Acid Milk Gels Research Articles

Effect of Kefiran and Milk Proteins Addition on the Rheological Behavior of Glucono-delta-Lactone Induced Milk Gels

The properties of kefiran as a texture modifier was evaluated providing new perspectives in understanding the rheology of fermented dairy products and the possibility of using this generally recognized as safe polysaccharide as an additive in dairy and other foods. For this purpose, the effect of kefiran, Sodium Caseinates (SCN) and Whey Proteins Concentrates (WPC) addition on the rheological behavior of glucono-delta-lactone (GDL) induced milk gels was evaluated. The acidified milk samples were prepared from homogenized and pasteurized full fat (3.5%) bovine milk with or without the addition of kefiran, SCN and WPC at 1.5% concentration. The incubation temperature was set at 37°C. According to the results, the incubation time required for the pH to reach 4.4 increased with increasing SCN and WPC concentration. Kefiran addition did not affect incubation time, however it did enhanced elasticity, apparent viscosity and thixotropic behavior of milk gels. The impact effect of kefiran on the rheological behavior of acid milk gels was more pronounced when compared to that of WPC, but lower to SCN effect. The protein matrix of GDL induced milk gels is governed by hydrophilic interactions in the presence of kefiran or WPC and by hydrophobic ones when SCN are added.

Clean Labels: The New Wave Sweeping the Fermented Milks Industry

Functional ingredients from naturally occurring food biopolymers are a trend catching up amongst food and beverage developers. This is driven by the demand of consumers for clean label ingredient listings on products. The clean label trend is extending into the yogurt market (acid gel) with consumers demanding hydrocolloids, stabilizers or thickeners are purged from ingredient listings. Substituting hydrocolloids in food systems especially yogurts could mean compromising on rheological properties and water holding capacities. However, structure functionality relationship of naturally occurring biopolymer caseins in milk can be exploited to overcome this foreseen loss of product characteristics. The following review is an attempt to delineate clean label solutions for the manufacture of acid milk gels.

Textural Properties of Acid Milk Gels: Effects of Dairy Protein Powder Rehydration State

In this study, the effect of dried dairy ingredient rehydration state on the textural properties and microstructure of acid milk gels was investigated. The control acid gel samples were made with 12% w/w of reconstituted skim milk powder. Milk base was enriched with whey protein isolates and/or micellar casein powders. Different rehydration times (5, 120, 240, 1,440 min) were considered before studying gel textural properties. This study showed that acid milk gels made with whey instead of casein resulted in an increase in firmness, gel strength, graininess but a decrease in syneresis susceptibility. Nevertheless, rehydration duration of casein powder in contrast to that of whey powder strongly altered the physical properties of the obtained acid milk gels. Moreover, prolonged hydration resulted in enhanced textural properties and decreased syneresis. Practical Applications Commercial preparation of yogurt involves different processing steps. Among them, protein fortification of the milk appears as a crucial step that attributes toward the change in textural properties. Milk protein powders are frequently dried in order to stabilize and transport. This is the reason why rehydration is an essential quality attribute of the powder. Consequently, rehydration is often realized by the industry before use and for a long time. This study showed that acid milk gels made with whey instead of casein resulted in an increase in firmness, gel strength, graininess but a decrease in syneresis susceptibility. Rehydration duration of casein powder in contrast to that of whey powder strongly altered the physical properties of the obtained acid milk gels. Prolonged hydration resulted in enhanced textural properties and decreased syneresis. These aspects are of high significance in the preparation of dairy products, i.e., different types of set and stirred yogurts.

Development of rheological and sensory properties of combinations of milk proteins and gelling polysaccharides as potential gelatin replacements in the manufacture of stirred acid milk gels and yogurt

Combinations of gelling polysaccharides (xanthan/locust bean gum [X/L], carrageenan and starch) and milk proteins (whey protein isolate [WPI], sodium caseinate and skim milk powder) were evaluated as potential gelatin replacers in acid milk gels. Gels with added X/L alone showed rheological (gelling and melting) and microstructural (typical casein network with thin strand-like structures) properties similar to those of gels with gelatin. Similar to the effect of adding gelatin, milk protein fortification enhanced water holding capacity (WHC) of the gels, with WPI being the most effective. Gels with combinations of polysaccharides (except carrageenan) and WPI were stronger and had higher WHC than gels with no stabilizer. In yogurt, the combination of WPI and X/L (WPI-X/L) produced similar effects on consistency, pseudoplasticity and apparent viscosity as gelatin and higher sensory scores for thickness and stickiness than gelatin; a lower score for smoothness was observed with WPI-X/L than with gelatin.

