Rheological properties and microstructure of high protein acid gels prepared from reconstituted milk protein concentrate powders of different protein contents

Structural dependence of concentrated skim milk curd on micellar restructuring

This study was conducted to establish an understanding of how milk concentration modulates the rennet curd structure. Rennet-induced gelation and renneting under slow acidification achieved using glucono-δ-lactone (GDL) and structural properties of reconstituted skim milk gels at two concentration levels (9 and 25 % total solids) were studied by measuring variations in (a) viscoelastic behaviour, (b) micellar size, charge density, diffusivity, and (c) hydrophobicity using dynamic rheometry, dynamic light scattering and fluorimetry, respectively. Concentrated milk showed a greater estimated hydrodynamic radius of casein micelles, lower zeta (ζ)-potential, ratio of serum to total Calcium (Ca) and charge density and increased surface hydrophobicity, all supporting the view that micellar restructuring particularly sub-particle transfer takes place and contributes to rapid gelation. Moreover, hydrophobic interactions occurred very quickly (within 5 min in combined gels, 10 min for renneting only), demonstrating their pivotal role during the flocculation stage. All gels exhibited a solid viscoelastic character as the elastic modulus (G') was greater than loss modulus (G″) while both G' and tan δ (G''/G') were frequency-dependent. Frequency sweeps classified the concentrated gels into three stiffness categories caused by the level of rennet or GDL as rigid, hard and soft, whereas an increased flow-like behaviour (high tan δ), restricted diffusion and excessive water retention revealed limited structural rearrangements (contraction & macrosyneresis) during curd ageing. Acidification increased the diffusion rate in control curd, thus, enhanced contractive rearrangements, macrosyneresis and curd strength. Findings suggest that micellar restructuring induced by milk concentration is the principal modulator of the curd structure.

Storage impact on egg white powder's physical and functional properties

BACKGROUND

Changes in storage temperature and time alter the functional properties of egg white powder (EWP) and determine its quality and shelf-life, finally affecting the consumer acceptance of the products made from EWP. In the present study, the EWP samples were stored at four different temperatures (-20, 4, 25 and 37 °C) for 60 days, and then the protein structural, physical and functional properties of EWP were measured and assessed further for correlation with storage conditions using heatmap.

RESULTS

The viscosity of the EWP solution increased after 30 days. Foaming ability and rheological properties increased first and then decreased compared to untreated samples with the prolonged storage time. Correlation analysis results indicated that the gel hardness, water holding capacity, foaming ability, emulsifying ability, particle size, dispersibility and viscosity of EWP were significantly related to storage time (P < 0.05). Only the gelation properties of EWP stored at 37 °C for 60 days changed significantly and were negatively related to its moisture content (P < 0.05). Additionally, the random coil content of EWP was positively correlated with particle size, moisture content, solubility and gel properties, whereas β-sheet was negatively correlated with them.

CONCLUSION

Compared to other temperatures, the functional properties of EWP were relatively stable under 4 °C. Therefore, the low temperature (4 °C) was selected as the most suitable storage temperature for EWP. The results of the present study could provide a theoretical basis for the shelf-life extension of EWP. © 2022 Society of Chemical Industry.

Colloidal and Acid Gelling Properties of Mixed Milk and Pea Protein Suspensions

The present study aims to describe colloidal and acid gelling properties of mixed suspensions of pea and milk proteins. Mixed protein suspensions were prepared by adding pea protein isolate to rehydrated skimmed milk (3% w/w protein) to generate four mixed samples at 5, 7, 9, and 11% w/w total protein. Skimmed milk powder was also used to prepare four pure milk samples at the same protein concentrations. The samples were analyzed in regard to their pH, viscosity, color, percentage of sedimentable material, heat and ethanol stabilities, and acid gelling properties. Mixed suspensions were darker and presented higher pH, viscosity, and percentage of sedimentable material than milk samples. Heat and ethanol stabilities were similar for both systems and were reduced as a function of total protein concentration. Small oscillation rheology and induced syneresis data showed that the presence of pea proteins accelerated acid gel formation but weakened the final structure of the gels. In this context, the results found in the present work contributed to a better understanding of mixed dairy/plant protein functionalities and the development of new food products.

