Effect of inulin and galactooligosaccharides on particle size distribution and rheological properties of prebiotic ketchup

ABSTRACT

The aim of this study was to evaluate the effects of different combinations of long-chain inulin and short-chain galactooligosaccharides mixed with different hydrocolloids on the physical/rheological attributes of prebiotic ketchup. Novel prebiotic ketchup was produced in which modified starch, xanthan, and guar gum was incorporated. Results showed that modified starch negatively influenced the physical properties of prebiotic samples and the optimum condition was 7.5% long-chain inulin and 2.5% galactooligosaccharides along with 0.4 % xanthan and 0.18% guar gum. Under these conditions, smaller hysteresis loop area, higher values of the linear viscoelastic region, larger G₀, η₀ in the creep test, and smaller sized suspended particles as compared to the other prebiotic samples were observed. In addition, galactooligosaccharides may interfere with the elastic behavior due to its high water solubility. Therefore, an appropriate amount (2.5%) of this ingredient may be used to produce a nutritive prebiotic ketchup with desirable textural properties. Environmental scanning electron microscopic images confirmed larger and inter-connected air bubbles entrapped into the semi-solid matrix of prebiotic sample produced under optimum condition.

KEYWORDS:

• Galactooligosaccharides
• ketchup
• long-chain inulin
• particle size distribution
• prebiotic
• rheological properties

Introduction

Ketchup is a popular tomato-based, low-calorie sauce^[1] usually consumed with fast foods.^[2,3] Considering its increasing consumption, new formulations with more nutritive ingredients will be in higher demand by the world market. Ketchup is a heterogeneous suspension, and several thickening agents including different types of hydrocolloids, are usually utilized in its formulation. These provide a viscous, consistent, and stable texture in this product.^[1,3,4] The effects of several hydrocolloids on the physical and rheological properties of ketchup have been studied.^[1,3–10] However, improvement of physical and nutritional attributes of this condiment is a desirable goal which can be achieved by incorporation of multi-functional thickening agents such as long-chain inulin in the formulation. Inulin can produce higher viscosity and better stability and might improve the nutritional value of the final product as well.^[11,12] Functional prebiotic products containing inulin have been widely investigated in different foods such as pasta,^[13] cheese,^[14,15] sponge cakes,^[16] dairy desserts,^[17,18] custards,^[12] chocolate,^[19] sausage,^[20] mayonnaise,^[21] and ketchup.^[22] Several works have also focused on the interactions between inulin and other hydrocolloids such as pectin and different types of starches.^[23,24]

Galactooligosaccharides (GOS) are indigestible prebiotics, structurally more stable at high temperatures,^[25] and therefore, may be used in heat-processed, low pH formulations such as ketchup. A few studies have reported the use of GOS to improve the mouthfeel and texture of products while also acting as a partial sugar substitute. Takasaki and Karasawa^[26] studied bread-making properties of frozen and unfrozen dough formulated with different concentrations of GOS syrup and reported that its addition had no negative effect on rheological properties of both dough types. Sin et al.^[27] studied the viscosity of GOS produced from soybean arabinogalactan and compared it with sucrose solutions. They concluded that, at low concentrations (brix < 50%), the viscosity of the soybean GOS was similar to that of sucrose solutions. The rheology of a dark prebiotic chocolate bar, formulated by partially replacing sucrose with GOS powder, was studied by Suter et al.^[28] Their results showed that the yield stress and apparent viscosity of the final products were dependent on the amount of GOS added. Fanaro et al.^[29] summarized the clinical and experimental data concerning the effects of a prebiotic mixtures of short-chain GOS and long-chain fructo-oligosaccharides on the intestinal flora and immune system and clearly demonstrated that this combination may exert a synergistic prebiotic effect on human infants. Thus, it could be hypothesized that this mixture of GOS and long-chain inulin may also exhibit synergistic technological/rheological behavior in food products.

