Table 2 shows the experimental values of extracting starch from yellow skin potatoes, indicating the processing conditions at 3000 rpm and 15 min as optimum for the highest yield of extracted starch. From the analysis of variance shown in
Table 3, the model was highly significant (
p < 0.05), which indicated that the models used to fit response variables were sufficient to display the relationship between the yield of starch and the independent variables. The “Lack of Fit
F-value” of 2.06 implies the lack of fit is not significant relative to the pure error. There is a 24.32% chance that a “Lack of Fit
F-value” this large could occur due to noise. Non-significant lack of fit is recommended for an adequate model. Both linear and square of X
1 (centrifugal speed, rpm) also showed a
p-value lower than 0.05. Furthermore, the linear and square of X
2 (centrifugal time, min) also showed a
p-value lower than 0.05. The interaction between X
1 (centrifugal speed, rpm) and X
2 (centrifugal time, min) gave
p < 0.05, which is considered significant. The
R2 of the models for potato starch yield (%) was 0.946.
Three-dimensional (3D) surface plots were assigned in order to study and determine the optimum conditions for independent and dependent variables [
24]. The equation in terms of coded factors can be used to make predictions about the response for given levels of each factor. By default, the high levels of the factors are coded as +1 and the low levels of the factors are coded as −1. The coded equation is useful for identifying the relative impact of the factors by comparing the factor coefficients. The quadratic polynomial model of coded factors is shown below:
As shown in
Figure 1, the effect of the variables and their interaction on predicted potato starch yield (%) was investigated. It showed that as X
1 (centrifugal speed) and X
2 (centrifugal time) increased, the potato starch yield increased. The optimum centrifugal speed and centrifugal time for maximum potato starch yield were 3000 rpm and 15 min, respectively.
3.1. Total Acidity in Yogurt Samples
Table 4 shows the effect of adding the extracted starch from the potatoes under the optimized condition on the total acidity in yogurt stored at 5 °C for 15 days. The results of the statistical analysis showed that Yp
1 and Yp
2 exhibited a maximum change in total acidity over storage and were significantly (
p < 0.05) higher than Y
G, Yp
3, and Yp
4, which was not significant (
p > 0.05) compared to Y
C. The mean values of total acidity were 1.35 ± 0.56, 1.33 ± 0.56, and 1.4175 ± 0.54 for Yp
1, Yp
2, and Y
C, respectively. This result was in agreement with those of Andic et al. [
25] and Anwer et al. [
26], who reported a significant relationship between the gradual increase in acidity of yogurt during the storage and the amount of lactic acid produced.
In spite of Yp
3 and Yp
4 showing a slight increase in the total acidity of manufactured yogurt during storage at 5 °C for 15 days, the statistical analysis showed that there were no significant differences (
p > 0.05) between Yp
3, Yp
4, and Y
G. This obtained result was in agreement with Kumar and Mishra [
27], who found that adding sufficient concentrations of starch could effectively reduce the amount of water, thus making it difficult for bacteria to metabolize lactose sugar and thereby reducing the amount of lactic acid produced.
3.2. pH of Yogurt Samples
The results in
Table 5 show that adding the starch extracted from yellow potato had a significant effect on the mean value of pH of yogurt samples. The statistical analysis implied that Yp
1 and Yp
2 presented the maximum decrease in pH values and were significantly (
p < 0.05) less than Y
G, Yp
3, and Yp
4, which was not significant (
p > 0.05) compared to Y
C. The mean values of total acidity were 4.16 ± 0.45, 4.205 ± 0.39, and 4.10 ± 0.47 for Yp
1, Yp
2, and Y
C, respectively. These findings were similar to those reported by Seelee et al. [
28] and Hassan et al. [
29], who declared that the pH value of yogurt decreased mainly because of the lactic acid produced during storage.
