# Influence of Processing Steps on Structural, Functional, and Quality Properties of Beef Hamburgers
## 1 Introduction  
The disintegration of muscle fibers during beef hamburger manufacturing affects their physicochemical properties. The term hamburger is protected by the guidelines of the German Food Code and must be prepared from 100% beef [1], thereby not exceeding the legal limit of 20% Vol. disintegrated muscle fibers [2]. Production of beef hamburgers usually involves four processing steps, namely pre-cutting during the first grinding, mixing, main grinding in the second grinding, and forming of the hamburgers [3]. Grinding is a mechanical way of communication [4] and is widely used for size reduction of meat [5]. During grinding, the meat is continuously conveyed towards the cutting set [6], extruded through the hole plates, and cut by the rotating knife [5], whereby the pressures of 6-8 bars act on lean meat. Due to increased grinder efficiency and material homogeneity and better ground meat quality, the size reduction is done gradually [4, 7]. In the pre-cutting (first grinding), meat is ground to 13-19 mm particle size [5], which is important to guarantee optimal conveying and comminution [4]. Mixing is performed to obtain an even distribution of meat and fat particles, often using paddle mixers with low energy input to ensure gentle handling [5]. In the main grinding step (second grinding), the meat is ground to the final particle size of approximately 3 mm [5]. Following, hamburgers are formed by filling the ground meat in a defined mold and applying pressure [8]. Meat is considered a plastic-elastic material that is able to store force acting on it to a certain degree [9; 10]. During hamburger manufacturing, meat is exposed to three main mechanical forces. First, wall friction forces are acting on the meat during grinding and mixing [6]. Second, pressure acts on the meat during forming and as it is forced through the hole blades. This pressure is higher, the smaller the hole plate diameter is [4], whereby the structure of the meat is disintegrated. It was found that the pressure within the hole plate is higher in beef meat than in pork meat and is higher when the fat content of the meat is higher [4]. Third, the meat is exposed to shear force due to conveying and due to cutting of the meat strands by the rotating knife at the end of the hole plate [6]. In this stage, the structural and mechanical properties of the meat are altered due to size reduction [4], and muscle fiber structures are degraded [11]. A mechanical rupture of the muscle fiber opens its internal structure, thus making proteins and intramuscular substances available for extraction [12]. Thereby, the degree of muscle fiber disintegration depends on the applied force and stress, the type of raw material, and the used technology. Hence, mechanical treatments, such as mixing or forming under pressure, are key parameters in muscle fiber disintegration [2] and significantly change the meat structure. During grinding, the ordered fibrillar structures and connective tissue are disrupted by mechanical force [13], thus leading to an increased amount of non-intact muscle fibers (_ANIC_). The _ANIC_ is evaluated by histological techniques [2], influences quality, functionality, and sensorial perception of beef hamburgers and is, thus, legally limited to 20 Vol.% in Germany [2; 11]. Histological approaches are used to quantify muscle fiber disintegration in muscle tissue to identify, characterize, and evaluate changes in the muscle fiber structure and enable the assessment of the meat in terms of legal rules [2]. As muscle fiber structures are disintegrated during grinding, resulting in leakage of meat juice, the meats drip loss will increase and meat quality is reduced [6]. The ability of meat to retain water is described as water-holding capacity [14], which is closely linked to the meat quality and the sensorial perceptions of the consumer, as it depends on muscle fibrillar changes during processing [14]. Thus, the amount of released water depends on the extent of muscle fiber disintegration during processing.