Milk Gels Research Articles

Changes in Protease Activity of Ginger Rhizome during Postharvest Storage

It has been reported that ripe ginger rhizomes contain proteolytic enzymes. In this study, we investigated the fluctuations in protease activity from ginger rhizomes stored after harvest for the utilization of proteases. The juice from ginger rhizomes that had been stored for 4 months after harvest exhibited the highest specific protease activity. Additionally, juices with high specific protease activity could form soymilk and cow milk gels. Electrophoresis experiments revealed that the protein bands in ginger rhizome juice were degraded by the proteases contained in the juice. This degradation was prevented by adding the cysteine protease inhibitor E-64.

Combined effect of acidification temperature and different acids on microstructure and textural properties of heat and acid-induced milk gels

Heat and acid-induced milk gels, such as Indian Paneer cheese, can serve as a meat substitute in various cuisines due to their non-melting and cookable features. This study investigated the effects of acidification temperature (60, 70, 80, or 90 °C) and different acidifiers (citric, lactic, or hydrochloric acids) on the composition, texture, and microstructure of these gels. Across all acids, increasing the acidification temperature reduced the yield of gel due to lower moisture retention. However, the hardness, springiness, and cohesiveness of the gels increased with temperature, influenced by the rate of protein aggregation and calcium recovery. At high temperatures (e.g., 90 °C), gels prepared by citric acid exhibited greater hardness and cohesiveness, but lower springiness compared to those made with lactic or hydrochloric acid. This effect may be attributed to the formation of a more compact gel network, characterized by fewer pores, and lower water mobility.

Impact of ultrasonic and heat treatments on the physicochemical properties and rennet-induced coagulation characteristics of milk from various species

This study investigates the effects of heat and ultrasonic treatments on the physicochemical parameters and rennet-induced coagulation properties of milk from a variety of species, including cow, goat, buffalo, and donkey. Milk samples were subjected to heat treatments at different temperatures (65 °C, 80 °C, 90 °C, 100 °C) and ultrasonic treatment at varying power levels (200 W, 400 W, 600 W, 800 W, 1000 W). The results revealed that changes in turbidity, particle size, zeta potential, secondary structure, and surface hydrophobicity were altered by both ultrasonic and heat treatments, as well as the kind of milk. Ultrasonic treatment of cow milk decreased α-helix content while increasing β-turn content. Under similar ultrasonic treatment, goat milk showed a considerable increase in β-sheet content, whereas β-turn and random coil contents decreased compared to control samples. Notably, the water-holding capacity of gels formed from all four types of milk increased significantly with the intensity of ultrasonic and heat treatments. The hardness of buffalo milk gels increased significantly after ultrasonic and thermal treatments, ranging from 63 °C for 30 min to 90 °C for 15 min, but the hardness of cow and goat milk gels increased in varying degrees compared to their control samples. Furthermore, gels from cow and goat milk had higher storage modulus (G’) and loss modulus (G’’) than those from buffalo and donkey milk, and changes in G’ and G’’ from the examined milk were altered by ultrasonic and heat treatments. These findings offer important insights into refining milk processing procedures to improve dairy product quality and usefulness.

The impact of calcium sequestration on the material and structural properties of acid milk gels and corresponding cream cheeses

SummaryWe report on the role of ethylenediaminetetraacetic acid (EDTA) in addition to the structural and material properties of acid milk gels formed via addition of glucono‐delta‐lactone (GDL) and these properties of corresponding analogue cream cheeses prepared from these gels. Increasing addition of EDTA to cheese milks prior to acidulation reduced the concentration of insoluble calcium in the milks. Acid gelation of milks, as measured by small deformation rheology, showed a decreased rate of gelation and lower relative elastic modulus of gels with increasing EDTA concentration. The sequestration of colloidal calcium phosphate from the casein micelle by the addition of EDTA led to the lower gelation rate and elastic modulus. The lower extent of ionic binding is considered the causative mechanism for formation of weaker gel structures. Cream cheeses prepared using acid gels with GDL acidulant were compositionally unaffected by EDTA concentration. However, variations in G′ observed in the acid gels containing increasing levels of EDTA correlated with large deformation material property changes in cream cheese analogues. In addition, the increasing inclusion of EDTA in cheese milks translated to reduced water‐holding capacity of the cream cheeses prepared from those milks. These results provide insights on the specific role of milk calcium on the structure, material and physical properties of acid milk gels and corresponding cream cheese analogues.

