Steady Shear Rheological Behavior and Thixotropy of Low-Calorie Pistachio Butter

Abstract

Stressing the importance of diet in the prevention of certain diseases, nutritional scientists have emphasized on the reduction of calories in consumers’ diets. The steady-shear flow behavior of three optimized low-calorie pistachio butter formulations containing fat replacers (xanthan gum, Reihan seed gum, and Balangu seed gum) and sweeteners (sucrose and isomalt) were investigated at different temperatures. All three samples presented a non-Newtonian shear thinning flow behavior. Bingham, power law, and Moore models were selected as the appropriate time-independent rheological ones. Increasing the temperature resulted in lower values of Bingham viscosity, Bingham yield stress, and consistency coefficient. Increasing the amount of pistachio paste and isomalt led to higher and lower viscosity values, respectively. All data were reasonably fitted by Arrhenius model and formula containing xanthan gum showed the highest temperature dependency. The structural recovery evaluation revealed a weak thixotropic behavior for all samples.

Keywords:

• Reduced-fat
• Pistachio butter
• Rheology
• Modeling
• Hydrocolloid

INTRODUCTION

Low-calorie pistachio butter is a novel food product, which could be considered as an answer to the worldwide consumers’ demand for consuming healthy foods.^[1] In its original form this product contains pistachio, sweetener, emulsifier, and flavor improver;^[2] but in the low-calorie version, it also contains hydrocolloids as a stabilizer and fat replacer.^[1] In spite of all the important roles played by the fats to improve the taste, texture, and aroma of food products, the high calorie content of fat and its contribution to obesity and subsequent probable heart diseases have raised some anxieties among consumers and nourishing scientists.

Fat replacers are generally implied to all the bulking agents or ingredients that somehow replace fat in a system. Most of the fat replacers are carbohydrate-based. These ingredients are plant polysaccharides, which include cellulose, gums, dextrins, fiber, maltodextrins, starches, and polydextrose.^[3] From a physical view, the low-calorie pistachio butter is an O/W emulsion and stabilization of its formula is dependent on the viscosity of the continuous phase. Having the ability to control the viscosity of a solvent, hydrocolloids have frequently been used as an appropriate stabilizer in reduced-fat foods. Developing the structure of low-fat foods by entrapping the oil inside a cellulosic gel network is another novel technique in this issue.^[4] Xanthan gum (XG) is a versatile hydrocolloid that can be used as a fat replacer in low-fat products due to its ordered molecular structure.^[5] Lallemantia royleana (with vernacular name Balangu) and Ocimum basilicum (with vernacular name Reihan) are mucilaginous natural plants that grow in different regions of Iran. Recent rheological studies have approved that their mucilaginous extract can be used as novel food hydrocolloids in food formulations.^[6,7] The strong desire for the low-calorie food products has also led to the discovery of new sweetener substitutes, particularly for diabetics and prone-obesity people.^[8] Isomalt is a sugar alternative which enables the development of products with specific characteristics. Besides the friendly manner with dental and gastro environments, isomalt can reduce the calorie in food products.^[9] Mixing the sweeteners is a common activity in food formulations and can improve the functionality, acceptability and economics of each single sweetener.^[10]

Rheology is widely known as a useful physical property in understanding the flow behavior of materials and consequently designing food processing equipment, handling system, quality control, and sensory evaluation.^[6,8] The rheological properties of some reduced-fat products, such as yoghurt,^[11] mayonnaise,^[12] soft-serve ice cream,^[13] sesame paste/date syrup blends,^[14,15] and peanut paste^[3] have been investigated as a function of the type and levels of fat replacers. The effect of different sweetener substitutes on the rheological behavior of food systems have also been the subject of different studies.^[8,16–18]

To obtain more knowledge on the rheological behavior of low-calorie pistachio butter in processing operations and as a function of different fat replacers and sweeteners, a series of rheological experiments were recently conducted.^[1,19,20] Following the previous experiments, the present study provides more knowledge about the steady shear behavior and thixotropy of low-calorie pistachio butter. In this article, therefore, we aimed to: (i) study the steady shear rheological behavior of low-calorie pistachio butter as a novel food product; (ii) evaluate the effect of ingredients on the parameters of different rheological models and the capability of the Arrhenius model in describing the effect of temperature on the variation of consistency coefficient; and (iii) investigate the potential of samples to recover their structure after the shearing process.

