Effect of rice flour incorporation on some physicochemical, color, rheological, bioactive and sensory properties of a new pestil formulation: one factor design approach and optimization

Abstract

Pestil is a special and a traditional food produced with wheat flour or a starch, water and molasses. In this study, a new pestil formulation was developed using different concentrations of rice flour (0–12%). One factor design (Response surface methodology) approach was used to determine the some physicochemical, rheological, bioactive, color and sensory properties of newly formulated pestil products. Also, an optimization study was performed to reveal the most-liked samples using the sensory analysis results. Dry matter levels ranged between 86.87 and 96.55 g/100g while the protein contents were in the range of 4.18–5.91 g/100g. Maximum and minimum total phenolic and antioxidant activity levels of the samples were observed for the samples coded as R4 and R5 with 1471.93–887.91 mg GAE/kg and 17701.65–12684.09 mg AAE/kg, respectively. The dynamic rheological properties of the pestil samples were significantly affected by the rice flour addition (p < 0.05). The optimization results showed that the best pestil formulation can be produced by incorporating 0.44 g/100g rice flour. In conclusion, the rice flour could be assisted to pestil production in the formulation for better pestil production.

Introduction

Pestil, which is known as a ‘orjik’ was firstly found about 200 year ago and the pestil was produced with different methods and called köme, kömbe or churchkhela in Anatolia (Kalkışım and Özdemir 2012; Yavuz 2019). Generally, the simple pestil formulation is composed of wheat flour or starch, water, honey, milk, mulberry molasses and sugar. If the churchkhela will be produced, the walnut or hazelnut can be added to the pestil formulation. At the present time, the mulberry molasses or ‘şıra’ (produced from mulberry by pressing) honey, milk and sugar are firstly mixed by a blender and then the mixture is boiled in the vacuum pan. Afterwards, the flour or starch is slowly added to the mixture by blending and then the viscosity of the mixture increased, and the product transformed to gel-like structure that is called as ‘herle’. The herle is spread to a cloth such as silk about 1 mm thickness for the drying. Finally, the dried pestil is removed from the cloth without tear about 24 h later by using wet sponge (Kalkışım and Özdemir 2012; Yıldız, 2013; Baltacı et al. 2016; Yuksel et al. 2020).

The pestil products have a good elasticity due to their constituents and production conditions. Especially, the elasticity of pestil is provided by the flour or starch concentration in the formulation. Some pestil producers prefer to use only wheat flour in the formulation while the other pestil producers use only starch (wheat, potato or corn starch) (Yavuz 2019; Yuksel et al. 2020). But rice flour generally is not used in the pestil formulations. In the present work, rice flour was used as an elasticity enhancer. In the pestil formulation, the starch such as wheat, corn or potato is used without flour and so the pestil texture can be harder because the starch purity is quite high like 99%. The starch of the pestil (without flour) can be exposed to the retrogradation and so the texture of the pestil can be harder. In the pestil production, if the wheat flour is used only, the pestil texture can be softer because of the lower starch content of the wheat flour (about 60–70%) (Yavuz 2019; Yuksel et al. 2020).

The rice flour is not used previously in the pestil formulations and so the present research could be useful for the pestil producers for a new pestil formulation prepared with the incorporation of rice flour. In this study, some physicochemical characteristics, color, bioactive and sensory properties of the pestil samples enriched with wheat and rice flours were determined and one factor design was used to reveal the effects of rice flour addition on the studied parameters. Also an optimization study was performed to determine the overall preference of the pestil formulation using sensory scores of the samples.

Materials and methods

Materials

Wheat (moisture 12.9%, protein 11.1%, 0.55% ash, 2.9% oil in dry matter) and rice flour (moisture 12.4%, protein 6.7%, 0.53% ash, 1.2% oil, 74.8 carbohydrate % in dry matter), milk (protein 3 g, carbohydrate 4.7 g, oil 3.3 g), flower honey (protein 0.3 g/100g, carbohydrate 82.4 g/100g, moisture 16.5 g/100g) and sugar were purchased from a local market in Malatya (Turkey). The mulberry molasses was procured from Koska Co. (Turkey).

