Food Processing with UHP Waterjets

Abstract

The use of UHP for food processing includes many applications such as cutting, peeling, pasteurization, and pumping through the orifice to affect food rheology. This paper focuses on food cutting applications using UHP waterjets. State-of-the-art food cutting systems are described including pumps, manipulators, sensors, cutting heads, and software. While UHP technology is commercially available at 621 MPa of pressure, most food cutting systems’ pressure is below 400 MPa. Highly focused waterjets are important for efficient slicing of food and thus diamond orifices with sharp entry edges are used in specially designed cutting using fast acting on/off valves. Automation is at an advanced level for fish, pin bone removal, poultry, meat, and vegetable processing systems where upstream sensor data are used with CNC controllers to determine the paths of the cutting jet(s) at relatively high production rates for portioning or trimming to tight specifications. Harvesting lettuce proved to be highly successful in improving the overall productivity and working environment ergonomics. An important advantage of the waterjet in increasing the shelf life of trimmed food is presented. For example, celery and lettuce shelf life increases by days over mechanical cutting. The use of salt as an abrasive material in abrasive waterjet cutting nozzles was found to be impractical for cutting meat with bone and more work is needed in this area. Bakery, cake, and sandwich cutting applications are utilized in actual plants in the USA and Europe. For example, small envelop cake cutting machines using relatively low-power jets are used for cutting cake into different shapes.

1. Introduction

A processed food is any food that has been altered in some way during preparation. Food processing can be as basic as freezing, canning, portioning, sterilizing, etc. What is relevant to this paper is the use of ultrahigh pressure (UHP) waterjet technology for food processing, although the use of UHP technology is not limited to cutting. The following is a list of current UHP technology applications in the food industry:

• Cutting: In this case, high-velocity waterjets, generated by pressures up to 621 MPa (90 Ksi), are used for the following:

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  Portioning: This is to slice through food product to produce food portions.

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  Excision: This is to cut undesirable food areas such as the fat off some meat cuts.

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  Trimming: This is to trim off some areas of the food such as the crowns of carrots.

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  Peeling: This is to remove the skin off some fruits or vegetables.

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  Stripping: This is to separate food from each other such as deboning operations.

  • This can be accomplished with different types of jets, not all of which have been commercially implemented.

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  Pure liquid jets: In this case, a liquid jet is used which is commonly water but sometimes an alternative liquid may be used (Ex. coconut oil is used for cutting some chocolates). This is the most commercially used waterjet tool.

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  Pure liquid fanjet: In this case, a fan jet is used either for peeling or stripping.

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  Abrasive waterjet: In this case, solid particles are entrained into the waterjet to increase its cutting ability as may be needed to cut through bones.

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  Air–Liquid Jets. Instead of abrasives, air is entrained into the waterjet to create a highly focused droplet jet.

Other jets such are cryogenic jets (using liquid nitrogen, for example) are possible to use, as stated by Dunsky and Hashish [1], but unlikely to be practical due to the cost and the special handling requirements.

• Pressurization: Processing food products by static pressurization is used to sterilize the food by killing harmful microorganisms and thus extending the shelf life and safety of the food.
• Pumping: In this case, the food, mainly liquids, may be pumped through the nozzle for the purpose of homogenization or other effects where high shear rates are required.

In this paper, we will focus on processing with UHP waterjets. Other papers to follow will address the areas of sterilization and pump-through processing.

2. Waterjet Characteristics

In this section, we list the advantages of waterjet for food cutting and address some of its technical characteristics that affect its food processing performance.

2.1. Advantages

There are many advantages, as stated by ref. [2], to using high-velocity waterjets for food cutting. Among these advantages are the following:

• The waterjet cutting edge is renewable and thus there is no cross contamination.
• Long-life waterjets have diamond orifices and thus there is no need to clean or sharpen blades or disks.
• The waterjet is flexible for cutting a wide range of food products.
• Omnidirectional cutting capability allows for easy shape cutting.
• A waterjet allows for a higher yield of produce as kerf widths are narrow.
• No heat or chemicals are used, only pure water.
• They have the ability to cut through multiple layers of food contents such as cady with nuts.
• The waterjet tool lends itself to automation and intelligent control.
• No metal detection equipment is needed such as that used when cutting with blades.
• High productivity with multiple jets on the same machine is possible.
• They have the ability to cut very soft food products without distortion.
• They have the ability to cut small portions of any shape.
• The waterjet tool can be used inside factories and in the field for harvesting.
• It is highly capable of cutting both soft and hard food items.
• It can easily be integrated into existing production lines, replacing conventional methods.
• The waterjet is approved by regulators such as the U.S. Food and Drug Administration.

Because of the above advantages, food cutting with waterjets was one of its early applications in the 1970s. Since then, the applications for food processing with waterjets have been significantly increasing and spreading worldwide. Before addressing some of the applications and systems used for cutting, we discuss the tools, i.e., the waterjet characteristics, that make them effective in the different applications.

2.2. Waterjet Coherency

An important characteristic of a water cutting jet is its coherency. Higher coherency increases the power density, and it has been observed that coherent jets are more effective in cutting a wide range of food products. Also, coherent jets can operate at longer stand-off distances, allowing more flexibility in food cutting applications. Figure 1 shows different waterjet coherency levels. The critical factors that affect the jet coherency (and its power density) include the following:

• Upstream tube diameter and length—The effects of the upstream tube diameter, d, and the level of turbulence above the orifice were studied. Results show that jet coherency improves when the tube size reaches a certain critical size so that the upstream Reynolds number is below a critical value. An upstream length of at least 20 tube diameters was found to also be important in producing coherent jets to allow for any turbulent eddies from the on/off valve passages to die out.
• Orifice edge geometry and condition—The upstream edge characteristic of an orifice is a most important factor that affects jet coherency. A sharp edge allows a vena contracta to form with maximum power density output. A fileted edge is not preferred as the water flow will attach to the edge, causing more friction and spreading. However, a minimal filet radius is needed to reduce sensitivity to small particles in the water. Water filters are typically used in upstream water pumps to increase the lifetime of the pump seals and the orifice. Sometimes inline filters are used inside the UHP water body above the orifice to catch any debris in the lines.

