Measurement of electrophoretic mobilities and zeta-potentials of particles from milk using laser Doppler electrophoresis

Some Remarks on Colloid Stability: Selected Examples Taken from the Milk Chain for Food Prepares

Different forces play key roles in the stability of food colloid dispersions. The focus here is on those controlling attraction and/or repulsion, which concur to stabilization, phase separation, coagulation and are quite evident in water-based systems. The combination of attractive and repulsive forces favors or hinders the association of colloid entities; such processes are often met in food technology. The above processes depend on the forces at work and colloid concentration in the medium (i.e., on interparticle distance). Worked examples deal with milk manipulation procedures, ending in cheese formation. The whole milk sequence is controlled by the combination of forces leading to aggregation and phase separation of casein and other milk components. Thereafter, one gets either fresh, for prompt consumption, or aged cheeses. The combination of attractive (van der Waals, vdW, and depletion) with repulsive (double layer, DL, but also steric) forces results in the dominance of aggregation versus dispersion modes in the milk transformation chain, which depends on the distance among colloid particles, on the amplitude of the mentioned forces, and on their decay. The combined role of double layer and van der Waals (vdW) forces is at the basis of the DLVO theory on colloid stability, which is properly modified when these forces overlap with steric stabilization and, eventually, with depletion. Steric effects are dispersive, and depletion ones favor colloid nucleation in a single phase. The milk manipulation chain is a worked example of the intriguing association features controlled by the mentioned forces (and of ancillary ones, as well), and indicates which forces favor the formation of products such as parmesan or mozzarella cheese but are not alien to the preparation of many other dairy products.

Exopolysaccharide from Streptococcus thermophilus as stabilizer in fermented dairy: Binding kinetics and interactions with casein of milk

Exopolysaccharides (EPSs) from lactic acid bacteria have great effect on the quality of fermented milk products. However, the mechanism for the quality improvement has not been well described. This study aimed to investigate the molecular binding kinetics and interactions between EPS obtained from Streptococcus thermophilus AR333 (EPS333) and casein of milk (CM) in a simulated acidifying process. The results indicated that EPS333 had a significant effect on the stability of casein micelles at acidic pH (6.0-4.5) according to the turbidity, ζ-potential, particle size and distribution analysis. The adsorption-desorption study by bio-layer interferometry identified the direct affinity binding between EPS333 and CM, the interactive moiety of casein was α-casein, rather than β- or κ-casein. Fluorescence quenching analysis revealed that the force types of interaction between EPS333 and CM were dynamically changeable during the acidifying process, mainly from electrostatic interaction at pH 7.0-6.5, to hydrophobic or hydrogen bonding at pH 6.5-5.5, and then transferred to electrostatic interaction again at pH 5.5-5.0. Conclusively, EPS333 could bind with CM directly via different binding forces during acidifying process to stabilize the properties of casein micelles.

Nanostructured Materials for Food Applications: Spectroscopy, Microscopy and Physical Properties

Nanotechnology deals with matter of atomic or molecular scale. Other factors that define the character of a nanoparticle are its physical and chemical properties, such as surface area, surface charge, hydrophobicity of the surface, thermal stability of the nanoparticle and its antimicrobial activity. A nanoparticle is usually characterized by using microscopic and spectroscopic techniques. Microscopic techniques are used to characterise the size, shape and location of the nanoparticle by producing an image of the individual nanoparticle. Several techniques, such as scanning electron microscopy (SEM), transmission electron microscopy/high resolution transmission electron microscopy (TEM/HRTEM), atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) have been developed to observe and characterise the surface and structural properties of nanostructured material. Spectroscopic techniques are used to study the interaction of a nanoparticle with electromagnetic radiations as the function of wavelength, such as Raman spectroscopy, UV–Visible spectroscopy, attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), dynamic light scattering spectroscopy (DLS), Zeta potential spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray photon correlation spectroscopy. Nanostructured materials have a wide application in the food industry as nanofood, nano-encapsulated probiotics, edible nano-coatings and in active and smart packaging.

Influence of Homogenization and Thermal Processing on the Gastrointestinal Fate of Bovine Milk Fat: In Vitro Digestion Study

Dairy lipids are an important source of energy and nutrients for infants and adults. The dimensions, aggregation state, and interfacial properties of fat globules in raw milk are changed by dairy processing operations, such as homogenization and thermal processing. These changes influence the behavior of fat globules within the human gastrointestinal tract (GIT). The gastrointestinal fate of raw milk, homogenized milk, high temperature short time (HTST) pasteurized milk, and ultrahigh temperature (UHT) pasteurized milk samples was therefore determined using a simulated GIT. The properties of particles in different regions of the GIT depended on the degree of milk processing. Homogenization increased the initial lipid digestion rate but did not influence the final digestion extent. Thermal processing of homogenized milk decreased the initial rate and final extent of lipid digestion, which was attributed to changes in interfacial structure. These results provide insights into the impact of dairy processing on the gastrointestinal fate of milk fat.