Preparation of pestil formulations

The rice flour at different concentrations (Table 1) and wheat flour (12 g/100g) were incorporated into the formulation for the production of pestil. The other ingredients of the pestil were 70 g/100g water, 9.0 g/100g sugar (saccharose), 1.5 g/100g flower honey, 6.5 g/100g milk and 3.0 g/100g mulberry molasses. The pestil production process was carried out at room temperature (25 ± 3 °C). Firstly, the “herle” which is a traditional name and that is known as the first phase of pestil production was prepared by using a cap on the boiling water, sugar, milk, mulberry molasses and honey. Afterwards, the rice and wheat flours (according to Table 1) were slowly added to the herle and continuously mixed. So the second phase of herle preparation was completed and the herle was lay out to silk cloth at 1 mm thickness and then dried at 50 °C for 24 h. Finally, the dried pestil samples were slipped from the cloth and then the samples were rested on the tray to dry (last dry process) at room conditions. After that, the pestil samples were exposed the related analyses.

Table 1.

Experimental one factor design used in the pestil production

Runs	Coded values	Uncoded values
	Rice flour (Xi)	Rice flour (g/100g)
R1	1.00	12.00
R2	0.00	6.00
R3	0.50	9.00
R4	1.00	12.00
R5	−1.00	0.00
R6	−1.00	0.00
R7	−0.50	3.00

The total level of the rice flour and wheat flour is 12 g/100g in the formulation

Physicochemical analysis, hydroxymethylfurfural content and color properties of the samples

Dry matter, ash and protein content of the pestil samples were determined according to the official procedures (AOAC 2000). The dry matter content of the samples was determined by the drying method using an oven (Nuve FN 120 Turkey). The ash content of the samples was designated by the dry burning method using a furnace (Protherm PLF115M, Turkey). The protein level of the pestils was determined by Kjeldahl method (N = 6.25). The water activity (a_w) content of the pestils was measured using an automatic a_w meter at 25 °C (Decagon, USA). The produced pestils were manually ground with a mortar and then L*(lightness), a* (± red–green) and b* (± yellow–blue) values of the samples were measured by using a colorimeter (Lovibond, England). Six replicates were performed for each analysis. Hydroxymethylfurfural (HMF) content of the pestil samples was designated according to the modified method of Küçük et al. (2007). The pestil sample (5 g) was weighed into a 50 mL flask and then was dissolved with 25 mL water. Afterwards, the Carrez I (0.5 mL) and Carrez II (0.5 mL) solutions were added and later distilled water was added to the mark. The sample solutions were filtered through 45 µm membrane filters. The sample solution (100 µL) was injected to the HPLC–UV system (Agilent 1100 series, USA) and then quantitative analysis was carried out. The C₁₈ column (Nucleosil, USA) was used for the separation of HMF and the mobile phase was selected as water–methanol (90:10 v/v, flow rate of 1 mL/min, wavelength at 285 nm). The alkali titration method was used for titration acidity of the pestil samples (TS 2002).

Determination of dynamic rheological properties of the pestil samples

Dynamic mechanical spectra of the pestil samples were analyzed. In this context, viscous and elastic properties of samples were investigated. For this aim, a stress/strain-controlled rheometer (Anton Paar MCR 102, Thermo Scientific, Germany) equipped with a parallel plate geometry was used (cone diameter 35 mm and gap size 1.000 mm). Firstly, a stress sweep test was performed to determine the linear viscoelastic range in the range of 0.1–10 Pa and then the frequency swept tests (0.1–10 and 0.1–50 Hz) were performed in the linear viscoelastic region (0.1–100 Hz) at 70 °C. As the rheological parameters, G′ (elastic modulus), G″ (viscous modulus), G^* (complex modulus) and tanδ (G″/ G′) values of the samples were determined (Kayacier et al. 2014).

Bioactivity analysis of the samples

Total phenolic and flavonoid contents of the samples

For the preparation of the extracts from the pestil samples, a 5 g of pestil sample was mixed with 20 mL of distilled water. Then the mixtures were subjected to homogenization using an Ultra-turrax homogenizer during 10 min and filtrated using a filter paper. This filtrate was used for the total phenolic content (TPC), total antioxidant, DPPH, FRAP and total flavonoid content (TFC) analysis. For the determination of TPC, a 100 µL of the extract sample was mixed with 4.5 mL of deionized water and then 100 µL of Folin–Ciocalteu was incorporated. Afterwards, this mixture was vortexed and left at 25 ± 3 °C (room temperature) during 10 min and then, 300 µL of 2% of Na₂CO₃ solution was added to the mixture. Finally, the samples were incubated for 30 min at (25 ± 3 °C) and then the absorbance values of the samples at 760 nm was recorded using a spectrophotometer (UV–Vis). The results were expressed as mg GAE/kg by using the gallic acid calibration curve (Kasangana et al. 2015).