Spatial arrangement of casein micelles and whey protein aggregate in acid gels: Insight on mechanisms

Skim milk used for the yoghurt manufacture contains 2 main colloidal particles, the native micellar casein (NMC) and the heat-induced whey protein aggregates (WPA). The aim of this study was to understand how these 2 colloids organize in space to form binary acid gels. While acid milk gels were considered homogeneous for scales typically above ∼10 μm, shorter length scales were examined to investigate the early stages of particle aggregation. The NMC and WPA were dispersed in milk permeate at different protein concentrations, separately or in an 80/20 w/w mixture (MIX). Acidification was performed at 35 °C with glucono-δ-lactone to achieve a final pH at ∼4.5 in 6 h. Acid gelation was studied by rheology, while the microstructure of the gels at pH 4.5 was studied by confocal scanning laser microscopy and transmission electron microscopy (TEM). Using Shih et al.'s (1990) model on the rheological data, it seemed that aggregation in the NMC and MIX mixtures was driven by the casein micelles and therefore differed from that of the WPA suspension. The different organizations were confirmed using image analysis of confocal or TEM images. The differences in gel formation were discussed in terms of the different interactive properties of the surface of these 2 colloids, together with their different internal structure.

Selection of EPS-producing Lactobacillus strains isolated from kefir grains and rheological characterization of the fermented milks

In this study the ability to produce exopolysaccharides during growth in milk of 28 Lactobacillus, previously isolated from kefir grain, was investigated and the rheological properties of the obtained fermented milks were also studied. During fermentation all microorganisms were able to produce polysaccharide with final concentration ranging from 20 mg L−1 to 370 mg L−1. Lactobacillus kefiranofaciens and Lactobacillus plantarum strains produced oligosaccharide with low degree of polymerization. The exopolysaccharides produced by the five Lactobacillus paracasei strains presented also a high molecular weight fraction. In consequence the acid milk gels obtained by fermentation with all the L. paracasei strains were the ones that presented the highest viscosities. Nevertheless the viscolestic characteristics of the resulting acid gels were different. L. paracasei CIDCA 83123 produces a milk having a viscous behavior whereas fermented milks with L. paracasei CIDCA 8339, CIDCA 83120, CIDCA 83121 and CIDCA 83124 has gel structure.The five L. paracasei strains isolated from kefir studied in the present work were the first described that produce high molecular weight exopolysaccharides. Since production of high molecular weight exopolysaccharides affects the viscosity of fermented milk these strains could have a successful application improving texture of fermented dairy product.

Effect of polysaccharides with different ionic charge on the rheological, microstructural and textural properties of acid milk gels

The effects of addition of polysaccharides with different ionic charge on rheology, microstructure, texture and water holding capacity (WHC) of acid milk gels were studied and compared to that of gelatin addition. Similar to gelatin, starch (neutral) and xanthan gum (anionic) did not prevent milk gelation in the first 30min of the acidification stage, even at high concentrations, and the typical casein network in acid milk gels could still be seen from electron micrographs; gelling and melting of these hydrocolloids were observed during the cooling and heating stages at specific concentrations. On the other hand, two neutral polysaccharides, guar gum (≥0.05%) and locust bean gum [LBG] (≥0.1%) inhibited milk gelation from the beginning of the acidification stage; the microstructure of the gel was modified greatly and no gelling/melting was observed during the cooling or heating stages. Another anionic polysaccharide, carrageenan, induced earlier milk gelation at low concentration (≤0.05%), but inhibited gelation entirely at high concentration (0.2%); inflections at ~27°C and 21°C were also observed during the cooling and heating stages at 0.05% concentration. The gel microstructure was not changed greatly, but showed smaller particle size at a carrageenan concentration of 0.05% than control sample. None of the polysaccharides showed as much improvement in WHC of the milk gels as gelatin did. Hence, xanthan and starch were found to be closer to gelatin in their effect on acid milk gels compared to guar gum, LBG and carrageenan.

