Experimental Investigation and Modeling for the Influence of Adding Date Press Cake on Drinkable Yogurt Quality

The extraction of date syrup produces a large quantity of by-product known as date press cake (DPC). This study aimed to utilize valuable ingredients of the DPC by adding 0 (Control), 2, 4, and 6% (g/100 g) of its powder to drinkable yogurt before fermentation. The physicochemical properties, texture profile, and sensory evaluation of the treated DPC-based drinkable yogurt (DPC drinkable yogurt) were measured after fermentation and 5, 10, and 15 days of storage at 4 °C. The modeling of the most critical quality attributes, i.e., pH, acidity, syneresis, water holding capacity (WHC), viscosity, and color difference (ΔE), was conducted to predict their values based on the DPC percentage and storage period. The DPC drinkable yogurt’s total solids, protein, and fat ranged between 11.19–11.83, 3.10–3.42, and 2.26–2.34%, respectively. Adding 2–6% DPC slightly increased the pH of DPC drinkable yogurt and decreased its acidity (p > 0.05) during storage. Increasing the DPC percent in DPC drinkable yogurt decreased the syneresis value, and WHC increased during storage. The color parameters and viscosity of DPC drinkable yogurt recorded the highest value at the end of the storage period for all treatments and increased steadily with the increase in DPC. The evaluation of the prediction models indicated that the predicted values were close to the actual experimental values for pH (R2 = 0.779), acidity (R2 = 0.973), syneresis (R2 = 0.961), WHC (R2 = 0.989), viscosity (R2 = 0.99), L* (R2 = 0.919), a* (R2 = 0.995), b* (R2 = 0.922), and ΔE (R2 = 0.921). The textural analysis indicated that increasing the concentration of DPC in the DPC drinkable yogurt increased hardness (g), springiness, cohesiveness, and gumminess and decreased adhesiveness and resilience during cold storage. The evaluation of sensory acceptance during the cold storage of the DPC drinkable yogurt was conducted by 30 expert panelists. Each panelist received four cups of 10 mL drinkable yogurt treatments at 5–10 °C. The evaluation results indicated that adding 2% of DPC was closest in overall sensory acceptability to the control sample (p < 0.05). This study revealed the potential use of DPC in drinkable yogurt as a natural, functional, and low-cost ingredient to improve the fiber content, physicochemical properties, and overall acceptability. Therefore, the fermented DPC-based yogurt drink has the potency to be a practical, value-added, and novel alternative to dairy-based yogurt.


Introduction
Food functionalization is one of the ever-growing markets, which require new bioactive ingredients. In addition, the bioactive components can be used to develop innovative DPC of date fruit (Khalas cv.) was obtained from Aldahaby Dates Factory, Al-Ahsa, Saudi Arabia. The DPC samples were dried under vacuum at 48 • C in an electric vacuum drying oven (LVO-2041P, Daihan Labtech Co., Ltd., Namyangju-si, Gyeonggi-do, Korea) for 72 h. They were then ground and sifted to obtain granules of 250 µm [30].
The drinkable yogurt was manufactured at the Agricultural Research Station pilot plant of King Faisal University. The freeze-dried starter culture used to ferment the yogurt was YC-X11 (Chr. Hansen company, Hørsholm, Denmark), containing Lactobacillus delbrueckii subsp. Bulgaricus and Streptococcus thermophiles. Cow's milk was prepared in equal quantities for each treatment. The pH and fat of the cow's milk used were 6.76 ± 0.04 and 2.26 ± 0.05 g/100 g, respectively. The DPC powder was added to T 0 , T 1 , T 2, and T 3 with concentrations (g/100 g) of 0, 2, 4, and 6%, respectively. Then, the pasteurization process was performed at 90 • C for 10 min, followed by the sudden cooling process to 40 • C. According to the starter data sheet, a starter culture (50 units) was added to the previous treatments, then incubated at 42 • C for 3 h (fermentation complete). Finally, the fermentation was stopped by cooling at 4 • C. The measurements were taken immediately after fermentation and after 5, 10, and 15 days of storage at 4 • C [31].

