Effect of Pressure Regulating Mechanism on Screw Pressing Efficiency and Production Capacity in Sea Buckthorn (Hippophae rhamnoides) Juice Extraction

Abstract

This study addresses the optimization of juice extraction from sea buckthorn (Hippophae rhamnoides) to meet the growing demand for healthy, natural food products. An upgraded screw press with a self-regulating pressure adjustment mechanism was developed and evaluated. Four spring types (wire diameters, 1–6 mm; maximum forces, 47.5–800 N) and screw rotation speeds ranging from 140 to 200 rpm were investigated for their effects on juice yield, compression density, internal pressure, and power consumption using sea buckthorn fruits. The results indicated that springs with higher maximum forces yielded greater juice outputs. The highest juice yield of 47% was achieved using a spring with a 3 mm wire diameter and 630 N maximum force (Spring 4) at 185 rpm. This configuration also demonstrated an optimal balance between compression density (948 kg/m³) and internal pressure (320 Pa) while maintaining the lowest power consumption (142 W). The internal pressure within the pressing chamber increased with both spring force and rotation speed. While Spring 2 generated the highest internal pressures (up to 570 Pa at 200 rpm), Spring 4 achieved moderate internal pressures, suggesting an effective pressure transmission. Spring 4 exhibited the lowest power consumption despite the high juice yields and compression densities. The study concludes that utilizing Spring 4 at 185 rpm optimizes the juice extraction efficiency while minimizing energy use. Equipment designs allowing the fine-tuning of pressure and rotation speed can significantly enhance the production efficiency in sea buckthorn juice extraction and potentially in other fruit-juice-processing applications.

1. Introduction

Currently, much attention is paid to the development of functional foods for healthy eating. The main task of functional food is to provide a positive physical effect on the human body and thereby strengthen its health. One of the priority directives, solving the nutrition problems of the population, is the use of local wild raw materials. Natural compounds of plant origins have a very active effect on the enzyme systems of detoxification of the body, contribute to the neutralization and excretion of many toxicants of an endo- and exogenous nature, and contribute to the normalization of the internal environment of the body and increasing the efficiency of its adaptive mechanisms [1,2,3]. One such type of biologically valuable raw material available in the Republic of Kazakhstan is sea buckthorn.

Sea buckthorn (Hippophae rhamnoides) holds a special place among berry crops due to its chemical composition. The presence of water-soluble and fat-soluble biologically active substances significantly affects the functional properties of the finished product [4,5]. From sea buckthorn juice, a nutritious beverage with a high content of suspended solids and a very high content of vitamin C and carotenes is obtained [6,7].

Different juice production methods, such as diffusion, extraction, and pressing, are employed to obtain this juice. The pressing method is one of the main and most common methods. During the pressing process, the cellular structure is disrupted, which varies from fruit to fruit. As a result, the processing costs, processing time, and production costs are reduced. To study the process of pressing (screw pressing) in the production of various food products, including juices, many scientists have conducted research on screw presses [8,9,10,11,12,13,14].

The scientific literature provides limited data on the performance of screw presses for processing sea buckthorn and even less information on designs featuring a working body with a self-regulating pressure mechanism for uniform pressure distribution during raw material pressing. This gap leads to significant power losses due to low juice yields and the inability to apply optimal pressure for the effective separation of the liquid fraction.

Considering this, there is a need for a universal press that is suitable for juice extraction under low-power production conditions to accelerate the juice separation process from wild sea buckthorn without using chemical methods. Utilizing the improved experimental press equipment eliminates the necessity for manual effort, intensifies the juicing process, and increases the efficiency of the equipment.

The purpose of this research is to enhance the efficiency of juice extraction from sea buckthorn (Hippophae rhamnoides) fruits by developing and evaluating an upgraded screw press equipped with a self-regulating pressure adjustment mechanism.

2. Materials and Methods

2.1. Sampling

In this study, the wild sea buckthorn—growing in the East Kazakhstan region of the Republic of Kazakhstan in Zaisan, Sauyr-Tarbagatai, in foothills near small lakes and in large forest areas—was used (Figure 1) [15].

