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.     

Rheological,microstructural and sensory characterization of low-fat and whole milk set yoghurt as influenced by inulin addition

The effect of inulin addition (0–4%) upon texture and microstructure of set yoghurt with different levels of fat (0.2%–3.5%) was investigated. A two-factor experimental design with four treatments was used for data analysis. Skimmed milk with various inulin and cream concentrations was standardized to 4% protein content, homogenized, heated to 92 °C and fermented at 42 °C until a pH of 4.6 was reached. The chemical composition, pH, consistency and microstructure properties of the yoghurts were analysed after 6 days of storage at 5 °C. The statistical analysis showed that inulin and fat significantly affected the rheological and sensory results. Higher yield stress, “firmness” and “creaminess” values were observed in yoghurt produced with higher inulin additions, whereas the pH value was not affected. A significant correlation was found between yield stress and sensory determined firmness (r = 0.91). The microstructure examined by confocal laser scanning microscopy (CLSM) was only slightly affected by the concentrations of inulin in the range studied, possibly due to weak protein interactions between the inulin and the milk protein network.     

Microstructure and viscoelastic properties of low-fat yoghurt structured by monoglyceride gels

Three monoglyceride gels (MG gels) made up by 5 g of monopalmitin/monostearin blends in ratios of 0.10:0.87, 0.30:0.65 and 0.43:0.50, and 0.05 g of an anionic co-surfactant per 100 mL of water were prepared. Differential Scanning Calorimetry (DSC) results indicated that when cooled below the Krafft temperature (T_krafft) the three MG gels were predominantly in the α-gel phase, and were used under this condition to prepare three low-fat yoghurt variations (Y_0.1MP,0.87MS, Y_0.3MP,0.65MS, and Y_{0.43MP,0.50MS}). Scanning electron micrographs showed that Y_0.1MP,0.87MS and Y_0.3MP,0.65MS structure was characterized by aggregated fused casein micelles, which had attached to their surface MG gels, with comparatively smaller interspaced voids than those showed by the control full-fat yoghurt (Y₁₀₀). Y_0.1MP,0.87MS and Y_0.3MP,0.65MS exhibited higher viscoelastic parameters over the whole frequency experimental sweep than Y₁₀₀, Y_{0.43MP,0.50MS}, and low-fat control yoghurt (Y₅₀).     

Effect of blends of short and long-chain inulin on the rheological and sensory properties of prebiotic low-fat custards

Long-chain inulin (TEX) and short-chain inulin (CLR) combined in different proportions (25:75, 50:50, and 75:25) were added (7.5 g/100 g) to low-fat custards and their rheological and sensory properties were studied. The effect of the inulin blends on the rheological properties depended on the presence of λ-carrageenan (0.01 g/100 g). Slight variations were observed when inulin blends were added to custards without λ-carrageenan, whereas in samples with carrageenan, there was an important increase in thixotropy, consistency and elasticity. In both cases (samples with and without carrageenan), the differences perceived by assessors indicated that the blends 50:50 and 25:75 TEX:CLR were those affording greater thickness, sweetness and vanilla flavour intensity to the custards. Addition of inulin blends to low-fat samples did not manage to fully emulate the rheological behaviour of full-fat custard. However, the low-fat sample with the inulin blend 50:50 TEX:CLR added was perceived to have the same creaminess and thickness as the full-fat sample. The use of this blend combined with carrageenan gave thicker, creamier custard, which was preferred over the full-fat custard.     

Effect of microfluidization of heat-treated milk on rheology and sensory properties of reduced fat yoghurt

The effects of microfluidization at 150 MPa (MFz) and conventional homogenization at 20/5 MPa (CH) of heat-treated milk on the rheology and sensory properties of non- (0.1%) and low- (1.5%) fat stirred yoghurts were compared. Homogenization conditions clearly affected the sensory properties of reduced-fat yoghurts, but the effect was highly dependent on fat content. MFz of heat-treated milk yielded products with very different sensory profiles from the conventional yoghurts. For non-fat yoghurts, MFz of heat-treated milk enhanced the perception of buttermilk and soft cheese flavours, and natural yoghurt aroma and flavour, but also increased the intensity of undesirable mouthfeel characteristics such as chalkiness, mouth-dryness and astringency. For low-fat yoghurts, MFz significantly improved creaminess and desirable texture characteristics such as smoothness, cohesiveness, thickness, and oral and spoon viscosity. These differences in sensory profiles, especially textural properties, were partially related to rheological properties, particularly flow behaviour. MFz of heat-treated milk resulted in non- and low-fat yoghurts with higher yield stress, more pronounced hysteresis effect and higher viscosity than those of CH yoghurts of similar fat contents. These findings suggest that microfluidization may have applications for production of high-quality yoghurt with reduced-fat content.     

