Peanut-Shelling Technologies and Equipment: A Review of Recent Developments

Abstract

Peanut is an important oil crop and cash crop, with a wide range of applications in many fields such as the food industry, light industry, and chemical industry. Mechanized shelling is a necessary part of the post-production processing of peanuts, and it is also the key to determining the quality of peanut products. Reducing shelling damage is an effective way to improve the quality and comprehensive benefits of peanut products. Consequently, it is of great significance to strengthen the research on damage reduction in mechanized peanut-shelling. China is a large peanut producer, but the research on mechanized shelling started relatively late, and the existing technology is not compatible with the high-quality shelling requirements of farmers. This paper reviews the status of mechanized peanut-shelling technology, compares the technical characteristics and equipment development of the world’s important peanut producing countries, it summarizes and proposes the suggestions to reduce loss from the aspects of varieties, agronomy, technology, and technical equipment; further deepen innovative research; and strengthen the construction of peanut-shelling socialized service systems. It is expected to provide reference for effectively reducing damage and improving quality of China’s mechanized shelling, and promoting the sustainable development of peanut shelling industry.

1. Introduction

Peanut is a globally important oilseed crop and a high-quality protein resource, with a global peanut cultivation area of about 2.95 × 10⁷ hectares and a production of 5.42 × 10⁷ tons, mainly distributed in China, the United States, India, Nigeria, Senegal, etc. [1,2,3]. Peanuts are popular because they are rich in dietary fiber, unsaturated fatty acids, vitamins B3, B6, pantothenic acid, and magnesium, which can improve memory, slow down stress, prevent stroke and cardiovascular disease, and are beneficial to human health [4,5]. Peanuts are versatile, not only for oil production but also for making peanut butter, peanut flour, peanut protein and snacks, beverages, etc. Peanut shells, peanut skins, peanut meal, and peanut seedlings contain proteins, fibers, antioxidants, polyphenols, and many other functional compounds, which can be used as animal feed, in the production of biofuels, as adsorbents, and as functional ingredients [6,7,8].

China is the largest producer, consumer, and importer of peanuts, with a peanut planting area of 4.82 × 10⁶ hectares, more than 16% of the global total, ranking second in the world, and an output of 1.86 × 10⁷ tons, accounting for about 40% of the world’s total output, ranking first in the world [9,10,11]. China’s peanut production plays an important role in the global peanut production industry, and its peanut quality directly affects the quality of the world’s peanuts and their products. In 2023, China imported about 700,000 tons of peanuts, which is a major trade commodity with countries such as Sudan and Senegal, and exported about 400,000 tons, which is also an important foreign exchange-earning agricultural product for Chinese exports to the European Union, Southeast Asia, Japan, and South Korea. In addition, peanuts are also the second largest oilseed crop in China, closely related to China’s oilseed safety and the quality of its products [12,13,14].

Peanut shelling is an indispensable and critical part of its production. From the perspective of the use of peanuts, it can be divided into the following three categories: edible, oil, and seed; for any one of these uses the peanuts must be shelled to obtain peanut kernels. Therefore, the operational effect of peanut-shelling equipment directly affects the quality and industrial benefits of peanuts and their products. Although peanut-shelling technology has been applied in peanut-producing countries, the working quality of shelling equipment has always been a hot issue of concern, and reducing shelling damage and improving the quality of peanut kernels has been a worldwide problem [15] (pp. 226–240). Damaged peanut kernels not only find it difficult to meet the requirements of China’s peanut high-quality production, seed production, and export of foreign exchange, which greatly reduces the economic benefits of peanut production, but also the damaged kernels are highly susceptible to Aspergillus flavus, which produces aflatoxins, a strong carcinogen, which greatly affects the food safety of peanuts and endangers human life and health [16,17,18,19,20]. Therefore, scientists from several countries around the world have conducted a large number of studies to reduce shelling damage. Although these studies continue to promote the progress of peanut-shelling technology, the quality and efficiency of peanut-shelling machines are not high, and it is still common to produce a large number of instances of incomplete shelling, seed coats cracked, kernels broken, damaged, and half-grain after shelling; the relevant technologies still need further research and breakthrough innovation [21,22]. Therefore, this paper starts from the principles, development process, and technical characteristics of peanut shelling, comprehensively analyzes the technical characteristics of peanut-shelling equipment in the world’s peanut-producing countries, especially the development process and equipment application status of mechanized shelling technology in the United States, and compares the relevant problems in China’s peanut-shelling technology development. It also summarizes the technical characteristics of peanut shelling, analyzes the advantages and disadvantages of various technologies, and proposes the improvement direction of peanut-shelling equipment to provide effective reference and support to reduce peanut-shelling damage, improve peanut-shelling quality, and promote the healthy and rapid development of the peanut industry.

2. The Key Technology Status of Mechanized Peanut Shelling

The effect of mechanized peanut shelling is affected by many factors, and the constant upgrading of its technology and equipment has benefited from the continuous breakthroughs made by a large number of scholars in the physical and mechanical characteristics of peanuts, process research, structure types of key components as well as the movement parameters, and technological integration of the whole machine.

2.1. Research on the Shelling Law and Material Characteristics of Peanuts

Peanut shelling is the result of the interaction between varieties and key components of shelling, as variety characteristics have a significant impact on the quality of peanut shelling. Peanut varieties are complex, with a variety of pod shapes and sizes, including single-grain, normal, axe shape, gourd shape, bee-waist shape, cocoon shape, hockey-stick shape, and beaded shape. The kernel shapes also vary, including round, cylindrical, and oblong (Figure 1), and there are obvious differences in shelling characteristics and great differences in adaptability of mechanized shelling [23,24]. Wynne J.C. et al. [25] evaluated the shelling quality of a variety of peanuts, which provided a reference for the evaluation of peanut breeding suitable for mechanized shelling.

