Development of yoghurt powder using microwave-assisted foam-mat drying

Abstract

Yoghurt powder is widely used in industries of confectionery and baking. The production of yoghurt powder can be made by several drying methods, including freeze, spray, microwave vacuum, convective and foam-mat. In this study, the effect of varying concentrations of egg albumin (EA) on foam and powder characteristics of yoghurt were determined. Besides, microwave-assisted foam-mat drying of yoghurt was carried out to examine the effects of three microwave output powers (100, 180 and 300 W) on powder properties of yoghurt. Increased in EA concentration resulted in an increase in foam expansion and decrease in foam density. Higher foam stability (after 120 min.) was observed for foams containing 10 and 15% EA (both found as 88.24%). As powder properties, wettability and solubility times were significantly decreased with the addition of EA, while water holding capacity was increased. Change in EA concentration was significantly (p ≤ 0.05) effective on all powder properties dried at 100 W. Carr Index and Hausner Ratio values were in the range of 0.99–13.89 and 1.01–1.17, respectively. Microwave powers were significantly (p ≤ 0.05) effective on the flowability of powders containing 0, 5 and 10% EA. Yoghurt powders showed mostly excellent flow characteristics (for all concentrations of EA and microwave output powers).

Introduction

Yoghurt is a fermented dairy product, obtained by lactic acid fermentation of milk (Hayaloglu et al. 2007) by addition of starter cultures Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus (Mckinley 2005). Yoghurt is a probiotic food product which provides health benefits and therefore, it is widely consumed because of its nutritional value (Hnin et al. 2020). On the other hand, its shelf life is about 1 day in 25–30 °C, 5 days in 7 °C and 10 days at 4 °C (Kumar and Mishra 2004), therefore, some methods are used to extend its shelf life such as straining yoghurt for removal of water, sun-drying of yoghurt etc. (Say et al. 2015). Drying of yoghurt to produce powder provide less volume, reduced transportation, packaging and storage costs (Koç et al. 2014). Drying of yoghurt can be made by freeze-drying, microwave, spray and convective drying. Yoghurt powder can be used in the production of confectionery, baked goods, yoghurt flavored chocolates, wafers and in preparation of soups (Hayaloglu et al. 2007).

Foam-mat drying is a method that uses whipping of semi-liquid or liquid food to produce stable foam by incorporation of air. Foaming agents are used to provide stable foams for longer times especially for subsequent drying (Abbasi and Azizpour 2016). Methylcellulose, guar gum, soy lecithin, glycerol monostearate (GMS) and egg white (egg albumin) are widely used as foaming agents in the production of food foams. Foam-mat drying provides a larger drying surface area due to the incorporation of air inside the foam. Therefore, it usually decreases the drying time. Besides, reconstitution and powder properties of obtained powders are good and superior to spray and drum dried products due to the honeycomb structure (Malik and Sharma 2019).

In literature, there are many studies on yoghurt drying by freeze-drying (Sharma et al. 1992), spray drying (Koç et al. 2014), microwave vacuum drying (Kim and Bhowmik 1994), convective air drying (Hayaloglu et al. 2007) and foam drying (Carvalho et al. 2017; Malik and Sharma 2019). However, combined drying techniques such as microwave and foam-mat have not been used to determine the effects on powder and reconstitution properties of yoghurt. The objectives of this study are to investigate the effects of the concentration of foaming agent on foam and powder properties of yoghurt by using microwave-assisted foam-mat drying, and in addition, to determine the effects of different microwave output powers on powder properties of yoghurt.

Material and methods

Materials

Light yoghurt (in 100 g of yoghurt, fat (g): 1.4, carbohydrate (g): 6.2, protein (g): 4.0) and eggs used in the experiments were procured from a local market in Antalya, Turkey. Distilled water was used for the analysis of powder properties.

Preparation and drying of yoghurt foam

Coagulum of raw yoghurt was broken by manual stirring. Fresh egg albumen (EA) (obtained from eggs at 25 °C) was used as a foaming agent in varying concentrations of 0 (control sample), 5, 10 and 15% (weight of EA/weight of yoghurt). Yoghurt foam was obtained by mixing with a mixer (Harmony 550, Fakir Hausgerate, Germany) for 3 min at maximum speed. The foam was spread on a flat plate (11 cm in diameter) and dried at 5 mm thickness at microwave power intensities of 100, 180 and 300 W using a domestic microwave oven (GW73E, Samsung, South Korea). Drying was stopped below 10% of moisture content. Dried yoghurt flakes were collected and ground to a powder using a grinder for 1 min. (PRG 266, Premier, Turkey). The obtained powders were sieved with a sieve with 0.5 mm of aperture. Then, powders were kept in polyethylene bags and kept at 4 °C until further analysis.

