bs_bs_banner

Journal of Food Process Engineering ISSN 1745-4530

RHEOLOGICAL PROPERTIES OF ALOE VERA (ALOE BARBADENSIS MILLER) JUICE CONCENTRATES NISHANT R. SWAMI HULLE, KIRAN PATRUNI and P. SRINIVASA RAO1 Agricultural and Food Engineering Department, Indian Institute of Technology, Kharagpur, West Bengal 721302, India

1

Corresponding author. TEL: 91-3222-283164; FAX: 91-3222-282244; EMAIL: [email protected] Received for Publication January 17, 2014 Accepted for Publication March 24, 2014 doi:10.1111/jfpe.12093

ABSTRACT Flow characteristics of aloe vera juice concentrates (1.5–5.5°Brix) were determined over a range of temperatures (15–55C) by using coaxial cylinder geometry. Aloe vera juice exhibited non-Newtonian fluid behavior which was well described by power law model. Functional groups were characterized by Fourier-transform infrared (FTIR) spectroscopy indicating the presence of O-acetyl and shifting of –CH bonds. Effect of test temperatures at different shear rates on viscosity was evaluated using Arrhenius-type equation and the activation energy values were evaluated which ranged between 12.148 and 53.073 kJ/mol. Effect of concentration was evaluated using power type and exponential type equations. An expression for combined effect of temperature and concentration was developed. Small amplitude oscillation sweep measurement was also carried out and the frequency sweep test exhibited viscoelastic nature of juice.

PRACTICAL APPLICATIONS Aloe vera is commercially very important crop and is being used in food, medicines and cosmetics. Rheological properties are important parameters and are used to assess the functional and structural properties of food products. In this study, the flow and deformation characteristics of aloe vera juice concentrates were evaluated as a function of temperature and concentration. An expression for the combined effect of concentration and temperature on the viscosity was evaluated. Oscillatory sweep experiments can be used to assess the viscoelastic properties, which are useful to develop the products of desired functionalities. The changes in functional groups due to concentration can be assessed using FTIR spectroscopy analysis.

INTRODUCTION Aloe vera is a tropical or subtropical plant belonging to Liliaceae family. It is characterized by lance-shaped leaves arranged in a rosette pattern with jagged edges and sharp points. It is being used worldwide since ancient times for its medicinal and therapeutic properties (Capasso et al. 1998). There are more than 360 known species of aloe vera, of which Aloe barbadensis Miller is widely used for manufacturing cosmetics, functional foods and drugs (Eshun and He 2004; Rodriguez et al. 2010; Ahlawat and Khatkar 2011). Two important major components of aloe vera leaf include innermost portions of parenchyma cells containing aloe vera gel and yellow latex containing anthraquinones. The raw juice of aloe vera contains more than 98% moisture and Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.

the remaining portion is composed of polysaccharides (more than 60% w/w), phenolic compounds, organic acids, enzymes, vitamins and minerals (Tanaka et al. 2012). The polysaccharides containing glucomannans, mannans and pectins of different molecular weights of aloe vera are responsible for its biological activities in vivo, as well as in vitro (Yaron 1993; Chow et al. 2005). It is also reported that polysaccharides of aloe vera with specific molecular weights have different applications (Agarwala 1997). Utilization of aloe vera gel in food product formulations is increased in recent years in products such as health drinks, beverages and yoghurt due to its bioactive components (He et al. 2005). The physicochemical and structural elucidation of these bioactive components has been reviewed by Ni et al. (2004). In view of its highly perishable nature, the 1

RHEOLOGICAL PROPERTIES OF ALOE VERA CONCENTRATES

importance is now being given to increase shelf stability of aloe vera gel and to retain its functional properties using appropriate processing technology. Rheological properties are necessary to understand the structural and functional properties of the food products. They are also used for design and development of processing equipment and quality control. Changes in viscoelastic properties can affect the energy requirement in processing operations. In rheological studies, parameters such as temperature, concentration and composition need to be considered. Lad and Murthy (2013) reported the gel characteristics of aloe vera juice at different temperatures and found that aloe vera had elastic behavior throughout the frequency range (0.1–10 Hz) showing shear thinning flow characteristics. Vega-Galvez et al. (2011) and Opazo-Navarrete et al. (2012) studied the effect of high-pressure processing and reported that the rheological properties were unaffected during high-pressure processing. The flow behavior of the fluid foods varies from Newtonian to non-Newtonian and to it can be described using steady shear measurements and by fitting viscometry data to commonly used mathematical models such as Newtonian, power law (Ostwald–de Waele), Herschel–Bulkley and Casson models (Rao 2007). Small amplitude oscillatory shear tests have been used to measure rheological properties such as elastic modulus (G′) and viscous modulus (G″) at low shear without altering the structural characteristics of samples and are useful to understand viscoelastic properties. These properties can be used to assess long-term stability of products (Alvarez and Canet 2013). Oscillatory measurements along with steady shear measurements can be used in combination to derive useful relationships at low frequencies and shear rates (Ahmed et al. 2007). Literature review showed that there was very little information available on rheological properties of aloe vera juice at different temperatures and concentrations. Aloe vera contains more that 98% moisture, so it can be concentrated like fruit juices for various applications; concentration has other advantages such as reduction of transportation, packaging and storage costs. Rheological properties of different fruit juice concentrates such as orange (Falguera and Ibarz 2010), pear juice (Ibarz et al. 1989), clarified mango juice concentrate (Singh and Eipeson 2000), apple and pear (Ibarz et al. 1987), and pummelo juice (Chin et al. 2009) are available. Limited work has been conducted on the rheological properties of concentrates of aloe vera juice. The objectives of this study were to study the flow properties of aloe vera juice as a function of concentration and temperature using steady shear measurements. The obtained experimental data were used to evaluate the activation energy and combined effect of concentration and temperature using multiple regression analysis. Furthermore, deformation properties were studied as a function of concentration. 2

N.R. SWAMI HULLE, K. PATRUNI and P.S. RAO

MATERIAL AND METHODS Sample Preparation Freshly harvested aloe vera leaves were obtained from research farm of Agricultural and Food Engineering Department, IIT Kharagpur, India. About 3-year-old leaves were selected according to uniform size, shape and color. They were washed thoroughly and disinfected using 50 ppm sodium hypochlorite solution. Then the aloe leaves were kept vertically for 1 h to remove undesirable yellow colored sap. Hand filleting was done to separate the gel fillet portion and was cut into small pieces and blended in a domestic blender (HL 1631, Philips, Kolkata, India). The fluid obtained was then filtered through muslin cloth and immediately kept in the refrigerator at 4C and was termed as aloe vera juice. However, according to literature, this fluid was also termed as aloe gel (Vega-Galvez et al. 2011) and aloe gel juice (Chang et al. 2006). The aloe vera juice concentrates were prepared using lab scale rotary evaporator (Sohag Laboratory Equipments, Ambala, Haryana, India) at 50 ± 3C and 150 mm Hg pressure. The evaporation flask was rotated at constant rotational speed of 120 rpm, in a constant temperature hot water bath. Samples of desired concentration were obtained and minimal adjustment of concentrations was done by using distilled water, and samples were kept under chilled condition (4C) for further rheological analysis.

