The breakdown properties of heat-set whey protein emulsion gels in the human mouth

Differently structured whey protein emulsion gels were formed by heating at different concentrations of NaCl. The formation of gels was monitored by oscillatory rheometry. The large deformation properties relevant to breakdown properties in the human mouth were measured by a uniaxial compression test and fracture wedge set test using a texture analyzer. A panel of 8 subjects was used to examine the in-mouth behaviours of gels including mastication parameters, degree of fragmentation and oil droplet release. The results showed that in general the gel hardness increased with increasing NaCl concentration. The gels containing 10/25 and 100/200 mM NaCl were characterized as being soft and hard, respectively. These soft and hard gels had different breakdown patterns in the mouth. On the other hand, sensory experiments showed the gel with 10 mM NaCl needed a significantly lower number of chewing cycles (19.4 ± 2.1) compared with gels with higher NaCl. The values of median size of particles in masticated gels containing 10, 25, 100 and 200 mM NaCl were about 4.00, 2.85, 1.05 and 0.95 mm, respectively, which suggested that higher hardness led to greater fragmentation in the human mouth. The fragmentation of the gel was highly correlated with functions of the mechanical properties. There was no obvious coalescence of the oil droplets during oral processing and only very few oil droplets were released from protein matrix during mastication.     

Rheological and structural characterization of gels from whey protein hydrolysates/locust bean gum mixed systems

The gelling ability of whey proteins can be changed by limited hydrolysis and by the addition of other components such as polysaccharides. In this work the effect of the concentration of locust bean gum (LBG) on the heat-set gelation of aqueous whey protein hydrolysates (10% w/w) from pepsin and trypsin was assessed at pH 7.0. Whey protein concentrate (WPC) mild hydrolysis (up to 2.5% in the case of pepsin and 1.0% in the case of trypsin) ameliorates the gelling ability. The WPC synergism with LBG is affected by the protein hydrolysis. For a WPC concentration of 10% (w/w), no maximum value was found in the G′ dependence on LBG content in the case of the hydrolysates, unlike the intact WPC. However, for higher protein concentrations, the behaviour of gels from whey proteins or whey protein hydrolysates towards the presence of LBG becomes very similar. In this case, a small amount of LBG in the presence of salt leads to a big enhancement in the gel strength. Further increases in the LBG concentration led to a decrease in the gel strength.     

Gelation behaviour and rheological properties of acid-induced soy protein-stabilized emulsion gels

The protein concentration dependence on the rheological properties of acid-induced gels formed with unheated and heated soy protein-stabilized emulsions (UHSPE and HSPE) was investigated at different acidification temperatures. Pre-heat treatment on soy protein solutions resulted in a higher storage modulus (G′) and a shorter gelation time (t_gel) of acid-induced emulsion gels. A maximum in tan δ was observed in the UHSPE gels but no maximum was detected in the HSPE gels. Increasing the acidification temperature decreased the G′ and t_gel. The dependence of the G′ on the protein concentration (c) can be scaled with a power law: G′ ∼ c^A. The exponent (A) increased with pre-heat treatment and acidification temperature. The experimental data.fitted the fractal scaling model (G′ ∼ φ^A) and the simple time- scaling model above very well for the acid-induced soy protein-stabilized HSPE gels with varying oil volume fraction. The large deformation and fracture properties were significantly affected by soy protein concentration, pre-heat treatment, acidification temperature and volume fraction of oil droplets (p < 0.05).     

Effects of ultra-high pressure homogenization on the properties and structure of interfacial protein layer in whey protein-stabilized emulsion

The effect of high pressure homogenization on the properties of whey protein adsorbed on emulsion interface was determined by competitive adsorption with Tween 20, Fourier transform infrared (FT-IR) spectroscopy, and RP-HPLC. The amount of whey proteins desorbed by Tween 20 increased at higher pressure. The results of FT-IR spectroscopy showed that higher homogenization pressures led to decrease of α-helix and increase of β-sheet indicating the formation of fewer interactions with the lipid phase and more interaction between adsorbed whey proteins, respectively. In RP-HPLC profile, the retention time of whey protein desorbed from high pressured emulsion was shorter than that from low-pressured emulsion. From these results, we have shown that higher pressures cause the formation of a more compact interfacial protein layer. Finally, “high pressure” emulsions are more stable than “low-pressure” ones, because of the protein layer formed by protein-protein interactions as well as the decrease of droplet size.     

