Effects of high hydrostatic pressure (HHP) on the rheological properties of Aloe vera suspensions (Aloe barbadensis Miller)

Innovative Food Science and Emerging Technologies 16 (2012) 243–250

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Effects of high hydrostatic pressure (HHP) on the rheological properties of Aloe vera suspensions (Aloe barbadensis Miller) Mauricio Opazo-Navarrete a, Gipsy Tabilo-Munizaga a,⁎, Antonio Vega-Gálvez b, Margarita Miranda b, Mario Pérez-Won b a b

Food Engineering Department, University of Bio Bio, PO Box 447, Chillán, Chile Food Engineering Department, University of La Serena, PO Box 559, La Serena, Chile

a r t i c l e

i n f o

Article history: Received 19 December 2011 Accepted 15 June 2012 Editor Proof Receive Date 24 July 2012 Keywords: Aloe vera High pressure treatment Structure investigations CSR-examination Oscillation test

a b s t r a c t The effects of high hydrostatic pressure (HHP) on the rheological properties of Aloe vera suspension during storage at 4 °C were investigated. Aloe vera suspension was pressurised at 300, 400 and 500 MPa for 1, 3 and 5 min. Aloe vera suspension exhibited shear-thinning behaviour, and the experimental data fit the Herschel–Bulkley model well (R2 > 0.99). The samples treated at 300 MPa for 1, 3 and 5 min and 400 MPa for 1 and 3 min showed a decrease in their yield stress (σ0) values (p b 0.05), while the samples treated at 300 MPa for 1, 3 and 5 min and 400 MPa for 1 min showed an increase in the consistency coefficient (K) and a decrease in the flow index (n) values (p b 0.05). The Aloe vera suspension exhibited thixotropy and behaved like a weak-gel (G′ > G″) during storage at 4 °C regardless of the pressure treatment. In conclusion, the HHP treatment does not modify the gel properties and the effect on the rheological properties of Aloe vera suspension is dependent of the pressure–time treatment applied. Industrial relevance: Given the many beneficial effects of Aloe Vera gel in the health and food industries, its demand has increased considerably. However, its use is affected by its highly perishable nature. The application of high hydrostatic pressure (HHP) is presented as an innovative technology to maintain the original rheological properties of Aloe vera suspension, leading to an enhancement of its functional attributes. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Aloe vera (Aloe barbadensis Miller) is a member of the Aloaceae family (Eshun & He, 2004). Aloe vera gel is the colourless gel contained in the inner part of the fresh leaves. This gel, the parenchymatic tissue of the Aloe leaves, contains over 98% water, and more than 60% of the dry matter is composed of polysaccharides (Garcia-Segovia, Mognetti, Andrés-Bello, & Martínez-Monzó, 2010) as pectins, cellulose, hemicellulose, glucomannan, acemannan and mannose derivates (Bozzi, Perrin, Austin, & Arce, 2007). With the recent resurgence of herbal products as part of ‘the green movement’, Aloe vera is witnessing a new renaissance across the world (Vega-Gálvez et al., 2011) as food industries today seek to meet the consumers' demand for a healthy lifestyle. The potential use of Aloe vera gel as a functional food or ingredient due to its beneficial properties in treating constipation, coughs, ulcers, diabetes, headaches, arthritis and immune-system deficiencies (Eshun & He, 2004; Vogler & Ernst, 1999) makes this plant an interesting alternative for the food industry; it represents an opportunity to open new lines of products with significant added value and high acceptance by consumers. Because of its therapeutic and functional properties and hence its beneficial effects on

⁎ Corresponding author. Tel.: +56 42 253030; fax: +56 42 253066. E-mail address: [email protected] (G. Tabilo-Munizaga). 1466-8564/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2012.06.006

humans, the use of Aloe in the formulation of food products has increased (Miranda, Maureira, Rodríguez, & Vega-Gálvez, 2009). The potential use of Aloe vera products often involves some type of processing, e.g., blanching, dehydration and freezing. Improper processing procedures may cause irreversible modifications to the polysaccharides, affecting their original structure, which may promote important changes in the physiological and pharmacological properties of these active compounds (Eshun & He, 2004; Femenia, García-Pascual, Simal, & Rosselló, 2003; Garcia-Segovia et al., 2010; Gulia, Sharma, Sarkar, Upadhyay, & Shitandi, 2010). For these reasons, the leaf needs to be processed with the aim of retaining every bioactive component of the Aloe vera gel. HP processing could preserve the nutritional value and the delicate sensory properties of fruits and vegetables due to its limited effect on the covalent bonds of low molecular-mass compounds such as colour and flavour compounds (Oey, Lille, Van Loey, & Hendrickx, 2008). However, pressures ranging from 100 MPa to 1 GPa can cause a variety of changes relevant to the preservation and processing of foods, including the destruction of microorganisms, an alteration of enzyme activity, all of which lead to changes in functional properties, keeping its original freshness, flavour, taste and colour changes which are minimal (Varela-Santos et al., 2012). In recent years, many researchers have reported on the applications of HHP in different areas of food processing (Briones, Reyes, Tabilo-Munizaga, & Pérez-Won, 2010; Briones-Labarca,

