Quality assessment of yogurt enriched with different types of fibers

ABSTRACT This study evaluated the effect of the addition of four different types of dietary fibers on the rheological, physicochemical and sensory characteristics of yogurt. The four types of fibers (inulin, pea, oat and wheat) were added in the yogurt formulation in different proportions (1%–2.5%) using classical technology adapted to laboratory conditions. The obtained results showed that, the most viscous samples were obtained with wheat fibers addition (1% and 1.5%), while the best viscous characteristics were obtained for the samples with oat fibers addition (2% and 2.5%). The lowest syneresis value (38.86 ± 0.2) were observed for the samples with 1.5% pea fibers addition. Yogurt samples with the highest acceptance scores were samples with 2% wheat fibers and respectively with 2.5% pea fibers addition. All the tested fibers were compatible with the yogurt-manufacturing process. Therefore, the fibers addition in yogurt could be considered an alternative to incorporate dietary fibers in the human diet.


Introduction
Yogurt is one of the most consumed healthy and nutritious foodstuff worldwide (Shi et al., 2017;Zhi et al., 2018). Yogurt has a better digestibility of proteins than milk and many latent positive effects on health by providing the human body prebiotic and probiotic bacteria. Additionally, by incorporating fibers in yogurt, researchers have achieved a mean of increasing fibers consumption in all sectors of the populace and they have developed a functional food with an extensive array of beneficial effects. Several studies reported prebiotic fortification by adding dietary fibers in yogurt. Consumption of highfiber yogurt may prevent or reduce obesity, diabetes, cancer, hypercholesterolemia, gastrointestinal disorders, colonic diverticulosis and constipation, ulcerative colitis, hyperlipidemia, hypertension, coronary artery disease, but also promote intestinal microflora and gastrointestinal immunity (Dello Staffolo, Sato, & Cunha, 2017;Dhingra, Michael, Rajput, & Patil, 2012;Hoppert et al., 2013;Ramirez-Santiago et al., 2010;Sah, Vasiljevic, McKechnie, & Donkor, 2016;Tomic et al., 2017).
Many researchers reported that the rheological properties of yogurt are affected differently depending on the type of fiber source (Luana et al., 2014;Raju & Pal, 2014;Dello Staffolo, Bertola, Martino, & Bevilacqua, 2004;Hashim et al., 2009). The role in increasing the water holding capacity, in stabilization of high fat yogurt, in enhancing viscosity characteristics and the gel forming ability are properties of fibers that allow the development of fiber-enriched yogurt with improved texture and reduced syneresis (Dello Staffolo et al., 2017;Balthazar et al., 2016;Lazaridou, Serafeimidou, Biliaderis, Moschakis, & Tzanetakis, 2014;Espírito-Santo et al., 2013). According to Glibowski and Rybak (2015), the inulin addition in skimmed yogurt contributed in obtaining an yogurt with similar texture properties when compared with control full-fat yogurt. Sanz, Salvador, Jiménez, and Fiszman (2008) reported that yogurt fortification with oat fibers allowed the development of a product with no significant diminution in the sensorial quality, but with slight diminution in texture quality. However, Hashim et al. (2009) reported that the addition of 0.5% oat β-glucan or inulin and guar gum were effective in improving serum retention and viscoelastic properties of yogurt.
Oat and wheat fibers are the most frequently used auxiliary materials in the dairy industry, leading to fortified finished products. Oat fibers (containing β-glucan, an indigestible polysaccharide) were proven to increase immunity, to improve anticancer activity and lower blood cholesterol, lipids and blood glucose. Adding oat fibers in yogurt fostered the creation of a good fermented product, with insignificant drop in flavour quality and only a minor decline in texture quality (Dello Staffolo et al., 2004;Sanz et al., 2008).
According to the EFSA (2010), "wheat fibers help to accelerate intestinal transit." Wheat bran is extremely rich in fibers, as well as in minerals such as potassium, phosphorus and magnesium. Wheat dextrin, extracted from wheat starch, is widely used to add fibers in processed foods especially for its contribution in lowering cholesterol (lowdensity lipoprotein [LDL] and total cholesterol) and in reducing the risk of type 2 diabetes and coronary heart disease. Wheat fibers improve the products quality characteristics in dietetic foods, meat products, pasta, bread, baked goods, cheese and other dairy products (Krasaekoopt & Watcharapoka, 2014).
Inulin is an indigestible carbohydrate which is a natural prebiotic agent, obtained industrially mainly from chicory and Jerusalem artichoke (Delgado & Bañón, 2018). In the dairy industry, inulin has been used to replace fat, while improving taste and texture of low-fat/non-fat dairy products. The texture achieved by using inulin is attributed to its propensity to form small aggregates of microcrystals that seal a great amount of water, thus leading to a fine creamy texture (Crispín-Isidro, Lobato-Calleros, Espinosa-Andrews, Alvarez-Ramirez, & Vernon-Carter, 2015;Tseng & Zhao, 2013).
Trying to create functional foods with improved functionality and health benefits has become of great interest for researchers worldwide, fortifying yogurt with pea fibers being an appealing alternative. Pea fibers (insoluble dietary fibers extracted from pea hulls) are proved to ensure significant cardiovascular benefits, predominantly via cholesterol reducing means. It promotes satiety thus helping in weight-loss. Pea fibers blend wonderfully with other ingredients and they are good bulking agents. Due to their quality, they have the potential to be used in a wide range of applications: from bakery products to dairy industry. Their high dietary fiber content may increase the nutritional profile of the finished product, thus, by fortifying yogurt with pea fibers it completes its healthy properties (Bitaraf,  Manufacturers are directly interested in using natural ingredients as an alternative to chemical stabilizers in making dairy products. Taking into account this fact the one of the goals of this study was to determine the type and amount of fibers which can be recommended for the industrial production of yogurt in order to improve the quality of the final product. The aim of this study was to evaluate the effect of the addition of four types of fibers (inulin, pea, oat and wheat) on the rheological, physicochemical and sensory characteristics of yogurt.

