Gelation of Mixed Gels Containing κ-Carrageenan and Skim Milk Components

ABSTRACT: The gelation characteristics of mixed gels containing κ-carrageenan and skim milk or milk fractions (skim milk permeate or retentate) obtained by ultrafiltration were examined. Increasing the skim milk solids content of mixtures containing carrageenan increased setting temperatures and gel strength. The milk proteins contributed to gel strength but did not influence the setting temperature of mixtures. The binding of denatured whey proteins to casein micelles affected gel network formation of milk-carrageenan mixtures containing 10% milk solids. Network formation in mixed gels containing carrageenan and milk or milk fractions was initiated by the carrageenan component and dictated primarily by the ionic content of the mixtures.     

Proteolysis,protein distribution and stability of UHT milk during storage at room temperature

Proteolytic degradation and distribution of caseins and whey proteins between the soluble and colloidal phases were studied in six batches of commercial UHT milk (three skim and three whole milks) during storage at 25 ± 2 °C. For that purpose, at 30 day intervals, milk samples were ultracentrifuged and the pellets and supernatants analysed by capillary electrophoresis and SDS‐PAGE. Samples were also visually examined for signs of gelation. Extensive proteolytic degradation of the micellar fractions and severe changes in the electrophoretic pattern of the proteins present in the serum fractions were observed in all the batches. A higher proportion of denatured whey proteins not attached to the micelle surface was found in the skim milk samples as compared with the whole milk samples that could provide less resistance against gelation. In addition to β‐Lg, para‐κ‐casein was also found in the serum fraction. A high proteolytic activity against κ‐casein could be responsible for the hydrolysis of serum‐liberated κ‐casein or could have enhanced the liberation of β‐Lg–para‐κ‐casein complexes through proteolysis of micellar κ‐casein. © 1999 Society of Chemical Industry     

Microstructure of acid–induced skim milk–locust bean gum–xanthan gels

The microstructure of acid skim milk gels (14% w/w milk protein low heat powder) with or without addition of locust bean gum (LBG), xanthan gum (XG) and LBG/XG blends was determined by transmission electron microscopy (TEM), phase-contrast light microscopy (PCLM) and scanning electron microscopy (SEM). Three polysaccharide concentrations (0.001%, 0.02% and 0.1%, w/w) were used for binary mixtures. In the case of ternary mixtures, three LBG/XG weight ratios were used (4/16, 11/9 and 16/4) at 0.02% total polysaccharide concentration. Control acid skim milk gels were structured by a homogeneous network of casein particles (0.1–0.7 μm in diameter) and clusters immobilizing whey in small pores (1–5 μm in diameter). Filamentous structures and small aggregates were observed at the surface of casein particles. Low concentration of LBG or XG (0.001% w/w) did not affect markedly the microstructure of acid skim milk gels. Conversely, LBG or XG at 0.02 or 0.1% concentration and LBG/XG blends at the three ratios selected had a great influence on the gel microstructure. Although the size and surface structure of the casein particles were not modified by the presence of polysaccharides, the primary casein network appeared very compact with a decrease of pore size and a large increase in the porosity of the network at the supramolecular level (sponge-like morphology). The effect is stronger for gels containing LBG and XG used at higher concentration and less apparent for gels containing LBG/XG blends. Skim milk/XG gels were highly organized into fibrous structures whereas skim milk/LBG gels were more heterogeneous. These structures were discussed in the light of volume-exclusion effects (demixing) and specific interactions between casein micelles and polysaccharides. At the three weight ratios, skim milk/LBG/XG gels displayed both jagged “coral-like”, “veil-like” and filamentous structures. These structures could originate from a secondary network constituted by the known LBG/XG synergistic interactions.     

