J Food Sci Technol DOI 10.1007/s13197-015-1849-5
ORIGINAL ARTICLE
Effect of drying methods on rheological and textural properties, and color changes of wild sage seed gum Fakhreddin Salehi 1 & Mahdi Kashaninejad 1
Revised: 9 March 2015 / Accepted: 21 April 2015 # Association of Food Scientists & Technologists (India) 2015
Abstract The effect of different drying methods, oven drying (40–80 °C), freeze drying (−40 °C) and vacuum oven drying (100 mbar, 50 °C) on the rheological and textural properties, and color changes of wild sage seed gum were investigated in this study. The results showed that the apparent viscosity of gum solution (0.6 % w/w), was varied from 0.162 to 0.344 Pa×s (60 s−1), and freeze-dried gum exhibited the highest viscosity value among all dried gums. Standard error (SE) and correlation coefficient (R) methods were selected for carrying out the error analysis due to the best fit model and the Heschel-Bulkley’s model was found the suitable to describe the flow behavior of gum solution over the whole experimental range. The amounts of hardness, stickiness, consistency and adhesiveness of wild sage seed gum gel (3 % w/w) were changed from 45.7 to 78.2 g, 9.8 to 17.0 g, 340.4 to 794.8 g.s, and 91.4 to 159.2 g.s at different drying condition. The rise in temperature had a negative effect on the color changes (high ΔE) and brightness (low L*) of gum solution. Keywords Polysaccharide . Rheology . Texture . Wild sage seed gum
Introduction Many mucilaginous seeds around the world have been introduced to be used as accessible, cost effective and natural sources for producing food hydrocolloids. These * Fakhreddin Salehi
[email protected] 1
Faculty of Food Science & Technology, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran
hydrocolloids are water-soluble ingredients used as thickener, gel former, foam, improve the rheological properties, extend shelf life, encapsulate flavors and emulsion stabilizing agents in a wide range of foods and pharmaceuticals (Mirhosseini and Amid 2012; Salehi et al. 2014; Salehi and Kashaninejad 2014a). The genus Salvia (Labiatae) contains more than 700 species of mucilaginous endemic plant. Wild sage (Salvia macrosiphon) seeds are round, small, with a mucilage layer which could swell in water, giving viscous suspension properties which are comparable with commercial food hydrocolloids (Salehi et al. 2014). Some potential of the wild sage seed gum as a new source of hydrocolloid have been recently investigated by Razmkhah et al. 2010. Farahnaky et al. (2013) studied the effect of various salts, NaCl (0.5–3 %), CaCl2 (0.5–3 %) and Na2HPO4 (0.2–0.6 %), and pH condition (3– 9) on rheological properties of Salvia macrosiphon hydrocolloid solutions. They reported that all variables had significant impacts on rheological parameters. The rheological characteristics play a significant role in the process design (fluid flow, pump sizing, extraction, filtration, extrusion and purification, pasteurization, evaporation and drying process) (Marcotte et al. 2001). The viscosity of polysaccharide gums is largely influenced by the particle size and distribution, molecular weight and ratio of soluble to insoluble matters (Larrauri, et al. 1994). The textural and rheological properties of plant gums depend on the method and condition of extraction, purification and drying (Amid and Mirhosseini 2012). The drying process can provide a broad range of molecular weight depending on the type and condition of drying, thus varying the viscosity (Nep and Conway 2011). Amid and Mirhosseini (2012) reported that difference among the viscosity of all durian seed gums, dried at different drying methods (oven, freeze drying, spray drying and vacuum oven drying), could be due to the difference between their molecular
J Food Sci Technol
weights. They found that all dried durian seed gums showed pseudoplastic (shear-thinning) flow behavior. Nep and Conway (2011) also demonstrated that the grewia gum showed different degree of viscosity varying from 0.20 to 0.32 Pa×s depending on the drying method. They reported that air-dried grewia gum showed higher viscosity than spraydried and freeze-dried grewia gum. In addition, the color of the dried product is an important quality factor, which is affected by the drying conditions. The most commonly used drying techniques applied for plant gums included oven drying (Wang et al. 2010; Salehi and Kashaninejad 2014b), spouted bed dryers (Cunha et al. 2000), microwave vacuum drying (Sundaram and Durance 2008), freeze drying (Barresi et al. 2009; Moreira 2009), vacuum drying (Wang et al. 2009) and spray drying (Nep and Conway 2011; Amid and Mirhosseini 2012). There is no study available in the literature that investigating the effect of different drying techniques and temperature on the rheological and textural properties of wild sage seed gum. Therefore, the purpose of present study was to investigate the influence of different drying techniques (oven drying (40-80 °C), freeze drying and vacuum oven drying) on the rheological and textural properties, and color changes of wild sage seed gum.