Materials
The yogurt samples were obtained in laboratory conditions, using the following raw materials: cow's milk with 3.5% fats, 4.5% carbohydrates, 3% proteins; lactic bacteria cultures (Lactobacillus bulgaricus and Streptococcus thermophillus) were supplied by Danisco Romania S.R.L. The different fibers used in the experiments were provided by Enzymes & Derivates Romania: inulin fibers (I), pea fibers (P), oat fibers (O) and wheat fibers (W). Inulin fibers (I) are presented as a white homogeneous powder with min. 95% fibers, max. 8% moisture and low energetic value (1.3 kcal/g). We used the long-chain type inulin (high performance, highly polymerised, HP). Pea fibers (P) are presented as a white powder with min. 48% fibers, 39% starch, min. 7% protein, max. 8% moisture. Oat fibers (O) are presented as a dark white homogeneous powder with min. 96% fibers, max. 8% moisture and the water retention capacity of 4.8 g H 2 O/g. Wheat fibers (W) are presented as a white homogeneous powder with min. 97% fibers, max. 8% moisture and the water retention capacity of 4.9 g H 2 O/g."

Experimental conditions for yogurt samples preparation
For the yogurt production, the cow's milk was first pasteurized at 90°C for 5 minutes and cooled to 45°C. After cooling, the milk was inoculated with the starter culture. In all samples, milk was directly inoculated with 0.02% (w/v) starter culture of Lactobacillus bulgaricus and Streptococcus thermophillus. After a strong stirring for the evenly distribution of the culture, the inoculated samples were transferred over the inulin, pea, oat and, respectively, wheat fibers, which were previously dosed directly into the yogurt jars in varying proportions from 1% to 2.5%. The fermentation process was carried out at 43°C, until a pH of 4.6 was reached. Subsequently, the finished products were stored at 4°C -6°C for the next 24 h.

Physicochemical analysis
The titratable acidity of yogurt samples was expressed in Thörner degrees, after sample titration with NaOH 0.1N. The portable F2 Standard METTLER TOLEDO pH-meter was used for the pH measurement in different stages of samples preparation: during fermentation, in finished product and during storage. Colour has been determined using a Konica CR400 Chromameter (Konica Minolta, Japan). The samples were measured against a white spectrum. The colour intensity, hue angle and ΔE* were calculated using the following equations: where: L* (100 = white; 0 = black), a* (+, red,green), b* (+yellow;blue), C ab *chroma, h ab *hue angle and ΔE*colour difference.
The syneresis determination was performed by the means of a Spin MPW 223E Centrifuge, with respect to the method used by Gengatharan, Dykes, and Choo (2017): 10 ml of sample volume; sample centrifugation at 639 × g for 10 min, at 4 ± 1°C; the clear supernatant scaling and applying the below formula for obtaining the syneresis percent of samples: All analyses were carried out in triplicate.

Rheological analysis
The Modular Advanced Rheometer System (Thermo Haake Mars) was used to study the rheological properties of yogurt samples. The samples were allowed to rest for 10 minutes at 8°C on the Ti40mm geometry plate, before conducting the analyses. The samples were subjected to frequency dependency experiments from 0.1 to 10.0 Hz, at 8°C. The storage modulus (G'), the loss modulus (G"), the loss tangent (tan δ) and the complex viscosity modulus (|η*|) at 1 Hz frequency were monitored. Also, there were conducted viscosity tests depending on the shear rate (0.02-100 s −1 ) and depending on time (10 minutes at a constant shear rate of 100 s −1 ). The rising curves of viscosity at different shear rates, from 0.02 to 100 s −1 , for the yogurt samples, were adjusted to Bingham, Ostwald de Weale, Casson and Herschel-Bulkley models. Three determinations of each test were conducted for every sample (Mathias, Carvalho Junior, Carvalho, & Sérvulo, 2011). The Haake RheoWin Data Manager software was used for obtaining the graphical representation of viscosity curves and the values in the tables.