Effect of protein composition on the rheological properties of acid-induced whey protein gels

Protein dispersions with different ratios of α-lactalbumin to β-lactoglobulin were heat-denatured at pH 7.5 and then acidified with glucono-δ-lactone to form gels at room temperature. Heat treatment induced the formation of whey protein polymers with reactive thiol group concentrations ranging from 1 to 50 μmol/g, depending on protein composition. During acidification, the first sign of aggregation occurred when the zeta potential reached −18.2 mV. Increasing the proportion of α-lactalbumin in the polymer dispersions resulted in more turbid gels characterized by an open microstructure. Elastic and viscous moduli were reduced, while the relaxation coefficient and the stress decay rate constants were increased by raising the proportion of α-lactalbumin in the gel. After one week of storage at 5 °C, gel hardness increased by 12%. The effect of protein composition on acid-induced gelation of whey protein is discussed in relation to the availability and reactivity of thiol groups during gel formation and storage.     

Effect of whey protein enzymatic hydrolysis on the rheological properties of acid-induced gels

A protein dispersion blend of β-lactoglobulin and α-lactalbumin was heat-denatured at pH 7.5, hydrolyzed by α-chymotrypsin and then acidified with glucono-δ-lactone to form gels at room temperature. Heat treatment induced the formation of whey protein polymers with high concentration of reactive thiol groups (37 μmol/g). The reactive thiol group concentration was reduced by half after 40 min enzymatic hydrolysis. It was further reduced after enzyme thermal deactivation. During acidification, the first sign of aggregation for hydrolyzed polymers occurred earlier than for non hydrolyzed polymers. Increasing the hydrolysis duration up to 30 min resulted in more turbid gels characterized by an open microstructure. Elastic and viscous moduli were both reduced, while the relaxation coefficient and the stress decay rate constants were increased by increasing the hydrolysis duration. After one week storage at 5 °C, the hardness of gels made from hydrolyzed polymers increased by more than 50%. The effect of polymer hydrolysis on acid-induced gelation is discussed in relation to the availability and reactivity of thiol groups during gel formation and storage.     

Effect of whey protein concentrate addition on the physical properties of homogenized sweetened dairy creams

The influence on their whipping properties of homogenization at first and second stage pressures of 3.5/1.5 MPa and addition of whey protein concentrate (WPC) powder at three different (0.7, 1.4, and 2.1 wt percentage) concentrations to sweetened and homogenized creams was studied. Homogenization of cream significantly decreased maximum overrun and made the foam microstructure less open, while increasing whipping time, cream and foam lightness (Hunter L -value) and apparent viscosity. It also resulted in a less elastic foam structure with an increased drainage. Addition of WPC decreased the amount of maximum overrun, foam drainage and its lightness in parallel with developing a more compact microstructure. It increased the whipping time, apparent viscosity of unwhipped creams and foams, and resulted in a less elastic foam structure. The apparent viscosity of whipped cream with 2.1 wt percentage WPC, however, was lower than that of whipped cream with 1.4 wt percentage WPC, due most probably to the start up of gel formation at 2.1% WPC concentration in sweetened cream when it was sheared. Fresh foam whipped from sweetened cream with 2.1 wt percentage WPC also tended to have a slightly but not statistically significant lower elastic modulus (G') than fresh foam whipped from sweetened cream with 1.4 wt percentage WPC. This concentration can be considered as the critical value for gel formation in sweetened creams enriched by whey proteins when sheared. This study indicated the potential of WPC powder for reducing foam drainage from whipped homogenized sweetened cream.     

