Texture and sensory characterization of functional yogurt supplemented with flaxseed during cold storage

Abstract In this study, flaxseed was used as a functional ingredient in yogurt formulations. The goal of this study was to produce prebiotic yogurt supplemented with flaxseed and investigation of its texture and sensory properties. Yogurt samples containing 0%–4% flaxseed was produced and stored at refrigerator (4–5°C) for 28 days. Textural properties were determined by texture analysis, and sensory characteristics were assessed by 26 trained panelists. Addition of flaxseed to yogurt samples increased the hardness, gumminess, chewiness, cohesiveness, and springiness values in produced yogurt samples. However, adhesiveness level was reduced in a sample enriched with flaxseed. By increasing flaxseed concentration, the color of samples was significantly different than the control sample; L* value was diminished and a* and b* value increased. Sensory scores including taste and mouthfeel, appearance, and overall acceptance showed reduction trend in samples containing a high level of flaxseed. In general, results showed that the addition of 2.63% flaxseed into yogurt samples lead to produce functional food with satisfactory texture, sensory characteristics that sustained these properties until 17.17 days after cold storage.


| Textural profile analysis of yogurt samples
Textural parameters were measured by using texture profile analysis (TPA) (Zwick Company, Ulm, Germany) with mechanical compression of samples and the back extrusion test in four cycles with the cylindrically shaped probe (diameter of 40 mm). TPA instrument measured different parameters such as hardness, chewiness, gumminess, springiness, cohesiveness, and adhesiveness. The analyzer was connected to a computer that documented data via a software program called test software testXpert ® II.

| Sensory evaluation
Five-point hedonic scale test (including 1 = dislike very much; 2 = "dislike", 3 = "neither like nor dislike", 4 = like and 5 = like very much) was used for evaluating sample acceptability. Sensory assessment of yogurt samples was done after 1, 14, and 28 days of cold storage. Twenty-six persons as panelists (that were members of the staff and students of the Hamedan University of medical science, Iran) assessed the sensory properties of yogurt samples.
For this experiment, the cups that contained 100 ml of yogurt sample at 10°C were provided. Each sample was assessed by a person with three repeats. Yogurt samples were assessed for flavor, mouth feels, appearance, non-mouth feel properties, and overall acceptability.

| Color evaluation
Color measurement was done similar to the previous study with some modifications (Khodadadi, Ardebili, Eyvazzadeh, Zargari, & Moradi, 2014). Yogurt samples were placed in the floor of the aluminum dark chamber with 30 × 40 × 40 cm dimensions. A digital camera (Canon, Japan) was located on the roof of the chamber, and four 60-w halogen lamps were placed in chamber inner corners. The samples were placed on the floor of the chamber. The images captured by mentioned camera were transferred to a computer, and its color was measured by image processing software (Photoshop CS 5 Portable) according to the Hunter Lab format that is L* (brightness), a* (+ red to -green component) and b* (+yellow to -blue component). Also, the color change was calculated as ∆E according to the following equation:

| Optimization
For optimization, responses such as hardness, chewiness, springiness, cohesiveness, and independent variable including flaxseed concentration and storage time were selected in maximum and adhesiveness and gumminess in minimum. Another variable was selected according to Table 2.

| Evaluation of hardness
Hardness or firmness is the most commonly assessed parameter for yogurt texture analyses that it was defined as the necessary force to attain a given deformation. This factor is a critical texture property for yogurt like products. The findings obtained from ANOVA are shown in   (Mudgil, Barak, & Khatkar, 2017).
In this study, we found that flaxseed increased the yogurt hardness while some studies have been shown that the addition of functional compounds into yogurt lead to a decrease in hardness.
Azari-Anpar, Tehrani, Aghajani, and Khomeiri (2017), Azari-Anpar, Payeinmahali, Daraei Garmakhany, and Sadeghi Mahounak (2017) showed that the addition of Aloe Vera gel into yogurt decreased samples harness so that the lowest of hardness was related to the sample containing 5% Aloe Vera gel. The occurrence of salicylic acid and antimicrobial agents in Aloe Vera gel decreased starter culture bacteria growth (Azari-Anpar, Tehrani, et al., 2017). Also, Michael, Phebus, and Schmidt (2010) found that plant extracts including olive, onion, citrus, and garlic decrease the yogurt hardness or firmness (Michael et al., 2010). In another study, it was observed that the addition of 2-2.5% partially hydrolyzed guar gum (PHGG) to yogurt samples did not influence on hardness while values greater than 2.5% reduced hardness (Mudgil et al., 2017).
TA B L E 2 Optimization of independent variables and responses for flaxseed-enriched yoghurt production

| Evaluation of adhesiveness
Adhesiveness or stickiness is the required work for prevailing attraction force between foodstuff surface and various substances coming into contact with them. In fact, adhesiveness is the force required to separate the material that sticks to the teeth during eating (Delikanli & Ozcan, 2017). Adhesiveness had an inverse relationship with yogurt eating quality. As can be seen from the ANOVA data in  (2001) reported that the addition of amaranthus seeds into yogurt samples decrease the adhesiveness amount (Grega et al., 2001). In another study, it was revealed the replacement of milk fat with maltodextrin resulted in the increment of yogurt adhesiveness (Domagała, Sady, Grega, & Bonczar, 2006).
In this study, we found that the adhesiveness decreased by increas-