The analysis of foam properties

Foam density

The density of foams was measured according to the method used by Ng and Sulaiman (2018). The density of yoghurt foam was measured a mass per volume of foam expressed in g cm⁻³ as shown in Eq. (1).

$$\text{Foam density =}\frac{\text{mass of foam (g)}}{{\text{volume of foam (cm}}^3)}$$ 1

Foam expansion

Foam expansion indicates that the ability of air incorporated in the yoghurt foam structure. Yoghurt foam density was expressed by Durian (1995) and determined as shown in Eq. (2).

$$\mathrm{Foam}\mathrm{expansion}=\frac{{\mathrm{V}}_1-{\mathrm{V}}_0}{{\mathrm{V}}_0}\times 100$$ 2

where V₁ was final volume of foamed yoghurt, cm³, V₀ was the initial volume of mixture, cm³.

Foam stability

Foam of the yoghurt samples was inserted into 50 mL of a graduated cylinder and left at room temperature (25 °C) for 2 h. Every 30 min, the volume reduction was measured (Ng and Sulaiman 2018). By using Eq. (3), stability was expressed as a percentage, where V_t is the volume of foam after 30 min., V₀ is the initial foam volume.

$$\mathrm{Foam}\mathrm{stability}=\frac{{\mathrm{V}}_{\mathrm{t}}}{{\mathrm{V}}_0}\times 100$$ 3

The analysis of foam mat dried yoghurt powder

Moisture content

Moisture contents of yoghurt foams and powders obtained after the drying process were measured by AOAC (1990) method.

Water holding capacity

The water holding capacities of yoghurt powders were determined according to Stone et al. (2015). Water holding capacity was determined by suspending 0.5 g of powder in 5.0 g of water in a screw cap centrifuge tube. Samples were vortexed (Heidolph, Heidolph Instruments GmbH & Co. KG, Schwabach, Germany) for 10 s every 5 min for a total of 30 min and then centrifuged (Sigma 2-16KL, Germany) at 10,000 rpm for 15 min at 25 °C. The supernatant was carefully removed and the remaining powder was weighed. Water holding capacity was calculated by dividing the weight gained by the powder by the original powder weight (%).

Wettability

Wettability was measured according to the method of Malik and Sharma (2019). 1 g of powder was poured on 10 mL distilled water at ambient temperature and wetting time of powder in seconds was recorded.

Solubility

The solubility time of yoghurt powder was carried out according to Goula and Adamopoulos (2008). 2 g of powder was poured in 50 mL distilled water containing beaker at 25 °C and mixed with a magnetic stirrer (Heidolph, Heidolph Instruments GmbH & Co. KG, Schwabach, Germany) at 500 rpm. The time required for the powder to dissolve completely was recorded in seconds.

Bulk and tapped density

Bulk density is determined by measuring the occupied volume of 2 g yoghurt powder in 100 mL graduated cylinder without being tapped (Eq. (4)). On the other hand, tapped density is measured after tapping the graduated cylinder containing powder 80 times before recording volume of the powder (Eq. (5)).

$$\text{Bulk density}\left({\mathrm{\rho }}_{\text{B}}\right)=\frac{\text{mass of powder (g)}}{{\text{volume of powder (cm}}^3)}$$ 4

$$\text{Tapped density}\left({\mathrm{\rho }}_{\text{T}}\right)=\frac{\text{mass of powder (g)}}{{\text{final tapped volume (cm}}^3)}$$ 5

Flowability and cohesiveness

The flowability and cohesiveness of the powders were evaluated in terms of Carr index (CI) (Eq. (6)) and Hausner ratio (HR) (Eq. (7)) by using bulk (ρ_B) and tapped (ρ_T) density determination, respectively (Asokapandian et al. 2016). The classification of flowability based on CI and HR values is given in Table 1.

$$\text{CarrIndex}\left(\text{CI}\right)=\frac{{\mathrm{\rho }}_{\text{T}}-{\mathrm{\rho }}_{\text{B}}}{{\mathrm{\rho }}_{\text{T}}}$$ 6

$$\text{Hausner Ratio}\left(\text{HR}\right)=\frac{{\mathrm{\rho }}_{\text{T}}}{{\mathrm{\rho }}_{\text{B}}}$$ 7

Table 1.

