By Eben van Tonder
7 July 2026
Introduction
In an earlier article, we examined the broader science of burger patty formation, thermal behaviour, and manufacturing systems [1]. That article described how proteins denature during cooking, how fat behaves during heating, and how the structure of a patty determines its eating quality.
This article addresses a specific and commercially significant problem that was not resolved there, namely why some patties bulge in the centre during cooking while others do not. The problem appeared in our own production when we changed from a hand press to a casing and bandsaw system. The change introduced a structural defect that the old method had prevented without us fully understanding why. This article explains the mechanism and shows how formulation can partially correct it without a mechanical repress step.
What We Did Before and What Changed
Previously, we pressed ground beef into round patties using a hand press. The patties cooked flat. We then moved to a different system. Ground beef is packed into a cylindrical plastic casing, frozen solid, and then cut into 100g patties using a bandsaw. The patties began to bulge in the centre during cooking. The question is why.
Fibre Orientation. The Core of the Problem
To understand bulging, you need to understand what happens to the physical structure of the meat during manufacturing and during cooking.
Muscle tissue is made of long fibres. These fibres run in a direction. In whole muscle, they run along the length of the muscle. In ground meat, they are broken into shorter pieces by the grinder, but each particle still has an internal orientation. The direction in which these fibre fragments are arranged within a formed patty determines how the patty behaves when it is heated.
During cooking, proteins within the fibre contract. This is the same process that causes a steak to shorten along its length when it is grilled. When proteins denature and coagulate between approximately 55 and 80 degrees Celsius, the structures that hold the fibre in its extended state collapse and the fibre shortens [2]. The patty responds to this shortening by deforming in the direction the fibres are pointing.
What the hand press did to the fibres. Imagine a round patty lying flat on a table. The top and bottom faces are the cooking surfaces. Now press down on that patty with a flat round tool. The force goes straight down and the patty cannot move downward because the table is there. The only direction the material can go is outward. The meat flows sideways in every direction from the centre toward the edge, like water spreading when you press down on it. This outward sideways flow is called radial flow. It moves from the centre of the patty outward toward the circumference, like the spokes of a wheel point from the hub to the rim.
As this flow happens, the fibre fragments are swept along in the same direction. They align with the flow. After pressing, the fibres lie flat, parallel to the cooking surface, and point outward from the centre toward the edge. When these fibres contract during cooking, they try to shorten. Because they are pointing radially outward, their contraction pulls the patty inward from the edges toward the centre. This tightens the patty. The centre cannot rise because there is no upward force component in the direction the fibres are facing. The patty holds its shape [2, 3].
What the casing and bandsaw do to the fibres. When ground beef is pushed through a filler into a cylindrical plastic casing, the meat flows along the length of the tube. It moves from the filler nozzle toward the closed end of the casing. The fibre fragments align with this longitudinal flow. After filling and freezing, the fibres inside the frozen cylinder are oriented along the long axis of the cylinder, pointing from one end to the other. The bandsaw then cuts across the cylinder at right angles to its length. Each cut produces a patty. That patty has fibres pointing perpendicular to the cut face. Because the cut face becomes the cooking surface, the fibres in the patty are perpendicular to the cooking surface. In the context of the patty lying flat on a grill, these fibres are pointing straight up and down.
When these fibres contract during cooking, they try to shorten in the vertical direction. The outer rim of the patty is constrained by its own surface area and by contact with the grill. The centre has no such constraint. Therefore the centre lifts. This is the geometric origin of bulging [2, 4].
What Comes to the Surface When You Press a Patty
Anyone who has hand-pressed burger patties knows that moisture appears on the surface of the patty during pressing. What that moisture actually is deserves explanation, because it is central to why the hand press prevented bulging for a second reason beyond fibre orientation.
When ground beef is mixed with salt, the salt dissolves in the water naturally present in the meat. This creates a concentrated salt solution within the meat. Myosin, the primary structural protein in muscle and the protein most responsible for binding and gel formation in comminuted meat products, is soluble in this salt solution [5]. As mixing continues, myosin molecules are extracted from the myofibrils and dissolve into the salt solution. The result is a viscous, sticky, protein-rich aqueous phase distributed throughout the mixture. This is why properly mixed ground beef with salt feels tacky. The tacky quality comes from dissolved myosin [6].
