Invited review: Modeling milk stability

Novel insights into the stability of milk and milk products during storage and processing result from describing caseins near neutral pH as hydrophilic, intrinsically disordered, proteins. Casein solubility is strongly influenced by pH and multivalent ion binding. Solubility is high at a neutral pH or above, but decreases as the casein net charge approaches zero, allowing a condensed casein phase or gel to form, then increases at lower pH. Of particular importance for casein micelle stability near neutral pH is the proportion of free caseins in the micelle (i.e., caseins not bound directly to nanoclusters of calcium phosphate). Free caseins are more soluble and better able to act as molecular chaperones (to prevent casein and whey protein aggregation) than bound caseins. Some free caseins are highly phosphorylated and can also act as mineral chaperones to inhibit the growth of calcium phosphate phases and prevent mineralized deposits from forming on membranes or heat exchangers. Thus, casein micelle stability is reduced when free caseins bind to amyloid fibrils, destabilized whey proteins or calcium phosphate. The multivalent-binding model of the casein micelle quantitatively describes these and other factors affecting the stability of milk and milk protein products during manufacture and storage.

Analysis of Volatile Compounds in Processed Cream Cheese Models for the Prediction of "Fresh Cream" Aroma Perception

Controlling flavor perception by analyzing volatile and taste compounds is a key challenge for food industries, as flavor is the result of a complex mix of components. Machine-learning methodologies are already used to predict odor perception, but they are used to a lesser extent to predict aroma perception. The objectives of this work were, for the processed cream cheese models studied, to (1) analyze the impact of the composition and process on the sensory perception and VOC release and (2) predict "fresh cream" aroma perception from the VOC characteristics. Sixteen processed cream cheese models were produced according to a three-factor experimental design: the texturing agent type (κ-carrageenan, agar-agar) and level and the heating time. A R-A-T-A test on 59 consumers was carried out to describe the sensory perception of the cheese models. VOC release from the cheese model boli during swallowing was investigated with an in vitro masticator (Oniris device patent), followed by HS-SPME-GC-(ToF)MS analysis. Regression trees and random forests were used to predict "fresh cream" aroma perception, i.e., one of the main drivers of liking of processed cheeses, from the VOC release during swallowing. Agar-agar cheese models were perceived as having a "milk" odor and favored the release of a greater number of VOCs; κ-carrageenan samples were perceived as having a "granular" and "brittle" texture and a "salty" and "sour" taste and displayed a VOC retention capacity. Heating induced firmer cheese models and promoted Maillard VOCs responsible for "cooked" and "chemical" aroma perceptions. Octa-3,5-dien-2-one and octane-2,3-dione were the two main VOCs that contributed positively to the "fresh cream" aroma perception. Thus, regression trees and random forests are powerful statistical tools to provide a first insight into predicting the aroma of cheese models based on VOC characteristics.

Evaluation of processed cheese viscoelastic properties during sterilization observed in situ

Sterilized processed cheese is a specific dairy product with a prolonged shelf life intended for regular retail offer but also as food provisions for armies during peacetime, as well as during crisis and emergency situations, and for storage in state material reserves. Storage requirements are usually defined as ≤25°C for at least 24 mo. One of the ways to achieve such a shelf life is sterilization. Therefore, the aim of the work was to describe, for the first time in the available scientific literature, in situ changes in the viscoelastic properties of spreadable melt (34% wt/wt DM content, 45% wt/wt fat in DM content, and 14% wt/wt protein content) during an increase in temperature (target temperature 122°C), holding at sterilization temperature (20 min) and subsequent cooling (to ~30°C). While increasing to the target sterilization temperature, a significant decrease occurred in the storage and loss moduli values. Both moduli started to increase again during the target sterilization temperature period and during the whole cooling phase. The values of the storage and loss moduli were significantly higher at the end of the cooling of the sterilized product, and conversely, the phase angle value was lower compared with the melt before sterilization. As a result of sterilization, an increase occurred in the levels of markers of the Maillard reaction complex and lipid oxidation processes. The value of hardness, corrected stress, and elongational viscosity also increased compared with nonsterilized products. As a result of sterilization, the flavor worsened and sterilized processed cheeses showed darker (brownish) color. However, even after sterilization, the products were evaluated as acceptable for consumers and maintained their spreadability.