Several studies have been published on the evaluation of rheological/textural properties of tomato products like tomato juice,^[30] tomato paste,^[31–34] tomato sauce,^[35] ketchup,^[2,36,37] and ketchup-processed cheese mixtures.^[38] In addition, the effects of various types of hydrocolloids,^[1,3,4,6,10] processing parameters, and the structural characteristics of tomato paste on the rheological/viscoelastic behavior of ketchup have been investigated.^[34,39] According to Wang,^[40] one of the most important factors for selecting prebiotic ingredient is chemical stability under food processing conditions. The results of previous studies have shown that heating at low pH’s has the most severe effect on reducing the prebiotic activity. Therefore, for a food product such as ketchup, the critical point is to select suitable prebiotics which could tolerate this condition and render them available for bacterial metabolism and prebiotic activity. Stability of long-chain inulin and short-chain GOS at high temperatures and low pH’s has been proven previously.^[25,41–43] In addition, Otles^[44] reported that short-chain GOS and long-chain FOS mixture may exhibit better prebiotic properties in increasing fecal bifidobacteria and lactobacilli compared with the placebo group.

The main objective of this research was to evaluate the effects of different combinations of long-chain inulin, short-chain GOS and different hydrocolloids on the flow behavior, dynamic and transient viscoelastic properties, particle size distribution, and microstructure of prebiotic ketchup formulations. The results of this study might advance the technological information for producing a new food condiment with improved functionalities that adheres to the healthy lifestyles of today’s conscious consumers.

Materials and methods

Materials

Long-chain inulin (≥23 monomers, Frutafit TEX, Sensus, Netherlands), commercial food grade GOS (90% purity, mixture of oligosaccharides of DP: 2, 3 and 4, Anhui, China), acetylated potato distarch phosphate (Emjel EP300; Emsland, Germany), xanthan gum (GRINDSTED xanthan 200, Danisco, Denmark), and guar gum (GRINDSTED GUAR 250, Danisco, Denmark) were used in the preparation of ketchup samples. Other ingredients of the basic formulation were supplied by different companies as follows: tomato paste (Dashte Neshat, Iran), vinegar (Liagol Khazar, Iran), sugar (Hekmataneh sugar industries,Iran), salt (Shams, Iran), glucose syrup (Shahdineh aran, Iran), and flavoring agents (Givaudan, Switzerland).

Preparation of inulin suspension

For each formulation specific amounts of inulin powder was suspended in water to achieve the desired concentration (Table 1). For example, to prepare a 2-kg batch of ketchup with K5 formulation, 150 g of inulin powder was suspended in 985.4 g of water and mixed with a mechanical stirrer (Brown, 600 Watt). Suspensions were heated and sheared at 80°C in a water bath for 10 min to form a gel and then cooled to 25°C.

Table 1. Formulation of ketchup samples.

	Samples
Ingredients (%)	K1	K2	K3	C1	C2	^†C3	K4	K5
Tomato paste	20	20	20	20	20	20	20	20
Vinegar	8	8	8	8	8	8	8	8
Salt	2	2	2	2	2	2	2	2
Sugar	12.5	10	12.5	13	13	13	12.5	10
Glucose syrup	—	—	—	7.5	7.5	7.5	—	—
Flavorings	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15
Inulin	3.75	7.5	7.5	—	—	—	3.75	7.5
GOS	3.75	2.5	—	—	—	—	3.75	2.5
MS	1.5	1.5	0.75	0.75	1.5	1.5	—	—
Xanthan	—	—	—	—	—	0.4	0.4	0.4
Guar	—	—	—	—	—	0.18	0.18	0.18
Water	48.35	48.35	49.1	48.6	47.85	47.27	49.27	49.27

^†C3: commercial product.

Ketchup samples preparation

Different combinations of inulin, GOS, modified starch (MS), xanthan, and guar gum were added to the basic ingredients of ketchup samples as shown in Table 1. For inulin-containing samples, the prepared inulin suspension was initially mixed with tomato paste and then with sugar, glucose syrup (in control samples), salt, vinegar, and ketchup flavorings. Different amounts of GOS, xanthan, guar gum, and MS were added and mixed with an electrical blender (Brown, 600 Watt). Each ketchup sample was prepared in a 2 kg batch, heated to 60°C, homogenized at 240 bar (APV, Gaulin GmbH), and finally pasteurized in a water bath at 80°C for 10 min and immediately poured into 200 mL glass jars. After cooling to 25°C, the samples were stored in a refrigerator until analysis.