Chemical Analysis The physicochemical composition of aloe vera juice concentrates was analyzed and reported in Table 1. The total soluble solids were measured using digital refractometer (PAL-1, Atago, Tokyo, Japan). pH was measured using digital pH meter (Tritrino plus 877, Metrohm, Herisau, Switzerland). Moisture content was estimated by vacuum drying at 60C until constant weight was reached and results were expressed on wet basis (Garcia-Segovia et al. 2010). The crude ash content was estimated by incineration in muffle furnace at 550C (Miranda et al. 2009). The crude protein was estimated using Kjeldahl method (Vega-Galvez et al. 2011). The carbohydrate composition in polysaccharide in aloe juice samples was estimated using calorimetric assay as described by Hu et al. (2003).

Fourier-Transform Infrared (FTIR) Spectroscopy Analysis FTIR spectra for concentrate samples were measured by using a spectrometer (NICOLET 6700, Thermo Scientific, Madison, WI, USA) to observe the changes in the peaks and assess the functional characteristics of the concentrates. The Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.

N.R. SWAMI HULLE, K. PATRUNI and P.S. RAO

RHEOLOGICAL PROPERTIES OF ALOE VERA CONCENTRATES

TABLE 1. PHYSICOCHEMICAL COMPOSITION OF ALOE VERA JUICE CONCENTRATES USED FOR EXPERIMENTS Aloe vera juice concentrates (°Brix) Parameters

1.5

2.5

3.5

4.5

5.5

Moisture (%), w.b. pH Ash (%), d.b. Lipid (%), d.b. Protein (%), d.b. Total carbohydrates (%), d.b.

98.45 ± 0.35 4.20 ± 0.02 22.50 ± 0.50 4.55 ± 0.10 7.40 ± 0.22 52.91 ± 5.05

97.14 ± 0.23 4.12 ± 0.01 20.01 ± 0.15 4.26 ± 0.12 7.35 ± 0.31 49.54 ± 2.33

96.02 ± 0.10 4.05 ± 0.01 18.53 ± 1.07 4.23 ± 0.22 7.27 ± 0.15 48.36 ± 1.45

95.43 ± 0.22 4.04 ± 0.01 20.62 ± 0.62 4.18 ± 0.12 7.12 ± 0.16 49.02 ± 2.12

94.22 ± 0.32 4.03 ± 0.01 19.00 ± 0.16 4.24 ± 0.24 7.74 ± 0.24 48.41 ± 1.65

aloe vera concentrates were directly loaded for FTIR spectra measurement and the frequency range selected was from 700 to 4,000 cm−1 and measurements were done at ambient conditions (25 ± 2C).

Rheological Measurements and Analysis Steady shear and oscillatory sweep experiments were carried out using a stress-controlled dynamic rheometer (Bohlin Gemini 200, Malvern Instruments Ltd, Malvern, U.K.) and data acquisition by Bohlin software, version 6.51.0.3. Coaxial cylinder (C25 – cup and bob) geometry was chosen due to its greater sensitivity with low viscosity materials and fluid suspensions (Kok 2010). The diameter of the cup was in proportion to the bob size as described by the DIN Standard. The free surface of the sample was always covered by solvent evaporation trap to prevent the evaporation of the sample. The measurement device was equipped with a temperature control unit (Peltier plate) that gives effective temperature control over extended time (accuracy ± 0.1C). A 10 mL sample of aloe juice was loaded into the cylinder and was allowed to equilibrate at set temperature (15–55C). Sample temperature was controlled by Peltier system attached with circulation unit (range −40 to 180C). All the rheological measurements were carried out in triplicate. Dynamic oscillatory measurements were carried out in the frequency range from 0.1 to 10.0 Hz. Initially, amplitude sweep test was used to determine linear viscoelastic region (LVR). On the other hand, the frequency sweep mode was used to evaluate viscoelastic properties of juice samples at a constant stress selected from LVR. Values of viscous modulus (G″) and elastic modulus (G′) were obtained from the frequency sweep test.

Shear stress-shear rate models

σ = μγ n σ = K (γ ) n σ = σ 0 + K (γ ) 0.5 0.5 0.5 σ = σ 0 + (Kγ )

Statistical Analysis All the experiments were performed in triplicate. Analysis of variance was carried out using Microsoft Office Excel 2007 version.

Microscopic Images The microscopic images were observed using a bright field microscope (DM750, Leica Microsystems, Glattbrugg, Switzerland). A drop of around 0.1 mL was spread on a microscope slide and the sample was covered with another slide and images were taken using Leica DFC295 camera mounted on the microscope. Digitalized images were recorded using LAS Software, version 3.5.0 (Leica Microsystems). Images of areas of interest of the raw aloe vera juice sample were captured at suitable magnification. Microscopic analysis of concentrate samples was not performed, because it is required to dilute the sample for examination and it may change the structure of the sample which may not be correlated with rheological analysis.

RESULTS AND DISCUSSION FTIR

TABLE 2. RHEOLOGICAL MODELS USED FOR ANALYSIS OF FLOW CHARACTERISTICS OF ALOE VERA JUICE

Newtonian Ostwald–de Waele (or power law) Herschel–Bulkley Casson

For steady shear measurements, rheometer was set to predetermined temperatures (15, 25, 35, 45, 55C) followed by programmed shear changing from 0.1 to 100 s−1. Parameters such as apparent viscosity, shear rate and shear stress were obtained from the software. The experimental shear rateshear stress data of aloe vera juice concentrates were analyzed using various rheological models (power law and Casson) given in Table 2. Goodness of fit was assessed using correlation coefficient (R2) and Chi-square (χ2) values.

