwartz, USA). Drying was done for up to 300 min at 70 °C and flow rate of 2 m/s. The distance between the trays was about 10 cm. To have a homogenous moisture migration from top and bottom of each sample, the samples were placed on a metal mesh and then transferred to the drier. At the beginning of drying and after 5, 10, 25, 40, 65, 90, 160 and 300 min of drying nine pistachio samples were taken for further tests and their moisture contents were calculated gravimetrically, by taking into account their initial and final weights and initial moisture contents. The average standard deviation of moisture contents was about 3%. 2.1.3. Rehydration To obtain rehydrated pistachio cuts of known moisture, completely dried samples (as obtained at the end of drying in the previous section) were placed in closed boxes over a super saturation solution of NaCl until the moisture content of samples reached 4.38%, 8.31%, 15.14%, 21.53%, 30.31%, 35.22%, 40.59% and 61.33% (these moisture levels were chosen to match the moisture contents of the nine samples taken after drying) by frequent weighing of the samples and using the Eq. (1) (Hii et al., 2009):
X i ¼ ðM i M ds Þ=M ds 100
ð1Þ
where Xi is moisture content (dry basis, db), Mds and Mi are weights of dry solid and sample at time i, respectively. The average standard deviation of moisture contents was about 3%. 2.2. Textural analysis Pistachio texture was tested using a Texture Analyzer (Texture Analyzer, TAPlus, Stable Microsystems, Surrey, England) with a 30 kg load cell. Texture profile analysis (TPA) was performed using a cylindrical aluminum probe with diameter of 100 mm. Double compression test was done to 15% of sample initial height at a test speed of 1 mm/s and a waiting time of 10 s between the two compressions. Texture Exponent Lite software provided by the equipment’s manufacture was used to analyze force–time graphs and five textural parameters were derived from each TPA: hardness (peak force of first compression), gradient (slope of force– deformation), compression energy as area under first peak SSE until maximum (Farahnaky et al., 2012), springiness, the ratio of the time recorded for the second bite of compression until peak force to that of the first bite, cohesiveness as the ratio of the area under the second to that of the first compression (Bourne, 2002). Measurements of texture were carried out at room temperature (25 °C) and for each sample 6 replicates were performed. 2.3. Modeling of texture data The resulting textural parameters were fitted into three experimental models (Eq. (2) (linear), Eq. (3) (quadratic), and Eq. (4) (exponential)) to examine the suitability of these equations in predicting texture–moisture changes in pistachio at a wide range of moisture contents.
y ¼ ax þ b
ð2Þ
y ¼ ax2 þ bx þ c
ð3Þ
y ¼ b þ yo expx=a
ð4Þ
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341
where y refers to texture parameter in a defined moisture content, yo is initial texture parameter, x is moisture content and, a, b, c, are constant parameters. Goodness of the fits was calculated using R2 (determination coefficient), RMSE as root mean square error (Eq. (5)), SSE, sum square error (Eq. (6)), v2 the chi-square (Cox et al., 2012), (Eq. (7)) and E% the percent mean relative deviation modulus (Eq. (8)) between the experimental and the predicted values (Peleg, 1998; Deng et al., 2011).
RMSE ¼
SSE ¼
v2 ¼
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 Xn ðX exp X pre Þ2 i¼1 N
n 1X ðX exp X pre Þ2 N i¼1
Pn
E% ¼
X pre Þ2 NZ
i¼1 ðX exp
100 N
Pn
X pre j X exp
i¼1 jX exp
ð5Þ
ð6Þ
ð7Þ
ð8Þ
where Xexp is defined as the experimental texture value, Xpre is the texture value predicted by the model, N is the observations number and Z is the number of constants. 2.4. Stress relaxation test Stress relaxation tests of pistachio cuts were conducted using a Texture Analyzer by compressing each pistachio cut to 15% of its original height. After compression the force was recorded for 120 s during which the probe was still. The tests were performed at room temperature on 6 replicates for each sample. The force– time data obtained from stress relaxation tests were fitted into Maxwell model (Mohsenin, 1970; Sherman, 1970) with n = 3 represented by Eq. (9) to analyze the pistachio textural behavior under compress. Maxwell model consists of n components that each component is comprised of a dashpot (viscous) and spring (elastic) elements (Yildiz et al., 2013).
