Engineering properties of tender sorghum (hurda) panicles and grains for the design of mechanized processing equipment

Introduction

Sorghum (Sorghum bicolor L. Moench) is an important cereal crop widely cultivated in semi-arid and tropical regions because of its exceptional tolerance to drought, high temperature, and lowfertility soils. Besides its adaptability, sorghum has attracted considerable attention as a nutritious cereal owing to its high carbohydrate content, moderate protein level, dietary fiber, minerals, and diverse bioactive compounds, including phenolic and antioxidants. The crop is increasingly recognized as a sustainable food source and an important ingredient for developing functional and gluten-free food products (Xiong et al., 2019; Abah et al., 2020) [1,2] . In India, tender sorghum grains, commonly known as hurda, are harvested at the milk to softdough stage and consumed as a traditional seasonal food, particularly in Maharashtra and adjoining regions. At this stage, the grains possess high moisture content, a soft endosperm, pleasant sweetness, and superior eating quality compared with mature sorghum grains. These unique characteristics make sorghum (hurda) grains an important value-added product with growing commercial potential. However, the high moisture content and immature cellular structure considerably alter the engineering behavior of the grains, influencing their dimensional, gravimetric, frictional, and flow properties during handling and processing (Mate et al., 2021) [3] . Engineering properties of agricultural materials constitute the fundamental database required for the design, development, and optimization of post-harvest processing equipment. Parameters such as grain dimensions, geometric mean diameter, sphericity, thousand-grain weight, bulk density, true density, porosity, angle of repose, coefficient of static friction, and terminal velocity are extensively used in the design of hoppers, conveyors, elevators, cleaning systems, grading equipment, storage bins, pneumatic separators, and other grain handling machinery. Furthermore, these properties facilitate mathematical modeling of grain behavior, optimization of machine components, and prediction of material flow characteristics, thereby improving processing efficiency while minimizing product losses and mechanical damage (Mohsenin, 1986; Gely and Pagano, 2017) [4,5] . Numerous investigations have reported the engineering properties of mature sorghum grains over different moisture ranges and demonstrated that moisture content significantly influences grain size, shape, density, porosity, angle of repose, coefficient of friction, and thousand-grain weight (Mwithiga and Sifuna, 2006; Gely and Pagano, 2017) [6,5] . These studies have primarily focused on dry sorghum intended for storage, transportation, milling, and conventional post-harvest operations. Since mature grains possess substantially lower moisture content and higher kernel hardness than tender grains, the engineering data generated for mature sorghum cannot be directly applied to the design of equipment intended for processing high-moisture tender sorghum (hurda) grains. Although a few studies have reported selected physical properties of tender sorghum (hurda) grains, the available information remains limited and is largely restricted to basic characterization under varying moisture conditions. Comprehensive engineering data specifically intended to support the design and optimization of mechanized processing equipment for tender sorghum grains are still scarce. This lack of reliable engineering information limits the scientific design of feeding, conveying, extraction, cleaning, and handling systems for mechanized Sorghum hurda processing (Mate et al., 2021) [3] . Therefore, the present investigation was undertaken to determine the engineering properties of tender sorghum (hurda) grains at the edible maturity stage. The generated engineering database is expected to provide scientifically validated design parameters for the development and optimization of mechanized processing equipment intended for handling, conveying, cleaning, and processing tender sorghum grains.

Materials and Methods

Sample Collection

The milky stage sorghum (hurda) grains of various locally grown variety Gulbhendi were procured from group of Sorghum growers, Mr. Kiran Narayan More, District Ch. Sambhaji Nagar for conducting the experiments on sorghum (hurda) grains extraction. The sophisticated equipment's and machineries viz: oven, moisture meter, tachometer, variable frequency drive etc. were used from AICRP on Post Harvest Engineering & Technology, Dr. PDKV, Akola. The average moisture content of the tender grains varied between 60 and 70% (w.b.), representing the edible maturity stage commonly used for sorghum (hurda) grains preparation.

Determination of Engineering Properties

The evaluated characteristics were classified as moisture, physical, gravimetric, frictional, and aerodynamic properties. All measurements were conducted in triplicate, and mean values were used for analysis and equipment design.

Moisture Content

Moisture content of tender sorghum (hurda) grain and sorghum (hurda) panicles was determined by the standard oven-drying method. A 20 g sample was dried at 103°C for 17 h, cooled in a desiccator, and weighed. Moisture content was calculated on a wet basis (Mate et al., 2021) [3] .

………………… (3.1)

Physical Properties 2.2.2.1 Panicle Dimensions and Weight

Panicle length and maximum width were measured using a digital vernier caliper with a least count of 0.01 mm. Individual sorghum (hurda) panicle weight and sorghum (hurda) grain weight per panicle were measured using a precision electronic balance with an accuracy of 0.01 g. Grain percentage was calculated as the ratio of grain weight to panicle weight, according to (Mate et al., 2021) [3] .