The classification of flowability based on CI and HR values (Asokapandian et al. 2016)

Flowability	Carr index (CI)	Hausner ratio (HR)
Excellent	0–10	1.00–1.11
Good	11–15	1.12–1.18
Fair	16–20	1.19–1.25
Passable	21–25	1.26–1.34
Poor	26–31	1.35–1.45
Very poor	32–37	1.46–1.59
Very, very poor	> 38	> 1.60

Statistical analysis

Statistical analysis on the results were conducted by the analysis of variance (ANOVA) (p ≤ 0.05) by using SPSS software (version 21.0) (IBM Software, NY, USA). Duncan’s multiple range tests were carried out to determine the effects of EA concentration and microwave output power on all responses. All experiments were replicated and analyses were duplicated. Results were given as average ± standard deviation.

Results and discussion

Effect of EA concentration on the properties of yoghurt foam

Foam density and expansion were calculated based on Eqs. (1) and (2), respectively. There were significant (p ≤ 0.05) effects on foam density and foam expansion with varying EA concentration. Increasing EA concentration caused a decrease in foam density, in contrast to foam expansion (Table 2.). Similar results were observed in the study of Falade et al. (2003) in which foam-mat drying of cowpea was made using fresh egg albumin at various concentrations as a foaming agent. Egg albumen addition significantly increased foam expansion due to the incorporation of air into the sample and formation of bubbles. When compared to the control sample, for yoghurt foam with 15% EA, almost 4 times higher foam expansion was observed. Similar to the study of Ng and Sulaiman (2018) in which egg albumin and fish gelatin as foaming agents were used and the effects on foaming properties of beetroot were studied.

Table 2.

Foam properties of yoghurt

Egg albumen concentration (EA) (%, w/w)	Foam density (g cm−3)	Foam expansion (%)
0	1.02 ± 0.02c	9.26 ± 2.62a
5	0.91 ± 0.01b	17.82 ± 1.63b
10	0.77 ± 0.01a	33.35 ± 1.46c
15	0.76 ± 0.01a	35.83 ± 0.76c

Each column followed by different superscripts is significantly different (p ≤ 0.05)

On the other hand, a decrease in foam density was observed because of the expansion in foam volume. It was reported that foamed materials have a lower density than non-foam materials (Ratti and Kudra 2006). Air bubbles were trapped in the foam during whipping and volume were risen with lowering the density. At low concentration of the foaming agent, high foam density was caused due to restriction of movement of the foaming agent from the aqueous phase towards the air-aqueous interface (Karim and Wai 1999). During drying, low density of foam could provide better water removal due to the larger surface area.

Foam volume observed for 120 min and foam stability in percentage is shown in Fig. 1. Foam stability shows the ability of binding water and in foam mat drying, it matters greatly because foam should able to preserve its structure during the drying process (Khamjae and Rojanakorn 2018). In this study, even though all yoghurt foams held their initial volumes for 30 min, foams prepared with 10 and 15% EA held their initial volumes for 60 min. Samples with 10 and 15% EA had higher foam stability values (88.24%) at 120 min than control and 5% EA containing samples. These results could be observed because of the increase in protein concentrations increases the interfacial thickness of film which provides coating and stabilizing of air bubbles. Very low difference in stability values between the control sample and foams containing foaming agent was observed. It was stated by Ng and Sulaiman (2018), the foam could be collapsed due to the thinner film formation in lower concentrations of foaming agent and gasses could be escaped which results in shrinkage and volume reduction.

Fig. 1.

Foam stability of yoghurt foams containing different concentration of EA

Effect of EA concentration on the powder properties of yoghurt

Powder properties of foam-mat dried yoghurt are illustrated in Table 3. Moisture contents of yoghurt powders were found between 4.80 and 9.98% (wet basis, w.b.) which were microbiologically safe. Similar final moisture contents were obtained by Qadri and Srivastava (2017) in which study, they dried guava pulp with microwave-assisted foam-mat drying (0.04 and 0.07 g H₂O/g dry matter). Moisture contents of powder containing EA were increased when microwave output power increased from 100 to 300 W. The reason could be that the crust formation at higher microwave powers which did not let water removal from the product. In addition, Çalışkan Koç and Dirim (2018) revealed that the percentage of the solid part decreased with the increase in the moisture content of the powders and the density value of the powders decreased due to the bigger density value of the solid part.