DPC, Milk, and DPC Drinkable Yogurt Analysis
All measurements were carried out in three replicates for the milk used, DPC, and DPC drinkable yogurt treatments. Total solids (TS), protein, fat, ash, and pH were determined for the milk used for yogurt preparation by Lactoscan Funke-Gerber D-12105, Berlin, Germany. DPC drinkable yogurt treatments were analyzed in terms of total solids, protein, and fat, while DPC was analyzed in terms of moisture, protein, fat, ash, crude fiber, and water binding capacity. The solubility and color parameters were determined according to the standard methods of AOAC [32]. Meanwhile, the moisture, fat, ash, and dietary fibers were estimated by the gravimetric method No. AOAC 934.01. Kieldahl measured protein No. AOAC 976.05, and the obtained values were expressed as total nitrogen multiplied by 6.38 and 6.25 to obtain the total protein content in DPC drinkable yogurt treatments and the DPC. Fully automated grude and detergent fibre analysis (Fibertec TM 8000, FOSS, Hilleroed, Denmark) was used to determine dietary fiber. The total carbohydrates were calculated mathematically (100 -(moisture% + fat% + protein% + ash%). The changes in chemical characteristics were tracked by analyzing the DPC drinkable yogurt treatments immediately after fermentation and cooling and then after 5, 10, and 15 days of storage at 4 • C in terms of each of the following.
2.2.1. pH and Acidity pH was measured with a Thermo Orion 3 Star pH Benchtop Meter (Fisher Scientific, Instruments, Pittsburgh, PA, USA), which was calibrated with a pH 4.00 and 7.00 buffer solution (Thermo Fisher Scientific, Waltham, MA, USA). Titratable acidity as a percentage expressed as lactic acid was also estimated according to the method described in AOAC [32].

Syneresis, Water Holding Capacity, and Viscosity
Syneresis or whey separation was determined according to the method described by Cichońska et al. [33] using a centrifuge (Hermel-Z233 M-2, Hermle Labortechnik GmbH, Wehingen, Germany). The 40 g samples were mixed and centrifuged at 4 • C and 16,125× g (× g is times gravity) for 20 min. The relationship between the centrifuge speed in revolutions per minute (RPM) and relative centrifugal force (RCF) was calculated as follows: g = 1.118 × 10 −5 R S 2 (1) where g is the relative centrifugal force (RCF), R is the rotor radius in centimeters, and S is the centrifuge speed in RPM. After complete expulsion, the separated serum was poured and weighed. The syneresis values of three replicates for each treatment were calculated as a percentage according to the following equation: where S is the syneresis percentage, m 1 is the mass of the separated serum after centrifugation in grams, and m 2 is the initial mass of the yogurt before centrifugation in grams. Water holding capacity (WHC) values of the yogurt treatments were estimated according to the method described by Feng et al. [34]. First, a yogurt sample (10 g) is centrifuged for the three replicates at 1500 × g for 10 min; then, the filtrate is poured, and the precipitate is weighed. Then, WHC is calculated according to the following equation: where WHC is the water binding capacity (%), m ys is the mass of the yogurt sample, and m s is the mass of the sediment.
The viscosity values were estimated according to the method described by Cichońska et al. [33] using rotary viscometer LV DV-II+Pro (Brookfield Engineering, Middleboro, MA, USA) at 4 • C and a spindle (S64) with a rotation of 100 rpm by applying a constant shear speed (0.05 s −1 ). The readings were taken in the 15th second of measurement. Three replicates of yogurt treatments were taken in centipoise units (cP).

Color Parameters
The color characteristics of the drinkable yogurt treatments were measured according to the method described by Hunter and Harold [35] using the Hunter Lab color meter (Hunter Associates Laboratory Inc., Reston, VA, USA). The Hunter Lab color meter was calibrated before the measurements with black and white plates, where the reading was taken in (L*, a*, b*). The value of L* indicates the extent of lightness or luminance/darkness, ranging between the value of black and 100 for white. a* value expresses redness/greenness; the positive value of redness falls into negative greenness values. b* expresses yellowness/blueness and the positive values for yellow and negative for blueness. L* (brightness, 100 = white, 0 = black), a* (+, red; −, green), and b* (+, yellow; −, blue). The color difference (∆E) was calculated based on the International Commission on Illumination (CIE) lab using the following equation: where ∆E is the color difference of the fruit, L* is fruit lightness, a* is greenness-redness, b* is blueness-yellowness. Texture profile analysis (TPA) features, which include hardness, adhesiveness, springiness, cohesiveness, gumminess, and resilience [36], were assessed by a double stress test using a texture analyzer (model: TA.XTplusC, Stable Micro Systems Ltd., Godalming, UK). Before the TPA analysis, the test samples were left at 10 • . A 25 mm diameter perplex cylindrical probe was used to measure the textural profile of the yogurt samples at 10 ± 0.5 • C. The TPA analysis was performed by compressing twice using the probe for 10 mm penetration. In the first stage, the samples were compressed, and the probe's speed was fixed at 5 mm/s during the samples' pretest, compression, and relaxation. The load cell was 5 kg, and the trigger force was 0.1 N. The typical textural profile (force-time) curve was obtained with one complete run. The hardness, adhesiveness, springiness, cohesiveness, gumminess, and resilience of yogurt samples were calculated by the software included with the texture meter used.