Gao, Xuemei et al. [27] pointed out that the peanut geometrical dimensions affecting the parameters of the components are mainly the width and thickness of the pods and seeds. Xu Hongmei et al. [28] found that under the action of loads in different directions, peanut pods exhibit two forms of shell rupture: longitudinal rupture and transverse rupture (Figure 2). Longitudinal rupture generally occurs at the bonding edge of the two sides of the pod shell, and transverse rupture tends to occur at the thinnest part of the shell at the loading point of the cross-section firstly, and then cracks outward along the webbing of the pods.

Na et al. [29] divided the mechanical damage characteristics of kernels into broken and half-grained, severely damaged kernels, slightly damaged kernels, complete seed coat shedding, partial seed coat shedding, and invisible internal damage (Figure 3).

Davidson J.I. et al. [30,31] not only described the probability distribution of peanut seed size with mathematical relations to provide a numerical reference for peanut-shelling sieve requirements and equipment performance, but also studied the effect of mechanical shelling of peanuts on seed germination, and concluded that only the cylinder rotation speed had a significant effect. Lü et al. [32] found that the pods with poor fluidity also had poor scattering and a higher mechanical damage rate. Blankenship P.D. et al. [33] found that the shelling quality was poor when ventilating or adding water to restore the moisture content of peanuts before shelling, and the moisture content of peanuts should be maintained during storage to ensure shelling quality. Lamb, M.C. et al. [34] shelled peanut pods with a moisture content of 10.49% and 18.49%, respectively, and the results showed that there was no significant difference in the number or weight of unshelled pods between the two operations. Xu et al. [35] explored the static friction coefficient between pods and Q235 steel, large-hole sieves, and small-hole sieves under different moisture contents and found that it exhibits a similar trend, and increases with the increase in moisture content. Many scholars have also found through experiments that moisture content, peanut variety, loading speed, and placement method all have significant effects on the rupture force of pods and breakage force of kernels [36,37,38,39,40,41].

2.2. Research on the Peanut-Shelling Process

The post-harvest processing technology of peanuts is also extremely important for shelling quality. Fresh peanuts have a high moisture content, which makes them prone to mold and black spots, and they need natural drying or mechanical drying to reduce their humidity, facilitating shelling and long-term storage [42]. However, during the drying process, it is necessary to control the degree of moisture to avoid damaging the pectin composition and cellular structure of the peanut seed coat, preventing seed coat cracking and kernel damage, and reducing the risk of aflatoxin contamination [43]. Studies have shown that the lowest mechanical damage of kernels occurs when the moisture content is controlled at 12.0% to 13.5%, which helps the pod shells to fully rupture while retaining the malleability of the kernels [44]; therefore, in order to ensure the shelling effect, peanut pods can be soaked and wetted to adjust the moisture content to a level suitable for shelling before the actual shelling. The required rupture force of the pods can also be significantly reduced by sulfuric acid treatment, but it has not been widely promoted due to food safety concerns. Then, peanut pods are subjected to a de-sorting treatment to remove impurities such as soil and stones, and also undergo clearing and grading to improve pod homogeneity and reduce kernel damage during shelling. After shelling, kernel-shell separation and kernel grading are achieved by negative pressure wind-sorting, gravity separation, and color sorting to differentiate between kernels of different qualities [45]. Some shelling plants in United States will also connect the processes of cleaning, grading, and shelling in series, and design the process flow to form a complete set of peanut-shelling and processing lines, and set up different shelling process routes for oil, food, or seed [46,47,48]. In addition, when storing peanut pods or seeds, it is also necessary to strictly control the storage environment, temperature, and humidity to prevent the infection and destruction of Aspergillus flavus and birds, beasts, insects, and reptiles. By upgrading the above process technology, the probability of mechanical damage during peanut shelling can be reduced, and the efficiency and quality of shelling can be greatly improved to ensure food safety.

2.3. Research on the Principles of Peanut Shelling

Peanut shelling is the process of breaking the shell of peanut pods, taking out the kernel with the help of mechanical external force, and separating the kernel and the hull by various sieving techniques. Most of its principles are based on squeezing, rubbing, and kneading; the current mainstream shelling equipment is based on different structural types under this principle [49,50,51,52], as shown in

Table 1. Classic peanut-shelling structures, principles, and features.

Type	Structural	Principle
Rubber rollers, squeezing and rubbing type		Peanut pods are subjected to friction from two horizontally placed rubber rollers relatively rotating at different speeds, which produces a rubbing action and normal squeezing action to shell the peanuts.
Toothed roller & toothed plate, shearing and rubbing type		It consists of a toothed curved plate and an external toothed roller. The pods are sheared and rubbed by the steel teeth between the roller and the plate to remove the shell. The kernel breakage rate is relatively high.
Rubber cylinder & concave sieve, squeezing and kneading type		It consists of a closed rubber cylinder and a woven concave sieve. Peanut pods are brought into the gap between the cylinder and sieve through the rotation of the cylinder and are squeezed and kneaded to remove the shells.
Rods & concave sieve, striking and kneading type		It consists of an open-type cylinder made of flat steel rods and a steel concave sieve. The pods are struck by the rotating rods and kneaded by the high-speed rotating cylinder to remove the shells, and the performance is ideal.
Grinding disc, pressing and rubbing type		It consists of an upper fixed grinding disc and a lower rotating grinding disc. The pods are shelled by friction and squeezing between the two grinding discs.
Floating rubber plate & sieve, pressing and rubbing type		It consists of a flexible washboard connected by connecting rods to achieve horizontal reciprocating motion and a braided flat sieve, and the pods are shelled by the pressing and rubbing action of the rubber plate. The shelling effect is not good.

. After many years of comprehensive consideration of productivity and operational quality, the most common equipment for peanut shelling is the striking and kneading type currently. In addition, there are some new shelling methods, such as the gas explosion method, ultrasonic method, etc. [53,54,55].

2.4. Research on Peanut-Shelling Equipment

Based on its four types of shelling equipment, United States scholars studied the effects of different shelling materials (polyurethane, high-density polymer, steel-aluminum alloy, tungsten carbide, and heat-treated steel), mechanism types (perforated metal sieve, steel grid sieve, “T”-shaped steel grid sieve, and cast-iron cylinder), and various variables (peanut variety or type, concave sieve type, feeding method, rod width, and cylinder–concave sieve spacing, etc.) on the shelling performance.