EðtÞ ¼ Ee þ
n X
Ei expðt=si Þ
337
both dried and rehydrated pistachios were lower at the high moisture level (61.33%, Fig. 1a) than those of low moisture (4.38%, Fig. 1b). For more detailed information, force–time curves were analyzed and main textural parameters (hardness, gradient and compression energy) of the dried and rehydrated samples at different moisture levels are presented in Fig. 2. The texture hardness of pistachio cuts increased with moisture decrease. This could be related to the fact that higher moisture contents will result in more swollen and softer structures as reported by Hii et al. (2012) who compared fresh and dehydrated cocoa beans and reported similar findings. Reduction of extra and intracellular spaces, canals and cavities, rapid loss of moisture content and case hardening have been referred to as the reasons of hardness increase with moisture decrease (Deng et al., 2014; Chang et al., 2011; Valencia-Pérez et al., 2008). This trend was also observed for rehydrated pistachio cuts in which the samples of higher moisture levels had lower hardness values. Cox et al. (2012) reported the texture softening of seaweed texture on rehydration at different temperatures and García-Segovia et al. (2011) have pointed out that mushroom fibers softening due to filling cavities with water is related to softer texture of rehydrated samples. The inverse relationship of texture hardness with moisture was in line with the findings of other researchers on broccoli, mushrooms and chestnuts (Sanjuán et al., 2001; Hernando et al., 2008; Moreira et al., 2008). Similar results can be drawn from compression energy and gradient values. One interesting and general phenomenon observed for textural parameters is that the studied textural parameters were greater for dried samples than those of rehydrated samples at same moisture level and all textural parameters showed similar trends (Fig. 2). Compression forces of all dried samples are greater than those of rehydrated samples with the same moisture level, for example the compression peak forces of dried and rehydrated samples at
ð9Þ
i¼1
where E(t) is the modulus decaying at time t, t is time of decay (s),
s(s) is the relaxation constant for ith Maxwell element, Ei is the decay modulus of each element, and Ee is the equilibrium or residual modulus (Cespi et al., 2007). The decay stress (Ei) is the indicator of materials elasticity (Khazaei and Mann, 2005) and Ee, the equilibrium modulus, represents the stiffness and strength of the components (Campus et al., 2010; Tang et al., 1998). 2.5. Statistical analysis The statistical analysis of the results was conducted by performing the analysis of variance to determine significant differences (P < 0.05) among the samples. Duncan test was then used for grouping of the samples using SPSS software (version 16). For stress relaxation test, the force-relaxation time data were fitted to Maxwell model by minimizing sum of squares using Solver (Excel, Microsoft Office 2007, Microsoft Ltd.). 3. Results and discussion 3.1. Textural measurements Force–time graphs of TPA test of dried and rehydrated pistachio cuts with moisture content (db) of 61.33% and 4.38% are presented in Fig. 1a and b, respectively. As expected, compression forces of
Fig. 1. Compression curves of dried and rehydrated pistachio kernels with moisture contents (db) of 61.33% (a) and 4.38% (b).
