Low Amplitude Oscillatory Rheology Research Articles

Influence of heat treatment of goat milk on casein micelle size, rheological and textural properties of acid gels and set type yoghurts.

Acid gels and yoghurts were made from goat milk that was heated at 72°C/30 s, 85°C/5 min, and 95°C/5 min, followed by acidification with starter culture at 43C until pH 4.6. The rheological and textural properties of acid gels and yoghurts were analyzed using dynamic low amplitude oscillatory rheology and back extrusion texture analysis, respectively. The effect of goat milk heat treatment on the mean casein micelle diameter and protein profile was also determined by dynamic light scattering and SDS PAGE electrophoresis, respectively. The shortest gelation and fermentation time was recorded for yoghurt prepared from milk heated at 85°C/5 min. Also, the pH of gelation, the storage moduli (G') and yield stress were higher for this yoghurt, compared with the other two. Textural properties of goat milk yoghurts such as firmness and consistency were strongly affected by milk heat treatment, and the highest values were recorded for yoghurt produced from milk preheated at 85°C/5 min, as well. The largest casein micelles were measured after 85°C/5 min treatment and their size decreased at higher temperature, despite higher denaturation of whey proteins at the most intense heat regime, indicating the structure changes that influence on the acid gelation.

Gelation of milks of different species (dairy cattle, goat, sheep, red deer, and water buffalo) using glucono-δ-lactone and pepsin

Dynamic low-amplitude oscillatory rheology was used to study the gelation properties of skim milk gels made at 37°C, using glucono-δ-lactone alone (acid gels) or a combination of glucono-δ-lactone and porcine pepsin ("combination gels"). The protein contents of the skim milks increased in the order goat milk < cattle milk < buffalo milk < sheep milk < deer milk, whereas the average casein micelle diameters increased in the order cattle milk < buffalo milk < goat milk < sheep milk ≃ deer milk. The gelation pH (4.55-4.73) of all milks were close to the isoelectric pH (4.6) of casein, except for buffalo milk, which had a significantly higher gelation pH (5.72). The storage moduli (G') of the acid gels increased with time in the milks of all species except for buffalo milk, for which a double peak in G' was observed. The final storage moduli after 6 h (G'final) increased in the order goat milk < cattle milk < sheep milk < deer milk < buffalo milk. In general, for the combination gels, the G'final values and the gelation pH increased to variable extents, except for goat milk. Confocal scanning laser microscopy showed that goat milk and cattle milk formed gels with more open protein networks compared with the dense clustered protein networks of the milks with high protein content (buffalo, sheep, and deer milks). This study indicates that milks from different species respond differently under the action of an acid precursor and pepsin. These results can be used to provide a better understanding of curd making and the digestion properties of noncattle milks.

Investigating the properties of high-pressure-treated, reduced-sodium, low-moisture, part-skim Mozzarella cheese during refrigerated storage