China’s peanut-shelling equipment is mainly based on the open cylinder-concave sieve of the striking and kneading type, and the solid cylinder-concave sieve of the squeezing and kneading type. The striking and kneading type utilizes rigid striking and shearing functions to shell, with high efficiency but a high breakage rate, applicable to large-scale shelling for oil extraction or food processing; the squeezing and kneading type utilizes extrusion and friction functions to shell, with a low breakage rate but prone to be clogged, resulting in low efficiency, applicable to the small batches of farmers shelling for seed. The shelling cylinder and concave sieve are the key components of these types of peanut-shelling equipment, and scholars have carried out a lot of research to improve the quality of shelling [56,57,58,59,60], as summarized below.

2.4.1. The Main Structural Types of Key Components

The shelling cylinder is the core component of shelling equipment. Its types are mainly divided into open and solid types. The open cylinder generally has 2~4 rods/scrapers uniformly distributed around the rotating shaft, rods in the shape of square, round, flat, threaded, etc., the material of wood, iron, rubber, nylon, polyurethane, etc., or flexible materials, bumps, edge plate, etc. attached to the outside of the rods/scrapers. The solid cylinder’s outer surface is divided into two types, smooth and with teeth; the material also is wood, rubber, polyurethane, etc. (Figure 4). The open cylinder is more widely used than the solid cylinder. In addition, there are also special rollers, such as spiral and taper.

The shelling cylinder interacts with the concave sieve to shell the peanuts and separate the kernels from the crushed husks. The sieve is generally semi-circular or minor arc-shaped, and the material is mostly cast iron. In order to improve the screening efficiency and sieve surface utilization coefficient, and reduce the wear and tear on the kernels, the area of the sieve holes gradually increases, developing into woven, perforated, and grid forms (Figure 5), or being optimized by adding sieve ribs, or changing the shapes of grids and punches. However, woven screens have been phased out because of poor rigidity, easy deformation, and high kernel breakage rates. Punched screens, although strong, have a low sieve surface utilization coefficient and low shelling efficiency, and are less commonly used. In comparison, the grid concave sieve has superior performance and has been widely applied.

2.4.2. The Type of Shelling Equipment and the Impact of Its Operational Quality

Gao et al. [61] optimized the striking and kneading peanut-shelling equipment by designing the shelling cylinder as an adjustable rotating play-bar mechanism, which can change the play-bar type and number, and change the spacing of the shelling cylinder-concave sieve grouping, as shown in Figure 6. Peanuts were shelled under the high-speed striking and kneading of the play-bar between the cylinder and concave sieve. The adjustable mechanism improves the adaptability of the machine to peanut varieties, but the all-metal shelling cylinder increases kernel breakage to some extent. The machine performed best when shelling BaiSha peanuts with 11.87% moisture content.

Gao et al. [62] created a small independent parallel shelling equipment with three rotary stages and a single row of three drums, which can utilize multi-stage drums to improve the adaptability of pod varieties, sizes, and multi-processing, and to reduce the frequency of adjustment and cleaning of the machine, and also uses thick flexible rods to avoid the rigid impacts and to reduce the shear effect, which is applicable to small batches of seed-breeding shelling, as shown in Figure 7. The machine has the best performance when shelling peanuts with moisture content of 16~18% of the Baisha 1016 cultivar.

Hao et al. [63] developed a grinding disc type peanut-shelling equipment. The shelling structure is cone-disc shaped; the upper cone disc is fixed, the lower cone disc rotates, forming a ring conical cavity shelling chamber, and the surfaces of the upper and lower cone discs are pasted with prickly rubber skin. Pods are moved downward between the upper and lower cones by centrifugal force, friction, and gravity. The large pods rupture their shells at the upper end of the two cones, and the small pods fall into the narrow area along the cone and are shelled by the squeezing and rubbing force, as shown in Figure 8. The grinding disc type design achieves targeted shelling for both large and small pods, However, when shelling, the pods are prone to pile up at the bottom of the grinding disc, which increases the damage rate, and the rubber spines wear out from extended operation, thereby reducing the shelling quality. The best performance of this machine is in the shelling of Huayu 23 peanuts with a moisture content of 15% to 18%.

Lu et al. [64] designed a vertical tapered drum type peanut-shelling equipment. The shelling principle is similar to the grinding disc type, but the realization structure is different. After feeding, the pods fall into the tapered drum and the conical grid sieve along the flow regulation plate, where they rotate downward following the hulling rib strip on the outer surface of the tapered drum, and in the process are subjected to extrusion, friction, shear, and other combined forces to shell, as shown in Figure 9. This machine utilizes the principle of laminar flow and thin-layer shelling to increase the effective volume of the shelling chamber and improve the efficiency of the separation of shells and kernels. However, there is the same problem that the pods tend to accumulate at the bottom of the tapers. The machine has the best performance in shelling Silihong peanuts with moisture content of 9% to 11%.

Li et al. [65] developed a reciprocating kneading peanut-shelling equipment, which consists of the crank rocker mechanism with many round rubber dots fixed at the bottom and a concave woven sieve. The crank rocker mechanism enables reciprocating swing, which is used to lay the pods flat, and uses the round rubber dots reciprocating and rubbing the pods to shell, as shown in Figure 10. The machine is simple in structure and easy to operate, but the woven sieve mesh is not easy to replace, and its adaptability to different peanut varieties is poor. Moreover, the round rubber dots suffer from significant wear and tear due to prolonged use. The machine had the best performance for shelling Yuhua 22 peanuts with a moisture content of 11% to 14%.

