Physical, morphological, and mechanical properties of raw and steamed cashew nuts (Anacardium occidentale L.)

ABSTRACT

Cashew (Anacardium occidentale L.) plays an important role in global agriculture, particularly in commercializing the cashew kernel. Nevertheless, kernel extraction poses a challenge due to the nut variable shape and size. Characterizing the properties of the cashew nut becomes a pivotal input for the advancement of processes and machinery. This work presents a comprehensive characterization of raw and steamed cashew nuts from Puerto Carreño, Colombia, with focus on morphological, physical, and mechanical properties. Cashew nut samples from different farms in the region were collected. Morphological properties, including size and shape, were analyzed by measuring length, width, and thickness for 100 nuts from each supplier, both in their raw state and after steaming. Geometric diameter and sphericity were calculated, and qualitative shape characterization was performed. Physical properties, such as mass, bulk density, true density, and moisture content, were also measured. Angle of repose and coefficient of friction were determined to assess flow and frictional behavior. Additionally, compressive mechanical properties were evaluated through uniaxial compression testing along three orientations. The findings indicated significant differences in morphological and mechanical properties between raw and steamed cashew nuts. Conversely, the results demonstrated that cashew nuts from Vichada region exhibit a noteworthy degree of dispersion and larger dimensions than those from Asia and Africa, comparable to those from Brazil. The findings guide machinery design in the cashew industry and offer insights into the physical, morphological, and mechanical properties of cashew nuts in Puerto Carreño, contributing to understanding and enabling quality assessment, processing optimization, and product development.

KEYWORDS:

• Cashew nut
• morphological properties
• nut size
• physical properties
• mechanical properties

Introduction

Cashew trees (Anacardium occidentale L.) are an important food crop in global agriculture and trade, which originated in South America, specifically northeastern Brazil, and were introduced to India and Africa in the 16th century, resulting in global spread and the evolution of diverse variations within the species. While the pseudofruit dominated early interest, modern international cashew trade revolves around the commercialization of the cashew kernel and its precursor, the in-shell cashew nut or Raw Cashew Nut.

Between 2015 and 2021, the global production of Raw Cashew Nut increased significantly, reaching a total of 24.37 million tons. This was matched by consistent export market performance, with an aggregate export volume of 12.17 million tons over the same time period. Notably, Ivory Coast accounted for 36.87% of this export landscape, followed by the United Republic of Tanzania at 13.26%, Ghana at 12.66%, Nigeria at 7.44%, Guinea-Bissau at 6.45%, Burkina Faso at 5.49%, Benin at 5.43%, and Indonesia at 4.03%.^[1]

In contrast, the market for cashew kernels, also known as shelled cashew, grew at a rate of 20% during this time period. This increase was accompanied by a significant increase in exports, which totaled 4.27 million tons. Vietnam emerged as the dominant exporter, with 62.07% share, followed by India at 21.21%, the Netherlands at 6.23%, the United Arab Emirates at 2.77%, Germany at 2.69%, Brazil at 2.36%, Ivory Coast at 1.77%, Ghana at 1.70%, Indonesia at 1.50%, and Mozambique at 1.20%.^[1]

This significant increase in demand for cashew is driving exploration into untapped cultivation potential in a variety of geographic regions, including Colombia. This South American country, endowed with fertile landscapes and favorable climatic conditions conducive to cashew growth, has begun to establish itself as a promising contender in cashew production, displaying a production volume of 6,510 tons between 2019 and 2021, accompanied by cumulative Raw Cashew Nut exports of 229 tons during the same period.^[1] Notably, the Vichada Department has emerged as a cashew production hotspot, with the highest production rates. This region has seen the birth of numerous initiatives, ranging from cultivation to post-processing, with a particular emphasis on marketing the cashew kernel, which has the highest value in the markets.^[2]

Within cashew kernel production, efficient processing is a critical link in the value chain. The transformation of Raw Cashew Nut into cashew kernels requires a series of processes, each of which has a significant impact on the final product’s quality and market value. The process initiates with the collection under the cashew trees,^[3] followed by sun-drying and a period of rest for the raw cashew nuts, and then moves into a phase of softening the nut’s shell. This critical softening process is primarily accomplished through two primary methods: frying or steaming. In the case of Raw Cashew Nut frying, the nuts are immersed in a bath of palm oil or cashew shell oil, heated to temperatures ranging from 190°C to 210°C for 90 seconds, and then cooled.^[4–6] The steaming method, on the other hand, involves the use of steam autoclaves, which subject the Raw Cashew Nut to pressures of 7 Bar for 28 to 34 minutes, followed by an 18-hour cooling period.^[5,7] Due to quality requirements, the steaming process has been preferred over frying in recent times. In the frying process, certain attributes, such as color, flavor, and texture, may be compromised due to reactions between sugars and amino acids. Moreover, this method generates thick fumes, contributing to environmental pollution. On the other hand, the steaming process preserves the mentioned characteristics of the almond. Notably, almonds maintain their white color, a crucial factor for exportation, as international buyers often favor this quality. Then, after cooking, the shelling process emerges as a complex juncture, influenced by the irregular shape and size variations of the nuts, the presence of Cashew Nut Shell Liquid (CNSL) content, and the inherent hardness of the kernel.^[8]

The physical properties – such as mass, volume, density, and angle of repose – are closely intertwined with critical aspects of cashew nut storage, transportation, and handling capabilities, significantly impacting the dynamics of movement and the materials essential for manufacturing processing equipment.^[8] Morphological aspects are commonly associated with the overall dimensions of the nut and its variability, directly influencing the design and sizing of equipment involved in the shell-opening process and subsequent separation.^[9] Likewise, mechanical properties facilitate the identification of relationships between pre-opening treatments and the mechanical requirements of the equipment necessary for the cashew nut processing^[6,10] ensuring optimal conditions for obtaining almonds with the highest quality standards. Addressing these challenges requires a comprehensive understanding of the physical, morphological and mechanical properties of cashew nuts.