To determine the TFC of the pestil samples, a 500 µL of the extract samples and 3200 µL of methanol (30% v/v) were mixed. Afterwards, 150 µL of 0.5 M sodium nitrite solution and 150 µL of 0.3 M aluminum chloride were added to the mixture and incubated for 5 min. One mL of NaOH solution (1 M) was incorporated into this mixture and the final mixture was vortexed again and then the samples were incubated for 10 min at room conditions (25 ± 3 °C). The absorbance values of the samples were measured at 506 nm using a spectrophotometer (UV–Vis). The results were expressed mg QE/kg by using quercetin calibration curve (Kasangana et al. 2015).

Antioxidant activity of the samples

For the determination of the antioxidant activities of the samples, three different antioxidant analyses namely Radical scavenging activity (DPPH), phospomolybdenum test and ferric reducing antioxidant power (FRAP) test were used. In this context, for the test, 500 µL of the samples was mixed with 2500 µL of deionized water and then 1000 µL of molybdate reagent (by dissolving 8.25 g sodium molybdate, 4.0 g dipotassium hydrogen phosphate, and 4.0 g potassium dihydrogen phosphate in 500 mL of water) was incorporated. After mixing the samples, the sample tubes were placed in a waterbath at 95 °C for 90 min for the incubation. At the end of the incubation (for about 30 min), the samples were removed from the bath and allowed to the temperature of the samples to drop to room temperature about 25 ± 3 °C. The absorbance values of the samples were recorded at 695 nm using a spectrophotometer (UV–Vis). The antioxidant activity of the samples was calculated as mg ascorbic acid equivalent (mg AAE) per kilogram of pestil samples (mg AAE/kg) (Parmer 2012).

DPPH radical scavenging activity of the samples was determined according to modified method of Uysal et al. (2014). For this aim, sample (100 µL) was mixed with 3000 µL of DPPH solution (0.1 mM in methanol) and this mixture was vortexed for a while. All of these samples were incubated for 30 min at dark conditions. At the end of the incubation, the absorbances of the samples were measured at 517 nm (UV–Vis). The DPPH radical scavenging performance of the samples was calculated by using Eq. 1. Ascorbic acid calibration curve was used to calculate the radical scavenging activity values of the samples as mg AAE/kg;

$$\%Inhibition=\left(\frac{Ac-As}{\mathit{Ac}}\right)\times 100$$ 1

where A_c is the absorbance of the control sample and A_s is the absorbance of the sample.

For the FRAP test, 250 µL of each sample diluted with distilled water was mixed with 2750 µL of FRAP reagent (2 mL; 0.01 mol TPTZ (2,4,6-tripyridyl-s-triazine) in 0.04 mol HCl, 0.02 mol FeCl₃·6H₂O and 0.3 mol acetate buffer) and the mixture was incubated for 30 min at 25 ± 3 °C. At the end of the incubation, the absorbance values of the samples were measured at 593 nm by a spectrophotometer (UV–Vis). Ferric reducing antioxidant capacity of the samples was calculated as mg ascorbic acid equivalent per kilogram of pestil samples (Uysal et al. 2014) (mg AAE/kg).

Sensory analysis

Pestils were served to trained panel consisting of twenty (ten members are female and panelists age were between 18 and 30 years old) members (faculty and graduate students of Inonu University, Food Engineering Department) for sensory analyses. A hedonic scale sensory analysis was selected for the evaluation of samples. The analysis include a scaling method to be color (1 = very brown, 9 = desired yellowness), firmness (1 = undesired texture, 9 = desired texture), taste/odor (1 = undesired, 9 = desired), stickiness (1 = undesired, 9 = desired) and overall acceptability (1 = dislike, 9 = like). The samples coded with random three-digit numbers were served to the panelists. Also between the samples, the drinking water was given to the panelists to rinse their mouths.