We proposed that the performance and sensory properties of reduced-Na, low-moisture, part-skim (LMPS) Mozzarella cheese could be extended by the application of high hydrostatic pressure (HHP) to cheese postmanufacture and thereby decrease microbial and enzymatic activity. Fermentation-produced camel chymosin was also used as a coagulant to help reduce proteolysis during storage. Average composition of the LMPS Mozzarella cheeses was 48.6 ± 0.6% moisture, 22.5 ± 0.4% fat, 24.5 ± 0.6% protein, and 1.0 ± 0.1% NaCl. Blocks of cheeses were divided into 3 groups randomly after manufacture and stored at approximately 4°C for 20 wk. The control group was not HHP treated. Two weeks after manufacture, 2 groups of cheese samples were treated with HHP at 500 or 600 MPa for 3 min and then returned to storage at approximately 4°C. Analysis was performed during 20 wk of storage after cheese manufacture. Texture profile analysis (TPA) and dynamic low-amplitude oscillatory rheology were used to monitor cheese functionality. Quantitative descriptive analysis was conducted with 9 trained panelists using a 15-point scale to evaluate texture and flavor attributes of unmelted cheese as well as cheeses melted on pizzas. Pressure treatments at 500 and 600 MPa resulted in approximately 1 and 2 log reduction in the numbers of starter culture, respectively, compared with the control when measured 1 d after HHP treatment. Starter numbers continued to decrease in all cheeses over the 20 wk of storage, but the decrease was larger in the HHP-treated cheeses. Even though the initial numbers of nonstarter lactic acid bacteria were the same in all cheeses, the numbers of these bacteria increased faster in the control cheeses. High-pressure treatment of LMPS Mozzarella cheese resulted in an initial (1 d after HHP treatment) increase in pH, but by 2 wk after HHP treatment there was no statistical difference in pH values between control and HHP-treated samples. Immediately after treatment, HHP-treated cheeses exhibited significantly lower TPA and sensory (unmelted) hardness. However, by 14 wk after pressure treatment, the 600-MPa HHP-treated cheese had significantly higher TPA compared with control or 500-MPa HHP-treated cheeses. Sensory panels also indicated that by 14 wk after HHP treatment, the 600-MPa treated samples were significantly firmer than the control or 500-MPa treated cheeses. Compared with control cheese, cheeses treated at 600 or 500 MPa exhibited lower water-soluble nitrogen values at 6 and 10 wk after pressure treatment, respectively. By 10 wk after pressure treatment, the levels of intact αS1-casein were significantly higher in all HHP-treated cheeses compared with the control. Pizza sensory panels indicated that 600-MPa treated cheese was significantly chewier and exhibited lower blister quantity and higher strand thickness compared with control cheeses. High hydrostatic pressure treatment of low-Na, LMPS Mozzarella cheese could result in the extension of its desired baking characteristics when the cheese is stored at refrigerated temperature.

ComDim for explorative multi-block data analysis of Cantal-type cheeses: Effects of salts, gentle heating and ripening

Common Dimension (ComDim) chemometrics method for multi-block data analysis was employed to evaluate the impact of different added salts and ripening times on physicochemical, color, dynamic low amplitude oscillatory rheology, texture profile, and molecular structure (fluorescence and MIR spectroscopies) of five Cantal-type cheeses. Firstly, Independent Components Analysis (ICA) was applied separately on fluorescence and MIR spectra in order to extract the relevant signal source and the associated proportions related to molecular structure characteristics. ComDim was then applied on the 31 data tables corresponding to the proportion of ICA signals obtained for spectral methods and the global analysis of cheeses by the other techniques. The ComDim results indicated that generally cheeses made with 50% NaCl or with 75:25% NaCl/KCl exhibit the equivalent characteristics in structural, textural, meltability and color properties. The proposed methodology demonstrates the applicability of ComDim for the characterization of samples when different techniques describe the same samples.

Rheological and physical properties of camel and cow milk gels enriched with phosphate and calcium during acid-induced gelation.

The rheological properties of acid-induced coagulation of camel and cow milk gels following the addition of calcium chloride (CaCl2) and hydrogen phosphate dehydrate (Na2HPO4*2H2O) were investigated using a dynamic low amplitude oscillatory rheology. For a considered condition, the final values of storage modulus (G') and loss modulus (G″) of camel milk gels were significantly lower than those of cow milk gels. The increase of the added CaCl2 levels improved significantly the gelation properties of camel and cow milk gels, since a reduction in the gelation time and an increase in the gel firmness were observed. Following the addition of Na2HPO4*2H2O at 10 and 20mM, no significant effect on the gelation rate and the firmness of camel milk gels was observed, while, a significant decrease in the gelation rate and firmness were observed for cow milk gels.

Rheological and textural properties of goat and cow milk set type yoghurts

Yoghurts were made from pasteurised and homogenised goat milk (2.5% protein), goat milk fortified with a milk protein isolate (5% protein) and cow milk (3% protein), by acidification with a starter culture at 43 °C until a pH of 4.6 was reached. The rheological and textural properties of gels and yoghurts were analysed using dynamic low amplitude oscillatory rheology and back extrusion texture analysis. Gelation and fermentation times of goat milk were longer, while gelation pH, storage moduli (G′) and yield stress values were lower, compared with those of cow milk. Textural properties of goat milk yoghurts such as firmness, consistency, cohesiveness and viscosity index were very poor. Consequently, the products could not be classified as set type yoghurts. Fortification of goat milk with a milk protein isolate contributed to a significant improvement of the rheological and textural properties of yoghurt.

