Introduction
Kimchi is one of the main Korean cuisine and a famous traditional side dish, which is prepared by the fermentation of vegetables such as cabbage, radish, and cucumber mixed with various seasoning salts, red pepper, garlic, and ginger pastes. It became popular globally due to its organoleptic, beneficial, and nutritional properties (Cheigh et al., 1994; Jung et al., 2014). Radish and cabbage are widely used as the main components of kimchi preparation. Radish is also seen on the Korean dining table as a salad or a variety of side dishes. It is an essential ingredient in soups, stews, and also for making a base broth for various dishes (Kim, 2016). Radish (Raphanus raphanistrum subsp. sativus) belongs to the Brassicaceae family and is an important vegetable crop worldwide, especially in Asia and Europe due to its wide adaptation, high yield, and abundant nutritional content (Umar et al., 2019). It can be consumed as a fresh vegetable or cooked condition and/or in processed conditions by pickling or drying. Due to its high medicinal and nutritional value, radish is suggested as an alternative treatment for various ailments including hyperlipidemia, coronary heart diseases, and cancer (Curtis, 2003). Every part of the radish is edible from the root to the flower. The green leafy tops of the radish are not only edible, but contain more vitamin C, protein, and calcium than its root. The green tops are highly nutritious and mineral-rich and should be valued more than their roots. Radishes and their green stems can be juiced for an excellent detoxifying drink. It can soothe the digestive tract and cleanse the entire body. Radishes can be categorized into four main types according to the cultivation seasons and a variety of shapes, lengths, colors, and sizes. , also called , is a variety of , is generally shorter, stouter, and sturdier than Daikon, and has a pale green shade down from the top.
Besides this, Chinese cabbage (Brassica rapa), another major component of Kimchi, is also a member of the Brassicaceae family and is cultivated widely throughout the world (Fordham and Hadley, 2003). It also contains around 18 intact glucosinolates components, vitamins, and minerals, which act as an alternative to medicines to prevent several diseases and promote human health (Yu et al., 2022). There are two major types of cabbage. The loose-leaf type known as Chihili or Mitchihili forms a long cylindrical head, 38-46 cm in height and 15-20 cm in diameter. The other, known as Wong Bok, forms a compact drum-shaped head that is slightly taller than its width. They grow up in the range of 20-25 cm in height and 15-20 cm in diameter. Chinese cabbage cultivars can differ greatly in plant size, mature period, and disease tolerance. The five commercial varieties of Chinese cabbage are Red dragon, Minuet, Rubicon, Bilko, and Tokyo bakena. The color is all green except the Red dragon type. Their head sizes and weight are between 25.4-30.48 cm tall and 2-5 kg (Huang et al., 2016). The size of the leafy head is an important indicator for measuring the yield and quality of Chinese cabbage (Wang et al., 2011).
Physical and strength properties, such as mass, height, width, diameter, length of root and stem, density and cutting diameter, assessment of radish and Chinese cabbage are important for industrial usage as well as enhance the mechanization rate through new field machinery development (i.e., transplanter, harvester) (Cetin et al., 2010). Usually, digital weight balance, digital caliper, rulers, and tapes are used for measuring the physical properties. The compressive strength is determined by the Universal Testing Machine (UTM), where force is applied until the failure of the target object. Moradi et al. (2017) tested the tomato stiffness using a UTM system, where a load cell of 50 kgf with an accuracy of 0.01N was used. Besides, the growth and physiological responses of Chinese cabbage and radish were analyzed for long-term exposure to elevated carbon dioxide and temperature by three cultivators (Choi et al., 2011). Venkatachalam et al. (2016) evaluated the tensile strength of bananas, which was between 15-20 MPa for fiber-reinforced composites and non-biodegradable matrices.
