Acid Milk Gels Research Articles

THE IMPACT OF HOMOGENIZATION AND MICROFILTRATION ON RENNET‐INDUCED GEL FORMATION

ABSTRACT The effects of the independent variables, milk homogenization pressure (p1), concentration factor of milk microfiltration (i) and pH on the rheological properties of rennet milk gels were studied. Nondestructive oscillatory rheometry was used to determine rennet coagulation time (RCT), curd firming rate (CFR) and cutting time (CT). A central composite design, comprising two levels of i (1 and 2), pH (6.4 and 6.6) and p1 (0 and 8 MPa), was applied. Second‐order polynomial models successfully described (R2 > 0.92) the relationship between processing parameters and rheological properties of the gels. pH had the most important influence on RCT, while CFR and CT were strongly influenced by i, pH and the interaction of i and pH. In contradiction to studies on active filler interactions for acid milk gels, a discrepancy was observed between results obtained by compression test and rheometry. Rennet gel firmness strongly decreased with a rise in p1 when measured using the compression test, whereby CFR increased with an increase in p1 when measured using rheometry. The latter result corresponds to higher storage modulus values after a certain time indicating higher gel stiffness. This effect was stronger for concentrated milk than for unconcentrated milk.PRACTICAL APPLICATIONSThe use of microfiltration (MF) and homogenization may reduce raw material and processing time in conventional cheese manufacture. However, MF markedly influences milk composition, and homogenization alters the particle size distribution of fat globules. Hence, both technologies may influence rennet‐induced gel formation, syneresis, cheese composition and quality. Curd firmness of homogenized milk is often too weak to resist the extensive curd treatment applied in semi‐hard cheese manufacture which causes loss of curd fines during the syneresis process and finally decreases cheese yield. MF leads to high curd firmness if cutting is not performed at the appropriate time, which unnecessarily extends processing time. The study of the effect of the individual treatments, as well as of the combination of both on rennet‐induced gel formation, is the first important step to evaluate their impact on further processing steps in cheese making. The combination of both technologies may overcome the antagonistic effect of the individual technology as low curd firmness due to homogenization can be compensated by MF that increases curd firmness and vice versa.

Changes in the rheological properties of stirred acid milk gels as induced by the acidification procedure

The aim of this study was to compare the rheological properties of stirred acid milk gels, acidified using two different acidification procedures. Pre-heated milks were acidified either by means of glucono-δ-lactone (GDL gel) at 20 °C or by incubation with lactic acid bacteria (lactic gel) at 38 °C. The latter procedure gave a harder acid set gel with a more heterogeneous structure. Once formed, the gels were filtered and stirred, then immediately characterised using the low-amplitude dynamic oscillation method for 20 h at 4 °C or creep measurement at 4 °C. The stirred gels were stored at 4 °C for 28 d, during which the pH and the viscosity at 64 s−1 were measured. After stirring, the lactic gel made at 38 °C had a higher instantaneous elastic modulus, a higher viscosity and showed a greater increase in the elastic modulus over 20 h compared with the stirred GDL gel. The viscosity of both stirred gels progressively increased during storage at 4 °C, but that of GDL acidified gels increased 3.5 times more than that of lactic gels. The procedure used to prepare the set acid gels had notable effects on the interactions formed within 20 h and 28 d of storage of stirred gels.

Effect of potato starch addition on the acid gelation of milk

Potato starch was added to skim milk at levels of 0–1.5%. The milks were heated and then acidified to form acid milk gels. The properties of the milks during acidification and the final properties of the acid gels were studied. The addition of starch resulted in a higher storage modulus, G′, in the final acid gels, and increasing the level of starch caused a linear increase in the final G′. Compared with acid gels prepared with no starch, the gelation time was reduced and the gelation pH was increased. However, the temperature and frequency dependences of the acid gels were not affected by the addition of starch. Furthermore, the breaking strain of the acid gels was not markedly affected by the addition of starch, whereas the breaking stress was dependent on the level of starch added. Confocal microscopy showed that the acid gels contained swollen starch granules embedded in a protein network, and that the protein network increased in density as the level of starch added increased.

TRANSGLUTAMINASE IN DAIRY PRODUCTS: CHEMISTRY, PHYSICS, APPLICATIONS

ABSTRACT Literature on the effects of microbial transglutaminase on various dairy‐based systems is discussed. Beginning with a short synopsis on the development of microbial transglutaminase as a functional tool for modifying foods, the principles of reactions catalyzed by transglutaminase and their structural implications, as well as the mechanisms of formation and cleavage of isopeptide bonds are reviewed. After summarizing the present knowledge on the specificity of microbial transglutaminase towards milk proteins, including reactions determined by individual lysine and glutamine residues, emphasis is placed on the effects of enzymatic cross‐linking on physicochemical properties in foods and, particularly, dairy‐based systems. Discussed are implications of cross‐linking on acidified milk gels including yogurt and effects on single milk protein fractions, with respect to several physicochemical properties including rheology and mechanical properties of these systems, but also syneresis, and emulsification behaviour.