(1) (2) (3) (4)

Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.

The effect of concentration (1.5–5.5 °Brix) of aloe vera juice on the functional groups present within it was evaluated through FTIR, where the wave number was plotted against transmission (Fig. 1). The FTIR spectra confirmed the presence of –OH stretching (corresponds to 3,207–3,144 cm−1), –CH stretching (corresponds to typical 2,926 cm−1), C = O 3

RHEOLOGICAL PROPERTIES OF ALOE VERA CONCENTRATES

FIG. 1. FTIR SPECTRA FOR DIFFERENT CONCENTRATIONS OF ALOE VERA JUICE

stretches of acetyl (corresponds to 1,720 cm−1), C-O-C stretches of acetyl groups (corresponds to 1,255 cm−1) and OH bending (corresponds to 1,050 cm−1). Strong and intensified bands at 1,050 and 3,207 cm−1 attributed to the presence of hydroxyl group as reported earlier by Ray and Aswatha (2013). Similar peaks were also noticed for bioactive polysaccharides such as acemannan and pectin (Femenia et al. 2003; Gentilini et al. 2014). Peak at 2,926 cm−1 corresponds to the –CH stretching that might be due to H-bond formation at higher concentrations (2.5–5.5 °Brix). However, medium intensity and broad-shaped bands were seen at 1,720 and 1,255 cm−1 which refer to C = O, and C-O-C stretching of acetyl and COOH groups in the sample (Femenia et al. 2003). Intensity of the peaks increased with increase in concentration, which might be due to the conversion of functional groups during concentration process. The presence of acetyl group also indicates the presence of biologically active compound, which helps molecule cross hydrophobic barriers in the cell (Chang et al. 2011). Similar band formation was reported in case of whole leaf aloe vera powder by Kim et al. (2009). The identification of strong bands in the range of 1,050 cm−1 mostly depicts the presence of monosaccharide units in the branched regions such as galactose and glucan units, and these observations were in accordance with the previous studies on aloe vera (Ray and Aswatha 2013; Gentilini et al. 2014).

Flow Characteristics of Aloe Vera Juice Concentrates The flow behavior of aloe vera juice concentrates was characterized by shear stress against shear rate plots. The shear stress versus shear rate plot for 2.5 °Brix at different temperatures has been shown in Fig. 2. At fixed temperature, 4

N.R. SWAMI HULLE, K. PATRUNI and P.S. RAO

FIG. 2. SHEAR STRESS-SHEAR RATE PLOT OF ALOE VERA JUICE (4.5°BRIX) FITTED TO POWER LAW MODEL AT DIFFERENT TEMPERATURES

the shear stress was not linearly dependent on shear rate values indicating the non-Newtonian behavior of the aloe concentrate. Viscosity of the juice decreased with increase in shear rate dictating the shear thinning behavior. This behavior was common for all the concentrations (1.5–5.5 °Brix) and temperatures (15–55C). The decrease in viscosity might be due to breakdown of structural units in the aloe juice because of hydrodynamic forces generated during shear (Rao 2007). This type of behavior is typical for fruit-based products and similar observations have been reported for strawberry juice (Sesmero et al. 2009), mango juice (Dak et al. 2006) and tamarind juice concentrates (Ahmed et al. 2007). Power law, which is commonly used to analyze the flow behavior of fluid foods, showed a good correlation between experimental and predicted data at all temperatures and concentrations (Table 3). Flow behavior index (n) values for all temperatures ranged between 0.31 and 0.85, demonstrating shear thinning behavior (n < 1). Increase in concentration resulted in decreased n values (P < 0.05), representing increase in the pseudoplasticity of aloe juice concentrates. However, the effect of test temperature on n was not significant (P > 0.05). Consistency index (K) was significantly influenced (P < 0.05) by the linear terms of concentration and temperature as well as their interaction term. K value increased and n values decreased with increase in temperature in general, indicating more concentration dependence at higher temperature. Similar results were observed by Karazhiyan et al. (2009) for Lepidium sativum seed hydrocolloid extract. The Casson model was used to evaluate the Casson yield stress which is also considered as apparent yield stress (Rayment et al. 1995). Good fit was obtained using Casson (R2 ≥ 0.943) and estimated model parameters are shown in Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.

N.R. SWAMI HULLE, K. PATRUNI and P.S. RAO

RHEOLOGICAL PROPERTIES OF ALOE VERA CONCENTRATES

TABLE 3. POWER LAW AND CASSON MODEL PARAMETERS FOR ALOE VERA JUICE AT DIFFERENT CONCENTRATIONS AND TEMPERATURES Power law

Casson

TSS (°Brix)

Temperature (C)

K (Pa·s)

n

R

χ

K (Pa·s)

σo (Pa)

R2

χ2

1.5

15 25 35 45 55 15 25 35 45 55 15 25 35 45 55 15 25 35 45 55 15 25 35 45 55

0.189 ± 0.002 0.151 ± 0.004 0.088 ± 0.002 0.058 ± 0.001 0.029 ± 0.001 3.179 ± 0.113 2.281 ± 0.063 1.783 ± 0.081 1.285 ± 0.100 1.103 ± 0.071 4.740 ± 0.204 4.803 ± 0.152 2.824 ± 0.104 2.703 ± 0.084 2.145 ± 0.066 6.903 ± 0.313 4.775 ± 0.180 3.253 ± 0.144 3.041 ± 0.052 2.302 ± 0.018 17.594 ± 0.412 14.895 ± 0.431 10.415 ± 0.156 8.306 ± 0.301 7.439 ± 0.200

0.710 ± 0.009 0.627 ± 0.015 0.685 ± 0.002 0.717 ± 0.005 0.809 ± 0.012 0.366 ± 0.013 0.404 ± 0.005 0.434 ± 0.017 0.469 ± 0.010 0.473 ± 0.020 0.409 ± 0.000 0.371 ± 0.018 0.451 ± 0.011 0.407 ± 0.009 0.432 ± 0.005 0.343 ± 0.001 0.399 ± 0.020 0.447 ± 0.010 0.389 ± 0.020 0.428 ± 0.011 0.265 ± 0.008 0.276 ± 0.010 0.330 ± 0.006 0.345 ± 0.009 0.347 ± 0.010

