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Ahari H, Massoud R, Sharifan & A. The Effect of Cuminum Essential Oil on Rheological Properties and Shelf Life of Probiotic Yoghurt. JNFS 2020; 5 (4) :296-305
URL: http://jnfs.ssu.ac.ir/article-1-292-en.html
Department of Food Sciences and Technology, Sciences and Research Branch, Islamic Azad University, Tehran, Iran.
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The Effect of Cuminum Essential Oil on Rheological Properties and Shelf Life of Probiotic Yoghurt
 
Hamed Ahari; PhD 1, Ramona Massoud; PhD 2 & Anousheh Sharifan; PhD *1
 
1Department of Food Sciences and Technology, Sciences and Research Branch, Islamic Azad University, Tehran, Iran.
2 Department of Food Sciences and Technology, Standard organization, Tehran, Iran.
 
ARTICLE INFO   ABSTRACT
ORIGINAL ARTICLE  
Background: Antimicrobial and antioxidant properties of the essential oils (EOs) are important in food industry. This study investigated the effects of EO from Cuminum cyminum on the preservation of yogurt containing Streptococcus thermophilus, Lactobacillus bulgaricus, and Bifidobacterium bifidum. Methods: The yogurt samples’ biological, physicochemical, and sensorial characteristics were evaluated at three levels of 1%, 2% and 3% of EO during the storage
(7, 14, and 21 days). Results: The bacterial activity declined significantly in all samples during the storage (P < 0.05). Adding EO decreased the rate of bacterial growth of both probiotic and starter strains over 21 days. The titratable acidity (TA) and syneresis were increased (P < 0.05), while the pH levels were decreased (P < 0.05) during the storage. The highest viscosity was 8600 mili paskal secent (mPa.s) in control sample in the first day and 5700 mPa.s for 1% cuminum cyminum essential oil (CEO) on the 21st day. The CEO had no effects on flavor, odor, and overall acceptability. Conclusion: Generally, the high level of phenols influenced the biological, physicochemical, and rheological properties of bioyogurts positively.
 
Keywords: Yoghurt; Probiotic; Cuminum; Essential oil
Article history:
Received:7 Dec 2019
Revised: 21 Jun 2020
Accepted: 21 May 2020
*Corresponding author:
sharyfana@gmail.com
Department of Food Science and Technology, Science and Research Branch, Islamic Azad University, Tehran, Iran.
 