Bifidobacterial growth–promoting effect of yak milk κ‐casein hydrolysates produced with different proteases

The growth‐promoting effects of κ‐casein hydrolysates produced by four different proteases (trypsin, alcalase, papain and chymosin) on bifidobacteria (Bb12 and BBMN68) viability were evaluated. Results showed that the obtained κ‐casein hydrolysates possessed better property for improving the viability of Bb12 and BBMN68 than native κ‐casein. The bifidobacterium viability and pH drop were dependent on the added hydrolysates. The addition of κ‐casein hydrolysates improved the viability of Bb12 and BBMN68 to a variable extent. The κ‐casein hydrolysate produced with papain (KCHP) is the best for Bb12 and κ‐casein hydrolysate produced with trypsin (KCHT) is the best for BBMN68. Proper amino acid profiles improved the viability of Bb12 and BBMN68. The bifidobacterial growth‐promoting capacity of κ‐casein hydrolysate may be due to its high content of Ala, Met, Tyr, Phe and Arg residues, especially Tyr which is richer in para‐κ‐casein than in casein glycomacropeptide (GMP). In addition, the sialic acid content of κ‐casein hydrolysate produced with the tested proteases did not show a direct relationship with its bifidobacterial growth–promoting effect. The obtained peptides possessed growth‐promoting activity for Bb12 and BBMN68 existed in para‐κ‐casein.     

Improvement of emulsifying properties of milk proteins with κ or λ carrageenan: effect of pH and ionic strength

The objective of this work was to determine the effect of λ‐carrageenan or κ‐carrageenan on the emulsion capacity, emulsion work and emulsion stability of milk proteins concentrate (MP) or sodium caseinate (SC) emulsions at different levels of pH and ionic strength. Incorporation of carrageenans to proteins emulsions resulted in an improvement of emulsifying properties at pH 6.0 and low ionic strength (0.2 m NaCl). Although emulsion capacity was high in MP than for SC, irrespectively of carrageenan employed, addition of λ‐carrageenan increased twofold emulsion work values (15 327 Ω s^?1 for MP and 11 455 Ω s^?1 for SC; around 6000 Ω s^?1 in the other treatments). Emulsion stability was high with λ‐carrageenan (9.8 s) than MP‐κ‐carrageenan or MP (7.45 and 7.40 s, respectively). Carrageenan improving of emulsion properties was because of the complex formation with MP, characteristic of this type of food system when pH was above of isoelectric point.     

Effects of beta-glucan addition to a probiotic containing yogurt

ABSTRACT:  This study investigated the effects of addition of β-glucan from 2 different cereal sources (oat and barley) on growth and metabolic activity of Bifidobacterium animalis ssp. lactis (Bb-12™) as determined by plating on a selective medium in yogurt during prolonged cold storage. These yogurt batches were compared to unsupplemented and inulin supplemented controls. All batches were also assessed for syneresis. Oat β-glucan addition resulted in improved probiotic viability and stability comparable to that of inulin. It also enhanced lactic and propionic acid production. The barley β-glucan addition suppressed proteolytic activity more than that from oat. These improvements were hindered by greater syneresis caused likely by thermodynamic incompatibility. Small amplitude oscillatory measurements of acidified model mixture of β-glucan/skim milk solids showed formation of casein gel within the β-glucan network. Binary mixtures of β-glucan and skim milk solids had apparent pseudoplastic and non-Newtonian behavior governed mainly by β-glucan contribution. Above critical concentrations, the mixtures underwent phase separation with the lower phase rich in protein. The phase diagram also showed that the addition of β-glucan may be possible at or below 0.24 w/w%.     

Acid Milk Gel Formation as Affected by Total Solids Content

Reconstituted skim milk with varying concentrations of total solids was coagulated using glucono-δ-lactone (GDL). Microscopic, turbidimetric and rheological procedures were used to examine mineral solubilization, buffering capacity, casein dissociation and micellar solvation during gelation. Total solids of the milk affected pH of the onset of gelation attributable to differences in colloïdal calcium phosphate in the casein particles during acidification. Firmness and elasticity of the resulting gel increased with total solids from a more direct contribution of dry matter during the last stage of acid milk gel formation.     