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Muñoz, & Maureira, 2011; Hemar, Liu, Meunier, & Woonton, 2010; Varela-Santos et al., 2012), focussing on food protein and its functional properties, modification, phase transition, gel rigidity and flow behaviour (Ahmed and Ramaswamy, 2004; Hemar et al., 2010). Knowledge of the rheological and/or textural properties of food products is essential for product development, quality control, sensory evaluation and the design and evaluation of processing equipment, especially in food suspensions it is difficult to link the rheological behaviour of these suspensions and the physical properties of the particles and to determine which type of interactions such as hydrodynamic, frictional and collisional forces between the particles determine the material behaviour (Lopez-Sánchez, P., & Farr, 2012; Ahmed et al., 2005). High-pressure processing can affect the rheological properties of food products such as fruits and vegetables, puree, pulp and juice (Landl, Abadias, Sárraga, Viñas, & Picouet, 2010). The physical structure of most high-moisture products remains unchanged after exposure to high pressure because shear forces are not generated by pressure (Ting & Marshall, 2002). For gas-containing products treated under high pressure, the colour and texture may be altered due to gas displacement and liquid infiltration. Physical shrinkage may occur as a result of the mechanical collapse of air pockets, while shape distortion may be related to anisotropic behaviour (Hogan, Kelly, & Sun, 2005). For foods not containing air voids, high pressure frequently results in minimal or no permanent change to textural characteristics (Ting & Marshall, 2002). The observed effects are dependent on the conditions of the high-pressure process as well as the type of fruit or vegetable (Oey et al., 2008). The effects of high hydrostatic pressure (500 MPa/1.5 min/20 °C) on processed apple puree enriched with a prebiotic showed that samples (treated and untreated) exhibited shear-thinning as the shear rates increased, which is characteristic of non-Newtonian fluids. With respect to the viscoelastic characteristics, all of the samples exhibited a more gel like behaviour with higher storage modulus values (G′) than loss modulus values (G″) (Keenan, Brunton, Butler, Wouters, & Gormlet, 2011). High-pressure processing from 500 to 700 MPa for 1 or 2 min at ambient temperature (20 °C) resulted in a significant increase in the viscosity of tomato puree during chilled storage while that treated at 300 MPa for 1 or 2 min retained its viscosity (Krebbers et al., 2003). The dynamic viscosity of watermelon juice subjected to a high pressure treatment at 900 MPa was similar to the untreated juice, indicating that high pressure treatment at 900 MPa helped to stabilise the dynamic viscosity of the juice (Zhang et al., 2011). The increase in viscosity due to pressure was attributed to an increase in the linearity of the cell walls and volumes of the particles due to the permeability of the cell walls (Oey et al., 2008). To the best of our knowledge, however, no research has been performed on the effect of high pressure on the rheological behaviour of Aloe vera suspension. Thus, the aim of this study was to describe the correlations/ interactions between HHP treatment (and time depended) on structural relation in Aloe vera suspension (Aloe barbadensis Miller) using CSR-examinations and investigations in viscoelastic behaviour by oscillation tests. 2. Materials and methods 2.1. Extraction of Aloe vera gel Leaves of Aloe vera (Aloe barbadensis Miller) were provided by the INIA-Intihuasi (Coquimbo, Chile). Homogenous leaves were selected according to size, ripeness, colour and freshness. Acibar (a yellow-coloured liquid) was extracted by cutting the base of the leaves and allowing them to drain vertically for 1 h. The epidermis was removed, and the gel was extracted and triturated by a Philips Electric blender (HR1720, Amsterdam, The Netherlands) for homogenisation. Stabilisation of the Aloe vera suspension was achieved by allowing the suspension to stand for 24 h at refrigeration temperature (4 °C), after which time the suspension was packed in 150-ml polyethylene