Postal code: 1477893855
Tel: +226 70 35 62 27
 
Introduction
 
The literature showed that consumption of fermented dairy products had therapeutic and nutritional values (Vanegas-Azuero and Gutiérrez, 2018). Probiotics have beneficial effects that
can improve the intestinal microflora (Güler
and Gürsoy-Balcı, 2011
). Lactobacillus and Bifidobacterium are the most well-known probiotic strains (Lee and Salminen, 2009). The viability of probiotics in yogurt is frequently less than 6 log CFU/g during the storage (Massoud et al., 2015, Sarkar, 2008). On the other hand, the final flavor of yogurt is changed due to the addition of nonvolatile acids (lactic and pyruvic), volatile acids (butyric and acetic), miscellaneous compounds (amino acids), and carbonyl compounds (acetaldehyde and diacetyl) (Güler and Gürsoy-Balcı, 2011, Massoud et al., 2014). Additionally, other factors such as microbial growth, texture, and flavor may be affected in the presence of phenolic compounds because of their interactions with proteins during the fermentation and storage (Da Silva et al., 2017, Massoud et al., 2016).
Essential oils (EOs), defined as aromatic and oily liquids, are extracted from different plants (Otaibi and Demerdash, 2008). The antimicrobial properties of EOs would enhance the shelf life of certain foods (El–Nawawy et al., 1998). The EOs inhibits the growth of pathogens that may contaminate food and microorganisms responsible for food spoilage. Golestan et al. showed the antibacterial effects of Mentha spicata and Mentha aquatic EOs against Staphylococcus aureus, Lactobacillus reuteri, Bifidobacterium animalis, and Clostridium perfringens in kashk (a fermented dairy product)  (Golestan et al., 2016). Yangilar et al. successfully produced probiotic yogurt with ginger and chamomile EO (0.2 and 0.4%). The probiotic yogurt and the yogurt with ginger EO (0.4%) contained the highest rate of probiotics (B. lactis BB-12) (Yangilar and Yildiz, 2018). Some compounds from rosemary are responsible for the antimicrobial activity specially α-pinene, bornylacetate, and 1, 8-cineole (Hussain et al., 2010).
The aim of the present study was to determine the effects of EO extracted from Cuminum cyminum (CEO) on the pH values, survival of microorganisms, syneresis, rheological parameters, and sensory characteristics of bioyogurt containing Bifidobacterium bifidum during fridge storage.
Materials and Methods
Plant Material and EO Extraction: Cuminum cyminum L. (Green Cumin) was purchased from a local plant shop in Isfahan, Iran. The seeds of cumin were dried at 25 ˚C. Dried seeds of cumin were hydrodistilled for 3 hours with 500 ml of distilled water. The extracted CEO was collected and stored in tightly sealed glass vials in a refrigerator at 4 ˚C.
Gas Chromatography-Mass Spectrometry Analysis of the EOs: The chemical composition of the EO was investigated by a gas chromatograph (Agilent Technologies, USA) associated with
a mass spectrophotometer (Agilent Technologies, USA) with HP-5Ms capillary column (30.0m×0.25mm×0.25 μm). The initial temperature was 50 ºC for 2 minutes that subsequently elevated to 200 ºC at a rate of 3.5 ºC/min.
Primary culture preparation: Bifidobacterium bifidum PTCC 1644 was obtained from the Iranian Research Organization for Science and Technology. Lyophilized bacteria were transferred into a tube containing 10 ml of MRS broth that were incubated at 37 °C for 24 hours. The bacteria were then cultivated on MRS-bile agar (Merck, Germany) and anaerobically incubated at 37 °C for 48 hours.
Preparation of Bifidobacterium bifidum yogurt containing the CEO:  The raw milk (2.5% fat) was divided into seven 250 ml containers and heated to 85 °C for 20 min.  Bifidobacterium bifidum (140 μl), yogurt starter cultures YC-x11 (120 μl) (CHR Hansen, Denmark) with Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus added to each container after cooling to 44±1 °C. Afterwards, the CEO (1%, 2%, and 3%) was added into the containers and homogenized. The yogurt samples were placed in an incubator at 43 ± 1 ºC. Incubation ended when the samples reached pH = 4.6.
Bacterial count: 1 ml of the yogurt samples was diluted using 9 ml of sterile peptone water (Merck, Germany). Thereafter, 0.1 ml of dilutions was put on MRS-bile, MRS and M17 agars. The first two plates were incubated at 35-37 °C for three days under the anaerobic conditions, while the latter was subjected to incubation for 2 days at 35–37 ºC. The colonies were counted applying a colony counter (Vinderola and Reinheimer, 1999). 