Statistical analysis

In the present study, a one factor experimental design with two replicates at the center point was chosen for the modeling of processing variables (rice flour) and the predictive regression models were constructed. Table 1 shows the coded and uncoded values of the factors, levels and experimental design. Second-order polynomial equation of function X_i as stated below was fitted for each response analyzed:

$$Y=b_0+b_i{X}_i+b_{\mathit{ii}}{X}_{\mathit{ii}}^2$$

where Y is the estimated response; b₀, b_i, b_ii are constants. X_i and X_ii are processing variables. Uncoded values were utilized for performing all analysis and the number of tests could be limited to 7 as seen in Table 1. The experimental combinations were implemented in duplicate (1–4th and 5–6th runs as indicated in Table 1) in the center point of the model. The response surface analysis was carried out using Design Expert statistical package software (Version 7.0.0.a Stat Ease Inc. Hennepin, MN, USA).

Result and discussion

Some physicochemical characteristics and instrumental color (L*, a* and b*) properties of the pestil samples and also variance analysis results were given in Table 2. As is seen from the table, dry matter, ash, titratable acidity, a_w, protein, HMF, L*, a* and b* values of the samples were determined in this study. For the dry matter content of the samples, minimum and maximum responses for dry matter content (86.87–90.5 g/100g) were observed for the pestil samples prepared by 0 g/100g rice flour and 12 g/100g wheat flour, 12 g/100g rice flour and 0 g/100g wheat flour, respectively. Karaoğlu et al. (2020) reported that the dry matter content of pestils produced with whole grain flours ranged between 90.23 and 92.67 g/100g. Also, similar results were reported by Yuksel et al. (2020). The ash content of the samples increased with the addition of rice flour into the formulation and the maximum and minimum ash values were recorded as 1.12–0.67 g/100g, respectively. The main reason of the increment in the ash levels by the addition of the rice flour is the richer mineral composition of the rice flour compared to wheat flour (Sudha et al. 2007; Ahmed et al. 2015). As can be seen to Table 2, the titratable acidity values ranged from 0.20 to 0.33 SSA% and R5 and R6 have no any rice flour were the pestil samples showing the lowest acidity values compared to others. Karaoğlu et al. (2020) reported that the titratable acidity level of pestil produced by using whole grain flours was in the range of 0.40–0.73% citric acid equivalent. Similar results for the titratable acidity content (0.15–0.22 SSA%) were reported by Yuksel et al. (2020) for mulberry pestil produced with coconut flour. The protein content of the samples decreased with the incorporation of rice flour into the pestil formulation. According to the one factor design, the protein contents of the pestils were measured as in the range of 4.18–5.91. The highest protein content was monitored as 5.91 for the R6 sample containing 0 g/100g rice flour and 12 g/100g wheat flour. In the literature, the protein contents of the wheat flour (12.08–14.50%) were reported to be higher compared to rice flour (7.2–10.47%) (Kumar and Prabhasankar 2013; Ijah et al. 2014; Ahmed et al. 2015) and so the protein content of the samples in the present work was similar to the ones reported in the literature.

Table 2.

Some physicochemical characteristics and instrumental color properties of the pestil samples

Samples	Dry matter (g/100g)	Ash (g/100g)	Titratable acidity SSA%	Water activity (aw)	Protein (g/100g)	HMF (mg/kg)	L*	a*	b*
R1	86.87 ± 0.47	1.12 ± 0.04	0.30 ± 0.02	0.58 ± 0.01	4.32 ± 0.27	2.96 ± 0.11	44.12 ± 1.42	5.12 ± 0.60	22.06 ± 2.17
R2	88.59 ± 0.38	0.89 ± 0.03	0.24 ± 0.01	0.54 ± 0.01	4.98 ± 0.25	2.98 ± 0.05	41.35 ± 0.74	5.53 ± 0.51	21.63 ± 1.36
R3	88.13 ± 0.35	0.93 ± 0.06	0.26 ± 0.02	0.56 ± 0.01	4.63 ± 0.14	3.01 ± 0.17	42.16 ± 1.08	5.34 ± 0.47	21.78 ± 0.49
R4	87.41 ± 0.21	1.09 ± 0.02	0.33 ± 0.02	0.57 ± 0.02	4.18 ± 0.13	3.05 ± 0.14	42.89 ± 0.89	5.03 ± 0.58	22.87 ± 1.65
R5	90.03 ± 0.13	0.69 ± 0.04	0.22 ± 0.01	0.51 ± 0.01	5.76 ± 0.52	3.89 ± 0.17	39.76 ± 1.10	6.01 ± 0.31	20.43 ± 1.55
R6	90.55 ± 0.59	0.67 ± 0.03	0.20 ± 0.01	0.52 ± 0.01	5.91 ± 0.47	4.04 ± 0.26	40.52 ± 0.97	6.12 ± 0.36	21.01 ± 1.78
R7	89.08 ± 0.08	0.78 ± 0.04	0.24 ± 0.01	0.53 ± 0.01	5.33 ± 0.16	3.24 ± 0.10	41.03 ± 0.84	5.81 ± 0.62	21.32 ± 0.54