Sea buckthorn was purchased at an agricultural fair in Semey, Kazakhstan, from gardeners and farmers living in those areas.

2.2. Description of the Experimental Screw Press

The experimental screw press for juicing vegetable raw material is equipped with the following main parts: stand, mesh nozzle for extracting compressed juice, cone press screw located inside the stand (Figure 2), spring, nut, sliding tips, and electric motor. It is also equipped with a self-regulating pressure mechanism. The press screw and mesh nozzle are easily removable from the rack (Figure 3 and Figure 4).

The self-regulating pressure control mechanism in the working area on the juicing equipment provides a continuous change in the gap between the cone pressing screw and the mesh nozzle for releasing the compressed juice.

The pressure regulating mechanism consists of a spring, a lock nut, a check nut and two sliding lugs and washers, and a rubber sealing ring. Four types of springs with different characteristics were used in the experimental studies (Figure 5).

The screw press operates as follows. Raw material from the loading hopper is delivered into the pressing zone, where it is propelled toward the discharge outlet by the conical pressing screw. The weight of the loaded product is 3 kg, and the amount of juice in it is 55%; the rest consists of seed pits and pureed mixtures. The pressure required for pressing the raw material is generated by the conical pressing screw in conjunction with the conical screen along the direction of product movement. The juice is extracted through the openings of the conical screen. The pressure for juice extraction is regulated by a pressure adjustment mechanism. When the pressure in the working zone of the screw press increases, the tail end of the conical pressing screw exerts force on the spring of the pressure adjustment mechanism, compressing it. This action causes the screw to shift leftward (toward the pressure adjustment mechanism), increasing the gap between the conical screw and the conical juice-extracting screen and thereby reducing the pressure in the working zone of the press. This adjustment facilitates uniform juice extraction and allows the spent pulp to move freely toward the discharge outlet. When the pressure in the working zone decreases, the spring returns the screw to its original position (Figure 6).

Therefore, the design of the screw press enables increased productivity and reduced energy consumption while also enhancing the reliability and technological efficiency of the equipment.

2.3. Determination of Juice Yield During the Pressing Process

The juice yield is characterized by the juice release, i.e., the amount of liquid phase released during pressing. The juice yield is determined by Formula (1):

C = (A − B)/A × 100%.

(1)

where

• C–juice yield (%);
• A–mass of the product before pressing, kg;
• B–mass of extract after pressing, kg.

2.4. Determination of Vitamins by Method of Capillary Electrophoresis

Determination of the content of sea buckthorn extract was carried out on electrophoresis by Lumex “Kapel-105m” (St. Petersburg, Russia). The method of capillary electrophoresis is based on the separation of charged components in a complex mixture under the action of an additional electric field in a quartz capillary. The microsize of the analyzed solution ~2 kl is preliminarily introduced into a quartz capillary filled with the appropriate buffer, electrolyte. After applying a high voltage to the ends of the capillary, the components of the mixture, up to 30 kV, begin to move at different speeds, depending primarily on the charge and mass (depending on the value of the actual radius of ions) or, respectively, to reach the detection zone at different times. The sequence of peaks obtained is called an electrophoregram. The qualitative characteristic of the substances is the migration time, while the quantitative characteristic includes the peak height or peak area, which is proportional to the concentration of the substances.

The analysis is as follows: full length of the capillary, 60 cm; effective length (length from the entrance to the detector windows), 50 cm; working voltage applied to the electrode, +13 v; internal diameter of the capillary, 75 μm; detection, 254 nm; indirect temperature, 20 °C; sample introduction pressure, 300 mbar·s; composition of the working buffer, 10 mm benzimidazole, 5 mM tartaric acid, and 2 mM 18-crown-6. The sample preparation is based on the hydrolysis of the sample when mixed with the buffer solution [16].

