Properties Of Acid Milk Gels Research Articles

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.

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.

Tribo-rheological properties of acid milk gels with different types of gelatin: Effect of concentration

We investigated the effect of low concentrations (0.1 to 1%, wt/wt) of gelatin (types A and B) on the properties of acid milk gels in terms of rheology, tribology, texture, and water-holding capacity to better understand the role of gelatin in yogurt. The 2 types of gelatin showed similar effects on the properties of milk gels, with some minor differences, such as lubrication behavior at low concentrations. During acidification, gelatin at ≤0.4% caused an increase in the gel strength, and at higher concentrations it showed a negative effect. However, during cooling and annealing, we observed a positive effect on gel strength with 0.8 and 1% gelatin. Gelling and melting occurred at 0.8 and 1% concentrations of both types of gelatin. The addition of gelatin tended to decrease the storage modulus of milk gels and increase the apparent viscosity, pseudoplasticity, consistency, and yield stress. The firmness of the gels was decreased by gelatin at medium concentrations, but increased at high concentrations. Gelatin significantly enhanced the water-holding capacity of the gels; we observed no serum at concentrations ≥0.4%. With the addition of gelatin at concentrations ≥0.4%, the particle size of gels was greatly reduced, and their lubrication properties were significantly improved. This study showed that 0.4% was an effective concentration in acid milk gel; above this concentration, the properties of the milk gels were greatly changed. Tribology provided important information for understanding the role of gelatin in milk gels.

Effect of Ohmic Heating on the Formation and Texture of Acid Milk Gels

This study aimed to describe the effects of ohmic heat treatment (OHT) of milk on the formation and properties of acid milk gels. The influence of voltage gradient (25, 40, 55 V/cm), holding time (2, 16, 30 min), and final temperature (45, 65, 75, 85 °C) on rheological properties and particle size distribution was measured during the gelation process. Texture properties and syneresis of the samples were evaluated at the end of the gelation process. Scanning electron microscopy (SEM) micrographs of the acid milk gels were also taken at pH 4.6(i.e., pI or Isoelectric point of casein). Results indicated that oscillatory time sweep tests were more accurate than particle size measurements in detecting gelation onset. Complex modulus values of the final gel showed that OHT of the milk at 25 V/cm and holding at 85 °C for 16 min gave rise to a 21% higher structural strength compared to when using the conventional heat treatment (CHT) where temperature was 85 °C and holding time and come up time were 30 and 20 min respectively. In other words, by using the OHT milk, the same gel strength as that of CHT 85 °C sample was obtained by spending 185 min instead of 328 min incubation time (i.e., 40% less time). Higher voltage gradient lowered the final gel strength when temperature and holding time kept constant at 85 °C and 16 min respectively. Results indicated that the gel firmness enhanced and the amount of syneresis decreased when CHT was replaced by OHT in the manufacturing process. SEM revealed that the protein matrices of ohmic heating samples appeared to be more compact and denser with smaller pore size than observed in the conventional gel. Results indicated that OHT of milk successfully improves the quality of acid milk gels and obviate the need for increasing dry matter concentration or using additives.

Combined effect of transglutaminase and sodium citrate on the microstructure and rheological properties of acid milk gel

Combined effects of trisodium citrate (TSC), transglutaminase (Tgase) and heating were studied with relation to the mechanical properties of acid milk gels. Acidification of skimmed microfiltered milk samples was performed at 30 °C by addition of glucono – δ lactone to achieve final pH of 4.6 in 4 h. Increasing levels of TSC (0, 10 and 20 mM) were added to the samples, followed by addition of a constant level of Tgase (3 U/g of protein), and subsequent heating to 85 °C for 30 min. Prior to acidification, suspensions were analyzed in terms of amount of sedimentable material, hydration, hydrodynamic diameter (Dh) and ζ-potential (ζ) of the particles. Acid-induced gelation was studied using small deformation rheology followed by large deformation test. The water holding capacity of the gels (WHC) was performed at pH 4.6. Gels microstructure were investigated by confocal laser scanning microscopy at pH 4.6 and determination of mean pore size was performed by 2D-correlation analysis. Results showed that combined Tgase and TSC treatments had significant effect on rheological properties, WHC and gel microstructure. Higher stiffness of the gels was observed with Tgase and TSC at 10 mM, whereas a homogenous network distribution with smaller pore size was observed in the presence of Tgase and TSC at 20 mM.