2.3. Waterjet Power Density

The power density of the waterjet is defined as the power per unit area, and it correlates to the cutting capability of the jet. The denser the power, the more effective the jet becomes for cutting, as stated by Hashish [3]. Jet spreading, as shown above, will reduce the power density. However, we will develop an expression below for the power density to recognize the parameters that affect it. The waterjet velocity $V_w$ can be calculated from the pressure $P$ and the water density ${\rho }_w.$

$$V_w=C\sqrt{{\displaystyle \frac{2P}{{\rho }_w}}}$$

(1)

In the above equation, C is a lumped coefficient for the coefficient of velocity and the effect of water compressibility, as stated by Hashish [3]. This equation can be used to express the water flow rate Q_w as:

$$Q_w=C_d{A}_n\sqrt{{\displaystyle \frac{2P}{{\rho }_w}}}$$

(2)

In the above equation, $A_n$ is the cross-sectional area of the orifice while $C_d$ is its overall coefficient of discharge. Now, the waterjet power E_w can be determined from the following equation:

$$E_w=P{Q}_w$$

(3)

Using Equation (2) in Equation (3), the waterjet power can be expressed as:

$$E_w=C_d\sqrt{{\displaystyle \frac{2}{{\rho }_w}}}{A}_n{P}^{1.5}$$

(4)

Defining the jet power density $E_{wd}$ as the jet power per unit area, we obtain:

$$E_{wd}={\displaystyle \frac{E_w}{A_n}}=\sqrt{{\displaystyle \frac{2}{{\rho }_w}}}{C_{d}P}^{1.5}$$

(5)

For water, we replace $\sqrt{2/{\rho }_w}$ with $K_w$ as a water constant, and we obtain:

$$E_{wd}=K_w{C}_d{P}^{1.5}$$

(6)

Typical values of the waterjet power density are shown in Figure 2 below for $C_d$ values of 0.7, 0.6, and 0.5, and water density of 1.0 kg/dm³. Increasing the pump pressure from 400 MPa to 600 MPa, a 50% increase in pressure, will increase the power density by 83%. As can be noticed from the above equation, the power density depends on the pressure for a given liquid and orifice coefficient of discharge. The higher the coefficient of discharge, the higher the power density as more flow rate will flow out of the same orifice at the same pressure. This places important demand on orifice design. For example, an orifice with a $C_d$ of 0.7 has 16% more power and accordingly more power density than an orifice with $C_d$ = 0.6. This is important for food cutting as several orifices are typically used on a single waterjet cutting system using a single pump. Accordingly, power losses will affect the overall efficiency and productivity of the system. A “Food grade” orifice is one with a high $C_d$ value, a sharp edge, and is typically made of diamond.

Observe that the waterjet power density is in the range of 10⁵ W/mm² which is about the same or one order of magnitude less than lasers used for cutting. However, some high-power lasers may reach power densities a few orders of magnitude higher, for example, ~10¹⁴ W/mm².

2.4. Waterjet Force

One of the great advantages of power-dense waterjets is the low force they impart on the food while cutting and thus no crushing, mashing, or distortion will occur. The impact force $F$ of the waterjet can be calculated from the momentum equation using the water mass flow rate $m_w^{.}$:

$$F=m_w^{.}{V}_w={\rho }_w{Q}_w{V}_w$$

(7)

Expressing this in terms of pressure and power using the above relationships, we obtain:

$$F=C\sqrt{2{\rho }_w}{\displaystyle \frac{E_w}{\sqrt{P}}}$$

(8)

From this equation, we see that for a given power level, the jet force is inversely proportional to the square root of the pressure. So, for delicate foods when the force of the jet matters, increasing the pressure while keeping the power fixed (which means reducing the orifice size) will be an advantage. Figure 3 shows the jet impact forces at different pressures. It must be said here that the food material encounters much less force (~10 times less) than the full jet impact force while the jet is slicing through the material.

With mechanical food cutters, the viscoelastic properties of foods determine deformation, fracture, and friction during cutting and substantially affect the cutting behavior, especially at high cutting speeds, as stated by Schuldt et al. [4]. Accordingly, the rate of applying the cutting forces is important with mechanical cutters, but when cutting with waterjets, sensitivity to food material is much lower.

In the following sections, we first discuss the main waterjet food cutting system components and then discuss the different cutting applications using examples from actual industrial applications.

3. Waterjet Food Systems

There are several platforms in a food waterjet cutting system; some of these platforms are common to most food cutting systems such as UHP pumps, while others depend on the type of food being cut. These platforms can be divided into the following subsystems:

• UHP pumps and lines: This is the most upstream component in a waterjet system where the water enters the pump at ambient pressure and exits the pump into the plumbing system at a higher pressure. The UHP plumbing is used to transport pressurized water to the jet-forming nozzle(s). This plumbing system may consist of tubing, hoses, fittings, swivel joints, and rotary swivels
• Cutting heads: An on/off valve is used upstream from the nozzle to start and stop the UHP water flow to the jet-forming orifice. The orifice is a most critical component that transforms the pressure energy into kinetic energy. If abrasives are used, then a mixing tube is used to mix and accelerate the abrasives using the high-velocity jet.
• Motion System: The motion system in a food system depends on the food product to be cut. General examples are the following:

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  Cross cutting table: In this example, the waterjet(s) are traversed in one direction (X direction) and the food product is traversed on a feed screen in the Y direction.

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  Robotic arm: A robotic arm is used to manipulate the jet over a traveling or indexed screen carrying the food product, mainly meat such as chicken beef, fish, and pork.