Existing research has investigated various aspects of cashew nuts, including their morphological, physical, and mechanical properties. Qualitative and quantitative descriptors have been used to delineate the general and specific shapes of various nut parts,^[11–13] while dimensions and related attributes such as sphericity and mean geometric diameter have been evaluated while accounting for humidity and shell softening processes.^[6,12,14,15] Furthermore, researchers have concentrated on physical parameters with historical relevance to nut selection, such as mass, volume, true density, and bulk density.^[16–19] Meanwhile, mechanical properties, which are critical for designing effective processing equipment, have been studied using compression load tests under a variety of conditions.^[5,19,20]

A research gap emerges in this context, necessitating a deeper exploration of the physical and mechanical characteristics of cashew nuts, particularly in the Colombian context. Colombia’s desire to strengthen its cashew processing chain and achieve greater integration into the global cashew trade necessitates a deeper comprehension of the local raw nut and its processing requirements.^[21] Specifically, attention is directed toward Puerto Carreño, Vichada region, for being the largest planted area for cashew trees in Colombia, encompassing 4300 hectares with the potential for expansion to 7000 hectares. This considerable scale of cultivation establishes Puerto Carreño as a pivotal player in cashew nut production, not only within the country but also in South America. The extensive cashew plantations in this region offer a representative sample of the variations present in the Colombian cashew industry.

In this work a comprehensive characterization of cashew nuts produced in Puerto Carreño, Vichada region of Colombia is performed. The study aims to investigate the physical, morphological, and mechanical properties of the nuts, considering their potential implications for the cashew nut industry. By examining the size, shape, mass, density, moisture content, angle of repose, and coefficient of friction, it is established a detailed profile of the nuts’ physical properties. Furthermore, the morphological analysis enables the identification of qualitative features of the cashew nut produced in the Vichada region. Additionally, the mechanical characterization through uniaxial compression tests provides valuable insights into the nuts’ structural behavior, shedding light on their mechanical properties along different axes. The findings of this study contribute to the existing scientific knowledge of cashew nuts, supporting farmers, researchers, and industries in their pursuit of improved production, processing, and product development.

Materials and methods

Raw and steamed cashew nut sampling

To ensure a comprehensive characterization of cashew nuts produced in Puerto Carreño, a town located at 6°11’16“north latitude and 67° 28’ 57” west longitude, in the department of Vichada, part of Colombia in South America, with an elevation of 51 meters above sea level (masl) and an average temperature of 36°C, along with an annual precipitation average reaching 2233 mm, a sampling strategy was implemented. This strategy included cashew nut clones from various farms in the region. The collected samples comprised a mix of regional clones: Corpoica Mapiria Ao1, Corpoica Yopare Ao2, Corpoica Yucao Ao3. These clones, which have been developed by the Colombian Corporation for Agricultural Research (Corpoica – Agrosavia), are widely used in the country’s cashew nut industry due to their high yield and quality.^[22] The study identified these variants (Mapiria, Yucao, and Yopare) as exhibiting superior performance in terms of productivity, resistance to pests, and nutrient utilization. As a result, the local government initiated a widespread distribution of these genetically superior clones in the region. Farmers predominantly adopted these clones for grafting onto native trees as rootstocks, fostering a shift toward exclusive cultivation of these clones in Puerto Carreño.

The primary advantage distinguishing these tree clones from others lies in their robust disease resistance and their prolific nut production tailored for the specific conditions of the western plains in Colombia. The Corpoica Mapiria Ao1, a cashew tree variant, attains a height of up to 4.5 m. It showcases a capacity to yield approximately 14 kg of nuts, each with an average weight of 11 g and a count of 90 nuts per kilogram. Pertaining to their cashew kernels, an average weight of 3.3 g is observed, corresponding to roughly 30% kernel yield.^[22] Similarly, the Corpoica Yopare Ao2, another cashew tree variant, shares comparable characteristics with a height limit of 4.5 m. Its nut production reaches around 12 kg, featuring an average individual nut weight of 12 g and an aggregate of 83 nuts per kilogram. Analogous to the Corpoica Mapiria Ao1, this variant manifests an average kernel weight of 3.3 g, but with a slightly lower kernel yield of 28%.^[22] Diverging in stature, the Corpoica Yucao Ao3, yet another cashew tree variant, achieves a height of 5.5 m. Its nut output amounts to approximately 11 kg, each nut weighing approximately 13 g and totaling 77 nuts per kilogram. In terms of kernel attributes, these exhibit an average weight of 3.2 g, translating to an approximate kernel yield of 25%.^[22]

The cashew nut samples utilized in this study were predominantly collected during the period of January to March 2022, coinciding with optimal environmental conditions for nut production. Harvesting focused on mature trees, approximately five years old, with a spacing of 12 × 12 meters (69 trees per hectare). These trees, fertilized twice a year, were selected for their high nut yield. The harvesting process included depitting, which involves the removal of the false fruit (peduncle), followed by sun-drying for one to two days. This step was taken to ensure that the nuts could be stored for subsequent processing.

To obtain a representative sample of the cashew nuts produced in the region, a total of eight different farm suppliers were included in the study. 5 kg of raw cashew nuts were collected from each supplier. In addition, cashew nut clones were collected indistinctively in all farms to ensure randomness in the sample.

After collection, each sample was quartered according to Method-B from ASTM C702 standard^[23] to take a small representative sample from a large and inhomogeneous sample. This method is most often used for powders or granular aggregate but extended to other granular materials. Raw cashew nut subsamples for each characterization test were taken from each supplier and individually marked to maintain traceability. Once the raw cashew nut properties were measured, the samples were steamed under local conditions and procedures. This process was chosen because it effectively separated the kernel, improved its toughness, and did not alter its flavor. Additionally, steam cooking helped to preserve the quality of the cashew nut shell liquid, which is a valuable byproduct. The nuts were steamed for 20 minutes under a pressure ranging from 168 kPa to 306 kPa. This process is the standard method in the region for extracting and commercializing the cashew kernel. After steaming, the nuts were cooled to room temperature, and their properties were measured again.

This comprehensive sampling strategy allowed for a thorough analysis of the physical, morphological, and mechanical properties of cashew nuts produced in the region, including variations in properties across different clones, farms, and steaming processing.

Morphological properties

The size of cashew nuts from Puerto Carreño was analyzed by measuring the Length ($L$), Width ($W$), and Thickness ($T$) of 100 sampled nuts obtained from each of the eight different suppliers using the described sampling method, for a total of 800 nuts measured. The nuts were first measured in their raw form, and then measured again after steaming, to account for any changes in size and shape due to processing. Individual records were kept for each nut to analyze changes in size and shape before and after steaming. A sketch of the measured dimensions is shown in Figure 1. A digital caliper with a resolution of 0.01 mm was used to measure the nuts.