ROLE OF WHEY PROTEIN-CASEIN COMPLEXES ON YOGHURT TEXTURE

Properties of yoghurt extremely depend on the application of heat treatment to milk prior to manufacture. After heating the milk, denatured whey protein and casein makes complexes which dramatically improve the yoghurt texture such as firmness and water holding capacity etc. The formation of complexes, micelle bound or soluble whey protein/k-casein complexes, and their localization between casein micelles and serum depends on the physical properties (heat treatment, pH, compositions) of milk. We review the formation of these complexes and their effect on the physical, rheological and microstructural properties of acid milk gels. In order to investigate the interactions of denatured whey protein-casein, the formation of covalent and non-covalent bond namely thiol/disulphide, hydrophobic and electrostatic etc. are reviewed, with the emphasis on acid gelation and final yoghurt texture. This review summarizes the different factors and interactions on the formation of the micelle bound complexes or soluble denatured whey protein/k-casein complexes in heated milk and how these complexes affect the yoghurt properties. This information could be fundamental knowledge for developing a desirable yoghurt texture, may have potential interest for future research.

0502 Effect of incubation temperature on yield and molar mass of EPS during fermentation of milk by Streptococcus thermophilus DGCC 7785 and the impact on the rheological properties of acid milk gels

0502 Effect of incubation temperature on yield and molar mass of EPS during fermentation of milk by Streptococcus thermophilus DGCC 7785 and the impact on the rheological properties 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.

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.

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.

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.

Physical properties of acid milk gels: Acidification rate significantly interacts with cross-linking and heat treatment of milk

Rheological properties and microstructure of acid milk gels largely depend on the pre-treatment of the milk, including heating and enzymatic modification of the gel-forming proteins. It was the aim of our study to investigate the impact of the acidification rate on gels which were produced from milk reconstituted from powder containing casein cross-linked by microbial transglutaminase (mTGase), and on gels which were produced from cross-linked raw milk. 3–7% Glucono-δ-lactone (GDL) was used as acidulant. Gel stiffness was determined by dynamic small-deformation rheometry and penetration experiments, and forced syneresis and gel permeability served as indicators for gel microstructure.The measurements showed that acidification rate strongly interacts with casein oligomerization and heating. With increasing acidification rate the stiffness of gels from raw milk increased, but decreased if the casein was cross-linked with mTGase (oligomerization degree was approximately 25%, and the action of mTGase was stopped using N-Ethylmaleimide). Gels from heated milk became generally weaker if acidification was accelerated. However, casein cross-linking by mTGase enhanced gel stiffness at low acidification rate but decreased stiffness at high acidification rate. Rheological, syneresis and permeability results indicate that the aggregates formed through heating or cross-linking are responsible for the crucial alteration of gel properties and suggest that these act as structurally relevant nuclei which, depending on the time span which is given for gelation, assist in network formation or act as steric hindrances.

Physical properties of acid milk gels prepared at 37°C up to gelation but at different incubation temperatures for the remainder of fermentation

We investigated the effect of altering temperature immediately after gels were formed at 37°C. We defined instrumentally measurable gelation (IMG) as the point at which gels had a storage modulus (G′) ≥5Pa. Gels were made at constant incubation temperature (IT) of 37°C up to IMG, and then cooled to 30 or 33.5, or heated to 40.5 or 44°C, at a rate of 1°C/min and maintained at those temperatures until pH 4.6. Control gel was made at 37°C (i.e., no temperature change during gelation/gel development). Gel formation was monitored using small strain dynamic oscillatory rheology, and the resulting structure and physical properties at pH 4.6 were studied by fluorescence microscopy, large deformation rheology, whey separation (WS), and permeability (B). A single strain of Streptococcus thermophilus was used to avoid variations in the ratios of strains that could have resulted from changes in temperature during fermentation. Total time required to reach pH 4.6 was similar for samples made at constant IT of 37°C or by cooling after IMG from 37 to either 30 or 33.5°C, but gels heated to 40 or 44°C needed less time to reach pH 4.6. Cooling gels after IMG resulted in an increase in G′ values at pH 4.6, a decrease in LTmax, WS, and B, and an increase in the area of protein aggregates of micrographs compared with the control gel made at constant IT of 37°C. Heating gels after IMG resulted in a decrease in G′ values at pH 4.6 and an increase in LTmax values and WS. The physical properties of acid milk gels were dominated by the temperature profile during the gel-strengthening phase that occurs after IMG. This study indicates that the final properties of yogurt greatly depend on the environmental conditions (e.g., temperature, time/rate of pH change) experienced by the casein particles/clusters during the critical early gel development phase when bonding between and within particles is still labile. Cooling of gels may encourage inter-cluster strand formation, whereas heating of gels may promote intra-cluster fusion and the breakage of strands between clusters.