2.4.3. Research on Parameters of Component Structure and Working

The structural and working parameters of peanut-shelling equipment have a significant impact on the machinery’s performance. Wang et al. [66,67,68] conducted a large number of experiments and parameter optimization research. They firstly discovered that the working parameters of the vibration sorting device in the compound shelling equipment were set unreasonably, which led to a high impurity content and loss rate in material sorting, and the compound shelling effect of the equipment was not obvious. Through single factor and BBD experimental designs of the vibrating screen amplitude, the optimal working parameters for the vibrating screen amplitude, vibration frequency, and vibration arm angle were determined to be 3.8 mm, 485 Hz, and 35°; at this time, after sorting, the material’s impurity content was 2.18%, and the loss rate was 1.74%, which greatly improved the shelling effect. Then, addressing that the problem of parameter setting mismatch between the peanut-shelling feeding volume, shelling cylinder speed, and cylinder–concave sieve gap, resulting in a high shelling damage rate, to carry out parameter optimization, they concluded that with a feeding volume of 204.6 g/s, a shelling cylinder speed of 274.8 r/min, and a cylinder–concave sieve gap of 24.7 mm, the best effect of shelling work is achieved, with a shelling damage rate of 3.44%, and a net rate of removal of 96.0%. After that, the coupling effect between various shelling influencing factors was considered, using the Hertz theory and BBD design method to perform comprehensive multi-objective optimization of the 6BH-800 peanut-shelling equipment. It was concluded that the elastic modulus of the shelling cylinder material is 10 MPa, the curvature radius of the key shelling components is 12.77 mm, the speed of the shelling cylinder is 277.48 r/min, and the cylinder– concave sieve gap is 24.24 mm are the best working parameter combinations, and a shelling damage rate of 4.89% and a shelling efficiency of 97.91% were achieved.

3. Overview of Peanut Shelling Mechanization in Foreign Countries

Peanut cultivation abroad is dominated by the United States, India, Nigeria, Argentina, and Brazil, and the degree of mechanization of peanut shelling is different. The United States is a rare large-scale peanut-planting country in the developed countries; its peanut-shelling equipment and varieties are highly integrated, and the technical performance is relatively stable.

3.1. Mechanized Peanut-Shelling Technologies and Research Status in the United States

3.1.1. Peanut Production Situation

The peanut planting area of the United States is about 6.67 × 10⁵ hectares, ranking sixth in the world, with a yield of 2.52 × 10⁷ tons, accounting for about 5% of the world’s production, ranking fourth in the world. Peanut planting is mainly concentrated in the southeastern states: Georgia, Alabama, Texas, and Florida, respectively, accounting for 55%, 10%, 10%, and 9% of the peanut planting area in the United States. There are four main types of varieties produced, Runner, Virginia, Spanish, and Valencia, and they are used for different purposes according to their characteristics; about 57% are used to make peanut butter, peanut candies, peanut foods, etc., 19% are used for export trade, only 14% are used for oil extraction, and about 10% are used for seed production [69,70].

3.1.2. Development of Mechanized Peanut-Shelling Equipment

Mechanized peanut-shelling technology research in the United States started early and has a development history of more than 100 years. At the beginning of the 20th century, some simple wooden manual peanut-shelling equipment was introduced [71,72] and gradually matched with various power sources. By the late 1930s, dozens of manpower and power-operated peanut-shelling machines were invented [73,74,75,76,77,78,79,80,81,82,83], which gave birth to a variety of principles of peanut-shelling equipment and various structural forms of key shelling components [84]. Figure 11 shows the early peanut shelling equipment in the United States.

In the 1940s, the peanut planting area and production reached a historical peak, and the amount of shelling operations increased greatly. Under the promotion of ASPA, peanut shelling gradually developed into a centralized in operation, and the equipment has gradually developed towards efficient and large-scale. At the same time, some excellent shelling factories and shelling equipment manufacturers emerged, such as the American peanut company Birdsong Peanuts, Golden Peanut, and the American famous machinery manufacturing company LMC (Lewis M. Carter Manufacturing Company, Donalsonville, GA, USA). Figure 12 shows peanut-shelling line in the United States in the 1950s).

In the 1960s, there were four types of commercial shelling equipment popular in the United States, namely Appomattox Sheller, Medley Sheller, Hendrick Sheller, and Pearman Sheller (Figure 13); the difference lies in the different structures and materials of the shelling cylinder and the concave sieve. Among them, Medley and Hendrick had a shelling rate of about 60 percent, while Appomattox and Pearman had a shelling rate of about 70 percent [86]. During the same period, the efficiency of peanut-shelling equipment in Tift, Georgia, had reached 6 t/h [87]. In the 1970s, peanut-shelling equipment with a shelling efficiency of 10 t/h was gradually applied to production to meet the diversified needs of the market; small-scale laboratory shelling equipment also came into being (Figure 14a).

After years of development and market verification, the current U.S. commercial peanut-shelling equipment is dominated by LMC’s large-scale series shelling equipment, accounting for about 90% of its market share [88]. The shelling equipment is generally equipped with 4~5 shelling cylinders, whose core structure consists of three flanges clamping and fixing three rotating rods with corrugated edges at the outer end and a semi-circular concave sieve composed of “T”-shaped grid steel, the materials of which are all subjected to standard heat treatment (Figure 15). By setting the structural parameters of each shelling roller and concave plate sieve, parallel multi-stage shelling can improve shelling quality.

Table 2. Information about shelling equipment developed in USA.

Model	Structure	Characteristics
Single-cylinder shelling equipment [89]		Each shelling cylinder has three built-in hardened shelling bars; the spacing between the shelling bars and the concave grate is easy to adjust. The shelling grates or concave sieves are easy to change out and can be sharpened as they become dull with wear.
High-moisture shelling equipment		Used for shelling green peanuts to monitor the drying process; easy to operate and maintain, safe and efficient.
Multi-cylinder shelling equipment [90]		It is equipped with 3~5 shelling cylinders, a sheller shaker pan, hull aspiration, two deck screening shakers, gravity separators, and sheller grates of various sizes. Each shelling cylinder can be driven independently; the maximum capacity is up to 9 t/h.

shows the information of some shelling equipment developed by LMC.