Particle size distribution measurements

The volume mean diameter D [4,3] and D (0.5; the size of particle for which 50% of the sample is below this size) of each ketchup sample was measured using a Malvern Mastersizer 2000 laser diffraction particle size analyzer (Malvern Mastersizer 2000, England), according to the method of Juszczak et al.^[1] The refractive index (RI) and absorption value of the particles were adjusted 1.330 and 0.1, respectively. Each sample was analyzed in triplicate.^[45]

Characterization of rheological properties

Rheological properties of samples were analyzed in duplicate using the three tests described below.

1. Flow behavior analyses were carried out using a Physica rheometer (MCR 501, Anton Paar GmbH, Austria) and serrated surface parallel plates with diameters of 25 mm and a gap of 1 mm at 25°C. Although it was previously reported that at refrigerator-storage temperatures, a higher consistency and a lower serum separation may be observed;^[6] in this research all tests were performed at room temperature (25°C). The reason was that ketchup is usually kept at ambient temperature on the tables of fast-food restaurants and, therefore, the functional properties of the new ingredients can be assessed more realistically under these conditions than at refrigeration temperatures. Before measurements, the ketchup samples were loaded in the measuring system and kept at room temperature for 5 min. In thixotropic tests, the shear rate was first ramped up from 0 to 300 s^–1, and after a wait of 45 s, it was ramped down at the same rate to zero. The hysteresis area (Aup–Adown) was measured using Rheoplus software (Anton Paar, Austria). The experimental data of flow behavior tests was fitted to the Herschel–Bulkley model (τ = τ₀ +K γⁿ) to calculate its three main parameters: τ₀ the yield stress (Pa), K the consistency index (Pa sⁿ), and n (–) the flow behavior index.

2. Dynamic rheological analysis: The viscoelastic analyses were accomplished with the same geometry and temperature as explained for the flow behavior tests. First, strain sweep tests were performed at a frequency of 1 Hz to determine the range of the linear viscoelastic region; then frequency sweep were measured in the frequency range of 0.1–10 Hz for a constant shear strain of 0.6%.

3. Creep and creep recovery experiments: A fixed stress of 2 Pa was applied instantly under linear viscoelastic region, maintained constant for a period of 300 s, and then recovered in the same length of time. The creep and creep recovery results were obtained by Physica MCR 501 software according to the following equations, respectively:

(1)

(2)

where J (t) is the overall compliance at any time, J₀ is the instantaneous compliance, J_m is the viscoelastic compliance, λ is the mean retardation time, η₀ is the zero shear viscosity, and J_max is the maximum creep compliance.^[46]

Microstructure analysis

An environmental scanning electron microscope (ESEM; XL30 Philips) was used to study the microstructure of the commercial and the best selected prebiotic ketchup produced under optimum conditions, at a voltage of 15 kV, pressure of 2.9 mbar, and 500× magnification.

Statistical analysis

Statistical analysis was conducted using the Minitab (Version 16) software. One-way analysis of variance (ANOVA) and Tukey’s test (p ≤ 0.05) were used to estimate the significance of differences between the means and to calculate the values of the least significant difference, respectively.

Results and discussion

Particle size distribution

The stability of dispersions is strongly influenced by particle size and this is even more important in a product such as ketchup which is very susceptible to syneresis. The stability can be improved by adding hydrocolloids and by homogenization.^[11,38] The particle size distribution of different ketchup samples is shown in Fig. 1. Two peaks were identified. The first was at 100 μm, which may belong to the raw materials of the tomato^[39] and modified potato starch.^[1] This peak in samples containing only MS (C1, C2) appeared with approximately 37% higher volume in comparison with starch-free samples (K4, K5; Fig. 1a). The second peak is observed in the range of 3–4 μm only in the samples formulated with xanthan and guar gum (K4, K5, and C3; Fig. 1b). It can be noted that the size of this peak for K5 was approximately twice bigger than that of the commercial product (C3). This result suggests that when compared with starch, inulin is more effective in producing smaller suspended particles in the final product when combined with xanthan and guar gum. In another word, in the presence of starch (K2), the number of large particles that may be formed in ketchup suspension is higher. When starch is replaced by other hydrocolloids (xanthan and guar gum, K5), particle size parameters [d(0.5), D_4,3] are decreased (Table 2). D_[4,3], or the mean diameter of particles, is usually considered as a good index of large particles. Therefore, from the results shown in Table 2, it may be concluded that the lowest D_[4,3] values belong to C3 and K5: the commercial ketchup and the sample formulated with 7.5% inulin, 2.5% GOS, 0.4% xanthan, and 0.18% guar gum, respectively. It appears that (1) inulin alone does not perform very successfully in reduced fat/emulsion type food products, and (2) only in combination with other additives, especially xanthan and guar gum, can it provide better textural characteristics. This was previously reported by Bortnowska and Makiewicz,^[47] Brennan and Tudorica,^[48] and Veen and Budemann.^[49] Moreover, the results of the present study confirm this cooperative performance for suspension type/oil-free products such as ketchup where inulin is used as a prebiotic as well as a texture-improving agent.