2.5. Determination of Compression Density of Sea Buckthorn

The experiment to determine the compression density of sea buckthorn was carried out on hydrostatic scales. The experimental studies on density determination were carried out using the method described in [17].

2.6. Determination of Energy Characteristics and Pressure of Experimental Press Equipment

A measuring bench was used to determine the energy characteristics of the experimental press equipment. Measuring devices consisted of a voltmeter, ammeter, and phase meter (device for measuring “cos φ”). All instruments were connected to the electrical circuit control system and to the electric motor of the experimental press equipment for sea buckthorn juice pressing. The sea buckthorn was loaded into the pressing equipment to measure current, voltage, and cos φ. Then, the electric motor was started. Next, the corresponding values of the electrical quantities indicated in the instruments were recorded. After that, the measurement results were processed by statistical methods [18].

The pressure inside the screw press was measured using a PASCO Xplorer GLX data logger (PASCO Scientific, Roseville, CA, USA) and a compatible PASCO pressure sensor (PASCO Scientific, Roseville, CA, USA). A pressure port was installed near the pressing zone and fitted with a stainless-steel fitting and membrane to protect the sensor. The sensor was connected directly to the GLX or via a PASCO analog adapter (PASCO Scientific, Roseville, CA, USA). The calibration was performed by applying known pressures and recording corresponding voltages to establish a calibration curve. The data collection parameters (sampling rate, units) were set on the GLX before operation. During pressing, real-time pressure readings were recorded and analyzed to optimize process parameters.

2.7. Statistics

Measurements were repeated three times, and the results were summarized as the mean value together with its respective standard error. The statistical analysis was carried out using Excel 2021 (Microsoft Office, Redmond, WA, USA). To determine if there were significant differences between samples, a one-way analysis of variance (ANOVA) was employed. The results were considered statistically significant if the p-value was lower than 0.05.

3. Results and Discussion

3.1. Physico-Chemical Properties of Sea Buckthorn

The physico-chemical properties of sea buckthorn vary significantly due to growth in different areas as well as ecological and geographical conditions. According to the scientific research, the main physical and chemical parameters in 3 different samples of sea buckthorn fruits collected in the East Kazakhstan region were determined. The fleshiness of fruits was 85% on average, peel—8.73%, and seeds—6.27%. The total lipid content in the crude mass was 6.8% for flesh, 8.5% for peel, and 6.1% for seeds. The nutritive value of sea buckthorn fruits averaged 5.4 g of lipids, 5.7 g of carbohydrates, 2 g of dietary fiber, 2 g of organic acids, 0.7 g of ash, and 84.2 g of water. The energy value of sea buckthorn fruit was 76.2 kCal.

Sea buckthorn is distinguished by its rich vitamin profile, which contributes to its nutritional and therapeutic significance [19]. Comparing the vitamin composition of sea buckthorn juice with the Recommended Daily Intake (RDI) reveals that sea buckthorn is exceptionally rich in vitamin C and vitamin B9 (folate) (

Table 1. The vitamin content of sea buckthorn.

Vitamin	Content, mg/L	Recommended Daily Intake, mg/day [22]
B1	0.33 ± 0.01	1.5
B2	0.50 ± 0.01	1.8
B6	0.76 ± 0.02	2.0
C	1540.54 ± 30.11	90
B3	0.26 ± 0.00	20
B9	8.60 ± 0.13	0.4

). A 100 mL serving of sea buckthorn juice contains approximately 154 mg of vitamin C, which is about 171% of the RDI of 90 mg, thus exceeding the daily requirement for this vitamin. It also provides around 860 µg (0.86 mg) of vitamin B9 per 100 mL, equating to 215% of the RDI of 400 µg. However, the levels of other B vitamins are comparatively low: vitamin B1 at 0.033 mg (approximately 2.75% of its 1.2 mg RDI), vitamin B2 at 0.050 mg (3.85% of 1.3 mg RDI), vitamin B3 at 0.026 mg (0.16% of 16 mg RDI), and vitamin B6 at 0.076 mg (5.85% of 1.3 mg RDI) per 100 mL serving. Therefore, while sea buckthorn juice significantly surpasses the daily requirements for vitamin C and folate, it contributes minimally to the intake of other B vitamins. This composition indicates that sea buckthorn provides a substantial amount of essential vitamins, particularly vitamin C and several B vitamins, which are crucial for various metabolic processes and overall health maintenance [20,21]. The high levels of these vitamins make sea buckthorn a valuable ingredient for nutritional and functional food products.