Furthermore, the results revealed that Yp
3 and Yp
4 displayed a negligible decrease in the pH of manufactured yogurt during storage at 5 °C for 15 days. The Yp
3 and Yp
4 treatments had more capability to resist pH changes due to their ability to prevent lactose conversion [
30]. The statistical analysis showed that there was no significant difference (
p > 0.05) between Yp
3, Yp
4, and Y
G.
3.3. Syneresis of Yogurt Samples
As shown in
Table 6, adding different concentrations of extracted starch had highly significant results, decreasing syneresis in manufacturing yogurt during storage at 5 °C for 15 days. This study indicated that Yp
1 and Yp
2 exhibited the minimum reduction in syneresis with the passage of time and were significantly (
p < 0.05) higher than Y
G, Yp
3, and Yp
4, which was not significant (
p > 0.05) compared to Y
C. The mean values of syneresis were 4.32 ± 1.40, 4.38 ± 1.39, and 4.45 ± 1.39 for Yp
1, Yp
2, and Y
C, respectively. This result was in accordance with Isleten et al. [
31] and Guven et al. [
32], who observed that the lowest values of syneresis were obtained during storage compared to the first day of production.
In contrast, both Yp
3 and Yp
4 treatments displayed preferable results in terms of the reduction in syneresis values during storage at 5 °C for 15 days. This significant reduction can be ascribed to the ability of a high concentration of starch to increase the concentration of an adsorbing polymer. Previous results of Hasan et al. [
33] were in agreement with this present investigation. Moreover, the statistical analysis emphasized that there were no significant differences (
p > 0.05) between Yp
3, Yp
4, and Y
G.
3.4. Microbiological Analysis of Yogurt
The data regarding microbial population changes of all yogurt samples are given in
Figure 2. The results revealed that the total bacteria count in all yogurt samples increased throughout the storage time. Adding different concentrations of optimized extracted starch had a significant effect, decreasing the microbial content compared with the control sample (Y
C). This study indicated that Yp
1 and Yp
2 exhibited the minimum reduction in microbial content with the passage of time and were significantly (
p < 0.05) higher than Y
G, Yp
3, and Yp
4, which was not significant (
p > 0.05) compared to Y
C. The mean values of the total bacteria count (log10 CFU/mL) during storage at 5 °C for 15 days were 4.5 ± 0.23, 4.6 ± 0.11, and 4.8 ± 0.18 for Yp
1, Yp
2, and Y
C, respectively. However, both Yp
3 and Yp
4 treatment displayed preferable results in terms of the reduction in microbial content during storage at 5 °C for 15 days. Moreover, the statistical analysis emphasized that there were no significant differences (
p > 0.05) between Yp
3, Yp
4, and Y
G. This present investigation was not in agreement with previous results of Hasan et al. [
33] and Dave et al. [
34], who confirmed that there were no significant effects due to different concentrations of stabilizers.
In addition, the results also indicated that coliform bacteria were not found during storage. This result was in accordance with Hasan et al. [
33] and Ganesh [
35], who confirmed the absence of coliform bacteria because of good storage requirements and avoiding contamination.
3.5. Sensory Evaluation
The results in the sensory investigation included appearance, texture, flavor, and acidity, as shown in
Figure 3,
Figure 4,
Figure 5 and
Figure 6. The statistical analysis emphasized that there was no significant difference (
p > 0.05) between Yp
3, Yp
4, and Y
G for all sensory parameters. This study also indicated that Yp
1 and Yp
2 had the lowest scores in terms of all sensory parameters with the passage of time and were significantly (
p < 0.05) lower than Y
G, Yp
3, and Yp
4; however, it was not significant (
p > 0.05) compared to Y
C. This finding was similar to those reported by Malik et al. [
7], who confirmed that yogurt samples remained satisfactory during storage at 5 °C for 15 days for all sensory parameters due to the different concentration of starch extracted from
Trapa bispinosa. In contrast, this present study was not in agreement with Sameen et al. [
12], who said that there was no statistical difference between adding a different concentration of starch in manufacturing yogurt and the control sample for all sensory parameters.