Sensory Evaluation
Sensory attributes were assessed by 30 panelists (10 women and 20 men, aged 20 to 60 years) according to the method described by Wang et al. [37]. The acceptance test was conducted under the illumination of the sensory evaluation room maintained at 25 • C in the Department of Food and Nutritional Sciences, College of Agricultural and Food Sciences, King Faisal University. Sensory evaluations were carried out by 30 professional panelists, including teaching staff of the food science and nutrition department, university dairy pilot plant staff, and selected staff of dairy companies in Al-Ahsa 31982, Saudi Arabia. Each panelist received four cups of 10 mL drinkable yogurt treatments at a temperature of 5-10 • C, a sensory assessment sheet, and a water bottle for mouthwash provided between each sample assessment. A 5-point hedonic scale (1 = I don't like it, and 5 = I really like it) was used. Samples were evaluated based on color, texture, flavor, and overall acceptability [38,39].

Statistical Analysis
The results were statistically analyzed through the SAS program in four analyses during the storage period (0, 5, 10, and 15 days) of the prepared drinkable yogurt treatments (0, 2, 4, and 6% DPC) following a randomized complete block design. Duncan's multiple range test (MRT) was used to determine the variance between treatments within the 0.05 level of significance, where the averages in the same columns with capital letters denote a significant difference within the level of significance (p < 0.05). Design Expert software (DX Version 13, Stat-Ease, Inc., Minneapolis, MN, USA) was used to graphically analyze the experimental data and model the influence of DPC addition and storage time on the physicochemical properties of the DPC drinkable yogurt.

Physicochemical Analysis of the Milk Used and DPC
As the results reveal in Table 1, the total solid, protein, fat, and ash contents (g/100 g) of the cow's milk used in preparing yogurt drink treatments were added to the DPC. Additionally, DPC contents (gm/100 g) of moisture, total protein, fat, ash, crude fiber, total sugars, water holding capacity (WHC), solubility, pH, and color determinants were listed. The values of color are consistent with the shape appearance of the DPC used during drying and after grinding ( Figure 1). DPC drinkable yogurt treatments include the T0 treatment (the control sample free of DPC), while T1, T2, and T3 treatments contain 2%, 4%, and 6% (g/100 g) of DPC, respectively. The results in Table 2 show that the percentages (g/100 g) of total solids, protein, and fat for T0 were 11.19, 2.97, and 2.26; T1-11.83, 3.10, and 2.34; T2-12.92, 3.3, and 2.3; and T3-13.45, 3.42, and 2.35, respectively. Significant increases (p > 0.05) in total solids for DPC drinkable yogurt treatments due to the solid contents of DPC and the slight increases in fat and protein are due to the few ranges of DPC. These results agree with the studies, which supplemented yogurt with different sources of dietary fiber [11,[43][44][45].  The results of Khalas DPC analysis agreed with Al-Farsi et al. [40], d the DPC protein of Omani varieties ranged from 3.62 g/100 gm in Al-Shahal DPC to 5.23 g/100 gm in Mabseeli DPC. In contrast, the fat content ranged from 5.02 g/100 g in the Mabassili DPC to 5.90 g/100 g in the Um-sellah DPC. The dietary fiber ranged between 77.75 and 80. 15  DPC drinkable yogurt treatments include the T 0 treatment (the control sample free of DPC), while T 1 , T 2 , and T 3 treatments contain 2%, 4%, and 6% (g/100 g) of DPC, respectively. The results in Table 2 show that the percentages (g/100 g) of total solids, protein, and fat for T 0 were 11.19, 2.97, and 2.26; T 1 -11.83, 3.10, and 2.34; T 2 -12.92, 3.3, and 2.3; and T 3 -13.45, 3.42, and 2.35, respectively. Significant increases (p > 0.05) in total solids for DPC drinkable yogurt treatments due to the solid contents of DPC and the slight increases in fat and protein are due to the few ranges of DPC. These results agree with the studies, which supplemented yogurt with different sources of dietary fiber [11,[43][44][45]. Table 2. Physicochemical analysis of DPC drinkable yogurt treatments during storage period at 4 • C.

Treatments
Total Solids (g/100 g) Protein (g/100 g) Fat (g/100 g) 1 11.83 ± 0.03 c 3.10 ± 0.10 c 2.34 ± 0.01 a T 2 12.92 ± 0.05 b 3.30 ± 0.04 b 2.34 ± 0.02 a T 3 13.45 ± 0.13 a 3.42 ± 0.01 a 2.35 ± 0.02 a The indicative values of the parameters are the means (±SD) of three replicates. Lowercase letters a, b, c, and d for the horizontal comparison between the treatments and the significance of the difference within (p < 0.05) limits. T 0 , T 1 , T 2 , and T 3 treatments refer to the percentage of DPC added to drinkable yogurt at 0, 2, 4, and 6%, respectively.

