Rheological Characterization of Low Fat Sesame Paste Blended With Date Syrup

Abstract

In this paper, the time-dependent flow behavior of reduced fat sesame paste with date syrup were assessed using Brookfield rotational viscometer as a function of fat substitutes (guar gum, xanthan, and starch) at three levels and shearing time. Two models were used to predict the time-dependent behavior, known as, first-order stress decay with a zero equilibrium stress and Weltman models. It was found that time-dependent behavior of samples was adequately fitted with these models. All blends exhibited the thixotropic behavior and thixotropy of samples increased with increasing shear rate. For Weltman model, the A and B constants appeared in the range of 189.2–730.6 Pa and 0.090–1.571 Pa, respectively. While for first-order stress decay model, the initial shear stress (τ₀) was in the range of 197–794 Pa, and the breakdown rate constant (K) varied in the range of 0.002–0.018 s⁻¹. The presence of fat substitutes generally resulted in an increase of viscosity and thixotropy of blends in comparison with the blank.

Keywords:

• Rheology
• Weltman model
• Kinetic model
• Starch
• Xanthan
• Guar

INTRODUCTION

Sesame paste, namely Tahin or Tahineh in Turkey and Ardeh in Iran, is a local food in the Middle East and other East Asian countries, which is made from ground, de-hulled and roasted sesame seeds without adding or removing any of its constitutes. Halwa-Ardeh (semi-solid paste) is also a traditional food in Iran, which is produced by blending the sesame paste with a proper sweetener such as honey, grape juice concentrate (pekmaz), mulberry juice concentrate (pekmaz), date syrup, and sugar beet molasses. Sesame paste is of a high nutritive value; it is rich in lipids (54–65 kg/100 kg); proteins (17–27 kg/100 kg); carbohydrates (6.4–21 kg/100 kg); dietary fiber (9.3 kg/100 kg); niacin (4.5 mg/100 g); thiamin (1.08 mg/100 g) and some minerals such as: calcium (100 mg/100 g); iron (9 mg/100 g); and phosphorous (807–840 mg/100 g).[1–4] Date is a natural source of energy due to its high sugar content nearly to 85% of total solids. Date syrup as a sweetener also contains valuable nutrients. Averaged chemical composition per 100 kg sample of concentrated date juice (°Brix of 82) was reported as follow: moisture content, 16.5 kg; proteins, 1.45 kg; glucose, 38.2 kg; fructose, 39.4 kg and ash, 1.6 kg.[5]

As long as health professionals continue stressing the importance of diet in the prevention of certain diseases, consumers will look for ways to reduce fat/calorie in their diets.[6] A survey conducted in 1993, by CCC, found that 136 million American adults consume low fat or fat free products, and that more than 6 in 10 consumers want more reduced-fat choices available. As a result, more than 1,000 new low-fat products have been introduced annually since 1990.[7]

Hydrocolloids (e.g., xanthan, carrageenan, pectin, guar, starch, and gelatin) are generally used in food applications due to their functional properties.[8] For several years, fat substitutes have been used to partially or fully replace fat. These have generally originated from hydrocolloids and categorized into three main groups including carbohydrate-based fat substitutes, protein-based fat substitutes and lipid-based fat substitutes.[9] The carbohydrate-based fat substitutes are derived from cereals, grains, and plants that include digestible and non-digestible carbohydrates (Starch derivatives, gums). They provide some of the functions of fat by binding water, and providing consistency/texture, mouth coating, and opacity.[10] Fat reduction and replacing it with a proper fat substitute affect on physico-chemical properties and sensory characteristics of product. Consumers usually prefer a product with high quality similar to reference product. Therefore, it is very important to investigate the best condition of fat substitute used in product formulation. It seems that tahineh or sesame paste is a water-in-oil emulsion, but sesame paste products (such as tahin/pekmaz blend and tahin/date syrup blend) can be regarded as an oil-in-water emulsion. Tahin contains the oil phase (dispersed phase) and fruit juice concentrate contains the water phase (continuous phase). Consumer acceptance of butter-like products is highly dependent on spreadability on another material such as bread, which requires information about viscosity and flow behavior of product.[11]

The literature review has been showed that there is limited number of scientific papers about the rheological properties of sesame paste products,[2,4,11] however, much research works has been published for sweeteners,[12–14] and food hydrocolloids.[15–17] There is negligible information available about the rheological properties of low fat sesame paste/date syrup blends, with the exception of Razavi, Habibi Najafi & Alaee work that had been studied the time-independent rheological behavior of sesame paste/date syrup blends as a function of fat substitutes and temperature.[18] While the viscosity of hydrocolloids and food emulsion systems can be significantly affected by variables such as shear rate, temperature, concentration, and time of shearing. Therefore, the aims of this work were[1] to investigate the effect of different fat substitutes (starch, xanthan gum and guar gum) at three concentration levels on the time-dependent rheological properties of reduced fat sesame paste/date syrup blend as a new low fat product; and[2] to evaluate the time dependency of reduced fat sesame paste/date syrup under shearing conditions using two models; first order stress decay model, with a zero equilibrium stress value and Weltman model.