Rheology of stirred acidified skim milk gels with different particle interactions

Steady-shear rheology as well as small and large amplitude oscillatory shear rheology was used to quantify particle interactions in stirred, acidified skim milk gels. Gels were made from milk using either high-pressure treatment up to 600 MPa or heat treatment at 90 °C and subsequent acidification. This allowed gels with different particle interactions to be studied. Upon increased rate of steady-shear, a marked decrease in viscosity occurred at a certain stress level, which was considered as an apparent yield stress. At this stress level the rheological properties of the stirred gels changed from a behaviour characteristic for a gel to behaviour of a dispersion of interacting particles. A gel to solution transition was also assessed in stirred gels by using large amplitude oscillatory shear. Stirred acidified milk gels with different particle interactions could be distinguished by quantification of the apparent yield stress or the shear stress where elastic and viscous moduli coincided. Particle interactions in the stirred gels were gradually enhanced by high-pressure treatment up to 600 MPa shown by gradually increased shear viscosity and apparent yield stress, as observed by steady-shear. Oscillatory shear measurements revealed that high-pressure treatment up to 600 MPa increased moduli in stirred gels and enhanced the shear stress recorded at the gel to solution transition. However, similar rheological properties were found in stirred gels when these were made either from milk subjected to high-pressure treatment at 600 MPa or heat treatment at 90 °C.

Influence of fat on the perceived texture of set acid milk gels: a sensory perspective

Correlation of sensory perception of texture with instrumental parameters is key to understanding how specific structure formation can influence the consumer acceptability of a food product. Experimental acid milk gels with different solids-non-fat content (10–20%, w/v), with and without added fat (0–4% fat, w/v), were manufactured and characterised using quantitative descriptive analysis, confocal microscopy and small/large deformation rheology. Confocal micrographs of the gels showed that the gel structures were in agreement with the perceived sensory textural differences, in that the addition of fat, up to 4% (w/v), caused major changes in the microstructure of the network and in the overall perception of texture. This was observed mainly at low total solids levels (10–14%); no significant changes in microstructure or sensory perception of texture were detected at high total solids levels (above 18%), regardless of fat addition. The main effects of increasing fat content in the gels were a decrease in the mean pore size and an increase in the average cluster size. Added fat also caused the gels to become firmer, more resistant to penetration, more cohesive and sticky, creamier and less compressible before fracture (less ‘give’). Both instrumental and quantitative microstructural image analysis results correlated with perceived texture, and, when used in combination, these data sets generated an estimated model with satisfactory predictive ability for textural parameters as assessed by a trained panel (pred r 2=96.3%).

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.

Casein micelles as hard spheres: limitations of the model in acidified gel formation

In many situations casein micelles can be treated as adhesive spheres and milk gels are often considered as particle gels. In this study of acidified milk gels, produced over a wide range of acidification conditions using glucono-δ-lactone as acidulant, we show that adhesive sphere and particle gel theories are of limited applicability. Internal micellar integrity and the rates of change of structure and composition with pH play a role in defining the viscoelastic properties of the resulting gels. The ongoing changes in internal micelle structure enacted as pH is lowered are discussed in terms of the recent dual-binding model for casein micelle structure and assembly.

Sensory and instrumental textural characteristics of acid milk gels

Texture is a group of physical properties that derives from the structure of food and the way its constituent ingredients interact. Understanding the textural properties of a food product can be achieved by examination of its rheological behaviour and microstructure, so long as they can be related to the perception of texture by consumers. In this research, skim milks acidified with glucono-δ-lactone (GDL) and representing a range of textures were assessed using descriptive sensory analysis (non-oral) and instrumental techniques to investigate the correlation between the two types of data. All sensory attributes could be used to discriminate between the gel samples. Heat treatment of the milks prior to acidification appears to be the main factor producing the textural differences between acid milk gels, with solids content (10–20%) as a secondary factor. Sensory differences were in agreement with differences revealed using instrumental techniques. Confocal laser scanning micrographs supported the sensory results and showed that gels made from unheated milk had large protein clusters with big pores, while gels from heated milks had a more defined, branched structure with smaller pores. The effect of solids content on the microstructure of the gels could be seen mostly by the degree of interconnectivity between clusters of protein. Correlation analysis revealed that, with the exception of surface water (whey separation) and smoothness, all other sensory descriptors could be satisfactorily modelled with the instrumental information available, with canonical R2 above 85%.