Sea buckthorn demonstrates a considerable antioxidant capacity and is rich in bioactive compounds. The antioxidant activity, assessed using the DPPH radical scavenging assay, was found to be 526 mg/g in methanol extracts and 363 mg/g in water extracts, indicating a higher efficacy of methanol in extracting antioxidant constituents. The total phenolic content (TPC) was measured at 117 mg/g in methanol extracts and 133 mg/g in water extracts, suggesting that water extracts may contain more phenolic compounds than methanol extracts (

Table 2. Antioxidant capacity of sea buckthorn.

DPPH (mg/g)		Total Phenolic Content (TPC, mg/g)	β-Carotene (mg/100 g)
Methanol	526 ± 6	117 ± 2	48.9 ± 0.8
Water	363 ± 5	133 ± 2

). Additionally, the β-carotene content was determined to be 48.9 mg per 100 g of sea buckthorn, highlighting its potential as a significant source of this provitamin A carotenoid. These findings underscore the nutritional and functional value of sea buckthorn, making it a promising candidate for inclusion in health-promoting food products and nutraceutical applications.

3.2. Study of Juice Yield During Pressing

Generating the required pressure in the press compartment is directly related to the pressure control mechanism—the spring. The pressing screw moves forward and backward through a self-regulating pressure control mechanism involving a spring-loaded system, not by reversing the motor’s rotational direction. As the raw material accumulates and pressure increases in the pressing zone, the tail end of the conical pressing screw compresses the spring in the pressure adjustment mechanism. This compression allows the screw to shift axially backward (toward the pressure adjustment mechanism), increasing the gap between the screw and the mesh nozzle, which reduces the pressure. When the pressure decreases, the spring expands, pushing the screw forward to decrease the gap and increase the pressure. This automatic adjustment reduces energy consumption by preventing the motor from exerting unnecessary force against high resistance. By maintaining optimal pressure levels without manual intervention or motor strain, the system enhances the energy efficiency and reduces mechanical wear, thereby decreasing the overall energy consumed during screw press operation.

When investigating the process associated with different diaphragm holes, the minimum amount of juiciness index of sea buckthorn product in the diaphragm hole was found to be δ = 6 mm. When the diaphragmal hole is further reduced, the secreted juice and other parts of the extract side by side come out together in the mesh nozzle and cause clogging of the mesh nozzle. Therefore, the matched size of diaphragm holes, δ = 6 mm, was selected.

The data indicate that springs with higher maximum deformation forces generally yielded higher juice outputs (Figure 7). Specifically, Spring 4 consistently produced the highest juice yields across all rotation speeds, reaching up to 47% at 185 rpm. This suggests that a spring force of around 630 N is optimal for the mechanical pressing of sea buckthorn fruits. In contrast, Spring 3, with the lowest force of 47.5 N, resulted in the lowest juice yields, averaging around 35–41%. This underlines the importance of sufficient pressure in the pressing zone to break down fruit tissues effectively and release the juice. The rotation speed of the screw was varied from 140 rpm to 200 rpm. An increase in rotation speed is generally correlated with an increase in juice yield up to a certain point. For instance, with Spring 4, the juice yield increased from 41% at 140 rpm to 47% at 185 rpm (p < 0.05). However, at 200 rpm, a slight decline to 46% was observed.