pH and acidity measurement: The pH values were measured using a pH meter (Swiss, Metrohm 632). Acidity based on lactic acid amount was measured with titration method using 0.1 N sodium hydroxide and phenolphthalein (ISIRI 695).
Syneresis: 10 g of yogurt was put into the centrifuge tubes and weighted (W0). The supernatant was removed after centrifuging at 350 G at 10 °C for half an hour. The syneresis of yogurt was calculated based on the following formula (Amatayakul, 2005):
% Rs = We /Wg × 100
Where, we is the weight of whey released from yogurt and Wg is the initial weight of yogurt.
Rheological properties: The flow behavior parameters, including apparent viscosity, consistency index, and flow index were measured using co-axial cylinders (Rheostress 300, ThermoHaake, Germany). The analysis was performed in triplicate with the shear rate varying from 0 to 22 s−1 in up-down-up steps. Thixotropic index refers to the area between the upward and downward shear stress curves. The Herschell-Bulkley model was utilized for fitting data  (Karsheva et al., 2013):
σ=σ0+˙n
Where, σ is the shear stress (Pa), σ0 is the yield stress (Pa), k is the consistency index (Pa.sn), γ is the shear rate (s−1), and n is the flow index (dimensionless). The apparent viscosity, η, was described as the ratio of shear stress, σ, to shear rate, γ.
Sensory evaluation: 10 experienced panelists evaluated the aroma, flavor, texture, and overall acceptance. All organoleptic attributes were rated by a five-point hedonic scale from one to five; strongly dislike (1) to strongly like (5)  (Ahmed, 2011).
Data analysis: The cell counts and physiochemical properties were expressed as mean ± standard deviation (SD) of three replicates using SPSS (version 20.0, SPSS Inc.). Analysis of variance test and Duncan’s multiple range tests were used to examine the significant difference. A P-value of less than 0.05 was considered as highly significant.
Results
The chemical composition of the cuminum cyminum essential oil (CEO): The compounds of the CEO are shown in Table 1. The major 3 constituents of the CEO out of 18 were Cumin aldehyde (55%), γ-terpinene (14.01%), and Limonene (10.04%). The mean total phenol content of the CEO was 31.41 ± 0.25 mg gallic acid/g.
Bacterial activity: Microbiological counts of B. bifidum, L. bulgaricus, and S. thermophilus are presented in Table 2. The probiotic bacteria and starter cultures significantly were reduced over 21 days of storage at 4 °C (P < 0.05). In this study, the viability of S. thermophiles and L. bulgaricus showed the maximum decrease of 0.80 and 0.98 log10 CFU/g in the control sample during the storage time, respectively.
Titrable acidity (TA) and pH: The addition of EO did not change the TA and pH of control. Storage period had significant changes in both TA and pH (P < 0.05) (Table 3). TA enhanced in all samples during refrigeration over 21 days and pH decreased in all samples within 21-day storage at 4 ºC (P < 0.05). Treatment with 2% and 3% CEO exhibited a significantly lower pH and higher TA.
Syneresis: It is determined by the balance between attraction and repulsion forces in casein network (Amatayakul, 2005) . In Table 3, syneresis remarkably increased throughout the storage at 4 ºC in the following order (lowest to highest): Control (1.42%), Y1 (1.73%), Y2 (2.64%), and Y3 (2.82%).
Flow behavior: The flow behavior of all yogurt samples is summarized in Table 4. The samples showed pseudoplastic fluid characteristics owing to the flow behavior index (n) values below 1 (R2 > 0.99). Adding the EO reduced n values considerably from 0.501 (control sample) to 0.240 (sample with 3% CEO) on the first day (P< 0.05). The refrigerator storage only changes n values of the control sample significantly (P < 0.05). The higher concentrations of EO did not change the flow behavior towards the dilatant fluid (n > 1). The consistency index notably increased from
8.24×10-2 Pascal secend (Pa.sn) (control sample) to 10.63×10-2 Pa.sn (sample with 1% CEO) on the first day of storage (P < 0.05).
Sensory evaluation: The overall scores of aroma, odor, texture, and acceptance for the yogurt samples are demonstrated in Table 5. The control samples had the highest mean score of flavor, odor, texture, and acceptability. The yogurt samples containing 2% and 3% CEO represented the lowest sensory scores. The refrigerator storage did not affect the organoleptic properties significantly (P > 0.05).
 