0.979 0.998 0.997 0.999 0.994 0.996 0.996 0.995 0.996 0.995 0.998 0.997 0.999 0.998 0.998 0.998 0.998 0.997 0.997 0.997 0.993 0.999 0.998 0.997 0.999

0.044 0.080 0.089 0.016 0.041 3.755 3.397 3.344 2.551 2.342 5.726 5.476 0.949 2.102 1.629 4.997 5.028 1.508 3.902 3.091 12.330 5.363 6.422 15.470 03.319

0.052 ± 0.001 0.041 ± 0.005 0.039 ± 0.008 0.033 ± 0.002 0.032 ± 0.000 0.070 ± 0.004 0.070 ± 0.010 0.070 ± 0.020 0.069 ± 0.002 0.065 ± 0.006 0.101 ± 0.001 0.101 ± 0.002 0.092 ± 0.004 0.077 ± 0.003 0.075 ± 0.005 0.107 ± 0.002 0.098 ± 0.003 0.094 ± 0.002 0.076 ± 0.001 0.078 ± 0.003 0.120 ± 0.005 0.111 ± 0.002 0.113 ± 0.001 0.102 ± 0.002 0.097 ± 0.001

0.214 ± 0.001 0.157 ± 0.001 0.067 ± 0.002 0.057 ± 0.000 0.014 ± 0.001 4.175 ± 0.002 2.963 ± 0.004 2.275 ± 0.005 1.597 ± 0.010 1.354 ± 0.008 6.357 ± 0.015 4.833 ± 0.020 3.824 ± 0.017 4.123 ± 0.033 2.833 ± 0.011 6.658 ± 0.200 6.367 ± 0.132 4.162 ± 0.084 3.987 ± 0.054 2.991 ± 0.060 18.839 ± 1.032 16.822 ± 0.735 12.898 ± 0.502 10.992 ± 0.530 9.967 ± 0.605

0.985 0.988 0.984 0.996 0.991 0.963 0.964 0.964 0.967 0.964 0.981 0.985 0.987 0.980 0.978 0.984 0.976 0.987 0.965 0.971 0.957 0.947 0.979 0.968 0.979

0.003 0.009 0.007 0.001 0.003 0.092 0.089 0.089 0.077 0.075 0.098 0.075 0.052 0.058 0.061 0.089 0.113 0.055 0.100 0.088 0.157 0.244 0.096 0.169 0.098

2.5

3.5

4.5

5.5

2

2

Values presented are means ± standard error of triplicate values. TSS, total soluble solids.

Table 3. According to Fernandez et al. (2008), the appearance of yield stress in liquid samples indicates the presence of a cross-linked or interactive structure in the sample which must be broken down so that flow can occur at that particular shear rate. Higher yield stress values were obtained at lower temperatures and higher concentrations and vice versa; yield values ranged between 0.014 and 18.839 Pa. It might be due to the fact that increase in concentration favors the H-bond formation as witnessed in FTIR spectra. The results are in good agreement with the data reported by Yaron (1991) where increased apparent yield values of aloe vera were obtained with increase in polysaccharide concentration. The appearance of yield stress in aloe vera juice concentrates indicated its applicability in preparation of stable suspensions. Opazo-Navarrete et al. (2012) reported the yield stress values of aloe vera suspensions ranging between 0.89 and 1.25 Pa. These values were higher than fresh aloe juice (0.014–0.214 Pa) used in this study. This variation in yield values is mainly due to changes in composition which is dependent on field of cultivation, period and weather conditions, type of processing employed, type of instrument and shear rate range used for measurements. Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.

The rheological properties of aloe juice are attributed to high molecular weight polysaccharides containing mannan, and once extracted from plant their viscous characteristics degrade rapidly (Yaron 1991, 1993; Ni et al. 2004). Hence, care should be taken while processing of aloe vera juice, and factors such as holding time before processing, concentration and interaction of polysaccharides, dissolved solids and also processing method should be analyzed considering the quality of the aloe vera product (Nindo et al. 2010).

Effect of Temperature The temperature dependence of viscosity is well described by Arrhenius-type equation (Tonon et al. 2009; Nindo et al. 2010):

η = η∞ ∗ exp ( Ea RT )

(5)

where η is the measured viscosity (Pa·s), η∞ is constant, Ea is the activation energy (kJ/mol), and R is the universal gas constant (8.314 J/mol/K). The model constants of Eq. (5) were evaluated by plotting a log viscosity at different shear rates against 1/T (K−1) as shown in Fig. 3 for aloe 5

RHEOLOGICAL PROPERTIES OF ALOE VERA CONCENTRATES

N.R. SWAMI HULLE, K. PATRUNI and P.S. RAO

TABLE 4. ACTIVATION ENERGY VALUES AT DIFFERENT CONCENTRATIONS AND SHEAR RATES TSS (°Brix)

Shear rate (s−1)

η0

Ea (kJ/mol)

R2

1.5

5 50 100 5 50 100 5 50 100 5 50 100 5 50 100

1.4612E-09 8.6770E-08 3.0713E-07 0.0003 0.0023 0.0016 0.0008 0.0014 0.0017 0.0012 0.0008 0.0008 0.0078 0.0120 0.0095

53.073 39.166 34.498 23.76 13.864 13.636 22.419 17.013 15.187 23.731 19.507 17.511 18.687 12.801 12.148

0.990 0.966 0.959 0.998 0.944 0.996 0.934 0.990 0.932 0.979 0.989 0.960 0.989 0.939 0.928

2.5

3.5

4.5

5.5 FIG. 3. EFFECT OF TEMPERATURE ON THE APPARENT VISCOSITY OF ALOE VERA JUICE CONCENTRATE (TSS 4.5°BRIX) AT DIFFERENT SHEAR RATES

concentrate (4.5 °Brix). It indicated that viscosity of aloe vera juice concentrates decreased with increase in temperature over the range of shear rates. This might be due to increased mobility of macromolecules due to temperature rise, causing less resistance to flow. The change of magnitude of viscosity was more at shear (5 s−1) compared to small difference at 50 and 100 shear rates (s−1). Processing involves wide variations in the temperature, hence it is very important to evaluate the effect of temperature on the rheological properties (Rao 2007). Interaction effect of temperature and shear rate was also evaluated and was statistically significant for aloe vera juice at all concentrations (P < 0.05). The Arrhenius constants were evaluated within temperature range from 15 to 55C and are shown in Table 4, along with regression coefficients. The activation energy values ranged between 12.148 and 53.073 kJ/mol. The effect of shear rate on activation energy was also evaluated; it was found that activation energy decreased with increase in concentration significantly (P < 0.05) indicating pronounced effect of temperature on the viscosity at low concentrations. Similar results have been reported earlier by Karazhiyan et al. (2009) for L. sativum seed hydrocolloid extract and

TSS, total soluble solids.