Although radish and Chinese cabbage have several food and health beneficial properties, the cultivation rates of these crops are reducing in many countries, such as Japan (19.5% yield and 18% acreage of daikon radish decreased from 2006 to 2018) (LLC, 2021) for the shortage of labor and time-intensive cultivation process, increasing percentage of the aged farmer, and high labor wage (Chowdhury et al., 2020). Mechanization of radish and Chinese cabbage cultivation process is essential. However, there are some semi-automatic or automatic transplanters and harvesters in the market, new machines are developing with more automatic features. Adequate knowledge about the physical properties of the target crop is very essential for this purpose. Moreover, the evaluation of the physical properties of upland crops (i.e., radish and Chinese cabbage) are essential for industrial usage, such as handling, transporting, cleaning, grading, packing, storing, and processing. Therefore, the objective of this study was to investigate the physical and strength properties of radish and Chinese cabbage for automatic harvesting as well as industrial use.
Materials and Methods
Measurement of physical properties of radish and Chinese cabbage
The commonly cultivated commercial varieties of radish and Chinese cabbage were surveyed. Thirty pieces of Daikon radish (Raphanus sativus var. longipinnatus) and Napa Chinese cabbage (Brassica rapa subsp. pekinensis), fifteen for each, were purchased with stem and root. The collected radish and Chinese cabbage were cultivated in Yeoju and Taebaek city, respectively. As the properties of harvested crops degrade with time, the radishes and cabbages were tested as soon as possible after harvesting with minimum degradation of physical, chemical, and biological properties. Digital caliper (Mitutoyo CD-15CPY, Kawasaki, Japan), digital weighing balance (CAS-KSC 1313, Seoul, Korea), ruler, marking tape, soft pen, camera, glove, and kitchen knife were used for the physical measurement of vegetables (Fig. 1-2). The experiment was conducted at the non-destructive Biosensor Laboratory in Chungnam National University, Daejeon, Republic of Korea on 9th, November 2019. A serial number of each vegetable was recorded by marking tape to measure physical dimensions before the experiment.
To obtain the basic data of the physical properties of radish, such as mass, height, diameter, and width of the bulb, length of root and stem were recorded (Fig. 3) (Cetin et al., 2010). Similarly, mass, height, diameter, and width of cabbage head, length of root, and stem for Chinese cabbage were also measured (Fig. 4) (Kanamitsu and Yamamoto, 1996; Lee et al., 2013).
Measurement of strength properties for radish and Chinese cabbage
In this research, a UTM (M18-16253-EN, Instron, Norwood, USA) was used to investigate the strength of radish and Chinese cabbage. A circular combined blade, aluminum mold for vegetables, and UTM were used to determine possible compression force. UTM mainly consists of a load cell, up and down the control button, emergency stop button, extension indicator, and plotters. An aluminum mold of 100×80×100 mm (length-width-height) was fabricated to fix the probe with UTM. Among several types of cutting blades, the circular (disk-type) blade is commonly used in upland crop harvesters (Cao et al., 2022; Alatyrev et al., 2020). The dimension of the circular blade used in this study was 160 ×1.5 mm in diameter and thickness, and the radius of the polygon hub was 15 mm (Fig. 5). A load cell of 50 kgf with an accuracy of ±0.25% was installed on the UTM to measure the compressive force of the specimens instantaneously. All the radish and Chinese cabbage were tested. The circular combined blade was fixed with the UTM machine and the zero setting was adjusted firstly. The cutting speed of the UTM was set at 10 mm/min through Bluehill 3 program. Grain of solid samples for setting speed: 1.25 mm/min ± 50%, and fruits and vegetables for setting speed: 2.5mm~30 mm/min were used for the experiment (ASABE, 2017).
Later, the radish was chopped with a kitchen knife into 100×80×50 mm (length-width- height) to obtain an equal size of the specimens during the compression test. The specimen was fitted with an aluminum mold and the movement of the specimen was adjusted using a control button, finally cut by the circular combined blade. After the compression force test by UTM, the actual cutting diameter of the stem for radish was measured up to fifteen pieces and recorded in an excel file. Likewise, the compression force test of root cutting diameter for Chinese cabbages was determined as the same method for radishes as displayed in Fig. 6-7. Compressive strength was calculated using equations 1 and 2.