• A CNC controller is typically used to control the interpolating motion of the machine. A robotic arm system may also be used as a manipulator.
• Controls, Software, and Sensors: A CNC controller is typically used to control the interpolating motion of the machine. The software is used to enable operators to interface with the machine controller and is specifically tailored for the application. Sensors are used to guide the motion based on certain portioning criteria.
• Other: There are other subsystems for food processing such as sorting, packaging, and labeling but these are outside the scope of this paper.

3.1. UHP Pumps and Plumbing

Two main classes of pumps are used in UHP waterjet cutting. These are intensifier-type and direct drive pumps. Intensifier pumps are the most used in the food cutting industry due to the demand for responsiveness, reliability, and an extended lifetime. Figure 4 illustrates the principle of pressure intensification used in these pumps. A low pressure acting on a large area results in a higher pressure acting on a smaller area. Conventional variable output hydraulic pumps are used to generate about 20 MPa of pressure over a piston of an intensifier. A plunger with 20-to-33-times less area attached to that piston is used to transmit this hydraulic force to water, thus pressurizing it to 400 MPa or 600 MPa, respectively. An important component of the intensifier pump is a pressure attenuator which is a pressure vessel used to store energy to compensate for pressure drop during intensifier position reversal. A pump will consist of one or more intensifiers, most commonly two intensifiers. Figure 5 shows examples of intensifier pumps with a single intensifier with 621 MPa (90 Ksi) of operating pressure capability. Direct drive pumps are used when the system is of a small size such as a cake cutting system and when the pressure is relatively low, say around 200–300 MPa, to be more reliable.

UHP plumping, shown in Figure 6, consists of tubing, coils, fittings, hoses, and swivel joints. Small-diameter (6 mm OD) tubing is commonly used due to its relative flexibility in the form of whips and coils. An alternative option, though rare in food systems, to achieve flexibility is to use swivel joints, and in this case, larger tubing (~9.5 mm OD) may be used. The tubing is typically autofrettaged to increase its lifetime. UHP hoses offer great flexibility and ease of replacement when the pressures used are relatively low (less than 345 MPa). Another approach to increase the lifetime of UHP components is not to close and open the valve, but rather a deflector is used to interrupt the jet instead of shutting it off. Also, fittings for pressures like 690 MPa have been developed, as stated in Raghavan et al. [5], for an extended lifetime and are mostly used in UHP food sterilization systems.

3.2. Cutting Heads

Cutting heads are the tools that are mounted on the system manipulator to perform the cutting by generating high-velocity cutting jets. The cutting head includes the on/off valve, the water body, and the orifice.

On/Off valves, see Figure 7a, are either normally closed or normally open based on the design of the actuator which is mostly operated pneumatically. Standard shop air (~0.4 MPa) is used to actuate the valve in either case. The critical parts in UHP valves are the poppet and seat which are designed to accomplish at least 100,000 cycles and thus the seals, material selection, and material treatment are all of great importance to meet the severe UHP tribological demands. In normally closed valves, air acts upon the actuator piston against a spring to allow for the lifting of the poppet off the seat (containing a hole) and enables the flow to take place. It closes when the air pressure is released. In normally open valves, air pressure is used to close it. For food cutting, the on/off cycle time must be minimal for efficient operation. A typical opening time for these valves is about 48 ms and a close time of about 160 and 115 ms is normal for the normally closed and normally open valves, respectively. Due to the relatively high forces for opening and closing, solenoid-type valves are not suitable. The orifice in the cutting head is where the pressure potential energy is converted to kinetic energy and as described above, is of critical importance to the performance of the waterjet. Diamond (either natural, synthetic, or PCD) may be used with a lifetime of about 1000 h. Sapphire orifices are not preferred due to their relatively short lifetime. Hashish [6] developed a quick-change orifice mount, so an orifice could be replaced in seconds without having to disassemble the holding nut. This offers potential for automatic orifice replacement. Vacuum sensors have also been used in the aerospace industry, as stated by Hashish et al. [7], to detect changes in orifice health, but this has not been applied in food systems.

Other types of cutting heads are related to the entrainment of different media into a high-velocity waterjet. These media could be air or solid particulate material to act as abrasives. Figure 7b shows a standard abrasive water jet (AWJ) cutting head that can be used for this purpose.

The air entrainment results in high-speed focused droplet jets which were found to be more effective for cutting some food products than standard coherent waterjets. Entrainment of solid particles included entrainment of sugar, salt, and ice.

3.3. System Manipulators, Support Tables, Sensors, and Software

There are many types of manipulator and feed systems used in the food cutting industry based on the food being cut and the required degree of automation. Cartesian, articulated arm, cylindrical, SCARA, polar, and delta-type robots have all been used in the food industry. In addition, custom manipulators have been built for certain classes of food such as cake and pizza cutting. There are also several types of food support and feed tables such as belts, trays, robotic grippers, and steel mesh. The following are examples to illustrate the flexibility of the waterjet tool adapting to all these types of variations.

A cutting table is what the food is placed on and supported by to be cut. For example, a tray may be used to hold a cake which is then placed under a robot or manipulated under a jet. Cross feed belts, whether made from plastic or stainless-steel mesh, are widely used in high-volume production and may include one or more waterjets that are either stationary or traversed in one or two directions, X and Y, while the food is fed in the X direction. Figure 8 shows schematic examples of these configurations which may be used for vegetables, meat, fish, or poultry.

The use of multiple waterjets to trim, slice, and cut is the simplest form of manipulation where the food is either fed on a belt drive under a bank of properly spaced stationary waterjets or the waterjet bank is manipulated linearly to cut a stationary food. Example trimming and cutting systems are shown in Figure 9 where lettuce and celery are trimmed and/or cut into smaller sizes, a cake sheet is cut with multiple jets, and a meat block is sliced into steak.