3.1.3. Peanut Shelling Socialized Service System

The rapid development of the American peanut-shelling industry also benefits from a sound socialized service system, such as the American Peanut Shellers Association (APSA), the National Peanut Buying Points Association (NPBPA), the Southern Peanut Farmers Federation (SPFF), and the United States Peanut Federation (USPF), which organically unified peanut sowing, harvesting, processing, and product sales into a complete system, and laid a solid foundation for the U.S. to deeply integrate peanut shelling with peanut variety characteristics, promote advances in technology and equipment, and facilitate the gradual formation of large-scale, integrated, and serialized production lines [91]. Figure 16 and Figure 17 show factory peanut shelling and its assembly-line work in the United States.

3.2. Mechanized Peanut-Shelling Technologies and Research Status in Other Countries

India and Nigeria have a long history of peanut cultivation, their planting area, and production are in the forefront of the world, and they play a pivotal role in the development of the world peanut industry. The status quo of their shelling technology and equipment is of reference significance for the research and improvement of mechanized peanut husking.

3.2.1. Research on Peanut Production and Shelling Mechanization in India

India widely cultivated peanuts in the second half of the 19th century [93], and has become the world’s first peanut planting area and second peanut production country. Peanut production exceeds 7 million tons, accounting for about 14% of the world’s output. India’s peanuts are mainly planted in western, southern, and eastern coastal areas, such as West Bengal, Gujarat, Andhra Pradesh, and Tamil Nadu [2]. Some achievements have also been made in research into mechanized peanut-shelling in India. Gojiya D.K. et al. [94] studied the physical and mechanical properties of peanut kernels. Mishra A. et al. [95] first developed a low-cost, manually operated pedal-powered peanut sheller. Nilesh B. [96] designed an economical peanut sheller suitable for agricultural use (Figure 18a). Darshan Gowda C.P. et al. [97] manufactured an electric peanut sheller and selected and designed its parts (Figure 18b). Ramachandra Raju K. et al. [98] developed a multi-functional peanut sheller with a built-in roaster. Prathamesh S.G. et al. [99] designed a peanut sheller suitable for seed production and small industry (Figure 18c,d).

There are many small and medium-sized peanut-shelling equipment enterprises in India, mainly including Shree Rajaram Agro Industries Private Limited (SRRAI), Chetan Agro Industries, Shri Sai Industries (SSI), A P S Industries, and so on. Large shelling equipment is often equipped with a unique circular separator or a structure equipped to facilitate tractor transport. However, the overall level of peanut-shelling equipment in India is not high, and models and characteristics of some shelling equipment are shown in the

Table 3. Some peanut-shelling equipment models and features in India.

Company (Location)	Model	Structure	Characteristics
Shree Chamunda Agro Engineering Works (Rajkot, Gujarat, India)	Peanut breaker machine [100]		Semi-automatic operation; capacity: 100 kg/h; size: 1016 × 356 × 838 mm; shelling rate: 90–95%; breakage rate: 2%.
A P S Industries (Ahmedabad, Gujarat, India)	Combined peanut-shelling machine [101]		It consists of a peanut de-stoner and a peanut-shelling machine; Size: 1400 × 2500 × 2200 mm; capacity: 500 kg/h; weight: 700 kg; shelling ratio: ≥98%; breakage ratio: ≤5%; cleaning degree: ≥95.5%.
SRRAI (Bikaner, Rajasthan, India)	SRRAI- Tractor-1 [102]		Equipped with a round separator and tractor towing frame; capacity: 1200 kg/h; shelling ratio: 99%; power consumption: 5 kw/h.
SRRAI (Bikaner, Rajasthan, India)	SRRAI- Plant-14 [103]		It consists of three parts: feeder, shelling machine and round separator; fully automatic; sorting accuracy: 95%; size: height × length = 1980 × 3660 mm.

.

3.2.2. Research on Peanut Production and Shelling Mechanization in Nigeria

Nigeria is the largest producer of peanuts in Africa, with a peanut output of nearly 4.3 million tons, accounting for about 27% of Africa’s production and ranking third in the world [2], and about 78% of the country’s arable land is suitable for peanut cultivation, mainly in Kano State, north-central, north-west, and north-east regions. The peanut industry used to play an important role in the Nigerian economy, especially in the 1960s. Although the peanut industry has since declined due to the rise of the oil industry, peanuts are still an important part of Nigerian agriculture. The development of peanut shelling mechanization in Nigeria lags far behind that of China, India, the United States, and Argentina, and most of them are still dehulled by hand, and many scholars have developed and improved a variety of mechanical peanut-shelling equipment (Figure 19).

4. Overview of Peanut Shelling Mechanization in China

4.1. Peanut Production Situation

China ranks second globally in peanut planting area and first in production, playing a significant role in global peanut production. Peanut cultivation occurs in most provinces and regions across the country, with the exceptions of Tibet, Qinghai, and Hong Kong. It is predominantly concentrated in eastern and central China, with the top three planting areas being Henan, Shandong, and Guangdong. According to the latest data from the National Peanut Data Center, it can be found that a cumulative total of 3675 Chinese peanut varieties (lines) have been included, with 1067 varieties registered by the Ministry of Agriculture in previous years, and 732 newly registered varieties [107]. The consumption structure of peanuts is dominated by oil extraction, which accounts for more than half of the total national consumption, with about 40% for food, as well as feed, seed, and agro-industrial consumption.

4.2. Development of Mechanized Peanut-Shelling Equipment

The research on mechanical equipment for peanut shelling in China started late, and the earliest shelling equipment was hand-built by woodworkers and supplemented with tools such as wooden splints for shelling, which had low shelling efficiency and poor operational quality [108], as shown in Figure 20a. After the founding of the People’s Republic of China, farmers began to make shelling machines on their own initiative, and simple hand-cranked shelling machines appeared, such as a kind of hand-cranked wooden shelling machine used in the Yimeng mountainous area of Shandong Province, as shown in Figure 20b. Some agricultural tool factories also began to manufacture shelling equipment. The molding peanut sheller that began to be documented in China is the blower-type pedal peanut-shelling machine manufactured by the Garden Farming Tool Factory of Xiaogan County, Hubei Province, in 1958 [109], as shown in Figure 20c, but these items of equipment failed to solve the problems of operational quality and efficiency.