Effect of concentration, homogenization and stabilizing salts on heat stability and rheological properties of cow skim milk ultrafiltered retentate

Ultrafiltration (UF) of skimmed milk altered the composition of UF retentate and decreased the heat stability. Heat stability further reduced upon its subsequent homogenization or diafiltration. Poor heat stability of UF retentate restricts its processing at elevated temperatures. Therefore, this study was aimed to investigate the effect of protein concentration, homogenization and addition of stabilizing salts on the heat stability and rheological properties of UF retentates. Changes in the heat stability of fivefold homogenized UF retentate (5× HUFR) was studied in the pH range of 6.1-7.0. Disodium phosphate and trisodium citrate significantly increased the heat coagulation time (HCT) from 1.45 min (pH 6.41) to 120 min (at pH 6.5, 6.6, 7.0) and 80 min (pH 6.6), respectively. Significant reduction in ζ-potential of UF retentates was observed with an increase in calcium and reduction in pH during UF process. Rheological behaviour of retentates above threefold concentration exhibited Herschel-Bulkley behavior with linear increase in flow behavior index (n). Changes in the viscosity of the homogenized retentates were measured at the respective pH of maximum heat stability as a function of temperature (20-80 °C). Promising approaches that might improve the heat stability, solubility and other functional properties of protein rich powders have been discussed in this article.

The structure of infant formulas impacts their lipolysis, proteolysis and disintegration during in vitro gastric digestion

Milk lipids supply most of the calories necessary for newborn growth in maternal milk or infant formulas. The chemical composition of infant formulas has been optimized but not the structure of the emulsion. There is still a major difference between the native emulsions of milk fat globules and processed submicronic emulsions in infant formulas. This difference may modify the kinetics of digestion of emulsions in newborns and influence lipid metabolism. To check this, semi-dynamic gastric in vitro digestions were conducted on three matrices: a standardized milk emulsion containing native milk fat globules referred to as minimally-processed emulsion and two processed model infant formulas (homogenized or homogenized/pasteurized). Gastric conditions mimicked those reported in newborns. The minimally-processed emulsion was lipolyzed and proteolyzed slower than processed formulas. The difference in initial structure persisted during digestion. The surface of the droplets was the key parameter to control gastric lipolysis kinetics, the pattern of released fatty acids and proteolysis by faster hydrolysis of adsorbed proteins.

Cellular uptake of β-carotene from protein stabilized solid lipid nanoparticles prepared by homogenization-evaporation method

With a homogenization-evaporation method, β-carotene (BC) loaded nanoparticles were prepared with different ratios of food-grade sodium caseinate (SC), whey protein isolate (WPI), or soy protein isolate (SPI) to BC and evaluated for their physiochemical stability, in vitro cytotoxicity, and cellular uptake by Caco-2 cells. The particle diameters of the BC loaded nanoparticles with 0.75% SC or 1.0% WPI emulsifiers were 75 and 90 nm, respectively. Mean particle diameters of three BC loaded nanoparticle nanoemulsions increased less than 10% at 4 °C while they increased more at 25 °C (10-76%) during 30 days of storage. The oxidative stability of BC loaded nanoparticles encapsulated by proteins decreased in the following order: SC > WPI > SPI. The retention rates of BC in nanoparticles were 63.5%, 60.5%, and 41.8% for SC, WPI, and SPI, respectively, after 30 days of storage at 25 °C. The BC's chemical stability was improved by increasing the concentration of protein. Both the rate of particle growth and the total BC loss at 25 °C were larger than at 4 °C. The color of BC loaded nanoparticles decreased with increasing storage in the dark without oxygen, similar to the decrease in BC content of nanoparticles at 4 and 25 °C. Almost no cytotoxicity due to BC loaded nanoparticles cellular uptake was observed, especially when diluted 10 times or more. The uptake of BC was significantly improved through nanoparticle delivery systems by 2.6-, 3.4-, and 1.7-fold increase, respectively, for SC, WPI, and SPI, as compared to the free BC. The results of this study indicate that protein stabilized, BC loaded nanoparticles can improve stability and uptake of BC.

Formation of acid-heat-induced skim milk gels in the pH range 5·0–5·7: effect of the addition of salts and calcium chelating agents

The effects of pH and added salts or chelating agents on the gel strength and dynamic rheological properties of acid–heat-induced gels made from reconstituted skim milk (200 g solids/l) were investigated. Gel strength increased as pH was lowered between 5·75 and 5·15 except in the range pH 5·45–5·25 where a local maximum in gel strength was obtained at pH 5·35. Gel characteristics were affected by addition of salts or chelating agents but each of their effects was different, depending on the final pH of the milk gel. The addition of CaCl2 or chelating agents (Na2HPO4, disodium citrate or the disodium salt of EDTA) which affected micellar calcium phosphate, non-sedimentable casein and Ca2+ activity in different ways all resulted in decreased gel strength at pH 5·5. The addition of CaCl2 or MgCl2 caused a decrease in tanδ (ratio of the viscous modulus G″ to the elastic modulus G′) whereas disodium citrate or the disodium salt of EDTA addition caused an increase and Na2HPO4 addition did not cause a change. The addition of NaCl (up to 50 mM), which causes an increase in ionic strength but has no effect on non-sedimentable casein and Ca2+ activity, decreased gel strength but did not change tanδ. The addition of a range of other salts (KCl, NH4Cl, NaSCN, NaNO3 or Na2SO4) also decreased gel strength at pH 5·5.