This trend suggests that while higher rotation speeds enhance the shear forces and pressure exerted on the fruits, excessively high speeds may reduce the residence time of the fruit in the pressing zone, leading to an incomplete juice extraction. Therefore, an optimal rotation speed exists—identified here as 185 rpm—that balances the mechanical force and processing time. These results underscore the importance of fine-tuning the mechanical parameters to enhance the processing efficiency in food-engineering applications.

3.3. Determination of Compression Density of Sea Buckthorn During Pressing

The degree of compression is characterized by the ratio of pressing weight, product density, and modulus of elasticity when pressing dry materials [23]. In this research, the determination of the compression ratio is complicated by the fact that a liquid phase is extracted from sea buckthorn. When the liquid phase is separated from sea buckthorn, there is a change in volume, mass, and phase composition of the raw material. Consequently, the change in compression density of sea buckthorn is directly related to its moisture content. Compression density is a critical parameter that reflects the degree to which the fruit mass is compacted in the pressing chamber, directly influencing the juice extraction efficiency. Understanding how the compression density varies with mechanical adjustments provides deeper insight into the pressing process and aids in optimizing the equipment settings for maximum yield [24,25].

The springs with higher maximum forces (Springs 2 and 4) consistently produced higher compression densities across all rotation speeds. Spring 4, despite having a slightly lower maximum force than Spring 2, achieved the highest compression densities, reaching up to 951 kg/m³ at 200 rpm. The superior performance of Spring 4 suggests that factors beyond maximum force, such as spring stiffness and material properties, play significant roles. The design of Spring 4 may allow for more effective pressure transmission and better adaptation to the pressing mechanics of sea buckthorn fruits (Figure 8).

3.4. Determining the Pressure During Pressing

The pressure within the pressing chamber affects the degree of cell rupture in the fruit, thereby impacting the juice yield and extraction efficiency [26]. The data on the pressure inside the pressing chamber provide crucial insights into the mechanical dynamics of the sea buckthorn juice extraction process.

The springs with higher maximum forces—Spring 2 (800 N) and Spring 1 (750 N)—consistently generate higher internal pressures across all rotation speeds. Spring 2 produces the highest pressures, reaching up to 570 Pa at 200 rpm, which correlates with its higher maximum force compared to Spring 1. Spring 3, with the lowest maximum force (47.5 N), results in significantly lower internal pressures, highlighting its limited effectiveness in creating a sufficient mechanical force for optimal juice extraction. Spring 4 (630 N), while having a lower maximum force than Spring 1, still achieves moderate internal pressures, suggesting that its design and material properties contribute to its performance (Figure 9).

As the screw rotation speed increases from 140 rpm to 200 rpm, the internal pressure within the pressing chamber rises for all spring types (p < 0.05). This increase is more pronounced with springs that have higher maximum forces. The positive correlation between the rotation speed and internal pressure is attributed to the enhanced mechanical action and shear forces exerted on the fruit mass at higher speeds, intensifying the compaction and pressure within the chamber.

3.5. Determination of Power in the Pressing Process

The data on power expended during the pressing process provide crucial insights into the energy efficiency of the sea buckthorn juice extraction system under various configurations. As the rotation speed increases from 140 rpm to 200 rpm, the power required for pressing correspondingly rises across all spring types. Notably, Spring 1 consistently demands the highest power expenditure at each speed, starting from 158 watts at 140 rpm and reaching up to 176 watts at 200 rpm (p < 0.05). This is likely due to Spring 1’s higher force exertion within the pressing chamber, which increases resistance and, consequently, the energy required to overcome it. Conversely, Spring 4 consistently exhibits the lowest power consumption, ranging from 132 watts at 140 rpm to 150 watts at 200 rpm (p < 0.05) despite its ability to produce high juice yields and compression densities (Figure 10).