Rani and Bains (1987) for tomato ketchup. They reported that the activation energy values were dependent on the pectic substances in tomato. Activation energy values were correlated with flow behavior index and found that they were directly proportional to each other. Sharoba et al. (2005) reported that for most of the pseudo-plastic fruit products, with increase in pseudoplasticity temperature had less effect on viscosity.

Effect of Concentration The influence of concentration on viscosity of aloe vera juice was studied using power and exponential form of relationship as given below: (6)

η100 = η2 exp (a2C )

(7)

where η100 is apparent viscosity (Pa·s) at 100 s−1, η1 and η2 are constants (Pa·s), a1 and a2 are dimensionless constants, and C is concentration of aloe vera juice (°Brix). Estimated parameter values of Eqs. (6) and (7) are given in Table 5.

η100 = η2exp(a2C)

η100 = η1C a1

η100 = η1C a1

Temperature (C)

η1 (Pa·s)

a1

R2

η2 (Pa·s)

a2

R2

15 25 35 45 55

0.0023 0.0023 0.0014 9.6229 × 10−4 5.7528 × 10−4

2.7939 2.5903 2.83278 2.94805 3.15547

0.995 0.999 0.997 0.999 0.999

0.0040 0.0036 0.0026 0.0018 0.0012

0.8125 0.7686 0.8199 0.8474 0.8904

0.997 0.994 0.997 0.996 0.999

TABLE 5. EFFECT OF CONCENTRATION ON THE APPARENT VISCOSITY OF ALOE VERA JUICE AT DIFFERENT TEMPERATURES

η1, a1: constants determined from power law type model; η2, a2: constants determined from exponential type model.

6

Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.

N.R. SWAMI HULLE, K. PATRUNI and P.S. RAO

RHEOLOGICAL PROPERTIES OF ALOE VERA CONCENTRATES

model Eq. (9) proposed by Ibarz et al. (2009) to predict the rheological behavior within the studied temperature and concentration range:

E η = K 2 exp ⎛ a + K 3C + K 4C 2 ⎞ ⎝ RT ⎠

(9)

where η is viscosity (Pa·s); K2, K3 and K4 are model constants, R is gas constant, and T is absolute temperature (K). Apparent viscosity values were chosen at different shear rates (5, 50 and 100 s−1). Multiple linear regression analysis was conducted on the linear form of Eq. (9) to obtain model constants. Both the model fit and the parameters were significant with correlation coefficient of 0.954. The obtained model equation was

23.272 η = 1.934 × 10−6 exp ⎛ + 2.332C − 0.223C 2 ⎞ (10) ⎝ RT ⎠

FIG. 4. ZERO SHEAR VISCOSITY OF ALOE VERA JUICE AS A FUNCTION OF TEMPERATURE AND CONCENTRATION

Good fit was obtained for both the models as evident from R2 values (R2 > 0.9937). Viscosity of aloe juice concentrates increased with increase in concentration as expected. The reason might be due to change in concentration which restricts the molecular movement between water and solute. This movement is generally held by several intra- and intermolecular spacing and hydrogen bonds (Azoubel et al. 2005). Similar model equations have been used to describe the relation between concentration and apparent viscosity by Ibarz et al. (1992) for black currant juice and Chun and Yoo (2004) for rice flour dispersions. Zero shear viscosity was calculated in order to investigate the transition from dilute to concentrate regimes using Moore model (Moore 1959) as shown below:

η = η∞ + (η0 − η∞ ) (1 + Kγ )

(8)

where η∞ and η0 are high and low shear viscosities. It is capable of predicting flow properties over wide range of shear rates. The variation of zero shear viscosity at different temperatures as a function of concentration was shown in Fig. 4. It was observed that viscosity increased at slower rate up to a concentration of 4.5 °Brix, after which it increased suddenly which may indicate the transition from liquid to semisolid regime where the biopolymers at higher concentration participate in the formation of entanglements (McConaughy et al. 2008).

Combined Effect of Temperature and Concentration The combined effect of temperature and concentration on apparent viscosity was evaluated by single exponential Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.

The surface plot for combined effect of concentration and temperature on viscosity at different shear rates is shown in Fig. 5. Response surface plots indicated that temperature and concentration both affected the viscosity, where concentration had a pronounced effect. Similar trends for combined effect of temperature and concentration for other fruit juices were reported earlier (Belibagli and Dalgic 2007; Magerramov et al. 2007; Shamsudin et al. 2009; Yilmaz et al. 2010, 2011). In general, these equations are very helpful in processes such as pumping requirements, mixing of different fluid products involving simultaneous heat and mass transfer, where single equation is used to predict the dependency of viscosity on concentrations and temperatures.

Dynamic Shear Rheology In dynamic rheological analysis, stress sweep was performed to find out the LVR for the aloe juice concentrates at a frequency of 10 ω (rad/s) and critical stress point was determined to get the point at which sample begins to flow. Figure 6 shows the experimental data of critical stress plotted as a function of concentration. It was found that critical stress increased monotonically with increase in concentration as expected. This increase might be due to interaction between pectic substances and formation of networks between biomolecules. In addition, shifting of peaks in FTIR spectra at 2,926 cm−1 indicates the formation of weak forces of interaction such as H-bond. Fresh juice (1.5 °Brix) did not show any LVR region, hence was not subjected to frequency sweep tests. However, weak gel characteristics of fresh aloe vera gel have been reported recently (Vega-Galvez et al. 2011; Opazo-Navarrete et al. 2012; Lad and Murthy 2013). This difference was due to changes in the compositions and other components, as this plant gel is highly dependent on climatic and environmental conditions (Hamman 2008; Ramachandra and Rao 2008). 7

RHEOLOGICAL PROPERTIES OF ALOE VERA CONCENTRATES

N.R. SWAMI HULLE, K. PATRUNI and P.S. RAO

FIG. 5. SURFACE PLOTS REPRESENTING THE COMBINED EFFECT OF TEMPERATURE AND CONCENTRATION ON THE VISCOSITY OF ALOE VERA JUICE AT DIFFERENT SHEAR RATES: (A) 5 S–1; (B) 50 S–1; (C) 100 S–1