Rheological, texture and sensory properties of low-fat mayonnaise with different fat mimetics

Application of whey protein isolate and low-methoxy pectin-based fat mimetics in mayonnaise was studied. Fat was partially substituted by different fat mimetics at levels of 50%, respectively, which the fat mimetics were referred to as PFM1 (microparticulate pectin gel), PFM2 (pectin weak-gel), and PFM3 (microparticulated combination of WPI and pectin). The full fat (Ff) (100% oil) mayonnaise without fat mimetic was used as a control experiment. Physicochemical, rheological, texture analysis, and sensory evaluation of the Ff and low fat (Lf) mayonnaises were performed. The results indicated that all Lf mayonnaises had significantly lower energy content, but higher water content than their Ff counterpart. In terms of texture, the formulation with pectin weak-gel as fat mimetic showed similar texture values as those of the Ff sample. Both Ff and Lf mayonnaises exhibited thixothopic shear thinning behaviour under steady shear tests and were rheologically classified as weak gels under small amplitude oscillatory shear tests. Sensory evaluation demonstrated that mayonnaises substituted with low-methoxy pectin were acceptable. This study shows good potential for pectin weak-gel and microparticulated pectin gel to be used as a fat mimetic in mayonnaise.     

Using fractal image analysis to characterize microstructure of low-fat stirred yoghurt manufactured with microparticulated whey protein

Differences in the microstructure of low fat yoghurt manufactured with microparticulated whey proteins used as fat replacer were investigated. Images were obtained by confocal laser scanning microscopy and studied using a technique for image analysis that combines an initial 2D-wavelet compression followed by fractal analysis and inspection of the fractal curves by principal components analysis (PCA). One commercial and three experimental microparticulated ingredients with different chemical characteristics were used in the yoghurt formulations and compared to both full and low fat yoghurts without fat replacer. The results showed that the amount of native and soluble whey proteins present in the microparticles had a positive influence on the structure of the formed gel. The created structure, dominated by dense aggregates and low amount of serum, had an increased degree of self similarity or fractality with yoghurts in which fat was present.     

Influence of partially demineralized milk proteins on rheological properties and microstructure of acid gels

Innovative clean label processes employed in the manufacture of acid gels are targeted to modify the structure of proteins that contribute to rheological properties. In the present study, CO₂-treated milk protein concentrate powder with 80% protein in dry matter (TMPC80) was mixed with nonfat dry milk (NDM) in different ratios for the manufacture of acid gels. Dispersions of NDM and TMPC80 that provided 100, 90, 70, and 40% of protein from NDM were reconstituted to 4.0% (wt/wt) protein and 12.0% (wt/wt) total solids. Dispersions were adjusted to pH 6.5, followed by heat treatment at 90°C for 10 min. Glucono-δ-lactone was added and samples were incubated at 30°C, reaching pH 4.5 ± 0.05 after 4 h of incubation. Glucono-δ-lactone levels were adjusted to compensate for the lower buffering capacity of samples with higher proportions of TMPC80, which is attributable to the depletion of buffering minerals from both the serum and micellar phase during preparation of TMPC80. Sodium dodecyl sulfate-PAGE analysis indicated a higher amount of caseins in the supernatant of unheated suspensions with increasing proportions of CO₂-treated TMPC80, attributable to the partial disruption of casein micelles in TMPC80. Heat treatment reduced the level of whey proteins in the supernatant due to the heat-induced association of whey proteins with casein micelles, the extent of which was larger in samples containing more micellar casein (i.e., samples with a lower proportion of TMPC80). Particle size analysis showed only small differences between nonheated and heated dispersions. Gelation pH increased from ?5.1 to ?5.3, and the storage modulus of the gels at pH 4.5 increased from ?300 to ?420 Pa when the proportion of protein contributed by TMPC80 increased from 0 to 60%. Water-holding capacity also increased and gel porosity decreased with increasing proportion of protein contributed by TMPC80. The observed gel properties were in line with microstructural observations by confocal microscopy, wherein sample gels containing increasing levels of TMPC80 exhibited smaller, well-connected aggregates with uniform, homogeneous pore sizes. We concluded that TMPC80 can be used to partially replace NDM as a protein source to improve rheological and water-holding properties in acid gels. The resultant gels also exhibited decreased buffering, which can improve the productive capacity of yogurt manufacturing plants. Overall, the process can be leveraged to reduce the amount of hydrocolloids added to improve yogurt consistency and water-holding capacity, thus providing a path to meet consumer expectations of clean label products.     