The use of XY motion to manipulate the nozzle may be accomplished with a cartesian system or by robotic motions such as a delta or six-axis arm. These robots are particularly important when the jet needs to be tilted as in the case of cutting fish to remove the pin bone. Figure 10, Marel of Garðabær, Iceland, shows a two-bridge system for fish portioning and pin bone removal by tilting the nozzles which move on one axis only. Since the nozzles are only moving on one axis, the waterjet catcher system is simply a slot under every nozzle to catch the jet after it cuts. The nozzle can also be tilted to cut at an angle as shown in Figure 10.

An alternative approach is to manipulate the nozzle in two axes, X and Y, over the linearly fed food. The system shown in Figure 11 uses a single cross bar for the Y motion with two bridges, and one on each side for the X motion. In this case, each jet has two degrees of freedom as shown in Figure 11.

To add tilt angles as an additional degree of freedom to the XY motion, a delta-type robotic arm has been used for fish trimming and portioning, as shown in Figure 12. Two delta robotic arms are used while the food (fish) is traveling in the X direction on steel mesh. The use of two nozzles allows for rapid production rates with each jet cutting certain zones on the fly. Either UHP hoses or tubing coils can be used. The jet tilting is used for cutting at angles and removing the pin bones.

For cake cutting, several manipulator types are being used. Six-axis articulated arm robots, as shown in Figure 13, offer great flexibility and possible cutting at angles. The robot is commonly hung over a cross-feed table where cakes (or other food) are linearly fed. High cutting speeds to the order of 1 m/s are used. However, for relatively low production rates, SCARA-type or rotary-linear manipulator systems have been found suitable for cake cutting with a minimal machine envelop size. In a rotary-linear manipulator, as shown in Figure 14a, the nozzle is held stationary while the cake is traversed in both linear and rotary motions to cut any shape or pattern that is programmed on the machine controller. In SCARA-type systems, as shown in Figure 14b, the nozzle and a point catcher are linked to move together using the same drive motor and thus there is no need for a slot or tank catcher. This system can cut a cake at a speed of about 0.5 m/s using medium waterjet pressures of around 200 MPa.

Software technology is important in food processing facilities and can operate at different levels: the food processing software level and the ERP level. Food processing software addresses the flow of the food through the processing line starting with receiving the food and ending with packaging and labeling. This software will obviously depend on the food being cut.

For cake cutting, for example, simple shape cutting software may be used to select the shape to be cut. Figure 15 shows a graphical user interface (GUI) to simplify the cutting process.

Malone et al. [8] described a robot fish-cutting system which incorporates complex sensing with knowledge-based control. The system is used to remove the lateral pin bones from fish filets by means of a robot-manipulated waterjet cutter. A sensing system is used to obtain geometric and positional information about each filet. A knowledge-based control system uses a priori knowledge of the fish species and information from the sensing system to generate the appropriate robot commands. The successful deboning of fish filets demonstrated the viability of this approach which is now in practice in many fish processing facilities.

In these facilities, complex software is used to interface the sensor data with the machine software. In this case, a vision module is used to scan the characteristics of the food traveling on a feed belt. For example, high-speed X-Ray cameras (such as Inspirex R40LF-800, by Mettler Toledo, Columbus, OH USA) may be used to accurately locate the bones [9]. A 3D vision camera is used to measure fish or chicken weight distribution. Analysis of X-ray and 3D images of fish traveling through the machine on a conveyor belt allows data on the precise position of the filets to be fed to two robots deploying water jets. They trim away the areas containing pin bones and cut the filets into predetermined portion sizes with exceptionally high accuracy. Figure 16 shows different portioning patterns that the software will facilitate programming.

Alitavoli and McGeough [10] proposed an expert system for meat cutting applications with waterjets. An interactive acquisition methodology was addressed in the design and structure of the food processing knowledgebase. The acquired knowledge is then to be formalized into an acceptable format for the system. It is believed that the field of artificial intelligence will play a significant role in food processing in the future. Today, digital manufacturing execution systems (MESs) are implemented in many food processing facilities not only to manage requirements of safety regulations but also to maximize production performance. The food processing software is a layer in the MES that unites machines and production planning. With food processing software in an MES, orders are automatically transferred to the production equipment, as well as material consumption, and the number of produced items are reported back and can be accessed by the company’s ERP system.

4. Selected Cutting Applications

There are many food product applications making use of the advantages of waterjets listed above. In this section, we select some of these application groups and discuss some of the observations and findings when cutting with waterjets.

4.1. Beef, Pork, and Lamb

Meat cutting has the highest processing costs of all food products, according to McGeough [11], as it is highly labor intensive with skilled operators who are usually certified by standards and regulations. Although the saws are regularly sterilized, avoidance of possible cross contamination cannot be completely guaranteed. When the waterjet is incapable of cutting harder parts of the meat, such as bone or larger cross-sections, abrasive waterjet cutting may be required. However, typical hard abrasives such as garnet used in waterjet cutting are unacceptable for food processing. Alternative acceptable abrasives including salt, sugar, and starch crystals have been found to be insufficiently effective.

Alitavoli and McGeough [10] suggested (i) the use of a conveyor for the transfer of carcasses between stations; (ii) that all major cutting operations be performed in one installation with an omni-directional waterjet head; and (iii) the utilization of a stationary multi-nozzle cutting head that performs repetitive operations as the carcass is moved on the conveyor. Their findings are related to experiments on the waterjet cutting of beef, chicken, lamb, and pork. These tests revealed that waterjet cutting may be applied successfully to beef and pig meat, although further trials are necessary, to explore the potential advantages of this technique for industry, especially for cutting through bone using an abrasive waterjet.

Heiland et al. [12] reported on cutting beef chuck. They indicated that the removal of objectionable tissue and bone required that the waterjet be guided through intricate paths and patterns. Inertia limitations of the robot arm that controlled the waterjet lowered the high linear speeds that were otherwise possible with 12.5 mm thick slices.