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.

Native vs. Damaged Milk Fat Globules: Membrane Properties Affect the Viscoelasticity of Milk Gels

The storage modulus G′ of rennet and acid milk gels filled with milk fat globules was measured as a function of the fat globule surface composition (native milk fat globule membrane, caseins and whey proteins, or a mixture of the three due to mechanical treatments) and surface area (i.e., the fat globule size). By different technological procedures, it was possible to obtain fat globules of constant surface composition but various sizes, and vice-versa, which had never been done. For both rennet and acid gels, a critical fraction of the fat globule surface covered by caseins and whey proteins was identified (∼40%), beyond which G′ increased. Below this threshold, the gel viscoelasticity was unaffected by mechanical treatments. When the diameter of native milk fat globules decreased, the G′ of rennet gels increased slightly, whereas that of acid gels decreased sharply. For both types of gels, G′ increased when the diameter of partially disrupted fat globules decreased. For recombined globules completely covered with caseins and few whey proteins, G′ increased with fat globule surface area for rennet gels whereas it decreased for acid gels. With the help of confocal microscopy and in the light of general structural differences between rennet and acid gels, a mechanism is proposed for the effect of fat globule damage and diameter on G′, depending on the interactions the globules can undergo with the casein network.

Effect of SDS on Acid Milk Coagulability

ABSTRACT To study the importance of hydrophobic interaction on the mechanism of acid milk gel formation, milk coagulation process was monitored at 30 °C in presence of various levels of sodium dodecyl sulphate (SDS). As a function of the SDS concentration, acid milk coagulability was either enhanced or reduced. Main pH‐induced biochemical changes were preserved despite the presence of SDS (such as pH‐induced demineralization and pH‐induced protein solubilization). It could be assumed that SDS‐modified casein micelles ability to coagulate by lowering of pH might seem to be governed essentially by the level of SDS‐induced k‐casein micellar dissociation, at natural milk pH.

Monitoring of Acidified Milk Gel Formation by Ultrasonic Shear Wave Measurements. High-Frequency Viscoelastic Moduli of Milk and Acidified Milk Gel

In the present paper we describe the application of the thickness shear mode resonator technique for the measurement of viscoelastic parameters of milk and acidified milk gels in the megahertz frequency range. The technique provides information on the viscoelasticity of milk and milk gels in the time scale of 10–8 to 10–7 s. The length scale of the measurements, determined by the depth of penetration and the wavelength of the shear wave, falls between the submicron and micron range. In milk acidified by glucono-δ-lactone we observed an increase in the high- and low-frequency loss modulus, G”, below pH 5.1, indicating aggregation of casein particles into clusters. There was a sharp rise in high- and low-frequency storage (G′) and loss (G″) moduli between pH 4.85 and 4.7, as a result of gel network formation in acidified milk. Both G″ and G′ of milk gel in the megahertz frequency range are several orders higher than those we obtained at low frequencies (0.02 to 10Hz) using dynamic rheology. The high-frequency (5 to 25MHz) viscosity of milk was found to be the same as at low frequencies. Overall, our results demonstrate the high sensitivity of the ultrasonic shear wave measurements to the changes in the rheological parameters in acidified milk during gelation.

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.

A Rheo-optical Investigation into the Viscoelastic Moduli of Acidified Milk Gel

Abstract Diffusing wave spectroscopy (DWS) in the backscattering geometry was employed to observe the evolution of the intensity correlation function during the acidification of skimmed milk by gluconic-δ-lactone (GDL). At the stage when the formation of casein particle gel is largely complete the correlation function at shorter decay times reveals the local structural arrest of the casein micelles, whereas at longer delay times it illustrates the hindered slow motion of casein micelle aggregates. We use the principles of the approach suggested by Mason, Gang and Weitz, linking the optically measured mean square displacement, <Δr2(t)>, of the microscopic particles in a dense colloid to its viscoelastic properties, to provide an estimate of the frequency dependent viscoelastic modulus of the acidified milk gel (AMG). We compare the viscoelastic moduli measured by the conventional mechanical rheometry with the optically measured ones. The results of the two different experimental methods are found to be in reasonable agreement.