338
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341
polymers will come in closer contact leading to formation of stable hydrogen and other non-covalent interactions. Therefore during drying gradually and continuously more micro pores will become unavailable. On hydration however when water comes in contact with dried food, less available sites will become interacted with water molecules and more free water or water of high mobility will be present. Therefore on rehydration water first fills the open spaces and then the more external zones (Moreira et al., 2008). This will result in differences in physical response of dehydrated and rehydrated pistachio cuts to deformation force and manifests as texture hysteresis. Springiness and cohesiveness of dried and rehydrated pistachios at different moisture levels are presented in Table 1. Reducing moisture content from 61.33% to 4.38% decreased both springiness and cohesiveness of dried samples and rehydrated samples. The springiness and cohesiveness of high moisture pistachio cuts before drying was 0.913 and 0.804, respectively. This indicates that at high moisture pistachio texture is more elastic and its viscous component is minor, therefore texture destruction after 15% compression has been rather low and texture integrity remained high. Comparison of springiness and cohesiveness of dried and rehydrated pistachios shows reduction of these parameters in rehydrated samples as compared to their corresponding samples with same moisture level. For example at moisture level of 8.31% springiness of dried and rehydrated samples were 0.719 and 0.488 and cohesiveness was 0.452 and 0.337, respectively. This is revealing that by drying and rehydration of pistachio, not only texture hardness but also textural behavior (in terms of elastic or viscous domination), is subject to change. Capillary voids filled with moisture can be the cause of cohesiveness increase by moisture increase during rehydration (Kotwaliwale et al., 2007; Khin et al., 2007). When food biopolymers absorb water it appears that they become more elastic. The springiness of rehydrated cells can decrease due to reduction in ability of cells to recover their initial form (Khin et al., 2007). 3.2. Texture modeling
Fig. 2. The curves of textural parameters (compression energy (a), gradient (b) and hardness (c)) changes for experimental and predicted values (from exponential model) for both dried and rehydrated pistachio kernels. Bars show stdev for six replicates.
61.33% moisture level were about 30 and 6 N (Fig. 1a and b), respectively. Differences in texture hardness, compression energy or gradient between a pistachio sample dried to a certain moisture level and a sample completely dried and then rehydrated to the same moisture level is appearing to be consistent and may be called ‘‘texture hysteresis’’. From Fig. 2 it might be concluded that texture hysteresis of pistachio is most observed in the moisture range of 15–30% where texture differences between the dried and rehydrated samples are greater. Texture hysteresis is referring to the fact that when a food is dried and then rehydrated to the initial moisture level, then texture difference will exist between the fresh and the rehydrated samples of same moisture. This is similar to moisture hysteresis when desorption and adsorption curves of food materials are drawn on isotherm graphs in which if the lines do not overlap, then moisture hysteresis is confirmed (Ansari et al., 2011). Generally, during drying water is removed from foods and as a result food components including polysaccharides and protein
Hardness, gradient and compression energy data were fitted into linear, quadratic and exponential models to quantify texture changes and differences between dried and rehydrated samples. R2, RMSE, SSE, v2 and E% were calculated from the experimental and predicted data to choose the best model with higher value of R2 and lower values for other parameters (Table 2). Values of R2 ranged from 0.14 to 0.995 and confirmed that the exponential model was the best for modeling pistachio texture data with the highest R2 and the lowest values for RMSE, SSE, v2 and E%. Fig. 2a –c shows the predicted (lines) and experimental data Table 1 Springiness and cohesiveness of dried and rehydrated pistachio kernels at different moisture levels. Moisture content (%)
0 4.38 8.31 15.14 21.53 30.31 35.22 40.59 61.33
Springiness
Cohesiveness
Dried pistachio
Rehydrated pistachio
Dried pistachio
Rehydrated pistachio
0.402a ± 0.07 0.599a ± 0.03 0.719a ± 0.06 0.746a ± 0.11 0.778a ± 0.06 0.805a ± 0.06 0.866a ± 0.04 0.861a ± 0.05 0.913a ± 0.01
0.402a ± 0.07 0.423b ± 0.03 0.488b ± 0.04 0.519b ± 0.01 0.642a ± 0.02 0.625b ± 0.06 0.728b ± 0.04 0.759a ± 0.07 0.804b ± 0.02
0.231a ± 0.02 0.376a ± 0.01 0.452a ± 0.06 0.648a ± 0.02 0.642a ± 0.01 0.695a ± 0.03 0.683a ± 0.08 0.712a ± 0.00 0.758a ± 0.02
0.231a ± 0.02 0.255b ± 0.01 0.337b ± 0.02 0.324b ± 0.01 0.392b ± 0.05 0.474b ± 0.02 0.531b ± 0.07 0.601b ± 0.06 0.640b ± 0.01
Different letters show significant differences between the dehydrated and rehydrated samples at same moisture content for each parameter (a < 0.05).