MATERIALS AND METHODS

Preparation of Formulae

In the previous study, the formulation of low-calorie pistachio butter on the basis of three hydrocolloids as fat replacers (Reihan seed gum (RSG), Balangu seed gum (BSG), and xanthan gum (XG)) and two sweeteners (sucrose and isomalt) were investigated.^[21] On the basis of the conditions constructed using central composite design (CCD), 20 formulae were suggested for each fat replacer. The samples were all evaluated by sensory and steady shear rheological measurements and then optimized using MINITAB statistical software (version 13.2, MINITAB^TM, USA). The results of the optimization process are presented in Table 1.^[20] In the present study, complementary measurements were conducted on the three optimized formulations. The O’hadi pistachio was obtained from Kashmar city, Khorasan province in Iran. Isomalt was provided from Sim-Sim Com., Mashhad, Iran. Sucrose and vanilla were obtained from local markets. Potassium sorbate, lecithin, and xanthan gum were purchased from Merck (Germany), Acros (Belgium), and Sigma-Aldrich (Germany) companies, respectively. The formulations were prepared based on a previous study.^[1] BSG and RSG were extracted using the optimized methods proposed by other researchers.^[22,23]

TABLE 1 Optimized levels for preparation of 400 g low- calorie pistachio butter sample^[20]

		Amount of ingredients (g)
Formula	Gum type	Pistachio paste	Water	Fat replacer	Sucrose	Isomalt
BO1	RSG	102.629	153.944	0.092	102.629	34.186
BO2	BSG	104.892	157.337	0.136	26.223	104.892
BO3	XG	97.952	146.927	0.400	69.213	79.018

Note: BO1, BO2, and BO3 represent formulae containing Reihan seed gum, Balangu seed gum, and xanthan gum as fat replacers, respectively.

Rheological Measurement

After the preparation process, the samples were kept at 4°C for 24 h. Rheological measurements at different temperatures (5, 25, 45, and 65°C) were performed using a controlled stress Bohlin CVO rheometer (Bohlin Instruments Inc., UK) equipped with a 15-mm parallel plate measuring system and 1.5-mm gap. Specimens were loaded onto the geometry using a spatula and the plate was slowly moved down to the specified measuring position. The excess specimen was completely trimmed off using the spatula. The exposed specimen was coated with silicone oil to prevent the samples from dehydration. Rheological properties were described in terms of viscosity versus shear rate in the shear rate range between 0.1–300 s⁻¹. Each time a new sample was used for rheological measurements and all the measurements were conducted at least in duplicate. To quantify the thixotropic recovery rate of samples, a 3-step thixotropy test was conducted. The samples were subjected to an initial low-shear conditioning step (1 s⁻¹), then a high-shear structure-breakdown step (100 s⁻¹), and finally a recovery step at the original 1 s⁻¹ shear rate. Each step took 1200 s. The proportion of viscosity recovered at the end of the final step was then calculated as an indication of the thixotropic behavior.

Rheological Modeling

The flow curves of low-calorie pistachio butter samples were described by Power law (or Ostwald-Waele), Bingham, Herschel-Bulkley, Sisko, and Moore models.^[24] The Power law model (Eq. 1) gives a good description of fluid flow behavior in the shear rate range available by most rheological instruments; therefore, it has been extremely used in studies on food operations:

(1)

where (Pa.s) is apparent viscosity, (s⁻¹) is shear rate, k (Pa.sⁿ) is consistency coefficient, and n (dimensionless) is the flow behavior index. However, one drawback of this model comes with its poor fitting for data obtained at a wide range of shear rate.^[25] By including the yield term in the Power law model, the Herschel-Bulckly model is obtained (Eq. 2) as follows:

(2)

Where, is yield stress (Pa), k is consistency index (Pa.s), and n is the dimensionless flow behavior index.