Kinetics of pepsin-induced hydrolysis and the coagulation of milk proteins

Hydrolysis-induced coagulation of casein micelles by pepsin occurs during the digestion of milk. In this study, the effect of pH (6.7-5.3) and pepsin concentration (0.110-2.75 U/mL) on the hydrolysis of κ-casein and the coagulation of the casein micelles in bovine skim milk was investigated at 37°C using reverse-phase HPLC, oscillatory rheology, and confocal laser scanning microscopy. The hydrolysis of κ-casein followed a combined kinetic model of first-order hydrolysis and putative pepsin denaturation. The hydrolysis rate increased with increasing pepsin concentration at a given pH, was pH dependent, and reached a maximum at pH ∼6.0. Both the increase in pepsin concentration and decrease in pH resulted in a shorter coagulation time. The extent of κ-casein hydrolysis required for coagulation was independent of the pepsin concentration at a given pH and, because of the lower electrostatic repulsion between para-casein micelles at lower pH, decreased markedly from ∼73% to ∼33% when pH decreased from 6.3 to 5.3. In addition, the rheological properties and the microstructures of the coagulum were markedly affected by the pH and the pepsin concentration. The knowledge obtained from this study provides further understanding on the mechanism of milk coagulation, occurring at the initial stage of transiting into gastric conditions with high pH and low pepsin concentration.

Heat-Induced Interaction of Milk Proteins: Impact on Yoghurt Structure

Heating milk for yoghurt preparation has a significant effect on the structural properties of yoghurt. Milk heated at elevated temperature causes denaturation of whey protein, aggregation, and some case gelation. It is important to understand the mechanism involved in each state of stabilization for tailoring the final product. We review the formation of these complexes and their consequence on the physical, rheological, and microstructural properties of acid milk gels. To investigate the interactions between denatured whey protein and casein, the formation of covalent and noncovalent bonds, localization of the complexes, and their impact on ultimate gelation and final yoghurt texture are reviewed. The information regarding this fundamental mechanism will be beneficial to develop uniform quality yoghurt texture and potential interest of future research.

Fat-free fermented concentrated milk products as milk protein-based microgel dispersions: Particle characteristics as key drivers of textural properties

The popularity of fat-free fermented concentrated milk products, such as fresh cheeses and high-protein yogurt, has increased over the recent years, attributed to greater availability and improvements in taste and texture. These improvements have been achieved through modifications and new developments in processing technologies, for example, higher heat treatment intensities and incorporating different membrane filtration technologies. Though numerous processing parameters are discussed in the literature, as well as reasons behind the developments, detailed examinations of how process modifications affect the final textural attributes of these products are lacking. To draw links between processing parameters and texture, we review the literature on fat-free fermented concentrated milk products from the perspective of fermented milk protein-based microgel particles as the basic structural unit. At each main processing step, relationships between process parameters, micro- and macrostructural and sensory (textural) properties are discussed.An overview of particle characteristics that drive structural changes at each processing step is developed in relation to textural characteristics. Using this approach of assessing relationships between structural characteristics of concentrated dispersions of fat-free fermented milk protein-based microgel particles and processing parameters provides a basic context for the selection of optimal parameters to achieve a desired texture.

Physicochemical and microstructural properties of fermentation-induced almond emulsion-filled gels with varying concentrations of protein, fat and sugar contents