339
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341 Table 2 Statistical parameters of linear, quadratic and exponential models of textural parameters of dried and rehydrated pistachio kernels. Model
Parameter
Linear
R2* RMSE* SSE*
Dried pistachio Compression energy
Gradient
Hardness
Compression energy
Gradient
Hardness
0.901 0.208 0.043 0.065 0.075
0.824 0.545 0.293 0.440 0.091
0.953 0.172 0.030 0.045 0.035
0.624 1.594 2.550 3.825 1.774
0.703 0.986 0.968 1.452 0.752
0.721 0.725 0.523 0.784 0.443
0.140 2.653 7.062 10.594 1.345
0.502 0.972 0.951 1.427 0.089
0.186 1.414 2.004 3.007 1.083
0.443 1.069 1.143 1.714 0.144
0.317 1.387 1.905 2.858 0.513
0.431 1.125 1.257 1.886 0.475
0.927 0.261 0.069 0.103 0.197
0.878 1.172 1.375 2.063 0.259
0.936 0.240 0.060 0.090 0.084
0.995 0.091 0.008 0.012 0.037
0.891 0.674 0.462 0.693 0.212
0.991 0.165 0.027 0.041 0.076
v2* E%* Quadratic
R2 RMSE SSE
v2 E% Exponential
Rehydrated pistachio
R2 RMSE SSE
v2 E%
R2, is coefficient of determination and the higher the better (R2 = 1 is the highest). The standard error of estimate (SEE) and root mean square error (RMSE) were calculated to give indications of the precision of the estimation of the experiment, the lower these values the better. X2 (chi-square) is an indication of sum of errors taking into account the number observations and model constants and the lower the better. The mean relative percentage deviation modulus (E%), with a modulus value below 10% is indicative of a good fit for practical purposes. *
Table 3 Constant rate (a) and residual texture (N) (b) of textural parameters in exponential model of dried and rehydrated pistachio kernels. Modeling parameters
a b
Compression energy
Gradient
Hardness
Dried pistachio
Rehydrated pistachio
Dried pistachio
Rehydrated pistachio
Dried pistachio
Rehydrated pistachio
46.34a ± 2.24 6.14a ± 0.35
9.24b ± 0.26 0.91b ± 0.04
66.50a ± 5.62 7.33a ± 0.89
17.57b ± 0.97 2.29b ± 0.37
81.36a ± 8.39 10.06a ± 1.18
10.93b ± 1.24 1.89b ± 0.60
Different letters show significant differences between the samples in terms of each parameter in each row (a < 0.05).
points. Constant parameters of exponential model, a as the texture changes rate constant and b (residual texture) were analyzed statistically and presented in Table 3. The constants rate of changes, a, obtained from hardness, gradient and compression energy were greater for dried samples as compared with rehydrated ones. This confirms the superior texture changes in rehydrated samples over the moisture range, for example a parameters from hardness values were 81.36 and 10.93 for dried and rehydrated pistachio cuts, respectively. Also residual texture parameter (b) of rehydrated samples (10.06 N) was significantly different from that of dried samples (1.89 N) as obtained for hardness data. 3.3. Stress relaxation test Stress relaxation graphs and modeling parameters are presented in Fig. 3 and Table 4. Ee, as an indication of residual modulus Table 4 Parameters of Maxwell model for dried and rehydrated pistachio kernels at different moisture levels. Moisture content (%)
Dried pistachio
Ee (N)
Fig. 3. Stress relaxation tests of dried and rehydrated pistachio kernels with moisture contents (db) of 61.33% (a) and 0% (b). Dots are experimental results and lines are modeled (Maxwell, n = 3) data.