Table 1. Beef cutting observations, from Heiland et al. [12].

Slice Thickness	Cutting Speed	Kerf Loss	Remarks
mm	m/min
12.5	25	<2%	clean cut through full thickness
25	5	<2%	clean cut through full thickness
50	1	>5%	Lower half is rough
100	0.2	>10%	Lower half extremely ragged

shows cutting speeds for different chuck thicknesses. This study demonstrated the applicability of robotic waterjet excision of objectionable material from slices of beef chuck. Data were used in a cost study showing that the waterjet cutting system with an annual production rate of 7.6 million kg would save at least USD 0.5 million over a manual method employing 20 meat cutters each in two shifts.

4.1.1. Abrasive Waterjet

Alitavoli and McGeough [10] showed that the addition of abrasives, such as sugar and salt, can substantially increase the depth of cutting in meat samples, especially when bone was present. Wang and Shanmugam [13] confirmed this in a more recent study on AWJ cutting of beef, pork, and lamb meat with and without bone using salt particles, producing a narrow kerf of less than 1 mm, which is less than the traditional cutting processes with saws. They also showed that meat can be cut at room temperature to eliminate the freezing or chilling costs which are needed if saws are used. Figure 17 shows results of cutting through bone. It was concluded that AWJ cutting is a viable technology for meat cutting and researchers recommended a study to assess the effect of salt on increasing shelf life. They suggested that ice particle AWJ should further be developed with harder ice particles.

4.1.2. Ice-Waterjet

McGeough [11] studied cutting meat bone with ice-waterjets. He found that plain waterjets penetrate only the outside of the bone. The addition of ice-abrasives enabled cutting that was 40% deeper into the bone, the marrow of which became flushed out. In contrast, the use of a garnet abrasive in the jet enabled clean cutting through the bone without loss of marrow. He concluded that the ice jet would be initially limited to large-scale commercial applications due to the initial cost of waterjet equipment.

The author conducted the cutting of food using CO₂ particles entrained in high-velocity waterjets. He arrived at a similar conclusion to that of McGeough [11] that the performance of these ice-waterjets is not effective for commercial use.

4.2. Fish

Fish filet cutting has been discussed above, showing the great benefits of automated waterjet fish portioning and pin bone removal. Franklínsdóttir [14] conducted a parametric study on cod and salmon fish cutting. She found that cutting speed is the most influential factor. The second strongest parameter was the orifice diameter where the performance of an orifice diameter of 0.12 and 0.15 mm was much better than that of the larger orifice diameters of 0.17 and 0.20 mm in terms of quality of cut and minimized waste. The waterjet pressure in the range from 200 MPa to 350 MPa affected performance but not significantly; however, higher pressures would have shown a difference. The cutting speed should be within a range from 300 and 600 mm/s for tail portions and loins and from 450 and 900 mm/s for belly flaps for optimum performance. Optimum ranges of transverse speed for each specific part were set with the goal of increasing automation and meeting certain standards of cutting quality and performance not considering connective tissues and bones.

Kasperowicz et al. [15] identified minimum pressure requirements to cut different portions of whole trout fish. Their research showed that the muscle lobe of rainbow trout can be divided into areas of different strengths against a pressure waterjet, as shown in Figure 18. The most durable elements of the muscular lobe turned out to be inter- and intramuscular fibers located within the lateral line and caudal fin. During the cutting tests, it was observed that the waterjet has the property of selective cutting of tissues. This means that by setting a specific supply pressure value using the appropriate nozzle diameter, it is possible to cut soft tissue, that is, muscle, fat, and peritoneal membranes, without affecting harder tissues, for example, intermuscular fibers or bones. Proper use of such a property may bring great benefits to the fish processing industry.

It must be mentioned here that instead of pressure, traverse speed may be used for selective cutting. Some areas will require a high speed while other areas will require slower speeds, as Franklínsdóttir [14] identified.

Lobash et al. [16] patented a process to cut frozen fish to avoid the use of knives, that can cause a significant amount of portion breakage, and cut portions from a frozen fish slab that can be economical but have cut portions with an attractive appearance that can be pictured on the wrapper of a frozen fish package with greater appeal over fish portions with only straight edges.

4.3. Poultry

Cutting of poultry is one of the early high-volume waterjet automated cutting applications. A vision-controlled UHP waterjet portioning machine developed by Design System Inc., Redmond, WA, USA (now JBT FoodTech, Chicago, IL USA), is reported to handle up to 1500 kg of boneless filets per hour, as stated by Morris [17], using 0.076 mm and 0.127 mm orifices at about 380 MPa of pressure.

Bansal and Walker [18] observed that a hard-frozen chicken filet had different physical characteristics, and it was generally harder to cut with the UHP waterjets. On the other hand, at room temperature, the meat was too soft, which adversely affected the cutting operation, resulting in irregular slices, and the soft meat slices did not stay together on the cutting table. They also identified that an orifice size of 0.127 mm was best suited for cutting chicken breast filets in the pressure range of 179 to 224 MPa with a cutting speed of 100 mm/s or slower. Good cutting results were obtained for chicken breast filets when chilled to about 0 °C as the meat was firm to the touch but pliable.

Today, large-chain fast food companies are using waterjets for cutting chicken nuggets using several parallel nozzles on X and Y axes for rapid production. These operations are proprietary to the companies and thus no specific data are available.

Air-Waterjet

The entrainment of air in an AWJ nozzle creates a highly focused droplet jet that was tested for cutting chicken legs across the bones and tendons after trying regular waterjets at 345 MPa and a speed of 0.7 m/sec. While tendons can be cut at slower speeds, it was desired to keep the speed as high as possible. When air was entrained in an AWJ (instead of abrasives, see Figure 19), the tendons were cut at 345 MPa, but marginally. Tests conducted at 517 MPa of pressure showed that tendons can be cleanly cut.

