The sensory acceptance of food materials is based on two key factors: taste and food structure. In the emerging field of alternative food materials, meat analogs have been hailed for decreasing greenhouse emissions, meeting demand for more sustainable diets, and contributing to animal welfare. While efforts to imitate taste have largely succeeded, matching food structure is still a struggle. Extrusion is the most widely applied approach: fibrous structure is obtained top-down, e.g. by introducing structure into bulk material with shear forces. In contrast, bottom-up approaches produce µm scale fibers, e.g. through a wet- or electrospinning process and subsequent assembly of the individual elements. Characterization of rheological properties using, e.g., Anton Paar’s MCR rheometer, is crucial for both approaches, to optimize food structuring, and ensure product quality consistency.
Optimizing the mouthfeel of meat analogs
Rheometry may help improve design and mouthfeel of meat analogs
Between 1950 and 2000, annual meat production increased from 45 to 229 million tons, and in 2006 it was expected to rise further, to 465 milllion tons by 2050. News about the first 3D-printed vegan steaks and rising-star, plant-based-“meat”-producing companies in the last decade (e.g., impossiblefoods, beyondmeat) have fuelled a sense that meat alternatives are a recent trend but soy-based products (e. g. tofu) have a longstanding history both in Asia and Europe[2,3,4]. In the 19th and 20th centuries, meat alternatives were first documented as motivated by ethical reasons and as the result of food shortages during war. However, only the impact of methane emissions on global warming, increasing demand for sustainable diets, and, last but not least, growing awareness about animal welfare have led to a massive increase in demand for meat analogs made from alternative foods.
Nevertheless, sensory acceptance remains a key prerequisite in the establishment of new products. The two main factors for dietary products are: taste and food structure. While imitation meat taste has been successfully demonstrated, the structuring of analog meat to mimic the fibrous texture of naturally grown meat remains elusive.
And yet there is hope. Rheometry may help improve process design and characterization of meat analog mouthfeel. Common analytical methods for testing food structure are mainly descriptive and empirical. Beyond that, it has been demonstrated that rheometry is an ideal analytical method to support both quality control and R&D in the meat analog industry.
It tastes right, but does it feel right?
Several food materials like fruit, fungi, and dairy serve as raw materials for meat replacement. One of the classical meat alternatives is tofu, also known as bean curd, which basically consists of soybean protein and is produced by coagulating soy milk and then pressing it into blocks of varying stiffness. It is well-known for its efficient absorption of added flavors resulting in a simulatory taste of meat. However, consumer acceptance of these products is comparatively low in Western countries due to the sensoric deviations compared to meat itself. That’s why efforts are increasingly being made not only to imitate taste, but also to develop products structurally analogous to meat.
Food structuring can be realized in various ways. A key feature is fibrous elements that mimic the structure of whole muscle meat. Such fibrous morphology can be elicited via two fabrication routes: The bottom-up approach produces individual fibrous structural elements which are assembled in larger-scale units (Figure 1). These elements range in scale from nano to micro, and resemble muscle cells, myofibrils, sarcomeres with proteins, and tissue. Candidate materials are produced in a cell-culturing process, through fungi biomass production, or through fiber production involving wet- or electrospinning. For both processes, shear rheology and extensional rheology determine process parameters for fiber production. Subsequent enzymatic or binding assembly of aligned primary elements yields the higher order structure, and the anisotropy and fibrous perception the consumer expects for meat. In contrast, the top-down approach satisfies the meat structure on greater-length scales only (Figure 1). Anisotropy of various plant-based raw materials is introduced by extrusion, unidirectional freezing, and the mixing of proteins and hydrocolloids. For example, non-soluble biopolymers in water can be stretched by shear load, washed, and subsequently dried to obtain the meat analog (e. g. Valess©). Such products resemble the meat structure only on a small scale and are thus commonly used for minced meat.
Extrusion is the most widely applied method for meat analog production, even though the technology is energy-intensive, not very well defined, and still mostly based on empirical knowledge. For extrusion cooking with high water content, vegetable protein powder and water are mixed and homogenized, followed by heating and pressurization and subsequent transport through a narrow pipe. This pipe is watercooled from the outside and the resulting temperature gradient leads to shear rate gradients that favor shear banding, thus introducing a fibrous structure. In addition, spinodal decomposition (i.e. immiscibility of water-rich and protein-rich domains) has been found to be an additional prerequisite for fiber-bearing, meat-like structures obtained from the water-rich extrusion process. For extrusion process design, it is necessary to know the temperature- and pressure-dependent viscosity, structural phase changes like the spinodal decomposition temperature (transition from homogeneous liquid to a two-phase mixture with protein- and water-rich domains), thermal diffusivities, and chemical potentials. During the shear process, droplets become elongated under both shear and extensional effects, acquiring fibrous geometry. Breakup is known to be controlled by the viscosity ratio between components and a matrix, while certain extrusion rates favor fibers with an ideal length-to-diameter ratio. This process can be monitored via rheo-microscopy using instruments such as Anton Paar’s RheoOptics rheo-microscope. Further, scaled-down experiments allow investigation of size effects and also reduction of the sample volume required for measurements.
In addition, cone-cone and couette geometries, inspired by the use of rheometers, have been employed to apply large shear forces to obtain fibrous structures with a similar approach. This technique has been scaled up to pilot plants. However, the extrusion approach currently remains the only sufficiently robust and commercially viable production process[10,16].
The all-important ‘first bite’
After production is completed, the ‘first bite’ is crucial. The fracture properties of meat influence the texture and resulting sensory appearance. The mechanical properties and tenderness of meat products, both natural meat and meat analogs, are often characterized by compression, cutting (Warner-Bratzler Shear Force device, WBSF), and tensile tests. Oriented samples representing orientations both parallel and perpendicular to the fiber orientation are required.
Alternatively, the fracture behavior during this first bite can be related to the yield and flow point determined with a rheometric oscillatory test called an amplitude sweep. Here, e.g., minced meat is expected to show significant differences compared to bulk meat.
While aspects of mouthfeel like grittiness and astringency cannot be solely attributed to rheometric properties but are better described by tribological measurements which simulate the (lubricated) friction between two sliding surfaces, e.g. tongue and palate, rheological characterization is an inescapable process in the production of meat analogs, ideally complemented by the tribological measurements.
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