1. Introduction
Cassava (
Manihot esculenta Crantz), or tapioca, is a shrubby plant with upright stems reaching 2–5 m. It has massive roots underneath the stems and is rich in fiber and starch [
1,
2]. Cassava is propagated using stems cut into 13–17 cm long seed stems. These seed stems are then planted horizontally, obliquely, or directly after trenching. Tuber stems and cassava leaves are both utilized to process over 2000 product varieties, such as starch, alcohol, and organic chemicals [
3,
4,
5]. This versatile crop has applications in the food, medicine, and light industries [
6,
7]. Cassava is a significant renewable biomass energy crop and a crucial raw material for producing fuel ethanol [
8]. Cassava cultivation holds practical and strategic importance in ensuring food security and addressing the energy supply–demand imbalance [
9]. Cassava is predominantly grown in the Guangxi, Guangdong, and Hainan provinces in China, Yunnan, and Fujian. Cassava is mainly concentrated in Guangxi, accounting for approximately 60% of the country’s total output. China has now emerged as a significant cassava importer, with Guangxi alone importing around 5.5 million tons of fresh cassava raw materials from ASEAN countries annually, leading to a 50% dependence on foreign trade [
10].
Numerous studies have focused on cassava’s chemical and nutritional compositions [
11,
12,
13,
14]; however, more research is needed on its geometrical and mechanical properties. Cassava serves as a crucial provider of vitamins, carbohydrates, and minerals and is used in diverse sectors, including food, pharmaceuticals, textiles, animal feed, and biomass energy [
15,
16]. Cassava is rich in mineral elements and contains calcium, iron, zinc, potassium, magnesium, and manganese in quantities comparable with many legumes, excluding soybeans. Notably, its calcium content ranges from 19 to 176 mg/100 g, while its potassium content is 271 mg/100 g, significantly surpassing that of other cereals and rhizome crops. The carotene content in fresh root tubers ranges from 0.454 to 1.069 mg/100 g, with the color intensity corresponding to carotene levels [
17]. Brazilian red meat cassava varieties exhibit the highest carotene contents, followed by the yellow and white varieties [
18]. Changlin’s research delved into the structural aspects of cassava seeds, investigating its seed dormancy mechanisms. The mechanical barriers or inhibitors of its seed coat have been identified as critical factors that induce seed dormancy. Changlin also proposed effective methods to break seed dormancy and determined the optimal germination temperature for cassava seeds [
19].
The MPC are the basis for designing various cassava harvesting and processing machinery, conducting dynamic simulation studies, and establishing mathematical models. However, few theories and test methods exist on the MPC, both at home and abroad, and the scientific nature of the design of related harvesting and processing machinery needs to be improved. Yang Wang et al. studied the geometrical and MPC tubers and measured the bending, tensile, and shear mechanical properties of South China No. 205 cassava tubers but did not discuss the influencing factors of the MPC or their relationship [
20]. Zhong studied the MPC stalks; designed relevant mechanical property test methods; and measured the compression, shear, bending force, loading direction, and diameter of the stalks [
21,
22]. Yan Mei studied the MPC, such as cutting and compression, and came up with the influencing factors, such as variety, cutting speed, cutting angle, and MC [
23]. Dong Dong Ying designed and tested a tester for the MPC stems cut at multiple angles [
24].
Although scholars have conducted some research on the MPC at the macro level, they have not considered the linkage between the mechanical and geometrical properties of cassava, and studies on the effects of a single factor are not in-depth enough; thus, further research on these parameters and influencing factors is needed. In addition, few studies have been conducted on cassava moisture content and the mechanical properties of cassava, and no studies on cassava moisture content as well as the geometrical properties of cassava have emerged. In addition, no studies have been published on the relationship between MC and the effect of the geometric and MPC. Based on this, the present study focused on measuring the variation in the geometric and mechanical properties of two cassava varieties, the traditional variety “HRMUS KIMUYU” and the new cultivar “Sinsen 48”, which has not yet been studied, at different MCs, including their three-dimensional dimensions; geometrical characteristics; surface area; and various mechanical properties such as firmness and shear strength. The objective of this study was to investigate the relationship between cassava moisture content and the geometric properties as well as the mechanical properties of cassava. The findings of this research can be utilized to design, optimize, and assess cassava harvesting and processing equipment.
2. Materials and Methods
The test materials ‘Newly elected No. 48’ (starting now, referred to as A) and ‘Jin Yue HRMUS’ (now referred to as B) were cultivated in Guilin City during the 2022 growing season. This experiment used a randomized complete block design at the Mechanical Engineering Laboratory of Guangxi Normal University. In October of the same year, cassava was harvested using mechanical equipment. After harvesting, the cassava was cleaned and dried, with uniformly sized tubers free of damage or wormholes selected as test samples. In order to guarantee that the two types of cassava have some similarity, we tested their chemical composition (starch, reducing sugar, anthocyanin, protein, dietary fiber, and vitamin C), and the results of the test were as follows: the content of starch in cassava of class A is about 688 mg/kg, the content of reduced sugar is about 36.47 mg/kg, the content of anthocyanin is about 159.41 mg/kg, the content of protein is about 8.13 g/kg, the content of dietary fiber is about 15.3 g/kg, and the content of vitamin C is about 403.6 mg/kg. The starch content in cassava of class B is about 672 mg/kg, reduced sugar content is about 36.02 mg/kg, anthocyanin content is about 177.34 mg/kg, protein content is about 8.44 g/kg, dietary fiber content is about 13.4 g/kg, and vitamin C content is about 432.7 mg/100 g (all of the above contents are homogeneous); the two types of cassava are not very different in chemical composition, and they have a certain degree of similarity.