The power consumption for each spring type shows a gradual increase as rotation speed rises, with the most significant jump occurring between 185 rpm and 200 rpm. When evaluating the influence of different springs on screw press operation, the key factor is achieving an optimal balance between the pressing force and mechanical resistance within the pressing chamber. Spring 4, despite having a lower spring force (630 N) than Springs 1 and 2, provides a more stable and appropriately calibrated pressure on the raw material. This balanced pressure ensures that the sea buckthorn berries are efficiently compressed to release juice without excessively increasing frictional resistance. As a result, the motor encounters less opposing torque, and the power consumption decreases. If the motor faces less opposing torque, it does not need to apply as much force to overcome resistance [27].

Under these conditions, Spring 4 maintains a consistently high juice extraction rate because it neither under-presses (which would reduce yield) nor over-presses (which would elevate energy demand), thus achieving the highest juice yield at the lowest energy expenditure. In contrast, Spring 3 has the smallest spring force (47.5 N) and a much lighter construction. While one might assume this would save energy by minimizing resistance, it fails to maintain the required pressure threshold for efficient juice extraction. The insufficient pressing force leads to suboptimal contact between the screw and the product. Consequently, the material does not undergo uniform compression, and the motor’s efficiency decreases due to irregular material flow and potential slippage. Although it requires less force initially, this lack of proper compaction and consistent mechanical loading actually leads to higher relative energy costs compared to the well-matched force provided by Spring 4. This integrated analysis underscores the necessity of considering the energy efficiency alongside the juice yield and compression density when refining pressing techniques in food engineering, ultimately contributing to more sustainable and cost-effective industrial practices.

The experimental screw press we developed, incorporating various spring types and a self-regulating pressure mechanism, differs from other presses not only in its design approach but also in its methods for optimizing the juice extraction process. For example, ref. [28] focuses on adjusting the screw flight geometry to minimize raw material rotation and enhance productivity. Our press builds on this concept by using an elastic element (spring) for dynamic pressure stabilization, automatically adjusting the gap and thus reducing energy consumption. The work in [29] employs a screw jack for pressing, simplifying the construction. In contrast, our design maintains a consistently optimal pressing mode through the spring’s elasticity. The work in [30] targets improvements in feeding screw parameters to boost productivity when processing whole fruits by altering the flight pitch. The belt-crushing double-screw squeezer [31] integrates fruit crushing and juice extraction into a single system, employing two screws and a pneumatic adjusting device to regulate the outlet clearance and thus fine-tune the juice yield. This design emphasizes efficiency, reduced manual handling, and adaptability to varying raw materials. While the squeezer integrates crushing and pressing into a single system, thereby increasing the efficiency and throughput, it introduces added mechanical complexity and depends on external pneumatic systems. The study by Hebbar et al., (2008) focuses on a single-screw auger-based extractor. Their research evaluates performance metrics such as juice yield, extraction efficiency, and losses for peeled and unpeeled fruits [32].

3.6. Mathematical Description of Pressure Self-Regulation Mechanism for Press Equipment

Mathematical modeling of the pressure self-regulation mechanism in a screw press provides a tool for optimizing spring characteristics and operational parameters. By converting forces and deformation into equations, engineers can predict performance, determine the ideal spring stiffness, and minimize energy consumption. It also enables adjusting of the operating speed, press gap, and other variables to maintain optimal pressure for different materials. This systematic approach replaces trial-and-error with data-driven decisions, saving time, improving efficiency, and extending equipment life. Ultimately, it ensures a stable, energy-efficient operation while maintaining the desired product quality and throughput [33].

In the mechanism of pressure self-regulation, we use helical cylindrical compression springs of class III of the 2nd grade made of steel of round cross-sections. The main parameters of the spring are accepted in accordance with the interstate standard GOST 13775-86 [34]. However, some parameters of pressure regulation require a coordinated calculation due to the specifics of the mechanism operation. The following forces are applied during pressing: Fp—pressure force, N; Fs—shear force, N; and Fe—elastic force, N (Figure 11).

The pressure force, F_p, is the force generated by the pressure value during pressing. Consequently, its trajectory direction will be oriented in the direction of the product and if, for solids, we define it by a simple equation, as shown below:

$$F_P=P\cdot S,\mathrm{N}$$

(2)

where

• P—pressing pressure, Pa;
• S—area affected by the force, m³.