Variance analyses	Dry matter g/100g	Ash g/100g	Titretable acidity SSA%	Water activity (aw)	Protein g/100g	HMF mg/kg	L*	a*	b*
A	89.7*	197.1*	31.9*	84.5*	429.9*	86.1*	39.6*	379.1*	21.7*
A2	0.2	0.7	1.4	0.2	1.1	28.3*	0.6	0.1	9.842E−004
Model	44.9*	98.9*	16.6**	42.4*	215.5*	57.2*	20.1*	189.6*	10.8**
Lack of fit	0.62	5.75	0.85	0.05	0.22	1.92	0.14	0.14	0.16
R2	0.95	0.98	0.89	0.95	0.99	0.96	0.91	0.99	0.84

A Rice flour, R² determination coefficient

*p < 0.01, **p < 0.05

The HMF contents of the pestil samples varied between 2.96 and 4.04 mg/kg and it was observed that the HMF content of samples decreased with the addition of the rice flour into the pestil formulation (Table 2). In Turkey, the HMF content of pestil products was limited to be 50 mg/kg by Turkish Standard Institution (Yuksel et al. 2020; TSE 2000) and so the presented results for HMF in the current work were suitable according to the standards. Some studies reported that the HMF levels of the pestils were 22.45 mg/kg (Baltacı et al. 2016), 11.21–21.55 mg/kg (Yuksel et al. 2020) and 18.00 mg/kg (Yıldız et al. 2011) which showing that the HMF contents of the pestil samples were quite lower than the maximum level stipulated by TSE.

Maximum and minimum lightness (L*), redness (a*) and yellowness (b*) values of the pestil samples were 42.89, 6.12 and 22.87 and 39.76, 5.03 and 20.43, respectively. Maximum instrumental color properties (L*, a* and b*) of the samples was determined for the R4 and R6 samples containing 12 g/100g rice flour and 0 g/100g wheat flour, 0 g/100g rice flour and 12 g/100g wheat flour, respectively. The L* and b* levels of the samples increased with the addition of rice flour into the pestil formulation while the a* level decreased. The color characteristics of the pestil samples could be affected by the ingredients used in the pestil manufacturing process and also the followed methodology for the fabrication. Also, the lightness value of the rice flour was reported to be higher than the wheat flour while the redness and yellowness levels of the rice flour were lower compared to wheat flour (Wanyo et al. 2009). There are two important reactions occurred during the fabrication of the pestils that affect the color characteristics of the final products which are Maillard and caramelization reactions (Yuksel et al. 2020; Karaoğlu et al. 2020). These reactions products occurred during the manufacturing process show a significant effect on the final products and caused an important desired or undesired changes on the color values of the samples. Taking into account all of these reasons, the instrumental color properties of the manufactured pestil samples in the current research could be affected from these reactions and rice flour concentration. The linear interaction (A) of dry matter, ash, titratable acidity, a_w, protein, HMF, L*, a* and b* values were determined to be significant (p <  0.01) and the constructed models can be used to predict these values of the samples depending on the rice flour concentration. Also, the quadratic effects (A²) of HMF were determined to be significant (p < 0.01). Determination coefficients of some physicochemical and instrumental color parameters of the pestil samples were calculated to be quite high (R² = 0.84–0.99, Table 2).