Figure 1. Measured dimensions of Cashew nuts.

The geometric mean diameter ($D$) of both raw and steamed nuts was calculated using Equation 1(1) $D={\left(L\cdot W\cdot T\right)}^{\frac{1}{3}}$(1) Additionally, the sphericity ($\phi$) of the cashew nuts was determined using Equation 2(2) $\phi =\frac{D}{L}$(2) .^[24]

(1) $D={\left(L\cdot W\cdot T\right)}^{\frac{1}{3}}$(1)

(2) $\phi =\frac{D}{L}$(2)

Shape qualitative characterization

On the other hand, the qualitative features of cashew nuts from Puerto Carreño were also characterized using the method proposed by the International Board for Plant Genetic Resources (IBPGR).^[25] This method enables the identification and description of specific qualitative aspects of plant genetic resources, including color, shape, and size, through a list of descriptors and coding classification. Specific qualitative aspects of cashew nuts from Puerto Carreño were identified using this list of descriptors and coding classification.^{[16,18,26,27]}

For this study, the sampled cashew nuts collected from local suppliers and farms were visually inspected and compared to the descriptors provided by IBPGR to characterize their morphological features. The main descriptors measured in this study included nut shape, shape of the nut base, suture of the nut, flanks of the nut, stylar scar on the nut, shape of the nut apex, and the relative position of the suture and apex. The data fields for these descriptors in the IBPGR were numbered as 34, 51, 52, 53, 54, 55, and 56, respectively. Correspondingly, the IBPGR codes assigned to these descriptors were 4.2.7, 6.2.25, 6.2.26, 6.2.27, 6.2.28, 6.2.29, and 6.2.30, respectively.^[11] These descriptors were assessed through visual analysis, employing a random selection using the quarter method.

Mass, bulk density and true density

To measure the mass of raw and steamed cashew nuts, an electronic balance (OHAUS, USA; Model: PA214) with a precision of 0.001 g was used. Each nut’s mass was recorded individually for every nut from each of the eight suppliers. In total, 100 nuts per supplier were measured, resulting in a total of 800 nuts measured in this study.

The bulk density of raw and steamed cashew nuts was determined by filling a wooden box with internal dimensions of 100 mm x 100 mm x 100 mm, resulting in a total internal volume of the box ($V_b$) of 1000 cm³. The mass of both the empty box ($m_b$) and the filled box ($m_t$) were measured using an electronic balance with a precision of 0.001 g. The bulk densities (${\rho }_b$) were then calculated using Equation 3(3) ${\rho }_b=\frac{m_b-m_t}{V_b}$(3) .^[28] Bulk density was determined for 10 samples for raw and steamed cashew nuts, for a total of 20 measurements.

(3) ${\rho }_b=\frac{m_b-m_t}{V_b}$(3)

The true density of raw and steamed cashew nuts was determined using the displacement method with ethanol as the liquid medium.^[24] Ten raw and ten steamed cashew nuts were randomly selected from each supplier using the curtail method, and their masses ($m_n$) were measured individually. Each nut was then placed into 60 ml of ethanol in a calibrated beaker, and the liquid displaced by the nut ($V_n$) was recorded with a precision of 1 ml. The true density was calculated using Equation 4(4) ${\rho }_t=\frac{m_n}{V_n}$(4) .

(4) ${\rho }_t=\frac{m_n}{V_n}$(4)

Moisture content

The moisture content of raw and steamed cashew nuts was measured using the Whole Pod Method proposed in ASAE/ASABE S410.3 standard for moisture measurement in peanuts.^[29] For each supplier, 10 samples of 200 g of both raw and steamed cashew nuts were randomly selected using the quartering method. The initial mass ($m_0$) of each sample was recorded using an electronic balance (RADWAG Balances and Scales, Poland; Model: PS4500.R2) with a precision of 0.01 g. The samples were then introduced to a forced-draft oven (EQ, Model: HDN-30A-EQ) at 130°C for 4.5 hours. Afterward, the samples were taken out of the oven and cooled to room temperature, and their masses were recorded again ($m_f$). The whole pod moisture content ($M{C}_P$) was calculated.

Angle of repose

The angle of repose $\beta$ is the angle between the horizontal plane and the mound formed by the nuts after a free fall on static surface of a given material. This angle is directly related with the friction forces between the nuts and defines the filling or emptying behavior of granular materials. There are four main methods for measuring the angle of repose: funnel method, box with emptying removable side, revolving cylinder and sliding.^[30–33]

In this work, to determine the angle of repose for raw and steamed cashew nuts the box with removable side method was selected.^[34] The wooden box shown in Figure 2 was used, with dimensions of 300 mm x 200 mm x 400 mm. Four different materials were used as the base of the box, including galvanized steel, stainless steel, medium-density fiberboard (MDF), and ultra-high molecular weight polyethylene (UHMWPE), to investigate changes in the angle of repose. 1 kg of nuts from each of the eight selected suppliers in the region were mixed to create a sample of 8 kg. The testing box was filled with the sample, and the slide gate was moved up to allow the nuts to flow freely and form a mound. The angle between the horizontal plane and the pile of nuts was measured using a digital protractor goniometer Medigauge 900,100 with a precision of 0.05°. The procedure was repeated five times, for every base material and for raw and steamed cashew nuts.

Figure 2. Repose angle measuring box with removable side. Dimensions in mm.

Coefficient of friction

The coefficient of friction between raw and steamed cashew nuts and various materials was determined using the Tilting Table Test method, as proposed in the USBR 6258–09 procedure.^[35] The device used in the experiment is shown in Figure 3 which features a tilting platform capable of changing the angle of inclination and its contact surface.

Figure 3. Tilting Table for Coefficient of friction measuring.

To begin, a group of 50 nuts was randomly selected from a mixture of cashew nuts from the eight suppliers using the quartering method. From this sample, arbitrarily selected subgroups of 12 nuts were placed on a wooden box without a lid or bottom and positioned on the platform surface. The tabletop was gradually tilted until the specimen box started to slide. The angle of tilt at this point was measured and recorded as the angle of static friction ($\theta$). The coefficient of friction ($\mu$) was then calculated using Equation 5(5) $\mu =\frac{sin\theta }{cos\theta }=tan\theta$(5)

This process was repeated 10 times for both raw and steamed cashew nuts with four different contact surface materials: galvanized steel, stainless steel, MDF, and UHMWPE.