Table 1. The chemical composition of the cuminum cyminum essential oil (CEO)
 
Compound Retention index Peak area
α-Thujene 930 1.05
α-pinene 938 0.47
Sabenene 981 0.37
β-pinene 987 5.38
Myrcene 995 1.07
α-phellandrene 1000 0.94
p-cymene 1013 0.60
Limonene 1028 10.04
1,8-cineole 1031 0.10
γ-terpinene 1072 14.01
m-cymenene 1085 0.20
trans-pinocarveol 1130 0.07
Cumin aldehyde 1167 55.00
p-cymene-8-ol 1182 0.40
α-terpineol 1188 0.20
Myrtenol 1195 0.10
o-cumenol 1196 0.10
Carvacrol 1299 5.40
 
 
Table 2. The survival of the starter and probiotic bacteria in the different yogurts during storage
 
Yogurt samples Storage time (days) B. bifidum L. bulgaricus S. thermophilus
Control
 
 
 
1 8.18 ± 0.01f 6.92 ± 0.05d 7.01 ± 0.00e
7 8.32 ± 0.03g 7.17 ± 0.02e 6.64 ± 0.09c
14 7.98 ± 0.09e 6.10 ± 0.01b 6.21 ± 0.03b
21 7.71 ± 0.02d 6.12 ± 0.02b 6.03 ± 0.01ab
Y1
 
 
 
1 8.16 ± 0.01f 6.62 ± 0.05cd 7.12 ± 0.01f
7 8.01 ± 0.07e 6.84 ± 0.09d 7.14 ± 0.01f
14 7.80 ± 0.09d 6.72 ± 0.01d 6.72 ± 0.00cd
21 7.48 ± 0.01c 6.98 ± 0.06d 6.88 ± 0.03d
Y2
 
 
 
1 8.09 ± 0.01e 6.84 ± 0.04d 7.04 ± 0.02e
7 7.74 ± 0.07d 7.19 ± 0.04e 7.17 ± 0.05f
14 7.24 ± 0.06c 7.03 ± 0.00de 6.84 ± 0.06d
21 6.01 ± 0.09a 6.82 ± 0.03d 6.95 ± 0.04e
Y3
 
 
 
1 8.01 ± 0.02e 7.25 ± 0.08ef 6.94 ± 0.01e
7 7.53 ± 0.06cd 8.00 ± 0.06g 7.60 ± 0.01g
14 6.84 ± 0.05b 7.72 ± 0.01f 6.80 ± 0.02d
21 5.85 ± 0.07a 7.50 ± 0.06f 6.82 ± 0.00d
Mean and standard deviation values with different letters are significantly different (P < 0.05). Probiotic yogurt with no essential oil: Y1 (1% cuminum cyminum essential oil), Y2 (2% cuminum cyminum essential oil  and Y3 (3% cuminum cyminum essential oil)
 

 
Table 3. The effect of  essential oils and storage on the titrable acidity , pH, and syneresis of the probiotic yogurt samples
 
Yogurt samples Storage time (day) Titrable acidity (%) pH Syneresis (%)
Control
 
 
 
1 0.82 ± 0.01b 4.64 ± 0.06a 3.93 ± 0.14d
7 0.85 ± 0.01b 4.53 ± 0.01ab 4.59 ± 0.41c
14 0.89 ± 0.03c 4.42 ± 0.01bc 4.81 ± 0.33c
21 0.91 ± 0.03c 4.34 ± 0.08c 5.35 ± 0.16b
Y1
 
 
 
1 0.79 ± 0.05a 4.62 ± 0.05a 4.01 ± 0.50d
7 0.83 ± 0.01b 4.51 ± 0.04ab 4.54 ± 0.71c
14 0.87 ± 0.01c 4.40 ± 0.01bc 4.36 ± 0.00c
21 0.90 ± 0.01c 4.35 ± 0.01c 5.74 ± 0.83b
Y2
 
 
 
1 0.79 ± 0.01a 4.61 ± 0.01a 3.75 ± 0.41d
7 0.82 ± 0.01b 4.55 ± 0.01b 4.82 ± 0.00c
14 0.83 ± 0.01b 4.41 ± 0.09bc 5.09 ± 0.85c
21 0.88 ± 0.02c 4.37 ± 0.01c 6.39 ± 0.08b
Y3
 
 
 