pH and Acidity of DPC Drinkable Yogurt
Increasing the percentage of DPC between 2 and 6% caused slight increases in pH ( Figure 2a) and a slight decrease (p > 0.05) in acidity ( Figure 2b). Generally, storage over 14 days affected the pH and acidity values, as the pH values decreased, and the acidity increased. The pH values for T 0 , T 1 , T 2 , and T 3 were 4.51, 4.53, 4.55, and 4.63 at the beginning of the storage period, while they were 4.37, 4.39, 4.41, and 4.43, respectively, at the end of the storage period. The acidity of T 1 had the greatest value and that of T 3 the lowest value at the beginning and end of storage compared to the control sample, which recorded 0.61 and 0.76% at the beginning of storage and 0.77 and 0.81% at the end of storage, respectively. Compared to the control sample, with increasing the DPC addition, the pH values increased, and the acidity ratios decreased with the increase in DPC concentrations. However, with the progression of the storage period, the pH values decreased, and the acidity percentages increased. This increase in pH with DPC addition can be attributed to its ability to retain water and thus dilute the concentrations of lactic acid and other organic acids produced by the starter culture. This may increase the pH, especially in yogurt fortified with dietary fiber [29,46]. The acidity estimation results for all treatments were kept at 0.87%, as recommended for yogurt [47]. The pH values of all yogurt treatments increased with increased DPC and water absorption from its fibers. Some research studies have reported that adding different dietary fiber sources affected the pH and acidity of fortified yogurt [44,48]. On the other hand, not all the fruit peels or their residues affected yogurt fermentation. No differences were recorded in the acidity degree of the yogurt to which papaya peel flour was added during storage [49]. At the same time, the acidity of yogurt containing passion fruit peel powder was much higher than in the respective controls, which is the behavior expected by the metabolism of lactic acid bacteria [50]. Factors such as total soluble solids, storage temperature, and additives can also reduce the pH of yogurt [51] due to post-acidification and increased activity of lactic acid bacteria. Adding fruits or pulp to yogurt stimulates starter culture bacteria to increase acidity and lower the pH compared to some peels or fibers of these fruits [48,52].
which papaya peel flour was added during storage [49]. At the same time, the acidity of yogurt containing passion fruit peel powder was much higher than in the respective controls, which is the behavior expected by the metabolism of lactic acid bacteria [50]. Factors such as total soluble solids, storage temperature, and additives can also reduce the pH of yogurt [51] due to post-acidification and increased activity of lactic acid bacteria. Adding fruits or pulp to yogurt stimulates starter culture bacteria to increase acidity and lower the pH compared to some peels or fibers of these fruits [48,52].

Syneresis, WHC, and Viscosity of DPC Drinkable Yogurt
The results in Figure 3a indicate that the storage time and DPC addition slightly affected the syneresis values, which decreased with the increase in DPC addition and during the storage period. The highest ability to retain whey (the lowest degree) was noted immediately after fermentation. T0 was significantly higher for syneresis (p < 0.05) than T1, T2, and T3 during storage. Syneresis values increased during storage time, but the rate of increase was lower for DPC drinkable yogurt treatments. The rate ranged during the beginning and end of the storage period between 50.26 and 57.06 for the control sample, while for the T1, T2, and T3 samples, the ranges were 48.03-53.26, 45.03-52.23, and 43.1-51.33, respectively. Thus, the increased amounts of DPC added reduced the syneresis. T3 was the lowest syneresis value (p < 0.05); this decrease may be attributed to an increase in total solids, as mentioned by Mahdian and Tehrani [53]. Recent studies [6,11,24,54,55] reported that fortification of yogurt with different dietary fibers supports the viscosity and thickening properties of the yogurt gel. The decrease in syneresis in the DPC drinkable yogurt treatments may be attributed to gummy sugars in the fibers, which can trap water