The influence of the protein, fat and sugar in almond milk on the formation of the acidic gel was investigated by determining their physicochemical and microstructural properties. The protein, fat and sugar in the almond milk were varied from 2% to 6%, 0.8%-7% and 0.6%-7%, respectively and fermented using Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophiles cultures to form a gel structure. Both protein and fat increased the gel strength, viscosity (stirred gel) and lightness of almond yoghurts as the concentration increased. The addition of protein content increased the cohesiveness (from 0.70 to 1.17), water holding capacity (from 28.75% to 52.22%) and D_4,3 value of particle size (from 32.76 μm to 44.41 μm) of almond yoghurt. Fat reduction decreased the firmness (from 6.56 g to 4.69 g), D_4,3 value (from 88.53 μm to 18.37 μm), and water holding capacity (from 48.96% to 27.66%) of almond yoghurt. With sugar addition, almond yoghurt showed increased adhesiveness, decreased lightness and a low pH, with no significant difference in firmness, particle size, and flow behaviour. The confocal images provided evidence that the fortified protein contents homogeneously entrapped fat globules resulting in a more stable gel network and increased fat content led to large fat globule formation resulting in a harder gel network, while the added sugar did not significantly affect the gel network. The results suggested that the protein fortification enhances the texture of almond yoghurt. The fat content of 7% with 3.5% protein showed poor consistency and gel strength of yoghurt. Sugar mainly contributed to bacterial metabolism during fermentation.

Investigation of the flowability, thermal stability and emulsification properties of two milk protein concentrates having different levels of native whey proteins

Milk protein concentrate-85 (MPC85) is a dairy ingredient which has a diverse range of applications in food products. The technofunctional properties of two MPC85 samples having similar gross composition but different levels of native whey protein (WP), i.e., MPC85S1 and MPC85S2 with 16.6 and 6.0 g native WP/100 g protein, respectively, were compared. Rheometeric analysis showed that under an applied normal stress of 1.0-15.0 kPa, the compressibility, the air permeability and the cohesiveness of MPC85S2 was higher compared to MPC85S1. Differential scanning calorimetry showed that protein denaturation in MPC85S1 began at 63 °C while for MPC85S2 it began at 70 °C. The heat coagulation time (HCT at 140 °C) for 4.2% (w/v, on a protein basis) reconstituted MPC85S1 and MPC85S2 was 2.2 and 2.7 min, respectively. While a higher lightness for MPC85S1 was evidenced using colourimeter analysis, the colour stability on oven drying at 95 °C for MPC85S2 was higher than MPC85S1. The emulsion produced with MPC85S1 flocculated after 1 d and phase separation occurred after 14 d. In the case of MPC85S2, flocculation began after 4 d while phase separation was observed at 33 d. The viscosity of MPC85S2 (4.2% (w/v) protein) was higher than MPC85S1. This study showed differences between the flowability, viscosity, colour properties, thermal stability (in powder and in reconstituted format), emulsification and buffering capacity for MPC samples having two different levels of WP denaturation. The results demonstrated that the MPCs studied having two different levels of WP denaturation could be targeted for different functional applications. The minimal/maximum level of denaturation required to induce technofunctional property differences requires further study.

The effect of ultrafiltration on the acid gelation properties of protein-standardised skim milk systems

In this study, we investigated the impact of ultrafiltration (UF) on the acid gelation of milk using two protein-standardised milk systems, consisting of either skim milk and retentate (SR) or permeate and retentate (PR), over different seasons in New Zealand. The composition and the physicochemical properties of the two systems before heating were comparable, whereas the levels of heat-induced α-lactalbumin denaturation and the association of the casein micelles with α-lactalbumin were significantly lower in PR than in SR. PR displayed superior acid gelation properties compared with SR, which was most pronounced in the late season. The structural modifications of the whey proteins and casein micelles that were induced by UF and the re-equilibration of calcium in the milk mixtures may have affected the acid gelation properties of the milk by influencing the denaturation and micelle association of the whey proteins. The results suggest that UF has the potential as a tool for tuning the acid gelation properties of milk.

