Development and Evaluation of Modified Rotor–Stator Knives for Enhanced Fine Grinding of Chicken Meat–Bone Raw Material in Colloid Mill

Abstract

The growing demand for cost-effective, high-quality protein ingredients in the meat industry highlights the need for advanced processing methods capable of producing uniform, functional meat–bone pastes from poultry by-products. This study investigates the optimization of colloid milling parameters for the fine grinding of chicken meat–bone by-products, with a focus on improving particle size distribution, rheological properties, and processing efficiency. A modified rotor–stator system with teeth inclined at 20° and a reduced pitch (0.5 mm) was compared to a conventional configuration (45° inclination, 1.5 mm pitch). Experiments were conducted at rotor speeds ranging from 1000 to 4000 rpm, with a fixed clearance of 0.1 mm. The results showed that the modified design significantly enhanced grinding efficiency, reducing the proportion of bone fragments > 1 mm and yielding over 70% of particles under 0.1 mm at 3000 rpm. Viscosity and shear stress measurements indicated that grinding at 3000 rpm yielded a dynamic viscosity of 71,507 Pa·s and a shear stress of 43,531 mPa·s, values that were significantly lower (p < 0.05) than those observed at other tested speeds, thereby producing a paste consistency with the most favorable balance of elasticity and flowability. At 4000 rpm, the temperature rise (up to 32 °C) led to partial denaturation of muscle proteins, accompanied by emulsion destabilization and disruption of the protein gel matrix, resulting in reductions in the viscosity and water-binding capacity of the paste. Comparative analysis confirmed that tool geometry and rotor speed have critical effects on grinding quality, energy use, and thermal load. The optimal operating parameters, 3000 rpm with modified rotor–stator teeth, achieve the finest, most homogeneous bone paste while preserving protein functionality and minimizing energy losses. These findings support the development of energy-efficient grinding equipment for the valorization of poultry by-products in emulsified meat formulations.

1. Introduction

The domestic meat industry in Kazakhstan has experienced significant growth in production volumes and diversification of product lines in recent years [1]. As a critical component of the national food system, the meat sector is responsible for supplying high-quality protein products that meet increasingly stringent safety and sensory requirements. One particularly promising area for innovation lies in the valorization of slaughter by-products, especially poultry bones, which constitute a substantial portion (20–25%) of the carcass after primary processing [2]. These include vertebrae and flat breast and back bones, as well as smaller fragments from drumsticks and thighs. The effective utilization of such by-products can significantly enhance the economic viability of meat-processing operations and contribute to a more circular economy in food production [3,4]. Mechanically deboned meat (MDM), obtained after secondary pressing of residual carcass material, often contains bone particles up to 2–3 mm in size. These inclusions are organoleptically unacceptable in most processed meat products, such as sausages and deli meats, due to their gritty texture and negative impact on mouthfeel [5,6].

To address this challenge, there is a clear need for advanced processing equipment capable of reducing bone material to a fine paste with particle sizes below 0.1 mm, making it imperceptible during consumption [7]. Colloid mills, which are commonly used for the fine comminution of relatively soft materials such as nuts, plant tissues, and offal, offer a potential solution [8]. However, when applied to harder materials like bone tissue, these systems require significant design modifications. Specifically, specialized tooth geometries and enhanced wear resistance of cutting edges [9].

Colloid mills utilize a high-speed rotor–stator mechanism to generate intense shear forces in a narrow gap, fragmenting raw material into fine particles. During milling, the rapidly rotating rotor and stationary stator act together to create a high shear field, enabling significant particle size reduction [10]. However, conventional rotor–stator designs often lack the ability to achieve uniform, ultra-fine grinding of tougher materials such as bone, and may leave unacceptable coarse fragments in the final paste. Addressing these limitations through improved tool geometry is essential for producing homogeneous meat–bone emulsions and enhancing the overall efficiency of meat by-product processing.