Sensory analysis
For the sensory analysis, the yogurt samples were evaluated by a consumer panel formed by students and academic staff of the Food Engineering Faculty from 'Ștefan cel Mare' University of Suceava. Each sample was equally dosed (30 mL) in a glass beaker. The glasses were presented to tasters at 10 ± 1°C, with their coding number. The scoring method was used as a quality assessment system, according to SR 6345:1995, on a scale of 0-20 points. Thereby, there were attributed points from 0 to 5 with their weighting factor (importance coefficient) to each sensorial feature. The weighting factor of 0.5 was assigned to appearance, colour, consistency and smell and the importance coefficient was assigned to taste. For each characteristic, there was calculated the non-weighted average score, by adding the points given by the tasters to the arithmetic mean. Afterwards, by multiplying each non-weighted average score of each sensory feature with the corresponding weight factor, there was calculated the weighted average score. By summing all weighted average scores corresponding to the sensory attributes of each analysed sample, the total weighted average score was obtained. The sensory assessment of samples' quality was performed on the basis of the overall average score, on a scale of 0-20 points and compared to control sample.

Results and discussion
3.1. The effect of fibers addition on the rheological properties of yogurt The most important information on the yogurt samples structure was given by the conducted viscoelasticity tests. Four groups of experiments (yogurt with inulin, pea, oat and wheat fibers) were subjected to measurements at different frequencies for assessing the elastic modulus (G'), the viscosus modulus (G''), the phase angle (tan δ) and the complex viscosity (|η*|). In Table 1, there are presented the values of G', G'', tan δ and |η*| at 1 Hz for the yogurt samples. The highest consistency, gel firmness and dynamic viscosity modules were observed for I 4 sample (yogurt with 2.5%  Table 2 show the values of the regression coefficient r for models adjusted for the upward viscosity curves of yogurt samples. The smallest correlation index, r = 0.3373, was observed for I 1 sample (yogurt with 1% inulin fibers), according to Bingham model. The highest r index, r = 1 was showed by O 4 sample (yogurt with 2.5% oat fibers) according to the Herschel-Bulkley model, which describes the plastic and viscous behaviour (nonnewtonian, power law type). The r > 0.99 values confirm the appropriate model to characterize the rheological behaviour of yogurts. Figures 1-4 show the graphical representation of samples' viscosity versus time.
From these figures, it can be observed that the most viscous samples were those with wheat fibers addition at low concentration (1% and 1.5%), while for higher concentrations (2% and 2.5%) the best viscous characteristics were for the samples with oat fibers addition.

The effect of the fibers addition on the physicochemical properties of yogurt
The obtained results for the physicochemical properties of the yogurt samples, such as acidity, pH and syneresis are included in Table 3.
The acidity and the pH did not vary much for samples with different fibers addition, probably due to the fact that total free amino acids are released and also consumed by the starter culture (p > 0.05). An increased level of total amino acids in fermented products with oat addition was reported by Zhu, Miller, Nelson, and Glahn (2008). The acidity remained practically unchanged (p > 0.05) for yogurt containing inulin. This was also observed by Crispín-Isidro et al. (2015), who reported that acidity did not change independently of inulin concentration (2%-6%) due to a buffering effect of the protein.
The syneresis had a bigger magnitude, of 8.21 units, the smallest value (38.86 ± 0.2) (p ≤ 0.05) being recorded for P 2 sample (yogurt with 1.5% pea fibers addition) and the highest value (47.07 ± 0.01) (p ≤ 0.05) being observed for W 2 sample (yogurt with 1.5% wheat fibers addition). As it can be observed, an increased level of fibers addition lead to a decrease in the syneresis values, due to water holding capacity of fibers that absorbed the whey released by the gel structure. A decrease of this value compared to control samples was also reported by Raju and Pal (2014). Furthermore, according to Rinaldoni, Campederrós, and Pérez-Padilla (2012) and Crispín-Isidro et al. (2015), the syneresis was improved compared to the control sample, due to the fact that the fibers have the ability to interact         through hydrogen bridges with charges moieties on the surface of the protein, thus controlling syneresis. The colour parameters of yogurt samples are showed in Table 4.
The colour differences between samples were not significant (p > 0.05), the highest ΔE* value (colour difference) related to control sample was 1.65 for I 4 sample (yogurt with 2.5% inulin fibers addition). These results are in disagreement with those obtained by Raju and Pal (2014): fibers fortification leading to noteworthy changes of yogurt colour; but in agreement with those obtained by Dello Staffolo et al. (2004) who saw no substantial differences between the control yogurt and yogurt containing wheat, bamboo and inulin fibers.