Effect of dextran and dextran sulfate on the structural and rheological properties of model acid milk gels

Various types of polysaccharides are widely used in cultured dairy products. However, the interaction mechanisms, between milk proteins and these polysaccharides, are not entirely clear. To explore the interactions between uncharged and charged polysaccharides and the caseins, we used a model acid-milk-gel system, which allowed acidification to occur separately from gelation. The effect of adding uncharged dextran (DX; molecular weight ~2.0×106 Da) and negatively charged dextran sulfate (DS; molecular weight ~1.4×106 Da) to model acid milk gels was studied. Two concentrations (0.075 and 0.5%, wt/wt) of DX or DS were added to cold milk (~0°C) that had been acidified to pH values 4.4, 4.6, 4.8, or 4.9. Acidified milks containing DX or DS were then quiescently heated at the rate of 0.5°C/min to 30°C, which induced gelation, and gels were then held at 30°C for 17 h to facilitate gel development. Dynamic small-amplitude-oscillation rheology and large-deformation (shear) tests were performed. Microstructure of gels was examined by fluorescence microscopy. Gels made with a high concentration of DX gelled at a lower temperature, but after 17 h at 30°C, these gels exhibited lower storage moduli and lower yield-stress values. At pH 4.8 or 4.9 (pH values greater than the isoelectric point of caseins), addition of 0.5% DS to acidified milk resulted in lower gelation temperature. At pH 4.4 (pH values less than the isoelectric point of caseins), addition of 0.5% DS to acidified milk resulted in gels with very high stiffness values. Gels made at pH 4.8 or 4.9 with both concentrations of DS had much lower stiffness and yield-stress values than control gels. Microstructural analysis indicated that gels made at pH 4.4 with the addition of 0.5% DX exhibited large protein strands and pores, whereas gels made with 0.075% DX or the control gels had a finer protein matrix. At higher pH values (>4.4), gels made with 0.5% DX had a finer structure. At all pH values, gels made with 0.5% DS exhibited larger pores than the control gels. This study demonstrated that low concentrations of uncharged DX did not significantly affect the rheological properties of model acid milk gels; high concentrations of DX resulted in earlier gelation, possibly caused by depletion-induced attractions between casein particles, which altered the microstructure and created weaker gels. At pH values <4.6, negatively charged DS produced stiff casein gels, which might be due to attractive crosslinking by electrostatic interactions between DS and caseins at pH values below the isoelectric pH of casein (i.e., positively charged casein regions interacted with negatively charged DS molecules).

Calcium and phosphorus equilibria during acidification of skim milk at elevated temperature

The solubilisation of calcium and phosphorus from casein micelles was studied during skim milk acidification with glucono-delta-lactone. Dynamic measurements of storage modulus (G′) as a function of time were recorded during the formation of acid milk gels with a final pH of 4.6. An abnormal change in slope of G′ (G′shoulder) as a function of time was observed at 50 and 60 °C with decreasing pH. During the appearance of G′shoulder immediately after onset of gelation there was a large change in the concentrations of serum and micellar calcium and phosphorus for change of 0.1 pH units. The onset of gelation at pH values, where the colloidal calcium phosphate is not fully solubilised, is coupled to casein micelle disintegration, and will create increased bonding between caseins forming the final acid gel with a concomitant and sudden solubilisation of calcium and phosphorus from the casein micelles.

Bovine chromosomal regions affecting rheological traits in acid-induced skim milk gels