This study focuses on the investigation and refinement of colloid mill design solutions aimed at the ultra-fine grinding of poultry bones. By modernizing the rotor–stator knife mechanism, the research seeks to improve the efficiency of bone processing and promote full valorization of raw materials. Ultimately, the proposed technological advancements are expected to facilitate the production of high-quality meat emulsions free from detectable bone inclusions, thus broadening the range and enhancing the sensory attributes of meat-based food products.

2. Materials and Methods

Samples

The raw material used for grinding consisted of meat-and-bone residues obtained from broiler chickens of the Ross 308 cross, aged 40–50 days, which were taken from the commercial poultry company LLP “Ardager”, Semey, Kazakhstan. Following manual deboning of the carcasses, the remaining bones—including vertebrae, breast, drumstick, and thigh bones—typically retained up to 8% adherent muscle tissue. The chemical composition of the poultry bone fraction was as follows: moisture content 65.6%, protein 16.8%, fat 12.1%, and ash 5.5%.

Chicken carcasses with a total mass of 50 kg were initially subjected to deboning and sinew removal, with all bones carefully separated. The recovered bones were first coarsely ground using a power crusher. The resulting bone material was then divided into four equal portions, each weighing 6 kg. Each portion was processed separately in a colloid mill. First, the control knife assembly was installed, and each batch was milled at a different rotor speed. Afterward, the experimental knife assembly was installed, and the remaining batches were similarly milled at the same set of rotational speeds. This procedure allowed a systematic comparison of grinding effectiveness between the control and experimental rotor–stator geometries under identical processing conditions.

Grinding was performed at four different rotor speeds: 1000 rpm, 2000 rpm, 3000 rpm, and 4000 rpm. The variation in rotational speed was achieved by using an electric motor equipped with a frequency converter. Two types of rotor–stator knife assemblies were used in the study. The control assembly represented a factory-standard configuration with a tooth inclination angle of 45° and a tooth pitch of up to 1.5 mm. The control tool configuration was selected because it is a widely used industry standard in commercial colloid mills intended for grinding relatively soft food materials such as nuts and other non-fibrous or less resistant raw ingredients. While this geometry is effective for the fine grinding of softer products, it is not optimized for hard inclusions such as bone, because hard particles with sizes of more than 0.1 mm are not acceptable in food systems.

The experimental assembly was designed with a modified tooth geometry, featuring a 20° inclination angle and a reduced tooth pitch of up to 1 mm.

Description of Fine Grinding Equipment

A general view of the grinder is shown in Figure 1. The device operates according to the principles of centrifugal grinders. The throughput capacity of the machine is 80 kg/h (

Table 1. Technical characteristics of colloid mill.

Indicator	Characteristic
Capacity, kg/h	80
Electric power, kW	4
Overall dimensions, cm	64 × 41 × 90
Weight, kg	150
Hopper volume	8 L

). The structure of the grinder includes a supporting frame, a loading hopper, a working assembly (rotor–stator), and a discharge chamber. Figure 1b provides a detailed depiction of the individual components and assemblies of the grinder.

Determination of mass and volume flow rates

The mass flow rate is determined by the following formula:

$$M={\displaystyle \frac{m}{t}}·3600$$

where M—mass flow rate, kg/h,

m—mass of loaded raw material,

t—grinding time, s.

The volumetric flow rate is determined by the following formula:

$$V={\displaystyle \frac{V_s}{t}}·3600$$

where V—volume flow rate, m³/h,

V_s—volume of loaded raw material,

t—grinding time, s.

Determination of particle size distribution of bone particles

The dried solid residue after alkaline treatment was passed through a set of laboratory sieves with nominal mesh diameters of 2 mm, 1 mm, 0.5 mm, and 0.1 mm. In emulsified meat products, a particle size below 0.1 mm is generally considered acceptable, as larger bone fragments negatively impact texture and sensory quality. A 50 g suspension was placed on the top sieve and sieved in a vibrating analyzer for 10 min at an amplitude of 1.5 mm. The mass of material retained on each sieve and in the pallet was weighed with an accuracy of ±0.01 g. The mass fraction of a fraction was calculated as the ratio of the mass of residue on a given sieve to the original weight, expressed as a percentage. Each determination was performed three times; the results were then averaged [11].