Chemometric insights into milk-carrageenan breaking and gel strength

The relationship between the molecular structure and composition of carrageenan and its milk gelation properties has been studied using chemometric and machine learning tools. Carrageenan types—κ-carrageenan and κ/ι-hybrid— were analyzed for their gel and breaking strengths, crucial for their industrial application as food gelling and thickening agents. Four analytical platforms, Fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) spectroscopy, size-exclusion chromatography with multi-angle light scattering detection (SEC-MALS), and inductively coupled plasma mass spectrometry (ICP-MS), were employed to characterize the molecular structure and composition aiming to predict milk-carrageenan breaking strength.Both single and multi-block predictive modeling were applied to predict functionality, challenging the conventional approach that relies only on single analytical platforms for prediction.Support Vector Machine (SVM) trained on FT-IR spectra, achieved the most accurate predictions, indicating its potential as an efficient alternative to traditional characterization methods by requiring only measurements directly on the carrageenan powder rather than the laborious functionality testing.In examining multi-block modeling, particularly through Sequential and Orthogonalized PLS (SO-PLS), the study evaluated the added value of incorporating further analytical blocks. While adding SEC-MALS and ICP-MS data did not significantly improve prediction models, their inclusion enriched the causal understanding of carrageenan's structure-function relationship.

Effect of heat treatment and extraction pH on macroscopic and molecular properties of dromedary milk gel induced by Urtica dioica extracts

Effect of heat treatment and extraction pH on macroscopic and molecular properties of dromedary milk gel induced by Urtica dioica extracts

Two-step aging dynamics in enzymatic milk gels

Two-step aging dynamics in enzymatic milk gels

Milk serum peptidomics revealed the age gelation of direct UHT milk

Age gelation is undesirable for direct UHT (dUHT) milk, which is closely related to protein hydrolysis. However, little information is available for the role of serum peptides during the age gelation. In this study, the composition and protein morphology of serum phase were characterized by RP-HPLC, ICP-MS and TEM. The results showed significant increases in soluble proteins, free amino acids, calcium, and phosphorus from casein micelles, indicating protein hydrolysis and peptide release into the serum phase. 23,466 peptides derived from caseins and other proteins were identified in serum phase by peptidomics. The serum peptide profiles of age gelation milk changed dramatically. Peptide fingerprinting revealed that plasmin and cathepsin contributed to the protein hydrolysis during age gelation, with a significant increase in their activity observed. 23 characteristic peptides were ultimately selected as potential indicators for age gelation. These findings provide new insights into the age gelation of UHT milk.

Influence of Heating Temperature and pH on Acid Gelation of Micellar Calcium Phosphate-Adjusted Skim Milk.

Micellar calcium phosphate (MCP) plays an important role in maintaining the structure and stability of the casein micelle and its properties during processing. The objective of this study was to investigate how heating (10 min at 80 or 90 °C) at different pH levels (6.3, 6.6, 6.9, or 7.2) impacted the acid-induced gelation of MCP-adjusted milk, containing 67 (MCP67), 100 (MCP100), or 113 (MCP113) % of the original MCP content. The unheated sample MCP100 at pH 6.6 was considered the control. pH acidification to pH 4.5 at 30 °C was achieved with glucono delta-lactone while monitoring viscoelastic behaviour by small-amplitude oscillatory rheology. The partitioning of calcium and proteins between colloidal and soluble phases was also examined. In MCP-depleted skim milk samples, the concentrations of non-sedimentable caseins and whey proteins were higher compared to the control and MCP-enriched skim milk samples. The influence of MCP adjustment on gelation was dependent on pH. Acid gels from sample MCP67 exhibited the highest storage modulus (G'). At other pH levels, MCP100 resulted in the greatest G'. The pH of MCP-adjusted skim milk also impacted the gel properties after heating. Overall, this study highlights the substantial impact of MCP content on the acid gelation of milk, with a pronounced dependency of the MCP adjustment effect on pH variations.