Figure 1. Particle size distributions of ketchup samples: A: All of samples, B: C1, C2, K4, K5, C: C3, K5.

Table 2. Particle size parameters and γ _LVE of ketchup samples^†.

Sample^‡	D[4,3] (µm)	D(0.5) (µm)	LVE γ (%)
K1	112.80 ± 2.48^b	96.67 ± 1.41^a	0.62 ± 0.15^c
K2	98.03 ± 4.00^bc	85.01 ± 5.00^a	1.07 ± 0.56^bc
K3	109.36 ± 7.00^bc	90.00 ± 7.06^a	1.00 ± 0.00^bc
C1	106.00 ± 5.46^bc	93.00 ± 4.17^a	0.84 ± 0.22^bc
C2	110.09 ± 1.00^bc	90.04 ± 8.00^a	1.81 ± 0.48^abc
C3	91.35 ± 3.00^c	71.00 ± 3.20^b	2.65 ± 0.71^a
K4	116.43 ± 6.48^b	88.00 ± 4.33^a	2.15 ± 0.00^ab
K5	95.01 ± 13.17^bc	64.21 ± 3.00^b	2.15 ± 0.00^ab

^†The results are expressed as mean ± standard deviation and values with different superscript letters represent significant differences (p < 0.05).

^‡K1 (3.75% Inulin, 3.75% GOS, 1.5% MS); K2 (7.5% Inulin, 2.5% GOS, 1.5% MS); K3 (7.5% Inulin, 0.75% MS); C1 (0.75% MS); C2 (1.5% MS); C3 (0.4% Xanthan, 0.18% Guar, 1.5% MS); K4 (3.75% Inulin, 3.75%, GOS, 0.4% Xanthan, 0.18% Guar); K5 (7.5% Inulin, 2.5%, GOS, 0.4% Xanthan, 0.18% Guar).

Rheological properties

Flow behavior tests

The results of the flow behavior tests are shown in Fig. 2.

Figure 2. Flow curves for ketchup samples.

All samples showed pseudoplastic behavior (n = 0.322–0.486) with consistency coefficient (k) and yield stress (τ_y) in the range of (8.60–18.70 Pa.sⁿ) and (18.40–60.75 Pa), respectively. Yield stress is usually considered as an important parameter for ketchup quality.^[2] In this research, the maximum values (49.5–57.8 Pa) belonged to the prebiotic samples formulated with xanthan and guar gum, which were comparable to the commercial product. Additionally, using a higher concentration of inulin resulted in significantly greater consistency indices, such that with the same amount of GOS and MS, using twice concentration of inulin in K2 caused the consistency index to increase by approximately 40% as compared to K1. Arcia et al.^[17] observed that the addition of a higher concentration of long-chain inulin led to a higher consistency coefficient value in prebiotic dairy dessert. In this research, the lowest consistency (4.6 Pa.sⁿ) was observed in C1 which contained only the lowest level (0.75%) of MS. This result is in agreement with the range of consistency coefficient reported by Juszczak et al.^[1] for ketchups formulated with different types of starch. Compared with the commercial ketchup, the smallest hysteresis loop area was observed in samples containing prebiotics xanthan and guar gum. According to Achayuthakan and Suphantharika,^[50] the presence of xanthan in the system may reduce the thixotropic behavior and provide more quick structural recovery in the product after shearing takes place. This may be related to the unusual rigid rod-like conformation of xanthan. A lower hysteresis loop area range and even rheopectic behavior has been reported for some types of German and Egyptian commercial ketchup.^[2] The rheopectic behavior might be due to the large amount of native starch used in those commercial products which was not present in the formulations of the current research.