Syneresis, WHC, and Viscosity of DPC Drinkable Yogurt
The results in Figure 3a indicate that the storage time and DPC addition slightly affected the syneresis values, which decreased with the increase in DPC addition and during the storage period. The highest ability to retain whey (the lowest degree) was noted immediately after fermentation. T 0 was significantly higher for syneresis (p < 0.05) than T 1 , T 2 , and T 3 during storage. Syneresis values increased during storage time, but the rate of increase was lower for DPC drinkable yogurt treatments. The rate ranged during the beginning and end of the storage period between 50.26 and 57.06 for the control sample, while for the T 1 , T 2 , and T 3 samples, the ranges were 48.03-53.26, 45.03-52.23, and 43.1-51.33, respectively. Thus, the increased amounts of DPC added reduced the syneresis. T 3 was the lowest syneresis value (p < 0.05); this decrease may be attributed to an increase in total solids, as mentioned by Mahdian and Tehrani [53]. Recent studies [6,11,24,54,55] reported that fortification of yogurt with different dietary fibers supports the viscosity and thickening properties of the yogurt gel. The decrease in syneresis in the DPC drinkable yogurt treatments may be attributed to gummy sugars in the fibers, which can trap water and be released to the DPC during the milling process [25]. These results are in agreement with the results of Arabshahi-Delouee et al. [56] for yogurt with flaxseed press cake, Karaca et al. [24] for yogurt with apricot press cake, Pérez-Chabela et al. [26] for yogurt with mango and potato peels powder, Rojas-Torres et al. [57] for yogurt with butternut squash, and Diep et al. [27] for yogurt with tamarillo. Figure 3b display the effect of DPC treatments and storage period on the WHC. With the increase in DPC percent, the WHC values decreased during storage. For example, the WHC values for treatments T 0 , T 1 , T 2 , and T 3 were 51.8, 56.76, 58.66, and 58.9 at the beginning of the storage period, while they were 47, 48.96, 52.1, and 54.13 at the end of the storage period (p < 0.05). These results are in agreement with the results of Güler-Akın et al. [58] for yogurt with oat and inulin fibers, Karaca et al. [24] for yogurt with apricot press cake, and Diep et al. [27] for yogurt with tamarillo. Figure 3b display the effect of DPC treatments and storage period on the WHC. With the increase in DPC percent, the WHC values decreased during storage. For example, the WHC values for treatments T0, T1, T2, and T3 were 51.8, 56.76, 58.66, and 58.9 at the beginning of the storage period, while they were 47, 48.96, 52.1, and 54.13 at the end of the storage period (p < 0.05). These results are in agreement with the results of Güler-Akın et al. [58] for yogurt with oat and inulin fibers, Karaca et al. [24] for yogurt with apricot press cake, and Diep et al. [27] for yogurt with tamarillo. The viscosity results (Figure 4) showed the highest values at the end of the storage period for all DPC yogurt treatments-T1, T2, and T3-compared to the control sample T0. In addition, the viscosity values increased directly with the DPC increase. For example, T3 recorded 2452.33 and 2962.66 cp at the beginning and end of the storage period, respectively. The findings are in agreement with the results of Karaca et al. [24] for yogurt with apricot press cake, Varnait et al. [6] for yogurt with blackberry press cake, and Diep et al. [27] for yogurt with tamarillo.
On the other hand, the results of the study conducted by Tseng et al. [59] found that the use of 3% of red grape peels after fermentation weakened the viscosity of yogurt. As the addition to yogurt takes place after or before the fermentation process, Cichonska et al. [33] stated that the statistical analysis of their results showed the differences in the viscosity of yogurt when milled flaxseed was added after fermentation and before fermentation compared to the control sample. The viscosity of yogurt significantly increased when ground flaxseeds were added after fermentation, while the viscosity decreased when they were added before fermentation. The viscosity results (Figure 4) showed the highest values at the end of the storage period for all DPC yogurt treatments-T 1 , T 2 , and T 3 -compared to the control sample T 0 . In addition, the viscosity values increased directly with the DPC increase. For example, T 3 recorded 2452.33 and 2962.66 cp at the beginning and end of the storage period, respectively. The findings are in agreement with the results of Karaca et al. [24] for yogurt with apricot press cake, Varnait et al. [6] for yogurt with blackberry press cake, and Diep et al. [27] for yogurt with tamarillo.

Color Parameters of DPC Drinkable Yogurt
The color measurement value determinants given by Hunter Lab Device L*, a*, b* for DPC drinkable yogurt treatments during the storage period are shown in Figures 5 and 6. The L* values ranged between 69.83 and 78.45 for DPC yogurt treatments vs. the control treatment of 96.75 in the first storage period ( Figure 5). In addition, they ranged between 45.92 and 57.54 compared to the control treatment and 72.6 at the end of the storage period. T3 containing 6% of DPC was the darkest treatment according to the L* values, which amounted to 69.83 at the beginning of the storage period. The degrees of whiteness decreased significantly during the storage period for all treatments (p < 0.05).
Regarding a*, the results indicated that the control sample during the first and last storage period was negative, ranging from −4.24 to −1.91, which indicated its tendency toward greenness, and the degree of greenness decreased at the end of the storage period On the other hand, the results of the study conducted by Tseng et al. [59] found that the use of 3% of red grape peels after fermentation weakened the viscosity of yogurt. As the addition to yogurt takes place after or before the fermentation process, Cichonska et al. [33] stated that the statistical analysis of their results showed the differences in the viscosity of yogurt when milled flaxseed was added after fermentation and before fermentation compared to the control sample. The viscosity of yogurt significantly increased when ground flaxseeds were added after fermentation, while the viscosity decreased when they were added before fermentation.