| Evaluation of cohesiveness
Cohesiveness or consistency is an important textural parameter of yogurt and shows its acceptance from the consumer's point of view.
Cohesiveness is defined as the forces of inner bond links, which maintain the product as a perfect, and it is expressed as the force content that can cause to deform a material before it is broken. As can be seen from Figure 1 and Table 1, flaxseed concentration and storage time had a significant impact on the samples cohesiveness (p < 0.01). Flaxseed incorporation (4%) into the yogurt samples increased cohesiveness value from 0.61 (control sample) to 0.68 N.
Since cohesiveness indicates the strength of the internal bonds in yogurt structure, therefore high value of cohesiveness showed that yogurt structure in samples containing flaxseed is more strength and firmer compared to control sample (Salvador & Fiszman, 2004).
The protein matrix had an important role in cohesiveness (Tunick, 2000). Our result is opposite to the findings obtained for yogurt containing Aloe Vera foliar gel that decrease in cohesiveness reported in the final product (Azari-Anpar, Tehrani, et al., 2017; Azari-Anpar, Payeinmahali, et al., 2017). Also, it was reported that the addition of 3.0%, 4.0%, and 5.0% of dried grape pomace into the yogurt samples reduced cohesiveness amounts (Mohamed, Zayan, & Shahein, 2014). Domagała et al. (2006), showed that there is no significant difference between cohesiveness of control yogurt and yogurts containing fat or maltodextrin (Domagała et al., 2006). The increment in cohesiveness during cold storage was reported in another study that confirms our finding. do Espírito Santo, Perego, Converti, and Oliveira (2012) found that the cohesiveness amount of yogurt containing passion fruit peel powder, increased during storage time (do Espírito Santo et al., 2012). Flaxseed influenced on internal bonds in yogurt structure, thus it reduced adhesiveness and increased cohesiveness (Bhat, Deva, & Amin, 2018).

| Evaluation of gumminess
Gumminess is defined as the energy required to break a semisolid food into fragments until it is ready to swallow (Dar & Light, 2014;Domagała et al., 2006;). It is a defect. The range of gumminess was 0.43 and 1.73 N ( Gumminess has an undesirable effect on appearance and texture.  (Domagała et al., 2006). However, there are opposite results with our results. So that, Nikoofar, Hojjatoleslami, and Shariaty (2013) observed that fortification of yogurt with Quince seed mucilage decreased gumminess of the final product (Nikoofar et al., 2013).

| Evaluation of springiness
Springiness is the rate and extent to which a deformed material returns to its initial condition after the force is eliminated. Springiness depends on different agents such as heat treatment, protein interaction, elasticity, and degree of unfolding of protein (Delikanli & Ozcan, 2017). The influence of flaxseed addition to yogurt on textural profile properties of springiness is presented in (Table 2) integrity of yogurt and addition of flaxseed in yogurt, increase texture integrity, so this is a suitable reason for the higher springiness observed in yogurt containing flaxseed compared to control sample. Mudgil et al. (2017) Studies were in agreement with our result; they showed that springiness of the yogurt samples increased by the increase in (partially hydrolyzed guar gum) PHGG level (Mudgil et al., 2017). Ayar and Gurlin (2014) found that springiness has enhanced in the yogurt samples fortified with carrot during 1st and 10th days of storage time (Ayar & Gurlin, 2014). However, there are opposite studies with our studies, for instance, Mudgil et al. (2017) showed that springiness of yogurt samples decreased with an increase in incubation time (Mudgil et al., 2017).

| Evaluation of chewiness
The chewiness is the time or work needed for masticating a sample for decrease it to a state ready for consuming; it is related to firmness, cohesiveness, and elasticity (Dar & Light, 2014).

| Sensory evaluation
Sensory assessment helps to define the product properties which are prominent concerning the product acceptability for the customer. In this study, the influence of flaxseed concentration and storage time on the sensorial properties of the flaxseed-enriched and control yogurt samples is represented by the response surface plots for better conception in Figure 2. Sensorial properties such as taste, flavor, appearance, mouthfeel, and overall acceptability were considered to evaluate the sensory quality of the final product. The addition of dried grape pomace, rice, and bare bran into the yogurt samples reduced the appearance, Flavor, texture, and overall acceptability score compared with control yogurt (Hasani, Sari, Heshmati, & Karami, 2017;Hasani et al., 2016;Mohamed et al., 2014).
Although sensory attribute score of flaxseed enriched yogurt samples is lower than control sample, it seemed that consumers preferred functional food with potential health advantages due to their nutritional information increment . As well as, the addition of a flavoring agent into flaxseed enriched yogurt could improve the sensory characteristic of these products.

| Color
The white color of milk, and also yogurt, is resulted in the light dispersion of fat globules and casein micelles (Walstra, Geurts, Walstra, & Wouters, 2005). In this study, we found that by addition of flaxseed, the yogurt color changed significantly (p < 0.01). Although, storage time had no significant effect on the color intensity of yogurt samples ( Figure 3). The L* value of the yogurt samples ranged from 56 to 68 (Table 2). The lightest (56) and the darkest (68)  increased with the addition of CFE during storage period (Noh et al., 2013).

| Optimization
The object of optimization is to obtain flaxseed-enriched yogurt product with the good rheological, sensory, and color properties. So,