MATERIALS AND METHODS

Materials

Commercial sesame paste (Ardeh) samples were obtained from Simorgh Company, Mashhad, Iran. The averaged chemical compositions of sesame paste given by the supplier were: proteins, 24 kg/100 kg; oil, 60 kg/100 kg and carbohydrates, 8 kg/100 kg sample. Date syrup (°Brix of 60 and pH = 4.67) was prepared by pilot plant vacuum evaporation of data juice at the Department of Food Science & Technology, Ferdowsi University of Mashhad, Iran.

A blank sample was prepared by mixing sesame paste and date syrup in the proportion of 50:50 wt/wt. In order to be labeled fat modified, products must contain at least 25% less fat than a reference product.[19] Therefore, the fat content of the blank sample was reduced by 25% wt. Low-fat sesame paste/date syrup blends were then produced by partially replacing fat with fat substitutes. The fat substitutes used in this study were guar gum (Higum, 4500F, Rhodia), xanthan gum (Rhodigel Ulter, Rhodia) and starch (art: 1252. Merck Com.). Based on primary experiments in this research, three levels of concentration for each fat substitute were employed: xanthan gum (0.01, 0.015, and 0.02%wt), guar gum (0.1, 0.15, and 0.2%wt), and starch (0.75, 1.25, and 1.75%wt).

Rheological Measurements

The flow behavior of samples were measured at 45 and 55°C using the Brookfield rotational viscometer (Model RVDV-II, Brookfield Engineering Inc., USA) equipped with spindle model RV7 at the speed range of 1–80 rpm. Samples were sheared at an increasing-decreasing (upward-downward) order of spindle speed (N). Three readings for apparent viscosity (η_a) were taken per sample at 30 s intervals. Shear rate, , was calculated based on the manufacturer's instruction as follow:

(1)

Then shear stress, τ (Pa), obtained using the equation:

(2)

The time-dependent rheological properties were investigated by shearing samples at three constant shear rates (20, 40, and 60 rpm) for a period of 10 min. Then the apparent viscosity was measured as a function of step changes in shearing time from 1 to 10 min. Then the time dependency of apparent viscosity of samples was evaluated by fitting the experimental data with two models:

1. Weltman model:[20]

(3)

where, t is the shearing time (min), A & B are constant parameters, which characterize the time dependent behavior.

2. First order stress decay model, with a zero equilibrium stress value (kinetic model)[2]:

(4)

where, τ₀ is the initial shear stress value and K is the breakdown rate constant.

RESULTS AND DISCUSSION

Flow Behavior

The shear stress-shear rate relationship of fat sesame paste/date syrup blend for all fat replacers, fat substitution levels, and temperatures (25, 3, 45, and 55°C) reported non-linear, indicating that the sesame paste/date syrup blends behave as a non-Newtonian fluid.[18] The flow curve (shear stress-shear rate rheogram) of reduced fat sesame paste/date syrup blend contain 0.01% substitution of Xanthan gum at two temperatures (45 and 55°C) both in the upward (increasing shear rate) and downward (decreasing shear rate) directions is shown in Figure 1. As can be seen, the presence of hysteresis loops is an indication of shearing history effect (time-dependent behavior) on the structure of blends. The results of this research showed the presence of hysteresis loops similar to Fig. 1 for all cases studied. It can be also found that the downward values of shear stress are lower than upward values, such that the apparent viscosity has decreased in downward pattern. In other words, there is an irreversible, shear induced, permanent deformation (thixotropic or structural decay behavior) affecting the molecular structure of blends structure. The flow behavior of sesame paste/date syrup blends is in agreement with findings of Abu-Jdayil et al.[2] that investigated the flow behavior of sesame paste (tahineh) in the forward and backward direction and reported the presence a non-linear relationship and hysteresis loop for all conditions studied, indicating a thixotropic behavior for tahineh.[2]

Figure 1 Hysteresis loop of the flow curves of low fat sesame paste/date syrup blend at different temperatures for fat substitution with 0.01% xanthan gum.

Time-dependent Properties

Time dependency of the apparent viscosity of sesame paste/date syrup blends was assessed by applying the two models (Weltman model and first order stress decay model) for a 10-min period and at different values of constant shearing rate (20, 40 and, 60 rpm). The parameters obtained by fitting the shear stress (τ)–time data are given in Tables 1 to 6. A zero value of B (the coefficient of breakdown) in Weltman model and K in first order stress decay model indicates the apparent viscosity of the fluid is independent of time. The results of this research showed that the values of B and K as a function of shear rate, fat replacer and fat substitution level is non-zero and for both models, the shear stress decreases with increasing time of shearing (1–6), which means that sesame paste/date syrup blends exhibit a thixotropic behavior. Time dependent flow properties have been also reported by other workers.[21–23]

Table 1 The parameters of the Weltman model for a shearing period of 10 min evaluated as a function of starch substitution at 25°C ∗.