As a special case of Herschel-Bulkley model, the Bingham model was applied on the experimental data as well (Eq. 3):

(3)

Where is Bingham yield stress (Pa) and is Bingham viscosity (Pa.s). In the case of very shear-thinning fluids and for covering the high shear rates, the Sisko model (Eq. 4) and Moore model (Eq. 5) can be used as follows:

(4)

where (Pa.sⁿ)and n_s are the consistency index and the flow behavior index of the Sisko model, respectively, and (Pa.s) stands for the so-called high shear rate limiting viscosity.

(5)

Where, and are zero shear rate viscosity and relaxation time, respectively. An Arrhenius-type model was used to evaluate the temperature dependency of viscosity of samples:

(6)

where is the proportionality constantm, is the activation energy (kJ/mol), R is the universal gas constant (kJ/mol K), and T is absolute temperature (K).

Statistical Analysis

The statistical analysis of the experimental data was performed using SigmaPlot software (Version 12.0). Drawing the graphs and fitting the models were performed using Excel 2007 and MATLAB softwares (version 7.6.0.324, USA), respectively. To evaluate the goodness of fit, the R² and RMSE statistics were used. The RMSE values <10%, 10–20%, 20–30%, and >30% represents the excellent, good, fair, and poor fitting ability, respectively.

RESULTS AND DISCUSSION

Flow Behavior Characterization

Figure 1 shows a representative rheogram of a low-calorie pistachio butter sample containing RSG at different temperatures. It is obvious that the sample showed a curvature downwards on the shear rate axis, i.e., a non-Newtonian shear thinning flow behavior, at all temperatures. A similar trend was observed for two other formulations (figures not shown). Increasing the shear rate makes the hydrodynamic forces become dominant and disrupt the flocks and consequently reduces viscosity.^[26] In an earlier study on different formulations of low-calorie pistachio butter, similar behavior has been reported.^[19] Other researchers have also reported the shear thinning behavior of sesame butter at different conditions of study.^[14,27−29] This pseudoplastic behavior has also been reported for commercial pistachio butter.^[30] The high-shear rate Newtonian plateau was clearly observed for all temperatures; however, the low-shear rate one was not covered by the shear rate range studied (Fig. 1). Similar trends were observed for the two other formulae as well. A representative rheogram including the forward and backward curves has been shown in Fig. 2.

TABLE 2 The power law, Bingham, and Moore model parameters for low-calorie pistachio butter formulae containing Reihan seed gum (BO1), Balangu seed gum (BO2), and xanthan gum (BO3) as fat replacers