乳固形物浓度对浓缩型酸乳酪流变性质和感官品质的影响

研究了乳固形物浓度对浓缩型酸乳酪(Labneh)品质的影响，对采用传统工艺制备的新鲜Labneh样品分别进行流变分析和感官评定。流变分析表明，浓缩型酸乳酪具有较强的弹性结构；随着乳固形物浓度的增加，浓缩型酸乳酪的硬度和粘性都会明显下降；扫描电镜观察微观结构表明，其结构致密而均匀，而且酪蛋白颗粒的尺寸比对照组小；感官评定表明，当乳固形物浓度提高到接近质量分数15．50％时浓缩型酸乳酪细腻光滑、可涂抹性较理想。     

MILK GEL STRUCTURE. XI. ELECTRON MICROSCOPY OF GLUCONO-δ-LACTONE-INDUCED SKIM MILK GELS1

Gels were prepared from fresh and reconstituted skim milk (10–30% total solids) using glucono-δ-lactone as the acid precursor within the pH range of 4 to 6. Firmness of glucono-δ-lactone-induced skim milk gels increased with (a) decrease in pH of skim milk having the same total solids content, (b) increase in total solids at a given pH, (c) pre-heating skim milk to 90°C, and (d) adding glucono-δ-lactone at a higher temperature. Electron microscopy revealed that in condition (a) larger sizes of casein micelles (particles) were associated with a higher firmness whereas in conditions (b), (c), and (d) the increased firmness was correlated with decreased micelle sizes. The relation between rheological behavior and microstructure of glucono-δ-lactone-induced skim milk gels was more dependent upon the micellar arrangement than upon the size and shape of the casein micelles. Conditions which led to large casein micelle clusters and chains in conditions (b), (c), and (d) resulted both in severe syneresis and in weaker gels. An unusual phenomenon was observed in skim milk gels made at pH 5.5, particularly at the lower total solids content (10%): the casein particles consisted of a solid core surrounded by an outer lining, 0.03–0.05 μm thick, which resembled a membrane-line structure; there was a free annular space of 0.05 to 0.08 μm between the lining and the core. The existence of this structure was confirmed by thin-sectioning and freeze-fracturing techniques.     

Influence of pH and NaCl on rheological properties of rennet-induced casein gels made from UF concentrated skim milk

Milk concentrates are used in the production of cast cheese. The effects of pH (5.19–6.21) and NaCl concentration (0, 1.75% and 3.50%, w/w) on the rheological and microstructural properties of rennet-induced casein gels made from ultra filtered skim milk (19.8%, w/w casein) were investigated. Low pH and high NaCl concentration reduced the development rate of the gel elasticity after coagulation of the ultra filtrated skim milk. Strain at fracture and stress at fracture from uniaxial compression of casein gels 48 h after coagulation showed maximum and minimum values at pH ∼5.8 and 5.29, respectively. Young's modulus from uniaxial compression of the same gels was almost constant between pH 5.52 and 6.21 but much lower at pH 5.28. Addition of NaCl resulted in increased Young's modulus in the interval pH 6.21–5.52. As pH decreased, the level of colloidal calcium phosphate decreased concomitantly, giving less cross-links in the casein network and partly explaining the changes in the rheological properties. Increased ionic strength by adjusting pH and addition of NaCl also influenced rheological results. The microstructure examined with confocal laser-scanning microscopy was unaffected by the changes in pH and concentrations of NaCl in the range studied as revealed by image analysis and calculations of two- and three-dimensional data from micrographs.     

Rheological properties and microstructure during rennet induced coagulation of UF concentrated skim milk

Rennet induced coagulation of ultrafiltrated (UF) skim milk (19.8%, w/w casein) at pH 5.8 was studied and compared with coagulation of unconcentrated skim milk of the same pH. At the same rennet concentration (0.010 International Milk Clotting Units g⁻¹), coagulation occurred at a slower rate in UF skim milk but started at a lower degree of κ-casein hydrolysis compared with the unconcentrated skim milk. Confocal laser scanning micrographs revealed that large aggregates developed in the unconcentrated skim milk during renneting. Following extensive microsyneresis the protein strands were shorter and thinner in gels from UF skim milk. Moreover, during storage up to 60 days (13 °C), the microstructure and the size of the protein strands of the UF gel changed only slightly. Hoelter–Foltmann plots suggested that the coagulation rate was reduced in the UF skim milk due to a high zero shear viscosity of the concentrate compared with the unconcentrated skim milk.     