In 1965, the scientific research related to peanut-shelling machines was emphasized by the state, and the corresponding scientific research topics were designed by the competent authorities [111]. With the implementation of The Household Contract Responsibility System and the improvement of peanut income, the technical demand for shelling equipment tends to flourish, and small electric shelling equipment has come out. Until the 1990s, the contradiction between the rapid development of the peanut industry and the inefficient manual shelling was prominent, shelling equipment has been rapidly developed, and shelling equipment of various structures has emerged, such as open-type cylinder, solid-type cylinder, double- and multiple-cylinder, compound shelling, etc. The component materials have gradually been updated, and the shelling process has been further improved, which can realize the wind separation of shells and kernels, reelection, sorting, etc. [112,113,114]; however, the mechanical damage rate is as high as 20–25%, with poor shelling efficiency and large operational losses.

Nowadays, China has developed a combined unit of shelling equipment and de-stoning and de-impurity, cleaning, and grading devices, with an output of up to 6~8 t/h, but it is still dominated by small and medium-sized shelling equipment, with an output of 2~3 t/h. Meanwhile, scholars are still carrying out research and development of seed peanut-shelling equipment and multifunctional and integrated medium- and large-sized shelling equipment, which integrates the functions of automatic loading, de-stoning and de-impurity, cleaning and grading, and bagging, etc., intending to continuously reduce the mechanical damage rate and use it for seed production [115,116,117].

Table 4. Some peanut-shelling equipment models and features in China.

Companies (Location)	Model	Structure	Characteristics
KMEC (Anqiu, China)	BK-200 peanut-shelling machine [118]		Capacity: 200 kg/h; power: 2.2 kw; weight: about 40 kg size: 630 × 600 × 930 mm
Yantai Lingyuan Peanut Machinery Co. Ltd. (Yantai, China)	LY-20000 Eco-friendly peanuts shelling machine group [119]		Capacity: 6000 kg/h; weight: 2100 kg; power: shelling machine 28 kw, stone remover 9.5 kw; loss ratio: ≤0.2%; shelling rate: ≥99.5%; breakage rate: ≤2%; number of operators: 3–4 people
Huojia County Yuhang Machinery Factory (Hujia, China)	20000 model four-cylinder peanut-shelling machine [120]		Capacity: 5000–8000 kg/h; shelling rate: ≥99; breakage rate: ≤5% loss rate: ≤0.5%; clean rate: ≥99% operating moisture: 6.3%~12% size: 3400 × 1860 × 3360 mm
Nanjing Institute of Agricultural Mechanization, Ministry of Agriculture and Rural Affairs (Nanjing, China)	6BH-800 Peanut- shelling machine [121]		Capacity: 400 kg/h; breakage rate: ≤5% shelling rate: ≥95 clean rate: ≥96%
	Peanut seed shelling and sorting complete equipment set [121]		Used for peanut stone removal, shelling, and cleaning; capacity: 0.5–1 t/h; breakage rate: ≤5%; impurity rate: ≤1%

below shows the information of some peanut-shelling equipment and production units in China.

5. Comparison and Analysis of the Development of Peanut Shelling Mechanization in China and the United States

5.1. Comparison of Mechanized Peanut-Shelling Technologies between China and the United States

(1)

Comparison of equipment adaptability

Both China and the USA are major peanut-producing countries, but the national conditions of the two countries are different, and the peanut planting regions and varieties have their own characteristics. China’s peanut cultivation is widely distributed; each region has its own specific peanut variety, and the number of cultivated varieties reaches thousands, with significant differences in characteristics, and the diversity of peanut varieties poses extremely high requirements for the variety adaptability of shelling equipment. The peanut planting area in the United States is relatively concentrated, and peanut varieties are selected according to the technical requirements of mechanical harvesting or processing. There are few types of peanut varieties, and whether they are suitable for mechanized shelling is one of the indicators to be considered in the early stage of variety breeding. Varieties that are not suitable for mechanized shelling processing will not be planted in the market. As a result, the level of integration between U.S. peanut agricultural machinery and agronomy is relatively high, and the equipment can well meet the shelling operations for oil and edible peanuts of the existing varieties in its domestic market.

(2)

Comparison of the level of shelling technology and equipment

In China, peanut shelling is narrowly defined as the process of separating peanut kernels from the pods, and China’s peanut planting area is scattered, most peanuts are planted by small farmers, and the average planting area per household is not large, which cannot result in large-scale centralized production and shelling. In contrast, peanut shelling in the United States is a sophisticated process that encompasses more than just removing the shells. It also includes stages like sorting and grading unshelled pods before shelling, cleaning and grading kernels after shelling, aflatoxin sampling, packaging, and storage. It has gradually developed into a centralized shelling production line factory; moreover, there is a substantial investment in peanut-shelling plants, consolidated peanut purchasing stations, and established peanut corporations with an annual shelling capacity of 300,000 tons [122,123].

China’s peanut production model determines that its production equipment is mainly small and medium-sized single shelling machines, which have poor adaptability and making it difficult to achieve high-quality shelling. In contrast, US commercial peanut-shelling equipment not only includes shelling functions but also pod- and kernel-processing units, and weighing and bagging systems. It has set up different shelling processes and equipment for oil, seed, or edible processing, and the oil shelling process is relatively simple. Seed shelling reduces the operational productivity and increases processing steps for kernels such as color sorting and grading, and selecting broken kernels for oil extraction and leaving intact kernels for seed production. Edible processing has a higher level of technology, and the pods need to go through multi-level sorting and grading, and then the kernels are processed to carry out spot checks on the content of Aspergillus flavus, etc., to strictly control food safety. In addition, US shelling companies also adopt temperature and humidity control technologies in kernel storage facilities that control the temperature below 55 °F (about 12.78 °C) and the relative humidity at 55~70%, so that the peanut kernels can be stored for one year without affecting the taste or quality. They also use clean energy and energy-saving technologies, using solar power to supply the energy needs of the shelling plant [124].