The production of fermented milk products has increased worldwide during the last decade and is expected to continue to increase during the coming decade. The quality of these products may be optimized through breeding practices; however, the relations between cow genetics and technological properties of acid milk gels are not fully known. Therefore, the aim of this study was to identify chromosomal regions affecting acid-induced coagulation properties and possible candidate genes. Skim milk samples from 377 Swedish Red cows were rheologically analyzed for acid-induced coagulation properties using low-amplitude oscillation measurements. The resulting traits, including gel strength, coagulation time, and yield stress, were used to conduct a genome-wide association study. Single nucleotide polymorphisms (SNP) were identified using the BovineHD SNPChip (Illumina Inc., San Diego, CA), resulting in almost 621,000 segregating markers. The genome was scanned for putative quantitative trait loci (QTL) regions, haplotypes based on highly associated SNP were inferred, and the additive genetic effects of haplotypes within each QTL region were analyzed using mixed models. A total of 8 genomic regions were identified, with large effects of the significant haplotype explaining between 4.8 and 9.8% of the phenotypic variance of the studied traits. One major QTL was identified to overlap between gel strength and yield stress, the QTL identified with the most significant SNP closest to the gene coding for κ-casein (CSN3). In addition, a chromosome-wide significant region affecting yield stress on BTA 11 was identified to be colocated with PAEP, coding for β-lactoglobulin. Furthermore, the coagulation properties of the genetic variants within the 2 genes were compared with the coagulation properties identified by the patterns of the haplotypes within the regions, and it was discovered that the haplotypes were more diverse and in one case slightly better at explaining the phenotypic variance. Besides these significant QTL comprising the 2 milk proteins, 3 additional genes are proposed as possible candidates, namely RAB22A, CDH13, and STAT1, and all have previously been found to be expressed in the mammary gland. To our knowledge, this is the first attempt to map QTL regions for acid-induced coagulation properties.

Impact of formulation and saliva on acid milk gel friction behavior.

Rheological analysis is commonly used to evaluate mechanical properties in studies of food behavior. However, rheological analysis is often insufficient to describe food texture as evaluated by descriptive sensory analysis. Additionally, traditional rheometry does not account for changes in food behavior as a function of saliva incorporation into the food during mastication. The objectives of this study were to evaluate friction behavior of acid milk gels with and without the addition of saliva, and to determine relationships between frictional behaviors and mechanical and sensory behaviors. Acid milk gels were prepared with 12.5% total solids comprising nonfat dry milk, whey protein isolate, waxy maize starch, and gelatin in different ratios. The addition of starch was found to have significant impact on acid milk gel frictional behavior. Addition of saliva resulted in a change in frictional behavior over the entire sliding speed range measured. Correlations were found between rheological, tribological, and sensory behavior, suggesting that an underlying mechanism may impact both viscosity and friction behavior. Additional study is needed to further explore the links between food structure, rheology, tribology, and sensory texture. Application of tribology in food science allows measurement of friction behavior of foods. Matching both rheological and tribological behavior is important to creating reduced-fat or reduced-sugar products with similar mouthfeel to the original product.

High-protein fat-free acid milk gels: Control of protein composition and heat treatment

Skim milk supplemented with calcium caseinate (Ca-Cas) and whey protein isolate (WPI) at different ratios was preheated at 72–90 °C for 7 min and subsequently acidified for the production of set-style and stirred acid gels. Upon heat-treatment of the suspensions, the native whey protein content decreased and particle size increased with increasing temperature and WPI content. Consequently, the pH of gelation, firmness of the set gel, viscosity and particle size of the stirred gel all increased as the content in denatured whey protein increased. However, for preheating temperatures of 80 or 90 °C, the viscosity and firmness of the gels reached a plateau, although almost all whey proteins were denatured and largest particles were formed. These results open perspectives for the production of high-protein yoghurts without unusual firmness properties.

Microstructure and stability of skim milk acid gels containing an anionic bacterial exopolysaccharide and commercial polysaccharides

The structure and stability of acid milk gels containing an anionic exopolysaccharide (EPS) produced by Lactobacillus rhamnosus RW-9595M in combination with different commercial polysaccharides were studied. Gel internal structure was visualised using confocal scanning laser microscopy; protein, EPS and polysaccharides were labelled using different fluorophores to resolve their specific location. EPS at 0.01% resulted in lower syneresis (2.5%) than control (3.7%); micrographs showed EPS co-localised with protein and reinforcing network connectivity. However, 0.05% EPS increased syneresis to over 17%, although no significant changes in gel strength were detected. Pectin or agar–agar alone at 0.05% resulted in very low syneresis (<1.8%). Both polysaccharides were found to form a tenuous secondary yet more ramified network, effectively reducing pore size. Mixing pectin or agar–agar with EPS at 0.05% resulted in a loss of their functionality. Anionic polysaccharides with stiff molecular structures, such as xanthan or κ-carrageenan, induced extensive protein self-aggregation and syneresis.