Strain amplitude sweep test

All ketchup samples showed solid viscoelastic behavior (G’ > G”) with the linear viscoelastic strain (γ_LVE) in the range of 0.60–2.65% (Table 2). The prebiotic samples formulated with inulin, xanthan, and guar gum (K4 and K5) showed viscoelastic behavior closer to that of the commercial ketchup (C3) and thereby confirmed the results obtained from particle size analyses. For the other prebiotic samples (K1, K2, and K3), however, the much lower γ_LVE, may be due to the negative interaction between MS and non-starch polysaccharides such as inulin.^[23,34]

Frequency sweep, creep, and creep recovery tests

Figure 3a

Figure 3. A: Elastic G’ (Pa) and loss modulus G” (Pa), B: Complex viscosity (η*) as a function of the frequency (Hz) in ketchup samples.

shows changes in storage modulus (G’) and loss modulus (G”) as functions of frequency. In all ketchup samples, G’ was more than G”; and both parameters showed slight frequency dependency due to weak gel behavior. No cross-over point was observed between G’ and G” at the studied frequency range, which indicated the stability of samples. These results are in agreement with those of previous studies.^[1,2,39] The complex viscosity (η*) versus frequency curves for ketchup samples are shown in Fig. 3b. According to these results, η* was found to be a decreasing function of the frequency as reported by Yilmaz et al.^[38] Compliance variations versus time in the creep and creep recovery tests of the ketchup samples are shown in Fig. 4. The parameters obtained in these tests and analyzed by Rheoplus software and are given in Tables 4 and 5 G₀ (elastic modulus), G₁ (viscoelastic shear modulus), and η₀ (viscosity in zero shear stress) are criteria for the structural strength and gel network. These factors were found to be lower in C1 and increased by increasing the amounts of hydrocolloids. Dolz et al.^[51] showed that using hydrocolloids such as MS, xanthan, locust bean gum, and a blend of xanthan and locust bean gum as the gel-forming compounds could strengthen structural and elastic properties (G₀, G₁) of food emulsions. Hydrocolloid type and concentration are also important. To better interpret the functions of the four different hydrocolloids used in this research, the rheological properties of different pairs of formulations were compared as follows.

Figure 4. Compliance variations versus time (t) in creep and creep recovery tests of ketchup samples.

Table 3. Rheological Parameters of frequency sweep tests of ketchup samples at 0.1 and 10 (HZ).^†

Sample^‡	G′ (Pa) 0.1 10 (HZ)	G″ (Pa) 0.1 10 (HZ)	tan (δ) 0.1 10 (HZ)	η* (Pa.s) 0.1 10 (HZ)
K1	817.0 ± 259.0^bc; 1955 ± 502^bc	188.0 ± 73.5^c;599.5 ± 151.0^bc	0.23 ± 0.02^d;0.33± 0.00^ab	1345 ± 417.2^bc; 32.5 ± 8.34^bc
K2	2840 ± 523.3^a;7270 ± 792^a	710.0 ± 126.0^abc;1945.5 ± 318.2^ab	0.25 ± 0.00^bcd;0.27 ± 0.01^c	4660 ± 863.0^a; 119.5± 13.40^a
K3	1016.5 ± 175.0^abc;3525 ± 290^abc	241.0 ± 37.0^bc;722.0 ± 90.5^abc	0.24 ± 0.00^cd;0.20 ± 0.00^d	1665 ± 290.0^abc; 57.30 ± 5.00^abc
C1	442 ± 229.1^c; 1142 ± 478^c	112.0 ± 70.0^c;351.0 ± 151.3^c	0.22 ± 0.03^cd;0.31 ± 0.00^ab	725.5 ± 381.1^c; 19 ± 8.00^c
C2	776 ± 83.4^bc; 3155 ± 78^abc	203.5 ± 11.0^c;606.5 ± 15.0^abc	0.27 ± 0.01^abcd;0.19 ± 0.00^d	1265 ± 134.4^bc; 51.10 ± 1.13^abc
C3	2675 ± 1124.3^ab; 7930 ± 2758^a	825.0 ± 318.2^ab;1950.0 ± 749.5^ab	0.31 ± 0.01^abc;0.24 ± 0.00^c	4355 ± 1718.3^ab; 130 ± 45.33^a
K4	1585 ± 262.0^ab;4235 ± 757^ab	520.0 ± 99.0^abc;1405.0 ± 304.1^ab	0.33 ± 0.00^ab;0.33 ± 0.01^a	2655 ± 445.5^ab; 71.10 ± 13.01^ab
K5	3165 ± 983.0^a; 8660 ± 1782^a	1038.0 ± 215.0^a;2390.0 ± 452.5^a	0.33 ± 0.03^a;0.28 ± 0.00^bc	5305 ± 1591.0^a; 143 ± 30^a