The total phenolic and flavonoid contents, antioxidant activity, DPPH radical scavenging activity and FRAP levels of the samples and also the variance analysis results were tabulated in Table 3. As can be seen in the table, the total phenolic content of the samples increased with the increase in rice flour concentrations in the pestil formulations while the total phenolic content of the samples showed a clear decrement with the addition of wheat flour into formulation. Also, the results for the antioxidant activity, DPPH, FRAP and total flavonoid content of the samples showed a similar increment with the increase of rice flour in the formulations. Maximum levels for the total phenolic content, antioxidant activity, DPPH, FRAP and total flavonoid content of the samples were determined for the R4 sample containing 12 g/100g rice flour and 0 g/100g wheat flour while minimum levels for the same parameters was observed for the R5 sample containing 0 g/100g rice flour and 12 g/100g wheat flour. These results could be explained by the highest bioactivity of the rice flour compared to wheat flour because the rice flour showed higher phenolic compound levels compared to wheat flour (Chen and Ho 1997; Wanyo et al. 2009). In the literature, the phenolic content of rice flour was reported to be in the range of 1.05–1.95 mg GAE/g (Chinma et al. 2015) while the total phenolic content of wheat flour was between 177 and 257 µg GAE/g (Vaher et al. 2010). The linear interactions (A) of processing variable on total phenolic content, antioxidant activity, DPPH, FRAP and total flavonoid content were determined to be significant (p < 0.01) and the model can be used to predict these values of samples depending on the rice flour concentration. Also, the quadratic effects (A²) of the related variables were not significant (p > 0.05). Determination coefficients of total phenolic content, antioxidant activity, DPPH, FRAP and total flavonoid content of the pestil samples were calculated to be high (R² = 0.89–0.96, Table 3).

Table 3.

Bioactive properties of the pestil samples

Samples	TPC (mg GAE/kg)	AA (mg AAE/kg)	DPPH (mg AAE/kg)	FRAP (mg FeSO4/kg)	TFC (mg QE/kg)
R1	1353.97 ± 77.4	16,343.94 ± 496.1	257.20 ± 57.5	13,027.94 ± 762.6	2950.29 ± 147.3
R2	1065.94 ± 107.7	14,318.86 ± 766.9	198.73 ± 36.5	9931.28 ± 223.3	1978.68 ± 104.5
R3	1129.59 ± 88.9	15,051.34 ± 261.5	236.12 ± 43.4	11,914.21 ± 322.0	2517.21 ± 180.4
R4	1474.93 ± 118.6	17,701.65 ± 767.4	329.91 ± 72.5	14,506.78 ± 770.3	3025.16 ± 249.5
R5	887.91 ± 51.4	12,684.09 ± 465.7	101.34 ± 8.27	7964.39 ± 409.8	1701.69 ± 65.7
R6	968.03 ± 91.2	13,785.19 ± 985.0	122.88 ± 6.24	8667.15 ± 495.5	1396.34 ± 102.1
R7	984.98 ± 124.8	13,971.37 ± 261.4	168.00 ± 47.4	9229.95 ± 104.4	1928.85 ± 118.4

Variance analyses	TPC (mg GAE/kg)	AA (mg AAE/kg)	DPPH (mg AAE/kg)	FRAP (mg FeSO4/kg)	TFC (mg QE/kg)
A	60.8*	32.1*	45.1*	91.1*	105.7*
A2	5.0	1.8	0.01	2.6	2.0
Model	32.9*	17.0**	22.5*	46.8*	54.0*
Lack of fit	0.52	0.19	0.08	0.09	0.71
R2	0.94	0.89	0.92	0.96	0.96

A, Rice flour; R², determination coefficient; TPC, total phenolic content; AA, antioxidant activity; DPPH, 2,2-diphenyl-1-picrylhydrazyl; FRAP, ferric reducing antioxidant power; TFC, total flavonoid content