Insolubility in milk protein concentrates: potential causes and strategies to minimize its occurrence

Milk protein concentrates (MPCs), which are produced from skim milk following a series of manufacturing steps including pasteurization, membrane filtration, evaporation and spray drying, represent a relatively new category of dairy ingredients. MPC powders mainly comprise caseins and whey proteins in the same ratio of occurrence as in milk. While bovine MPCs have applications as an ingredient in several protein enriched food products, technofunctional concerns, e.g., reduced solubility and emulsification properties, especially after long-term storage, limit their widespread and consistent utilization in many food products. Changes in the surface and internal structure of MPC powder particles during manufacture and storage occur via casein-casein and casein-whey protein interactions and also via the formation of casein crosslinks in the presence of calcium ions which are associated with diminishment of MPCs functional properties. The aggregation of micellar caseins as a result of these interactions has been considered as the main cause of insolubility in MPCs. In addition, the occurrence of lactose-protein interactions as a result of the promotion of the Maillard reaction mainly during storage of MPC may lead to greater insolubility. This review focuses on the solubility of MPC with an emphasis on understanding the factors involved in its insolubility along with approaches which may be employed to overcome MPC insolubility. Several strategies have been developed based on manipulation of the manufacturing process, along with composition, physical, chemical and enzymatic modifications to overcome MPC insolubility. Despite many advances, dairy ingredient manufacturers are still investigating technical solutions to resolve the insolubility issues associated with the large-scale manufacture of MPC.

Updating insights into the rehydration of dairy-based powder and the achievement of functionality

Dairy-based powder had considerable development in the recent decade. Meanwhile, the increased variety of dairy-based powder led to the complex difficulties of rehydrating dairy-based powder, which could be the poor wetting or dissolution of powder. To solve these various difficulties, previous studies investigated the rehydration of powder by mechanical and chemical methods on facilitating rehydration, while strategies were designed to improve the rate-limiting rehydration steps of different powder. In this review, special emphasis is paid to the surface and structure of the dairy-based powder, which was accountable for understanding rehydration and the rate-limiting step. Besides, the advantage and disadvantage of methods employed in rehydration were described and compared. The achievement of the powder functionality was finally discussed and correlated with the rehydration methods. It was found that the surface and structure of dairy-based powder were decided by the components and production of powder. Post-drying methods like agglomeration and coating can tailor the surface and structure of powder afterwards to obtain better rehydration. The merit of the mechanical method is that it can be applied to rehydrate dairy-based powder without any addition of chemicals. Regarding chemical methods, calcium chelation is proved to be an effective chemical in rehydration casein-based powder.

Effect of pH and heat treatment conditions on physicochemical and acid gelation properties of liquid milk protein concentrate

Milk protein concentrates (MPC) are typically dried high-protein powders with functional and nutritional properties that can be tailored through modification of processing conditions, including temperature, pH, filtration, and drying. However, the effects of processing conditions on the structure-function properties of liquid MPC (fluid ultrafiltered milk), specifically, are understudied. In this report, the pH of liquid MPC [13% protein (70% protein DM basis), pH 6.7] was adjusted to 6.5 or 6.9, and samples at pH 6.5, 6.7, and 6.9 were subjected to heat treatment at either 85°C for 5 min or 125°C for 15 s. Sodium dodecyl sulfate PAGE was used to determine the distribution of caseins and denatured whey proteins in the soluble and micellar phases, and HPLC was used to quantify native whey proteins as a measure of denaturation, based on the processing conditions. Both heat treatments resulted in substantial whey protein denaturation at each pH, with β-lactoglobulin denatured more extensively than α-lactalbumin. Changes in liquid MPC physicochemical properties were monitored at d 1, 5, and 8 during storage at 4°C. Viscosity increased after heat treatment and also over time, regardless of pH and heating conditions, suggesting the role of whey protein denaturation and aggregation, and their interactions with casein micelles. The MPC samples processed at pH 6.9 had a significantly higher viscosity than those heated at pH 6.5 or 6.7, for both temperature and time conditions; and samples processed at 85°C for 5 min had higher viscosity than those heated at 125°C for 15 s. Particle size analysis indicated the presence of larger particles after 5 and 8 d of MPC storage after heating at pH 6.9. Acid-induced gelation of the liquid MPC led to significantly higher gel firmness after processing at 85°C for 5 min, compared with 125°C for 15 s. Also, gels made from MPC adjusted to pH 6.5 had higher storage moduli, with both time and temperature combinations, demonstrating the role of pH-dependent association of denatured whey proteins with casein micelles in gel network formation. These findings enable a better understanding of the processing factors contributing to structural and functional properties of liquid MPC and can be helpful in tailoring milk protein ingredient functionality for a variety of food products.