	Starch %
	0			0.75			1.25			1.75
Shear rate (s^−1)	A	−B	R^2	A	−B	R^2	A	−B	R^2	A	−B	R^2
4.18	189.2	0.278	0.75	290.5	0.253	0.73	316.3	0.539	0.73	341.1	0.495	0.64
8.36	254.0	0.462	0.89	466.4	0.641	0.79	492.6	0.871	0.72	556.7	0.944	0.73
12.54	316.3	0.824	0.91	608.9	0.824	0.78	616.6	1.571	0.79	730.6	1.214	0.70
∗A (Pa) and B (Pa) parameters were obtained by fitting experimental data to Weltman model (τ = A + B In t).

Table 2 The parameters of the Weltman model for a shearing period of 10 min evaluated as a function of xanthan gum substitution at 25°C ^∗.

		Xanthan gum %
		0			0.01			0.015			0.02
Shear rate (s^−1)	rpm	A	−B	R^2	A	−B	R^2	A	−B	R^2	A	−B	R^2
4.18	20	189.2	0.278	0.75	290.1	0.154	0.71	289.3	0.330	0.98	348.6	0.143	0.88
8.36	40	254.0	0.462	0.89	426.3	0.228	0.79	476.9	0.545	0.85	530.25	0.259	0.72
12.54	60	316.3	0.824	0.91	554.2	0.243	0.60	600.8	0.845	0.95	648.5	0.385	0.81
∗A (Pa) and B (Pa) parameters were obtained by fitting experimental data to Weltman model (τ = A + B In t).

Table 3 The parameters of the Weltman model for a shearing period of 10 min evaluated as a function of guar gum substitution at 25°C ^∗.

		Guar gum %
		0			0.01			0.015			0.02
Shear rate (s^−1)	rpm	A	R^2	−B	A	−B	R^2	A	−B	R^2	A	−B	R^2
4.18	20	189.2	0.75	0.278	210.0	0.090	0.72	380.7	0.295	0.64	365.3	0.354	0.86
8.36	40	254.0	0.89	0.462	489.0	0.306	0.92	529.2	0.480	0.98	518.2	1.210	0.85
12.54	60	316.3	0.91	0.824	597.1	0.394	0.98	665.9	0.749	0.99	—	—	—
∗A (Pa) and B (Pa) parameters were obtained by fitting experimental data to Weltman model (τ = A + B In t).

Table 4 The parameters of the First-order stress decay model, with a zero equilibrium stress value, for a shearing period of 10 min evaluated as a function of starch substitution at 25°C ^∗.

		Starch %
		0			0.75			1.25			1.75
Shear rate (s^−1)	rpm	τ0	K	R^2	τ0	K	R^2	τ0	K	R^2	τ0	K	R^2
4.18	20	197.0	0.007	0.81	304.9	0.008	0.70	342.0	0.014	0.77	368.2	0.014	0.79
8.36	40	265.8	0.008	0.73	500.7	0.010	0.69	534.7	0.014	0.79	597.6	0.014	0.73
12.54	60	330.8	0.008	0.70	654.0	0.012	0.71	648.2	0.018	0.75	794.0	0.015	0.77
∗τ0 (Pa) and K (s^−1) parameters were obtained by fitting experimental data to First-order stress decay model (τ = τ0 e ^−Kt).

Table 5 The parameters of the First-order stress decay model, with a zero equilibrium stress value, for a shearing period of 10 min evaluated as a function of xanthan gum substitution at 25°C ^∗.

		Xanthan gum %
		0			0.01			0.015			0.02
Shear rate (s^−1)	rpm	τ0	K	R^2	τ0	K	R^2	τ0	K	R^2	τ0	K	R^2
4.18	20	197.0	0.007	0.81	298.1	0.005	0.68	438.5	0.005	0.72	353.8	0.002	0.40
8.36	40	265.8	0.008	0.73	438.5	0.005	0.72	494.2	0.006	0.53	538.9	0.002	0.25
12.54	60	330.8	0.008	0.70	570.3	0.005	0.81	623.9	0.006	0.42	665.2	0.004	0.51
∗τ0 (Pa) and K (s^−1) parameters were obtained by fitting experimental data to First-order stress decay model (τ = τ0 e ^−Kt).

Table 6 The parameters of the First-order stress decay model, with a zero equilibrium stress value, for a shearing period of 10 min evaluated as a function of guar gum substitution at 25°C ^∗.