When you press this mixture, you apply mechanical pressure. That pressure forces the aqueous phase toward the surface of the patty. The protein-rich liquid moves to the surface in the same way that squeezing a wet sponge forces water outward. What appears on the surface is not simply water. It is the myosin-rich brine. You can verify this by touching the surface of a freshly pressed patty. The surface is distinctly tacky and slightly sticky compared to the interior. This is dissolved myosin.
When the patty is cooked, this surface layer of dissolved myosin denatures. Myosin denaturates between approximately 54 and 58 degrees Celsius [2]. As it denatures, it forms a continuous protein film over the cooking surface. This film is cohesive and relatively stiff. It acts as a structural skin. This skin provides resistance to the upward movement of the centre. It is not merely decorative. It is a functional structural element [3, 5].
The bandsaw-cut patty has none of this. The top and bottom cooking surfaces are fresh machine-cut faces. They have no extracted protein layer. The only surfaces that received any mechanical working are the cylindrical sides of the casing, which are not the cooking surfaces. The two surfaces that matter most structurally have no surface protein film. This is the second reason the casing-and-bandsaw system produces bulging where the hand press did not.
How Formulation Reduces Bulging Without Repress
A re-press step after the bandsaw is not available in this production context. The patties are packed and sold frozen directly after cutting. However, the following formulation decisions reduce the tendency to bulge, although they do not fully eliminate the geometric cause.
Reducing fat from 30% to 15 to 20%. Higher fat content interrupts the continuity of the protein matrix. Fat particles are hydrophobic and do not participate in the protein network. At 30% fat, a significant proportion of the structure is occupied by fat that contributes no structural cohesion. During cooking, this fat liquefies. Liquid fat has no structural stiffness. The protein matrix therefore has more voids and discontinuities at 30% fat than at 15 to 20% fat. A denser, more continuous protein matrix provides greater resistance to deformation during the cooking process [7, 8].
Additionally, the ratio of extractable protein to total product weight is higher in a leaner patty. More myosin is available per unit volume. The protein network that forms during cooking is therefore stronger [2, 5].
Reducing added water from 20% to 15%. Excess free water within the patty produces steam during cooking. Steam generates internal pressure. Internal pressure in the centre of the patty, where heat penetrates last and concentrations of steam can develop, contributes to outward and upward pressure on the centre. Reducing added water reduces this steam effect. Furthermore, with less water diluting the protein solution within the patty, the concentration of dissolved myosin in the aqueous phase is higher. The protein gel that forms on heating is denser and provides more structural resistance [9, 10].
Removing TVP and starch. Textured vegetable protein and starch are filler materials. They do not contribute to the structural protein network. Starch gelatinizes during cooking between approximately 60 and 70 degrees Celsius, producing swelling and water uptake that generates local volume change within the patty [11]. This volume change can contribute to internal pressure in the centre. Removing both materials gives a more homogeneous protein matrix without internal swelling elements and without the localized pressure effects of starch gelatinization.
Proper protein extraction. This is the most significant formulation variable. Adequate mixing with the correct salt concentration, typically 1.5 to 2% sodium chloride, at temperatures below 4 degrees Celsius, extracts the maximum amount of myosin into solution [5, 6]. This dissolved myosin forms an interconnected binding matrix throughout the entire patty. When this matrix gels during cooking, it provides structural cohesion from the inside. A patty with thorough protein extraction resists deformation significantly better than one where extraction was incomplete [3, 5]. The work of the Bundesforschungsanstalt für Fleischwirtschaft, now the Max Rubner-Institut in Kulmbach, on protein functionality in comminuted products confirms that extraction efficiency is the primary determinant of structural integrity in formed meat products [12].
Adding fat at the end, ground finely at 3mm. This is counterintuitive but important. If fat is mixed into the lean meat from the beginning and salt is added simultaneously, the fat particles coat the surfaces of the lean meat particles. This coating reduces the contact area between protein molecules and therefore reduces the extraction of myosin into solution [13]. The fat physically interferes with the protein-protein interactions that produce a cohesive matrix.
By extracting protein from the lean fraction first, with salt and without fat present, and then adding the fat at the end of mixing, the protein network is fully established before fat is incorporated. The fat particles are then coated by the already-dissolved protein rather than blocking protein dissolution. This sequence protects extraction efficiency. Grinding fat to 3mm rather than coarser produces smaller, more uniformly distributed fat particles. Smaller particles interrupt the protein matrix at a finer scale and create fewer large structural discontinuities [13, 14].