Determination of dynamic viscosity and shear stress

The dynamic viscosity (η, Pa·s) and shear stress (τ, mPa) of the meat and bone paste were determined using a BOYN Digital Viscometer Model LVDV-2T (BOYN Instrument Co., Ltd., Shanghai, China). First, 100 mL of the tested paste was introduced into a 250 mL beaker thermostat and kept there until the temperature stabilized at 18–22 °C (thermometer control ± 0.2 °C). The measuring spindle (diameter 12.5 mm) was immersed so that the mark was covered by the sample, and then a constant rotation speed of 6 rpm was set. After 60 s from start-up, the readings displayed on the instrument display were recorded: the dynamic viscosity in Pa·s and the corresponding shear stress in mPa. For each sample, three parallel measurements were performed and the mean value and relative standard deviation were calculated (RSD ≤ 3% was considered acceptable). Before each new series, the spindle and beaker were thoroughly cleaned with warm water and neutral detergent and dried.

Determination of Water-Binding Capacity

The determination of water-binding capacity (WBC) was carried out using a gravimetric filter paper pressing method. In this procedure, a weighed portion of the meat–bone paste sample was placed between sheets of qualitative filter paper (Whatman Grade 1, diameter 150 mm) and subjected to standardized light mechanical pressure for a fixed duration, allowing free water to be released from the matrix. The moisture expelled from the sample was absorbed by the filter paper, forming a clearly visible wet spot. The diameter and area of this wet spot were subsequently measured, and the quantity of separated water was estimated based on a calibration curve correlating the stained area to moisture content. The WBC was then calculated as the difference between the initial sample moisture and the amount of water released, expressed as a percentage of the total water content. This method allows for indirect assessment of the matrix’s ability to retain water under low mechanical stress [12].

Statistics

All quantitative data are presented in the format of the mean ± standard deviation (SD) calculated from a minimum of three parallel measurements. ANOVA (analysis of variance) was used to evaluate differences between the groups; differences were considered statistically significant at a probability level of p < 0.05. Calculations and graphing were performed in Microsoft Excel 2016 (Microsoft Corp., Redmond, WA, USA).

3. Results

3.1. Main Design Characteristics of Cutting Part of Colloid Mill

The machine used for the ultra-fine grinding of food materials is equipped with a rotor and a stator of different geometric configurations, which facilitate high-speed rotation. The processed material is subjected to a combination of forces, including its own weight, aerodynamic resistance, and centrifugal acceleration [13]. By adjusting the gap between the stator and the rotor, the material experiences intense shear stresses, frictional forces, impact loads, and high-frequency vibrations. The combined action of these mechanical forces results in grinding, crushing, and thorough mixing of the components, ultimately yielding a product with the desired properties [14].

Rotor Design

The moving grinding assembly of the colloid mill consists of a rotor composed of three truncated-cone cylinders mounted on a central shaft. Each cylinder features a central bore to facilitate installation, and the external surfaces of the cylinders are equipped with teeth inclined at 20°. A key design feature of the rotor is the intentional variation in both diameter and tooth pitch across the three cylinders. The lowermost cylinder, positioned closest to the base, has the largest diameter and the smallest tooth pitch, providing initial coarse grinding. The middle cylinder possesses an intermediate diameter and an intermediate tooth pitch, enabling further reductions in particle size. The uppermost cylinder exhibits the smallest diameter but the largest tooth pitch, contributing to the final stage of grinding. When assembled on the shaft, the cylinders form a stepped truncated cone whose diameter progressively decreases toward the top. This gradient in tooth density along the rotor length promotes a sequential, controlled reduction in particle size as the bone material advances through the milling zone, ensuring the efficient comminution and preparation of a homogeneous paste.