(5) $\mu =\frac{sin\theta }{cos\theta }=tan\theta$(5)

Mechanical properties

The compressive mechanical properties of raw cashew nuts were measured under uniaxial loading using the ASAE S368.4 standard, Compression Test of Food Materials of Convex Shape.^[36] Each cashew nut was individually positioned between parallel plates, and a uniaxial compression load was applied using a universal testing machine (Shimadzu, Japan; Model: AGS-X). During the test, the compressive load and displacement were measured at a crosshead speed of 1.8 mm/min. Three different orientation positions of the nut were tested to determine the compressive properties along the length ($x-axis$), thickness ($y-axis$), and width ($z-axis$), as shown in Figure 4.

Figure 4. Compression test directions. (a) length ($x-axis$), (b) thickness ($y-axis$), and (c) width ($z-axis$).

Then, the procedure was repeated for steamed nuts to determine the changes in the mechanical properties after processing. 5 cashew nuts, selected using the quartering method, were tested for each axis and for four suppliers in both conditions, raw and steamed, for a total of 120 nuts tested. Load-displacement curves were obtained for each nut and for the three loading axes. Mean fracture load for both conditions and for each supplier were determined and compared to determine differences between suppliers and a comparison conducted to assess the effect of the steaming process on the mechanical properties of the cashew nut.

Statistical analysis of data

Each measured property was categorized in groups based on the supplier and the processing condition, either raw or steamed, and then analyzed statistically using Minitab 19 software (2023 Minitab, LLC. USA). The first step in the analysis involved applying Grubbs’ test to each group to detect outliers in each data set, which was assumed to come from a normally distributed population. Atypical values were then rejected and removed from all the groups. Next, the Shapiro-Wilks normality test was applied to each group to confirm that the data sets were well-modeled by a normal distribution.

For each property and group, descriptive statistic as the mean and standard deviation were calculated individually for each group and for all the sample. Then, an analysis of variance (ANOVA) and a Tukey test were applied between suppliers and processing conditions to determine if there were significant differences in the property means.

These statistical analyses were conducted to infer the type of relation between suppliers and the differences in properties due to the steaming process. Also, a paired sample T-test was applied for the mean dimensions and mass to determine if the steaming process change the nut properties. These statistical analyses were conducted to infer the type of relation between suppliers and the differences in properties due to the steaming process. A 95% confidence level was used for all the statistical analyses reported.

Results and discussion

Morphological properties

The length ($L$), width ($W$), and thickness ($T$) of a representative sample of both raw and steamed cashew nuts of Puerto Carreño region of Vichada, Colombia were measured. Tables 1 and 2 present a consolidated summary of the means of the main dimensions measured, grouped by supplier. Additionally, the geometric mean diameter ($\mathrm{D}$) and sphericity ($\phi$) are reported. At the bottom of the tables, the maximum and minimum values of all the samples are reported. Finally, the means for all the sample are provided.

Table 1. Dimensions and mass of raw cashew nuts from Puerto Carreño.

Supplier	Mass(g)	Length (L) (mm)	Width (W) (mm)	Thickness (T) (mm)	$D$(mm)	$\phi$(%)
Supplier 1	9 ± 1	33 ± 1	25 ± 1	20 ± 1	26 ± 1	78 ± 2
Supplier 2	10 ± 1	34 ± 2	26 ± 1	23 ± 2	27 ± 1	80 ± 2
Supplier 3	9 ± 1	34 ± 2	27 ± 1	21 ± 1	26 ± 1	78 ± 2
Supplier 4	8 ± 1	33 ± 2	26 ± 1	21 ± 2	26 ± 1	80 ± 3
Supplier 5	12 ± 1	36 ± 2	29 ± 1	24 ± 2	29 ± 1	81 ± 3
Supplier 6	9 ± 1	35 ± 2	26 ± 1	21 ± 1	27 ± 1	77 ± 2
Supplier 7	9 ± 1	34 ± 2	25 ± 1	21 ± 1	26 ± 1	77 ± 2
Supplier 8	8 ± 1	32 ± 2	25 ± 1	21 ± 2	26 ± 1	79 ± 3
Minimum	3.50	25.64	20.66	12.59	21.72	64.46
Maximum	15.88	40.70	31.85	33.43	32.50	98.48
Total mean	9 ± 1	34 ± 1	26 ± 1	22 ± 1	27 ± 1	79 ± 2

Table 2. Dimensions and mass of steamed cashew nuts from Puerto Carreño.

Supplier	Mass Steamed	Length (L) (mm)	Width (W) (mm)	Thickness (T) (mm)	$D$(mm)	$\phi$(%)
Supplier 1	9 ± 1	33 ± 2	25 ± 1	20 ± 1	26 ± 1	78 ± 2
Supplier 2	10 ± 1	34 ± 2	26 ± 1	23 ± 2	27 ± 1	80 ± 2
Supplier 3	9 ± 1	34 ± 2	26 ± 1	22 ± 1	26 ± 1	78 ± 2
Supplier 4	8 ± 1.	34 ± 2	25 ± 1	21 ± 1	26 ± 1	78 ± 2
Supplier 5	12 ± 1	37 ± 1	28 ± 1	25 ± 1	29 ± 1	79 ± 2
Supplier 6	9 ± 1	35 ± 2	27 ± 1	21 ± 1	27 ± 1	77 ± 3
Supplier 7	9 ± 1	34 ± 2	25 ± 1	21 ± 1	26 ± 1	77 ± 2
Supplier 8	9 ± 1	32 ± 2	25 ± 1	21 ± 2	26 ± 1	80 ± 3
Minimum	3.60	25.80	20.56	14.49	22.16	61.78
Maximum	15.85	50.56	30.90	27.62	32.02	91.89
Total mean	9 ± 1	34 ± 1	26 ± 1	22 ± 1	27 ± 1	78 ± 1

The mean length, width, and thickness of raw cashew nuts harvested in Puerto Carreño were found to be 33.87 mm, 25.95 mm, and 21.58 mm, respectively. These results indicate that, on average, cashew nuts from Puerto Carreño are larger in size than those from some traditional producer countries, such as Ivory Coast, Nigeria, Ghana, Indonesia, India, and Burkina Faso. The reported average sizes for these countries ranged from 28.0 mm to 33.7 mm for length, 17.81 mm to 25.40 mm for width, and 14.13 mm to 18.38 mm for thickness.^{[10,15,18,37]} However, when compared to other cashew nut producers from South America, the nuts from Puerto Carreño are smaller in average. For example, in Brazil, the reported sizes range from 34.86 mm to 46.35 mm for length, 22.56 mm to 28.46 mm for width, and 18.78 mm to 22.86 mm for thickness.^[38,39] See Figure 5.