Practical Steps to Reduce Bulging Without Mechanical Intervention
The following steps address the bulging problem through formulation and process adjustments that are compatible with a pack-and-freeze workflow.
First, reduce fat content to 15 to 18% of product weight. This is achievable by adjusting the lean-to-fat ratio in the blend.
Second, reduce added water to no more than 15% of green weight. This reduces internal steam pressure during cooking.
Third, remove starch and TVP from the formulation. These contribute no structural benefit and introduce internal swelling and pressure effects during cooking.
Fourth, adopt a two-stage mixing protocol. Mix lean meat with salt and allow protein extraction to proceed for three to five minutes in a temperature-controlled mixer below 4 degrees Celsius. Confirm extraction by checking that the mixture is sticky and forms strings when pulled. Add fat only after this extraction is confirmed. Mix gently after fat addition for no more than one additional minute.
Fifth, ensure fat is ground to 3mm before addition. This requires a separate fine-grinding step for the fat fraction.
Sixth, review salt concentration. If salt is below 1.5%, protein extraction will be incomplete regardless of mixing duration. A salt level of 1.5 to 1.8% is the practical target for a burger patty of this type.
Seventh, fill the casing at the lowest filling pressure the equipment permits. Higher filling pressure increases the degree of axial fibre alignment. Slower filling at lower pressure allows some randomization of fibre orientation, which partially reduces the geometric bias toward vertical fibre alignment [4].
These steps do not eliminate the geometric cause of bulging. The bandsaw cut will always produce a patty with predominantly vertical fibre orientation and without a surface protein film on the cooking faces. However, a denser and stronger internal protein matrix provides greater resistance to deformation. The combination of these formulation measures materially reduces the severity of bulging under commercial cooking conditions.
References
[1] van Tonder, E. Burger Patty Formation, Structure, Thermal Behaviour, and Industrial Manufacturing Systems. EarthwormExpress. https://earthwormexpress.com/the-meat-factory/meat-science-research/burger-patty-formation-structure-thermal-behaviour-and-industrial-manufacturing-systems/
[2] Tornberg, E. (2005). Effects of heat on meat proteins. Implications on structure and quality of heat-treated meat. Meat Science, 70(3), 493-508.
[3] Huff-Lonergan, E. and Lonergan, S.M. (2005). Mechanisms of water-holding capacity of meat. The role of postmortem biochemical and structural changes. Meat Science, 71(1), 194-204.
[4] Barbut, S. (2015). The Science of Poultry and Meat Processing. University of Guelph. Chapter 11. Comminuted and Restructured Products.
[5] Xiong, Y.L. (1994). Myofibrillar protein from different muscle fiber types. Implications of biochemical and functional properties in meat processing. Critical Reviews in Food Science and Nutrition, 34(3), 293-320.
[6] Acton, J.C. and Saffle, R.L. (1969). Preblended and prerigor meat in sausage emulsions. Food Technology, 23(3), 367-371.
[7] Keeton, J.T. (1983). Effects of fat and NaCl/phosphate levels on the chemical and sensory properties of pork patties. Journal of Food Science, 48(3), 878-881.
[8] Carballo, J., Fernandez, P., Barreto, G., Solas, M.T. and Jimenez Colmenero, F. (1996). Morphology and texture of bologna sausage as related to content of fat, starch and egg white. Journal of Food Science, 61(3), 652-655.
[9] Hamm, R. (1975). Water-holding capacity of meat. In Cole, D.J.A. and Lawrie, R.A. (eds), Meat. Butterworths, London.
[10] Lawrie, R.A. and Ledward, D.A. (2006). Lawrie’s Meat Science. 7th edition. Woodhead Publishing.
[11] Sajilata, M.G. and Singhal, R.S. (2005). Specialty starches for snack foods. Carbohydrate Polymers, 59(2), 131-151.
[12] Bundesforschungsanstalt für Fleischwirtschaft, now Max Rubner-Institut. Kulmbach research programme on comminuted meat product technology. Published technical reports series, Kulmbach, Germany.
[13] Claus, J.R. and Hunt, M.C. (1991). Low-fat, high added-water bologna formulated with texture-modifying ingredients. Journal of Food Science, 56(3), 643-647.
[14] Srinivasan, S. and Xiong, Y.L. (1996). Gelation of beef myofibrillar proteins treated with oxidative stress. Journal of Food Science, 61(4), 748-752.