Influence of whey protein concentrate on the rheological characteristics of dough, microstructure and quality of unleavened flat bread (parotta)

In order to explore the possibility of using WPC as a functional ingredient in Indian traditional product – South Indian Parotta investigations were made to study the effect of replacement of wheat flour with 5, 10 and 15% WPC on the farinograph, extensograph, amylograph characteristics of wheat flour, quality of parotta and microstructure of baked parotta. The results showed an increase in farinograph stability, extensograph resistance to extension up to 10% WPC and a decrease in the farinograph water absorption, extensograph extensibility, amylograph peak viscosity, cold paste viscosity, breakdown and setback values with an increase in the level of WPC from 0% to 15%. The quality characteristics of parotta showed that the spread ratio decreased and shear force values increased significantly above 5% level. Control parotta and parotta with 5% WPC were soft, possessed thin and transparent layers whereas parottas beyond 5% WPC had thick, fused and opaque layers. The parottas with 5% WPC were rated good. The quality characteristics of parotta were adversely affected beyond 5% level of WPC. The microstructure of the top and middle layer of baked parotta with 5% WPC showed that there was a disruption in the continuity of the gluten matrix.     

Rheology and microstructure of myofibrillar protein-plant lipid composite gels: Effect of emulsion droplet size and membrane type

Composite gels were prepared from 2% myofibrillar protein (MP) with 10% imbedded pre-emulsified plant oils (olive and peanut) of various particle sizes at 0.6 M NaCl, pH 6.2. Dynamic rheological testing upon temperature sweeping (20-70 °C at 2 °C/min) showed substantial increases in G′ (elastic modulus) of MP sols/gels with the addition of emulsions, and the G′ increases were inversely related to the emulsion droplet size. Furthermore, gels containing emulsified olive oil had a greater (P < 0.05) hardness than those containing emulsified peanut oil. Regardless of oil types, MP-coated oil droplets exhibited stronger reinforcement of MP gels than Tween 80-stablized oil droplets; the latter composite gels had considerable syneresis. Light microscopy with paraffin sectioning revealed a stable gel structure when filled with protein-coated oil droplets, compared to gels with Tween 80-treated emulsions that showed coalesced oil droplets. These results suggest that rheological characteristics, hardness, texture, and water-holding capacity of MP gels were influenced by type of oils, the nature of the interfacial membrane, and the size of emulsion droplets.     

Effect of whey and pea protein blends on the rheological and sensory properties of protein-based systems flavoured with cocoa

Whey and pea protein combined in different proportions (100W:0P, 75W:25P, 50W:50P, 25W:75P, 0W:100P) were used to prepare protein-based systems flavoured with cocoa and containing κ-carrageenan or κ-carrageenan/xanthan gum as thickeners. Steady and dynamic shear rheological properties of samples were measured at 10 °C and sensory differences were evaluated. Protein-based systems exhibited a shear-thinning flow behaviour that was fitted to the simplified Carreau model. Samples showed different viscoelastic properties, ranging from fluid-like to weak gel behaviour. For both types of system (with and without xanthan gum) viscosity, pseudoplasticity and elasticity rose on increasing the pea protein proportion in the blend. The sample with only whey protein obeyed the Cox-Merz rule, while in the rest of the samples complex viscosity was higher than apparent viscosity. Regarding sensory properties, the protein blend ratio mainly affected sample thickness, which rose as pea protein proportion increased. However, at the same time, the chocolate flavour and sweetness decreased and the off-flavour increased.     

Rheological properties of rennet casein-whey protein gels prepared at different mixing speeds

The rheological properties at small (oscillatory shear) and large (uniaxial compression) deformations of heat-induced gels (80 °C for 20 min, pH 7.3) containing 25% rennet casein (RCN), 2.5% disodium phosphate and 0%, 2.3% or 6.3% of whey protein isolate (WPI) were measured for samples cooked in a torque-rheometer at mixing speeds within a range of 20–200 rpm (shear rates: ∼15–230 s⁻¹). In addition, microstructure analyses were performed, separately staining RCN and WPI, by Confocal Scanning Laser Microscopy (CSLM). Both small and large deformation tests indicated that increasing addition of WPI prior to the cooking process of RCN resulted in gels exhibiting higher storage and deformability moduli than WPI-free samples. Increasing shear rates during cooking also affected the rheological properties of RCN–WPI gels, and stronger gels were formed as the shear rate during cooking was increased. Despite the data dispersion among replicates, the effect of shear rate on gel strength were evident for RCN gels with 6.3% WPI and relatively clear for gels with 2.3% WPI; however, the trend was uncertain for WPI-free RCN gels. Possible explanations for this observation are that when increasing WPI levels in the presence of RCN and heat, disulfide-thiol exchange reactions between denatured WPI and κ-casein (κ-CN) are increased and possibly promoted by shear rate, resulting in stronger and more cross-linked gel structure. CSLM results were not conclusive to support this hypothesis.     