Figure 5. Cashew nut length ranges and mean from different countries.

Also, the cashew nuts harvested from the Puerto Carreño region exhibited a considerable wider range in size, with lengths varying from 17 mm to 50 mm. This wide range poses a significant challenge for the processing industry. In comparison to other countries, the length of cashew nuts from Puerto Carreño is particularly diverse. While traditional producer countries like Ivory Coast, Nigeria, Ghana, Indonesia, India, and Burkina Faso reported compact length ranges, the cashew nuts from Puerto Carreño demonstrate a broader spectrum of sizes.

The large size of the cashew nuts in Puerto Carreño is explained by the combination of favorable environmental conditions found naturally in the region (humidity, hours of sunlight per day, UV index, among others). Also, production practices adopted in Vichada such as soil preparation processes, fertilization, and irrigation cycles, among other factors, promote nut growth and high yields.

On the other hand, an analysis of variance (ANOVA) and Tukey tests were performed to investigate the presence of significant differences in nut size among the various suppliers. The Tukey test revealed a p-value below 0.05 when comparing the length, width, and thickness of cashew nuts, indicating the absence of similarity among nuts harvested by each supplier. Specifically, the statistical analysis revealed that in general cashew nuts exhibited distinct sizes among suppliers. Specifically, Figure 6 illustrates the Tukey test results for length of the cashew nuts of each supplier studied and compared in pairs. In this figure, each pair of suppliers is characterized by a confidence interval. If the interval does not encompass the zero line, it indicates a significant difference between the corresponding pair of suppliers. Consequently, suppliers 1 and 2 exhibit dissimilarity with all other suppliers. Suppliers 3, 4, and 5, on the other hand, share similarity with each other. And finally, suppliers 6, 7, and 8 demonstrate similar length. This observed variation in cashew nut sizes can be attributed to the varying levels of technification in crop cultivation implemented by the different suppliers in the region.

Figure 6. Graphical display of pair-wise comparisons from Tukey’s HSD for the cashew nut length measured in Puerto Carreño. Any confidence intervals that do not contain 0 provide evidence of a difference in the suppliers.

After processing the cashew nuts, they were remeasured. The mean length, width, and thickness of steamed cashew nuts from Puerto Carreño were 34.28 mm, 25.80 mm, and 21.79 mm, respectively. This was larger than the average sizes of steamed cashew nuts from other countries such as India, Nigeria, and Indonesia which had lengths, widths, and thicknesses in the range of 27.98 mm to 32.13 mm, 21.91 mm to 24.42 mm, and 14.13 mm to 18.63 mm, respectively.^[5,8,40,41]

To test if the steaming process caused a significant change in nut dimensions, a paired T-test with 95% confidence was performed. The results showed p-values lower than 0.05 for the comparison of length, width, and thickness before and after steaming. Consequently, after the steaming process, there are notable change in the mean dimensions of the nut. Specifically, the length mean transitions from 34 ± 1 mm to 34 ± 1 mm, the width mean shifts from 26 ± 1 mm to 26 ± 1 mm, and the thickness undergoes a change from 22 ± 1 mm to 22 ± 1 mm. Using the T-test with the assumption of equal variance at a significance level of 0.05, it is determined that (mean raw – mean steamed) is significantly different from zero. These statistical tests indicated a significant difference in the measures, revealing an increase in the length and thickness of the nuts and a decrease in the width of the nuts. Similar studies have also reported changes in cashew nut size after processing.^[5] These changes are attributed to water absorption and an increase in moisture content during the steaming process.

This result showed that the cashew nuts harvested in Puerto Carreño were significantly larger than those from other countries in Africa and Asia and similar to those in South America. This finding has significant benefits for the region, as larger nut sizes are known to be appreciated and have an increasing demand, especially for the complete kernel in jumbo sizes, according to the International Nut and Dried Fruit Council.^[42]

Additionally, the geometric mean diameter and sphericity of both raw and steamed cashew nuts from Puerto Carreño are reported. The geometric mean diameter and sphericity averages for raw cashew nuts were 26.65 mm and 78.74%, respectively, while those for steamed cashew nuts were 26.79 mm and 78.22%, respectively. The geometric mean diameter showed a significant increase due to the boiling process, but sphericity did not show a significant change. The geometric mean diameter is used to predict nut behavior in various environments and the ease with which unwanted materials can be separated during the cleaning process. It provides useful information about the size distribution of nuts and their ability to interact with various sieves or separators. Understanding the sphericity of nuts is also important for designing efficient screening systems. When considering sieve or separator design, sphericity refers to a nut’s ability to occupy the same volume as a sphere. It aids in determining the best shape and dimensions for the screening elements in order to achieve effective separation. The geometric mean diameter and sphericity both play important roles in evaluating the physical properties of nuts and optimizing processing operations.

Shape qualitative characterization

Furthermore, a morphological characterization was conducted on a total of 72 cashew nuts obtained from eight suppliers in Puerto Carreño. The descriptors provided by the International Board for Plant Genetic Resources (IBPGR) were used for this analysis. The findings revealed the prevalence of certain dominant class descriptors throughout the entire sample, highlighting the consistency of certain morphological traits across suppliers. The main descriptors evaluated, and their corresponding results can be found in Table 3.

Table 3. IBPGR Qualitative characterization of cashew nuts form Puerto Carreño.

IBPGR	Descriptor Name	Class	Class Name
34	Nut Shape	1	Kidney-Shape
51	Shape of Nut Base	1	Round
52	Suture of Nut	2	Angular
53	Flanks of Nut	5	Round
54	Stylar Scar on Nut	7	Large
55	Shape of Nut Apex	2	Intermediate
56	Relative Position of Suture an Apex	1	Suture projection in front of apex

This morphological characterization serves as a qualitative comparison tool to describe the shape and distinctive features of the cashew nuts. While there is a lack of studies indicating superior characteristics over others in this regard, it is worth noting that such features become particularly relevant during the transformation processes. It is also important to consider that studies focusing on the specific traits of the cashew kernel are relatively scarce in comparison.

