# Freeze-Thaw Pre-Treatment of Cassava Tubers to Improve Efficiency of Mechanical Peeling
## 3 Results and Discussion
### Characteristics of Cassava Tubers
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Source** & \
\begin{tabular}{c} **Sum of** \\ **Squares** \\ \end{tabular} &
\begin{tabular}{c} **Degree of** \\ **Freedom** \\ \end{tabular} & **Mean Square** & \(F\)**Value** & \(p\)**Value** \\ \hline Intercept & 3333.06 & 20.00 & 166.65 & 100.03 & \textless{}0.0001 \\ X\({}_{1}\)-Rotational speed of & 242.44 & 1.00 & 242.44 & 145.52 & \textless{}0.0001 \\ X\({}_{2}\)-Peeling time & 134.14 & 1.00 & 134.14 & 80.51 & \textless{}0.0001 \\ X\({}_{3}\)-Thawing temperature & 3.91 & 1.00 & 3.91 & 2.35 & 0.1597 \\ X\({}_{4}\)-Incubation time & 1170.61 & 1.00 & 1170.61 & 702.65 & \textless{}0.0001 \\ X\({}_{1}\)X\({}_{2}\) & 5.27 & 1.00 & 5.27 & 3.16 & 0.1090 \\ X\({}_{1}\)X\({}_{3}\) & 8.98 & 1.00 & 8.98 & 5.39 & 0.0453 \\ X\({}_{1}\)X\({}_{4}\) & 0.25 & 1.00 & 0.25 & 0.15 & 0.7062 \\ X\({}_{2}\)X\({}_{3}\) & 52.94 & 1.00 & 52.94 & 31.78 & 0.0003 \\ X\({}_{2}\)X\({}_{4}\) & 81.73 & 1.00 & 81.73 & 49.06 & \textless{}0.0001 \\ X\({}_{3}\)X\({}_{4}\) & 193.85 & 1.00 & 193.85 & 116.35 & \textless{}0.0001 \\ X\({}_{1}\)\({}^{2}\) & 193.16 & 1.00 & 193.16 & 115.94 & \textless{}0.0001 \\ X\({}_{2}\)\({}^{2}\) & 60.03 & 1.00 & 60.03 & 36.03 & 0.0002 \\ X\({}_{3}\)\({}^{2}\) & 54.87 & 1.00 & 54.87 & 32.94 & 0.0003 \\ X\({}_{4}\)\({}^{2}\) & 80.95 & 1.00 & 80.95 & 48.59 & \textless{}0.0001 \\ X\({}_{1}\)X\({}_{2}\)X\({}_{3}\) & 66.69 & 1.00 & 66.69 & 40.03 & 0.0001 \\ X\({}_{1}\)X\({}_{2}\)X\({}_{4}\) & 256.22 & 1.00 & 256.22 & 153.80 & \textless{}0.0001 \\ X\({}_{1}\)X\({}_{3}\)X\({}_{4}\) & 198.61 & 1.00 & 198.61 & 119.21 & \textless{}0.0001 \\ X\({}_{2}\)X\({}_{3}\)X\({}_{4}\) & 74.87 & 1.00 & 74.87 & 44.94 & \textless{}0.0001 \\ X\({}_{1}\)\({}^{2}\)X\({}_{2}\) & 14.87 & 1.00 & 14.87 & 8.92 & 0.0153 \\ X\({}_{1}\)\({}^{2}\)X\({}_{3}\) & 20.77 & 1.00 & 20.77 & 12.47 & 0.0064 \\ Residual & 14.99 & 9.00 & 1.67 & – & – \\ Lack-of-fit & 8.20 & 4.00 & 2.05 & 1.51 & 0.3268 \\ Pure error & 6.79 & 5.00 & 1.36 & – & – \\ Correction total & 3348.05 & 29.00 & – & – & – \\ \hline \hline \end{tabular} \(R^{2}\), 0.995; Adjusted \(R^{2}\), 0.986; and \(p<0.05\) indicates significance at the 95% level.  
\end{table} 
Table 5: ANOVA for reduced cubic equation for the effect of FTP, the rotational speed of the brushes, and the peeling time on the peel loss (\(PL\)) of cassava tubers. Based on the model, an optimum peeling process was predicted for a peeled surface area of 99.5% and a peel loss of 19% at a rotational speed of 1000 rpm, peeling time of 3.4 min, thawing temperature of 59 \({}^{\circ}\)C, and incubation time of 90 s. The optimum peel loss of 19% of mechanical peeling was higher than the \(PL_{man}\) of 14.5% obtained in manual peeling. The reason can be explained by variability in size and shape of cassava tubers. According to other studies, the proportion mass of peels in the tubers ranged from 15 to 20% [11; 20]. Therefore, the peeled surface area of 99.5% and peel loss of 19% can be appropriate for the optimum peeling process in this study. Reaching 100% peeled surface area would increase the peel loss as well, which is not acceptable for cassava industries. To validate the prediction of the model, three trials were conducted at optimal conditions. The peeled surface area and the peel loss were \(94.9\pm 2.6\%\) and \(21.7\pm 2.3\%\), respectively. Figure 3: (**a–d**) Surface plots indicating the effect of the rotational speed of the brushes \(v\) (rpm), peeling time \(t_{p}\) (min), thawing temperature \(T\) (\({}^{\circ}\)C), and incubation time \(t_{t}\) (s) on the peel loss \(PL\) (%) of cassava tubers, while keeping the other variables constant. (**e**) Normal probability plot of the residuals of PL. (**f**) Plot of residuals versus predicted values of PL. The rotational speed of the brushes and the peeling time were the most important variables for the peeling process. These findings are in line with other studies [20; 24; 25; 26; 27].