La importancia de la educación en el desarrollo

The amount of oil released during baking is an important quality indicator for cheese melting, as it has been shown to influence aspects of cheese melt behaviour and changes in appearance during cooking (i.e. the extent of surface browning and blistering)(Rudan and Barbano 1998, Everett and Auty 2008, Ma, Balaban et al. 2014). For these samples, processing time within the RVA was additionally varied to explore the effect of processing on AMC melt properties (Figure 5.11).

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Cooking of AMC_NaCas was seen to result in no oil being released to cheese surface. The structural arrangement of the active filler particles within the AMC (as observed from confocal micrographs), in which fat droplets were relatively uniformly distributed within the protein network, limited droplet-droplet contact, and thus partial coalescence during storage was prevented. In addition, strong interactions between active fat fillers and protein matrix minimised any possible oil phase separation during cheese baking. Fat droplets therefore remained stable in cheese matrix on melting, and accordingly no oil was released.

The amount of oil released from AMC_Tween was determined as the highest of the AMC samples, as shown in figure 5.11. It was observed that 25.7 ± 1.8 % of the total fat concentration was released to the cheese surface for an AMC_Tween sample that had been prepared through 30 minutes shearing within the RVA. The oil-off increased to 34.3

±1.6 % when extending residence time in the RVA to 45 min; however, oil-off was seen

to then decrease to a value of 17.3 ± 0.7 % for a sample that had been shear for 60 minutes during preparation in the RVA. As stated previously, fat globules covered with Tween 20 did not bind to the protein matrix but were localized in the serum pockets, where they became phase concentrated, underwent subsequent partial coalescence during storage and full coalescence during cooking.

The relaxation of the protein matrix during baking could be expected to allow localised domains of coalesced fat to become interconnected, resulting in increasing phase separation of protein and fat phases, and eventually allowing channelling of the liquid oil to the surface of the cheese during baking. The cohesion of protein network and the size of fat-serum channels were likely to both be influenced by residence time within the RVA, which in turn was seen to impact on the extent of oil-off.

This was also observed for the AMC_NC samples, where oil-off was seen to be highly dependent on RVA residence time, reducing from 11.6 ± 2.5 % to 0.5 ± 0.1 % when residence time was increased from 10 minutes to 30 min. The extent of oil-off decrease followed a log curve relative to residence time, but falling within the extremes observed below AMC_Tween and above AMC_NaCas. This intermediate oil-off behaviour again supported the findings in CLSM (Figure 5.2), rheology (Figure 5.8c) and flowing extent in

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baking (Figure 5.10), in considering the that behaviour of fat fillers in AMC_NC presented a dynamic balance between inactive and active fillers, transitioning progressively towards ‘active’ filler properties with increased residence time during RVA preparation. The investigation of Michalski, Cariou et al. (2002) found the transition to active fat globules occurred from the membrane damage fraction above 40% when covered with caseins and whey proteins in mechanical treatment. Oil release in AMC_NC would be expected to follow the same mechanism as proposed for AMC_Tween, although it should be additionally considered that any individual fat globules in AMC_NC acting as active fillers would serve to strengthen cheese texture to stop matrix flowing/melting. In this context, while it was interesting to observe that free oil in AMC_AMF declined with increased residence time, this trend was actually seen to be less impactful in relation to changes to the extent of flow behaviour as a consequence of processing. Instead, the magnitude of oil-off could not be related to the extent of AMC_AMF flow. From a structural perspective, oil-off was maximised when at low residence times for RVA mixing, resulting in the dispersion of fat as relatively large pockets within the protein network. As considered previously, these could become interconnected during melting of the protein network allowing release of oil to the surface. With increasing residence time, homogenisation of the fat phase resulted in a reduction in droplet size, and more uniform distribution of droplets within the protein network. On melting the greater separation of droplets would reduce the incidence of coalescence and accordingly less oil would be released. The fact that the amount of oil released had little impact on the extent of flow implied that liberation of a specific minimum concentration of oil appeared essential for flow to occur in the first place (given that AMC_NaCas for which no oiling off occurs did not flow), but that beyond this concentration further release of oil had little impact on the extent of flow. This may support the earlier observation that cheese flow was in fact governed by the formation of a surface layer of fat that promoted flow by inhibiting surface drying of the cheese.

To investigate the specific role of fat droplet partial coalescence on oil-off, AMC_Tween was made using canola oil, which would not be expected to undergo partial coalescence under AMC processing and storage conditions, since there was no solid fat present within the emulsion at these temperatures. Accordingly, oil-off measurements were

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made for AMC_Tween comprising canola oil droplets with stored at 4 °C for 9 days. For

canola AMC_Tween stored at 4 °C the oil-off during baking was reduced (12.4 ± 1.7 wt.%)

when compared to the AMC_Tween prepared with milk fat (25.7 ± 1.8 wt.%) stored at

the same temperature (4 °C). The fact that oil-off was not completely inhibited in canola

AMC_Tween indicated that even though partial coalescence may not have taken place, some other form of emulsion destabilisation leading to coalescence must have occurred either during processing, storage or heating that would enable the release of free oil during baking.

Figure 5.11: Free oil after 10 minutes baking at 170 °C was compared in AMCs within four types of fat fillers: manipulated fat globules covered with NaCas (ƹƹ) or Tween 20 (ƽ); the anhydrous milk fat without emulsifier (x) and fat globules from natural cream (▲). X-axis is residence time of AMCs in RVA processing.