In previous studies conducted in countries like India,^[11] similar descriptors were identified for cashew nuts harvested in that region. However, it is important to note that certain descriptors exhibited more variability compared to others. For instance, in the case of descriptors 51, 52, and 55, multiple classes were observed within the population. This variability can be attributed to the presence of over 500 different varieties of clones in India, which differs from the situation in Puerto Carreño, where only four clones are present. This discrepancy in the number of clones between the regions can significantly impact the consistency of specific morphological traits observed in cashew nuts. The genetic diversity inherent in a larger number of clones can lead to greater variation in descriptors related to nut shape and other characteristics.

In conclusion, the observed variations in cashew nut morphological properties, particularly dimensions, carry significant implications for the industrialization of cashew nut production. The length, width, and thickness of raw cashew nuts from Puerto Carreño were found to be larger on average compared to some traditional producer countries that design and produce the currently available processing machinery. This diversity in size, coupled with the considerable range observed, poses challenges for the processing industry, especially in deshelling processes, where forces must be carefully controlled to prevent damage to the valuable kernel. Furthermore, the differences in cashew nut sizes among suppliers indicate varying levels of technification in crop cultivation, emphasizing the need for tailored processing approaches. Results also shed light on the impact of processing, specifically steaming, on nut dimensions. The increase in length and thickness highlights the dynamic nature of cashew nuts during processing. These changes are crucial considerations for the design of tools used in the deshelling process, specifically tailored to accommodate the cashew nut sizes found in the Puerto Carreño region.

Mass, bulk density and true density

The mass of both raw and steamed cashew nuts from Puerto Carreño was measured for a sample size of 100 nuts obtained from eight different suppliers in the region. The mean and 95% confidence interval for both raw and steamed nuts are presented in Tables 1 and 2 respectively. In addition, the minimum and maximum measures are reported, along with the mean and confidence interval for the entire sample of 800 nuts.

The results reveal that the mean mass of raw cashew nuts from Puerto Carreño is 9.37 g. Compared to other producer countries such as Ivory Coast, Nigeria, Ghana, India, Burkina Faso, and Indonesia,^[10,15,41] the measured nut is heavier. These countries show a range of 3.82 g to 9.10 g. However, the cashew nut variety from Brazil has a range of 11.00 g to 15.73 g,^[38,39] making the nut from Puerto Carreño lighter in comparison (see Figure 7).

Figure 7. Cashew nut mass ranges and mass mean from different countries.

On the other side, while Agrosavia’s study aimed at developing cashew clones has yielded nuts averaging around 11–13 grams,^[43] the current investigation unveils a broader spectrum of nut sizes, ranging from 8.5 to 16.8 grams, with a mean of 9.37 grams. While the potential for larger nut sizes exists within the ambit of these clones, prevailing cultivation practices have restrained their growth, resulting in slightly smaller dimensions. However, this presents a promising juncture, as the enhancement of cultivation practices within the region holds the potential to unlock larger nut sizes. The prospect of optimizing cultivation techniques emerges as a pathway to harnessing the full scope of these cashew clones and yielding nuts of more substantial proportions.

The cashew nuts harvested from the Puerto Carreño region exhibited a significantly wider range in mass, with masses varying from 3.5 g to 15.8 g, mirroring the findings for length. In comparison to other countries, the mass of cashew nuts from Puerto Carreño shows remarkable diversity. Traditional producer countries such as Ivory Coast, Nigeria, Ghana, Indonesia, India, and Burkina Faso reported more compact mass ranges, whereas the cashew nuts from Puerto Carreño demonstrate a broader spectrum of masses.

Additionally, Tukey tests were conducted to investigate the presence of significant differences in nut mass among the suppliers. The Tukey test comparing the mass of cashew nuts between suppliers yielded a p-value below 0.05, indicating the absence of similarity among nuts harvested by each supplier. Notably, in general the suppliers exhibited cashew nuts with statistically different masses. Specifically, Figure 8 illustrates the comparison in pairs of suppliers. It shows that suppliers 1 and 3 exhibit dissimilarity with all other suppliers, while suppliers (2, 4, 5), (4, 5, 6), (5, 6, 7), and (6, 7, 8) demonstrate similarity among themselves. It is crucial to note, however, that most comparisons in the Tukey test yield statistically significant differences.

Figure 8. Graphical display of pair-wise comparisons from Tukey’s HSD for the cashew nut mass measured in Puerto Carreño. Any confidence intervals that do not contain 0 provide evidence of a difference in the suppliers.

After steaming the nuts, they were remeasured and found to have a mass of 9.39 g. A paired T-test with 95% confidence was performed, which showed a p-value below 0.05 for the comparison of masses before and after steaming. Overall, after the steaming process, the mean mass shifts from 9 ± 1 g to 9 ± 1 g. Under the T-test assumptions at a 0.05 significance level with equal variance, it is established that (mean raw – mean steamed) is significantly different from zero. The statistical tests indicated a significant difference in the measures, revealing an increase in mass of the nuts. These changes are attributed to water absorption and an increase in moisture content during the steaming process. These results are in contrast to mass changes observed with other processing conditions, such as roasting, where the mass of the nuts decreases due to CNSL and humidity loss.^[8]

The bulk density and true density measurements of raw and steamed cashew nuts from Puerto Carreño are presented in Table 4. The bulk density of raw nuts was found to be 600.82 kg/m³, while steamed nuts had a bulk density of 570.40 kg/m³. These results are consistent with bulk densities found in other producer countries, such as Nigeria, India, and Indonesia.^{[5,8,28,41,44]} Statistical analysis of the samples using a T-test yielded a p-value below 0.05, indicating that there is a significant difference in bulk density between raw and steamed nuts. This decrease in bulk density is attributed to the expansion of the nuts during the boiling process, resulting in a larger volume for the same mass. Similar behavior has been reported for boiled cashew nuts in India, where changes from 596.67 kg/m³ to 523.79 kg/m³ have been observed.^[5]

Table 4. Densities and moisture content of raw and steamed cashew nuts from Puerto Carreño.