4.4. Vegetables

There is a wide range of vegetables being harvested and processed with waterjets and perhaps these were the first foods to benefit from UHP waterjet processing. In the following sections, we group some vegetables for discussion and observations.

4.4.1. Lettuce and Celery Harvesting and Trimming

Schield [19] and Schield and Harriott [20] were perhaps the first to study using waterjets for food cutting, especially lettuce harvesting. This was inspired by using waterjets for debarking and the need for an improved lettuce harvesting method. They found that an orifice-type nozzle is better than cone-type nozzles and conducted lab tests to simulate harvesting conditions. They observed that cleanness of cut is inversely related to increased jet diffusion, i.e., jet coherency is important. They calculated that at a forward speed of about 5 km/h, 15.3 cu m of lettuce would be harvested per hour and thus waterjet harvesting is highly viable, and that power and water requirements are feasible for field harvesting systems. Cantwell et al. [21] compared waterjet and blade cutting of lettuce and concluded that better salad-cut romaine quality was obtained with a waterjet using a food-grade nozzle. While both the blade and waterjet caused cell damage, the initial bacterial counts were lower with waterjet cutting. Figure 20 shows a comparison of cut lettuce discoloration with a waterjet and a blade. Béguin et al. [22] showed that because the internal liquid of injured cells is removed by the water flow, browning of the waterjet-shredded lettuce is markedly reduced compared to any commercial cutting techniques.

Recently, Taylor Farms (Salinas, CA, USA) started using waterjet automated romaine lettuce harvesting with great ergonomic benefits to the operators, productivity, yield, and cut quality for enhanced shelf life. For example, an automated harvester by Ramsey Highlander, as studied by Maconachy and Offerdahl [23], and shown in Figure 21, uses several side-firing jets powered by a KMT high-pressure pump to cut six heads of lettuce at the same time and reduces labor by 30 workers.

While work is in progress on harvesting celery with waterjets, celery trimming and cutting with waterjets has been an established practice in the industry, for example, the Duda Farms in Florida ship about 1 million kg of fresh-cut celery every week, due to the ease of cutting and increased shelf life that are similar to lettuce. Orr and Spingler [24,25] reported on the effect of using a 0.18 mm waterjet at 207 MPa on prolonging the shelf life of fresh pre-cut celery and other vegetables like carrots. The results for celery cutting were compared to other methods shown in

Table 2. Comparison of celery cutting methods.

Cutting Method	Microflora Count	Days of Acceptable Sensory Quality (2)
	CFU/g (1)	10 °C
Kitchen paring knife	2 × 1011	12
Commercial rotary blade cutter	4 × 1011	13
Single Edge razor blade	1 × 109	24
Sharp thin blade knife sawing motion	5 × 106	25–40
Waterjet	5 × 104	25–40

(1) Colony Forming Units, (2) at 10 °C.

below. It was concluded that waterjet slicing minimizes bruising and tissue damage which results in increased shelf life for the fruits and vegetables they tested.

4.4.2. Potatoes, Onions, Carrots, and Sugarcane

Potatoes and onions are “round” vegetables and are challenging to slice with waterjets as the standoff distance will vary across the vegetable and thus the cuts may not be consistent from edge to middle. However, carrots are more “cylindrical”, with reasonable size diameters, making them easy for waterjet cutting.

Becker and Gray [26] evaluated waterjet slicing of potatoes. They found that, in general, the best cuts were obtained with the smallest orifice, regardless of water pressure or cutting speed. As orifice size increased, effects of water pressure and cutting speed became more apparent, with the most subsurface cellular damage occurring at lower cutting speeds and intermediate water pressures. When higher cutting speeds are desired, the smallest orifice (0.076 mm) was acceptable if it completely cuts through the sample. Because of the atomized spray from the waterjet column, standoff distances should always be minimized, preferably 0.5 cm or less.

Recently, the author conducted exploration tests on cutting potatoes at 600 MPa of pressure with an 0.13 mm orifice size and a cutting speed ranging from 0.13 to 0.85 m/s. The cuts at 0.64 m/s were found to be acceptable with minimal striations over 75 mm which was the potato diameter.

The process of trimming green onions and leeks is similar to the trimming of celery and lettuce and similar systems are used for this purpose. The main motivation is extended shelf life due to less cell destruction and high productivity. Figure 22 shows the surfaces cut with a waterjet and a blade after 6 days with browning starting on the blade-cut surface earlier than when a waterjet is used.

Onion slicing with waterjets has been tested using a UHP nozzle manifold, as shown in Figure 23. The spacing between the jets was minimized without compromising the ability of the manifold to work at 600 MPa of pressure. While onions can be sliced at lower pressures, the use of higher pressures results in lower forces for the same jet power and thus a smaller-diameter orifice and less kerf waste. Minimizing the forces is important to keep the onion slices stuck together during slicing and feeding out of the slicing area. The slice thickness can be reduced based on the angle of the manifold with the direction of travel. For example, an angle of 45 degrees will cut the slice thickness in half.

Similarly to celery, waterjet cutting of carrots is well-established; for example, baby carrots are actually crowned and cut carrots. Tatsumi et al. [27] studied carrot slicing with waterjets and treatment with a NaCl solution to evaluate reducing the amount of white tissue development on carrot stick surfaces. The NaCl treatment caused a weight loss of 4–10% (not commercially acceptable). Waterjet slicing resulted in the striation of surface tissue and left loose layers of cells, as observed with the scanning electron microscope. These cells dehydrated rapidly and formed as much white tissue as on carrots sliced with a culinary knife. However, they realized that based on the findings of Becker and Gray [26] on cutting potatoes, the whitening and loose layers of cells might have been avoided or eliminated by using a smaller waterjet orifice or adjusting cutting speeds. They did not explore using a waterjet with sodium chloride to gain the benefits of both in eliminating the whitening effect. Posselius and Conklin [28] demonstrated that carrot crowning with high-pressure waterjets is highly productive at a rate of 15 to 20 ton/hour for 9 cm diameter carrots at a pressure of 276 MPa. The need for a vision system for accurate crowning was suggested. The waterjet cutting of the crown before the lye peeling bath is a significant advantage that is enabled by the waterjet trimming process, which reduces the time in the lye bath and the good carrot tissue loss.