1 0.78 ± 0.01a 4.63 ± 0.01a 4.05 ± 0.50d
7 0.81 ± 0.01b 4.51 ± 0.01ab 5.79 ± 0.50b
14 0.84 ± 0.07b 4.42 ± 0.08bc 6.13 ± 0.25b
21 0.87 ± 0.07c 4.39 ± 0.01bc 6.87 ± 0.14a
Mean and standard deviation values with different letters are significantly different (P < 0.05). Probiotic yogurt with no essential oil: Y1 (1% cuminum cyminum essential oil), Y2 (2% cuminum cyminum essential oil  and Y3 (3% cuminum cyminum essential oil)
 
 
Table 4. Rheological parameters from the Herschel–Bulkley model and thixotropic index for the yogurt with the essential oils during storage
 
Variables Storage time (day) Study groups
Control Y1 Y2 Y3
Behavior index 
 
 
1 0.50 ± 0.001d,B 0.24 ± 0.002c,A 0.24±0.004bc,A 0.24 ± 0.002b,A
7 0.50 ± 0.003c,B 0.24 ± 0.001b,A 0.24 ± 0.001b,A 0.24 ± 0.003b,A
14 0.49 ± 0.001c,B 0.24 ± 0.001b,A 0.24 ± 0.002b,A 0.24 ± 0.001b,A
21 0.41 ± 0.004c,A 0.24 ± 0.003b,A 0.23±0.003ab,A 0.23 ± 0.001b,A
Consistency index (×102 Pascal- secnds) 1 8.24 ± 1.05a,B 10.63 ± 2.12b,B 11.27 ± 2.05c,A 10.71 ± 1.00b,AB
7 8.00 ± 2.02a,A 10.22 ± 1.04b,A 11.25 ± 2.17c,A 10.60 ± 1.08b,A
14 8.45 ± 1.46a,C 10.89 ± 1.43b,C 11.39 ± 1.26c,B 10.98 ± 2.01b,C
21 8.32 ± 3.08a,B 10.71 ± 2.35b,BC 11.25 ± 2.07c,A 10.85 ± 2.73b,B
Apparent viscosity (×102 Pa.s)
 
1 0.86 ± 0.01c,B 0.69 ± 0.02b,B 0.49 ± 0.01a,B 0.42 ± 0.02a,B
7 0.80 ± 0.02c,A 0.64 ± 0.01b,AB 0.45 ± 0.00a,AB 0.39 ± 0.02a,B
14 0.78 ± 0.01c,A 0.60 ± 0.02b,A 0.41 ± 0.03a,A 0.34 ± 0.00a,AB
21 0.75 ± 0.00c,A 0.57 ± 0.00b,A 0.38 ± 0.01a,A 0.30 ± 0.01a,A
Thixotropic index (Pa.s-1)
 
1 21.50 ± 1.16c,B 14.70 ± 1.11b,B 9.25 ± 0.12ab,B 7.25 ± 1.18a,A
7 20.06 ± 1.13c,AB 13.78 ± 1.08b,AB 9.12 ± 0.12ab,B 6.15 ± 1.14a,A
14 18.40 ± 1.12b,A 14.18 ± 1.17b,B 8.13 ± 1.03a,AB 6.04 ± 1.12a,A
21 19.64 ± 1.10c,AB 11.45 ± 0.17b,A 6.28 ± 0.06a,A 7.00 ± 0.12a,A
Mean and standard deviation values with different letters are significantly different (P < 0.05). Probiotic yogurt with no essential oil: Y1 (1% cuminum cyminum essential oil), Y2 (2% cuminum cyminum essential oil  and Y3 (3% cuminum cyminum essential oil)
 
 
Table 5. Sensory attributes of the control and supplemented yogurts according to a 5-point hedonic scale during the storage
 
Variables Storage time (day) Study groups
Control Y1 Y2 Y3
Flavor
 
1 4.9  ± 0.03b,A 2.6 ± 0.07a,A 1.3 ± 0.06a,A 1.0 ± 0.04a,A
21 4.7 ± 0.00b,A 2.8 ± 0.01a,A 1.4 ± 0.05a,A 1.0 ± 0.04a,A
Odor
 