Color Parameters of DPC Drinkable Yogurt
The color measurement value determinants given by Hunter Lab Device L*, a*, b* for DPC drinkable yogurt treatments during the storage period are shown in Figures 5  and 6. The L* values ranged between 69.83 and 78.45 for DPC yogurt treatments vs. the control treatment of 96.75 in the first storage period ( Figure 5). In addition, they ranged between 45.92 and 57.54 compared to the control treatment and 72.6 at the end of the storage period. T 3 containing 6% of DPC was the darkest treatment according to the L* values, which amounted to 69.83 at the beginning of the storage period. The degrees of whiteness decreased significantly during the storage period for all treatments (p < 0.05). Regarding the color difference (ΔE) value, the results indicated that the samples in the first storage period were zero because this is the baseline (control), as shown in Figure  7, accompanied by the visual color at the beginning and end of the storage period ( Figure  8). The highest values were found at the end of the storage period for all DPC drinkable  Regarding the color difference (ΔE) value, the results indicated that the samples in the first storage period were zero because this is the baseline (control), as shown in Figure  7, accompanied by the visual color at the beginning and end of the storage period ( Figure  8). The highest values were found at the end of the storage period for all DPC drinkable yogurt treatments. In addition, ΔE values increased directly with increasing DPC percent- Regarding a*, the results indicated that the control sample during the first and last storage period was negative, ranging from −4.24 to −1.91, which indicated its tendency toward greenness, and the degree of greenness decreased at the end of the storage period compared to its beginning (Figure 6a). On the other hand, all DPC yogurt treatments-T 1, T 2 and T 3 -recorded significant (p < 0.05) positive values-3.3, 5.04, and 4.98, for T 1 , T 2 and T 3, respectively-at the beginning of the storage period, and the values were 3.69, 5.46, and 6.35 at the end of the storage period. This indicates a tendency toward redness, which may be attributed to the reddish-brown color of the DPC. Thus, the storage period significantly (p < 0.05) affected the a* values, as the values increased, as did their tendency to redden more at the end of the storage period compared to its beginning.
The b* values of DPC drinkable yogurt treatments revealed yellowing at the beginning and end of the storage period (Figure 6b). The control sample recorded significantly lower values (p < 0.05) than all treatments at the beginning and end of the storage period, whereby it scored 15.35 at the beginning of storage and 12.72 at the end of it. T 3 was the most yellow, with 20.47, followed by T 2 with 18.49 and T 1 with 16.9, compared to the control T 0 , which reached 15.35 at the beginning of the storage period.
Regarding the color difference (∆E) value, the results indicated that the samples in the first storage period were zero because this is the baseline (control), as shown in Figure 7, accompanied by the visual color at the beginning and end of the storage period ( Figure 8).
The highest values were found at the end of the storage period for all DPC drinkable yogurt treatments. In addition, ∆E values increased directly with increasing DPC percentage and storage period. For example, after five days, T 0 , T 1 , T 2 , and T 3, ∆E recorded 21.13, 30.7, 37.52, and 40.39. This did not change significantly subsequently for another 10 days, and then, values of 24.42, 40.1, 51.79, and 48.18, respectively, were recorded after 15 days. Generally, all DPC yogurt treatment values-T 1 , T 2 , and T 3 -were significantly (p < 0.05) decreased with increasing DPC and storage period at 4 • C. Figure 8 shows the visual color of DPC drinkable yogurt treatments during the storage period.     Rojas-Torres et al. [57] indicated that yogurt samples supplemented with thickeners from butternut squash seeds were affected in terms of glossiness, which they attributed to the size of the fat granules and protein. Łopusiewicz et al. [11] found that adding Camelina press cake to yogurt affected the a* (redness) value of the yogurt treatments, which the study attributed to the presence of a carotenoid pigment; however, the values decreased during the storage period due to the decrease in the stability of the dye.
Additionally, a decrease in a* (redness) values was observed in yogurt supplemented with flaxseed press cake as a result of fermentation [60]. In a study where ginseng root extract was added to yogurt, the results showed that the color values were affected by the Rojas-Torres et al. [57] indicated that yogurt samples supplemented with thickeners from butternut squash seeds were affected in terms of glossiness, which they attributed to the size of the fat granules and protein. Łopusiewicz et al. [11] found that adding Camelina press cake to yogurt affected the a* (redness) value of the yogurt treatments, which the study attributed to the presence of a carotenoid pigment; however, the values decreased during the storage period due to the decrease in the stability of the dye.
Additionally, a decrease in a* (redness) values was observed in yogurt supplemented with flaxseed press cake as a result of fermentation [60]. In a study where ginseng root extract was added to yogurt, the results showed that the color values were affected by the addition, as the L* values decreased from 93.96 to 90.99, while a* and b* values increased from 2.92 and 2.30 to 5.91 and 11.11, respectively. Darkness and decreased luminosity L* compared to the control samples combined with increased b* values resulted in a yellowish color [55]. Alqahtani et al. [61] mentioned the effect of yogurt with tomato pomace on color measurements during storage. The results indicated a decrease in the whiteness values of L* and increases in the values of a* and b*. The results of Mkadem et al. [28] in enhancing the fermented milk drink with dried dates showed its effects on the color characteristics, especially a* and b*, where the a* value increased from −1.8 ± 0.8 to 4.5 ± 0.2, while the b* value increased from 3.1 ± 0.1 to 12.1 ± 0.1. In contrast, the value of the gloss L* decreased, which was attributed to the dyes of the date dryer. Adding dried tamarillo plant fiber (5-15%) to yogurt decreased the L* value, while the value of a* increased with the increase in the addition percentage [27]. The study attributed this to the natural pigments of the anthocyanins in tamarillo. In a study mixing microencapsulated guava pomace fibers with yogurt, the L* value was unaffected, and the values of a* and b* within the treatments were close; this was due to the contents of beta-carotene inside the guava pomace fibers used [62]. Figure 9 displays the correlation between the DPC treatments, storage period, and physicochemical properties of DPC drinkable yogurt. Significant positive correlations exist between the DPC treatments and pH, WHC, viscosity, a*, b*, and the DPC drinkable yogurt's color difference (∆E). ments and acidity, syneresis, and L* value. Regarding the storage period, th icant positive correlation between the DPC treatments and acidity, synere and ΔE of the DPC drinkable yogurt and a significant negative correlation DPC treatments and pH, WHC, L*, and b* value of the DPC drinkable yogur a slight positive correlation existed between the a* value and storage period  On the other hand, there is a significant negative correlation between the DPC treatments and acidity, syneresis, and L* value. Regarding the storage period, there is a significant positive correlation between the DPC treatments and acidity, syneresis, viscosity, and ∆E of the DPC drinkable yogurt and a significant negative correlation between the DPC treatments and pH, WHC, L*, and b* value of the DPC drinkable yogurt. In addition, a slight positive correlation existed between the a* value and storage period.