Acid-Induced Gelation of Enzymatically and Nonenzymatically Cross-Linked Caseins-Texture Properties, and Microstructural Insights

Casein gels consist of a fractal organized network of aggregated casein particles. The gel texture thereby depends on the structure, the spatial distribution, and the interaction forces of the network's elementary building blocks. The aim of this study was to explore the technofunctional consequences of a possible specificity of Maillard reaction-induced cross-linking reactions on casein with respect to texture and microstructure of acid gels. Therefore, sodium caseinate glycated with lactose in the dry state (60 °C, a_w 0.5) was compared with casein samples cross-linked with methylglyoxal, with glutaraldehyde, or via microbial transglutaminase, respectively, at similar levels of protein cross-linking as confirmed by size-exclusion chromatography under denaturing conditions. Casein gels prepared by acidification with glucono-δ-lactone were characterized concerning pH kinetics during gelation, mechanical texture properties under large deformation, and water-holding capacity, while viscometric properties of casein suspensions were obtained prior to gelation. The gel microstructure was captured by confocal laser scanning microscopy and evaluated by means of image texture analysis. All protein cross-linking reactions studied led to an enhanced gel strength which was accompanied by an increased interconnectivity of the gel network and a decrease in apparent pore sizes. Gels with more densely packed strands, as was the case for enzymatically modified casein, exhibited pronounced mechanical stability. The spontaneous destabilization of the gel network upon prolonged glycation reactions, which was not obviously displayed by microstructural features but connected to an increased viscosity and pronounced pseudoplastic flow of the unacidified suspension, suggests a limitation of particle rearrangements and the weakening of interparticle protein-protein interactions by additional structure attributes formed during the early Maillard reaction (glycoconjugation).

Effect of seasonal variations on the acid gelation of milk

We investigated the effect of seasonal variations on the acid gelation properties of bovine milk in a seasonal-calving New Zealand herd for 2 full milking seasons. We tested the formation of acid gels in 2 milk systems: unstandardized skim milk and standardized whole milk (4.6% protein, 4.0% fat). For unstandardized skim milk, late-season milk acid gels had a longer gelation time and a lower gelation pH than early- and mid-season milk acid gels, but we found no consistent seasonal variation in the final storage modulus. For standardized milk, late-season milk had the most inferior acid gelation properties during the year, including the lowest final storage modulus, the lowest gelation pH, and the longest gelation time. Standardization alleviated but did not eliminate the prolonged gelation time of late-season milk. These results indicated that the physicochemical properties of seasonal milk contributed greatly to its acid gelation, independent of differences in protein content. Standardization was not adequate to stabilize the acid gelation properties of late-season milk. Desirable acid gelation properties correlated with lower glycosylated κ-casein content, lower β-lactoglobulin:α-lactalbumin ratio, lower extent of whey protein-casein micelle association, and lower total calcium and ionic calcium content. We discuss the possible effects of the correlating variables on the acid gelation properties of seasonal milk. Natural variations in the glycosylation degree of κ-casein might play an important role in acid gel structural development by altering the electrostatic and hydrophobic interactions among the milk proteins.

Acid induced gelation behavior of skim milk concentrated by membrane filtration

This work reports a detailed study of the effect of ultrafiltration (UF) and diafiltration (DF) on the acid-induced gelation behavior of fresh milk retentates (2× and 4×). Concentrates were heated at 80°C for 15 min, and compared to unheated samples. The use of extensive DF caused a significantly greater amount of protein (both caseins and whey proteins) in the supernatant fraction, compared to UF retentates at the same concentration, both in unheated and heated samples. DF retentates showed higher pH of gelation, compared to the corresponding UF retentates. The development of tan δ is reported for the first time as a function of colloidal calcium release, and the protein gelation behavior discussed in light differences in composition of the soluble fraction. The results demonstrate how processing history can affect compositional changes and the gelation behavior of fresh milk retentates. Membrane filtration is a widespread unit operation in the dairy industry, employed either to prepare fresh concentrates for further processing, or ingredients with specific functional properties. This work describes in detail the effect of processing history during membrane filtration on the rheological properties of acid induced gels and will help in optimizing formulations and prepare the right ingredients for the right application. It will also be possible to determine new ways to define processing quality of the milk protein concentrates, as it relates to their ability to form texture in fermented dairy products.