The freeze drying was carried out by using a freeze dryer (Operon freeze-dryer, Operon Co Ltd, Korea). The cups of wild sage seed gum dispersions were transferred into freeze dryer chamber and frozen at −40 °C for 48 h. The extracted wild sage seed gum was dried in a vacuum oven dryer for 48 h (Vacuum oven VO, Memmert Universal, Schwabach, Germany). The vacuum and temperature were maintained at 100 mbar and 50 °C, respectively. The dried sample was milled and passed through a 35 mesh sieve. Then, the milled powder was weighed and stored in the air-tight bottle prior to the investigation.
Sample preparation for texture, viscometer and image analysis Hydrocolloid solutions were prepared by dispersing the hydrocolloid powder in distilled water by a stirrer, and two levels of concentration were employed: 0.6 % w/w for evaluation of shear rate dependency and image processing, and 3 % w/w for investigation of the textural properties of wild sage seed gums. The prepared solutions were kept for 24 h at 25 °C to complete hydration of the gum.
Analytical tests
Materials and methods Extraction of gum The cleaned wild sage seeds were soaked in distilled water (for 20 min, pH=7, 25 °C and water/seed ratio 20:1 w/w). Separation of the hydrocolloid from the swollen seeds was achieved by passing the seeds through an extractor (Panasonic, MJ-J176P, Japan) equipped with a rotating plate that scraped the gum layer on the seed surface. The extracted mucilage was then filtered and collected (Salehi et al. 2014; Salehi and Kashaninejad 2014a). The extracted mucilage with concentration of 0.6 % w/w was considered as the control sample (CS).
Apparent viscosity The viscosity of crude and dried wild sage seed gum was measured using a rotational viscosimeter (Brookfield, model RVDV- II+ pro, USA). The rheological parameters of wild sage seed gum at different logarithmic shear rate of 0.6 to 120 s−1 were studied using spindle No.S02 at 18 rotations and 25 °C. Different viscous flow models (Power law, Bingham, Herschel-Bulkley, Casson and Vocadlo) were used to fit the experimental shear stress-shear rate data of wild sage seed gum solution (Steffe 1996; Rao 1999; Salehi et al. 2014; Salehi and Kashaninejad 2014b). A Power law model
Drying methods In the current study, the effect of three different drying techniques on the textural and rheological properties, and color changes of wild sage seed gum was investigated. Three drying techniques including freeze drying, vacuum oven drying and oven drying (40-80 °C) were selected. The dispersions of wild sage seed gum (50 g; 10 mm depth) were dried in an air forced oven (convection oven, Memmert Universal, Schwabach, Germany) at different temperature including 40, 50, 60, 70 and 80 °C for 48 h.
τ ¼ k pγ˙ np
ð1Þ
Where τ is shear stress (Pa), kP is the power law consistency coefficient (Pa×sn), γ˙ is the shear rate (s−1) and nP is the power law flow behavior index (dimensionless). B. Bingham’s model τ ¼ τ 0B þ ηBγ˙
ð2Þ
Where ηB is called the Bingham plastic viscosity (Pa×s) and τ0B is the Bingham yield stress (Pa).