Improved emulsion stabilizing properties of whey protein isolate by conjugation with pectins

Functional properties of glyco-protein conjugates of the anionic polysaccharide pectin with whey protein isolate, obtained by dry heat treatment at 60 °C for 14 days, have been investigated in O/W emulsions containing 20% (w/w) soybean oil and 0.4% (w/w) protein both at pH 4.0 and 5.5. Emulsion stabilizing properties of mixtures and conjugates were compared at five protein to pectin weight ratios by determining changes in droplet size distribution and extent of serum separation with time. The results indicated that the dry heat-induced covalent binding of low methoxyl pectin to whey protein, as shown by SDS-PAGE, led to a substantial improvement in the emulsifying behaviour at pH 5.5, which is near the isoelectric pH of the main protein β-lactoglobulin. At pH 4.0, however, a deterioration of the emulsifying properties of whey protein was observed using either mixtures of protein and pectin or conjugates.The observed effects could be explained by protein solubility and electrophoretic mobility measurements. The protein solubility at pH 5.5 was hardly changed using mixtures of protein and low methoxyl pectin or conjugates, whereas at pH 4.0 it was decreased considerably. Electrophoretic mobility measurements at pH 5.5 revealed a much more pronounced negative charge on the emulsion droplets in the case of protein–pectin conjugates, which clearly indicated that conjugated pectin did adsorb at the interface even at pH conditions above the protein's iso-electric point. Hence, the improved emulsifying properties of whey protein isolate at pH 5.5 upon conjugation with low methoxyl pectin may be explained by enhanced electrosteric stabilization.Comparing two different commercial pectin samples, it was clearly shown that the dextrose content during dry heat treatment of protein–pectin mixtures should be as low as possible since protein–sugar conjugates not only resulted in increased brown colour development, but also gave raise to a largely decreased protein solubility which very badly affected the emulsifying properties.     

Effect of blackberry and raspberry juice on whey protein emulsion stability

The effect of raspberry and blackberry juice on oil-in-water (o/w) emulsion oxidative stability was investigated with different concentrations of whey proteins and rapeseed oil. The extent of lipid oxidation was measured by determining conjugated diene hydroperoxides and thiobarbituric acid reactive substances and that of protein oxidation by loss of natural tryptophan fluorescence and formation of protein carbonyl compounds. In addition, the anthocyanin colour stability and emulsion turbidity were measured. The increasing concentration of whey proteins and berry juices led to enhanced stabilization of the interface formed during emulsification. The anthocyanin concentration and colour decreased during oxidation leading to whiter emulsions. Raspberry juice provided a better overall antioxidant protection towards lipid and protein oxidation compared to blackberry juice. The antioxidant activity of berry juices towards lipid oxidation varied with concentration. The antioxidant activity increased with increasing concentration of berry juices. In conclusion, red berry juice anthocyanins, as well as other phenolic compounds, act as antioxidants improving the oxidative stability of whey protein emulsions. However, the antioxidant protection is different towards protein and lipid oxidation, and is also affected by other components present.     

Kinetics of chemical marker formation in whey protein gels for studying microwave sterilization

The kinetics of 4-hydroxy-5-methy-3(2H)-furanone (M-2) formation in a model food system (20% whey protein gel) was determined for studying cumulative time–temperature effects in high-temperature-short-time processes. M-2 was formed from -ribose and amines through non-enzymatic browning reactions and enolization under low acid conditions (pH > 5). The order of the reaction for M-2 formation was determined by non-linear regression analysis and further confirmed by graphical method. M-2 formation followed a first-order kinetics and the rate constant temperature dependence was described using an Arrhenius relationship. The reaction rates and activation energy were determined using two-step, multi-linear and non-linear regression analyses. This study also demonstrated the use of M-2 formation in determining the cumulative heating effect in a model food system subjected to 915 MHz microwave heating.     

Relating the effect of saliva-induced emulsion flocculation on rheological properties and retention on the tongue surface with sensory perception

Perception of food emulsions can often not be directly related to the structure of the products before consumption. Taking into account the changing product structure upon oral processing might increase understanding of the relation between perception and product properties.     