^†The results are expressed as mean ± standard deviation, and values with different superscript letters represent significant differences (p < 0.05). ^‡K1 (3.75% Inulin, 3.75% GOS,1.5% MS); K2 (7.5% Inulin, 2.5% GOS,1.5% MS); K3 (7.5% Inulin, 0.75% MS); C1 (0.75% MS); C2 (1.5% MS); C3 (0.4% Xanthan, 0.18% Guar,1.5% MS); K4 (3.75% Inulin, 3.75%, GOS, 0.4% Xanthan, 0.18% Guar); K5 (7.5% Inulin, 2.5%, GOS, 0.4% Xanthan, 0.18% Guar).

Table 4. Creep factors for ketchup samples.^†

Sample^‡	J0 × 10^–4 (Pa^–1)	Jm × 10^–4 (Pa^–1)	Jmax × 10^–4 (Pa^–1)	G0 (Pa)	G1 (Pa)	η0 (Pa.s)	λ (s)	R^2
K1	29 ± 0.0^cde	22 ± 5.0^bcd	69 ± 7.0 ^cd	336.1 ± 0.8^abc	453.0 ± 104.3^ab	163220 ± 22217^bc	24.2 ±10.6^a	0.993
K2	18 ± 6.0^de	14 ± 3.0^cd	43 ± 9.0 ^d	576.4 ± 195.0^ab	721.5 ± 188.4^ab	261775 ± 8676^a	31.2 ± 8.9^a	0.998
K3	17 ± 5.0^e	10 ± 3.0^d	40 ± 7.0^d	605.0 ± 200.3^a	975.0 ± 346.2^a	237165 ± 33029^ab	32.1 ± 0.7^a	0.999
C1	100 ± 4.0^a	37 ± 1.0^ab	161 ± 7.0^a	100.0 ± 4.3^c	268.0 ± 13.2^b	120520 ± 12063^c	35.4 ± 1.0^a	0.996
C2	53 ± 3.0^b	38 ± 1.0^ab	117 ± 0.0^abc	188.6 ± 11.1^bc	259.4 ± 12.4^b	105865 ± 2765^c	19.0 ± 2.6^a	0.992
C3	34 ± 3.0^cd	36 ± 2.0^abc	96 ± 10.0^bcd	289.2 ± 29.0^abc	272.0 ± 18.0^b	116500 ± 20237^c	27.0 ± 3.4^a	0.997
K4	38 ± 5.0^bc	54 ± 13.0^a	129 ± 35.0^ab	314.5 ± 35.0^abc	191.0 ± 48.0^b	83100 ± 37930^c	26.3 ± 6.8^a	0.995
K5	22 ± 0.0^cde	25 ± 2.0^bcd	74 ± 5.0^bcd	444.6 ± 11.2^abc	393.0 ± 39.0^b	113892 ± 33415^c	28.0 ± 4.4^a	0.998

^†The results are expressed as mean ± standard deviation, and values with different superscript letters represent significant differences (p < 0.05). ^‡K1 (3.75% Inulin, 3.75% GOS,1.5% MS); K2 (7.5% Inulin, 2.5% GOS,1.5% MS); K3 (7.5% Inulin, 0.75% MS); C1 (0.75% MS); C2 (1.5% MS); C3 (0.4% Xanthan, 0.18% Guar,1.5% MS); K4 (3.75% Inulin, 3.75%, GOS, 0.4% Xanthan, 0.18% Guar); K5 (7.5% Inulin, 2.5%, GOS, 0.4% Xanthan, 0.18% Guar).