flexible pouches and stored under chilling conditions in a refrigerated room (4 °C) until further HHP processing. 2.2. High hydrostatic pressure treatment Packaged samples of Aloe vera suspensions were pressurised at ambient temperature (15 ±1 °C) in a 2 l processing unit (Avure Technologies Incorporated, Kent, WA, USA) at 300, 400 and 500 MPa for 1, 3 and 5 min and compared to untreated samples. Water was employed as a pressure-transmitting medium, and a ramp rate of 17 MPa/s was implemented; the decompression time was less than 5 s. After HHP treatment samples were stored at 4 °C until further rheological characterisation. All experiments were performed in triplicate. 2.3. Rheological measurements Rheological characterisation was performed after the pressure treatment using a controlled stress and strain rheometer, Physica MCR 300 (Anton Paar, Germany). The instrument was fitted with parallel plate (50 mm diameter; smooth) geometry. Samples leak out onto the centre of the base plate and the gap was set up to 0.8 mm between the two plates. The sample temperature was kept constant at 4 °C and internally controlled by a peltier plate system attached to a water circulation unit. Prior to each test, the sample was placed between the plates for 1 min to allow stress relaxation and temperature equilibration. Samples were changed before each measurement and sieved (0.05 mm) to uniform particle size. Flow curves of the Aloe vera gel samples were measured by varying the shear rate from 0.1 to 100 s−1. The resulting data were fit to the Newton, Herschel–Bulkley, Bingham, Power Law and Casson models. Rheological parameters (σ0, K, n, ηapp) were obtained by fitting the up curve to the Herschel–Bulkley model. The thixotropy behaviour was examined by estimating the hysteresis area between shear stresses of 0.1 and 100 s−1 in a controlled shear rate mode. The hysteresis area corresponds to the difference between the area enclosed by the up curve and the area enclosed by the different down curves in the shear rate range considered. The viscoelastic properties of Aloe vera gel were characterised using small amplitude oscillatory shear. The viscoelastic parameters, such as the storage modulus G′ (elastic modulus), the loss modulus G″ (viscous modulus) and tan δ (loss tangent), were calculated using the manufacturer's software (US200 Physica® version 2.01, USA). δ was determined for control samples and replicates. 2.4. Cox–Merz rule The Cox–Merz rule (Eq. (1)) studies the relationship between dynamic viscoelastic properties and steady-state rheological properties. The validity of the Cox–Merz rule was examined for each treatment by comparing the experimental values of η and those of |η*| vs. w in a double logarithmic plot. The relationship is given as:


jη ðwÞj ¼ ηðγ⋅ Þ for w ¼ γ ⋅

ð1Þ

where η* is the complex viscosity (Pa s), η is the shear viscosity (Pa s), w is the frequency of oscillation (rad/s) and γ⋅ is the shear rate (1/s). 2.5. Fourier transform infrared spectroscopy (FTIR) FTIR spectra of the Aloe vera gel were recorded on an IRPrestige-21 spectrometer (Shimadzu, Japan). For each measurement, a total of 128 scans were collected at 4 cm −1 resolution. The FTIR spectra of the Aloe vera gel were measured at wave numbers ranging from 4000 to 400 cm −1. The measurements were repeated three times and averaged to reduce baseline effects. Between measurements, the ATR was purged with water until no gel signal was detectable, and then the spectra were manipulated utilising built-in algorithms

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a 100

Effects of high hydrostatic pressure on rheological properties of Aloe vera gel were analyzed as 4 × 3 factorial in a completely randomised design. The experiment was replicated three times. Each replication considered seven samples for rheological parameters (σ0, K, n, ηapp, G′, G″ and δ). The factors included pressure with four levels (0.1, 300, 400 and 500 MPa) and time with three levels (1, 3, and 5 min). Significance testing was performed using Fisher's least significant difference (LSD) test, and the differences were taken to be statistically significant when the p-value was b 0.05. The multiple range test (MRT) included in the statistical program was used to prove the existence of homogeneous groups within each of the parameters analysed. All analysis was performed using Statgraphics Centurion XV Statistical Software (Statistical Graphics Corp., Herdon, USA).

Shear Stress (Pa)

2.6. Statistical analysis 10

10

1

1

0,1 0,1

1

b 100

0,1 100

100 control 300 MPa 400 MPa 500 MPa

Shear Stress (Pa)

Fig. 1(a), (b) and (c) shows the shear stress as a function of the shear rate for the Aloe vera suspension; all of the curves exhibited a non-Newtonian shear-thinning behaviour. This behaviour is typical of gels such as xanthan gum (Dolz, Hernández, Delegido, Alfaro, & Muñoz, 2007), and blueberry purees (Kechinski, Schumacher, Marczak, Tessaro, & Cardozo, 2011). The Herschel–Bulkley model was found to best fit all the curves (R 2 > 0.99) according to Eq. (2):

10

Shear Rate (1/s)

3. Results and discussion 3.1. Flow behaviour

control 300 MPa 400 MPa 500 MPa

Apparent viscosity (Pa s)

100 control 300 MPa 400 MPa 500 MPa

control 300 MPa 400 MPa 500 MPa

10

10

1

1

0,1 0,1

1

10

Apparent viscosity (Pa s)

of the software IR Solution. The signal was smoothed (9-point), normalised, cut and the second derivative applied (9-point).