Study on the rheological properties of heat-induced whey protein isolate-dextran conjugate gel

Effect of glycosylation on the rheological properties of whey protein isolate (WPI) during the heat-induced gelation process was evaluated. Significant changes in browning intensity, free amino groups content and SDS-PAGE profile showed that the conjugate of WPI and dextran (150 kDa) was successfully prepared using the traditional dry-heating treatment. For the conjugate, during the heating and cooling cycle, the curves of G′ and G″ were considerably shifted to lower values and their shapes varied comparing to the corresponding spectra of initial WPI and WPI + dextran mixture. After holding at 25 °C, G' reached a value of about 2200 Pa, only a tenth of the value that obtained in the initial WPI gel. Moreover, frequency sweep measurements revealed that the stiffness of gel was greatly reduced in the conjugate, although a typical elastic gel was still formed. All data showed that the rheological properties of thermal gelation could be modified upon the covalent attachment of dextran.     

Effects of two types of soy protein isolates,native and preheated whey protein isolates on emulsified meat batters prepared at different protein levels

The effects of substituting 1.5% of the meat proteins with low gelling soy protein isolate (LGS), high gelling soy protein isolate (HGS), native whey protein isolate (NWP), and preheated whey protein isolate (PWP) were compared at varying levels of proteins (12, 13 and 14%), with all meat control batters prepared with canola oil. Cooking losses were lower for all the non-meat protein treatments compared to the all meat controls. When raising the protein level from 12 to 14%, cooking losses increased in all treatments except for the NWP treatments. Using LGS increased emulsification and resulted in a more stable meat batters at the 13 and 14% protein treatments. Textural profile analysis results showed that elevating protein level increased hardness and cohesiveness. The highest hardness values were obtained for the PWP treatments and the lowest for the HGS, indicating a strong non-meat protein effect on texture modification. Non-meat protein addition resulted in lighter and less red products (i.e., lower red meat content) compared to the all meat controls; color affected by non-meat protein type. Light microscopy revealed that non-meat proteins decreased the frequency of fat globules' agglomeration and protein aggregation. The whey protein preparations and HGS formed distinct “islands” within the meat batters' matrices, which appeared to interact with the meat protein matrix.     

Cold,gel-like whey protein emulsions by microfluidisation emulsification: Rheological properties and microstructures

Novel cold, gel-like whey protein concentrate (WPC) emulsions at various oil fractions (φ; 0.2–0.6) were formed through thermal pretreatment (at 70 °C for 30 min) and subsequent microfluidisation. The rheogical properties and microstructures, as well as emulsification mechanism of these emulsions were characterised. The rheological analyses indicated that the gel-like emulsions exhibited shear-thinning and predominantly elastic gel behaviours, and the apparent viscosities and the mechanical moduli of the emulsions remarkably and progressively increased with increasing the φ from 0.2 to 0.6. Confocal laser scanning microscopy analyses confirmed close relationships between rheological properties and gel network structures at various φ values. The formation of the gel-like network structure was closely related to the high emulsifying efficiency by microfluidisation. This kind of novel gel-like emulsion might exhibit great potential and be applicable in food formulations, e.g. as a kind of carrier for heat-labile and active ingredients.     

Effect of addition of gelatin on microstructure of acidic milk gels and yoghurt and on their rheological properties

A study was made of the effect of the addition of gelatin on the microstructure of acid-heat-induced milk gels (90°C, pH=5.3) and yoghurt with and without the addition of 5% of milk solids, and a comparison was made with the microstructure of acidic milk gelatin gels obtained without heating (pH=5.3). It was seen that in the acid-heat-induced gels and in yoghurt the gelatin interacted with the network of milk proteins as a connection between the clusters formed, whereas it was the gelatin alone that was the basis of the formation of the gel when the milk did not reach the casein coagulation point (pH=5.3, unheated). The results of firmness tests indicated that the addition of 1.5% of gelatin developed fairly firm, deformable systems in all the cases studied, with a definite break point and almost total absence of syneresis. Dynamic rheology showed that the yoghurts with added gelatin exhibited more solid-like behaviour than the ones prepared without it.