Stator Design

The stationary grinding element, or stator, complements the rotor and likewise consists of three cylinders with central bores. The internal bores are machined to a conical profile that precisely matches the outline of the truncated-cone rotor. Teeth inclined at 20° are cut on the inner surfaces of these stator cylinders, providing optimal meshing with the rotor teeth and thereby maximizing the shear forces applied to the material.

During assembly, the rotor fits into the stator’s conical cavity, leaving a working gap between the opposing teeth. The gap between the rotor and stator is controlled using an adjustable threaded ring mechanism that holds the stator in place. By rotating this ring, the stator is moved closer to or further from the rotor, allowing precise regulation of the gap. Full closure of the gap (0 mm) is achieved after seven complete turns of the ring. The gap was initially set at a maximum of 3 mm and could be gradually reduced in controlled steps down to 0.1 mm. The setting was monitored using a calibrated scale on the adjustment mechanism to ensure accurate and reproducible gap widths, providing direct control over the fineness of the grinding process. The complete knife system—rotor plus stator—contains a total of 100 teeth, all set at a 20° inclination.

Working Mechanism

The colloid mill, incorporating the described rotor and stator components (Figure 2), operates on the principle of high-shear wet grinding. Poultry meat-and-bone raw material, preliminarily crushed to a size of approximately 3 mm, is fed into the machine through a loading hopper. From there, the material enters the working assembly containing the rotor–stator unit. The high-speed rotation of the rotor generates centrifugal acceleration, propelling the bone mass outward toward the periphery of the rotor.

As the material passes through the narrow, adjustable gap between the rotor and the stationary stator, it is subjected to a combination of intense mechanical forces. These forces include significant shear stresses generated by the high-speed relative motion between the rotor and stator surfaces, frictional forces arising from the contact between the bone material and the teeth of both components, impact forces resulting from the collision of bone fragments with these surfaces, and high-frequency vibrations induced by rapid rotation and interaction [15].

Larger bone fragments entering the mill initially undergo coarse grinding by the larger-diameter cylinder with a finer tooth pitch, followed by progressively finer comminution as the material moves toward the smaller-diameter cylinder with a coarser tooth pitch. The adjustable gap between the stator and rotor allows precise control over the intensity of these forces, ultimately determining the fineness of the resulting bone paste.

3.2. Changes in Performance of Colloid Mill Depending on Rotor Speed and Knife Design

This study examined the dependence of the colloid mill’s poultry meat-and-bone material grinding performance on both rotor speed (1000, 2000, 3000, and 4000 rpm) and the type of working tools (control and experimental). All experiments were conducted with a fixed gap of 0.1 mm using a constant initial raw material mass of 5 kg. The obtained data on grinding time, mass flow rate, and volumetric flow rate are presented in

Table 2. Influence of rotor speed on mass flow rate (kg/h) in colloid milling.

Rotor–Stator Type	Rotor Speed, rpm
	1000	2000	3000	4000
Control	264.71 ± 4.91 a	352.94 ± 3.99 b	439.02 ± 8.41 c	428.57 ± 4.27 c
Experimental	240.00 ± 4.45 a	333.33 ± 4.47 b	400.00 ± 4.70 d	375.00 ± 4.03 c

^a–d means within the same row with different letters differ significantly (p < 0.05).

and

Table 3. Volumetric flow rate (m³/h) at various rotor speeds.

Rotor–Stator Type	Rotor Speed, rpm
	1000	2000	3000	4000
Control	0.27 ± 0.00 a	0.36 ± 0.01 b	0.45 ± 0.01 c	0.44 ± 0.01 c
Experimental	0.24 ± 0.00 a	0.34 ± 0.01 b	0.41 ± 0.01 d	0.38 ± 0.01 c

^a–d means within the same row with different letters differ significantly (p < 0.05).

.