245

0,1 100

Shear Rate (1/s)

where σ is the shear stress (Pa), γ⋅ is the shear rate (s −1), σ0 is the yield stress (Pa), K is the consistency coefficient (Pa s n) and n is the dimensionless flow behaviour index. This model is the most commonly used tool for fitting the experimental data of non-Newtonian fluids (Severa, Nedomová, & Buchar, 2010). The rheological parameters obtained from the Herschel–Bulkley model (σ0, K, n) for the different treatments are provided in Table 1. The flow behaviour index (n) ranged in values between 0.16 and 0.25, providing evidence of shear-thinning behaviour (n b 1). This value indicates that the behaviour is far from Newtonian character. This response is characteristic of foods that have large numbers of particles in dispersion, such as pectic molecules and small particles of plant tissue, which probably interact to a greater or lesser degree and are responsible for the response between the applied strain and contrary effort to flow, which is not linear. Aloe vera suspension has a yield stress (σ0) ranging between 0.59 and 0.88 (Pa) that is higher (p b 0.05) in samples treated at 300 MPa regardless of time and at 400 MPa for 1 and 3 min. The yield stress represents the finite stress required to achieve flow. The presence of a yield stress in the samples of Aloe vera suspension is due to the large number of particles in dispersion in this suspension, which impede its flow. The determination of this parameter is very important because the presence of a yield stress implies that Aloe vera suspension has a high suspension ability, which is a useful property when being used as a stabiliser in food products such as mayonnaise and salad dressings, as observed for gum extracted from the Ocinum basilicum L. seed whose static yield stress ranged from 2.36 to 35.3 Pa for solutions of 0.5–2% BSG (Hosseini-Parvar, Matia-Merino, Goh, Razavi, & Mortazavi, 2010). This finding also explains the yield stress of the xanthan gum (Song, Kim, & Chang, 2006), with values that depend on its concentration; it is therefore commonly used in

c 100

100 control 300 MPa 400 MPa 500 MPa

control 300 MPa 400 MPa 500 MPa

10

10

1

1

0,1 0,1

1

10

Apparent viscosity (Pa s)

ð2Þ

Shear Stress (Pa)

⋅n σ ¼ σ 0 þ Kγ

0,1 100

Shear Rate (1/s) Fig. 1. Flow curves of Aloe vera suspension treated at 300, 400 and 500 MPa for (a) 1 min, (b) 3 min and (c) 5 min. The black symbols represent shear stress, and the grey symbols represent apparent viscosity.

colloidal systems, where the long-term stability is markedly increased with its addition (Hibberd, Howe, Mackie, Purdy, & Robins, 1987). The consistency coefficient (k), which corresponds to the apparent viscosity (ηapp ¼ σ = γ • ⋅ ), increased significantly (p b 0.05) at 300 MPa regardless of treatment time and at 400 MPa for 1 min compared with the other treatments, which all showed similar values to the untreated sample (p > 0.05). The increase in the consistency coefficient (K) of the pressure-treated samples could be due to an increase in pectic particle interactions, as high pressure promotes these types of interactions (Krebbers et al., 2003). Therefore, the increase in apparent viscosity, which is one of the most important rheological

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Table 1 Rheological parameters of Aloe vera suspension using the Herschel–Bulkley, Casson and Power Law models. Treatments

Rheological parameters Herschel–Bulkley

Casson n

2

Pressure (MPa)

Time (min)

ηapp (Pa s)

σ0 (Pa)

K (Pa s )

n (−)

R

0.1 300 400 500 300 400 500 300 400 500

– 1

0.31 ± 0.07abc 0.33 ± 0.02a 0.32 ± 0.03ab 0.26 ± 0.02d 0.33 ± 0.03ab 0.30 ± 0.06bc 0.26 ± 0.03d 0.32 ± 0.03ab 0.28 ± 0.04cd 0.31 ± 0.04abc

0.59 ± 0.15a 0.87 ± 0.18c 0.83 ± 0.15c 0.56 ± 0.27a 0.88 ± 0.23c 0.76 ± 0.09bc 0.61 ± 0.22a 0.86 ± 0.16c 0.64 ± 0.05ab 0.65 ± 0.05ab

7.0 ± .4a 14.8 ± 4.2d 12.1 ± 3.8cd 6.0 ± 2.1a 12.3 ± 2.6cd 8.8 ± 1.1abc 7.9 ± 2.3ab 11.2 ± 2.2bcd 6.8 ± 1.1a 8.8 ± 1.7abc