Table 3.

Powder properties of foam-mat dried yoghurt

Egg albumen (EA) (%, w/w)	Microwave output power (W)	Moisture content (%, w.b.)	Water holding capacity (%)	Wettability time (s)	Solubility time (s)	Bulk density (g cm−3)	Tapped density (g cm−3)	Flowability carr index (CI)	Cohesiveness hausner ratio (HR)
0	100	8.78 ± 0.01bAB	41.00 ± 9.13aA	919.50 ± 4.20aC	927.00 ± 5.77aC	0.50 ± 0.00cB	0.50 ± 0.00bA	0.99aA	1.01aA
	180	7.89 ± 0.47abC	66.25 ± 16.66aAB	933.00 ± 6.58bD	919.50 ± 3.70aC	0.40 ± 0.00aA	0.44 ± 0.00aA	10.00bA	1.11bA
	300	5.19 ± 2.93aA	98.25 ± 23.89bB	1075.50 ± 11.03cD	947.00 ± 9.80bD	0.48 ± 0.00bC	0.54 ± 0.00cC	12.50cB	1.14cB
5	100	9.93 ± 0.14aB	59.00 ± 23.80aA	911.50 ± 2.08abC	917.00 ± 3.27aC	0.45 ± 0.00bA	0.50 ± 0.00bA	9.09bB	1.10bB
	180	9.95 ± 0.58aD	49.00 ± 13.29aA	914.25 ± 2.87bC	916.50 ± 4.20aC	0.50 ± 0.00cB	0.56 ± 0.00cB	10.00cA	1.11cA
	300	9.98 ± 0.76aB	62.25 ± 12.76aA	910.25 ± 2.06aC	912.00 ± 2.45aC	0.42 ± 0.00aA	0.45 ± 0.00aA	8.33aA	1.09aA
10	100	6.22 ± 3.59aA	67.00 ± 14.47aA	651.00 ± 10.61aB	634.25 ± 6.99aB	0.63 ± 0.00cC	0.71 ± 0.00cC	12.50bC	1.14bC
	180	6.20 ± 0.11aB	77.00 ± 15.71aB	664.00 ± 0.82aB	682.25 ± 4.50cB	0.56 ± 0.00bC	0.62 ± 0.02bC	9.72abA	1.11abA
	300	7.50 ± 0.05aA	73.75 ± 16.46aAB	658.25 ± 11.84aB	669.00 ± 1.83bB	0.45 ± 0.00aB	0.50 ± 0.01aB	7.96aA	1.09aA
15	100	6.26 ± 0.39bA	103.00 ± 35.60aB	547.00 ± 1.63cA	535.00 ± 11.43aA	0.52 ± 0.00bB	0.56 ± 0.00bB	10.00aD	1.11aD
	180	4.80 ± 0.07aA	81.00 ± 20.41aB	545.00 ± 0.82bA	537.00 ± 11.43aA	0.56 ± 0.00cC	0.65 ± 0.04cC	13.89aA	1.17aA
	300	6.59 ± 0.34bA	79.00 ± 17.91aAB	543.00 ± 0.82aA	534.25 ± 11.03aA	0.45 ± 0.00aB	0.50 ± 0.00aB	9.09aA	1.10aA

Small letters indicate that samples are significantly different (p ≤ 0.05) at constant EA%. Capital letters indicate that samples are significantly different (p ≤ 0.05) at constant microwave output power. wet basis: w.b