An oral gel suitable for swallowing: The effect of micronization on the gel properties and microstructure of κ-carrageenan

κ-Carrageenan (κ-Car) is an important material for preparing food gels and hydrogels. However, κ-Car gel has issues with high hardness and low water-holding capacity. Modification strategy of micronization is proposed for the first time to explore its influence on texture properties and gelling process of κ-Car gel, and to investigate the feasibility of κ-Car as a food matrix with low strength. κ-Car undergoing 60 min of micronization, the d(0.9) decreased by 79.33 %, SBET and Vtotal increased by 89.23 % and 95.27 %. The swelling rate and degree of gelling process increased significantly, and the microstructure changed from loose large pores to dense small pores resembling a “honeycomb”. Importantly, the hardness of gel-60, Milk-60 and PNS-60 decreased by 72.52 %, 49.25 % and 81.37 %. In addition, WHC of gel-60, Milk-60 and PNS-60 was improved. IDDSI tests showed that κ-Car gels, milk gels and PNS gels can be categorized as level 6 (soft and bite-sized), except for PNS-60, which belongs to level 5 (crumbly and moist). Furthermore, the texture and bitter masking effect of milk gels and PNS gels were improved. In conclusion, this study demonstrated that micronization can be a novel approach to improve the gel properties of κ-Car, laying the groundwork for developing dysphagia foods.

Understand and characterize draining of acid milk gels using Magnetic Resonance Imaging and Time Domain-Nuclear Magnetic Resonance

A novel dedicated device is introduced to enable investigation of the filtration draining process of different acid-induced milk gels using Magnetic Resonance Imaging (MRI). This approach allows the spatio-temporal resolution of serum content and distribution in gels throughout draining. Its application for the characterization of five acid gels formulated using reconstituted or fresh milk and using different thermal treatments and/or lactic starters indicates its large potential for a better understanding of acid-induced gel draining in dairy industry. Self-diffusion and relaxation measurements using Time Domain-Nuclear Magnetic Resonance (TD-NMR) were performed on the gels following filtration draining to validate and supplement the MRI results, while gels obtained through centrifugation were examined in view of correlating T2 relaxation times to the water content of gels. The presence of lipids in reconstituted milk or the substitution of a casein fraction by microparticulated denatured whey proteins were shown to impact the structure of gels and their serum holding capacity.

Lactiplantibacillus plantarum enhances the texture of fermented milk by facilitating protein gelatinization through the action of a novel coagulation-promoting protease

Enzymatic modification can effectively improve the gel properties of protein-based food matrices. However, how bacterial proteases influence milk gels has been rarely reported. In the current study, the proteases isolated from two strains of Lactiplantibacillus plantarum, namely YM13–8.1 and YN22–3.1, demonstrated significant improvement in the textural properties of acid-induced gels characterized by increased hardness and a more compact, dense and fine-pored structure. The inclusion of milk-coagulating proteases resulted in a significant augmentation of hydrophobic interactions and disulfide bonds within the protein gels. Furthermore, both proteases exhibited rennet-like characteristics through κ-casein hydrolysis, with key hydrolysis sites identified as Tyr25-Ile26, Ala65-Ala66 and Lys112-Asn113. By utilizing MALDI-TOF-MS, whole genome sequencing and RT-qPCR analysis, we successfully identified the gene responsible for encoding the proteases as prtA, which has not been previously characterized. In conclusion, our findings demonstrate that the newly characterized protease prtA exerts a positive influence on milk gelling.

Rheological and microstructural characterization of lotus seed milks and their glucono-δ-lactone induced acid-set milk gels: 2. effect of oil addition

In a recent paper (Li, Li, Zhao, Cui, & Hemar, 2023) we showed that it was possible to manufacture lotus seed milks (LSM) and to use Glucono-δ-lactone (GDL) to prepare LSM acid-gels. LSM milks are known to have a very low lipid content, thus here the effect of soya oil addition is investigated in order to understand how LSM fortification with oil can affect its viscosity, and the mechanical and structural properties of the acid LSM gels. The oil was homogenized under conditions which resulted in oil droplets sizes in the range 0.1–0.4 μm. Heating the emulsions (80 °C, 30 min) resulted in a slight increase in particle size due to coalescence. The viscosity of emulsions was quasi-Newtonian and increased markedly with the increase in oil concentration. This was explained partly by the adsorption of the LSM aggregates at the interface of oil droplets, which increases considerably the effective volume fraction of the oil droplets. LSM emulsions when acidified by GDL formed colloidal gels with oil droplets embedded in the protein network, where the syneresis decreased with oil concentration, suggesting that the oil droplets should be considered as active fillers. The gel strength increased linearly with the oil concentration, but with a higher rate for the unheated emulsions than the heated ones. This is due to the decrease in the oil droplets number because of coalescence. This study demonstrates that homogenization of oil into LSM offers a route to manipulate the physico-chemical properties of the LSM emulsions and their acid gels.