Property	Raw	Steamed
Bulk density (kg/m3)	6011 ± 9	570 ± 13
True density (kg/m3)	1029.43 ± 0.04	922.40 ± 0.06
Moisture content (%)	10 ± 1	11 ± 1

The true density of cashew nuts from Vichada is also reported in Table 4. The mean density for raw nuts was found to be 1029 kg/m³, while steamed nuts had a mean density of 922 kg/m³. These values are consistent with reported densities of cashew nuts from Nigeria, India, Burkina Faso, and Indonesia which fall in the range of 934.6 kg/m³ to 1240 kg/m³ for raw nuts and 685 kg/m³ to 777 kg/m³ for processed nuts.^{[18,19,26,41]} A two-sample t-test was performed to compare the two samples, and the resulting p-value less than the significance level of 0.05, indicating that the samples are significantly different. The observed decrease in true density after steaming is explained by the change in volume during processing. Although the nuts absorb water and therefore increase in mass, the increase in volume is more significant.

Moisture content

Likewise, Table 4 shows the moisture content of both raw and steamed cashew nuts from Puerto Carreño. The mean moisture content of raw nuts stored at room temperature for 5 days after harvesting was found to be 10.25%. After steaming the nuts and allowing them to stabilize at room temperature for 10 days, the moisture content was remeasured and found to be 11.17%. A two-sample t-test was performed on the measurements, yielding a p-value lower than the significance level of 0.05. This result confirms that the two samples are statistically different. As expected, the steaming process increased the moisture content of the nuts. These moisture content values are consistent with those reported in the literature for different countries, with values ranging from 7.1% to 14%.^{[15,28,38,44]}

The moisture content of the nuts is critical in the cashew nut processing. According to the literature, high moisture content in the nut causes the shell to be more tenacious, making shelling more difficult. Furthermore, high moisture causes the nuts to have a higher angle of repose, obstructing their transport. High moisture content, on the other hand, makes the kernel more fragile due to the testa’s adherence to the nutshell. These findings highlight the importance of moisture control during the storage and processing of nuts in order to optimize their processing and final product quality.

Angle of repose

The angle of repose of cashew nuts with for four different contact materials were measured: galvanized steel, stainless steel, MDF, and UHMWPE. Results, shown in Table 5, for all four materials are in the range of 28.76° to 31.07° for raw nuts and 28.83° to 32.03° for steamed nuts. Although the results found are higher than those reported in the literature (ranging from 20.9° to 28.6°), it must be considered that the method used to determine the dynamic angle of repose is different. Previous studies used the funnel method, while this study used the box with removable side method. In addition, a paired t-test was performed to determine whether there are differences between the raw and steamed nuts. The analysis showed that the steaming process does not affect the angle of repose of the nuts against any of the materials tested.

Table 5. Angle of repose and coefficient of friction of raw and steamed cashew nuts from Puerto Carreño using different contact materials.

Contact Material	Angle of repose		Coefficient of friction
	Raw	Steamed	Raw	Steamed
Galvanized steel (°)	31.1 ± 0.7	32.0 ± 0.2	0.37 ± 0.01	0.39 ± 0.01
Stainless steel (°)	30.1 ± 0.6	28.8 ± 0.8	0.30 ± 0.01	0.32 ± 0.01
MDF (°)	30 ± 2	30 ± 1	0.32 ± 0.01	0.39 ± 0.01
UHMWPE (°)	29 ± 1	32 ± 2	0.30 ± 0.02	0.30 ± 0.01

The angle of repose obtained from the experimentation provide valuable insights for the design of a hopper or receiving element to ensure proper material flow without the need for additional stimuli such as extra loads. Additionally, these results offer a starting point for the design of transport elements such as channels or rails. This information can be used to optimize the efficiency of cashew nut handling processes.

Coefficient of friction

Also, Table 5 displays the coefficients of friction of both raw and steamed cashew nuts sourced from Vichada, Colombia, in contact with four different materials: galvanized steel, stainless steel, MDF, and UHMWPE. The static coefficients of friction for the cashew nuts on the tested materials range from 0.30 to 0.37 for raw nuts and from 0.30 to 0.39 for steamed nuts. A paired t-test was conducted, and the p-values were found to be below 0.05, which concluded that there were no significant differences in the coefficients of friction between raw and steamed nuts.

The values obtained in this study are higher than the values reported by.^[44] They reported a coefficient of friction in the range of 0.26 to 0.30 for galvanized steel for a nut with 10% moisture content. It should be noted that the pulley method was used in their study, which does not provide progressive and precise control over the force applied.

In conclusion, regarding the physical properties of the cashew nut produced in Puerto Carreño, the study emphasizes the importance of moisture control, as seen in the significant increase in moisture content after steaming, influencing the tenacity of the shell and the overall processing efficiency. The findings regarding bulk density, and true density provide valuable insights for optimizing processing operations. The decrease in bulk density after steaming indicates the expansion of nuts during boiling, impacting their behavior during handling and transportation. Additionally, the differences in true density highlight the change in volume during processing. The moisture content results further underscore the critical role of moisture control in the processing and final product quality of cashew nuts. Also, the angle of repose and coefficients of friction findings offer practical implications for the design of processing equipment, specifically for the design of loading and unloading hoppers and chutes. The relatively stable angle of repose after steaming suggests that the steaming process does not significantly affect the flow behavior of cashew nuts. The coefficients of friction data, while slightly higher than reported values, provide essential information for designing elements like hoppers and transport channels, contributing to the overall efficiency of cashew nut handling processes.

Mechanical properties

The characteristic uniaxial force-displacement curves for each supplier and loading axis are shown in Figure 9 for raw nuts and in Figure 10 for steamed nuts. All curves, for both processing states and the three loading axes, exhibit an initial compression stage that is almost linear until the first fracture occurs. At this point, the force at fracture is measured and recorded. After the first fracture, the $x-axis$ loading shows multiple consecutive fractures characterized by an increase in loading force followed by sudden drops when cracking occurs. In contrast, for the $y-axis$ loading, after the initial fracture, the load remains constant, forming a plateau, followed by a densification stage where the load increases exponentially. Finally, the $z-axis$ loading exhibits a similar behavior to the $x-axis$, with multiple fractures indicated by rises in load, subsequent fractures, and sudden drops in force.

Figure 9. Compressive mechanical behavior of raw cashew nuts.

Figure 10. Compressive mechanical behavior of steamed cashew nuts.