Model	Formulae	Temp. (°C)	K (Pa.sn)	n (–)	τ0 (Pa)	R^2	RMSE
Power law	BO1	5	7.145 ± 0.332	0.362 ± 0.083	—	0.994	0.316–0.415
		25	6.457 ± 0.223	0.402 ± 0.189	—	0.989–0.992	0.583–0.594
		45	4.952 ± 0.112	0.337 ± 0.103	—	0.995	0.510–0.512
		65	4.012 ± 0.210	0.315 ± 0.079	—	0.994–0.995	0.510–0.520
	BO2	5	6.347 ± 0.151	0.315 ± 0.124	—	0.992–0.995	0.549–0.566
		25	5.064 ± 0.196	0.387 ± 0.105	—	0.995	0.513–0.515
		45	3.241 ± 0.097	0.313 ± 0.086	—	0.995–0.998	0.298–0.312
		65	2.863 ± 0.099	0.308 ± 0.080	—	0.996–0.997	0.417
	BO3	5	5.255 ± 0.268	0.358 ± 0.087	—	0.996–0.998	0.405–0.412
		25	4.416 ± 0.326	0.352 ± 0.089	—	0.998	0.302–0.314
		45	2.867 ± 0.045	0.331 ± 0.085	—	0.994–0.996	0.310–0.501
		65	2.156 ± 0.085	0.320 ± 0.352	—	0.996	0.432
Bingham	BO1	5	1.431 ± 0.052	—	3.874 ± 0.058	0.949–0.952	1.117–1.275
		25	1.248 ± 0.006	—	2.680 ± 0.100	0.926	1.196–1.299
		45	0.751 ± 0.021	—	2.014 ± 0.058	0.939–0.945	0.703–0.729
		65	0.662 ± 0.074	—	1.889 ± 0.098	0.947	0.719
	BO2	5	1.289 ± 0.033	—	3.105 ± 0.109	0.946–0.948	1.122
		25	1.248 ± 0.041	—	3.105 ± 0.109	0.946	1.122
		45	0.687 ± 0.068	—	1.620 ± 0.021	0.927–0.933	1.135–1.149
		65	0.640 ± 0.062	—	1.368 ± 0.085	0.929–0.937	1.109–1.121
	BO3	5	1.216 ± 0.009	—	2.602 ± 0.005	0.954–0.961	1.006–1.208
		25	1.216 ± 0.009	—	2.021 ± 0.67	0.930–0.935	1.131–1.310
		45	0.591 ± 0.009	—	1.486 ± 0.103	0.922–0.929	1.186–1.199
		65	0.456 ± 0.038	—	1.156 ± 0.006	0.944–0.953	0.998–1.011
			η0 (Pa.s)	η∞ (Pa.s)	τ (s)
Moore	BO1	5	50.420 ± 1.263	0.651 ± 0.058	8.430 ± 0.058	0.990–0.993	0.452–0.613
		25	36.510 ± 0.985	0.577 ± 0.019	7.650 ± 0.061	0.996–0.997	0.280–0.308
		45	30.431 ± 1.689	0.412 ± 0.026	5.880 ± 0.021	0.998	0.136–0.206
		65	26.532 ± 1.003	0.330 ± 0.044	5.001 ± 0.023	0.997–0.999	0.115–0.158
	BO2	5	40.155 ± 0.899	0.596 ± 0.062	5.533 ± 0.021	0.991	0.476
		25	23.735 ± 1.026	0.534 ± 0.008	4.011 ± 0.005	0.996–0.997	0.245–0.330
		45	18.342 ± 1.056	0.349 ± 0.018	3.541 ± 0.010	0.997	0.125–0.226
		65	13.865 ± 0.865	0.296 ± 0.035	2.245 ± 0.002	0.993–0.995	0.269–0.306
	BO3	5	28.845 ± 0.786	0.504 ± 0.002	4.941 ± 0.005	0.991–0.995	0.353–0.591
		25	18.390 ± 0.896	0.475 ± 0.045	3.803 ± 0.003	0.994–0.998	0.314–0.413
		45	14.585 ± 0.984	0.285 ± 0.009	3.062 ± 0.002	0.996	0.156–0.166
		65	10.823 ± 0.090	0.216 ± 0.006	1.826 ± 0.004	0.998–0.999	0.110–0.216

TABLE 3 The Hersche-Bulkley and Sisko model parameters for low-calorie pistachio butter formulae containing Reihan seed gum (BO1), Balangu seed gum (BO2), and xanthan gum (BO3) as fat replacers