J Food Sci Technol
τ ¼ τ 0H þ k Hγ˙ nH
ð3Þ
Where kH is the consistency coefficient (Pa×sn), τ0H is the yield stress (Pa) and nH is the flow behavior index for Herschel-Bulkley model. D. Casson’s model 0:5 τ 0:5 ¼ τ 0:5 0C þ ηCγ˙
ð4Þ
Where ηC is the plastic viscosity (Pa×s) and τ0c is the yield stress (Pa) for Casson model. Modeling of data was using Curve Expert program version 1.34. Textural analysis The textural analysis of the gum sample was performed using a texture analyzer (TA-XT Plus, Stable Micro Systems Ltd., UK) with a 25 mm diameter cylindrical probe and a test speed of 1.0 mm s−1. The gums were filled in a bowl with a product height of 50 mm (d=50 mm, h=60 mm). Penetration test was performed up to 15 mm deformation. Other parameters were defined as: pre-test speed 1.0 mm s −1 , post-test speed 1.0 mm s−1 and trigger force 5 g. All experiments were performed at 25 °C. The texture profile measurements were used to analyze the parameters hardness, stickiness, consistency and adhesiveness. The results were processed with Texture Expert 1.05 software (Stable Microsystems). For data analyzing a typical time-force diagram was used. The maximum force was taken as an indication of gel hardness, which measures the strength of gel. Stickiness (g force) is defined as the maximum force necessary to overcome the attractive forces between the surface of the food and the surface of the probe with which the food comes in contact. The area under the curve up to the target deformation was taken as a measurement of consistency (g.s) and adhesiveness (g.s) is defined as the energy required overcoming attractive forces between the food and any surface it is in contact with (Salehi and Kashaninejad 2014b). Color measurement In this research, the gums samples were filled in a bowl with a product height of 50 mm (d=50 mm, h=60 mm) for color measurement. Sample illumination was achieved with two fluorescent lights (10 W, 40 cm in length), were placed in a wooden box (0.5×0.5×0.5 m3). The interior walls of the box were painted black to minimize background light. A color digital camera was located vertically at a distance of 20 cm from the sample (Panasonic, Model DMC-FS42, Japan). Since the computer vision system perceived color as RGB signals, which is device-dependent, the images taken were converted into L * (lightness/darkness),
a*(redness/greenness) and b* (yellowness/blueness) units to ensure color reproducibility. In the L*a*b* space, the color perception is uniform, and therefore, the difference between two colors corresponds approximately to the color difference perceived by the human eye (Salehi and Kashaninejad 2014b). The hue angle (°), hue=arctg (b*/a*), expresses the colour nuance and values are defined as follows: red-purple: 0°, yellow: 90°, bluish-green: 180°, and blue: 270° (Goncalves et al. 2007). Hue angle (H) of the films was calculated as follows (Bhat et al. 2013): . H ¼ t a n−1 b* a* when a* > 0 and b* > 0 . H ¼ 180 þ t a n−1 b* a* when a* < 0 . H ¼ 360 þ tan−1 b* a* when a* > 0 and b* < 0 The calculation of color changes (ΔE) for total color difference and C* (Chroma) for saturation were made with the following equations (Salehi et al. 2014): ΔE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi * 2 2 ΔL þ ðΔa* Þ2 þ Δb*
ð5Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 C ¼ ð a* Þ þ b*
ð6Þ
*
In this study, the image analysis of dried gum was performed using Image J software version 1.42e, USA. Standard error (SE) (Eq. 7) and correlation coefficient (R) (Eq. 8) were selected on the basis to carry out the error analysis due to the best fit model. . X . 1 2 SE ¼ ðY m −Y c Þ2 n−1 ð7Þ
10
Apparent viscosity (Pa×s)
C. Herschel-Bulkley’s model
CS
OD-40°C
OD-50°C
OD-60°C
OD-70°C
OD-80°C
FD
VO
1
0.1 0
20
40
60
80
100
120
Shear rate (1/s)
Fig. 1 Effect of different drying methods on apparent viscosity of wild sage seed gum solution as a function of shear rate for control sample (CS), oven drying (OD), freeze drying (FD), and vacuum oven drying (VO)
J Food Sci Technol
Apparent viscosity (Pa×s)
0.35
and Conway 2011). Shear rate dependency of the apparent viscosity of wild sage seed gum solutions at different drying process is shown in Fig. 1. It was found that the apparent viscosity of wild sage seed gum decreased as the shear rate increased (shear thinning or pseudoplastic behavior). The apparent viscosity clearly decreased from 4.2 to 0.102 Pa×s with increasing shear rate from 0.6 to 120 s−1 (80 °C). This value was higher than the viscosity reported for durian seed gum (0.065 Pa×s, 1 % and shear rate=1000 s−1) and P. flexuosa seed (1.9 Pa×s, 0.4 % and shear rate=64 s−1) solutions (Amin et al. 2007; Ibanez and Ferrero 2003). Wang et al. (2009) also reported the similar pseudoplastic (or shear-thinning) flow behaviour for freeze-dried, oven dried, spray-dried and vacuum