Valco et al. [29] studied the harvesting of sugar cane with waterjets and successfully cut sugarcane stalks in the laboratory with 0.36 mm jets at 400 MPa of pressure. It was found that the standoff distance and jet power levels are the most significant factors. They concluded that the requirements for a successful mobile waterjet field unit are not met, rendering harvesting with waterjets impractical. Thanomputra and Kiatiwat [30] conducted a simulation study using abrasive waterjets at 15 kW and a pressure of 360 MPa. The orifice and mixing tube were 0.25 mm and 0.76 mm, respectively. They also selected 80-mesh river-sand abrasives for their analysis. The experimental results showed that the system was able to cut sugarcane stalks completely at a much farther standoff distance by reducing the traverse speed. The study also showed that cutting sugarcanes of 30 and 120 mm diameters would require a traverse speed of 4.4 km/hr. and 1.1 km/hr., respectively. The results implied that limitations should be set to no more than a 210 mm standoff distance with a minimum traverse speed of 0.6 km/h.

There are many other vegetables that are being processed with waterjets worldwide. For example, waterjet cutters are being used in South and Central America for tipping green beans. Two 0.13 mm orifices powered by a 10 kW intensifier pump will trim about 1600 Kg per hour. Snapping green beans by hand is labor intensive and bruises and bends green beans which diminishes quality. There is currently additional interest in using waterjet technology for cutting asparagus, chunking melons, and capping strawberries.

4.5. Candy, Backed Food, and Sandwiches

Waterjet cutting in the candy and bakery industries has a great advantage due to the relatively low forces involved in the cutting process. This results in no distortion to the food as demonstrated by Figure 24 for cutting cake and candy. Observe that there is no crushing or effects on the edges. Also, please observe the ability to cut through different constituencies such as nuts, taffy, chocolate, etc.

Trieb [31] reported on the application of the diagonal cutting of egg and water cress sandwiches with high-velocity waterjets with 0.1 and 0.15 mm diameters, as shown in Figure 25. He indicated that pressures up to 300 MPa are sufficient, and that jet coherency is of great importance for no wet edges. The production line uses four jets that slice four sandwiches at a time and a robotic arm is used to pack four halves in one pack. This system produces 60 sandwiches per minute. The cutting table was initially equipped with a line catcher. All four waterjets sprayed into one vessel which caused a rather high noise level. When four single-catcher cup units were used and synchronized with the corresponding jet, the noise level dropped by more than 6 dBA.

Merle et al. [32] used slicing bread as a case study to identify the conditions that make the waterjet a perfectly clean tool. They concluded that these conditions are media filtration to 0.2 microns, the regular sanitation of the tubing, and controlling the ambient air to class 1000 or 100 based on the product. They also reported that the dominating factor for wetting the bread is pressure and then the cutting speed. Minimal moisture pick up was obtained at pressures in the range of 300 to 350 MPa and a speed in the range of 20 to 30 m/min.

Bakeries use a variety of methods to cut cake such as wire, rotary wheels, reciprocating blades, and ultrasonic blades. These means take time to clean out and cause distortion while cutting, leading to reduced yield. They are also limited to straight line cutting. Waterjets avoid all these issues with significantly more productivity and thus can be found in commercial markets with automated machines that are cost effective. As described in the section above, either single, dual, or multiple jets are used. The most important factors in cake cutting are to use highly coherent waterjets and to prevent the splash back from the support structure. These issues have been addressed in machines such as those provided by ABI LTD (Richmond Hill, ON, Canada), Xilix (Hutto, TX, USA), and Metronics Technologies (Noáin, Spain), using different approaches for the machines but with the same type of waterjet nozzles. Some examples of cutting cake are shown in Figure 26 below.

Boyle [33] quoted that a waterjet is an excellent choice when processing viscous, gelatinous, and multi-ingredient food products such as baked goods and confectionery such as Danishes. Cutting pizza is of course possible with excellent results but is not practical except on a large scale where pizza is frozen after cutting. In this case, every slice will be perfectly separated after heating.

5. Non-Through Cutting Applications

An application of UHP waterjets in the food industry is not to cut through the food but rather to remove the skin or of the food without damaging it such as peeling, or separate food from nonfood components such as deboning.

5.1. Deboning

The removal of meat off the bones after the main meat sections have been separated is a process that waterjets have been demonstrated in with excellent results. In this case, high-speed rotary jets are used to strip the meat off the bones and even mill some of the bone off for dog food, for example. There are no reports in the literature about this process but companies in Australia and the USA have tested this approach with good results.

Lambooij and Schatzmann [34] explored another application by conducting experiments on pig stunning. The results of this experiment suggest that waterjet stunning at 390 MPa, when combined with electro immobilization, may be a suitable method for slaughterhouses. However, further studies are required to improve the quality of meat.

5.2. Peeling and Skinning

Peeling is another application that waterjets have been used for. In 1982, the Ross Pickles company of the Newcastle upon Tyne, UK installed onion preparation machines which remove the onion skin using high-pressure water jets [35,36]. They claimed that the waterjet method of skinning is much gentler than any other method of machine peeling. It also cleans off any blemishes and ensures that the onions are transferred to the brine for maturing in scrupulously clean conditions, which greatly reduces any risk of the fermentation process going wrong.