(3)

Comparison of social service system and processing mode

China’s peanut-shelling industry mainly comprises small-scale decentralized operations. After harvesting and bagging peanuts, farmers generally wait for oil-extraction and food-processing enterprises or middlemen to purchase pods and seeds, but, due to lack of planning and organization, most farmers independently complete all aspects of peanut planting, harvesting, shelling, and sales. It is difficult for farmers and small enterprises to achieve large-scale production and management. Under the operation of the socialized service system, the peanut-shelling industry in the United States integrates the resources of various organizations, integrates planting, harvesting, weighing, cleaning, drying, inspection, grading, storage, shelling, and sales, actively advocates a strong agricultural policy, protects and improves the business environment of the peanut industry, and continuously promotes the centralization and assembly-line production of the peanut-shelling industry.

5.2. Analysis of the Development of Peanut Shelling Mechanization in China

After conducting systematic research on the progress of peanut-shelling technology and equipment in representative countries, and comparing and analyzing the characteristics and differences of peanut-shelling mechanization in China and the United States, the following several problems in the development of peanut shelling mechanization in China are summarized:

(1)

The integration of agricultural machinery and agronomy is insufficient, and the characteristics of varieties suitable for shelling are unknown

Peanut shelling involves multidisciplinary issues, especially in the two major disciplines of mechanics and agronomy. China’s peanut planting area is vast, the production mode is diverse, and the degree of standardization is low, resulting in a multitude of peanut varieties, and the maturity and humidity of peanuts at harvest are very different. The same equipment has uneven effects on peanut shelling with different varieties, maturity and humidity, so it puts forward extremely high requirements for the adaptability of shelling equipment to different varieties. At the same time, in the selection and breeding of new varieties, breeding experts are mainly oriented to high oil content and high yield, and less consideration is given to the indicators of mechanized shelling operations, and there is not enough research on the integration of peanut agricultural machinery and agronomy, which is not conducive to the improvement of shelling quality.

(2)

Lack of systematic research on the shelling process

An appropriate shelling process is an important means to ensure the quality of peanut shelling, but at present, there is insufficient systematic research on shelling-related processes in China, such as pre-treatment before dehulling (pod stone removal, impurity removal, cleaning, grading), pre-treatment for dehulling (pod humidity, storage temperature, humidity, and time), shelling treatment (structural types, structural parameters, and motion parameters of key components of shelling, operational processes of shelling), and post-shelling treatment (shell-kernel sorting, kernel grading, color sorting, bag storage), etc., which greatly restricts the quality of shelling. At the same time, there is no distinction in the shelling process for seeds, oil crops, or edible uses. Although Chinese scholars have carried out a lot of explorations on the aforementioned process technologies, there is still a lack of systematic research, and the efficiency of transforming the research results into practical production applications is not high, and there is no literature to fully explain the theoretical analysis and experimental verification of the whole shelling principles, process and technology.

(3)

Innovative research into shelling principles and technical equipment needs to be strengthened

There are various types of peanut-shelling equipment in China, but they generally operate on similar principles and have similar structures, and the innovative research on equipment automation and intelligence is insufficient. Most of the existing equipment uses mechanical rotary striking or kneading to realize pod shelling, which often suffers from issues such as the mixing of kernels with broken shells during discharge, low discharge rate of broken shells, poor cleanliness of working environment, incomplete shelling and peanut clogging, causing seed mix-ups during the seed-making shelling processes, and inconvenient cleaning. In addition, the gap specifications of the concave sieve grid supplies with the machines are limited, which cannot achieve the best matching of peanut varieties and shelling equipment, and will also affect Operational performance. Moreover, the indicator requirements of mechanized peanut-shelling are high, and the performance indicators identified in some of the literature or for the machines are inconsistent with the actual operation, and the actual operation of existing equipment often fails to meet these standard requirements. At the same time, the technical equipment of each link of shelling is also an important factor affecting the shelling quality, and there are still many related links in China’s peanut shelling without special equipment, which cannot fully satisfy demand. In addition, although there are many manufacturing enterprises related to peanut-shelling equipment in China, they generally face problems such as insufficient research and development capabilities, low technical content, and low-quality and extensive products, etc. Although many enterprises export their products to regions such as the Middle East and Africa, they have a low market share, weak influence, and lack well-known brand recognition.

(4)

It is difficult to achieve large-scale shelling production mode

Peanut planting in China is extensive and dispersed, large-scale and specialized production is limited; there are many farmers but the per capita planting area is small, and peanut-shelling methods are still dominated by scattered independent farmers shelling for self-reliance, and the amount of shelling is small, making it difficult to achieve large-scale production. Most farmers use hand tools or simple, small-scale shelling equipment, and there is a lack of processes such as pod impurity removal, cleaning, grading, and kernel cleaning and grading, resulting in unstable shelling effects and inconsistent product quality, limiting the shelling quality and the international competitiveness of products. The upgrading of shelling technology and development of complete sets of processing equipment is slow, and there are problems of low production efficiency and high labor intensity, which restricts the development of unified peanut seed supply and an intensive processing industry. At the same time, due to the late start of the research and development of mechanized peanut-shelling equipment in China, the investment of human and financial resources is still obviously insufficient, the breakthroughs in basic research and key technologies, innovation-driven and leading roles need to be further strengthened, and there is a lack of a socialized service system and effective platform for integrating the resources across the shelling industry, resulting in difficulties concentrating peanut-shelling operations and fragmentation of the industry.