0 4.38 8.31 15.14 21.53 30.31 35.22 40.59 61.33
a
23.08 21.00a 20.90a 15.42a 16.80a 12.02a 12.69a 5.40a 4.48a
Rehydrated pistachio
R2
s1
Ei a
1.90 2.39a 2.37a 1.63a 1.44a 1.24a 1.15a 0.86a 0.80a
a
30.23 25.25a 23.56a 21.38a 18.80a 17.40a 12.07a 12.41a 10.00a
0.94 0.95 0.88 0.87 0.91 0.93 0.94 0.94 0.92
Ee 23.08 6.39b 6.95b 5.24b 4.33b 2.05b 2.34b 1.92b 1.49b
R2
s2
Ei a
a
1.90 1.56b 1.77b 1.05b 0.74b 0.52b 0.34b 0.41b 0.23b
a
30.23 23.51a 22.82a 18.71b 16.15a 17.22a 14.52a 13.13a 7.04b
0.94 0.94 0.91 0.92 0.87 0.97 0.87 0.86 0.93
Different letters show significant differences between the dried and rehydrated samples in terms of each parameter in each row (a < 0.05).
340
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341
or texture at the end of stress relaxation, indicates that significant texture differences were found between dried and rehydrated samples for all moisture levels. For example residual texture was 21.00 and 6.39 N for dried and rehydrated pistachios at 4.38% moisture, i.e. the rehydrated samples had a weaker texture than the dried samples. Ei as the decay modulus of each element was greater for all dried samples than rehydrated samples with same moisture. The relaxation constant (s) showed significant differences at 61.33% moisture. Ei and Ee values confirmed that the tested materials had a solid-like structure (Bhattacharya, 2010).
4. Conclusion Double compression and stress relaxation tests were applied to dried and rehydrated samples to determine the mechanical behavior of samples and the results were modeled into linear, quadratic and logarithmic models and logarithmic was found as the best model. Hardness, gradient and compression energy of all pistachio samples increased with moisture reduction but significant differences between textural parameters for two processes were observed and the lower values for rehydrated samples were recorded at different moisture levels. This indicates that under a constant stress rehydrated pistachio kernels deform to a greater extent as compared to their corresponding dry samples at similar moisture levels. The extent of texture hysteresis of pistachio varied at different moisture levels and was most observed in the moisture range of 15–30%, therefore water mobility can be involved. It is evident from the results that thermal history of pistachio plays a key role in determining its textural and mechanical properties. Having pistachio kernels of same moisture content does not mean that textural properties will be the same even when other parameters such as variety are kept the same. In food formulations the delicate texture of pistachio kernel perceived by the consumers will depend not only on its moisture level but also on its thermal history during drying, storage and processing. Therefore, dried pistachio kernels will behave differently in high moisture food formulations as compared to fresh kernels. The outcome of this research is clearly showing that significant texture deteriorations or changes can occur when fruits are first dried and then rehydrated to the same moisture level and to gain structural characteristics similar to fresh fruits only moisture adjustment is not enough. Further research is suggested to evaluate the effect of thermal history and dehydration, rehydration processes and storage on textural properties of other food materials.