Designing microstructure into xanthan gum-enriched acid milk gels

Abstract Designing various microstructures through the interplay between phase separation and acid-induced gelation processes was investigated. Acid skim milk/xanthan gum mixed gels were prepared at different xanthan gum and glucono-δ-lactone (GDL) concentrations in order to modulate the extent of phase separation and the gelation rate, respectively. A gradual change in acid milk gel microstructure was found by increasing xanthan gum concentration, from homogeneous protein network, to fibrous structures and finally to isolated protein-rich aggregates. Addition of xanthan gum in the milk led to an apparent depletion–flocculation with protein-rich domains dispersed in a continuous phase enriched in xanthan. The sedimenting denser protein-rich domains could be trapped by protein gelation, forming specific fibrous structures at 0.05 wt.% xanthan gum. Manipulating the microstructure through modification of the gelation rate resulted in a design of a broad range of fibrous morphologies varying from 20 to 400 μm long, which could be used to generate novel textures for food products. Industrial relevance As a response to industrial demand by the consumer for novel acid milk gels with superior structural properties, mouthfeel and potential health benefits, we present here a way to manipulate textures by designing specific fibrous microstructures. Protein–polysaccharide mixtures that are subject to phase separation and gelation are interesting not only for the creation of major differences in structure and rheological properties of mixed systems, but also because of resulting protein-enriched products (up to 4 times more concentrated than conventional yoghurt). This approach should make it possible to improve the nutritional benefits as well as formulate a wide range of acid milk gels with innovative structures and sensory properties.

Combined confocal microscopy and large deformation analysis of emulsion filled gels and stirred acid milk gels

Abstract The microstructural breakdown properties of heat-induced whey protein (WP) gels prepared at pH 7.0 or 5.4 and containing emulsified sunflower oil (7%, w/w) were studied using notch propagation tensile testing in combination with dynamic confocal laser scanning microscopy (CLSM). In addition, the microstructural breakdown properties of stirred acid milk gels containing added emulsified oil (0–15%, w/w) or Konjac glucomannan (0.05%, w/w) were studied using compressive rheological deformation in combination with dynamic CLSM imaging. The structural breakdown properties (Young's modulus and stress/strain at fracture), the microstuctural behaviour of the protein phase, the emulsified oil phase and the pattern of notch propagation during large deformation tensile testing of the WP emulsion filled gel prepared at pH 7.0 (fine stranded gel) differed from that of the WP emulsion filled gel prepared at pH 5.4 (particulate gel). The protein aggregates, emulsified oil droplets and the Konjac phase in the stirred acid milk gels flowed on compression; however, the flow pattern changed from “frictional” flow with no emulsified oil or Konjac to “slip” flow in the presence of emulsified oil or Konjac. Also, compression of stirred acid milk gels below a certain critical height led to disintegration of the aggregated protein structure, serum release and reduced stability of the oil droplets in the stirred gels. The critical compression height was influenced by emulsified oil level and the presence of added Konjac.

Gelation of Cross‐Linked Casein under Small and Large Shear Strain

AbstractThe extent of cross‐linking by microbial transglutaminase of acid casein in phosphate buffer was varied, and its consequences on the rheology of chemically acidified gels were studied. Gel formation upon acidification was measured by dynamic rheometry and in rotational experiments at a shear rate of 0.002/s. Torque development during gelation under shear, and stiffness and fracture strain of the final gels was significantly affected by the amount of cross‐linked caseins. Moderately cross‐linked casein exhibited peculiar gelation behavior under shear as monitored with a perspex cylinder device and a video system. Macroscopically visible “threads” with a regular pattern were formed, which appeared to depend on amount and size of cross‐linked casein aggregates. From experiments with varying gelation temperature and acidulant concentration, it was observed that larger aggregates result in weaker gels when acidification rate is increased; in native systems, gelation rate did not show any effect.Practical ApplicationsTexture and rheology are important quality parameters of acid milk gels that can be modified, among others, by cross‐linking milk proteins using microbial transglutaminase. The present results demonstrate that it is crucial to control both the intensity of cross‐linking and acidification conditions to obtain products with the desired texture.