A comparative analysis of grinding time and productivity using the control (45° tooth angle, 1.5 mm pitch) and experimental (20° tooth angle, 0.5 mm pitch) working tools showed that in both cases, increasing the rotor speed from 1000 to 3000 rpm led to a noticeable reduction in processing time and an increase in mass flow rate. In the control setup, the grinding time decreased from 68 to 41 s, and the mass flow rate increased from 264.71 to 439.02 kg/h; in the experimental setup, the grinding time was reduced from 75 to 45 s, and the mass flow rate rose from 240 to 400 kg/h (Table 2).

However, further increasing the rotor speed to 4000 rpm did not significantly accelerate the grinding process (42 s in the control and 48 s in the experimental configuration), and the mass flow rate either slightly decreased (from 439.02 to 428.57 kg/h in the control) or showed no substantial growth (remaining within approximately 375–400 kg/h in the experimental version).

A similar trend was observed for the volumetric flow rate: in the control setup, it increased from 0.27 to 0.45 m³/h at 1000–3000 rpm and slightly decreased to 0.44 m³/h at 4000 rpm; a comparable peak at medium speeds was noted in the experimental setup as well (Table 3).

Thus, in both the control and experimental configurations, the greatest increase in productivity and reduction in grinding time were achieved when the rotor speed was increased from 1000 to 3000 rpm. Further increases, resulting in speeds of up to 4000 rpm, did not result in significant improvements in performance. On the contrary, in some cases, the mass flow rate slightly decreased, likely due to increased turbulence, flow redistribution, and possible “slippage” of larger particles. Statistical analysis (p < 0.05) confirmed that the differences in efficiency between 3000 and 4000 rpm were not significant; however, at 4000 rpm, the risk of tool wear and thermal heating of the product increases. Based on the obtained results, a rotor speed of 3000 rpm can be considered an optimal operating condition, as further increases to 4000 rpm do not yield statistically significant improvements in productivity or grinding fineness (p < 0.05), while energy consumption, tool wear, and heat generation are likely to rise [16].

3.3. Study of Effect of Rotor Speed and Knife Design on Temperature of Meat-and-Bone Paste

A series of experiments were carried out to evaluate the impact of rotor–stator geometry and rotor speed on the quality of poultry bone–meat paste. The rotor speed ranged from 1000 to 4000 rpm, with a fixed gap of 0.1 mm and an initial meat–bone mixture temperature of 2 °C. The control tool left bone fragments larger than 1 mm, whereas the experimental tool effectively eliminated larger fragments, likely because the shallower tooth angle created more shear and prolonged contact. Varying the rotor speed influenced the final paste temperatures.

At 1000 and 2000 rpm, the outlet temperature was moderately elevated (18–24 °C), while at 3000 rpm it dropped to around 8 °C in the control setup and 11 °C in the experimental setup. At 4000 rpm, the paste overheated to 26–32 °C, risking protein denaturation and reduced product quality. Industrial practice recommends a final temperature near 10–15 °C for optimal meat processing [17], making 3000 rpm the most favorable rotational speed tested (Figure 3). This temperature change can be explained by the longer period that the meat–bone mixture spends in the rotor–stator chamber at lower speeds, which permits greater heat accumulation and transfer to the product over an extended processing interval. In contrast, at 3000 rpm, the shorter exposure and increased throughput enable quicker passage through the mill and more effective heat dissipation, resulting in a lower final paste temperature.

At lower speeds, the time required to achieve fine grinding increases, and at 4000 rpm, there is little additional benefit in fragment size but a marked rise in temperature. Consequently, 3000 rpm provides the best balance of throughput, fine grinding, and adequate temperature control. The experimental rotor–stator geometry offers added assurance against the presence of coarse bone fragments, although preventing temperature buildup at higher speeds requires careful operation.

