3.1. Characteristics of Cassava Tubers
The characterization of cassava tubers in this study is presented in
Table 2. The mass proportion of the peels in the cassava tubers ranged from 13.3 to 15.6%, which is in accordance to the results (8.5 to 17%) reported by Ezekwe [
13]. The observed thickness of the peels ranged from 2.7 to 3.3 mm in this study. A wider thickness range for peels of 1.2 to 4.1 mm was reported by Adetan et al. [
11]. The range of diameter and length of cassava tubers varied from 53.1 to 90.9 mm and from 194 to 320 mm, respectively. The force necessary to penetrate the cassava tuber peels and cassava tuber flesh varied from 4.4 to 5.6 (N/mm
2) and from 2.8 to 3.8 (N/mm
2), respectively. Similarly, a peel penetration force of 3.3 to 5.47 (N/mm
2) for cassava tubers was reported by Adetan et al. [
11]. Other characteristic parameters including the weight of the tuber, the dry matter of the peel, and the dry matter of tuber in this study are similar to previous studies with slight differences [
11,
24].
A weight range of about 900 g (415–1287 g) was observed in the sample, associated with the mean ± SD of 733.4 ± 254.7. Sorting of the agricultural products prior to packaging or processing is a routine in post-harvest operations. Therefore, based on the properties of the normal distribution and after checking for normality, the weight range of the tubers was limited to 500–900 g, which as approximation of “mean ± 1 × SD” should cover more than two- thirds of the population.
3.2. Effect of the Rotational Speed of the Brushes, the Peeling Time, and FTP on the Peeling Process for Cassava Tubers
The RSM design matrix for the rotational speed of the brushes, the peeling time, and FTP are presented in
Table 3 together with the peeled surface area and the peel loss. The peeled surface area and peel loss ranged from 14.9 to 97.9% and 4.2 to 37.5%, respectively, by variation of condition parameters (rotational speed of brushes, peeling time, thawing temperature, and incubation time). The average peeled surface area and peel loss of cassava tubers was 64.1 and 18.7%, respectively.
The mathematical equation obtained from RSM for the peeled surface area (PSA) of cassava tubers is as follows:
where
PSA is the peeled surface area after FTP and the peeling process (%),
v is the rotational speed of the brushes (rpm),
tp is the peeling time (min),
T is the thawing temperature (°C), and
tt is the incubation time (s).
It was observed that increasing the rotational speed of the brushes, the peeling time, thawing temperature, and incubation time had a positive effect on the peeled surface area.
The effects of individual variables and their interaction on the peeled surface area are shown in
Table 4. The accuracy of the model was indicated by
R2 and adjusted
R2 of 0.890 and 0.813, respectively. The
MAPE was 13.8%. Speed of brushes, peeling time, and thawing incubation time significantly (
p < 0.05) influenced the peeled surface area of cassava tubers. Higher
p-values for the thawing temperature and some interaction terms suggested little impact on the peeled surface area of cassava tubers.
Figure 2 presents the surfaces plots for the peeled surface area as a function of the rotational speed of the brushes, peeling time, thawing temperature, and incubation time. The model was further verified with the normal probability plot for the externally studentized residuals. It was determined that most of the residuals were on a straight line (
Figure 2d). This indicates the normal distribution of data. Furthermore, the plot of residuals versus predicted values, as presented in
Figure 2e, shows no clear pattern among the data, which suggests the absence of biases.
The mathematical equation obtained from RSM for the peel loss (
PL) of cassava tubers is presented in Equation (4).
where
PL is the peel loss after FTP and the peeling process (%),
v is the rotational speed of the brushes (rpm),
tp is the peeling time (min),
T is the thawing temperature (°C), and
tt is the incubation time (s).
The results showed that increasing the rotational speed of the brushes, the peeling time and incubation time positively affected the peel loss. On the other hand, an increase in thawing temperature had a negative impact on the peel loss.
Table 5 presents the effects of the individual variables and their interaction on the peel loss. The accuracy of the model was indicated by
R2 and adjusted
R2 of 0.995 and 0.986, respectively. The
MAPE was 4.20%. Rotational speed of the brushes, peeling time, and incubation time significantly (
p < 0.05) influenced the peel loss of cassava tubers. Higher
p-values for the thawing temperature and some interaction terms suggested little impact on the peel loss of cassava tuber.
Figure 3 shows the surfaces plots for peel loss as a function of the rotational speed of the brushes, peeling time, thawing temperature, and incubation time. The model was further analyzed with the normal probability plot for the externally studentized residuals. Similar to the peel loss, the data was normally distributed (
Figure 3e) and there was no biases or clear patterns among the data (
Figure 3f).
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 °C, and incubation time of 90 s. The optimum peel loss of 19% of mechanical peeling was higher than the
PLman 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 ± 2.6% and 21.7 ± 2.3%, respectively.
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]. Based on Jimoh and Olukunle [
26], cassava tuber loss increased and the peeling efficiency decreased by increasing the rotational speed of the rollers. Similarly, Ademosun et al. [
24] stated that by increasing the rotational speed of the peeling chamber, the performance of the peeling machine was improved. Furthermore, Olukunle and Akinnuli [
27] found that increasing the rotational speed of the brushes increased the peeling efficiency from 48.4 to 88.7%. In addition, by varying the speed from 140 to 160 rpm, the peeling efficiency increased from 81.2 to 91.4% [
25].
The results indicate that FTP would improve the peeling of cassava tubers by softening and loosening the peels. The thermal shock induced by FTP resulted in a softening of the peels, and thus improved the peel separation. This finding is similar to results of Brown et al. [
16] and Thomas et al. [
17] where FTP has improved tomato peeling by loosening the peel.
There was no study to be found about the application of FTP on the peeling process of cassava tubers. Although the incubation time of the thawing treatment was short in this study, it is necessary to investigate the effect of FTP on the quality of cassava starch after the peeling process. The only thermal treatment applied to peel cassava tubers was a high-steam compression application [
28]. It was determined that the time of steam exposure was one of the main problems of this method, because a long exposure time would cook the cassava tubers which affects their firmness, adhesiveness, and the springiness of the tubers [
28].