Heat‐induced gels of soy protein and κ‐carrageenan at different pH values

Heat‐induced (90 °C/30 min) gelling of soy protein isolate (SPI) and κ‐carrageenan (κ‐CR) systems at pH values of 6.7 and 5.7 was evaluated. κ‐CR addition, increase in protein concentration and reduction in pH led to decreases in the initial gel structure forming temperature. Self‐supporting gels were not formed at concentrations of 8% (w/w) SPI or at concentrations below 0.3% (w/w) κ‐CR, but an increase in the concentration of SPI and κ‐CR led to an increase in the stress at rupture without influencing the deformability. Gel properties were a consequence of a simultaneous process of gelling and phase separation during heating. The non‐linear parameter of the Blatz, Sharda and Tschoegl (BST) rheological model allowed for the evaluation of the structural characteristics that in general corresponded to strain hardening behaviour. Strain weakening behaviour was observed at high biopolymer concentrations and at pH 6.7, which was associated with accentuated phase separation and a more discontinuous gel network.     

Amylose–Potassium Oleate Inclusion Complex in Plain Set‐Style Yogurt

Health and wellness aspirations of U.S. consumers continue to drive the demand for lower fat from inherently beneficial foods such as yogurt. Removing fat from yogurt negatively affects the gel strength, texture, syneresis, and storage of yogurt. Amylose–potassium oleate inclusion complexes (AIC) were used to replace skim milk solids to improve the quality of nonfat yogurt. The effect of AIC on fermentation of yogurt mix and strength of yogurt gel was studied and compared to full‐fat samples. Texture, storage modulus, and syneresis of yogurt were observed over 4 weeks of storage at 4 °C. Yogurt mixes having the skim milk solids partially replaced by AIC fermented at a similar rate as yogurt samples with no milk solids replaced and full‐fat milk. Initial viscosity was higher for yogurt mixes with AIC. The presence of 3% AIC strengthened the yogurt gel as indicated by texture and rheology measurements. Yogurt samples with 3% AIC maintained the gel strength during storage and resulted in low syneresis after storage for 4 wk.     

Diffusing wave spectroscopy and rheological studies of rennet-induced gelation of skim milk in the presence of pectin and κ-carrageenan

The effect of the presence of κ-carrageenan on casein micelle stability and their rennet-induced aggregation was studied using diffusing wave spectroscopy and rheology measurements. Different concentrations of κ-carrageenan, below the gelation regime, were added to skim milk at pH 6.7. No changes in casein stability and on the aggregation kinetics of casein micelles were observed at κ-carrageenan concentrations <0.015% (w/v). The effect of mixing different concentrations of high methoxyl pectin (HMP) (0.04, 0.12 and 0.18%, w/v) with 0.015% (w/v) κ-carrageenan in skim milk was then studied. It was observed that κ-carrageenan protects the system from HMP-induced destabilization. The study of the synergistic effect of κ-carrageenan and HMP on the structuring of casein micelles in milk opens new avenues for the development and control of processes where different mechanisms of aggregation and destabilization occur simultaneously.     

Selenium content of goat milk and its distribution in protein fractions.

This study reports on selenium distribution in goat milk. Skim milk was found to contain the major part (94%) of total milk selenium. The selenium distribution over casein and whey protein fractions depends on the separation method used, but irrespective of these methods, skim milk selenium is mainly associated with the casein fraction (greater than 69%). Approximately 9%, 7% and 24% of selenium is removed by dialysis (molecular cutoff 10-12 kDa) from skim milk, casein and whey respectively, indicating a major association of selenium with milk proteins. This observation is confirmed by selenium analysis of individual caseins and whey proteins isolated through ion-exchange chromatography and gel filtration. Selenium concentrations of the different isolated milk proteins show considerable variation (caseins: 294-550 ng Se/g; whey proteins: 217-457 ng Se/g).     

RHEOLOGICAL CHARACTERIZATION OF COMMERCIALLY PROCESSED FLUID MILKS

Flow profiles of commercially processed whole, two-percent, one-percent and nonfat milks with added solids as well as skim milk without added solids were measured using a rotational steady shear viscometer. Also tested were mixtures of concentrated skim milk added to skim milk over a range of 9.7% to 20.2% total solids. A shear rate range of 121 to 486 s^-1 was used with a cone and plate geometry. In all cases, linear plots of shear stress versus shear rate with small nonzero intercepts were obtained.     