*p < 0.01, **p < 0.05

The results of frequency sweep test (G′, G″, G^* and tanδ) obtained from rheological characterization of herle and variance analyses are tabulated in Table 4. As is seen from this table, the elastic (G′) and viscous modulus (G′′) of the samples increased with the increase of rice flour at 10–50 Hz while these parameters decreased with the increase in the wheat flour concentrations in the formulation. Tanδ values (10 Hz) of the samples showed an increase with the increase in wheat flour concentration (12 g/100g) while these values of the samples showed a decrement with the addition of rice flour. Also, the maximum tan δ value of the samples recorded at 50 Hz was monitored for the sample R3 containing 9 g rice flour and 3 g wheat flour while minimum tan δ value was for the sample R7 containing 3 g rice flour and 9 g wheat flour. Maximum complex modulus (G^*) value of the pestil sample was measured as 4767.7 Pa for the R6 sample containing 0 g/100g rice flour and 12 g/100g wheat flour while the minimum G^* value was recorded as 3261.1 Pa for the sample R1 containing 12 g/100g rice flour and 0 g/100g wheat flour in the formulation. The effects of linear interaction (A) of processing variable on the complex modulus, elastic modulus, viscous modulus and tan δ (10 and 50 Hz) of the samples were determined to be significant (p < 0.01 and p < 0.05) and the model can be used to predict the change of these values of the samples depending on the rice flour concentration. Also, the quadratic effects (A²) of rice flour were not determined to be significant (p > 0.05) except the tan δ values of (10 Hz) the sample (p < 0.05). Determination coefficients of the complex modulus, elastic modulus, viscous modulus and tan δ (10 and 50 Hz) of the samples were calculated to be quite high (R² = 0.79–0.99, Table 4). The dynamic rheological parameters of the samples (herle) were affected by the flour concentrations in the formulation. These results could be speculated with starch content of the rice and wheat flour. Because, viscosity of foods occurs with gelatinization reaction and the starch has the most important role for these reactions (Anuntagool et al. 2017; Ai and Jane 2015). As is examined the starch contents of these flours, the starch content of the rice flour and wheat flour were reported to be 84.66% (Puncha-arnon and Uttapap 2013) and 75.23% (Yuksel 2019), respectively and also the amylose content of these starches were stated as 25.01% for rice flour (Puncha-arnon and Uttapap 2013) and 35.36–33.50% for wheat flour (Wanyo et al. 2009), respectively. Thus, the starch concentration of the flours and also the levels of the amylose in the structure of the starch could affect the viscosity and also the other rheological parameters of the pestil samples.

Table 4.

Dynamic shear rheological parameters of samples (Herle)

Samples	G*	G′ (10 Hz)	G″ (10 Hz)	G′ (50 Hz)	G′′ (50 Hz)	Tan (δ) (10 Hz)	Tan (δ) (50 Hz)
R1	3261.1 ± 93.1	9280.6 ± 276.8	2105.8 ± 74.7	15,637.3 ± 210.8	5081.7 ± 42.5	0.23 ± 0.00	0.33 ± 0.01
R2	3972.8 ± 86.9	11,012.7 ± 214.1	2978.2 ± 76.9	17,920.8 ± 460.1	5893.7 ± 27.6	0.27 ± 0.01	0.33 ± 0.01
R3	3608.1 ± 167.5	10,701.6 ± 299.7	2859.1 ± 62.7	17,347.8 ± 280.9	5541.7 ± 179.7	0.27 ± 0.01	0.32 ± 0.00
R4	3412.6 ± 206.3	9678.9 ± 189.8	2072.0 ± 115.0	15,938.8 ± 218.6	5021.5 ± 181.6	0.22 ± 0.01	0.32 ± 0.01
R5	4682.9 ± 146.2	14,012.5 ± 192.3	3922.2 ± 144.3	20,898.8 ± 194.5	7104.2 ± 229.5	0.28 ± 0.01	0.34 ± 0.01
R6	4767.7 ± 111.2	14,436.7 ± 406.9	3821.4 ± 198.4	21,150.4 ± 262.6	7036.4 ± 237.5	0.27 ± 0.02	0.34 ± 0.01
R7	4307.8 ± 61.4	11,226.8 ± 222.7	3131.2 ± 176.8	17,991.8 ± 204.5	6165.5 ± 111.4	0.28 ± 0.02	0.34 ± 0.00

Variance analyses	G*	G′ (10 Hz)	G′′ (10 Hz)	G′ (50 Hz)	G′′ (50 Hz)	Tan (δ) (10 Hz)	Tan (δ) (50 Hz)
A	547.0*	48.4*	69.1*	53.4*	220.1*	20.1**	15.1**
A2	1.8	2.8	1.931E−003	1.3	3.1	8.1**	0.1
Model	274.4*	25.6*	34.5*	27.4*	111.6*	14.1**	7.6**
Lack of fit	0.05	9.32	30.25	24.13	17.60	1.1	0.90
R2	0.99	0.93	0.94	0.93	0.98	0.88	0.79