0.23 ± 0.02ab 0.16 ± 0.08d 0.17 ± 0.08d 0.25 ± 0.09a 0.17 ± 0.08d 0.20 ± 0.03bcd 0.23 ± 0.11abc 0.18 ± 0.08cd 0.23 ± 0.04ab 0.20 ± 0.05bcd

0.997 0.992 0.995 0.998 0.994 0.995 0.997 0.993 0.997 0.995

3

5

Power Law n

2

σ0 (Pa)

K (Pa s )

R

1.02 ± 0.15abc 1.25 ± 0.24e 1.18 ± 0.15de 0.89 ± 0.24a 1.22 ± 0.21e 1.08 ± 0.08cd 0.95 ± 0.27abc 1.18 ± 0.19de 0.93 ± 0.08ab 1.06 ± 0.08bcd

0.46 ± 0.05a 0.45 ± 0.04a 0.45 ± 0.05a 0.44 ± 0.03a 0.45 ± 0.05a 0.45 ± 0.05a 0.43 ± 0.04a 0.45 ± 0.05a 0.46 ± 0.04a 0.46 ± 0.04a

0.718 0.749 0.724 0.706 0.741 0.726 0.705 0.743 0.694 0.717

K (Pa sn)

n (−)

R2

3.05 ± 0.77ab 3.94 ± 0.85e 3.64 ± 0.45de 2.49 ± 0.72c 3.79 ± 0.68e 3.22 ± 0.46bd 2.68 ± 0.91ac 3.61 ± 0.61de 2.70 ± 0.32ac 3.20 ± 0.29bd

0.44 ± 0.01ab 0.40 ± 0.05c 0.41 ± 0.04c 0.46 ± 0.06a 0.40 ± 0.05c 0.42 ± 0.02bc 0.45 ± 0.07ab 0.40 ± 0.05c 0.45 ± 0.02ab 0.43 ± 0.03bc

0.974 0.980 0.974 0.969 0.977 0.976 0.969 0.979 0.969 0.973

a–d

Different letters in the same column indicate significant differences (p b 0.05) for each treatment.

parameters, could result from the formation of larger structures, but within colloidal dimensions, possibly formed by different associations between molecules and pectic particles. In addition, high pressure may also increase the degree of hydration of these molecules and particles using the same kind of interactions mentioned, which would result in an increase in consistency in pressure-treated Aloe vera suspension. However, this increase was found to depend on the pressure level and time of treatment, in contrast to reports by Krebbers et al. (2003) who showed that the application of different pressure levels (300, 500 and 700 MPa) induced a linear increase in viscosity, as confirmed by sensory analysis. High hydrostatic pressure (HHP) has an effect on rheological parameters such as yield stress (σ0) and the consistency coefficient (K) of Aloe vera suspension, thus requiring a higher stress to flow; this effect is reflected by an increase in the slope (Fig. 1) because HHP affects the solid–gel transition of the polysaccharide forming different gels (Mozhaev, Heremans, Frank, Masson, & Balny, 1994) depending on the level and time of pressurisation. The apparent viscosity (Pa s) of the Aloe vera suspension samples subjected to different HHP treatments (Fig. 1(a), (b) and (c)) decreases with shear rate (shear-thinning behaviour). Shear-thinning behaviour is very common in fruit and vegetable products, which results as the molecules become less dependent on each other and consequently offer less resistance to flow under an increase in shear rate (Steffe, 1996). In addition, thixotropy is a phenomenon in which reconstruction of the inner structure of a material being destroyed by shearing forces is time retarded. The thixotropic effect can be detected when the aloe samples are exposed first to increasing shear rates and then decreasing shear rates. The existence of a hysteresis loop indicates time-dependent fluid behaviour, and its area is a measure of the extent of thixotropy (Steffe, 1996). Because of the decrease in viscosity over time as well