Modeling of Physicochemical Properties of DPC Drinkable Yogurt
In order to model the physicochemical properties of DPC drinkable yogurt, the experiment outcomes were input into the Design Expert software for additional data analysis. The data of the two factors (DPC percent and storage period) were fitted with different models, i.e., linear, quadratic, and cubic, in order to model the physicochemical properties of DPC drinkable yogurt. The quadratic polynomial models most appropriately described the physicochemical properties of DPC drinkable yogurt, i.e., pH, acidity, syneresis, WHC, viscosity, and ∆E. The final predictive models for the two essential factors for pH, acidity, syneresis, WHC, viscosity, and ∆E are presented as Equations  (13) where DPC is the date press cake percentage, and SP is the daily storage period.
The standard deviation (Std. Dev.), mean value, coefficient of variation percentage (C.V.%), coefficient of determination (R 2 ), adjusted R 2 , and predicted R 2 Adeq precision criteria were used to evaluate the selected predicted models (Equations (5)-(13)) in terms of the pH, acidity, syneresis, WHC, viscosity, L*, a*, b*, and ∆E of DPC drinkable yogurt. The quadratic model emerged as the best because it exhibited a low standard deviation, high R-squared values close to 1, and low PRESS. The evaluation criteria of the prediction models are shown in Table 3. The evaluation criteria indicated that the selected prediction models could efficiently describe the experiments. Therefore, these models can be used to navigate the design space for the target physicochemical properties of DPC drinkable yogurt responses. The results of the quadratic models agree with Mohammed et al. [63]. Table 3. The evaluation criteria, i.e., standard deviation (Std. Dev.), mean value, coefficient of variation percentage (C.V.%), coefficient of determination (R 2 ), adjusted R 2 , predicted R 2 , and Adeq precision criteria, for the selected quadratic models for the target physicochemical properties of DPC drinkable yogurt.

Texture Profile Analysis of DPC Drinkable Yogurt
The texture profile analysis results (Table 4)     The indicative values of the parameters are the means (±SD) of three replicates. The uppercase letters A and B, represent the vertical comparison between storage periods. The lowercase letters a, b, c, and d represent the horizontal comparison between the treatments. Treatment T 0 , T 1 , T 2 , and T 3 refers to the percentage of DPC added to drinkable yogurt at 0, 2, 4, and 6%, respectively.
Although the springiness values increased with the increase in DPC, the increase was significantly inversely proportional (p < 0.05) to the progression of the storage period. Treatment T 3 recorded the highest value of 0.93 vs. 0.83 for the control sample at the beginning of the storage period, and the value was 0.79 vs. 0.71 for the control sample at the end of the storage period. This approach to characterization of springiness values was followed for each of the values of cohesiveness and gumminess. At the same time, the resilience values decreased with the increase in the addition of DPC and the progression of the storage period, with treatment T 3 recording a value of 0.31 vs. 0.64 for the control sample at the beginning of the storage period and 0.1 vs. 0.51 for the control sample at the end of the storage period. The results of the texture profile analysis in this study agree with those in previous studies [24,36,44,61,62,64].