Concentration dependent effects of dextran on the physical properties of acid milk gels

The effect of dextran from Leuconostoc mesenteroides (DEX500), added to milk prior to acidification with glucono-δ-lactone (GDL) or Streptococcus thermophilus DSM20259, was studied with respect to polysaccharide concentration. The incorporation of 5–30g/kg DEX500 significantly affected gelation behavior. Increasing DEX500 concentrations resulted in a linear increase of gel stiffness (GDL gels: R2=0.96; microbial acidification: R2=0.94; P<0.05) and 30g/kg DEX500 resulted in a 2-fold higher stiffness compared to gels without polysaccharide. The respective stirred gels depicted a significant reduction in syneresis, which decreased from 30.4% (0g/kg DEX500) to 22.0% (30g/kg DEX500) for chemically acidified gels after 1 d of storage. Physical characteristics of DEX500 in aqueous solution were helpful to explain its behavior in the complex system milk.

Addition of purified exopolysaccharide isolates from S. thermophilus to milk and their impact on the rheology of acid gels

We present a new approach for investigating the impact of exopolysaccharides (EPS) from lactic acid bacteria on the rheological characteristics of acid milk gels by decoupling microbial EPS synthesis from milk acidification. Free and capsular EPS of Streptococcus thermophilus ST-143 were isolated and purified separately from batch fermentation media to obtain EPS preparations with high purity (>85% EPS). Defined amounts of EPS were added to milk prior to acidification, which was induced by a chemical acidulant or by S. thermophilus strains. For the first time it is shown that the stiffness of acid milk gels increases almost linearly with the concentration of free EPS, whereas those derived from capsular EPS did not affect rheology of acid gels when added at the same amount. Both EPS fractions were determined to be uncharged, but differed distinctly in molar mass, which was 2.6 × 106 Da for free EPS, and 1.4 × 105 and 7 × 103 Da for the two fractions that were derived from the capsular EPS (ratio, 1:3). This supports the hypothesis that it is molar mass rather than EPS charge, which is responsible for the contribution to gel stiffness. For stirred yogurt, apparent viscosity increased significantly with addition of EPS and storage time at 6 °C. Depletion interactions between uncharged EPS of high molar mass and negatively charged casein micelles can be regarded as main factor which is responsible for these effects.

Structural mechanisms leading to improved water retention in acid milk gels by use of transglutaminase

Water retention in transglutaminase (TG)-treated acid milk gels was studied and linked with the gel formation dynamics. Heat-treated skim milk with and without pre-treatment by TG was acidified at 20 °C, 30 °C and 40 °C at constant glucono-δ-lactone (GDL) level to obtain different acidification rates. Formation dynamics and structural properties of acid-induced gels were followed by rheological and near-infrared light backscattering measurements as well as microscopy. TG-treated gels showed decreased tan δ values all through the acidification, which was pronounced around the gelation point. Backscattered light intensity was lowered in TG-treated gels compared to the controls indicating that TG-treated gels were comprised of smaller aggregates. Water holding capacity (WHC) was measured by using centrifugation at selected pH points (pH 5.2, 5.0, 4.8 and 4.6) during acidification. Both acidification temperature and TG treatment had significant effects on the water retention properties of the gels. Spontaneous syneresis observed at high acidification temperatures (≥30 °C) was prevented upon TG-treatment. WHC of TG-treated gels was significantly higher compared to the control gels at all pH points. TG-treated milk gels showed a homogeneous network formed of smaller aggregate and pore sizes at the gelation point and did not show any large-scale re-organisation thereafter. Transglutaminase is likely to act as a fixative of the protein network at an early stage of gelation and thereby limiting network rearrangements that take place in acid milk gels formed at high acidification temperatures leading to contraction and subsequent wheying off.

Changing the isoelectric point of the heat-induced whey protein complexes affects the acid gelation of skim milk

Heat treatment of milk at 85–95 °C for several minutes increases both the pH of gelation and firmness of acid milk gels, changes usually attributed to the formation of heat-induced whey protein complexes in the milk. To investigate the role of the electrostatic properties of the heat-induced complexes on the acid destabilisation of heated milk, heat-induced whey protein complexes were modified by succinylation or methylation, giving pI values ranging from 3.8 to 5.3, while size, secondary structure, surface hydrophobicity and thiol/disulfide distribution were kept reasonably constant. These complexes were added to whey-protein-free skim milk systems and the resulting acid gelation behaviour of the milk samples was evaluated. The pH of gelation of these milk samples increased significantly as the pI of the complexes increased. This demonstrated both the importance of the complexes in the acid-induced gelation of milk and the relevance of repulsive electrostatic interactions in the acid destabilisation of milk.