		Guar gum %
		0			0.1			0.15			0.2
Shear rate (s^−1)	rpm	τ0	K	R^2	τ0	K	R^2	τ0	K	R^2	τ0	K	R^2
4.18	20	197.0	0.007	0.81	212.1	0.002	0.69	386.5	0.002	0.31	396.8	0.008	0.74
8.36	40	265.8	0.008	0.73	495.4	0.002	0.48	538.8	0.003	0.37	549.7	0.010	0.61
12.54	60	330.8	0.008	0.70	606.4	0.003	0.58	680.4	0.003	0.34	—	—	—
∗τ0 (Pa) and K (s^−1) parameters were obtained by fitting experimental data to First-order stress decay model (τ = τ0 e ^−Kt).

The extent of thixotropy embodied by the magnitude of B or K, which for each fat substitutes, increased with increasing shear rate, at a fixed fat substitution level, but it varied differently with increasing fat replacer concentration, at a fixed shearing rate. Our finding that B or K increases with increasing shear rate is in agreement with that of Abu-Jdayil et al.[2] The larger B or K values the greater the departure from thixotropy behavior. Therefore, the samples containing starch were more thixotropic than blank sample and other low fat blends containing xanthan, and guar gums at all respected shear rates.

The value of breakdown coefficients, B and K, ranged from 0.090 to 1.571 Pa and between 0.002–0.018 s⁻¹, respectively for the Weltman and first-order stress decay models. While the initial shear stress values (A and τ₀) for these models, varied in the range of 189.2–730.6 Pa, and 197.0–794.0 Pa, respectively. Substitution of fat in the sesame paste/date syrup blends with each of fat replacer, and increasing in concentration of each fat substitute led to higher A and τ₀ values at all shear rates tested. Therefore, it can be concluded that the fat substitution with each fat replacer resulted in higher viscosity in comparison with the blank sample. It can be also found that the effect of starch on the consistency of sesame paste/date syrup blends was greater than other fat substitutes.

Time dependent shear stress profile of low fat sesame paste/date syrup blends containing 1.25% starch, 0.02% xanthan and 0.15% guar gum are shown in Figs. 2 to 4, respectively. It can be seen that the shear stress decreased with shearing time and shear stress-time profile almost fitted to both the Weltman and first order stress decay with a non-zero equilibrium stress value models, however, there is some discrepancy from the experimental data of blends containing 1.25% starch at low and high shearing rate by Weltman model predictions. The observations of our research are similar to the findings reported by Abu-Jdayil et al.[2] (2002), who studied the time dependency of sesame paste by three models of Weltman, first-order stress decay with a zero equilibrium stress value, and first-order stress decay with a non-zero equilibrium stress value.[2] It was concluded that these models could adequately describe the experimental data at high shearing time. Hydrocolloid solutions may exhibit time-dependent properties, mainly thixotropy, which means that the apparent viscosity or consistency decrease with time. Modified Weltman logarithmic model has been utilized to describe the thixotropic behavior of CMC and pectin solutions in flavored yogurt.[24–26]

Figure 2 Shear stress of low fat sesame paste/date syrup blends containing 1.25% starch at 25°C as a function of shearing time (10 min period) and shear rate (4.18, 8.36 & 12.54 s⁻¹), (Experimental data, °; Weltman model predictions, —; Kinetic model predictions, —).

Figure 3 Shear stress of low fat sesame paste/date syrup blends containing 0.02% xanthan gum at 25°C as a function of shearing time (10 min period) and shear rate (4.18, 8.36 & 12.54 s⁻¹), (Experimental data, °; Weltman model predictions, —; Kinetic model predictions, —).

Figure 4 Shear stress of low fat sesame paste/date syrup blends containing 0.15% guar gum at 25°C as a function of shearing time (10 min period) and shear rate (4.18, 8.36 & 12.54 s⁻¹), (Experimental data, °; Weltman model predictions, —; Kinetic model predictions, —).

CONCLUSIONS

Sesame paste/date syrup blends with or without a fat substitute are non-Newtonian time dependent semi-solid substances, which behave as thixotropic material. It is shown that the time dependency behavior of blends could be described by the Weltam and kinetic models. The magnitude of thixotropic breakdown coefficients (B and K) of all blends generally increased with increasing shear rate. Regarding the initial shear stress values (A and τ₀), it is concluded that the apparent viscosity of control increases using a fat substitute and also with increasing fat substitution level of each fat replacer, however the effect of starch is greater than others.

ACKNOWLEDGMENT

The authors would like to sincerely thank the Faculty of Agriculture, Ferdowsi University of Mashhad, Iran, for the laboratory facility of this research and from the Simorgh Company for financial support.

Notes

19. Nutrition Labeling Act of 1990, Publication No. 101–535, 104 Stat 2353.