Table 5. Creep Recovery factors for ketchup samples.^†

Sample^‡	Je × 10^–4 (Pa^–1)	Jv × 10^–4 (Pa^–1)	Je/Jmax (%)	Jv/Jmax (%)	R^2
K1	28.0 ± 0.0^c	15.1 ± 10.0^bc	67.21 ± 16^ab	39.000 ± 12.0^abc	0.990
K2	42.3 ± 4.0^bc	27.3 ± 11.0^bc	61.40 ± 12^abc	33.000 ± 16^bc	0.987
K3	31.5 ± 2.0^c	9.2 ± 5.0^c	76.04 ± 6.72^a	24.000 ± 7^c	0.994
C1	103.0 ± 13.0^a	58.5 ± 4.0^ab	64.00 ± 4.73^abc	36.330 ± 5.0^abc	0.987
C2	65.1 ± 1.0^b	53.0 ± 2.0^abc	55.27 ± 0.52^abc	45.000 ± 0.52^abc	0.971
C3	42.0 ± 5.0^c	54.0 ± 6.0^abc	42.20 ± 2.09^bc	58.000 ± 2.09^ab	0.999
K4	44.1 ± 6.0^bc	85.0 ± 28.0^a	35.00 ± 4.08^c	65.300 ± 4.08^a	0.992
K5	31.5 ± 0.0^c	43.0 ± 5.0^abc	42.48 ± 4^bc	57.515 ± 3.7^ab	0.991

^†The results are expressed as mean ± standard deviation, and values with different superscript letters represent significant differences (p < 0.05). ^‡K1 (3.75% Inulin, 3.75% GOS, 1.5% MS); K2 (7.5% Inulin, 2.5% GOS, 1.5% MS); K3 (7.5% Inulin, 0.75% MS); C1 (0.75% MS); C2 (1.5% MS); C3 (0.4% Xanthan, 0.18% Guar, 1.5% MS); K4 (3.75% Inulin, 3.75%, GOS, 0.4% Xanthan, 0.18% Guar); K5 (7.5% Inulin, 2.5%, GOS, 0.4% Xanthan, 0.18% Guar).

K1, K2

Compared to K1, K2 contained more long-chain inulin, less GOS, and the same amount of MS (Table 1). According to the results of the frequency sweep tests (Table 3, Fig. 3), the elastic modulus (G’) and complex viscosity (η*) of K2 were higher at 0.1 and 10 Hz, and tanδ at 10 Hz was significantly lower than those of K1. In the results of creep tests, K2 showed an approximately 60% higher η₀, i.e., lower pure viscous behavior, than K1 (Table 4). Meanwhile, K2 displayed a higher consistency index than K1 in flow behavior tests. Therefore, it seems that increasing the amount of inulin and simultaneously decreasing the amount of GOS can result in reinforcing elastic behavior. It is well known that long-chain inulin is a prebiotic compound with low water solubility and can be used as a thickener to improve structural properties by forming a strong gel network. The interactions between the MS and this type of inulin would also strengthen this structure.^[18,21] However, short-chain GOS (DP < 4) with high water solubility (>80%) and good water holding capacity^[52] may interfere with this phenomenon and inhibit the formation of a strong network in the food product, which would result in weakening of the elastic behavior of K1.

K2, K3

These samples contain the same amount (i.e., 7.5%) of inulin and different percentage of MS and GOS: (1.5, 2.5%) in K2 and (0, 0.75%) in K3 (Table 1). It means that K3 formulation contains half of the MS content and no GOS. The results of the rheological tests indicated that only tanδ at 10 Hz was significantly lower in K3 than in K2, that is the sample which was free of GOS exhibited stronger elastic behavior than K2, and long-chain inulin was sufficiently effective to strengthen the gel network in spite of lower amount of starch was used.

K3, C1

The amount of MS was similar in both samples but C1 was formulated without inulin. Eliminating inulin resulted in larger tanδ at 10 Hz for C1 and indicated weaker elastic behavior compared to K3 (Table 3). This was further confirmed by creep test. Also, the instantaneous compliance value (J₀) increased significantly and G₀, as pure elastic modulus, was 50% smaller for C1 relative to K3 (Table 4).

C1, C2

The effect of MS may be interpreted by comparing the rheological properties of these two samples, since only MS and no other hydrocolloid was used. Smaller tanδ (10 Hz) and J₀ values for C2 which contained a higher amount of MS were indications of stronger elastic behavior compared to C1; however, the tanδ values of these samples (0.192–0.307) lie approximately in the middle of the range reported by Juszcak et al. (0.14–0.37).^[1] This might be attributed to the use of lower amount of the rheologically weakest type of MS, i.e., potato starch used in the current research.