Influence of calcium chloride on the chymosin-initiated coagulation of casein micelles

Coagulation of para-casein micelles was monitored using a Platelet Aggregometer at 37·8 °C and pH 6·7. The macropeptide released by chymosin was determined quantitatively with fluorescamine. The aggregatability of para-casein micelles integrated over the complete hydrolysis of κ-casein was calculated from the amount of κ-casein hydrolysed during the clotting time (Tc). Low concentrations of CaCl2 enhanced the rate of κ-casein hydrolysis but increases in CaCl2 concentration above 8 mM caused marked decreases in this rate. Calcium chloride enhanced the ability of para-casein micelles to aggregate. Little aggregation of para-casein micelles occurred at 0·6 mM-CaCl2 even after all of the κ-casein was hydrolysed. As the concentration of CaCl2 was increased from 3 to 60 mM, aggregation of para-casein micelles took place at progressively lower levels of κ-casein hydrolysis and the percentage hydrolysis at the Tc decreased markedly from 71±7% to 26±8%. The combined influence of CaCl2 on the velocity of the chymosin-catalysed reaction and on para-casein micelle aggregatability accounted for its effect on the average rate of coagulation (calculated by the reciprocal of the Tc). Results are consistent with the hypothesis that electrostatic repulsions and ionic bonding are involved in the interaction between chymosin and κ-casein.

Estimation of casein micelles' surface energy by means of contact angle measurements

The surface energies of highly hydrated casein micelle layers isolated from variously pretreated skim milks have been determined by means of contact angle measurements. The long range Lifshitz-Van der Waals (LW) and the short range hydrogen bonding (SR) components of surface energy were determined using α-bromonaphthalene and water for contact angle measurements. Casein micelles isolated from untreated and heat treated milks showed similar surface energy values of about 63·5 mJ.m-2 with an LW component of 19·2 mJ.m-2 and an SR component of 44·3 mJ.m-2. The calculated attraction potential energy was − 0·7 mJ.m-2. Casein micelles isolated from renneted milk showed a surface energy of 33·0 mJ.m-2 with an LW component of 30·7 mJ.m-2 and an SR component of 2·3 mJ.m-2. The attraction potential energy of renneted micelles was nearly two orders of magnitude higher than those of micelles from other milks ( − 63·3 mJ.m-2). The SR component of interfacial energy accounted for 98% of this attraction potential. The importance of attractive forces in relation to casein micelle stability is discussed.

Electroacoustic determination of size and zeta potential of casein micelles in skim milk

Measurements of the zeta potential and particle size of casein micelles in skim milk suspensions at natural and lower pH have been made using the technique of electroacoustics. This technique requires no dilution or change of environment of the casein micelles. The zeta potential obtained at natural pH for a commercial skim milk suspension was −18 mV; it became less negative with decreasing pH. The median particle size observed at natural pH for a commercial skim milk suspension was 0·2 εm, in good agreement with previously reported values. The particle size increased as the pH was decreased.

Reformation of casein particles from alkaline-disrupted casein micelles

In this study, the properties of casein particles reformed from alkaline disrupted casein micelles were studied. For this purpose, micelles were disrupted completely by increasing milk pH to 10·0, and subsequently reformed by decreasing milk pH to 6·6. Reformed casein particles were smaller than native micelles and had a slightly lower zeta-potential. Levels of ionic and serum calcium, as well as rennet coagulation time did not differ between milk containing native micelles or reformed casein particles. Ethanol stability and heat stability, >pH 7·0, were lower for reformed casein particles than native micelles. Differences in heat stability, ethanol stability and zeta-potential can be explained in terms of the influence of increased concentrations of sodium and chloride ions in milk containing reformed casein particles. Hence, these results indicate that, if performed in a controlled manner, casein particles with properties closely similar to those of native micelles can be reformed from alkaline disrupted casein micelles.

Acid gelation properties of heated skim milk as a result of enzymatically induced changes in the micelle/serum distribution of the whey protein/kappa-casein aggregates

Changes in the acid gelation properties of skim milk as a result of variations in the micelle/serum distribution of the heat-induced whey protein/kappa-casein aggregates, induced by the combination of heat treatment and limited renneting, were investigated. No dramatic change in the zeta potential or the isoelectric point of the casein micelles was suggested, whether the aggregates were all attached to the casein micelle or not. Fluorescence intensity measurement using 8-anilino-1-naphthalenesulfonic acid (ANS) showed that the heat-induced aggregates were highly hydrophobic. Dynamic oscillation viscosimetry showed that acid gelation using glucono-delta-lactone (GDL) started at a higher pH value in prerenneted milk. However, no change in the gelation profile of skim milk could be related to the proportion of aggregates bound to the surface of the casein micelles. The results support the idea of an early interaction between the serum aggregates and the casein micelles on acidification.