Let us note that the area of the part of the body affected by the surface is equal to the area of the ring in our case:

$$\mathsf{\Delta }S=\pi \left(\mathsf{\Delta }{R}^2-\mathsf{\Delta }{r}^2\right),{\mathrm{m}}^2$$

(3)

Taking into account the total pressure on the pressing surface of the screw coils, we determine the pressure force by the following Equation (4):

$$\begin{array}{rr}& F_P=\rho gQ\cdot \left[{\displaystyle \frac{h\epsilon }{D\omega k^{\mathrm{’}}(1+\epsilon )}}+{\displaystyle \frac{\mu cL}{\xi d_E^{3}f}}\left({\displaystyle \frac{2n+1}{2n}}\right)\right]+{\displaystyle \frac{a}{k^{\mathrm{’}}-\left(k^{\mathrm{’}}-\frac{a}{q_B}\right)\mathrm{e}\mathrm{x}\mathrm{p}\left(\beta z\right)}}\\ & \end{array}\cdot \mathsf{\Delta }S, \mathrm{N}$$

(4)

The back-shear force, F_S, is the back-shear force along the product screw channel caused by exceeding the pressing pressure during the pressing process. Hence, its trajectory direction is in the direction opposite to the pressure force. We determine the backward shear force based on Archimedes’ force law:

$$F_S={\rho }_{\theta }\cdot g\cdot V_{\theta },\mathrm{N}$$

(5)

where

• ρ_θ—density of the sample, kg/m³;
• V_θ—volume of sample in the pressing channel, m³.

For the elastic force, F_E, in our case we are referring to the spring back force due to the backward shear force. Consequently, its trajectory direction is directed by the pressure force opposite to the reverse shear force (Figure 12). We determine the elastic force based on Hooke’s law:

$$F_E=k\cdot \mathsf{\Delta }l,\mathrm{N}$$

(6)

where

• k—spring stiffness, N/m;
• $\mathsf{\Delta }l$—distance at absolute spring deformation, m.

Figure 12. Scheme of forces of the pressure self-regulation mechanism. 1—pressing screw; 2—serrated nozzle; ${\delta }_i$—distance between gape nozzle and screw coil, m; $L_i$—distance of displacement of the pressure screw in the longitudinal axis, m; $l_i$—distance at absolute spring deformation, m, F_P—pressure force, N; F_S—shear force, N; F_E—elastic force, N; P—pressing pressure, Pa.

Figure 12. Scheme of forces of the pressure self-regulation mechanism. 1—pressing screw; 2—serrated nozzle; ${\delta }_i$—distance between gape nozzle and screw coil, m; $L_i$—distance of displacement of the pressure screw in the longitudinal axis, m; $l_i$—distance at absolute spring deformation, m, F_P—pressure force, N; F_S—shear force, N; F_E—elastic force, N; P—pressing pressure, Pa.

The absolute deformation of the spring can be characterized by both the length of the press screw channel along the longitudinal axis and the change in the gap between the gape nozzle and the screw coil.

Let us determine the forces acting on any point of the pressure self-regulation mechanism as a function of the variation of the gap between the gape nozzle and the screw coil depending on the operation of the press equipment.

Proceeding from Equation (6), the following equations are obtained:

$$F_E=k\cdot {\delta }_0, F_1=k\cdot {\delta }_1$$

(7)

It follows that if we write down the following expressions, then,

$${\displaystyle \frac{F_E}{F_1}}={\displaystyle \frac{k}{k}}={\displaystyle \frac{{\delta }_0}{{\delta }_1}}$$

(8)

Taking into account the invariance of the spring stiffness, i.e.,

$${\displaystyle \frac{F_E}{F_1}}={\displaystyle \frac{{\delta }_0}{{\delta }_1}}$$

(9)

Consequently, let us determine the force on which the pressure self-regulation mechanism acts as a function of the variation of the gap between the gape nozzle and the screw coil:

$$F_1={\displaystyle \frac{F_E\cdot {\delta }_0}{{\delta }_1}},N$$

(10)

Using such equal relationships, the forces acting on any point of the pressure self-regulation mechanism can be determined, even through the distance $L_i$ of displacement of the pressure screw along the longitudinal axis at the absolute spring deformation.