Amyloid fibril formation by αS1- and β-casein implies that fibril formation is a general property of casein proteins

Caseins are a diverse family of intrinsically disordered proteins present in the milks of all mammals. A property common to two cow paralogues, α_S2- and κ-casein, is their propensity in vitro to form amyloid fibrils, the highly ordered protein aggregates associated with many age-related, including neurological, diseases. In this study, we explored whether amyloid fibril-forming propensity is a general feature of casein proteins by examining the other cow caseins (α_S1 and β) as well as β-caseins from camel and goat. Small-angle X-ray scattering measurements indicated that cow α_S1- and β-casein formed large spherical aggregates at neutral pH and 20°C. Upon incubation at 65°C, α_S1- and β-casein underwent conversion to amyloid fibrils over the course of ten days, as shown by thioflavin T binding, transmission electron microscopy, and X-ray fibre diffraction. At the lower temperature of 37°C where fibril formation was more limited, camel β-casein exhibited a greater fibril-forming propensity than its cow or goat orthologues. Limited proteolysis of cow and camel β-casein fibrils and analysis by mass spectrometry indicated a common amyloidogenic sequence in the proline, glutamine-rich, C-terminal region of β-casein. These findings highlight the persistence of amyloidogenic sequences within caseins, which likely contribute to their functional, heterotypic self-assembly; in all mammalian milks, at least two caseins coalesce to form casein micelles, implying that caseins diversified partly to avoid dysfunctional amyloid fibril formation.

Influence of Emulsifying Salts on the Growth of Bacillus thuringiensis CFBP 3476 and Clostridium perfringens ATCC 13124 in Processed Cheese

Processed cheese is a dairy product with multiple end-use applications, where emulsifying salts play a fundamental role in physicochemical changes during production. Moreover, some of these salts may be a strategy to control spoilage and pathogenic microorganisms, contributing to safety and shelf life extension. This study aimed to evaluate the in vitro inhibitory activity of two emulsifying salts (ESSP = short polyP and BSLP = long polyP) against Bacillus thuringiensis CFBP 3476 and Clostridium perfringens ATCC 13124, and to compare the in situ effects of two emulsifying salts treatments (T1 = 1.5% ESSP and T2 = 1.0% ESSP + 0.5% BSLP) in processed cheeses obtained by two different methods (laboratory- and pilot-scales), during 45-day storage at 6 °C. C. perfringens ATCC 13124 growth was not affected in vitro or in situ (p > 0.05), but both of the treatments reduced B. thuringiensis CFBP 4376 counts in the tested condition. Counts of the treatments with B. thuringiensis CFBP 3476 presented a higher and faster reduction in cheeses produced by the laboratory-scale method (1.6 log cfu/g) when compared to the pilot-scale method (1.8 log cfu/g) (p < 0.05). For the first time, the inhibitory effect of emulsifying salts in processed cheeses obtained by two different methods was confirmed, and changes promoted by laboratory-scale equipment influenced important interactions between the processed cheese matrix and emulsifying salts, resulting in B. thuringiensis CFBP 4376 growth reduction.

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

The effects of varying the concentration of pentasodium triphosphate (PP) emulsifying salt [0, 0.6, 1.2, 1.5, and 1.8%, plus 0.9% of a mixture of citric acid (CA) and disodium phosphate (DSP) to adjust cheese pH to 5.85] on rheological, textural, physicochemical, and microstructural properties were studied in a processed cheese model system containing ~20% micellar casein concentrate, ~20% sunflower oil, and ~59% water. Special emphasis was placed on the unique casein fibrils recently described in a comparable processed cheese model system. Our results show that during processing (90°C, 17.37 rpm over 270 min) the apparent viscosity increased more and faster for formulations containing higher concentrations of PP, in analogy to the so-called creaming reaction, a general thickening of the molten cheese mass with prolonged processing. We found that 1.2% PP (plus 0.9% CA-DSP) appeared to be the threshold for the creaming reaction to take place. With increasing PP concentrations, cheese hardness increased in a sigmoidal fashion, and insoluble (protein-bound) calcium concentration decreased exponentially. Light micrographs of samples taken at the end of processing indicated initially large and dense casein aggregates within the matrix that disappeared with higher levels of PP, in parallel with the development of a finer emulsion. With transmission electron microscopy analysis on the same samples, the highly complex restructuring of the casein matrix was evident; casein fibrils had formed de novo at the periphery of the loosening casein aggregates. With higher levels of PP, amorphous areas were observed in place of the dense casein aggregates that appeared progressively void of protein, whereas fibril concentration increased throughout the rest of the matrix. Fibrils progressively attached to the surface of fat globules, thereby emulsifying them. Reverse-phase HPLC analysis of insoluble and soluble fractions indicated κ-casein to be the most likely constituent of the newly formed fibrils. The results of this study suggest that PP induced a concentration-dependent dissociation of caseins (through increased calcium chelation) and further led to their spatial separation. In essence, their chaperone activity was hindered, which resulted in amorphous aggregation on the one hand and fibril formation on the other.