Study of Particle Size Distribution

The data for the control knife configuration indicate that at lower rotor speeds (1000–2000 rpm), a substantial proportion of large bone particles remained in the paste (for example, at 1000 rpm, 24.32% of the particles had a size of 2 mm). Increasing the rotor speed to 3000 rpm reduced the fraction of such large fragments (2 mm) to 10.04%, and further to 7.47% at 4000 rpm. However, even at these higher speeds, more than half of the particles remained approximately 1 mm or larger, indicating that only partial fine grinding of the bone material was achieved (Figure 4). Particles exceeding 1 mm in diameter are generally considered unacceptable for emulsified meat products, as their presence imparts a gritty or sandy mouthfeel, diminishes product smoothness, and may lead to consumer rejection due to perceived defects in texture and overall eating quality. Thus, minimizing the proportion of such coarse particles is essential to ensure optimal sensory properties in the final product.

In contrast, the experimental rotor–stator configuration demonstrated a significantly greater capability for fine grinding. At 1000 rpm, only 3.67% of bone fragments had a size of 2 mm (compared to 24.32% in the control configuration). Increasing the rotor speed to 3000 rpm further shifted the particle size distribution toward finer fractions: only 1.67% of the fragments remained at 2 mm, while 70.30% were in the 0.1 mm fraction, and 6.24% belonged to the sub-0.1 mm category. At 4000 rpm, the proportion of the finest fraction slightly decreased (to 5.21% for particles smaller than 0.1 mm), but the share of coarse particles (≥1 mm) remained minimal.

A comparative assessment of the SEM micrographs presented in Figure 5 illustrates the pronounced effect of knife design on bone particle morphology after grinding. In Figure 5a, representing the control knife (45° tooth angle, 1.5 mm pitch), the bone fragments are predominantly coarse, with many particles ranging from 140 to 380 μm and a substantial number exceeding 200 μm in size. By contrast, Figure 5b, which depicts the experimental knife (20° tooth angle, 0.5 mm pitch), demonstrates a markedly finer and more homogeneous distribution, with most bone fragments measuring between 25 and 70 μm and the majority remaining below 100 μm. These microstructural images visually corroborate the quantitative particle size distribution data, confirming that the experimental knife configuration achieves more effective comminution of bone material. This improvement in particle size uniformity is critical for ensuring the functional and sensory quality of meat emulsions.

Thus, the working assembly with a tooth inclination angle of 20° and a 0.5 mm pitch provided more efficient and uniform grinding of bone inclusions compared to the control design, which left a noticeable portion of fragments ≥1 mm. Achieving a paste dominated by fractions smaller than 0.5 mm, and particularly by fractions under 0.1 mm, is critical for the production of emulsified sausages, pâtés, and other finely textured meat products that require the absence of perceptible bone particles to ensure acceptable mouthfeel and consumer safety. Furthermore, the experiments confirmed the critical role of rotor speed: increasing the speed from 1000 to 3000 rpm significantly reduced the proportion of coarse particles, whereas further increases (up to 4000 rpm) offered only marginal additional advantages and caused slight shifts within the finer size classes. Overall, the experimental rotor–stator configuration with the smaller tooth inclination angle (20°) demonstrated superior fine grinding efficiency, which is important for producing pastes free of noticeable bone inclusions.

The fineness of grinding for both meat and meat–bone mixtures is largely governed by the geometric and kinematic characteristics of the mill’s working elements. In rotor–stator systems (colloid mills), particle size is adjusted via the rotor’s tooth geometry, the stator’s orifice pattern, the number of cutting elements, and the inter-element gap [18]. Reducing tooth pitch, increasing edge sharpness, and narrowing the clearance (to as little as 0.1–0.5 mm) facilitate the production of a finely dispersed (colloidal) mass, but these modifications also impose higher power demands on the equipment.

Kinematic parameters, most notably rotor speed, directly modulate the rates of abrasion and fragmentation. As the rotational frequency increases from 1000 to 3000 rpm, the mean particle diameter decreases; however, speeds beyond 3000 rpm confer negligible gains in particle fineness while exacerbating frictional forces and blade wear. A further constraint arises from adiabatic heating: mechanical friction and plastic deformation can convert 20–30% of the input energy into heat, causing the paste’s temperature at discharge to rise by 5–10 °C and, under continuous operation, to reach 50–60 °C. Such thermal elevations impair protein functionality and reduce product viscosity. To maintain the requisite thermal conditions and achieve consistent, high-quality grinding, it is, therefore, essential to supply precooled raw material, employ water-jacketed housings, and, when necessary, operate in an intermittent mode [19].

