A transition from animal to plant-based protein is required to produce sufficient protein for the growing world population, while at the same time mitigates climate change. Especially the production of meat imposes a burden on the environment. Meat analogues, which are products that are similar to meat in its functionality, can help consumers to lower their meat consumption. The anisotropic, fibrous nature of meat is perhaps the most important characteristic of meat, which can be mimicked by structuring biopolymers, such as proteins and polysaccharides with the shear cell technology. The aim of this thesis is to obtain insight in the key mechanisms that play a role in the transformation of plant-based biopolymer blends into anisotropic/fibrous structures with shear cell technology. These two key mechanisms are the deformation of the two phases present in biopolymer blends, and the subsequent entrapment of this deformation during solidification. It was concluded that successful structure formation requires matching of the properties of the two phases. During structuring at elevated temperature, the two phases are deformed, while subsequent cooling ensures entrapment of the deformed dispersed phase(s) in the (continuous) phase. Ideally, the continuous and dispersed phase have different strength in the final product,.
Chapter 2 presents a method to determine the water distribution in soy protein isolate (SPI) – wheat gluten (WG) blends. The concentration of water in each separate phase was directly determined with time-domain nuclear magnetic resonance relaxometry (TD-NMR), and oscillatory rheology was used to indirectly asses the water distribution by determining the viscoelastic properties of the separate phases and the blend. It was shown that water distributes unevenly in SPI-WG blends: more water was absorbed by the SPI as compared to the WG phase. This methodology was developed for SPI-WG blends at room temperature and subsequently also applied to heated and sheared samples in Chapter 3. First, water distribution in the blend after a heat and/or shear treatment was assessed with TD-NMR and the outcomes were then used to predict the viscoelastic properties of the SPI and WG phase in the blend. This yielded insight in the deformability of the two phases in the blend. The viscoelastic properties were measured under conditions that are relevant for structure formation, i.e. during and after heating and shearing. It was shown that the water distribution was hardly affected by a heat or shear treatment, whereas the viscoelastic properties of the two phases changed significantly. The viscoelastic properties of SPI and WG became more similar due to water redistribution in the blend, which allows deformation and alignment of the dispersed phase during structuring.
Chapter 4 describes a study using a model blend that mimics soy protein concentrate (SPC). It consists of a relatively pure protein phase, soy protein isolate (SPI), and a soluble, more or less pure polysaccharide phase, pectin. This SPI-pectin blend formed fibrous materials at a similar heating temperature as SPC, being 140°C. Pectin formed the dispersed phase and was deformed when heated and sheared at optimal conditions. Chapter 5 extends the study on structure formation with SPI-pectin blends. Here, the deformation of the dispersed pectin phase and the influence of incorporated air were considered. The fibrous nature of these products appears upon tearing, and originates from detachment through or along the long side of the weak dispersed phase(s), being pectin and/or air. A model based on the rule of mixing was used to predict the mechanical anisotropy based on the volume fraction and the deformation of the weak, dispersed phase. The size and orientation of the dispersed phases, tailored by using different shear rates, were related to differences in fracture behavior when deforming the structures. Besides deformation, the strength and volume fraction of the weak phase(s) were important when composing a blend for fibrous structure formation. In Chapter 6, the behavior of the SPI and pectin phases in a blend was investigated by determining the viscoelastic properties while shearing and heating over time. A closed cavity rheometer (CCR) was used to determine these properties under similar conditions as used during fibrous structure formation. The addition of a small amount of pectin (2.2 wt.%) to a SPI dispersion (41.8 wt.%) resulted in viscoelastic behavior that changed in time during a shear treatment at elevated temperatures. Although one can clearly discern two distinct phases with SEM, the viscoelastic behavior of the SPI-pectin blend is more complex than that of a simple composite material.
Chapter 7 demonstrates the importance of the fractionation process on the structuring potential of soy proteins. An enriched soy protein fraction was obtained through an aqueous fractionation process. Those fractions could be used to make fibrous structures when: i) the soy protein fractions were toasted, which is a dry heating step, and ii) when a concentrate (75% protein) was combined with full fat flour, in such a ratio that the protein content was similar to commercial SPC. Toasting results in decreased protein solubility, increased water holding capacity and increased viscosity of the fractions, and these changes turned out to be important for fibrous structure formation.
Lastly, literature was reviewed to put all findings in perspective (Chapter 8). An overview is presented of all techniques that are commercially used and currently investigated to create meat-like structures. Structuring techniques are compared in their approach, being either bottom-up, which refers to assembly of structural elements that are then combined, or top-down, which refers to structuring of biopolymer blends using an overall force field. A bottom-up strategy has the potential to resemble the structure of meat most closely, by structuring the molecules including proteins into structural components (e.g. muscle cells) followed by assembly of individual structural components. A top-down strategy is more efficient in its use of resources and is better scalable, but can only create the desired structure on larger length scales. The techniques with a top-down strategy were further investigated by reviewing literature on similar processes outside this particular field of application, i.e. not meant to create fibrous structures. These insights were subsequently translated to the conditions as used in structure formation for meat analogues.
Chapter 9 concludes with a general discussion of all results presented in this thesis. The different chapters are integrated in design rules for fibrous structure formation. Furthermore, the complexity encountered when studying material and conditions during fibrous structure formation are discussed. Then, the potential and the challenges for understanding and applying fibrous structure formation with simple shear flow are summarized.
The overall societal goal of developing meat analogue food products is to help consumers in the transition from animal-based to a more plant-based diet. The scientific goal to obtain insight in fibrous structure formation with the shear technology as developed in this thesis is of importance, and can be the basis for developing the technology for the next generation meat analogues.