Emulsion formation

Emulsification time is defined as the time required for the formation of an emulsion as indicated by an abrupt increase in viscosity of the RVA graph during processed cheese manufacture (Kapoor & Metzger, 2005). The increase in the RVA viscosity can also be attributed to other interactions such as protein association at higher temperature during processing of the cheese (Lee et al., 2003). During RVA processing, it was observed that an increase in the Gellan-H concentration increased the time required to form the emulsion (Figure 25A). Gellan-H swells and absorbs water rapidly (Chandrasekaran, Radha, & Thailambal, 1992) during the initial cold dispersion (hydration for 40 min) of the ingredients. Consequently, the competition for water between casein and gellan gum has significant effects on the microstructure and hence the physical properties of processed cheese. As the emulsification of processed cheese is dependent on the interaction of trisodium citrate (TSC) with casein micelles (Gupta et al., 1984; Savello et al., 1989; Shimp, 1985), the resultant increase in the emulsification time suggests insufficient availability of water to hydrate the protein and/or the melting salt system. Therefore, increasing the amount of Gellan-H reduced the amount of TSC available to interact with the casein micelles and consequently reduced the conversion of rennet casein to more soluble paracaseinates (Lee et al., 2004). Figure 25A shows that the total time for emulsification of the samples containing Gellan-H (i.e. the time

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between the peak viscosity and the end of processing  10 min) decreased with

increasing concentration. However, the increase in RVA viscosity for Gellan-L (Figure 25B) was different from that for Gellan-H (Figure 25A). The time to achieve the peak viscosity did not change markedly; however, the viscosity increased noticeably with an increase in the Gellan-L concentration. The lower water-holding capacity of Gellan-L compared with Gellan-H (Huang et al., 2003) could result in a greater amount of moisture being available for casein hydration. Therefore, the increased efficiency of emulsification indicated by the greater increase in peak viscosity (Figure 25B) in the model processed cheese could have been due to an increased extent of casein hydration.

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Figure 25 RVA viscosity profile obtained during the manufacture of model processed cheeses containing 10.0% (wt/wt) protein, 30.00% (wt/wt) fat and 50.00% moisture: (A) Gellan-H; (B) Gellan-L.

Time ( min ) 0 2 4 6 8 10 12 RVA V isc osit y , c P 0 1000 2000 3000 4000 5000 6000 Time ( min ) 0 2 4 6 8 10 12 RVA V isc osit y, c P 0 1000 2000 3000 4000 5000 6000 No polysaccharide 0.5% Polysaccharide 1.0% Polysaccharide 2.0% Polysaccharide

A

B

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It is evident from Figures 26A and 26B that the fat globule particle size in the standard cheese formulation (10% (wt/wt) protein, 30.0% (wt/wt) fat) increased with an increase in the concentration of Gellan-H but decreased with an increase in the concentration of Gellan-L. The changes in the particle size of the cheese samples containing Gellan-L were in line with the published literature, which indicates that increasing the viscosity of the continuous phase generates higher shear stress during processing, which is therefore better able to reduce the fat particle size (Lee et al., 2004). The lower availability of water for protein hydration and the reduced action of TSC in the processed cheese samples containing Gellan-H would have resulted in poorly hydrated casein, which consequently would have decreased the ability of protein to diffuse to the fat interface, resulting in a poor emulsion and a greater fat globule particle size in the cheese matrix (Figure 26A). Another possible reason for not achieving efficient fat globule breakdown could have been retardation of the mixing efficiency at extremely high viscosity during the RVA processing at high polysaccharide (Gellan-H) concentrations. The observed behaviour was consistent with the work of Mounsey and O’Riordan (2008), who reported retardation in the emulsification properties with an

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Figure 26 Fat globule particle size in the cheese matrix for various concentrations of polysaccharides in model processed cheeses containing 10.0% (wt/wt) protein, 30.00% (wt/wt) fat and 50.00% moisture: (A) Gellan-H; (B) Gellan-L. Particle Size ( µm) 0.01 0.1 1 10 100 1000 10000 V olu m e (%) 0 2 4 6 8 Particle Size ( µm) 0.01 0.1 1 10 100 1000 10000 V o lu m e (%) 0 2 4 6 8 No polysaccharide 0.5% Polysaccharide 2.0% Polysaccharide 1.0% Polysaccharide

A

B

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It was also noticed while establishing the limits for the protein content (in Chapter 4) that, at 8.0% (wt/wt) protein, a sample containing greater than 2.0% (wt/wt) Gellan-H resulted in poor incorporation of the oil and a separated layer of oil was observed at the end of processing. A relatively lower concentration of protein available for emulsification of the fat could also have been a reason why fat separation was more evident at low (8.0% wt/wt) protein contents.

6.3.2 Microstructure

CLSM images of the processed cheese samples are shown in Figures 27A, 27B and 27C. The sample shown in Figure 27A contained no polysaccharide whereas the samples shown in Figures 27B and 27C contained 1.0% (wt/wt) Gellan-H and Gellan-L respectively. The protein matrices of all samples appeared to be continuous because no apparent change in the colour intensity of the background was observed. In the

presence of chelating agents, the calciumphosphate bridges that hold the protein units

together are broken down, allowing the protein units to dissociate. This dissociation gives a continuous (dissolved) structure of the proteins in the processed cheese system (Mounsey & O'Riordan, 2008; Savello et al., 1989; Shimp, 1985; Tolstoguzov, 1997).

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Figure 27CLSM images of model processed cheeses containing 10.0% (wt/wt) protein, 30.00% (wt/wt) fat and 50.00% moisture: (A) no polysaccharide; (B) 1.0% (wt/wt) Gellan-H; (C) 1.0% (wt/wt) Gellan-L. Protein is blue, fat is pink and polysaccharide is green. The scale bars represent 75 µm. Polysaccharide structures are visible as discrete entities in the protein matrix.

The micrographs show distinct differences in the protein matrices of the processed cheese samples containing Gellan-H and Gellan-L. Gellan-H appeared as distinct clusters (green) among the homogeneous protein matrix. The bright green particles visible in

C

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the micrograph were gellan gum, which appeared to be only partially dissolved because of the low availability of water in the matrix. Although the temperature used to manufacture the model processed cheese (85°C) was sufficiently high for Gellan-H to be soluble, the relatively short hydration time (40 min) and processing time (10 min) also implied limited time to complete the solubilisation of Gellan-H. The micrograph in Figure 27B shows that the Gellan-H gel clusters were dispersed within a continuous protein matrix. The pre-gel solution (during the hydration step) of Gellan-H, when processed under shear, resulted in the trapping of individual swollen granules in a dispersed phase within the continuous protein matrix. A closer look at these filament- shaped swollen clusters of Gellan-H revealed that the water attracted by the polysaccharide was observed against a dark background rather than a blue background (protein matrix), indicating a discontinuous serum phase with clear boundaries that was isolated from the continuous protein matrix. The CLSM image (Figure 27B) shows that polysaccharides do not become adsorbed on to the fat globule surface, but remain in the emulsion and modify the viscosity and water binding because of their extended, hydrated structure in the protein matrix.

The CLSM image of the sample containing Gellan-L (Figure 27C) did not show similar polysaccharide clusters in the continuous protein network. The smooth and continuous protein matrix indicated that the Gellan-L was well solubilised and dispersed in the processed cheese matrix.

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