Sensory Evaluation of DPC Drinkable Yogurt
The results of sensory acceptance regarding the color, texture, flavor, and general acceptance of DPC drinkable yogurt during the storage period at 4 • C showed that samples T 1 followed by T 2 and T 3 were the closest in overall sensory traits to the control sample (p < 0.05) over the progression of the storage period (Table 5 and Figure 12 [24] stated that the increase in the addition of apricot press cake fibers affected the arbitrators' preference for yogurt samples due to the increase in viscosity. At the same time, the results differed in terms of the effect of the storage period. Brodziak et al. [65] mentioned that adding sea buckthorn fruit to yogurt affected the organoleptic characteristics of addition and storage. The sensory characteristics were improved in yogurt samples with thickeners added from butternut squash seeds [57]. In another study [55] attempting to enhance yogurt with hydroponic ginseng extracts, the results showed that the treatment in terms of color was superior to the control sample at 1% concentration. At the same time, the preference decreased with increasing concentrations of it. At the same time, the texture and flavor were not affected by the addition of ginseng. With the addition of moringa to yogurt, the results of the study mentioned by Mendoza-Taco et al. [66] showed that the yogurt samples were not affected in relation to the sensory traits assessed. Meanwhile, in a study [67] attempting to enhance the vitality-boosting fermented camel milk drink with sugary date fibers and demonstrate its effects on sensory traits, the results indicated that adding 12.5% of sugary dates fiber was preferred by sensory traits assessors. The indicative values of the parameters are the means (±SD) of three replicates. The uppercase letters A, B, and C represent the vertical comparison between storage periods, while the lowercase letters a, b and c represent the horizontal comparison between the treatments, and the difference is significant within (p < 0.05). Treatment T 0 , T 1 , T 2 , and T 3 refers to the percentage of DPC added to drinkable yogurt at 0, 2, 4, and 6%, respectively.

Conclusions
Adding DPC powder before the fermentation of DPC yogurt slightly increased the pH and decreased acidity during the storage period. DPC addition slightly reduced syneresis values and increased the WHC during storage. Viscosity recorded the highest value at the end of the storage period for all treatments. Yogurt containing 6% DPC was the highest in viscosity. The L* values decreased significantly (p < 0.05) with increasing DPC, and the degrees of whiteness or glossiness decreased significantly during the storage period for all treatments. The a* values were positive for all DPC yogurt samples, and the b* values showed yellowness in all DPC treatments at the beginning and end of the storage. Texture profile data indicated that increased DPC and the storage period's progression increased the hardness (g), springiness, cohesiveness, and gumminess. Adding DPC reduced adhesiveness and resilience during storage. Yogurt containing 2% was the closest in overall sensory acceptability to the control sample (p < 0.05) and during the storage. The predicted values of the quadratic models were close to the actual observed values for the pH, acidity, syneresis, WHC, viscosity, and ΔE of the treated DPC drinkable yogurt. Generally, the results indicated the possibility of enriching yogurt drinks by 2-4% with DPC as an innovative functional additive with general acceptance. The results also support the idea of expanding the uses of DPC in yogurt, enhancing the circular economy for waste upgrading, and DPC can be proposed as a new functional fiber-based yogurt. Overall, these results gave a new idea regarding several aspects related to the upcycling vision, ranging from technological strategies to reusing agrifood by-products and obtaining functional ingredients for high-value-added food production. The sensory evaluation may require more participants in a further study before the proposed product can be applied on a large scale. Treatments T 0 , T 1 , T 2 , and T 3 refer to the percentage of DPC added to drinkable yogurt at 0, 2, 4, and 6%, respectively.
The addition of 0.5% of dried orange peel fibers did not have any significant effects (p < 0.05) on the sensory acceptance of the viability-boosting yogurt drink treatments compared to the control sample, while increasing the concentrations of dried orange peel fibers up to 2% had a negative effect on the sensory properties [68].

Conclusions
Adding DPC powder before the fermentation of DPC yogurt slightly increased the pH and decreased acidity during the storage period. DPC addition slightly reduced syneresis values and increased the WHC during storage. Viscosity recorded the highest value at the end of the storage period for all treatments. Yogurt containing 6% DPC was the highest in viscosity. The L* values decreased significantly (p < 0.05) with increasing DPC, and the degrees of whiteness or glossiness decreased significantly during the storage period for all treatments. The a* values were positive for all DPC yogurt samples, and the b* values showed yellowness in all DPC treatments at the beginning and end of the storage. Texture profile data indicated that increased DPC and the storage period's progression increased the hardness (g), springiness, cohesiveness, and gumminess. Adding DPC reduced adhesiveness and resilience during storage. Yogurt containing 2% was the closest in overall sensory acceptability to the control sample (p < 0.05) and during the storage. The predicted values of the quadratic models were close to the actual observed values for the pH, acidity, syneresis, WHC, viscosity, and ∆E of the treated DPC drinkable yogurt. Generally, the results indicated the possibility of enriching yogurt drinks by 2-4% with DPC as an innovative functional additive with general acceptance. The results also support the idea of expanding the uses of DPC in yogurt, enhancing the circular economy for waste upgrading, and DPC can be proposed as a new functional fiber-based yogurt. Overall, these results gave a new idea regarding several aspects related to the upcycling vision, ranging from technological strategies to reusing agrifood by-products and obtaining functional ingredients for high-value-added food production. The sensory evaluation may require more participants in a further study before the proposed product can be applied on a large scale.