GENERAL PRINCIPLES

Perceived Texture: Direct and Indirect Methods for Use in Product Development. F.R. Jack. Rheology of [Carrageenan Gels Containing Caseins. M.G. Lynch and D.M. Mulvihill. A Contribution to the Study of Staling of White Bread: Effect of Water and Hydrocolloid. S. Davidou, M. LeMeste, E. Debever. Relationship Between the Sponge Structure of Starch Bread and Its Mechanical Properties. C.J.A.M. Keetels, T. van Vliet. Cultivar and Maturity Effects on Muskmelon Colour, Texture and Cell Wall Polysaccharide Composition. V. Simmdjuntak, D. M. Barrett. Cultivar and Frozen Storage Effects on Muskmelon (Cucumis melo) Colour, Texture and Cell Wall Polysaccharide Composition. V. Simandjauntak, D.M. Barrett. Rheology of Milk Protein Gels and Protein‐Stabilized Emulsion Gels Cross‐Linked with Transglutaminase. E. Dickinson. Rheological and Biochemical Characteristics of High‐Pressure‐and Heat‐Induced Gels from Blue Whiting Muscle Proteins. M. Perez‐Mateos, H. Lourenco, P. Montero and A.J. Borderia. New Approaches to Understanding and Controlling Cell Separation in Relation to Fruit and Vegetable Texture. K. W. Waldron. Disulfide Bonds Influence the Heat‐induced Gel Properties of Chicken Breast Muscle Myosin. A.B. Smyth, D.M. Smith. Rheological Changes in Wheat Sourdough During Controlled and Spontaneous Fermentation. K. Wehrle and A. E. Arend.INSTRUMENTATION & METHODOLOGY Analyzing the Texture of Pasta for Quality Control. J. Smewing. Determining Rheological Properties of Cereal Products Using Dynamic Mechanical Analysis in Compression Mode. D. Weipert. Measurement of Extrusion Effects by Viscosity Profile Using the Rapid Visco Analyzer. P.J. Whalen, M.L. Bason, R.I. Booth, C.E. Walker and P.J. Williams. Instrumental Measurements of Raw and Cooked Gluten Texture. Z. Czuchajowska. Predicting a Hardness Measurement Using the Single Kernel Characterization System. C.S. Gaines, P.F. Finney, L.M. Fleege and L.C. Andrew. Tortilla Bending Technique: An Objective Method for Corn Tortilla Texture Measurements. E.L. Suhendro. Capillary Rheometry of Corn Endospermt Glass Transition, Flow Properties, and Melting of Starch. W. Zhang, K.C. Hoseney.INSTRUMENTAL MEASUfCEMENTS Dynamic Mechanical Properties of Frozen Meat. M.J. King.SENSORY EVALUATION Instrumental and Sensory Measurement of Beef Patty and Sausage Texture. M.M. Farouk.SENSORY MEASUREMENTS Analysis of Textural Changes in Hard Cheese during Mastication by Progressive Profiling. F.R. Jack, J.R. Piggott and A. Paterson. Physical, Compositional and Sensory Properties of French Fry‐Type Products from Five Sweetpotato Selections. W.M. Walter, Jr. Sensory Texture Profiling ‐ Historical and Scientific Perspectives. A. S. Szczesniak.FACTORS AFFECTING TEXTURE. Effect of Storage on Texture. AS. Szczesniak. Solute Infusion Effects on Texture of Minimally Processed Kiwifruit. V. Muntada. Rheological Properties and Microstructure of Acid Milk Gels as Affected by Fat Content and Heat Treatment. J.A. Lucey. Textural Properties and Sensory Quality of Processed Sweetpotatoes as Affected by Low Temperature Blanching. V.D. Truong, W.M. Walter. Texture of Glassy Corn Cakes as a Function of Moisture Content. Y. Li, K.M. Kloeppel and F. Hsieh.

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.

STABILITY OF MODEL ACID MILK BEVERAGE: EFFECT OF PECTIN CONCENTRATION, STORAGE TEMPERATURE AND MILK HEAT TREATMENT

ABSTRACTAcid milk beverages were made by homogenizing and diluting acid milk gels made with glucono‐b‐lactone. A central composite experimental design and response suqace methodology were used. Colloidal stability was assessed by the amount of surface whey produced during storage, the flow behaviour and particle size distribution. Using regression analysis a second order polynomial model satisfactorily predicted the effect of pectin concentration on whey separation (R2= 0.88). Whey separation was significantly decreased by pectin concentration and increased by the quadratic term for pectin concentration (P < 0.001). Neither of the other variables had a significant effect on any of the indices of colloidal stability. The prediction equations for the variables flow behaviour index and particle size distribution had much lower values (I 0.63). It is suggested that pectin prevented the aggregation of casein‐gel fragments; evidence for this was also provided by confocal scanning laser micrographs of beverages made with various concentrations of pectin.