Model	Formulae	Temp. (°C)	K (Pa.s^n)	n (–)	τ0 (Pa)	η∞ (Pa.s)	R^2	RMSE
Herschel Bulkley	BO1	5	15.464 ± 0.288	0.517 ± 0.004	45.730 ± 0.437	—	0.923–0.924	1.165–1.181
		25	13.990 ± 0.896	0.550 ± 0.091	40.870 ± 0.927	—	0.993–0.999	0.267–0.288
		45	11.384 ± 0.753	0.620 ± 0.021	37.164 ± 0.401	—	0.992–0.993	0.328–0.411
		65	10.309 ± 0.363	0.773 ± 0.139	35.504 ± 0.163	—	0.993	0.499–0.534
	BO2	5	13.321 ± 0.723	0.539 ± 0.881	38.237 ± 0.575	—	0.998	0.143–0.200
		25	12.278 ± 0.084	0.519 ± 0.599	33.219 ± 0.314	—	0.999	0.217
		45	11.056 ± 0.991	0.691 ± 0.924	31.730 ± 0.437	—	0.989–0.991	0.634–0.658
		65	11.238 ± 0.098	0.701 ± 0.261	28.071 ± 0.663	—	0.991–0.993	0.453–0.459
	BO3	5	12.397 ± 0.975	0.464 ± 0.835	27.594 ± 0.096	—	0.999	0.288
		25	11.601 ± 0.343	0.527 ± 0.172	21.124 ± 0.306	—	0.998–0.999	0.128–0.145
		45	10.876 ± 0.043	0.654 ± 0.003	15.986 ± 0.005	—	0.986–0.989	0.618–0.623
		65	10.054 ± 0.007	0.682 ± 0.002	−0.774 ± 0.365	—	0.957–0.962	1.019–1.100
Sisko	BO1	5	6.145 ± 0.397	0.464 ± 0.595	—	0.344 ± 0.819	0.975–0.992	0.878–0.999
		25	5.005 ± 0.508	0.490 ± 0.901	—	−0.354 ± 0.187	0.981	0.585–0.599
		45	3.250 ± 0.968	0.416 ± 0.376	—	−0.262 ± 0.126	0.994	0.429
		65	3.004 ± 0.163	0.985 ± 0.234	—	−0.532 ± 0.225	0.917–0.920	1.287–1.301
	BO2	5	5.316 ± 0.788	0.397 ± 0.261	—	0.456 ± 0.362	0.979–0.991	0.866–0.878
		25	5.874 ± 0.591	0.475 ± 0.924	—	−0.545 ± 0.576	0.983	0.754–0.768
		45	3.430 ± 0.363	0.380 ± 0.365	—	−0.708 ± 0.396	0.995–0.998	0.133–0.147
		65	3.336 ± 0.949	0.216 ± 0.546	—	−0.804 ± 0.163	0.996	0.146
	BO3	5	5.288 ± 0.009	0.384 ± 0.008	—	−4.121 ± 0.869	0.998	0.133
		25	4.631 ± 0.595	0.371 ± 0.579	—	−3.590 ± 0.912	0.996–0.997	0.140–0.148
		45	3.358 ± 0.005	0.326 ± 0.002	—	−3.765 ± 0.003	0.967–0.973	0.756–0.777
		65	2.857 ± 0.043	0.285 ± 0.043	—	−2.876 ± 0.387	0.976	0.862

FIGURE 2 A typical forward (----) and downward (-- --) shear stress-shear rate flow curve for the formula prepared using Reihan seed gum at 25°C.

FIGURE 1 A typical shear stress-shear rate flow curve for the formula prepared using Reihan seed gum (□: 5°C; ▵: 25°C; ○: 45°C; ×: 65°C).