Properties like wettability and solubility times, water holding capacity, Carr Index, Hausner Ratio, bulk and tapped densities etc. are important for commercial, economical and functional properties of powders. For drying with microwave output power of 100 W, an increase in EA concentration prompted an increase in water holding capacity and decrease in wettability and solubility times. Change in EA concentration was significantly (p ≤ 0.05) effective on all powder properties dried at 100 W. Powder containing 15% EA had the highest water holding capacity and the lowest wettability and solubility times when compared to others. For drying at 180 W, water holding capacities of yoghurt powders were increased by the addition of EA. However, the lowest water holding capacity was observed with 5% EA containing sample. Besides, these differences in the water holding capacities were not significantly (p > 0.05) effective. On the other hand, EA concentration was found to be significantly (p ≤ 0.05) effective on moisture content, wettability and solubility times, bulk and tapped densities for drying at 180 W. Water holding capacity of control sample dried at 300 W was higher than EA containing yoghurt powders, however, increasing EA concentration caused slightly increase in water holding capacity. Furthermore, change in EA concentration was significantly (p ≤ 0.05) effective on moisture content, wettability and solubility times, in contrast to water holding capacity. The more of EA concentration in yoghurt foams means more stable structure during the drying process where, more bubbles in the foam during the entire drying process. Therefore, air bubbles cause an increase in the porosity of the powder and its solubility (Abbasi and Azizpour 2016).

Wettability and solubility times of yoghurt powders were in the range of 543.00 ± 0.82–1075.50 ± 11.03 s and 534.25 ± 11.03–947.00 ± 9.80 s, respectively. Wettability and solubility times were significantly decreased with increasing EA concentration and similar behavior was observed by Malik and Sharma (2019). The lower wettability could be in relation with decrease of solubility due to denaturation of protein and indicates by lower dissolution rates (Fang et al. 2008).

Bulk density values were found between 0.44 and 0.63 g cm⁻³. Tapped densities were between 0.44 and 0.71 g cm⁻³. Higher bulk densities are desirable because of reduced transportation and packaging costs (Çalışkan Koç 2020). In the study of Malik and Sharma (2019), they investigated the effect of soy lecithin concentration on powder properties and drying efficiency of yoghurt foam. They observed the loose bulk density of yoghurt powders as 0.572 ± 0.01 g cm⁻³ and 0.574 ± 0.02 g cm⁻³ for control and treated sample with 0.4% soy lecithin, respectively. Packed bulk densities of control and treated samples were found as 0.848 ± 0.04 and 0.827 ± 0.01 g cm⁻³, respectively.

Bulk and tapped densities were significantly (p ≤ 0.05) affected by EA concentration for all microwave output powers (100, 180 and 300 W). Powder dried at 100 W and containing 10% EA had the highest bulk and tapped densities when compared to other powders. However, bulk density values of powders produced with 0 and 15% EA were not significantly (p > 0.05) different and lower than 10% EA containing sample when dried at 100 W. In contrast to these findings, bulk and tapped density values of lime juice powder obtained foam-mat drying were decreased by increasing ovalbumin concentration in the study of Dehghannya et al. (2018). Bulk densities of 10 and 15% EA containing powders dried at 180 W were the same, and for drying at 180 W, 15% EA containing powder had the highest tapped density, however there were not significantly (p > 0.05) different. For drying at 300 W, there were not significantly (p > 0.05) difference between powders containing 10 and 15% EA. The high content of egg white may be the reason for high bulk density values of yoghurt powders. It was reported when egg albumen concentration increased, bulk density values of pineapple powder were increased due to the higher molecular weight of proteins (Shaari et al. 2018).

The flow characteristics of foam mat dried yoghurt powders were evaluated in terms of Carr Index (CI) and Hausner Ratio (HR) which indicates the flowability and cohesiveness, respectively. For drying at 100 W, all powders showed an excellent flowability and cohesiveness values except powder containing 10% EA (good flowability and cohesiveness). Powders dried at 100 W were significantly (p ≤ 0.05) different and was significantly (p ≤ 0.05) effective on flowability and cohesiveness based on EA concentration. For drying at 180 W, only powder containing 15% EA showed good flowability characteristics and there were not significantly (p > 0.05) differences between powders in terms of EA concentration. At high power output of 300 W, the control sample showed the lowest flowability (good) in comparison with other powders containing EA (excellent).