as with shear rate, the forward and backward flow curves do not overlap each other. Instead, they form a clockwise hysteresis loop, the so-called thixotropic loop (Steffe, 1996). In our results, the up and the return curves were almost superimposed for all samples (data not shown) indicating that the structure was not significantly damaged during the 1st (up-sweep) flow measurement compared to the previous history of the sample. At low shear stresses, there is a competition between building up structure and flow, making it difficult to obtain a true flow measurement. Furthermore, these samples are highly time dependent (Lopez-Sánchez, Svelander, Bialek, Schumm, & Langton, 2011). The Aloe vera suspension shows thixotropic behaviour in the form of a hysteresis area (Pa s−1) (Table 2) regardless of the pressure treatment. Pressure treatment decreased (pb 0.05) the thixotropy values of Aloe vera suspension for almost all treatments with the exception of 400 MPa for 3 min. There is a decrease of 62% in thixotropy values compared to the untreated sample (Table 2). During storage time, the thixotropy value for samples treated at 500 MPa for 3 min increased (p> 0.05) while that for samples treated at 300 MPa for 1 min and 400 MPa for 5 min remained stable, thereby indicating that no detectable changes were present in the level of interaction between the particles responsible for establishing the structural network of Aloe vera suspension. The thixotropic values for all other treatments decreased significantly (pb 0.05) for samples stored for 50 days at 4 °C. 3.2. Viscoelastic properties For all treatments, the G′ values were greater than the G″ values (Fig. 2), showing a weak-gel behaviour with G′ > G″. Pressure treatment reduced the viscoelastic parameters (p b 0.05) of the storage modulus (G′) in samples treated at 300 MPa for 1 and 5 min and at

Table 2 Thixotropy values (Pa s−1) of pressurised Aloe vera suspension stored at 4 °C. Thixotropy (Pa s−1)

Treatment Pressure 0.01 MPa 300 MPa 400 MPa 500 MPa 300 MPa 400 MPa 500 MPa 300 MPa 400 MPa 500 MPa A–E

Time – 1 min

3 min

5 min

Day 1

Day 3 Aa

71.22 ± 15 42.70 ± 13DEab 55.44 ± 13BCDa 33.79 ± 18EFa 45.64 ± 23CDEabc 62.40 ± 5ABa 27.24 ± 8Fa 57.78 ± 13BCa 41.73 ± 19Fab 56.44 ± 4.4BCab

Day 7 ABCb

56.67 ± 13 68.40 ± 16Ee 61.31 ± 13BCDa 54.34 ± 6ABd 57.19 ± 8ABCa 48.93 ± 12Abc 52.96 ± 10ABd 71.07 ± 7Eb 66.16 ± 10CDc 56.20 ± 10ABCab

Day 15 ABCc

43.83 ± 17 45.13 ± 11ABCbc 52.87 ± 6Ca 37.99 ± 7Aab 43.53 ± 15ABCbcd 51.88 ± 14BCb 48.28 ± 10ABCcd 49.33 ± 17ABCac 39.99 ± 14ABa 40.81 ± 16ABCc

40.00 ± 5 54.96 ± 9CDEcd 56.39 ± 13DEa 58.81 ± 12Ed 57.27 ± 11Ea 47.99 ± 8ABCDbc 44.10 ± 9ABbc 41.43 ± 10Acd 46.94 ± 8ABCab 51.37 ± 9BCDEb

Different letters in the same column indicate significant differences (p b 0.05) for each treatment. Different letters in the same row indicate significant differences (p b 0.05) for storage time.

a–d

Day 20 Ac

Day 30 ABc

44.09 ± 7 60.56 ± 10DEde 39.61 ± 6Ab 51.64 ± 9BCcd 54.83 ± 12CDEab 47.90 ± 7ABCbc 44.09 ± 11ABbc 44.51 ± 12ABc 51.91 ± 10BCDb 61.81 ± 7Ea

Day 50 ACc

45.40 ± 5 40.93 ± 13CDab 39.90 ± 4CDEeb 44.26 ± 9ACbc 34.70 ± 4DEcd 40.02 ± 7CDEcd 49.95 ± 11ABcd 32.84 ± 4Ed 40.10 ± 9CDEa 53.82 ± 8Bab

42.30 ± 15Ac 32.39 ± 2BCDa 28.93 ± 9Dc 37.59 ± 6ABCab 32.81 ± 15BCDd 34.30 ± 11BCDd 39.38 ± 4ABb 39.72 ± 5ABcd 37.63 ± 9ABCa 30.93 ± 7CDd

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400 MPa regardless of treatment time (Fig. 2). The highest modulus (G′) was the untreated case (9.64 Pa). The loss modulus (G″) decreased for samples treated at 300 MPa for 1 and 5 min, at 400 MPa regardless of time and at 500 MPa for 1 min (p b 0.05). This result indicates that the pressure at certain levels has an effect on Aloe vera suspension, causing a decrease in the G′ and G″ parameters. Concerning the loss angle (δ), all pressurised samples showed no significant changes (p b 0.05) compared to the untreated sample, with values between 32.61° and 33.42°, demonstrating their viscoelastic nature. The Aloe vera suspension shows a weak-gel like behaviour with G′ > G″ during the entire storage time at 4 °C (Fig. 3(a), (b) and

control vs G` 300-1 vs Col 7 400-1 vs Col 22 500-1 vs Col 37 control vs G`` 300-1 vs Col 8 400-1 vs Col 23 500-1 vs Col 38