Sol gel transitions during acid gelation of milk containing modified waxy maize starch. Differences between chemical and bacterial acidification measured using rheological and spectroscopic techniques

Modified waxy maize starch (1%, w/v) was added to skim milk and the mixtures were heated and homogenized. Acidification was conducted at 40 °C, using either glucono-δ-lactone (GDL) or a commercial starter culture. The physico-chemical changes occurring during acidification were monitored using small oscillatory rheology, diffusing wave spectroscopy and ultrasound spectroscopy. A similar gelling behaviour was noted for GDL and bacterial-induced gels; however, a difference was noted in the values of storage modulus (G′). The presence of starch did not seem to affect the development of the gel structure, nor the mobility and positional correlations of the casein micelles during acidification. On the other hand, starch increased the final storage modulus, G′ of the acid milk gels. These results indicate the absence of direct interactions between micelles and the modified starch granules.     

Characterisation of the casein fraction of Ibérico cheese by electrophoretic techniques

The physicochemical characteristics of Ibérico cheese, a semi‐hard Spanish variety manufactured from mixtures of cow's, ewe's and goat's milk, were studied. The casein fraction and breakdown products of 6‐month‐old cheeses were characterised by various electrophoretic techniques: urea polyacrylamide gel electrophoresis (urea‐PAGE) at alkaline pH, isoelectric focusing (IEF) and two‐dimensional polyacrylamide gel electrophoresis (2DE). Proteins were separated in 2DE according to their charge/mass ratio by urea‐PAGE at alkaline pH in the first dimension and according to their isoelectric point by IEF in the second dimension. Some individual bands considered homogeneous by urea‐PAGE at alkaline pH (ie different grades of phosphorylation of α_s1‐casein and α_s2‐casein) or by IEF (ie overlapping of several bands of α_s2‐casein with γ‐casein bands) were found to be complex mixtures of casein components by 2DE. The two‐dimensional electrophoretic pattern was characteristic of the milk of each animal species included in Ibérico cheese. Capillary electrophoresis (CE) was also used to study the Ibérico cheeses. The high resolution of this technique allowed the identification of the main caseins of the different species (ie para‐κ‐casein, β‐casein, γ₂‐casein and γ₃‐casein). © 2002 Society of Chemical Industry     

Effect of manufacture and reconstitution of milk protein concentrate powder on the size and rennet gelation behaviour of casein micelles

Samples of raw skim milk, ultrafiltration/diafiltration retentate, concentrated retentate and milk protein concentrate powder (MPC80) from a single commercial production run were analysed using photon correlation spectroscopy. Measurements revealed insignificant differences in casein micelle size between the samples. In addition, there was no discernable difference between raw skim milk and MPC powder dissolved at 60 °C in the amount of casein remaining in supernatants from centrifugation at either 25,000 × g or 174,200 × g. Casein micelles did not appear to be altered during manufacture of MPC. The rennet gelation behaviour of reconstituted MPC was compared with raw skim milk. Reconstituted MPC did not coagulate unless supplemented with approximately 2 mm calcium chloride, which was attributed to the mineral removal during ultrafiltration/diafiltration. Addition of sufficient calcium could restore rennet coagulation kinetics and gel strength of reconstituted MPC to approximately that of raw skim milk.     

Evaluation of Alternative Methods to Increase Calcium Retention in Cottage Cheese Curd

The effect of several alternative methods including addition of rennet, addition of carrageenan and use of 2:1 (v/v) preconcentrated skim milk by ultrafiltration (UF) upon calcium retention, yield, composition and sensory properties of dry curd cottage cheese was investigated. Although each of the processing methods resulted in the manufacture of dry curd cottage cheese with different compositions and properties, none of them was satisfactory for increasing calcium retention. Added carrageenan bound additional whey proteins, added rennet interfered with curd syneresis and whey expulsion during cooking and use of UF preconcentrated skim milk resulted in an increase in yield, total solids and protein of the curd.