The force at the first fracture of both raw and steamed nuts is presented in Table 6 for the four suppliers. These results indicate a compressive force range of 410 N to 900 N for raw nuts and 220 N to 830 N for steamed nuts. A t-student test was used to compare these results, demonstrating a significant reduction in compressive strength following the steaming process. The results indicate that the steaming process influenced the compression force at fracture of the cashew nuts. In general, the force at fracture decreased after steaming, suggesting a reduction in the mechanical strength of the nuts. At the same time, force-displacement curves reveal that damage to the cashew nut occurs in distinct steps, commencing with the outer layer of the nut shell and propagating through its internal layers. It is crucial for nut processing, specifically deshelling, to identify and consider the fracture values. This identification aids in determining the forces necessary to open the shell during deshelling without surpassing these values to safeguard the kernel from undesired damage. Whole kernels hold higher value in the international cashew nut market.

Table 6. Compression force at fracture of raw and steamed cashew nuts from four suppliers at $x$, $y$ and $z$ orientations.

Force at Fracture		Supplier 1	Supplier 2	Supplier 3	Supplier 4
Raw	$x-axis$ (N)	451 ± 32	810 ± 30	575 ± 22	557 ± 11
	$y-axis$ (N)	452 ± 23	626 ± 59	499 ± 22	415 ± 30
	$z-axis$ (N)	810 ± 36	903 ± 17	861 ± 38	842 ± 42
	$x-axis$ (N)	382 ± 15	619 ± 68	365 ± 11	464 ± 23
Steamed	$y-axis$ (N)	223 ± 14	391 ± 19	229 ± 20	341 ± 31
	$z-axis$ (N)	662 ± 67	814 ± 56	636 ± 58	833 ± 35

Moreover, the findings reveal notable variations among the different suppliers. A Tukey test was conducted, comparing the results among suppliers, revealing significant differences among the suppliers with a 95% of confidence level. The specific effects varied among the suppliers.

Additionally, the compression force at fracture analysis revealed the presence of anisotropic mechanical behavior in cashew nuts. The variations observed in the force at fracture along different orientations ($x$, $y$, and $z$ axes) indicate that the nuts possess different mechanical properties depending on the loading direction. This anisotropic response can be attributed to the structural composition and organization of fibers or cells within the nuts. Understanding this anisotropy is crucial for proper handling, processing, and product design.

These findings emphasize the importance of considering supplier variations and processing techniques in the cashew nut industry. By understanding the mechanical behavior of cashew nuts from different suppliers and the impact of processing, manufacturers can make informed decisions regarding product quality, processing parameters, and packaging. Furthermore, these results can contribute to the development of strategies for optimizing cashew nut processing techniques and enhancing the overall quality and shelf life of cashew-based products.

Furthermore, the mechanical characterization of cashew nuts plays a pivotal role in designing effective tools for deshelling processes within the cashew industry. The revealed anisotropic mechanical behavior, showing variations in force at fracture along different orientations, highlights the structural composition of fibers within the nuts. This anisotropic response requires an approach in tool design, ensuring adaptability to diverse loading directions for the proper handling and processing of cashew nuts but is also indispensable for designing tools that can optimize deshelling efficiency while minimizing the risk of damaging the kernel. By incorporating these mechanical findings into tool design, manufacturers can enhance the precision and efficacy of deshelling processes, contributing to the production of whole and high-quality cashew kernels. This, in turn, aligns with the industry’s objective of meeting strict quality standards and catering to the preferences of the international cashew nut market, where the value of whole kernels is vital.

Conclusion

The morphological, physical, and mechanical properties of raw and steamed cashew nuts from multiple suppliers in Puerto Carreño, located in the Vichada region of Colombia, were measured in this study. The findings revealed that cashew nuts harvested in the Vichada region varied in size and exhibited a kidney-shaped morphology. Compared to nuts from other African and Asian countries, the cashew nuts from Vichada displayed similar sizes and mass, with some even larger in size. This discovery is significant as the preference for larger nut sizes, particularly for complete kernels in jumbo sizes, has contributed to an increased demand. Thus, the cashew industry in the Vichada region has the potential to establish strong competitiveness in the global market.

Furthermore, the cashew nuts exhibited lightweight characteristics, low moisture content, and favorable flowability and frictional resistance. The mechanical tests conducted revealed that cashew nuts possessed a relatively high compressive strength and displayed an anisotropic mechanical response and significant differences among suppliers. Additionally, the results indicated that the steaming process reduced their mechanical properties, enhancing the subsequent opening process of the nut.

The findings of this study carry important implications for the cashew nut industry. The physical properties provide valuable insights into the handling and storage requirements of cashew nuts, while the morphological features can guide breeding programs aimed at improving yield and quality. Moreover, the mechanical characterization offers valuable information for optimizing processing techniques and designing cashew-based products.

By enhancing our understanding of the physical, morphological, and mechanical properties of cashew nuts from the Vichada region, this research contributes to the advancement of the cashew nut industry. The knowledge gained from this study can inform strategies for cultivation, processing, and product innovation, leading to increased efficiency, improved quality, and expanded market opportunities. Ultimately, these advancements have the potential to benefit both producers and consumers, promoting a sustainable and thriving cashew nut industry.

Nomenclature

$D$	=	Geometric diameter of the nut
$L$	=	Length of the nut
$m_0$	=	Initial mass
$m_b$	=	Mass of empty box
$m_f$	=	Final mass
$m_n$	=	Mass of nuts
$m_t$	=	Mass of filled box
$M{C}_P$	=	Whole pod moisture content
$T$	=	Thickness of the nut
$V_b$	=	Volume of the box
$V_n$	=	Volume of nuts
$W$	=	Width of the nut
$\mu$	=	Coefficient of friction
$\phi$	=	Sphericity of the nut
${\rho }_t$	=	True density
${\rho }_b$	=	Bulk density
$\theta$	=	Angle of static friction

Acknowledgments

The authors wish to express their gratitude to Ministerio de Ciencia, Tecnología e Innovación de Colombia and OCAD de CTeI who carried out the viability, prioritization, and approval of this research with resources from Sistema General de Regalías - SGR in the call No. 6 of the Project “Aprovechamiento de los subproductos Agroindustriales en la producción del marañón en el departamento del Vichada - BPIN 2020000100571”. Likewise, the authors thank the government and the community of the department of Vichada in general for their interest and participation.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Sistema General de Regalías de Colombia [call No. 6 - BPIN 2020000100571].