A Rice flour, R² determination coefficient

*p < 0.01, **p < 0.05

The panel scores of the sensory analysis for the pestil samples are tabulated in Table 5 and Fig. 1. As is seen from this table, the color values of the samples were in the range of 5.9–6.9 and maximum color value was given for the R6 sample containing 0 g/100g rice flour and 12 g/100g wheat flour. The highest firmness score of the samples was determined to be 6.5 for R6 sample containing 0 g/100g rice flour and 12 g/100g wheat flour while the lowest firmness value was given was 5.6 for the R1 sample containing 12 g/100g rice flour and 0 g/100g wheat flour. The taste/odor and stickiness values of the samples were in the range of 5.5–6.8 and 5.7–6.7, respectively. As is seen from the overall acceptability scores of the samples, the highest value was 6.6 for the R6 sample containing 0 g/100g rice flour and 12 g/100g wheat flour while the lowest score was 5.8 for the R2 sample containing 6 g/100g rice flour and 6 g/100g wheat flour. Generally, the sensory scores of the samples decreased with the incorporation of rice flour into pestil formulation (Fig. 1). The effects of the linear interaction (A) of the rice flour on the color, firmness, stickiness and overall acceptability values of the samples were determined to be significant (p < 0.05) while the taste/odor values of the samples was not affected significantly (p > 0.05). The constructed regression model can be used to predict these values of samples depending on the rice flour concentration. Also, the quadratic effects (A²) of the processing variable did not showed a significant effect (p > 0.05). Determination coefficients of the color, firmness, stickiness and overall acceptability parameters of the samples were calculated to be moderately high (R² = 0.72–0.84) while the value for the taste/odor was calculated to be very low (R² = 0.03) (Table 4). In addition to that, an optimization study to reveal the best pestil formulation was performed and for this study, the sensory scores of the pestil samples enriched rice and wheat flour were used. The levels of the rice and wheat flours were calculated to be 0.44 g/100g and 11.56 g/100g, respectively and the desirability function score was calculated to be high (0.64) (Fig. 1).

Table 5.

Sensory analysis scores of the pestil samples

Samples	Color	Firmness	Taste/odor	Stickiness	Overall acceptability
R1	6.4 ± 0.84	5.6 ± 1.17	6.1 ± 0.99	5.9 ± 0.74	5.9 ± 0.57
R2	6.3 ± 0.82	5.8 ± 0.63	5.5 ± 0.53	6.1 ± 0.57	5.8 ± 0.42
R3	5.9 ± 0.74	6.2 ± 0.63	6.1 ± 0.74	5.7 ± 0.48	5.9 ± 0.57
R4	6.2 ± 1.03	5.9 ± 1.10	6.0 ± 0.67	6.1 ± 0.99	6.2 ± 0.63
R5	6.7 ± 0.95	6.5 ± 0.71	5.9 ± 0.74	6.3 ± 0.82	6.6 ± 0.52
R6	6.9 ± 0.57	6.5 ± 0.53	6.2 ± 0.79	6.7 ± 0.48	6.6 ± 0.52
R7	6.5 ± 0.53	6.4 ± 1.07	6.8 ± 0.42	6.1 ± 0.57	6.4 ± 0.52

Variance analyses	Color	Firmness	Taste/odor	Stickiness	Overall acceptability
Variance analyses
A	12.6**	10.3**	0.1	7.9**	14.5**
A2	5.2	0.1	5.693E−003	2.8	6.5
Model	8.9**	5.2	0.1	5.3	10.5**
Lack of fit	1.97	3.92	16.60	0.62	1.49
R2	0.82	0.72	0.03	0.73	0.84

A Rice flour, R² determination coefficient

*p < 0.01, **p < 0.05

Fig. 1.

Change in sensory analysis scores of the samples and desirability function of optimization process depending on the rice flour concentration

Conclusion

The pestil formulations were enriched with the rice flour to develop a new formulation. The addition of the rice flour into the pestil recipes provided a reduction of the HMF formation and an increment in the total phenolic content and antioxidant activity of the final products. According to optimization parameters, the best flour concentration for the pestil formulation was determined to be 0.44 and 11.56 g/100g rice flour and wheat flour, respectively. The results of the current research showed that the pestil samples enriched with rice flour may find opportunity to be commercialized as a healthy and new formulation.

Funding

The authors have not disclosed any funding.

Declarations

Conflict of interest

The authors have not disclosed any competing interests.