The ability of spore formers to degrade milk proteins, fat, phospholipids, common stabilizers, and exopolysaccharides

Spore formers are common spoilage-causing microorganisms in dairy products; however, their modes of spoilage (proteolysis, lipolysis, etc.) have not been described in detail for cultured dairy products such as sour cream and yogurt. The objective of the present study was to test the ability of spore-forming strains isolated from dairy environments for their spoilage-causing activities at typical sour cream (24°C) and yogurt (42°C) fermentation temperatures. A total of 25 spore-forming strains were isolated from different sources, including raw milk, pasteurizer balance tank, biofilms formed on heat exchangers, and milk powder. These strains were tested for proteolytic and lipolytic activities and for their ability to degrade phospholipids, common stabilizers (starch, gelatin, xanthan gum, pectin), and exopolysaccharides (EPS) at sour cream and yogurt fermentation temperatures. A higher percentage of positive strains was observed for selected activities at yogurt fermentation temperature compared with sour cream fermentation temperature. Identified proteolytic spore-forming strains, based on a skim milk agar method, were subsequently quantified for their level of proteolysis using non-casein nitrogen (NCN) content and sodium dodecyl sulfate-PAGE (SDS-PAGE). The proteolytic strains that showed the highest levels of proteolysis (highest percentages of NCN content) at 24°C were Bacillus mojavensis BC, Bacillus cereus DBC, Bacillus subtilis DBC, B. mojavensis DBC1, and Paenibacillus polymyxa DBC1. At 42°C the strains with the highest levels of proteolysis (highest percentages of NCN content) were B. subtilis DBC, B. mojavensis BC, B. mojavensis DBC1, B. cereus DBC, and Bacillus licheniformis DBC6. Results of SDS-PAGE demonstrated that proteolytic strains had primarily hydrolyzed β- and κ-CN. A viscometric method was used to evaluate the susceptibility of exopolysaccharides (EPS) to degradation by selected spore formers. This method helped to determine that EPS produced by commercial yogurt and sour cream cultures is susceptible to degradation by spore formers present in dairy environments.

Effects of milk heat treatment and solvent composition on physicochemical and selected functional characteristics of milk protein concentrate

Milk protein concentrate (MPC) powders (∼81% protein) were made from skim milk that was heat treated at 72°C for 15 s (LHMPC) or 85°C for 30 s (MHMPC). The MPC powder was manufactured by ultrafiltration and diafiltration of skim milk at 50°C followed by spray drying. The MPC dispersions (4.02% true protein) were prepared by reconstituting the LHMPC and MHMPC powders in distilled water (LHMPC_w and MHMPC_w, respectively) or milk permeate (LHMPC_p and MHMPC_p, respectively). Increasing milk heat treatment increased the level of whey protein denaturation (from ∼5 to 47% of total whey protein) and reduced the concentrations of serum protein, serum calcium, and ionic calcium. These changes were paralleled by impaired rennet-induced coagulability of the MHMPC_w and MHMPC_p dispersions and a reduction in the pH of maximum heat stability of MHMPC_p from pH 6.9 to 6.8. For both the LHMPC and MHMPC dispersions, the use of permeate instead of water enhanced ethanol stability at pH 6.6 to 7.0, impaired rennet gelation, and changed the heat coagulation time and pH profile from type A to type B. Increasing the severity of milk heat treatment during MPC manufacture and the use of permeate instead of water led to significant reductions in the viscosity of stirred yogurt prepared by starter-induced acidification of the MPC dispersions. The current study clearly highlights how the functionality of protein dispersions prepared by reconstitution of high-protein MPC powders may be modulated by the heat treatment of the skim milk during manufacture of the MPC and the composition of the solvent used for reconstitution.