The kinetics of milk gel structure formation studies by electron microscopy

The aim of this study is to enhance the comprehension of the mechanism of enzymatic gelation in milk by visualizing the evolution of its microstructure through transmission electron microscopy. In order to minimize the potential for artifacts during the preparation process and eliminate any possible difficulties in interpreting the resulting images, three distinct methods were employed in the research: shading the surface topography with vacuum deposition of heavy metal, negative staining of the specimen with a heavy metal solution and replicating a cleavage of a quick-frozen sample. The selection of time intervals for sampling the gel during its evolution is determined by the most probable significant modifications in the resulting gel. Based on the research, it has been shown that natural milk is a nonequilibrium system from the perspective of statistical thermodynamics. A notable observation is that the glycomacropeptides forming the hair layer on the surface of casein micelles are unevenly distributed, leading to the formation of micelle dimers and trimers. It has been determind that during the initial stage of enzymatic gelation in milk, clusters of loosely bound micelles are formed in areas with the highest concentration. The formation of micelle chains is absent at this stage due to the non-anisometric nature of micelles and the energetic disadvantage of their formation. It has been found that under the influence of enzymatic gelation near the gel point, a hierarchical process involving the transformation of the milk’s protein component is activated. The trigger mechanism for this process is a cooperative conformational transition in clusters of casein micelles, which initiates a chain of more energy-intensive reactions in the following sequence: hydro­phobic interactions → hydrogen bridges → electrostatic interactions → calcium bridges. The result is the conversion of loosely bound micelle clusters into denser aggregates, predominantly contributing to the formation of milk curd. It is worth noting that gelation in milk can be regarded as a process that reduces the free energy of the dispersed system. Understanding the correlation between the decrease in the free energy value during gelation and the physical properties of the finished cheese and other dairy products continues to be a relevant area of research.

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.

Production and characterization of flavored goat milk gels using zinc and calcium salts Producing functional foods

Goat milk gels were prepared using calcium and zinc salts. The viscosity of gel prepared by adding zinc chloride and flavored with banana and orange was higher than that of gel prepared using calcium chloride. WHC of gels prepared using zinc chloride and calcium chloride was high on the first day and then gradually decreased during storage time at 7 °C. The hardness of the sweetened, flavored goat milk gel prepared with calcium chloride was lower than that of the gel prepared with zinc chloride. The Sensory evaluation study showed that, in general, flavoring gels prepared from goat milk using zinc and calcium salts had a high degree of acceptability. Keywords: flavored gel; Zinc chloride; Goat milk; Rheological properties

Extent of κ-casein hydrolysis during renneting of bovine milk: A critical assessment of the analytical and estimation approaches.

Renneting is an enzymatic process that turns milk into curd which is then transformed into cheese. Rennet-induced coagulation of caseins (CNs) is the critical step during this process and the key is the primary hydrolysis of κ-CN's Phe105-Met106 bond by chymosin. This article comprehensively reviews the existing data on the extent/degree of κ-CN hydrolysis during renneting of bovine milk and critically evaluates its determination methods. The data show that under normal cheese-making conditions, milk gelation occurs at a degree of κ-CN hydrolysis <80%, which varies due to several factors including analytical and estimation approaches. The common approach involves isolating the macropeptides released, by precipitating whey proteins and residual CN in 1%-12% trichloroacetic acid (TCA), then assuming that the maximum amount obtained is 100% κ-CN hydrolysis. The drawback is that the estimated degree of κ-CN hydrolysis may be higher than the actual value as TCA partially precipitates the macropeptide fractions. Moreover, macropeptide isolation seems unnecessary based on current advances in chromatographic and electrophoretic techniques. The present work proposes a simple mass balance-based approach that will provide accurate estimates in future studies. The accuracy of measuring the degree of κ-CN hydrolysis has implications on the precision of the data in relation to its partitioning (% distribution between the curd and whey) which is essential for improving whey quality.