Evaluation of Rheological Models

Modeling provides a useful method for representing a large amount of rheological data in the form of a simple mathematical expression.^[24] An issue that has attracted considerable attention is the concept to choose the best model. It has been stated that there is no master model suitable for all different data. Choosing the best model to relate product viscosity to shear rate primarily depends on the intended application and use of a suitable instrument to determine the model parameters.^[25] Considering the low-shear rates in the case of storage stability and high shear rates for pumping situations are some examples in this case. To determine the most appropriate model in describing the flow behavior of samples over the studied shear rates and temperatures, the Power law, Bingham, Herschel-Bulkley, Moore, and Sisko models were evaluated (Table 2). Although the R² values for all models were close to one, the negative yield stress and infinite viscosity values in Herschel-Bulkley and Sisko models made these models inappropriate (Table 3). The appropriate mathematical models were Power law, Bingham, and Moore based on the R², RMSE values, and the existence of a yield stress. The flow behavior of full-fat pistachio butter revealed that the Herschel-Bulkley model was not an appropriate option for data obtained at 45 and 65°C.^[30] Bingham and Casson models have been previously applied in description of the rheological behavior of full-fat and reduced-fat peanut butters.^[16,31] The flow behavior of low-fat sesame paste was previously described by power law model.^[14]

The flow behavior indices of the Power law model confirm the pseudo-plastic behavior of three samples (n = 0.308–0.402). No definite trend was observed in the flow behavior indices with changing temperature (Table 2). The lowest k value was obtained for samples prepared by xanthan gum (Table 2). It is probably due to the low amount of pistachio paste presented in these formulae. The parameters obtained for the Bingham model at different temperatures and for three studied formulae are given in Table 2. In general, lower values of Bingham viscosity and Bingham yield stress were observed with increasing temperature. The viscosity showed 62.5, 50.3, and 53.7% reduction by increasing the temperature from 5 to 65°C for samples prepared using RSG, BSG, and XG, respectively. The samples containing RSG and XG showed the highest and lowest Bingham viscosities, respectively. Yield stress is an important factor in determining the limit of plug flow through the laminar flow in tubes. It is also important in determining the pumping requirements of fluids in pipes.^[8]

Low values of Bingham yield stress can make the stability of product emulsion questionable. There are, however, some positive points with yield stress in processing (e.g., pumping or mixing) and sensory characteristics (e.g., spread ability). This observation is the reason that both Power law and Bingham models are applicable mathematical models. The highest Bingham yield stress value was observed for samples containing RSG. It seems that the structure produced by this hydrocolloid was stronger than BSG and XG ones (Table 2). The role of high pistachio butter and, therefore, low free water should also be considered. Both limiting low shear rate and high shear rate viscosities of the Moore model show similar trends for all samples at different temperatures. Increasing the temperature resulted in lower viscosities for three samples (Table 2). The highest and lowest viscosity values were obtained for samples prepared using RSG, as it was expected (Table 2). The relaxation time parameter (τ) in Moore model is an indication of the rate of structure break down. The higher the τ value, the faster rate of structure breaks down. The samples prepared using RSG showed the highest relaxation time values (Table 2). It means that in comparison with two other formulae, the zero shear viscosity degrades at a much faster rate to approach the infinite shear viscosity.

Effect of Ingredients on the Rheological Parameters

By lowering the fat content of the spreads, the maintenance of the original water-in-oil emulsion faces some difficulties in stability.^[5] Hydrocolloids are of the most important ingredients that are expected to renew the stability and physical characteristics of low-fat spreads. It is probably due to their ability in developing intermolecular associations and improving textural characteristics. The highest value of consistency coefficient, Bingham viscosity, zero and infinite shear viscosities were obtained for formulae containing RSG, whereas their lowest values were measured for samples prepared using xanthan gum (Tables 2). It was previously reported that the viscosity of RSG is higher than Konjac, xanthan, and guar gums.^[6] BSG is a new natural hydrocolloid with excellent functional properties. Recent studies have also revealed its high viscosity forming capability.^[7] The type of fat replacer did not have a distinct effect on the flow behavior indices (Table 2). A similar trend was previously reported for low-fat sesame butter.^[14]

Pistachio is a source of fiber and cellulose, and these components can easily absorb water. It is therefore the low amount of pistachio paste that has made the k-value and viscosity values to be the lowest for samples containing xanthan gum as a fat replacer. Studying the rheological behavior of sesame paste, researchers reported that the higher levels of unhulled roasted sesame would result in an increase in the consistency coefficient of the product.^[29] They stated that the interfacial film formation caused from higher solid content led to higher k-value. A formula prepared using BSG has the largest amount of isomalt (Table 1). Isomalt does not absorb a noticeable amount of moisture.^[9] It seems that the high amount of isomalt has resulted in more fluidity of samples containing BSG.