发酵剂的筛选及其对搅拌型酸奶理化特性的影响

通过调查单菌株发酵特性、球菌/杆菌比例的选择、菌株复配后的性能,得到了4组优化的发酵剂配方。在不添加任何增稠剂的条件下,其发酵的酸奶在28d货架期内质构、后酸化程度表现优异且稳定。此外,为了预测酸奶货架期后酸化程度的强弱,比较了15组28d储存期实验与加速实验中所得的pH值及其变化趋势。结果表明,加速试验中趋于定值时的pH值可以作为判断后酸化程度强弱的先行指标。     

Effects of lactoperoxidase and hydrogen peroxide on rheological properties of yoghurt

The effects of activation of the lactoperoxidase (LPO) system by H2O2-NaSCN and hydrogen peroxide (H2O2) on the accessibility of sulphydryl groups (SH) in skimmed milk, and on the dynamic rheological properties of the resulting yoghurt were investigated. Four different concentrations of each reagent (20-80 mg H2O2-NaSCN/kg milk and 100-400 mg H2O2/kg milk) were compared. Clear negative correlations were noted between the accessibility of SH groups and both LPO activation rate and H2O2 concentration. Also the native PAGE pattern of the heat-treated samples showed that with increase in the H2O2-NaSCN and H2O2 concentrations, the level of interaction between beta-lactoglobulin (beta-lg) and kappa-casein (kappa-CN) decreased. The complex modulus (G*) of skimmed milk yoghurts declined gradually with the decrease in the concentration of accessible SH groups accordingly. Tan delta values of yoghurt samples were found to be different from the control, but close to each other, indicating that protein interaction forces taking place in the formation of gel networks of treated yoghurts were different from the control.     

乳清蛋白-黄油乳液凝胶对低脂酸奶理化特性及品质的影响

乳液凝胶是一种新型的脂肪替代物,乳清蛋白和黄油是乳品中常用的原、辅料,利用乳清蛋白和黄油制作的乳液凝胶在乳制品加工中具有良好的应用前景.制备不同蛋白和脂肪含量的乳清蛋白-黄油乳液凝胶颗粒(whey protein-butter emulsion gel particles,WPI-EG),研究其对低脂酸奶理化特性及感官...     

Physical properties of yoghurt manufactured with milk whey and transglutaminase

The effect of milk protein polymerization prior to the yoghurt fermentation process was evaluated by enzymatic reaction with microbial transglutaminase (Streptoverticillium mobaraense). Yoghurt samples were manufactured with 100% milk or by substituting milk with 20 or 30% of liquid milk whey, aimed at determining the use of natural milk whey in dairy products. Transglutaminase was added at a protein ratio of 0.5 U g⁻¹ to all samples and evaluated regarding rheological behavior, syneresis index and texture profile. The addition of enzyme transglutaminase contributed to syneresis prevention and increased the consistency index in yoghurt samples manufactured with milk whey. Yoghurt manufactured from 70% milk plus 20% milk whey, followed by enzymatic treatment, presented similar characteristics to traditionally manufactured yoghurt (C 100), with no alteration in the syneresis of the samples (p > 0.05) and presented texture parameters similar to the control yoghurt (C 100).     

Effects of different milk substitutes on pasting, rheological and textural properties of puddings

Commercial pudding powders are usually composed of starch, hydrocolloids, sugars, colorings and aromas, and are intended to be dissolved in milk. However, the use of milk substitutes is mandatory for people suffering from lactose intolerance and milk allergies, but these ingredients have an effect on pudding structure and rheological behavior that has been little studied. This work investigates the pasting, rheological and textural behavior of two commercial pudding powders dissolved in rice drink, light soy drink, partially skimmed milk (reference sample), and water (model system). The eight different puddings were evaluated both during preparation (heating and cooling phases) and a 3 day refrigerated storage period, by means of viscoamylographic tests, rotational and oscillatory measurements, and penetration tests. The two pudding powders, apparently similar in formulation, resulted in final products with large structural differences, that could be ascribed to different types and quantity of starch and carrageenans present in the formulation. Soy and rice drinks can not be indifferently used for pudding production, as the optimal texture of the final product is not always achieved. Generally, the properties of soy drink-based puddings were similar to those of the reference samples, while the characteristics of rice drink-based products were comparable to those of the water systems.     