1 4.5 ± 0.01b,A 2.0 ± 0.05a,A 1.0 ± 0.07a,A 1.0 ± 0.07a,A
21 4.4 ± 0.01b,A 2.1 ± 0.04a,A 1.1 ± 0.00a,A 1.0 ± 0.01a,A
Texture
 
1 5.0 ± 0.01b,A 4.1 ± 0.00b,A 3.1 ± 0.00a,A 3.1 ± 0.00a,A
21 5.0 ± 0.08b,A 4.0 ± 0.01b,A 3.0 ± 0.06a,A 2.8 ± 0.00a,A
Overall acceptability 1 4.8 ± 0.01b,A 3.1 ± 0.00a,A 2.1 ± 0.06a,A 1.1 ± 0.07a,A
21 4.7 ± 0.01b,A 2.9 ± 0.01a,A 1.9 ± 0.01a,A 1.0 ± 0.00a,A
Mean and standard deviation values with different letters are significantly different (P < 0.05). Probiotic yogurt with no essential oil: Y1 (1% cuminum cyminum essential oil), Y2 (2% cuminum cyminum essential oil  and Y3 (3% cuminum cyminum essential oil)
 
 
Discussion
Some studies were conducted over the chemical composition of EO obtained from Cuminum cyminum indigenous in Iran. The main component of EO is cumin aldehyde (46.10%) and
β-pinene (10.93%) (Mehdizadeh et al., 2017). Mohammadpour et al. identified 28 compounds in the CEO with α-pinene (29.2%), Limonene (21.7%), 1, 8-cineole (18.1%), and Linalool (10.5%) (Mohammadpour et al., 2012). The CEO’s chemical composition was determined with almost similar main constituents of Cumin aldehyde 25.2%, γ-terpinene 19.0%, ρ-mentha-1, 4-dien -7-al 16.6%, and ρ-mentha-1, and 3-dien-7-al 13.0% (Derakhshan et al., 2010).
Many studies reported the effects of different additives on the bacterial activity in yogurt. Blackcurrant polyphenol extract and cyanidin (cyanidin 3-o-β-glucopyranoside chloride) were added to yogurt (Sun-Waterhouse et al., 2013) .The findings indicated that some bindings took place  between phenolic compounds and milk proteins or polysaccharides in the yogurt (Papadopoulou and Frazier, 2004). The interaction between carbonyl compounds and phenolic compounds might also occur and form carbonyl–phenol bonds  (Hidalgo et al., 2017). In this study, adding EOs with different phenolic profiles altered the starter culture activity during the storage. This finding was in accordance with the study by (Da Silva et al., 2017, Hadad Khodaparast et al., 2007). revealed that the survival of yogurt starter culture decreased significantly by increase of the concentrations
of Ziziphora clinopodioides EO, (Hadad Khodaparast et al., 2007). Jimborean et al. reported that the yogurt incorporated with orange EOs showed an increased growth of the lactic acid bacteria (Jimborean et al., 2016). Phenolic compounds possess different antimicrobial properties  (Rauha et al., 2000). Additionally, the bacteria type may be related to the antimicrobial properties (Lee et al., 2006).  Bifidobacterium demonstrated a lower viability in the presence of CEO: 0.68-2.16 log10 CFU/g during storage. Marhamatizadeh et al. reported a relation between the growth of B. bifidum and the concentrations of olive and dill extract (Marhamatizadeh et al., 2013). In another study, they indicated that the bioavailability of lactic acid bacteria improved in the presence of coffee extract (Marhamatizadeh et al., 2014) .de Lancey et. al. noted that the presence of green tea extracts increased the survival of L. paracasei, L. acidophilus, and B. animalis ssp. lactis during the incubation at 37 ºC for three days (de Lacey et al., 2014) .
There are some studies on the effect of EOs
on physical and chemical characteristics of
yogurt. Ghalem and Zouaoui reported that the yogurt samples containing 0.36 g/l of Chamaemelum spp. extract showed an increased level of TA during the storage (Ghalem and Zouaoui, 2013b). Moritz et al. observed significantly higher TA in yogurt samples containing mint EOs (Moritz et al., 2012). Yangilar and Yildiz reported that the yogurt samples with ginger and chamomile EOs had higher acidity levels (Yangilar and Yildiz, 2018) .Joung et al. reported that yogurt samples with Nelumbo nucifera had lower pH  (Joung et al., 2016). Furthermore, pH varied within the range of 4.08 and 4.66 in yogurt samples with Chamaemelum extract. In another study, they showed unchanged levels of pH in the yogurt samples with R. officinalis EO during the storage period (Ghalem and Zouaoui, 2013a). The lactic acid production correlated to the viable counts of probiotic strain and prevented the growth of starter cultures due to acid production during the storage (Moritz et al., 2012). Shahdadi et al. also reported similar results; EOs reduced the lactic acid production, which in turn increased the survival of the probiotics (Shahdadi et al., 2015) .