Effect of Temperature

The logarithm of the apparent viscosity of pistachio butter samples versus the reciprocal of absolute temperature were evaluated at different shear rates (49.6, 155.6, and 299.2 s⁻¹). Straight lines with high R² values were obtained for all three samples, which indicate that the Arrhenius model has fit the data properly (Table 4). According to the activation energy values, the temperature dependency of viscosity for all samples was increased by increasing the shear rate from 49.6 to 299.2 s⁻¹. The high activation energy content indicates the large effect of temperature on viscosity.^[30] The highest activation energy was observed for a formula containing xanthan gum. It seems that its high water content has made the sample be more temperature sensitive. The reciprocal effect of fat content on the E_a values in a coconut milk sample has been reported before.^[32] Studying the rheological characteristics of commercial pistachio butter showed the high temperature dependency of the product.^[30]

TABLE 4 Arrhenius’s model parameters for the effect of temperature on apparent viscosity at 49.6, 155.6, and 299.2 s⁻¹ shear rates

Formulae	Shear rate (1/s)	Ea (kJ/mole)	R^2
BO1	49.6	1.319 ± 0.068	0.968
	155.6	2.788 ± 0.198	0.993
	299.2	3.719 ± 0.125	0.995
BO2	49.6	1.377 ± 0.056	0.987
	155.6	3.147 ± 0.099	0.989
	299.2	3.739 ± 0.169	0.971
BO3	49.6	1.678 ± 0.009	0.974
	155.6	3.030 ± 0.187	0.907
	299.2	4.900 ± 0.196	0.936

Note: BO1, BO2, and BO3 represent formulae containing Reihan seed gum, Balangu seed gum, and xanthan gum as fat replacers, respectively.

Thixotropy

All three formulae exhibited a short, rapid viscosity build up to a highly-structured state after shearing (Fig. 3). The proportion of recovered viscosity was 90.5, 78.9, and 82.3% for RSG, BSG, and XG, respectively. The samples containing Reihan seed gum and xanthan gum showed the highest and the lowest rate of structural break down (Fig. 3). It could previously be predicted by the values obtained for relaxation time parameters in the Moore model. Generally, all samples had slight thixotropic behaviors.

FIGURE 3 The structural recovery after shearing test for the formulae prepared using: (A) xanthan gum, (B) Balangu seed gum, and (C) Reihan seed gum.

CONCLUSION

The three optimized samples showed non-Newtonian shear thinning behavior. Bingham, Power law, and Moore models were the appropriate models for fitting the rheological data of low-calorie pistachio butter. Increasing the temperature resulted in lower values of all parameters representing viscosity and also consistency. The temperature dependency of all samples could be properly described by the Arrhenius mathematical model. The structure recovery evaluation presented a weak thixotropic behavior for all samples. For all low-calorie pistachio formulae, endemic fat replacers could provide higher viscosities in comparison to xanthan gum. This fact was predicted by all the evaluated rheological models. Developing the shear rate range—especially the lower limit—would shed more light on the rheological characteristics of this novel product. Evaluation of yield stress value could be useful for the prediction of emulsion stability characteristics.

Acknowledgment

The authors gratefully acknowledge the Food Rheology Laboratory of the Institute of Food Science, Department of Food Sciences and Technology, Boku University (Vienna, Austria) for providing the experimental facilities used in this work.