6. Summary and Development Recommendations

Peanuts are cultivated extensively around the world. The varying production conditions and planting patterns across different countries have led to the development of diverse peanut-shelling mechanization technologies. Some nations have embraced highly automated large-scale peanut-shelling equipment and have focused on breeding varieties that are suitable for mechanized shelling. They have also sought to optimize the shelling process, which is key to enhancing shelling technology and achieving a continuous reduction in peanut damage. However, the mechanization level of shelling in some major peanut-producing countries remains relatively underdeveloped and requires further improvement. After a comprehensive analysis of the aforementioned, and in consideration of China’s national context and the current state of peanut production, the following suggestions are proposed:

6.1. Promote Agricultural Machinery and Agronomy Integration and Strengthen Research on Varieties Suitable for Shelling

The effect of peanut shelling is influenced by a multitude of factors, including the material properties of the peanuts, the shelling process, the design of the equipment structure and operational parameters, and other elements. Despite extensive research by numerous scholars in China over an extended period, it is still not thorough enough, and there is still need to conduct further in-depth studies on the elements affecting shelling to expedite the large-scale, standardized production of peanuts. This includes a deeper investigation into the correlation between peanut-shelling damage and various characteristics such as pod features, mechanical properties of the shell and seed, seed fullness, constriction, and the mechanical properties associated with the junction of the seed coat and the kernel. Additionally, it is essential to identify and quantitatively characterize peanut varieties suitable for shelling, and actively engage in collaboration and dialogue with seed breeding specialists to select and promote peanut varieties that exhibit robust resistance to Aspergillus flavus infection, possess uniform geometrical characteristics in their pods and kernels, and are amenable to mechanized shelling and processing. The suitability for mechanized shelling should be incorporated as a key criterion in the appraisal and dissemination of new varieties. Furthermore, reinforcing the practical verification of shelling in production settings will ensure it serves as good early conditions for enhancing shelling performance.

6.2. Strengthen the Research on the Shelling Process and Key Parameters for Different Processing Purposes

Each processing link during peanut shelling significantly impacts the shelling effect. It is necessary to continuously improve the processing capacity of each link before, during, and after peanut shelling, and accelerate the development of complete assembly-line equipment. The methods of pod wetting and the operational process should be optimized to maintain the optimal shelling moisture content state in the pods. Drying treatments and storage practices should be improved to prevent excessive drying that can lead to chaffing of the seed coat and mold deterioration, thereby ensuring the quality, flavor, and food safety of peanuts. It is crucial to learn from advanced technologies in the United States, focusing on researching and applying control of the shelling environment’s temperature and humidity, aflatoxin detection, and energy-saving and environmentally friendly technologies. According to the different shelling needs of oil, seed, and food, set up different shelling processes, and formulate and implement unified product quality inspection standards. If the waste outlet is connected to the large collection room, the broken shells can be recycled and reused for feed production, soil improvement, biomass energy, environmental protection, building materials, etc. The technology for shelling should be continuously refined and integrated to enhance the peanut-shelling and deep processing industry chain. Concurrently, it is necessary to intensify research into the optimization machine structure and operational parameters, including cylinder speed, the spacing between the cylinder and concave sieve, the grid gap of the concave sieve, feeding speed, fan speed, etc. The goal is to minimize the machine shelling damage rate.

6.3. Continuously Achieve Theoretical Breakthroughs, Strengthen the Research and Application of New Materials, New Technologies, and Intelligent Means

China’s peanut-shelling equipment still faces issues such as high kernel breakage rates and low shelling efficiency. To address these challenges, it is imperative to significantly increase investment in research and development and to foster collaboration between industry, academia, and research institutions, and encourage enterprises and research units to conduct systematic and in-depth investigations into the mechanical behavior of peanuts during the shelling process, from the principles of shelling to equipment, materials, and structural design, exploring the impact of different shelling methods on peanut pods and kernels, elucidating the overall principles of peanut shelling and processing, achieving continuous theoretical breakthroughs, and leading to a reduction in kernel damage. Simultaneously, by drawing on the principles of LMC shelling equipment from the United States and its ultra-large structure, and considering the diverse characteristics of China’s peanut production modes and varieties, the development priorities will focus on the two main directions of high-quality shelling of small and medium-sized equipment and the development of complete sets of super-large shelling processing equipment. It is necessary to continuously improve the manufacturing capacity of advanced peanut-shelling equipment and peanut food processing technologies. The goal is to innovate the manufacture of new materials, technologies, and intelligent, efficient, and environmentally friendly peanut-shelling equipment that aligns with the distinctive features of China’s industry. Efforts should be directed towards establishing leading brands of peanut-shelling equipment and actively expanding the international market.

6.4. Establish a Socialized Service System Suitable for Chinese Peanut Production Characteristics

Drawing on the integrated development experience and operational mode of the U.S. peanut industry, which encompasses “production–purchase–storage–shelling–marketing”, and combining with the predominantly small and dispersed feature of Chinese farmers’ individual production methods, it is essential to innovate with Chinese characteristics and promote a transformation in the peanut production model. For instance, the government should take the initiative in formulating incentive policies and integrating resources from major peanut associations. This effort aims to construct a socialized service system that is well-suited to the characteristics of China’s peanut production. It should encourage agricultural product processing enterprises, supply and marketing cooperatives, and farmers’ professional cooperatives to collaborate and invest in large-scale peanut corporations. This will be accomplished, first of all, by carrying out pilot projects at provincial-city-county-town-village levels in the primary producing areas of Henan and Shandong, establishing peanut purchasing, storage, and shelling stations/centers that are appropriately scaled and adequately numbered, while concurrently enacting a reasonable and fair trading mechanism, such as the purchase and sales mechanism of ordered peanuts and ordered kernels, formulating a peanut quality inspection standard, and setting up a third-party agency to conduct testing. Front-end peanut farmers, who are dispersed across various regions, are solely responsible for the cultivation and harvesting of peanuts; then, these peanuts are uniformly procured and centrally shelled by mid-end stations/centers at various levels; finally, the shelled peanuts are sold to enterprises or customers at the back end who process and manufacture the final products. This modern peanut production model, characterized by a decentralized-centralized-decentralized approach, not only facilitates efficient and high-quality operations at every link of the peanut industry chain, but also promotes the integration and upgrading of advanced peanut-shelling and processing equipment. It enhances the overall shelling efficiency, quality, and technological level. Furthermore, it also achieves scaling and centralization in the peanut-shelling and processing industry, which is of utmost significance to the development of the peanut industry.