oven-dried flaxseed gums, respectively. Previous researchers also found that freeze-dried peach tree gum exhibited the pseudoplastic (or shear-thinning) flow behaviour at different concentrations (Simas-Tosin et al. 2010). As reported by Cui (2005), the viscosity of gum dispersions decreased with increasing the shear rate due to a decreasing number of chain entanglements at high shear rates. In general, the drying process led to decrease the viscosity of wild sage seed gum (Fig. 2). The freeze dried gum exhibited the highest viscosity among all dried gums. In the present study, the apparent viscosity of wild sage seed gum varied from 0.344 to 0.162 Pa×s (shear rate=60 s−1) depending on the drying method. The apparent viscosity decreased from 0.271 to 0.162 Pa×s with increasing temperature from 40 to 80 °C (shear rate=60 s−1, oven drying method). Wang et al. (2009) compared the viscosity of flaxseed gum dried by different techniques. They reported that the viscosity of flaxseed gum varied from 1.382 to 5.087 Pa×s. The significant changes of apparent viscosity could be due to the substantial impact of drying process on the chemical composition of wild sage seed gum. As also explained by Simas-Tosin et al. (2010), the presence of free, reducing oligosaccharides, phenolics and inorganic salts, beyond polysaccharide in the gum structure gives a more viscous solution. The effect of different drying methods on the viscosity of polysaccharide gum could be due
0.3 0.25 0.2 0.15 0.1 0.05 0
Fig. 2 Effect of different drying techniques on apparent viscosity of wild sage seed gum (shear rate=60 s−1); control sample (CS), oven drying (OD), freeze drying (FD), and vacuum oven drying (VO)
Where Υm is the measured value, Υc is calculated value for each data point, and n is the number of observations (Betts and Walker 2004). The correlation coefficient (R), Eq.8, gives information on the best fit model, having a value between (−1, 1) (Salehi and Razavi, 2010) vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u Xn u ½Y m −Y c 2 u R ¼ t1− X ni¼1 ½Y −T m 2 i¼1 m
ð8Þ
In Eq.8, Tm is mean of Υm and calculated by Eq.9: Xn Y i¼1 m Tm ¼ n
ð9Þ
Results and discussion Apparent viscosity The functional properties (viscosity and rheological properties) of gums are greatly sensitive to the drying process (Nep Table 1 The Power law and Bingham’s model models parameters for wild sage seed gum at different drying method
Drying method
CS OD-40 OD-50 OD-60 OD-70 OD-80 FD VO
°C °C °C °C °C
Power law model
Bingham’s model
kp (Pa×sn)
np
R
SE
τ0B (Pa)
ηB (Pa×s)
SE
R
2.62 2.92 3.03 2.58 2.68 1.72 3.17 2.41
0.33 0.24 0.22 0.24 0.19 0.25 0.27 0.32
0.998 0.985 0.967 0.959 0.968 0.990 0.985 0.997
0.18 0.32 0.46 0.53 0.31 0.18 0.42 0.16
4.35 4.59 4.67 4.16 3.87 2.76 4.94 4.03
0.09 0.04 0.04 0.04 0.03 0.03 0.07 0.08
0.963 0.874 0.837 0.800 0.82 0.896 0.874 0.934
0.76 0.94 1.01 1.14 0.71 0.56 1.19 0.89
* Control sample (CS), Oven drying (OD, 40–80 °C), freeze drying (FD), vacuum oven (VO)
J Food Sci Technol Table 2 The Heschel-Bulkley and Casson’s models parameters for wild sage seed gum at different drying method
Drying method
Heschel-Bulkley model τ0H (Pa)
CS OD-40 OD-50 OD-60 OD-70 OD-80 FD VO
°C °C °C °C °C
kH (Pa×sn)
Casson’s model nH
R
τ0C (Pa)
SE
ηC (Pa×s)
R
SE
1.29
1.64
0.42
0.999
0.10
2.96
0.19
0.991
0.37
0.19 0.85 1.40 0.18 0.32 0.36 0.89
2.46 1.87 1.11 2.6 1.81 3.08 3.15
0.28 0.33 0.43 0.21 0.15 0.30 0.27
0.998 0.987 0.998 0.988 0.992 0.999 0.998
0.13 0.31 0.08 0.20 0.16 0.07 0.15
3.50 3.62 3.14 3.12 2.07 3.64 2.79
0.11 0.10 0.11 0.08 0.09 0.15 0.17
0.939 0.912 0.886 0.903 0.952 0.939 0.975
0.66 0.76 0.88 0.53 0.39 0.84 0.55
* Control sample (CS), Oven drying (OD, 40–80 °C), freeze drying (FD), vacuum oven (VO)
to different proportions of soluble to insoluble matters. The drying methods significantly (p0) and the rise in temperature has a negative effect on the ΔE of gum. As shown in table 4, the ΔE values were varied from 0.63 to 15.57 at different drying temperatures and methods. It should also be noted that freeze dried gum had low ΔE values compared to the other samples (ΔE =0.63). The color of oven dried gum was darker (low L* value) compared to the freeze and vacuum oven dried samples. Table 4 reveals that increasing oven temperature from 40 to 80 °C caused a decrease in L* values from 42.1 to 31.6. Therefore, temperature control in oven drying allowed improvement of gum color. The chroma is measure of chromaticity (C*), which denotes the purity or saturation of the colour (Salehi and Kashaninejad 2014b). Chroma parameter values varied from 3.97 to 15.19 at different drying temperatures and methods. High temperature during drying of mint leaves lead to increase ΔE values (Therdthai and Zhou 2009). The L* values decreased by increasing drying temperature, which shows diminishing of brightness of gum solution.