Effect of microwave output power on the powder properties of yoghurt

Moisture contents of control sample decreased with increasing microwave output power from 100 to 300 W, in contrast to powders containing EA. Drying times of yoghurt foams were in the range of 56–12 min, 59–11 min, 48–12 min and 54–11 min for samples containing 0, 5, 10 and 15% EA and microwave output powers changing from 100 to 300 W, respectively. The moisture content of Nigela sativa powder (foam obtained with 15% of egg albumen, 0.69% of methyl cellulose, and 8 min of whipping time) were founds in a similar range with the values in this study (Affandi et al. 2017). The change in microwave output power was significantly effective on moisture contents of powder except containing 10% EA. Water holding capacity of powder produced a concentration of 15% EA decreased with increasing microwave output power and however, the change in microwave output power was not (p > 0.05) significantly effective on water holding capacity for powders containing EA.

Fang et al. (2008) reported that wettability is strongly affected by the surface composition (fats, proteins, and carbohydrates) and porosity. It is defined as the ability of powder particles to get over the surface tension between themselves and water. Solely, wettability times of control sample were negatively (p ≤ 0.05) affected by the increment of microwave powers which higher wettability times was not desirable. However, wettability times of powder with 15% EA were positively (p ≤ 0.05) affected by the increment of microwave output power.

One of the important parameter is solubility for evaluation of the behavior of the powders in the aqueous phase, because food powders must have high solubility to be useful and functional (Fang et al. 2008). Solubility times of powders containing 0 and 10% EA were significantly (p ≤ 0.05) affected by the change of microwave output power. It was reported that the increase in solubility with increasing temperature may be due to the increment of porosity of shrimp foam powder (Azizpour et al. 2016). Furthermore, for all powders, the microwave output power was (p ≤ 0.05) effective on bulk and tapped densities. Moreover, bulk and tapped density of powders were significantly (p ≤ 0.05) different at constant EA% (Table 3.). Meaning that the bulk density of microwave dried yoghurt powders is directly in relation with microwave output power. Drying at higher microwave power (at 300 W) lowered the bulk density values when compared with 100 W, this could be due to the solid structure formation at high microwave output power before the air get out from the foam (Varhan et al. 2019). Moreover, Freudig et al. (1999) claimed that the low tapped density values of the powders resulted in higher wettability times.

Flowability characteristics, CI and HR values were in the range of 0.99–13.89 and 1.01–1.17, respectively and significantly (p ≤ 0.05) affected by microwave output power for powders containing 0, 5 and 10% EA. Similar results were observed by Varhan et al. (2019) for microwave-assisted foam-mat dried fig powder. Dehghannya et al. (2019) reported that increased convective air drying from 50 to 70 °C improved flowability characteristics (CI and HR). The authors used methyl cellulose (1.5% weight/volume, w/v) and ovalbumin (2% w/v) as foaming agents to produce lime juice foam. They also reported that an increase in drying temperature caused an increase in water solubility index.

Two-way ANOVA was also applied to observe the effects of EA concentration and microwave output powers on powder properties of yoghurt foam. The results showed that the EA concentration and microwave output power showed significant interaction (p ≤ 0.05) on moisture content, water holding capacity, wettability and solubility times, tapped density, Carr Index and Hausner Ratio of yoghurt powder.

Conclusion

In this study, microwave-assisted foam-mat drying was successfully used to obtain yoghurt powder. The effects of varying egg albumin concentration on the foaming and powder properties of yoghurt were investigated. Moreover, the effects of microwave output power on powder properties were also determined. Foam expansion values were increased with the increment of EA concentration where foam density values were decreased, and there was not significant (p > 0.05) difference between yoghurt foams produced with 10 and 15% EA. On the other hand, powder properties are important in terms of economical, commercial and functional concerns. Wettability and solubility times were significantly decreased with the addition of EA while water holding capacity was increased. Flowability and cohesiveness are used to define flow characteristics of powder and are determined by measuring bulk and tapped densities. Most of the yoghurt powders including the control sample showed excellent flow characteristics where some of them had good flow characteristics. Yoghurt powder containing 15% EA and dried at 100 W could be chosen as best due to the excellent flow characteristics and desired powder properties (lower wettability and solubility times, higher water holding capacity, bulk and tapped density values). This study provides valuable information about foaming characteristics and powder properties of microwave-assisted foam-mat dried yoghurt containing varying EA concentration. Different foaming agents (soy lecithin, guar gum etc.) and drying techniques may be investigated with further studies.

Author contributions

ANY carried out literature survey and experiments, analyzed the data and wrote the manuscript.

Funding

The author received no financial support for the research, authorship, and/or publication of this article.

Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Compliance with ethical standards

Conflicts of interest

The author declares no conflict of interest.