增稠剂对搅拌型酸奶感官及流变特性的影响

针对不同增稠剂添加量对酸奶组织状态、加工情况、感官特性的影响,采用brookfield流变仪对破乳后酸奶进行了测定,分析增稠剂对黏度、触变性的影响。并结合感官评价对变性淀粉、琼脂、结冷胶、海藻酸钠进行了正交实验,经过实验筛选出最优配方案为:变性淀粉1.2%,琼脂0.08%,结冷胶0.06%,海藻酸钠0.2%,在该工艺条件下酸奶稳定性、口感达到最优状态。并对不同淀粉添加量的淀粉糊化效果进行观察,实验发现当淀粉添加量为1%~1.2%时,糊化效果最好。     

Transglutaminase cross-linking of milk proteins and impact on yoghurt gel properties

A novel yoghurt process was investigated in which milk proteins were covalently cross-linked by a microbial transglutaminase (TG) preparation containing glutathione (TG+GSH). As unheated milk is normally less reactive towards TG, TG+GSH was applied to enable non-inhibited cross-linking without requiring a pre-heat treatment beyond pasteurisation conditions. After the TG+GSH incubation phase, the enzyme was inactivated by heat treatment of the yoghurt milk prior to fermentation. During yoghurt fermentation, no negative effect of TG+GSH on fermentation time was found. Protein cross-linking by TG+GSH was enhanced, resulting in higher apparent viscosity and a higher degree of protein polymerisation than that given by TG without GSH. Furthermore, different ratios of casein to whey proteins (CWP ratios) were used to investigate the effect of both protein fractions on covalently cross-linked yoghurt gel structures. The results showed a relationship between extent of cross-linking, apparent viscosity and CWP ratio of the yoghurt gels. During storage for up to 6 weeks at 4 °C, no changes in rheological properties and degree of protein polymerisation were measurable for stirred yoghurt gels prepared from cross-linked milk proteins.     

Cell surface properties affect colonisation of raw milk by lactic acid bacteria at the microstructure level

Cell localisation in the matrix microstructure of fermented food is important for microbial activity and ripening. To evaluate the importance of physicochemical factors in these phenomena, the physicochemical behaviour of four strains of lactic acid bacteria were investigated after inoculation of raw milk or milk fractions. To discriminate between physicochemical and metabolic properties, media were (i) rapidly acidified to avoid cell division or (ii) bacteria were allowed to ferment milk. To evaluate the impact of milk components, cells were dispersed in protein or fatty globule fractions. From two Lactococcus lactis subsp. lactis biovar. diacetylactis strains selected from their different surface properties investigated in previous studies, after rapid acidification one was encountered in fatty globules and protein gels whereas the other was found in serum and on the surface of globules. After fermentation, the first one formed colonies in the lipid phase or next to it in the whey and the second formed colonies in the protein gel. Strains forming long chains (Lactobacillus bulgaricus and Streptococcus thermophilus) were always found in contact with fatty globules, where they developed. Our results suggest that localisation and subsequent spatial colonisation depend on the physicochemical properties of cell surfaces, on the structure in chains as well as on the metabolic ability to develop on proteins or lipids.     

Impact of oxidoreduction potential and of gas bubbling on rheological properties of non-fat yoghurt

The aim of this work was to study the effect of different gaseous conditions on the physico-chemical properties of yoghurt. Four conditions of oxidoreduction potential (Eh₇), +433 mV (milk gassed with air), +405 mV (ungassed milk), +283 mV (milk gassed with N2) and ?349 mV (milk gassed with N₂–H₂) were applied to milk. The rheological properties and microstructure of these yoghurts were determined by measuring apparent viscosity, whey separation (WS) and using confocal laser scanning microscopy. Exopolysaccharide (EPS) production by lactic acid bacteria (LAB) was also studied, and production was increased for extreme Eh₇ values: the highest values were obtained for air (oxidizing condition) and for N₂–H₂ (reducing condition). Yoghurts made under bubbled N₂–H₂ clearly led to low WS with higher gel aggregation and a lower proportion of pores.