Figure 7 shows a mechanical spectra describing the viscoelastic behavior of aloe vera juice at different concentrations under small amplitude oscillatory test. Oscillation stress of 0.1 Pa was selected from LVR region and was used in all the measurements. It was observed that both the viscous modulus (G″) and the elastic modulus (G′) increased significantly with increase in frequency (P < 0.05). At the beginning of the oscillation sweep test G″ was higher than G′, and after certain increase in frequency crossover

between G″ and G′ curves were observed indicating viscoelastic fluid behavior. In general, this type of frequency-dependent behavior is termed as “weak gels,” which is also indicated by positive slopes of log modulus versus log frequency plots (Ross-Murphy 1995). Colloidal dispersions such as globular proteins and water-insoluble particles of polysaccharides are reported to show this type of weak gel behaviors (Ikeda and Nishinari 2001). Similar

FIG. 6. VARIATION OF CRITICAL STRESS VALUES OF ALOE JUICE WITH CHANGE IN CONCENTRATION AT 25C

FIG. 7. MECHANICAL SPECTRA OF ALOE VERA JUICE CONCENTRATES (2.5–5.5°BRIX) AT 25C

8

Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.

N.R. SWAMI HULLE, K. PATRUNI and P.S. RAO

RHEOLOGICAL PROPERTIES OF ALOE VERA CONCENTRATES

FIG. 8. MICROSCOPIC IMAGES OF (A) CELLULAR (AT 20× MAGNIFICATION) AND (B) FIBROUS MATERIALS (AT 100× MAGNIFICATION) IN ALOE VERA JUICE, RESPECTIVELY

type of behavior was observed by Ahmed et al. (2007) for tamarind juice concentrates, and according to them this type of behavior shows the presence of entangled polymer networks which at low frequency forms rearrangement within the time period of oscillation, and at higher frequency again deforms showing both elastic and viscous behaviors. Lad and Murthy (2013) reported similar observations for aloe vera fibrous juice at different temperatures. This change in behavior from solution to gel is mainly observed from crossover points as observed in Fig. 7. The increase in concentration led to shift of crossover point to lower frequencies, which indicates that concentration of aloe vera juice influences the functional groups to participate in bond formations with weak forces of interaction and enhances the corresponding viscoelastic behavior. This change in functional groups can be clearly seen in FTIR spectra (Fig. 1), where increase in concentration led to increase in intensity of absorbance. The obtained results suggested that the viscoelastic properties of aloe vera are mainly dependent upon the concentration of the soluble solids such as water-soluble polysaccharides. This study also suggests that it is possible to attain desired textural properties from aloe vera by altering the concentration. The obtained G′ and G″ data were subjected to power law regression using the following equations to describe frequency dependence:

G ′ = K ′ (ω )n′

(11)

G ′′ = K ′′ (ω )n′′

(12)

The parameter values of K″ were higher than K′, indicating that G″ predominates in concentrate samples. Similarly, the exponent n′ of elastic modulus was always higher than n″ Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.

obtained from viscous modulus, indicating that the G′ increase with increase in frequency was higher than G″. The n′ (range 0.423–0.709) and n″ (0.192–0.323) values decreased with increase in concentration, which might be attributed to changes in the functional groups due to concentration process. Meza et al. (2010) reported similar observations for whey protein concentrate suspensions. This change in functional groups can be clearly seen in FTIR spectra (Fig. 1), where increase in concentration led to increase in intensity of absorbance. The obtained results suggested that the viscoelastic properties of aloe vera are mainly dependent upon the concentration of the soluble solids such as water-soluble polysaccharides. This study also suggests that it is possible to attain desired textural properties from aloe vera by altering the concentration.

Electron Microscopy Microscopic examination revealed the transparent cell wall materials and fibers dispersed in juice as shown in Fig. 8. Spiral fiber structures of various lengths were observed at many places in the sample (Fig. 8a). Aggregates of the cell wall like materials were observed including single as well as clumps of cells forming network-like structures (Fig. 8b). This variation in particle sizes and their interactions with other components may contribute to the complex viscous nature of juice. Processing changes the functionality due to structural breakdown which might affect the complex viscoelastic properties of aloe vera juice concentrates. According to Lad and Murthy (2013), during the rheological analysis increase in shear rates deforms the filamentous structures of aloe polysaccharide hydrogels affecting the viscoelastic moduli. 9

RHEOLOGICAL PROPERTIES OF ALOE VERA CONCENTRATES

CONCLUSIONS In the present study, coaxial cylinder geometry was used for flow characterization of aloe vera juice at different concentrations (1.5, 2.5, 3.5 4.5 and 5.5 °Brix) and temperatures (15, 25, 35, 45 and 55C). Aloe vera juice concentrates showed shear thinning behavior at all temperatures and concentrations tested and were well described by power law model. The consistency index increased with increase in concentration and followed exponential type of model. The flow behavior index and consistency index ranged between 0.265–0.809 and 0.029–17.594 (Pa·s), respectively. Temperature dependency of viscosity followed Arrhenius model and activation energy decreased with increase in concentration and ranged between 12.148 and 53.073 kJ/mol. The combined effect of concentration and test temperature on the viscosity of aloe juice was evaluated and a model equation was developed. FTIR spectroscopy indicated changes in functional groups and was also supported by rheological analysis. The viscoelastic property of aloe juice was enhanced with increase in concentration showing weak gel characteristics in range from 2.5 to 5.5 °Brix. The results obtained from this study are useful for aloe processing industries and their application in development of structured food or pharmaceutical products.

ACKNOWLEDGMENTS The authors wish to thank the National Agricultural Innovation Project, ICAR, New Delhi, for funding this work partially under the World Bank support, and the first author wish to thank CSIR, New Delhi, for providing research fellowship. The authors also thank Dr. P. P. Srivastav for providing facility for rheological experiments. REFERENCES AGARWALA, O.P. 1997. Whole leaf aloe gel vs. standard aloe gel (differences in processing techniques yields divergent properties, costs, applications). Drug Cosmet. Ind. 160, 22–28. AHLAWAT, K.S. and KHATKAR, B.S. 2011. Processing, food applications and safety of aloe vera products: A review. J. Food Sci. Technol. 48, 525–533. AHMED, J., RAMASWAMY, H.S. and SASHIDHAR, K.C. 2007. Rheological characteristics of tamarind (Tamarindus indica L.) juice concentrates. LWT-Food Sci. Technol. 40, 225–231. ALVAREZ, M.D. and CANET, W. 2013. Dynamic viscoelastic behavior of vegetable-based infant purees. J. Texture Studies 44, 205–224. AZOUBEL, P.M., CIPRIANI, D.C., EL-AOUAR, A.A., ANTONIO, G.C. and MURR, F.E.X. 2005. Effect of concentration on the physical properties of cashew juice. J. Food Eng. 66, 413–417.