Study of Rheological Properties

During the next stage of the study, the dynamic viscosity and shear stress of meat-and-bone paste were investigated. For the control working tool, it was found that at a rotor speed of 1000 rpm, the dynamic viscosity was 55,418 Pa·s and the shear stress was 38,256 mPa·s. At 2000 rpm, dynamic viscosity increased to 60,550 Pa·s and shear stress to 39,263 mPa·s. At 3000 rpm, these parameters reached 67,809 Pa·s and 45,907 mPa·s, respectively. However, at 4000 rpm, a slight decrease in dynamic viscosity to 62,354 Pa·s and in shear stress to 41,240 mPa·s was observed (Figure 6 and Figure 7).

With the experimental configuration, the dynamic viscosity and shear stress followed a similar trend. At 1000 rpm, the dynamic viscosity was 59,589 Pa·s and shear stress was 35,578 mPa·s. At 2000 rpm, the values increased to 61,158 Pa·s and 38,873 mPa·s, respectively. At 3000 rpm, the maximum dynamic viscosity, 71,507 Pa·s, and a shear stress of 43,531 mPa·s were recorded. A further increase to 4000 rpm resulted in a decrease in viscosity to 64,237 Pa·s and shear stress to 40,031 mPa·s. The dynamic viscosity of the meat–bone paste was evaluated across all grinding conditions. The typical viscosity range required for sausage emulsions is between 40,000 and 80,000 Pa·s, which ensures proper emulsion stability, texture formation, and ease of stuffing during processing [20].

The results demonstrate that as rotor speed increased from 1000 to 3000 rpm, both dynamic viscosity and shear stress consistently increased for both working tool types. This behavior is consistent with the general physical law that during the transition from coarse to fine grinding of meat and meat–bone raw materials, the rheological properties change markedly: the viscosity of the suspension and the yield strength increase [21]. This phenomenon is attributed to the reduction in particle size, leading to an increase in the total specific surface area and the formation of a denser network structure through intensified protein–fat and water-bonding interactions [22].

The change in rheological parameters depends on several factors. A finer particle size (higher dispersity) results in increased viscosity and shear resistance due to the stronger interparticle interactions within the pseudoplastic system [23]. The design characteristics of the grinding equipment, specifically the geometry of and the clearance between the rotor and stator, as well as the shape and density of the cutting teeth, critically influence the degree of grinding. The experimental working tool, with a tooth angle of 20° and a finer pitch of 0.5 mm, demonstrated more effective grinding and, hence, more pronounced rheological behavior compared to the control tool. Additionally, increasing rotor speed amplifies both stall and centrifugal forces, thereby promoting further particle size reduction and consequently enhancing viscosity and shear stress.

Moreover, the moisture content and the state of bound water within the paste affect the rheological behavior. Higher moisture levels enhance the particle–water interactions, contributing to a denser structural network and shifting the stress–shear rate curve toward higher values [24,25].

Fine grinding of meat-and-bone material not only reduces the particle size, but also systematically increases the dynamic viscosity and yield strength of the paste. These parameters directly correlate with the dispersity of the ground material, which is governed by the design and operational conditions of the grinder as well as by technological factors such as humidity and medium temperature. Among the studied conditions, a rotor speed of 3000 rpm provided optimal results, yielding a paste with favorable flow characteristics—moderately high viscosity ensuring elasticity and good cohesiveness without over-thickening. The experimental working tool proved superior in achieving fine and uniform grinding, contributing to the elastic, homogeneous consistency of the paste, which is particularly important for applications requiring stable emulsification and textural properties in processed meat products.