The effect of the fibers addition on the sensorial properties of yogurt
The sensory analysis complements the quality evaluation of the studied yogurt samples. The tasters' panel was formed by 9 persons who were previously trained. The final results of the sensorial evaluation of the yogurt samples are shown in Figure 5.
Yogurt samples with the highest acceptance scores, 18.8 and 19, were W 3 (2% wheat fibers) and respectively P 4 (2.5% pea fibers). The research of Crispín-Isidro et al. (2015) demonstrated an improvement in taste and texture for yogurt with inulin, while Kip, Meyer, and Jellema (2006) observed that inulin can be used to improve creaminess. Our results showed that yogurt with oat fibers received lower scores for sensorial analysis from the panel consumers in comparison with the yogurt enriched with the other fibers, confirming by the results presented in the study of Raju and Pal (2014). Hashim et al. (2009) studied the effect of fortification with date fiber and observed that the yogurt fortified with 3% date fiber resulted with similar sourness, sweetness, firmness, smoothness and overall acceptability as the control yogurt. As both fiber and yogurt are well known for their beneficial health effects, together will constitute a functional food with commercial applications.The results of Hashim et al. (2009) are also corroborated, since 1.32% oat fibers addition improved the body and texture of unsweetened yogurt and decreased the overall flavour quality. Tomic et al. (2017) found that addition of fibers from different sources (soy, rice, oat, corn, and sugar beet), at the level of 1.32%, in general led to lower overall flavor and texture scoresa grainy flavour and a gritty texture were intense in all samples except in those made with oat fiber. However, fiber size is also an important factor in yogurt formulation because of their impact on general acceptance.
The principal component analysis (PCA) has been conducted to evaluate the influence of different fibers on yogurt physicochemical, sensory and rheological parameters, from a descriptive point of view. The scores and compound loadings of the PCA are presented in Figures 6 and 7.
The two principal components (PC) explained 100% of the variations in data set, the PC1 explained 99%, while the PC2 explains 1%. It can be observed in the Figure 6 that the control sample (CS) is placed in the same dial with the sample with inulin; this fact reveals that the final product is not influenced significantly (p > 0.05) by the addition of inulin. The samples with wheat fibers are placed in opposition with the control sample, so that samples have different characteristics. Regarding the loadings, the parameters like G', acidity, h*, L* and syneresis have the biggest influence on the samples loadings. The G' is influencing strongly the PC1, while acidity is influenced by the acidity.

Conclusions
This study evaluated the influence of the yogurt enrichment with total dietary fibers from inulin, pea, oat and wheat on its the rheological, physicochemical and sensory characteristics. The four studied types of fibers (inulin, pea, oat and wheat) were added in the yogurt formulation in different proportions (1%-2.5%), with the classical production technology adapted to laboratory conditions. The most viscous samples were those with wheat fibers addition at low concentrations (1% and 1.5%), while for higher concentrations (2% and 2.5%) the best viscous characteristics were obtained for the samples with oat fibers addition. The acidity and the pH did not vary significantly for samples with different fibers addition. The lowest syneresis values were observed for the samples with 1.5% pea fibers addition, while the highest   Figure 5. Graphical representation of the sensory evaluation of yogurt samples with fibers addition: a) yogurt with inulin fibers addition; b) yogurt with pea fibers addition; c) yogurt with oat fibers addition; d) yogurt with wheat fibers addition.
Figura 5. Representación gráfica de la evaluación sensorial de muestras de yogurt a las que se adicionaron fibras: a) yogurt con fibras de inulina; b) yogurt con fibras de arveja; c) yogurt con fibras de avena; d) yogurt con fibras de trigo.  syneresis values were determined for the yogurt with 1.5% wheat fibers addition. Sensory speaking, yogurt samples with the highest acceptance scores were samples with 2% wheat fibers and respectively with 2.5% pea fibers addition.
The results showed that, different types of fibers incorporated in yogurt significantly affect its rheological properties as well as its composition, flavour and other sensory characteristics.
All the tested fibers were compatible with the yogurtmanufacturing process. Therefore, this study opens the way for testing other types of fibers obtained as a by-product of industrial food processing to obtain enriched yogurts and could be considered an alternative to incorporate fibers in the human diet.
Due to its nutritional properties, yogurt is one of most consumed healthy and nutritious foodstuffs worldwide. By incorporating fibers in yogurt, researchers have achieved a mean to increase fibers consumption in all sectors of the populace and they have developed a functional food with an extensive array of beneficial effects.

Disclosure statement
No potential conflict of interest was reported by the authors.