(c)), suggesting that Aloe vera suspension has elastic properties dominant over viscous properties. Pressure treatment regardless of time reduced the viscoelastic parameters (p b 0.05) of the storage modulus (G′) and the loss modulus (G″) immediately after pressure treatment compared with the untreated samples. Fig. 3(a) shows that the samples treated at 300 MPa for 1 min showed an increase in G′ from the 7 day to 20 day of storage at 4 °C; this result may be due to factors such as concentration, morphology and the shape of the particle agglomerations. These factors affect the elastic behaviour (G′ > G″) (Lopez-Sánchez et al., 2011) of polysaccharides and, therefore, it is suggested that high pressure affects Aloe vera suspension due to a change in the morphology of the polysaccharide particles. The treatments of 400 MPa and 500 MPa for 1 min show a decrease in the G′ and G″ values at day 7 of storage at 4 °C and then remained unchanged at day 50 of storage at 4 °C. These

a 12

G` (control) G` (300 MPa) G` (400 MPa) G` (500 MPa)

10

8

6

4

1 1

10 2

log w (1/s)

0

10

20

30

40

50

Storage time (days)

b G` (control) G` (300 MPa) G` (400 MPa) G` (500 MPa) G`` (control) G`` (300 MPa) G`` (400 MPa) G`` (500 MPa)

b 12

G` (control) G` (300 MPa) G` (400 MPa) G` (500 MPa)

10

10

G" G` (Pa)

log G", G` (Pa)

G" (control) G" (300 MPa) G" (400 MPa) G" (500 MPa)

10

G" G` (Pa)

log G", G` (Pa)

a

247

G" (control) G" (300 MPa) G" (400 MPa) G" (500 MPa)

8

6

4 1 1

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2

log w (1/s)

0

10

c c

G` (control) G` (300 MPa) G` (400 MPa) G` (500 MPa) G`` (control) G`` (300 MPa) G`` (400MPa) G`` (500 MPa)

12

30

40

G` (control) G` (300 MPa) G` (400 MPa) G` (500 MPa)

10

G" G` (Pa)

log G", G` (Pa)

20

50

Storage time (days)

10

G" (control) G" (300 MPa) G" (400 MPa) G" (500 MPa)

8

6

4 1 1

10

log w (1/s)

2 0

10

20

30

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Fig. 3. Viscoelasticity values of Aloe vera suspension pressure treated for (a) 1 min, (b) 3 min and (c) 5 min and stored at 4 °C for 50 days.

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parameters (G′ and G″) are higher in the samples treated with high hydrostatic pressure after 50-day storage at 4 °C compared to the untreated sample, where G′ and G″ significantly decrease by almost 52% and 39%, respectively. Likewise, samples' pressure treated for 3 and 5 min (Fig. 3(b), (c)) exhibit a decrease in their G′ and G″ values over time; however, the decrease in G′ is the most significant (pb 0.05) immediately after pressure treatment until day 7 of cold storage, after which it remains constant until the end of storage at 4 °C. The decrease in these parameters, especially in the G′ values, is due to the natural degradation of food produced by physical, chemical and microbiological factors, as reflected in the untreated and treated samples at 300 MPa for 1 min, which showed the largest reduction in G′. The tan (δ) ranged from 0.504 to 0.841 for the Aloe vera suspension stored at 4 °C for 50 days, which corresponds to δ values of 26.75° to 40.06°; this indicates that although viscoelastic behaviour was maintained during 50 days of storage, there was a loss of the elastic component, and thus the gel response neared viscous-like behaviour. 3.3. Cox–Merz rule Although initially developed for synthetic polymers, the Cox– Merz rule and/or its modified forms are applied to many liquid and semisolid foods (Yu & Gunasekaran, 2001). The Cox–Merz rule states that the apparent viscosity ηα = σ/γ at a specific shear rate (γ) is equal to the complex viscosity (η*) at a specific oscillatory frequency (ω), when γ = ω. It can be used to characterise the rheological