Freshly harvested cassava had an initial moisture content of about 65%. Subsequently, we dried the cassava to maintain its moisture content at ten moisture content levels, as applied to the two varieties, namely 80, 75, 70, 65, 60, 55, 50, 45, 40, and 35% (w.b.); uniaxial compression and shear tests were conducted in both loading directions, as shown in
Figure 1.
Using oven-drying for each variety at 110 ± 5 °C for twenty-four hours, the initial MC was measured. For various MC statuses, the samples were dried in a drying box at 85 °C (DHG-9051A electric thermostatic blast drying oven, Shanghai Yiheng Scientific Instrument Co., Ltd., Shanghai, China, with the relevant parameters detailed in
Table 1). Weight measurements were conducted every two hours until the target MC was attained. Before each drying session, approximately 50 kg of cassava was used for the experiment, and the MC was measured using Equation (1), described by Huimin et al. [
25], as illustrated in
Figure 2.
where
and
are the initial mass and dried mass of the samples (g), respectively, and
and
are the required MCs (%w.b.), respectively.
Three key cassava parameters, its dimension—width (
W, mm), length (
L, mm), and thickness (
T, mm)—were assessed using a General Vernier caliper NT13-20 (Tuofeng Measuring Instrument Co., Ltd., Hangzhou, China), with an accuracy of 0.02 mm, as illustrated in
Figure 2. The geometric average diameter, shape ratio, and surface area were evaluated, employing the subsequent equations:
where
is the geometric average diameter (mm),
is the shape ratio (%), and
is the surface area (mm
2).
Uniaxial compression and shear assessments were conducted using a CTX texture analyzer manufactured by Brookfield Company in the New York, NY, USA, with specific parameters delineated in
Table 2. The instrument boasted a mechanical sensor range extending to 1000 N, offering a precision resolution of 0.1 N. Testing protocols were applied in both vertical and horizontal orientations, positioning the specimen between two plates and subjecting it to a controlled compression and shear rate of 10 mm/min. Initial force measurements commenced at 0.5 N, with each trial meticulously repeated 20 times under ambient room conditions, as depicted in
Figure 3 and
Figure 4.
Uniaxial compression tests were conducted to assess the compressive firmness of cassava. The upper compression plate’s diameter was 175 mm (
Figure 3). Firmness was determined by dividing the compression force by the deformation value at the point of fracture.
where
is the firmness (N/mm),
is the compression force at the rupture point (N), and
is the deformation at the rupture point (mm).
Shear tests were conducted to assess the shear strength of cassava under shear stress [
26]. The upper shear plate used in the experiments had a thickness of 1 mm (
Figure 4). Shear strength is defined as the ratio of the force to the area at the splitting point.
where
is the shear strength (MPa),
is the shear force at the fracture point (N), and
is the stress area (mm
2).
This study examined how varying moisture levels and two cassava varieties influence cassava’s geometric properties. It explored the impact of ten moisture levels and two stress orientations and varieties on mechanical attributes, including fracture force, fracture displacement, firmness, and shear strength. Duncan’s multiple range test was employed to compare mean parameter values. Additionally, polynomial regression analysis was conducted using MC as the independent variable and fracture force as the dependent variable. Data analysis was performed using SPSS Statistics 20 (IBM, Armonk, NY, USA), with visualization and plotting executed using Origin 8.6 (Origin Lab, Northampton, MA, USA).
5. Conclusions
Our study underscores the significant effects of moisture content (MC), cassava variety, and loading orientation on the mechanical properties of cassava (MPC). We found that the MC of cassava, controlled by the drying process, ranged from 35% to 80% wet basis with increments of 5%. This range allowed us to evaluate the influence of humidity on key geometrical parameters, including length, width, and thickness, through a comprehensive correlation analysis. Notably, our results indicate that as MC increases, the compression rupture force (CRF) demonstrates a linear increase, while shear breaking force and shear strength exhibit a decrease with rising MC. These findings highlight the complex interplay between moisture levels and mechanical performance, emphasizing the need for the careful consideration of these factors in the design of harvesting machinery and food processing equipment. In the process of cassava processing, one can refer to the relationship between cassava moisture content and shear force, to explore the efficient processing time period after harvesting cassava. The evaluation of cassava properties presented in this study provides valuable reference data for establishing optimal conditions for harvesting, storage, and handling. However, it is important to acknowledge that our research focused on a limited set of geometrical and mechanical properties. Future investigations should expand upon these findings by exploring cassava contact parameters with various materials and examining the influence of roots and stems under real-world working conditions. Such research will further enhance our understanding of cassava behavior and improve the efficiency of processing technologies.