Drying method
ΔE*
L*
a*
b*
Hue value (°)
C*
CS OD-40 OD-50 OD-60 OD-70 OD-80 FD VO
5.44 8.59 8.81 7.05 15.57 0.63 6.56
46.87 (±3.03) 42.10 (±4.18) 40.86 (±4.12) 40.46 (±4.98) 39.82 (±4.96) 31.63 (±4.88) 46.26 (±3.14) 43.08 (±4.70)
−3.098 (±1.51) −4.588 (±1.47) −2.263 (±1.54) −1.108 (±1.54) −3.17 (±1.68) −3.419 (±2.03) −3.139 (±1.55) −3.13 (±1.59)
9.51 (±3.76) 7.37 (±3.17) 3.43 (±3.40) 3.81 (±4.14) 9.57 (±4.27) 6.37 (±4.74) 9.68 (±3.94) 14.87 (±4.28)
108.04 121.84 123.34 106.21 108.32 118.26 107.99 101.89
10.00 8.68 4.11 3.97 10.08 7.23 10.18 15.19
°C °C °C °C °C
* Control sample (CS), Oven drying (OD, 40–80 °C), freeze drying (FD), vacuum oven (VO); L: brightness (+ 100)/darkness (+0), a: redness (+120)/greenness (−120) coordinate, b: yellowness (+120)/blueness (−120) coordinate, C: chroma
J Food Sci Technol
Generally, hue angle is measured to represent visual attributes. From the results (Table 4), wild sage seed gum gels were in first quadrant of hue angle (90–180°) and were in the range of yellow hue to bluish-green hue. This hue value of gum gels is an indication that gum is desirable to be used for food packaging purposes, especially for light sensitive packaged materials (Bhat et al. 2013). Based on the results it can be concluded that freeze drying process can achieve better product quality.
Conclusions The aim of the present study was to investigate the effects of different drying methods on the rheological and textural properties, and color changes of wild sage seed gum. Three drying techniques including oven drying (40–80 °C), freeze drying (−40 °C) and vacuum oven drying (100 mbar, 50 °C) were used. The results indicated that all dried wild sage seed gums showed the pseudoplastic flow behavior (shear thinning fluid). The results revealed that the freeze-dried gum exhibited the highest viscosity among all dried wild sage seed gums. The best model is determined by analyzing standard errors and correlation coefficient values which determined from equations. It can be seen that the Heschel-Bulkley’s model was found the most suitable model to describe the flow behavior of wild sage seed gum solutions over the whole experimental range. The oven drying is a low cost drying technique as compared to freeze drying or vacuum drying. The present study reveals that the oven dried sample has good rheological properties. The amounts of hardness, stickiness, consistency and adhesiveness of wild sage seed gum gel (3 %) changed from 45.7 to 78.2 g, 9.8 to 17.0 g, 340.4 to 794.8 g.s, and 91.4 to 159.2 g.s at different drying condition. The results indicated that the freeze-dried gum exhibited the highest hardness, stickiness, consistency and adhesiveness. Recently, image analysis has been used as a promising approach to the objective assessment of a dried product’s quality. The present study also demonstrated that the color of wild sage seed gum was influenced by the drying process. The extracted mucilage samples were dried in oven at 80 °C subjected to the greatest color changes. The color of oven dried gum was darker (low L* value) compared to the freeze and vacuum oven dried samples.
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