Let us describe the change in the density and volume of the product in the channel of the pressing screw due to the change in the action of displacement forces applied to the spring of the pressure control mechanism. If, based on the Equation (5), we write down the displacement forces in two cases associated with the movement of the press screw,

$$\left.\begin{array}{c}{F}_1={\rho }_1\cdot g\cdot V_1\\ F_2={\rho }_2\cdot g\cdot V_2\end{array}\right\}),\mathrm{N}$$

(11)

If this is the case, it can be seen from the schematic in Figure 2 that the total displacement force is equal to the sum of these forces:

$$\begin{gathered} F_3=F_1+F_2, \\ {\rho }_1\cdot V_1={\rho }_2\cdot V_2 \end{gathered}$$

On this basis, let us find the volumetric change, $V_2:$

$$V_2={\displaystyle \frac{{\rho }_1\cdot V_1}{{\rho }_2}},$$

(12)

Given that the total volume is equal to the sum of the first volume and the second volume, we obtain the following equations:

$$V_3=V_1+{\displaystyle \frac{{\rho }_1\cdot V_1}{{\rho }_2}}\Rightarrow V_3=V_1\left(1+{\displaystyle \frac{{\rho }_1}{{\rho }_2}}\right)$$

(13)

It follows that the volume change in the screw ground coil would be as follows:

$$V_3=V_1\cdot {\displaystyle \frac{{\rho }_1\cdot {\rho }_2}{{\rho }_2}}$$

(14)

Hence, we determine the volumetric change $V_1$:

$$V_1={\displaystyle \frac{{\rho }_1\cdot V_3}{\left({\rho }_1+{\rho }_2\right)}},$$

(15)

Using Equations (1)–(4) respectively, it is possible to determine the density of the product in the channel of the pressing screw due to the change in the action of the displacement forces applied to the spring of the pressure regulating mechanism.

We can write the value of the forces in the pressing operation by using the following conditions:

$$F_P\ge F_S, F_S=F_E$$

(16)

For a better pressing process, the pressure force, F_P, must be greater than or equal to the shear force, F_S. If this condition is not met, the pressure required for the pressing process will not be enough. The shear force, F_S, and the counter-elastic force, F_E, are then equal to each other.

Because the force with which the spring is pressed, the spring force is counteracted again with the same force. Consequently, we must select a spring that can maintain the pressure force required for the optimum pressing pressure. In case the pressure force exceeds the optimum pressure limit required for pressing, it should be released by shear force. In other words, the presence of such a mechanism in the pressing equipment creates the conditions for the equipment to operate at an efficient capacity and to maintain the pressure required for the process at all times. In addition, it will avoid power losses and extend the service life of the equipment.

4. Conclusions

This research successfully demonstrates that the efficiency of juice extraction from sea buckthorn (Hippophae rhamnoides) can be significantly enhanced through the mechanical optimization of a screw press equipped with a self-regulating pressure adjustment mechanism. The research also demonstrates that the optimal juice extraction is achieved through a delicate balance of spring characteristics, screw rotation speed, compression density, and internal pressure, rather than by maximizing any single parameter. A spring with a 3 mm wire diameter and 630 N maximum force provided an optimal balance, delivering high performance with moderate internal pressure and reduced energy requirements. This configuration achieved the highest juice yield of 47% and a maximum compression density of 951 kg/m³, while maintaining lower power consumption compared to other springs tested. In future studies, exploring the scalability of the upgraded screw press in industrial settings and its applicability to other types of fruit or vegetable raw materials could extend the benefits of this research. This involves evaluating its long-term reliability, energy consumption, and overall throughput under continuous operation.