C2, C3

In both samples, 1.5% of MS was used, but C3 was formulated with xanthan (0.4%) and guar gum (0.18%) along with starch. Hydrocolloids such as xanthan and guar gum are generally added into concentrated fruits and vegetables juices to improve the consistency and prevent serum separation. Xanthan dispersions usually show a more elastic character as a weak gel, while guar dispersions present a different and somewhat opposite behavior as a concentrated viscous solution.^[53] Therefore, it is important to consider the interactions of these hydrocolloids with the tomato pulp in ketchup samples and with other ingredients, especially MS, which are included in complex food product systems. Starch-hydrocolloid interactions are strongly dependent on the type and level of hydrocolloid, starch origin, and starch/hydrocolloid ratio.^[54] Potato starch could make a strong interaction with guar gum.^[55] Therefore, one might expect significant variability in the rheological parameters of C3. However, according the results of Table 3, only tanδ (10 Hz) is significantly higher in C3 as compared with C2 and other parameters did not show any significant difference. It seems that interaction of xanthan with guar gum may have a balancing effect on rheological behavior of C3.

C3, K4

The amounts of xanthan and guar gum were kept constant in both formulations. In addition to these hydrocolloids, C3 and K4 contained MS and prebiotic compounds (inulin and GOS), respectively. K4 showed a higher tanδ (10 Hz), i.e., more viscous behavior than C3, which could be due to the presence of GOS in K4.

K4, K5

These samples were only different in prebiotics content, where a higher amount of inulin and a lower amount of GOS may have caused stronger elastic characteristics in K5 as shown by smaller tanδ (10 Hz) compared to K4. Finally it may be interesting to compare K2, K5, and C3. According to the results of Tables 3, 4, and 5, K5 was the only sample in which particle size, dynamic and transient rheological properties were not significantly different from those of the commercial product (C3). Since the same amount of prebiotics was used in the first two formulations and the only difference is related to xanthan and guar gum instead of MS in K5, it may be concluded that in prebiotic products, a mixture of xanthan and guar gum is more preferred than MS as a thickening agent for providing favorable textural and structural properties. Some degree of incompatibility may exist between inulin and MS.^[24] In addition, in tomato products such as ketchup, which naturally contain pectin, there is a negative interaction between pectin and long-chain inulin which can affect the functional properties of modified potato starch, leading to intensified viscous behavior.^[23] Based on the results of this study, xanthan and guar gum may be considered as suitable alternatives for MS to overcome the problems associated with prebiotic products. Therefore, the sample formulated by 7.5% long-chain inulin and 2.5% GOS along with xanthan and guar gum may be selected as the best combination.

ESEM micrographs

ESEM micrographs of C3 and K5 samples, shown in Fig. 5

Figure 5. ESEM micrographs of ketchup samples: A: C3, B: K5.

confirm textural similarities between the commercial and the optimized prebiotic ketchup. However, it seems that K5 has a more puffy texture which may be related to larger air bubbles entrapped in this product. Hydrophobicity of modified potato starch and its adsorption on air/water interfaces have been previously studied by other researchers.^[55,56] It has been reported that MS may stabilize air bubbles against aggregation and form a more steady texture,^[57] while the presence of inulin usually results in the occlusion of air bubbles by forming a more expanded texture due to the interconnection of air cells.^[16,58]

Conclusions

Taken together, the results of this study indicates that prebiotic mixtures of short-chain GOS and long-chain inulin can be included in ketchup formulations to produce a healthier, nutritious condiment such that it’s rheological and textural properties would not be significantly different from commercial products. It may be suggested that a mixture of xanthan and guar gum is a better choice than MS for reinforcing the structural integrity of the prebiotic ketchup formulation. Since presence of GOS may interfere with the elastic network, an appropriate amount (2.5%) of this ingredient should be used in prebiotic samples in order to provide desirable textural/rheological properties and also benefit from its nutritional advantages. This new formulation of prebiotic ketchup may be considered a suitable medium for producing synbiotic product containing probiotic bacteria in future studies.