Gelled emulsions and protein emulsion gels have been widely used as theoretical models to investigate the structure of protein-based semi-solid and solid foods (Cho, Lucey et al. 1999, Yang, Rogers et al. 2011, Dickinson 2012, Liu, Stieger et al. 2015). According to the interactions between emulsion droplets and the surrounding protein gel network, the droplets can broadly be classified as ‘active’ or ‘inactive’ (van Vliet 1988, Dickinson and Chen 1999). Active fillers are assumed to have strong interactions with gel matrix, for example protein-coated particles forming bonds to protein gels (Langley and Green 1989, Xiong and Kinsella 1991, Dickinson and Chen 1999, Liu, Stieger et al. 2015). In contrast, inactive fillers interact weakly or not at all with the surrounding gel network, such as for the non-ionic water soluble surfactant, Tween 20, which was used to stabilise emulsion droplets which were then demonstrated as acting as inactive lipid fillers in protein gels (Chen and Dickinson 1999, Dickinson and Chen 1999, Liu, Stieger et al. 2015).
The properties of such gel systems can be influenced according to whether the droplets are active or inactive. For example, Liu, Stieger et al. (2015) observed unbound (inactive) fat droplets led to more coalescence in their emulsion-filled gels compared to bound (active) fat droplets, and decrease of solid fat content (4-48 % in total fat) caused less coalescence of the unbound (inactive) fat droplets. Liu, Stieger et al. (2015) concluded that the negligible coalescence at low solid fat content is due to low sensitivity to rupture of the droplet interface upon deformation because of the low presence of fat crystals inside the droplets.
As indicated throughout this review, cheese can be considered as a particle-filled structure composed of continuous protein gel matrix and inter-dispersed fat globules (Rogers, McMahon et al. 2010, Yang, Rogers et al. 2011). Inert (inactive) fillers or structure breakers have been used to describe behaviour of fat droplets in cheese when native milk fat globule membrane is present on the surface of fat globules (Michalski, Cariou et al. 2002, Michalski, Gassi et al. 2003). This hypothesis of inactive fat fillers was evidenced by cryo-SEM images of fat globules surrounded by serum cavities in fresh Cheddar (Hassan and Awad 2005, Everett and Auty 2008). Fat globules covered with
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proteins, for example as a consequence of homogenization, are likely to interact with the cheese protein matrix and this can serve to reinforce the cheese structure (Guinee, Gorry et al. 1997, Rowney, Hickey et al. 2003). Interestingly, fat globules with limited interfacial coverage of proteins were considered as inactive fillers in the investigation of Michalski, Cariou et al. (2002) due to no significant increase in shear modulus. In this project study, the concept of active fillers and inactive fillers are used to investigate Mozzarella-type pizza cheese made using an alternate process to the traditional method of manufacture. Here, the detailed descriptions of active fillers and inactive fillers are concluded as follows:
Active fillers - filler particles form an association with the continuous phase (e.g. cheese protein matrix). The mechanical/material properties of the matrix are typically strengthened, which can be influenced further by the specific material properties of the dispersed phase particles.
Inactive fillers - filler particles lack association with the continuous phase. Shear modulus will be expected to decrease as fillers behave as structure breakers, suggesting a weakening effect on gel structure. However, this scenario can be complicated by considering different permutations for the structure and distribution of the fat phase within the gel network, as well as the specific material properties of the dispersed phase. The interfacial components adsorbed to the fat surface of emulsion droplets are a key factor in determining the filler behaviours and their contribution to the material properties of the gel system. Figure 2.9 shows the schematic models of active fillers and inactive fillers. Active filler particle interactions with the continuous phase can include various bonding mechanisms such as covalent or electrostatic bonds, or hydrophobic interactions.
Bond formation can enable applied shear forces to be transferred from continuous phase to active fillers, resulting in filler deformation (Figure 2.9A). However, in contrast inactive fillers deform only slightly under a small strain shear (Figure 2.9B) because of ‘lubricant’ effect of the aqueous layer (Suchkov, Popello et al. 1997). Yang et al. (2011) used particle-filled gel models (van Vliet 1988, Luyten and Van Vliet 1990) to explain Cheddar cheese rheological properties: during small deformation the structure within
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active fillers both the filler interface and matrix deform during small deformation, and overall stiffness would increase within firm fillers; the structure within inactive fillers is similar to the one filled with aqueous holes, because the mechanical forces exerted on the gel can only be passed onto the intermediate aqueous layers around the fillers, and only the intermediate aqueous layers are deformed.
Figure 2.9: Schematic models of cheese filled with (A) active fat fillers, (B) inactive fat fillers under small deformation. Yellow droplets are the filler particles; dark blue background is the continuous phase, and light blue around filler particle is the serum. Figures are modified from models of Chen and Dickinson (1999). The arrows show the small strain force.
Such structural effects are best studied by rheological analysis, which is a widely applied tool for relating colloidal interactions to material behaviours (Dickinson 1998). For cheese systems material properties are typically investigated through the determination of shear moduli (G’ and G’’) under conditions of low oscillatory strain. For filled gel systems, active fillers connected to the surrounding matrix result in increased elastic modulus (G’); while inactive fillers locating in aqueous pockets of protein network would decrease sample elastic modulus (G’) (van Vliet 1988, Dickinson and Chen 1999, Dickinson 2012). In the case of increased elastic modulus of (protein) gel matrix,
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Dickinson and Chen (1999) observed the less effect of active fillers, but the greater effect of inactive fillers (and vice versa).