Grain Dimensions

Thirty randomly selected healthy and undamaged sorghum (hurda) grain were used for dimensional measurements. The major, medium, and minor axes were measured using a digital vernier caliper with a least count of 0.01 mm and calculate according to (Mate et al., 2021) [3] .

Geometric Mean Diameter

The geometric mean diameter of tender sorghum (hurda) grain was calculated from the three principal dimensions according to Mohsenin (1986) [4] :

Where, l = length, mm b = breadth, mm t = thickness, mm

Sphericity

Sphericity was calculated from the principal grain dimensions according to Mohsenin (1986) [4] :

Where, a = longest intercept, mm b = Longest intercepts normal to a, mm c = Longest intercept normal to a and b, mm

Gravimetric Properties 2.2.3.1 Thousand-Grain Weight

Thousand-grain weight was determined by weighing 1000 randomly selected tender sorghum (hurda) grain using a precision electronic balance and expressed in grams and calculate according to (Mate et al., 2021) [3] .

Bulk Density

Bulk density was calculated as the ratio of the mass of sorghum (hurda) grain to the volume occupied by the grains without compaction (Mohsenin, 1986) [4] :

Bulk density (kg/m 3 ) = ……. (3.4)

True Density

True density was determined using the liquid displacement method and calculated according to Mohsenin (1986) [4] :

True density (kg/m 3 ) = …… (3.5)

Porosity

Porosity represents the percentage of air spaces present within a bulk sorghum (hurda) grain mass and was calculated from bulk density and true density values using the following relationship (Mohsenin, 1986) [4] :

Where, ρt =True density, kg/m 3 ρb= Bulk density, kg/m 3

Frictional Properties 2.2.4.1 Angle of Repose of Grains

The angle of repose of tender sorghum (hurda) grain was determined using the fixed-funnel method. The grains were allowed to discharge freely through the funnel to form a natural heap. The heap height and base diameter were measured, and the angle of repose was calculated using Eq. (3.7) according to (Mohsenin, 1986) [4] Ø = tan-1 2H/D ……………………………. (3.7)

Where, Ø = Angle of repose, degree H = Height of pile, mm D = Diameter of disc, mm

Coefficient of Static Friction

The coefficient of friction of tender sorghum (hurda) grain against a mild steel surface was determined using an inclined plane method. It was calculated using Eq. (3.8) according to (Mohsenin, 1986) [4] :

Where, μ = coefficient of static friction θ = angle of inclination at the initiation of grain movement, degree

Aerodynamic Properties 2.2.5.1 Terminal Velocity

Terminal velocity was determined using a vertical air column by gradually increasing the airflow until an individual tender sorghum grain attained a suspended state. The air velocity corresponding to the onset of suspension was recorded as terminal velocity and calculated according to (Mate et al., 2021) [3] , expressed in m/s.

Results and Discussion

The engineering properties of tender sorghum (hurda) panicles and sorghum (hurda) grains were determined at 66.52% moisture content (w.b.), corresponding to the milk stage commonly used for sorghum (hurda) grain preparation. The measured properties were grouped into physical, gravimetric, frictional, and aerodynamic characteristics. The consolidated results are presented in Table 1.

Moisture Content

The average moisture content of the tender sorghum (hurda) panicles and tender sorghum (hurda) grain was 66.52% (w.b.). This value represents the tender physiological condition at the milk stage used for sorghum (hurda) grain preparation. Moisture content influences the dimensional, gravimetric, frictional, and aerodynamic characteristics of cereal grains through changes in grain volume, mass, surface condition, and inter-particle interaction.

The observed moisture level was comparable with the range investigated for tender sorghum (hurda) grain by Mate et al.

(2021) [3] , who evaluated engineering properties of sorghum (hurda) var. Gulbhendi at approximately 62-70% moisture content (w.b.). Their results demonstrated that moisture variation significantly influenced grain dimensions, thousand-grain weight, density, porosity, angle of repose, and frictional behavior. Similar moisture-dependent changes in sorghum engineering properties have been reported by Gely and Pagano (2017) [5] . Thus, characterization at 66.52% moisture content provides representative engineering information for the tender sorghum hurda condition.

Physical Properties of Tender Sorghum Panicles

Dimensional and Shape Characteristics of Tender Sorghum Grains

The mean major, medium, and minor axes of tender sorghum grains (hurda) were 4.39 ± 0.29, 4.14 ± 0.14, and 3.03 ± 0.13 mm, respectively. The corresponding geometric mean diameter was 3.69 ± 0.18 mm. The relatively low standard deviations indicate limited variation in grain dimensions within the sampled material.