Study of Water-Binding Capacity (WBC)

The investigation of the effect of two types of working assemblies on the water-binding capacity (WBC) of meat-and-bone paste demonstrated that increasing the rotor speed from 1000 to 3000 rpm led to an increase in WBC, whereas further acceleration to 4000 rpm resulted in a decrease. For the control sample, the maximum WBC (63%) was achieved at 3000 rpm, after which it declined to 59%. In the experimental configuration, an even higher peak WBC of 68% was recorded at 3000 rpm, but at 4000 rpm, the value again decreased to 64% (Figure 8).

An increase in WBC in the range of 1000–3000 rpm indicates a more favorable structure of the ground paste, which better retains water. This effect is likely due to the fact that moderately elevated rotor speeds (around 3000 rpm) provide uniform and sufficiently fine grinding of bone and protein components without causing excessive overheating, thereby promoting optimal protein conformation and effective emulsification of the fat phase [26]. In contrast, at 4000 rpm, a partial deterioration in WBC (decline to 59–64%) is observed, which may indicate early signs of protein denaturation as well as undesirable structural and textural changes caused by excessive shear and localized heating.

Fine grinding of meat raw material enhances its water-holding capacity by increasing the specific surface area of the particles, exposing hydrophilic groups of proteins and collagen, and promoting the formation of a denser, gel-like structure composed of finely dispersed fragments, within whose network water is effectively entrapped [27]. From the perspective of meat processing, elevated water-holding capacity values are highly advantageous, as the addition of such meat-and-bone paste to sausage products enhances moisture retention in the meat batter, resulting in reduced cooking losses, improved juiciness of the final product, and a denser structure with minimal release of broth–fat emulsions [28]. For this reason, WBC values above 60% are generally considered favorable for emulsified or partially emulsified products (such as cooked and cooked-smoked sausages, frankfurters, etc.) [29].

Thus, the maximum water-holding capacity (63–68%) is achieved at a rotor speed of approximately 3000 rpm, with the experimental knife configuration showing a more pronounced peak (68%), further confirming the suitability of this geometry for enhancing the functional properties of the resulting paste. At 4000 rpm, the observed decline in WBC reflects undesirable “over-grinding” and structural degradation, negatively affecting the further use of the paste in sausage production.

Elevated temperatures during grinding can indeed reduce WBC, primarily by inducing partial denaturation of muscle proteins. This denaturation disrupts the native protein structure, causing aggregation and a reduction in available hydrophilic binding sites for water [30,31]. As a result, the protein matrix’s ability to retain water through capillary forces and chemical interactions diminishes. Additionally, excessive heating can compromise the gel-forming ability of myofibrillar proteins, further decreasing the paste’s capacity to hold water. Thus, temperature control during colloid milling is essential to preserve the functional properties of the resulting meat–bone paste.

4. Conclusions

The conducted experiments demonstrate that the modernized working assembly (rotor and stator with a tooth inclination angle of 20° and a reduced pitch of 0.5 mm) provides significantly higher grinding efficiency for bone components in poultry meat-and-bone material compared to the original design (45° inclination, 1.5 mm pitch). At the optimal rotor speed (approximately 3000 rpm), maximum productivity (mass and volumetric flow rate) was achieved, along with a shortened processing time. The granulometric composition of the paste showed a clear predominance of fine bone particle fractions (less than 0.5 mm), ensuring the absence of large fragments and improving the organoleptic properties of the final product. Under these conditions, the paste temperature remained within a range that does not induce protein denaturation or undesirable structural changes, resulting in more favorable rheological behavior. The increased water-holding capacity (WHC) indicates excellent moisture retention and, potentially, a more stable emulsion structure, which are critical factors for the production of sausages and similar products. Thus, the implementation of the modernized working assembly significantly enhances the quality and technological value of the meat-and-bone paste while maintaining an efficient and economically viable operating regime for the equipment. In future applications, using the modified knife assembly may improve process efficiency and enable greater use of meat by-products in value-added foods. Further research on scale-up, tool durability, and the economic feasibility of industrial adoption is planned.