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properties of materials with network-like structures and can indicate the shear/strain sensitivity of the structure (Augusto, Falguera, Cristianini, & Ibarz, 2011). Experimental data show considerable deviation between log γ⋅ vs. logη and logw vs. logη⁎ for Aloe vera suspensions studied regardless of the pressure level and time treatment and that the apparent viscosity was greater than the complex viscosity. This result means that all of the Aloe vera suspensions studied did not obey the Cox–Merz rule (equal magnitudes of η and η⁎ for equal values of γ⋅ and w). However, although the Cox–Merz rule was not followed in all of the cases, the two lines were parallel to each other, suggesting a linear relationship between the complex viscosity and apparent viscosity. Due to this linear relationship, Aloe vera suspension has oscillatory viscoelastic properties that can be directly used to predict changes in perceived texture in the mouth (Augusto et al., 2011; Bistany & Kokini, 1983); Gunasekaran & Ak, 2000). The apparent viscosity of Aloe vera suspensions studied regardless of the pressure level and time treatment was greater than the complex viscosity. This result means that all of the Aloe vera suspensions studied did not obey the Cox–Merz rule (equal magnitudes of η and η⁎ for equal values ofγ⋅ and w). However, although the Cox–Merz rule was not followed in all of the cases, the two lines were parallel to each other, suggesting a linear relationship between the complex viscosity and apparent viscosity. Due to this linear relationship, Aloe vera gel has oscillatory viscoelastic properties that can be directly used to predict changes in perceived texture in the mouth (Bistany & Kokini, 1983).

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3.4. FT-IR second derivative of Aloe vera suspension The FTIR-ATR spectra of the Aloe vera suspension in the characteristic region of polysaccharides (1200–800 cm−1) are presented in Fig. 4(a), (b) and (c). No apparent difference was observed between pressure treatments at 300 and 500 MPa regardless of time and at 400 MPa for 1 and 5 min (Fig. 4(a), (b) and (c)) compared with the untreated Aloe Vera suspension. However, samples treated at 400 MPa for 3 min (Fig. 4(b)) presented noticeable changes (greater absorption) compared with the other treatments; this difference occurs as a result of the time and pressure levels that modify water properties. Cheftel (1992) affirms that pressure provokes a decrease in water volume varying from 4% at 100 MPa to 15% at 600 MPa at 22 °C. This reduction would imply a concentration of the polysaccharides present in the gel thereby increasing their absorption by these amounts in this region (1200–1000 cm−1), which is dominated by ring vibrations overlapped with stretching vibrations of (C–OH) side groups and the (C\O\C) glycosidic bond vibration (Kačuráková, Capek, Sasinková, Wellner, & Ebringerová, 2000) and is seen in the second derivative of the spectrum. However, on day 50 of storage at 4 °C (Fig. 5(a), (b) and (c)), no differences are observed between treatments, which show similar spectra. This result is due to the reduced content of polysaccharides caused by the natural degradation of the Aloe vera suspension, resulting in lower absorption in this band, as shown by the decrease of the solid component (G′) in Fig. 2(a), (b) and (c), leading to a decrease in the viscosity of these samples (Fig. 1(a), (b) and (c)). 4. Conclusions Aloe vera suspension exhibits a thixotropic response and the non-Newtonian behaviour of type shear-thinning. The Herschel– Bulkley model was found to be adequate to describe the rheological behaviour of the Aloe vera suspension regardless of the pressure treatment studied. The application of HHP at 300 MPa regardless of the time and at 400 MPa for 1 and 3 min exerted a clear influence (p b 0.05) on the yield stress (τ0), leading to an increase in the value of this rheological parameter. The consistency coefficient (K) and flow behaviour index (n) for samples treated at 300 MPa regardless of time and at 400 MPa for 1 min exhibited different responses; while the first treatment yields an increase in these parameters, the second resulted in a decrease (pb 0.05). Aloe vera suspension exhibited viscoelastic behaviour with G′ being greater than G″ in both untreated samples and in those treated directly after pressure treatment and during cold storage at 4 °C, regardless of the treatment applied. The magnitude of the G′ modulus decreases during storage at 4 °C more quickly than that of G″, which decreased rapidly on the third day and then remained constant during the 50-day storage at 4 °C. Aloe vera suspension did not obey the Cox–Merz rule. The effect of HHP on the rheological properties of Aloe vera suspension is dependent of the pressure– time treatment. Based on the above results, treatment at 400 MPa for 3 min could help to maintain the rheological properties of Aloe vera suspension during storage at 4 °C, making HHP a promising alternative. Acknowledgments The authors gratefully acknowledge the financial support of the FONDECYT program (project No 1090228). References Ahmed, J., & Ramaswamy, H. S. (2004). Effect of high-hydrostatic pressure and concentration on rheological characteristics of xanthan gum. Food Hidrocolloids, 18, 367–373. Ahmed, J., Ramaswamy, H. S., & Hiremath, N. (2005). The effect of high pressure treatment on rheological characteristics and colour of mango pulp. International Journal of Food Science and Technology, 40, 885–895. Augusto, P. E. D., Falguera, V., Cristianini, M., & Ibarz, A. (2011). Viscoelastic properties of tomato juice: Applicability of the Cox–Merz rule. Food Bioprocess Technology. http://dx.doi.org/10.1007/s11947-011-0655-y.

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