The measured dimensions were comparable with values reported for mature sorghum under lower moisture conditions. Surpam et al. (2019) [7] reported mean grain dimensions of 4.39, 4.20 and 2.64 mm for length, width, and thickness, respectively, with a geometric mean diameter of 3.64 mm at 9.15% moisture content. The close agreement in geometric diameter indicates that the present tender sorghum (hurda) grain fall within the dimensional range reported for sorghum despite the substantial difference in moisture condition. Moisture-dependent dimensional changes have been reported in sorghum. Mwithiga and Sifuna (2006) [6] observed increases in principal grain dimensions and geometric mean diameter with increasing moisture content, while Sabar et al. (2020) [8] reported similar increases in grain length, width, thickness, and geometric mean diameter over a moisture range of 8.7-21.8% (w.b.). These changes are associated with moisture absorption and expansion of grain tissues. The mean sphericity of the tender sorghum (hurda) grain was 0.84 ± 0.03, indicating a relatively rounded grain shape. This value closely agrees with the value of approximately 0.834 reported by Surpam et al. (2019) [7] . Mate et al. (2021) [3] reported sphericity values of approximately 0.838-0.869 for tender sorghum over a comparable moisture range, placing the present observation within the reported range for sorghum (hurda) grain. The result therefore confirms that the tender sorghum (hurda) grain retain a comparatively rounded morphology at the examined moisture condition.

Gravimetric Properties of Tender Sorghum Grains

The thousand-grain weight was 20.59 ± 0.22 g, with values ranging from 20.22 to 21.06 g. The relatively low standard deviation indicates limited variation in grain mass within the sampled population. The observed value closely corresponds with the range of 20.41-21.14 g reported by Mate et al. (2021) [3] for tender sorghum over a comparable moisture range. Sabar et al.

(2020) [8] also reported an increase in thousand-grain weight with increasing moisture content in sorghum. The relatively high mass observed in the present tender grains can therefore be associated partly with their high moisture content. The mean bulk density was 625.00 ± 8.68 kg/m³, whereas the true density was 989.00 ± 3.59 kg /m³. The lower bulk density compared with true density reflects the presence of inter-granular void spaces within the grain mass. Mate et al. (2021) [3] reported bulk-density values ranging from approximately 662 to 610 kg/m³ and true-density values from 997 to 984 kg/m³ over increasing moisture contents in tender sorghum. The present values within this reported range and show comparable density characteristics.

The observed reduction in bulk density relative to true density is consistent with the moisture-dependent behaviour reported by Gely and Pagano (2017) [5] . Increasing moisture content causes grain expansion and modifies the packing arrangement, resulting in changes in the volume occupied by a given grain mass. The measured porosity was 36.39 ± 0.17%, indicating a substantial proportion of inter-granular void space. The value is also comparable with porosity values reported for sorghum (hurda) grain by Mate et al. (2021) [3] .

Frictional Properties of Tender Sorghum

The angle of repose of tender sorghum (hurda) grains was 33.23 ± 0.21°. The close numerical agreement indicates similar heapforming behaviour under the respective experimental conditions, although the two materials differ considerably in their physical form and structure. The grain angle of repose was comparable with values reported for sorghum under both tender and mature conditions. Surpam et al. (2019) [7] reported an angle of repose of approximately 35° at 9.15% moisture content, whereas Sabar et al. (2020) [8] observed values increasing from 39.84 to 43.19° over 8.7-21.8% moisture content. Mate et al. (2021) [3] , however, reported values of approximately 33.14-33.77° for tender sorghum over a comparable high-moisture range. The present value of 33.23° therefore agrees particularly well with observations for tender sorghum (hurda) grain. The coefficient of static friction of tender sorghum (hurda) grain against mild steel was 0.632. This value was considerably higher than the coefficient of approximately 0.20 reported by Surpam et al. (2019) [7] for mature sorghum at 9.15% moisture content. Gely and Pagano (2017) [5] reported an increase in the coefficient of friction with increasing moisture content, attributing the change to increased surface interaction and adhesion. Sabar et al. (2020) [8] similarly observed an increase in the coefficient of friction against mild steel with increasing moisture content.

Aerodynamic Properties of Tender Sorghum Grains

The mean terminal velocity of tender sorghum (hurda) grain was 4.64 ± 0.23 m/s. This represents the air velocity at which individual grains attained a suspended condition under the experimental setup. The observed value was lower than the terminal velocity reported for mature sorghum at considerably lower moisture contents. Sabar et al. (2020) [8] reported terminal velocities ranging from 7.06 to 7.99 m/s as moisture content increased from 8.7 to 21.8% (w.b.). The lower value obtained in the present study can be associated with the substantially higher moisture content and tender physiological condition of the grains. Moisture alters grain mass, dimensions, density, and projected area, thereby influencing aerodynamic behaviour. Overall, the measured properties were consistent with the established moisture-dependent behaviour of sorghum and showed close agreement with published observations for sorghum (hurda) grain. The present dataset therefore provides a comprehensive characterization of tender sorghum panicles and grains under a moisture condition representative of the commercial sorghum (hurda) grain harvesting stage.

Conclusion

The engineering properties of tender sorghum (Sorghum bicolor L.) (hurda) panicles and (hurda) grains (var. Gulbhendi) were characterized at 66.52% moisture content (w.b.

Acknowledgement

AcknowledgementsThe authors would like to express their sincere gratitude to ICAR-All India Coordinated Research Project on Post Harvest Engineering and Technology, Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola, Maharashtra for providing facilities and financial support to conduct the research.

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