References Al-Moghazy, M., Boveri, S., Pulvirenti, A., 2014. Microbiological safety in pistachios and pistachio containing products. Food Control 36, 88–93. AOAC, 2000. Association of Official Analytical Chemists, Official Methods of Analysis. AOAC, Washington DC. Ansari, S., Farahnaky, A., Majzoobi, M., 2011. Modeling the effect of glucose syrup on the moisture sorption isotherm of figs. Food Biophys. 6, 377–389. Bhattacharya, S., 2010. Stress relaxation behaviour of moth bean flour dough: product characteristics and suitability of model. J. Food Eng. 97, 539–546. Bourne, M.C., 2002. Food Texture and Viscosity,, .. Concept and Measurement, second ed. Academic Press, New York. Brooke, D.B., Bakker, F.W., Hall, C.W., 1997. Drying and Storage of Grains and Oilseeds. AVI Publishing, Van Nostrand Reinhold, New York, USA. Brufau, G., Boatella, J., Rafecas, M., 2006. Nuts: source of energy and macronutrients. Br. J. Nutr. 96, S24–S28. Campus, M., Addis, M.F., Cappuccinelli, R., Porcu, M.C., Pretti, L., Tedde, V., Secchi, N., Stara, G., Roggio, T., 2010. Stress relaxation behaviour and structural changes of muscle tissues from Gilthead Sea Bream (Sparus aurata L.) following high pressure treatment. J. Food Eng. 96, 192–198. Cespi, M., Bonacucina, G., Misici-Falzi, M., Golzi, R., Boltri, L., Palmieri, G.F., 2007. Stress relaxation test for the characterization of the viscoelasticity of pellets. Eur. J. Pharm. Biopharm. 67, 476–484. Chang, H.J., Xu, X.L., Li, C.B., Huang, M., Liu, D.Y., Zhou, G.H., 2011. A comparison of heat-induced changes of intramuscular connective tissue and collagen of beef
semitendinosus muscle during water bath and microwave heating. J. Food Process Eng 34, 2233–2250. Cox, S., Gupta, S., Abu-Ghannam, N., 2012. Effect of different rehydration temperatures on the moisture, content of phenolic compounds, antioxidant capacity and textural properties of edible Irish brown seaweed. LWT – Food Sci. Technol. 47, 300–307. Deng, Y., Wu, J., Su, S.Q., Liu, Z.D., Ren, L., Zhang, Y.L., 2011. Effect of far-infra red assisted heat pump drying on water status and moisture sorption isotherm of squid (Illex illecebrosus) fillets. Drying Technol. 29, 1580–1586. Deng, Y., Luo, Y., Wanga, Y., Yue, J., Liu, Z., Zhong, Y., Zhao, Y., Yang, H., 2014. Drying-induced protein and microstructure damages of squid fillets affected moisture distribution and rehydration ability during rehydration. J. Food Eng. 123, 23–31. Erbay, Z., Icier, F., 2009. Optimization of hot air drying of olive leaves using response surface methodology. J. Food Eng. 91 (4), 533–541. FAOSTAT, 2008. FAOSTAT Database Results, 2005.
A. Farahnaky, E. Kamali / Journal of Food Engineering 166 (2015) 335–341 Razavi, S.M.A., Taghizadeh, M., 2007. The specific heat of pistachio nuts as affected by moisture content, temperature, and variety. J. Food Eng. 79, 158–167. Ryan, E., Galvin, K., O’Connor, T.P., Maguire, R., O’Brien, N.M., 2006. Fatty acid profile, tocopherol, squalene and phytosterol content of Brazil, pecan, pine, pistachio and cashew nuts. Int. J. Food Sci. Nutr. 57, 219–228. Sacilik, K., Elicin, A.K., 2006. The thin layer drying characteristics of organic apple slices. J. Food Eng. 73 (3), 281–289. Sanjuan, N., Simal, S., Bon, J., Mulet, A., 1999. Modelling of broccoli stems rehydration process. J. Food Eng. 42 (1), 27–31. Sanjuán, N., Cárcel, J.A., Clemente, G., Mulet, A., 2001. Modelling of the rehydration process of broccoli florets. Eur. Food Res. Technol. 212, 449–453. Sherman, P., 1970. Industrial Rheology. Academic Press, New York.
341
Tang, J., Tung, M., Zeng, A.Y., 1998. Characterization of gellan gels using stress relaxation. J. Food Eng. 38, 279–295. Valencia-Pérez, A.Z., Garcia-Morales, M.H., Cardenas-Lopez, J.L., Herrera-Urbina, J.R., Rouzaud-Sandez, O., Ezquerra-Brauer, J.M., 2008. Effect of thermal process on connective tissue from jumbo squid (Dosidicus gigas) mantle. Food Chem. 107, 1371–1378. Venkatachalam, M., Sathe, S.S., 2006. Chemical composition of selected edible nut seeds. J. Agric. Food Chem. 54, 4705–4714. Yildiz, Ö., Yurt, B., Basßtürk, A., Toker, Ö.S., Yilmaz, M.T., Karaman, S., Dag˘lıog˘lu, O., 2013. Pasting properties, texture profile and stress–relaxation behavior of wheat starch/dietary fiber systems. Food Res. Int. 53, 278–290.