Short communication: Effects of nanofiltration and evaporation on the gel properties of milk protein concentrates with different preheat treatments

This study aimed to evaluate the effects of different concentration methods (nanofiltration and evaporation) and heat treatments on the gel properties of milk protein concentrate (MPC). The MPC gels were produced using glucono-δ-lactone (GDL) as an acidifier with different preheat treatments (30 min at 80°C and 5 min at 92°C). We then evaluated the effect of preheat treatments on MPC gel properties, including storage modulus (G'), loss tangent (tan δ), firmness, whey separation, and microstructure. The results indicated that without preheating, evaporation (EP)-MPC had higher G' and firmness, and lower tan δ and whey separation than nanofiltration (NF)-MPC. These results suggest that EP-MPC produced a better acid-induced gel than NF-MPC when no preheat treatments were performed. After preheating, however, except for a very small difference in the final G' (EP-MPC was higher), the 2 MPC did not differ significantly in firmness, final tan δ, or whey separation. Additionally, compared with the gel of unheated MPC, both preheat-treated gels (NF-MPC and EP-MPC) achieved increased G' and firmness and decreased tan δ and whey separation. The preheat-treated MPC also displayed a more flexible-stranded network. These findings demonstrate that, given a suitable heating treatment, NF-MPC compares favorably with EP-MPC in achieving desired gel properties.

Influence of partially demineralized milk proteins on rheological properties and microstructure of acid gels

Innovative clean label processes employed in the manufacture of acid gels are targeted to modify the structure of proteins that contribute to rheological properties. In the present study, CO₂-treated milk protein concentrate powder with 80% protein in dry matter (TMPC80) was mixed with nonfat dry milk (NDM) in different ratios for the manufacture of acid gels. Dispersions of NDM and TMPC80 that provided 100, 90, 70, and 40% of protein from NDM were reconstituted to 4.0% (wt/wt) protein and 12.0% (wt/wt) total solids. Dispersions were adjusted to pH 6.5, followed by heat treatment at 90°C for 10 min. Glucono-δ-lactone was added and samples were incubated at 30°C, reaching pH 4.5 ± 0.05 after 4 h of incubation. Glucono-δ-lactone levels were adjusted to compensate for the lower buffering capacity of samples with higher proportions of TMPC80, which is attributable to the depletion of buffering minerals from both the serum and micellar phase during preparation of TMPC80. Sodium dodecyl sulfate-PAGE analysis indicated a higher amount of caseins in the supernatant of unheated suspensions with increasing proportions of CO₂-treated TMPC80, attributable to the partial disruption of casein micelles in TMPC80. Heat treatment reduced the level of whey proteins in the supernatant due to the heat-induced association of whey proteins with casein micelles, the extent of which was larger in samples containing more micellar casein (i.e., samples with a lower proportion of TMPC80). Particle size analysis showed only small differences between nonheated and heated dispersions. Gelation pH increased from ˜5.1 to ˜5.3, and the storage modulus of the gels at pH 4.5 increased from ˜300 to ˜420 Pa when the proportion of protein contributed by TMPC80 increased from 0 to 60%. Water-holding capacity also increased and gel porosity decreased with increasing proportion of protein contributed by TMPC80. The observed gel properties were in line with microstructural observations by confocal microscopy, wherein sample gels containing increasing levels of TMPC80 exhibited smaller, well-connected aggregates with uniform, homogeneous pore sizes. We concluded that TMPC80 can be used to partially replace NDM as a protein source to improve rheological and water-holding properties in acid gels. The resultant gels also exhibited decreased buffering, which can improve the productive capacity of yogurt manufacturing plants. Overall, the process can be leveraged to reduce the amount of hydrocolloids added to improve yogurt consistency and water-holding capacity, thus providing a path to meet consumer expectations of clean label products.