The use of ice powder as an abrasive was attempted by Kluz and Geskin [37] for peeling some food products such as potatoes and carrots. Instead of a waterjet they used air. While results were qualitatively acceptable, the rates of peeling did not warrant commercial implementation. Perhaps the use of fan waterjets will be more productive and thus some research is recommended in this area.

Tests on the peeling of lemons, as shown in Figure 27, to extract skin oils were conducted to evaluate the feasibility of the waterjet peeling process. Similarly to a lathe process, lemons were held and rotated while a jet traversed. A fan-type jet was used to travel on top of a rotating lemon and along the axis of rotation. The middle picture shows the result when a traverse rate of 0.5 m/sec was used under a 245 MPa fanjet with an about 75–100 mm standoff distance. Slower speeds will result in more peeling.

For processing squid and the removal of skin, Singh and Brown [38] patented a process where the squid is treated by being severed into three longitudinally separate tentacle, eye or head, and body portions. The body portion is impaled on a peg and is rotated and subjected to external waterjets that remove the skin, which is discarded, and to internal waterjets that remove the viscera, which are also discarded. This leaves the squid body clean on the outside and inside.

6. Conclusions and Recommendations

6.1. Conclusions

The findings presented in this study lead to the following conclusions regarding the use of ultrahigh-pressure (UHP) waterjet technology for food processing applications:

• The demands placed on UHP waterjet systems in food processing are well within the capabilities of the current state-of-the-art technology. These systems exhibit high reliability, robustness, and adaptability for both plant-based and field operations.
• Waterjet cutting imparts minimal mechanical force while delivering high power density, allowing for the precise processing of delicate food products without deformation, crushing, or structural damage. This is particularly advantageous for products with soft or heterogeneous textures.
• Compared to conventional mechanical cutting tools such as blades, disks, and wire saws, UHP waterjets offer superior sanitary performance. The absence of contact surfaces eliminates the risk of cross-contamination, as each cut is executed with a clean, renewable cutting medium. Furthermore, waterjets require no tool sharpening or replacement, thereby reducing downtime and maintenance.
• The trimming of vegetables such as celery, lettuce, and green onions using waterjets has been shown to extend shelf life by minimizing cellular damage and reducing microbial contamination at the cut surface. This advantage directly translates to improved product quality and reduced waste in the supply chain.
• A wide range of waterjet cutting system configurations have been successfully deployed, including stationary jet arrays, single- and dual-axis platforms, delta robots, SCARA arms, and six-axis articulated robotic systems. This architectural versatility allows systems to be tailored to diverse food products, geometries, and throughput requirements.
• The integration of software and control technologies, including computer numerical control (CNC), graphical user interfaces (GUIs), and vision-guided sensing systems, plays a critical role in modern waterjet food processing. These systems enable real-time adaptive cutting and portioning, particularly in fish, poultry, and meat applications, and are increasingly integrated into higher-level manufacturing execution systems (MESs) and enterprise resource planning (ERP) platforms.
• Automated field harvesting using waterjets—most notably in lettuce processing—has been successfully implemented and has resulted in significant improvements in operational efficiency, workplace ergonomics, and produce quality. These outcomes validate the feasibility of UHP waterjets beyond in-plant applications.
• Although the waterjet technique has demonstrated promise for non-through-cutting applications such as peeling, skinning, and deboning, there are currently no commercially reported systems operating at scale in these domains. Further technological development, including the refinement of jet types (e.g., fan jets, air–liquid jets, and food-grade abrasive jets), is needed to unlock these applications.

In summary, UHP waterjet technology offers a clean, flexible, and high-performance toolset for food processing that not only meets stringent industrial and regulatory standards but also opens new avenues for product innovation and supply chain optimization.

6.2. Recommendation

Based on the findings and conclusions of this study, the following recommendations are proposed to advance the development and adoption of ultrahigh-pressure (UHP) waterjet technology in food processing:

• Development of High-Cycle On/Off Valves: Although current UHP on/off valve performance is acceptable, future advancements should focus on developing faster-response valves with an extended service life. This is particularly important for high-throughput food applications that require rapid actuation cycles, especially at elevated pressures.
• Utilization of Higher Pressures (Up to 621 MPa): While current systems often operate below 400 MPa, the application of higher pressures up to 621 MPa (90 Ksi) remains largely unexplored in food cutting. Leveraging these higher pressures may enable greater power density, narrower kerf widths, and reduced water consumption—all of which could enhance productivity and process efficiency.
• Exploration of Alternative Jet Modalities: Additional research is warranted to investigate non-traditional waterjet configurations, including the following:

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  Air–liquid jets: These can produce focused droplet streams that may be advantageous in cutting fibrous or layered food products.

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  Food-grade abrasive waterjets: The development of safe, consumable abrasive media (e.g., salt, sugar, starch) could expand waterjet applications to bone-in meats, frozen foods, and tough-skinned produce.

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  Fan-type jets: These may be particularly suitable for surface treatments such as peeling and deboning, offering broader coverage at controlled impact forces.

• Commercialization of Non-Through-Cutting Applications: While laboratory studies and pilot-scale demonstrations have shown the potential of waterjets for peeling, skinning, and deboning, these applications have not yet been widely commercialized. Industry and academia should collaborate to bridge this gap by conducting targeted R&D, optimizing system configurations, and validating hygiene and quality metrics.
• Integration with Advanced Vision and AI Systems: Future systems should integrate high-resolution vision technologies and AI-driven decision-making to enhance automation, especially for variable-shape and multi-component food items. This will be critical in advancing smart manufacturing and real-time quality control.
• Sustainability and Water Efficiency: Research into closed-loop water systems and reduced-flow nozzle designs is recommended to support environmental sustainability, especially in regions where water use is regulated or limited.
• These recommendations aim to guide future innovations in waterjet food processing technologies and support broader industrial adoption across new food categories and production settings.