(1)
(2)
where, σ is compressive strength (N/mm2); P is maximum cutting force (N); A is cross-sectional area of the specimen (mm2), and D is cutting diameter of the specimen (mm).
The average value of mass, height, diameter, width, density and length of root and stem for radish and Chinese cabbage were analyzed. Graphs for compression force and strength concerning the extended diameter of radish and Chinese cabbage were plotted. Regression analysis was performed to determine the correlation between compressive strength and extended distance for the vegetables.
Results and Discussion
Mean values with standard deviation, minimum and maximum for different physical dimensions of radish and Chinese cabbage were listed in Table 1. The physical properties of crops vary with species, cultivation method, environmental conditions, water-nutrition supply, and period of cultivation. For example, the bulb height, width, and density of the Daikon radish vary between 300-500 mm, 50-100 mm, and 780-980 kg/m3 (Obot et al., 2017), however, the maximum observed bulb height, widssth, and density in our experiment were 260 mm, 130 mm, and 684 kg/m3. Similarly, Cetin et al. (2010) observed the density of radish was found between 974.65-1,000.41 kg/m3. According to Clovegarden et al. (2020), the average calculated mass and length of the radish were in the range of 0.14-2.27 kg and 0.15-0.61 m, respectively.
Likewise, the maximum observed mass, height, and mass of Chinese cabbage were 1.78 kg, 343 mm, and 205 mm. Lee et al. (2013) observed the average mass of the Chinese cabbage was 3 kg and their diameter and height were in the range of 202.5 ± 10.4 mm to 243.8 ± 11.9 mm and 281.9 ± 1.46 mm to 321.3 ± 1.96 mm, respectively.
Besides, cutting speed, the average compressive force of blade, compressive strength, extended distance affected by the maximum compression force by UTM, cutting energy and standard deviation of vegetables were presented in Table 2.
The minimum and maximum compressive forces of radish and Chinese cabbage were between 19-48 and 108-188 N, respectively, as mentioned in Fig. 8. The lowest and highest compressive strength and extended distance of radish and Chinese cabbage were seen between 0.03-0.14 and 0.06-0.33 N/mm2 and 18.89-35.87 and 15.60-30.55 mm, respectively as described in Fig. 9. The linear regression analysis indicated that there was a negative relationship between maximum compressive strength (0.14 and 0.33 N/mm2) and maximum extended distance (35.87 and 30.55 mm) for radish and Chinese cabbage. It was reasonable to investigate the compressive strength of vegetables using a compressive test. Likewise, a compression test was conducted on packaged Roma tomatoes to study the effects of ripeness stage, vibration level, and container type using UTM. Three important parameters such as load, deformation, and stress were focused on all yield points of the fruit. Physical damage to the packages occurred during road transportation (Babarinsa and Ige, 2012).
Conclusion
The physical and strength properties of the upland crops (i.e., radish and Chinese cabbage) directly affect the mechanization rate and conveying characteristics of solid materials in industrial use also. The paper describes the strength of vegetables varies with extended distance when subjected to compressive force. It also shows the workability of vegetables with extended distances affected by the maximum compression force by UTM. It concludes that as the extended distance of the specimen increases, the compressive strength of the specimen decreases. According to the results, the radish and Chinese cabbage should have the strength to resist a compressive strength of 0.07 and 0.61 N/mm2. The strength correction factors between the physical properties of vegetables and strength should be further investigated. Furthermore, an empirical strength relationship is useful for the design of material and machine structure.
Acknowledgments
This work was supported by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry (IPET) through the Agriculture, Food and Rural Affairs Convergence Technologies Program for Educating Creative Global Leaders, funded by the Ministry of Agriculture, Food and Rural Affairs (MAFRA) (Project No. 320001-4), Republic of Korea.