10

N.R. SWAMI HULLE, K. PATRUNI and P.S. RAO

BELIBAGLI, K.B. and DALGIC, A.C. 2007. Rheological properties of sour-cherry juice and concentrate. Int. J. Food Sci. Technol. 42, 773–776. CAPASSO, F., BORRELLI, F., CAPASSO, R., DI CARLO, G., IZZO, A.A., PINTO, L., MASCOLO, N., CASTALDO, S. and LONGO, R. 1998. Aloe and its therapeutic use. Phytother. Res. 12, S124–S127. CHANG, X.L., WANG, C., FENG, Y. and LIU, Z. 2006. Effects of heat treatments on the stabilities of polysaccharides substances and barbaloin in gel juice from Aloe vera Miller. J. Food Eng. 75, 245–251. CHANG, X.L., CHEN, B.Y. and FENG, Y.M. 2011. Water-soluble polysaccharides isolated from skin juice, gel juice and flower of Aloe vera Miller. J. Taiwan Inst. Chem. Eng. 42, 197–203. CHIN, N.L., CHAN, S.M., YUSOF, Y.A., CHUAH, T.G. and TALIB, R.A. 2009. Modelling of rheological behaviour of pummelo juice concentrates using master-curve. J. Food Eng. 93, 134–140. CHOW, J.T.N., WILLIAMSON, D.A., YATES, K.M. and GOUX, W.J. 2005. Chemical characterization of the immunomodulating polysaccharide of Aloe vera L. Carbohydr. Res. 340, 1131–1142. CHUN, S.Y. and YOO, B. 2004. Rheological behavior of cooked rice flour dispersions in steady and dynamic shear. J. Food Eng. 65, 363–370. DAK, M., VERMA, R.C. and SHARMA, G.P. 2006. Flow characteristics of juice of “Totapuri” mangoes. J. Food Eng. 76, 557–561. ESHUN, K. and HE, Q. 2004. Aloe vera: A valuable ingredient for the food, pharmaceutical and cosmetic industries – A review. Crit. Rev. Food Sci. Nutr. 44, 91–96. FALGUERA, V. and IBARZ, A. 2010. A new model to describe flow behaviour of concentrated orange juice. Food Biophys. 5, 114–119. FEMENIA, A., GARCÍA-PASCUAL, P., SIMAL, S. and ROSSELLÓ, C. 2003. Effects of heat treatment and dehydration on bioactive polysaccharide acemannan and cell wall polymers from Aloe barbadensis Miller. Carbohydr. Polym. 51, 397–405. FERNANDEZ, C., ALVAREZ, M.D. and CANET, W. 2008. Steady shear and yield stress data of fresh and frozen/thawed mashed potatoes: Effect of biopolymers addition. Food Hydrocolloids 22, 1381–1395. GARCIA-SEGOVIA, P., MOGNETTI, C., ANDRES-BELLO, A. and MARTINEZ-MONZO, J. 2010. Osmotic dehydration of aloe vera (Aloe barbadensis Miller). J. Food Eng. 97, 154–160. GENTILINI, R., BOZZINI, S., MUNARIN, F., PETRINI, P., VISAI, L. and TANZI, M.C. 2014. Pectins from aloe vera: Extraction and production of gels for regenerative medicine. J. Appl. Polym. Sci. 131, 39760, 1–9. HAMMAN, J.H. 2008. Composition and applications of Aloe vera leaf gel. Molecules 13, 1599–1616. HE, Q., LIU, C.H., KOJO, E. and TIAN, Z. 2005. Quality and safety assurance in the processing of Aloe vera gel juice. Food Control 16, 95–104.

Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.

N.R. SWAMI HULLE, K. PATRUNI and P.S. RAO

HU, Y., XU, J. and HU, Q.H. 2003. Evaluation of antioxidant potential of Aloe vera (Aloe barbadensis Miller) extracts. J. Agric. Food Chem. 51, 7788–7791. IBARZ, A., VICENTE, M. and GRAELL, J. 1987. Rheological behaviour of apple juice and pear juice and their concentrates. J. Food Eng. 6, 257–267. IBARZ, A., PAGÁN, J., GUTIÉRREZ, J. and VICENTE, M. 1989. Rheological properties of clarified pear juice concentrates. J. Food Eng. 10, 57–63. IBARZ, A., PAGAN, J. and MIGUELSANZ, R. 1992. Rheology of clarified fruit juices. 2. Black-currant juices. J. Food Eng. 15, 63–73. IBARZ, R., FALGUERA, V., GARVIN, A., GARZA, S., PAGAN, J. and IBARZ, A. 2009. Flow behavior of clarified orange juice at low temperatures. J. Texture Studies 40, 445–456. IKEDA, S. and NISHINARI, K. 2001. Weak gel’-type rheological properties of aqueous dispersions of nonaggregated κ-carrageenan helices. J. Agric. Food Chem. 49, 4436–4441. KARAZHIYAN, H., RAZAVI, S.M.A., PHILLIPS, G.O., FANG, Y.P., AL-ASSAF, S., NISHINARI, K. and FARHOOSH, R. 2009. Rheological properties of Lepidium sativum seed extract as a function of concentration, temperature and time. Food Hydrocolloids 23, 2062–2068. KIM, S.A., BAEK, J.H., LEE, S.J., CHOI, S.Y., HUR, W. and LEE, S.Y. 2009. A novel method for air drying aloe leaf slices by covering with filter papers as a shrink-proof layer. J. Food Sci. 74, E462–E470. KOK, M.S. 2010. Rheological study of galactomannan depolymerisation at elevated temperatures: Effect of varying pH and addition of antioxidants. Carbohydr. Polym. 81, 567–571. LAD, V.N. and MURTHY, Z.V.P. 2013. Rheology of Aloe Barbadensis Miller: A naturally available material of high therapeutic and nutrient value for food applications. J. Food Eng. 115, 279–284. MAGERRAMOV, M.A., ABDULAGATOV, A.I., AZIZOV, N.D. and ABDULAGATOV, I.M. 2007. Effect of temperature, concentration, and pressure on the viscosity of pomegranate and pear juice concentrates. J. Food Eng. 80, 476–489. MCCONAUGHY, S.D., STROUD, P.A., BOUDREAUX, B., HESTER, R.D. and MCCORMICK, C.L. 2008. Structural characterization and solution properties of a galacturonate polysaccharide derived from aloe vera capable of in situ gelation. Biomacromolecules 9, 472–480. MEZA, B.E., VERDINI, R.A. and RUBIOLO, A.C. 2010. Effect of freezing on the viscoelastic behaviour of whey protein concentrate suspensions. Food Hydrocolloids 24, 414–423. MIRANDA, M., MAUREIRA, H., RODRIGUEZ, K. and VEGA-GALVEZ, A. 2009. Influence of temperature on the drying kinetics, physicochemical properties, and antioxidant capacity of Aloe Vera (Aloe Barbadensis Miller) gel. J. Food Eng. 91, 297–304. MOORE, F. 1959. The rheology of ceramic slips and bodies. Trans. Br. Ceram. Soc. 58, 470–494.

Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.

RHEOLOGICAL PROPERTIES OF ALOE VERA CONCENTRATES

NI, Y., YATES, K.M. and TIZARD, I.R. 2004. Aloe polysaccharides. In Aloes: The Genus ALOE (T. Reynolds, ed.) pp. 75–87, CRC Press LLC, Boca Raton, Florida. NINDO, C.I., POWERS, J.R. and TANG, J. 2010. Thermal properties of Aloe vera powder and rheology of reconstituted gels. Trans. ASABE 53, 1193–1200. OPAZO-NAVARRETE, M., TABILO-MUNIZAGA, G., VEGA-GÁLVEZ, A., MIRANDA, M. and PÉREZ-WON, M. 2012. Effects of high hydrostatic pressure (HHP) on the rheological properties of Aloe vera suspensions (Aloe barbadensis Miller). Innov. Food Sci. Emerg. Technol. 16, 243–250. RAMACHANDRA, C.T. and RAO, P.S. 2008. Processing of aloe vera leaf gel: A review. Am. J. Agric. Biol. Sci. 3, 502–510. RANI, U. and BAINS, G.S. 1987. Flow behaviour of tomato ketchups. J. Texture Studies 18, 125–135. RAO, M.A. 2007. Rheology of Fluid and Semisolid Foods: Principles and Applications, pp. 25–52, Aspean Publishers, Inc., Gaithersburg, MD. RAY, A. and ASWATHA, S.M. 2013. An analysis of the influence of growth periods on physical appearance, and acemannan and elemental distribution of Aloe vera L. gel. Ind. Crop Prod. 48, 36–42. RAYMENT, P., ROSS-MURPHY, S.B. and ELLIS, P.R. 1995. Rheological properties of guar galactomannan and rice starch mixtures – I. Steady shear measurements. Carbohydr. Polym. 28, 121–130. RODRIGUEZ, E.R., MARTIN, J.D. and ROMERO, C.D. 2010. Aloe vera as a functional ingredient in foods. Crit. Rev. Food Sci. Nutr. 50, 305–326. ROSS-MURPHY, S.B. 1995. Structure–property relationships in food biopolymer gels and solutions. J. Rheol. 39, 1451–1463. SESMERO, R., MITCHELL, J., MERCADO, J. and QUESADA, M. 2009. Rheological characterisation of juices obtained from transgenic pectate lyase-silenced strawberry fruits. Food Chem. 116, 426–432. SHAMSUDIN, R., DAUD, W.R.W., TAKRIF, M.S., HASSAN, O. and ILICALI, C. 2009. Rheological properties of Josapine pineapple juice at different stages of maturity. Int. J. Food Sci. Technol. 44, 757–762. SHAROBA, A.M., SENGE, B., EL-MANSY, H.A., BAHLOL, H.E. and BLOCHWITZ, R. 2005. Chemical, sensory and rheological properties of some commercial German and Egyptian tomato ketchups. Eur. Food Res. Technol. 220, 142–151. SINGH, N., I and EIPESON, W.E. 2000. Rheological behaviour of clarified mango juice concentrates. J. Texture Studies 31, 287–295. TANAKA, M., YAMADA, M., TOIDA, T. and IWATSUKI, K. 2012. Safety evaluation of supercritical carbon dioxide extract of aloe vera gel. J. Food Sci. 77, T2–T9. TONON, R.V., ALEXANDRE, D., HUBINGER, M.D. and CUNHA, R.L. 2009. Steady and dynamic shear rheological properties of açai pulp (Euterpe oleraceae Mart.). J. Food Eng. 92, 425–431.

11

RHEOLOGICAL PROPERTIES OF ALOE VERA CONCENTRATES

VEGA-GALVEZ, A., MIRANDA, M., ARANDA, M., HENRIQUEZ, K., VERGARA, J., TABILO-MUNIZAGA, G. and PEREZ-WON, M. 2011. Effect of high hydrostatic pressure on functional properties and quality characteristics of Aloe vera gel (Aloe barbadensis Miller). Food Chem. 129, 1060–1065. YARON, A. 1991. Aloe vera: Chemical and physical properties and stabilization. Israel J. Botany 40, 270. YARON, A. 1993. Characterization of Aloe vera gel before and after autodegradation, and stabilization of the natural fresh gel. Phytother. Res. 7, 11–13.

12

N.R. SWAMI HULLE, K. PATRUNI and P.S. RAO

YILMAZ, M.T., SERT, D. and DEMIR, M.K. 2010. Rheological properties of tarhana soup enriched with Whey concentrate as a function of concentration and temperature. J. Texture Studies 41, 863–879. YILMAZ, M.T., KARAMAN, S., CANKURT, H., KAYACIER, A. and SAGDIC, O. 2011. Steady and dynamic oscillatory shear rheological properties of ketchup-processed cheese mixtures effect of temperature and concentration. J. Food Eng. 103, 197–210.

Journal of Food Process Engineering •• (2014) ••–•• © 2014 Wiley Periodicals, Inc.