Quantitative image analysis of influence of polysaccharides on protein network formation in GDL-acidified milk gels

Exopolysaccharides (EPS) are commonly used to improve the texture of yogurt. These polysaccharides interact with casein micelles, the major protein in milk, via electrostatic and depletion mechanisms during fermentation by lactic acid bacteria (LAB). However, the relationship between the physicochemical properties and monosaccharide composition of EPS and their impact on yogurt texture is not yet fully understood. To address this knowledge gap, we studied the effects of polysaccharides commonly used as food additives on acid-induced milk protein networks. Confocal laser scanning microscopy (CLSM) was used to image the network microstructures. Image analysis, including Fourier transform, autocorrelation, and binarization-based techniques, was applied to quantify key structural features of the mixed milk protein/polysaccharide gels. These parameters were then related to the macroscopic properties of the model food matrices, such as elastic and viscous moduli and yield point. We found that the addition of neutral polysaccharides resulted in a concentration-dependent increase in structure factor, protein domain size, and pore fraction. In contrast, the presence of charged polysaccharides led to an increase in protein domain size, a decrease in pore fraction, and a decrease in elastic and viscous moduli. These results demonstrate the use of a quantitative image analysis method for selecting LAB with favorable EPS properties to improve yogurt texture.

Pressure-induced gelation of blended milk and pea protein suspensions

The pressure-induced (400 MPa for 15 min) gelation of blended skim milk (SMP) and pea protein concentrate (PPC) suspensions (15% w/w) at different ratios (SMP:PPC = 10:0, 8:2, 6:4, 5:5, 4:6, 2:8, 0:10) were studied. Blending milk and pea proteins reduced the gel strength markedly, indicating that milk and pea proteins do not interact with each other to the same extent. The secondary structure of the proteins was not significantly changed after HP treatment. The PPC dominating gels (0:10 and 2:8) had a large increase in surface hydrophobicity indicating higher exposure of hydrophobic clusters. Based on a differential solubility method it was established that the SMP dominating gels are govern mostly by electrostatic and hydrophobic interactions, while disulphide bonds, hydrogen bonds and electrostatic interactions contributed most to the PPC dominating gel networks. The weak 5:5 gel was established solely by hydrophobic interaction between milk and plant proteins. It is suggested that in the milk protein dominant gel (8:2), the milk proteins formed the primary gel structure likely filled with the pea protein secondary gel network and/or aggregates, whereas the inverse occurred in the pea protein dominant gel (2:8). Replacing milk proteins with pea proteins offers a great potential for produce innovative dairy products using high pressure processing, like for example dairy desserts with a neutral pH and textural properties similar to yogurt.

Revolutionizing goat milk gels: A central composite design approach for synthesizing ascorbic acid-functionalized iron oxide nanoparticles decorated alginate-chitosan nanoparticles fortified smart gels

Goat milk gels (GMGs) are popular food due to their high water content, low-calorie density, appealing taste, texture enhancers, stability, and satiety-enhancing characteristics, making them ideal for achieving food security and zero hunger. The GMGs were optimized using the central composite design matrix of response surface methodology using goat milk powder (35–55 g), whole milk powder (10–25 g), and potato powder (10–15 g) as independent variables. In contrast, complex modulus, flow stress, and forward extrudability were chosen as dependent variables. The maximum value of complex modulus 33670.9 N, good flow stress 7863.6 N, and good extrudability 65.32 N was achieved under optimal conditions. The optimized goat milk gel was fortified with ascorbic acid-coated iron oxide nanoparticle (magnetic nature) decorated alginate-chitosan nanoparticles (AA-MNP@CANPs), making it nutritionally rich in an economically feasible way—the decorated AA-MNP@CANPs characterized for size, shape, crystallinity, surface charge, and optical characteristics. Finally, the optimized fortified smart GMGs were further characterized via Scanning electron microscopy, Rheology, Texture profile analysis, Fourier transforms infrared (FTIR), and X-Ray Diffraction (XRD). The fortified smart GMGs carry more nutritional diversity, targeted iron delivery, and the fundamental sustainability development goal of food security.