The synersis is the most important parameter that influenced consumer’s acceptance level  (Gürsoy et al., 2010). Synersis is possibly due to the effect of low pH on casein particles, which improves the resistance of yogurt (Lucey and Singh, 1997) .Other factors resulting in synersis include high incubation temperature, low dry matter content, or high storage temperature (Lucey, 2004) .In a study, the synersis rate was elevated from 4.7% (v/w) to 8.3% (v/w) over 28 days (Panesar and Shinde, 2012a, b). In another study synersis increased by elevating the CEO concentration. The polyphenols would enhance rearrangements, which leads to a larger pore size in the gel matrix and higher synersis (Gürsoy et al., 2010) .Phenolic compounds can establish strong links with the primary metabolites, including proteins and carbohydrates (McManus et al., 1985).
The high flow behavior was also reported for the probiotic yogurt with grape extract (Da Silva et al., 2017) .The higher concentrations of EO did not change the flow behavior towards the dilatant fluid (n > 1)(Da Silva et al., 2017) .
According to Table 4, the EOs significantly mitigated the apparent viscosity of probiotic yogurts (P < 0.05), and the control sample showed the highest rate on the first day (i.e., 8600 mPa.s), while the sample containing 3% CEO had the lowest rate on the 21st day (i.e., 3000 mPa.s). The same trend was also observed by Vanegas-Azuero and Gutiérrez, 2018. Some reports pointed out that the positive contribution of the rearrangement of proteins increased during storage (Abu-Jdayil and Mohameed, 2002, Isleten and Karagul-Yuceer, 2006)
On the other hand, the results of thixotropic tests presented that the yogurt samples had a thixotropic non-Newtonian behavior. This phenomenon
results from the breakage of the gel when a shear force is used (Bourne, 2002). The thixotropic characteristic of the control sample was higher than other samples. The values of the hysteresis area were notably affected by increasing the content of EOs during the storage period (P < 0.05). Yogurts without any additive had notably higher sensory scores for color, appearance, flavor, texture, synersis, odor, acidity, and general acceptability than the controls (Vanegas-Azuero and Gutiérrez, 2018, Yangilar and Yildiz, 2018)
Conclusion
This study indicated that yogurt supplemented with CEO had a notable potential to deliver Bifidobacterium bifidum with sufficient population. The lowest pH and highest acidity rates belonged to groups with 1% CEO. Statistical analyses revealed that during the refrigerated storage, the bacterial counts, pH, sensory characteristics, and viscosity reduced, while the acidity values and sensory scores increased. Generally, the CEO contributed positively to the biological, physicochemical, and rheological properties of the bioyogurts, but not to the overall acceptance.
Acknowledgment
The authors acknowledge the Sciences and Research Branch of Islamic Azad University that cooperate for doing this work.
Conflicts of Interest
The team of authors declares that there is no conflict of interest.
Authors’ contribution
All authors contributed to data gathering, analyzing, and writing the manuscript and final revising it.
 
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Type of article: orginal article | Subject: public specific
Received: 2019/12/7 | Published: 2017/11/15 | ePublished: 2017/11/15

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