Stress Relaxation of Acid-induced Milk Gels

The effects of the addition of sugar (sucrose) or salt (KCl/NaCl mixture) on the rheological properties of acid milk gels formed under different process and composition conditions were studied using an experimental design and surface response analysis. The variables evaluated were: sodium caseinate, whey protein concentrate, carrageenan, sucrose or KCl/NaCl concentration, and the mixing temperature of the components. The stress relaxation measurements were made within the linear viscoelasticity domain, and the data obtained were fitted to the generalized Maxwell model. Five parameters were obtained from the fitting: three elastic elements and two viscous elements. In addition, the denaturation process of whey protein with different solute concentrations was evaluated using differential scanning calorimetry. Denaturation temperature increased with increases in the solute concentrations, but enthalpy values did not change independent of solute type and concentration. The results showed that the elastic and viscous moduli increased with sucrose concentration, i.e., the gel network was reinforced, while the opposite occurred with the addition of the saline mixture. Increasing ionic strength of saline mixture and the process temperature were the more important effects with respect to the rheological properties of gels with added saline mixture. On the other hand, the biopolymer concentrations showed the most significant effects on the properties of gels with added sugar.

Rheological properties of acid gels prepared from pressure- and transglutaminase-treated skim milk

The effect of heat treatment, transglutaminase treatment, and high hydrostatic pressure treatment of fresh skim milk on the rheological properties of acid milk gels, formed by acidification with glucono-δ-lactone, was investigated. Acid gels prepared from unheated skim milk had long gelation times, low gelation pH, and very low final G′ values. Heat treatment and pressure treatment of milk had similar effects in that both treatments decreased the gelation time, increased the gelation pH, and markedly increased the final G′ of the resultant acid gels. Transglutaminase treatment of milk prior to acid gel formation also markedly increased the final G′ of the acid gels; however, when transglutaminase treatment was performed during pressure treatment, a markedly higher final G′ was obtained than when only pressure or transglutaminase treatment was performed. It is proposed that there is increased cross-linking of the whey proteins, and an increased cross-linking between the whey proteins and casein when transglutaminase treatment is performed under high pressure. The introduction of a higher number of intermolecular cross-links will result in increased acid gel firmness and therefore an increased final G′.

Viscoelastic properties of acid milk gel as affected by fat nature at low level

Abstract The viscoelastic properties of acid milk gels containing small amounts of different fats were investigated. Skim milk was reconstituted from ultra low-heat skim milk powder and emulsions made with 2% (v/v) sunflower oil, olive oil, groundnut oil, or anhydrous milk fat using a pressure homogenizer. Acidification at 20 °C for 14 h to pH ∼4.6 was achieved by adding glucono- δ -lactone to the emulsion. Stress relaxation testing enabled determination of the firmness and the solid-like properties, i.e., elasticity. Regardless of the physical state of the fat, emulsion gels exhibited higher firmness than fat-free gels, despite the low fat level used. The firmness of the gels containing this small quantity of fat was more sensitive to temperature than was the firmness of fat-free gels. The relaxation time was higher in the presence of fat crystals. Modifications in the rheological properties of gels containing fat were attributed to fat droplets acting as active filler particles.

