Mechanisms of structure formation underlying the creaming reaction in a processed cheese model system as revealed by light and transmission electron microscopy

Abstract

The "creaming reaction," a general thickening of the molten cheese mass during the manufacture of processed cheese, which is often seen to occur in a stepwise fashion, affects the viscosity and texture of the finished product. Thus, this phenomenon is of critical importance for the processed cheese industry, yet mechanisms underlying the structure formation in this surprisingly complex and dynamic food system are only poorly understood. Using a model system consisting of micellar casein concentrate, vegetable oil, water, and a mixture of melting salts, we followed the characteristic viscosity profile with its primary and secondary increase over time. A rheometer equipped with a custom-made cup geometry was used, which served as a mini-reaction vessel to simulate the conditions during the manufacture of processed cheese. The mixture was subjected to constant heat (90°C) and stirring (7.93 rpm), comparable to processed cheese cooking, for up to 410 min. At specific time points, samples were taken, and the micro-and ultrastructure was investigated with light and transmission electron microscopy. Results from our extensive study uncovered the following key steps: (1) a decrease in fat globule size with concomitant increase in the number of fat globules, which were also more evenly distributed; (2) a progressive separation of the casein matrix into fibrillogenic and nonfibrillogenic fractions; (3) formation of fibrils and their higher-order structuring followed by their partial degradation; and (4) increasing interactions of the fibrils with the fat globule surface leading to a higher degree of emulsification. Of these different observations, results indicate that after the caseins dissociated under the influence of the melting salts, protein-protein interactions were the primary driver of the structure formation and thus contributed to the initial viscosity increase. Fat globules were involved in the structure formation at later time points. Therefore, fat-protein interactions in addition to continued protein-protein interactions were assumed to contribute to the secondary viscosity increase. An updated processed cheese creaming model is presented. The use of the term "texturization" instead of "creaming" is proposed.

Effect of pentasodium triphosphate concentration on physicochemical properties, microstructure, and formation of casein fibrils in model processed cheese

“…For samples containing 1.2% PP and higher, they also started to fluctuate toward the end of processing, an indication that parts of the molten mass periodically adhered to the stirrer and were randomly released, thus making the corresponding torque measurements less accurate. We observed a similar behavior previously (Vollmer et al, 2021). Whether an increase in PP concentration also led to a shorter time for protein dispersal and solubilization was difficult to assess, as clear inflection points were not evident in the apparent viscosity profile.…”

“…Chelating even more calcium (78% and 85%) by increasing the PP concentration to 1.5% and 1.8%, respectively, did not change the size of the fat globules noticeably (by qualitative assessment) but caused small areas without discernable internal structure to appear within the casein matrix (arrowheads in Figure 5E and F). These bore high resemblance to those observed in a comparable processed cheese model system containing 1.75% PP, 0.88% CA, 0.44% DSP, and 0.44% trisodium citrate (Vollmer et al, 2021).…”

“…The increase in apparent viscosity along the processing time-that is, the "creaming reaction," or the "texturization reaction," as Vollmer et al (2021) have recently suggested calling it-has been observed to occur in more or less distinct steps, depending on process and compositional parameters (Lenze et al, 2019;Vollmer et al, 2021), but no such steps were distinguishable in the current set of experiments. Rather, the viscosity increase was first gradual and then very rapid, particularly for higher concentrations of PP.…”

“…The magnitude of the creaming reaction in processed cheese heavily depends on compositional and processing conditions (Röck, 2010;Fu et al, 2018;Lenze et al, 2019). In a recent study, we investigated the underlying mechanisms leading to this viscosity increase, using high-resolution transmission electron microscopy, and found that it was mainly related to a restructuring of the casein matrix (Vollmer et al, 2021). This had already been postulated by others (Lee et al, 2003;Kawasaki, 2008;Heertje, 2014).…”

“…Although it would be natural to assume that these amorphous areas directly originated from coagulated casein micelles within the MCC powder particles as a consequence of the destabilizing effect of the emulsifying salts on the calcium-bridged protein network, we cannot make this conclusion, as we did not take samples during processing, only at the end, at 270 min. On the contrary, from experiments following the structure formation during the creaming reaction, we know that the MCC powder particles were completely hydrated, and nearly all casein micelles were dispersed and dissociated within 15 min of processing (when using 1.75% PP), after which the amorphous areas slowly appeared from a homogeneous casein matrix (Vollmer et al, 2021). Whether a similar behavior can be seen with lower concentrations of PP remains to be determined.…”