Effect of Controlled κ-Casein Hydrolysis on Rheological Properties of Acid Milk Gels

An experimental method based on the controlled chymosin-induced κ-casein hydrolysis of milk was proposed to modify micellar reactivity. Milk samples with a degree of κ-casein hydrolysis of 19, 35, and 51% were obtained. The physicochemical properties of partially converted casein micelles were determined. The net negative charge of casein micelles was reduced with increasing degree of κ-casein hydrolysis and a small but significant decrease in hydrodynamic diameter and micellar hydration were noted. Dynamic low amplitude oscillatory rheology was used to monitor the rheological properties of acid milk gels (GDL) made with partially chymosin-hydrolyzed milks in comparison with those of strictly acid and rennet gels. An increase in the gelation pH value was observed with increasing the degree of κ-casein hydrolysis. The moduli values (G′ and G″) reached 2h after the point of gel were, for all degrees of hydrolysis tested, significantly higher than those of strictly rennet and acid gels. Comparison of changes in ΔG′/Δt with time indicated differences in gel formation that could be related to the increased values of G′ obtained for acid gel made with chymosin-treated milk. At a given time after gelation (2h), increasing the degree of κ-casein hydrolysis in milk led also to an increase in the loss tangent and the serum holding capacity of acid milk gels suggesting a correlation between these two parameters.

Texture of acid milk gels: formation of disulfide cross-links during acidification

Denaturation of whey proteins during pasteurization of milk results in the formation of whey protein aggregates and whey protein-coated casein micelles. After cooling a substantial number of thiol groups remains exposed. Formation of larger disulfide-linked protein structures during acidification at ambient temperature was demonstrated by analytical methods. The time-dependent formation of these structures attributed significantly to the mechanical properties of acid milk gels, resulting in gels with an increased storage modulus and hardness. Addition of the thiol-blocking agent N-ethylmaleimide prevented the formation of disulfide-linked structures. The mechanical properties are shown to be the result of the contribution of denatured whey proteins to the protein network as such and the additional formation of disulfide bonds. Surprisingly, these sulfhydryl group-disulfide bond interchange reactions take place at ambient temperature and under acidic conditions. Therefore, the disulfide cross-linking is highly relevant for textural properties of acid-milk products, like yoghurt.

Rheological properties of acid gels prepared from heated milk fortified with whey protein mixtures containing the A, B and C variants of β-lactoglobulin

Abstract The effect of fortification of reconstituted skim milk with different levels of a whey protein mixture containing a 1:2 ratio of α-lactalbumin (α-la) and different genetic variants of β-lactoglobulin (β-LG) on the rheological properties of acid milk gels, formed by acidification with glucono-δ-lactone, was investigated. Milk samples were either unheated or heated at 80°C for 30 min before acidification. Acid gels prepared from unheated skim milk had very low G′ values, long gelation times and low gelation pH. Samples prepared from heated milk had markedly higher G′ values, a reduced gelation time and an increased gelation pH. The addition of increasing levels of whey protein mixtures containing β-LG B or β-LG C to the milk prior to heating and acidification caused an almost linear increase in the G′. In contrast, whey protein mixtures containing β-LG A caused a progressive increase in the G′ with added protein levels up to about 0.7% (w/w) but little further change at higher addition levels. A mixture of the A and B variants of β-LG gave an intermediate behaviour between those of the A and B variants. In all samples, the G′ value at 5°C was approximately twice that at 30°C so that the relative differences as a result of the β-LG genetic variants were similar for the two temperatures.

Rheological properties of acid milk gels as affected by the nature of the fat globule surface material and heat treatment of milk

Abstract The effects of different fat globule surface materials on the rheological properties of acid milk gels, formed by acidification with glucono-δ-lactone at 30°C, were investigated. Acid gels were prepared from recombined milks (∼3.4% fat) containing fat globules stabilized with low-heat, medium-heat or high-heat skim milk powder (SMP), sodium caseinate, whey protein concentrate (WPC), heated WPC or Tween 60. Unheated reconstituted skim milk (URSM) or milk heated at 80°C for 30 min (HRSM) was used in the preparation of recombined milks. In URSM or HRSM systems, gels containing fat globules stabilized by non-interacting materials (`structure breaker') (Tween and unheated WPC) had low storage moduli ( G′ ) compared with interacting materials (`structure promoter') (SMP, sodium caseinate and heated WPC). HRSM systems had higher G′ values than URSM. Gels containing fat globules stabilized by sodium caseinate or heated WPC had the higher G′ values than those containing SMP-stabilized fat globules in both URSM and HRSM systems.