Low-sodium Cheddar cheese: Effect of fortification of cheese milk with ultrafiltration retentate and high-hydrostatic pressure treatment of cheese

Low-sodium cheeses often exhibit an acidic flavor due to excessive acid production during the manufacturing and the initial stage of ripening, which is caused by ongoing starter culture activity facilitated by the low salt-in-moisture levels. We proposed that this excessive starter-induced acidity could be prevented by the fortification of cheese milk with ultrafiltration (UF) retentates (to increase curd buffering), and by decreasing microbial activity using the application of high-hydrostatic pressure (HHP) treatment (that is, to reduce residual starter numbers). Camel chymosin was also used as a coagulant to help reduce bitterness development (a common defect in low-sodium cheeses). Three types of low-Na (0.8% NaCl) Cheddar cheeses were manufactured: non-UF fortified, no HHP applied (L-Na); UF-fortified (cheese milk total solids=17.2±0.6%), no HHP applied (L-Na-UF); and UF-fortified, HHP-treated (L-Na-UF-HHP; 500 MPa for 3 min applied at 1 d post-cheese manufacture). Regular salt (2% NaCl) non-UF fortified, non-HHP treated (R-Na) cheese was also manufactured for comparison purposes. Analysis was performed at 4 d, 2 wk, and 1, 3, and 6 mo after cheese manufacture. Cheese functionality during ripening was assessed using texture profile analysis and dynamic low-amplitude oscillatory rheology. Sensory Spectrum and quantitative descriptive analysis was conducted with 9 trained panelists to evaluate texture and flavor attributes using a 15-point scale. At 4 d and 2 wk of ripening, L-Na-UF-HHP cheese had ~2 and ~4.5 log lower starter culture numbers, respectively, than all other cheeses. Retentate fortification of cheese milk and HHP treatment resulted in low-Na cheeses having similar insoluble calcium concentrations and pH values compared with R-Na cheese during ripening. The L-Na-UF cheese exhibited significantly higher hardness values (measured by texture profile analysis) compared with L-Na cheese until 1 mo of ripening; however, after 1 mo, all low-Na cheeses exhibited similar hardness values, which were significantly lower than R-Na cheese. Pressure treatment significantly increased maximum loss tangent (meltability) from rheology testing and decreased melt temperature. Sensory results indicated only very slight bitterness (<2.5 out of 15-point scale) was detected in all cheeses during the 6 mo of ripening. The L-Na-UF-HHP cheese did not significantly differ in bitterness and acidity from R-Na cheese during ripening. Pressures treatment of cheese at 500 MPa and cheese milk retentate fortification could be used to improve the quality of low-Na cheese.

Rheological and microstructural properties of rennet-induced milk protein gels formed with Orchis anatolica gum

The effects of Orchis anatolica gum (OaG) on the structural and rheological properties of rennet-induced milk protein gels were investigated. OaG, at 0–0.15%, was added to reconstituted skim milk solutions of identical pH (6.60) and protein content (3.5%). Analyses included dynamic low amplitude oscillatory rheology, forced syneresis (FS) and confocal laser scanning microscopy (CLSM). Addition of 0.10% OaG to reconstituted skim milk solutions decreased the gelation time from 20.2 to 13.4 min, increased the gel firmness from 12.8 to 22.2 Pa and increased FS from 24.4 to 40.7% at 60 min. OaG affected formation of milk protein gel structure by increasing viscosity of the aqueous phase and inducing phase separation, with the balance of these effects dependant on the concentration of OaG. These insights on thermodynamic compatibility between OaG and milk proteins could provide unique functional properties to milk protein gels and facilitate innovation in cheese product development.

Rheological characterization of gel-in-oil-in-gel type structured emulsions

We report the fabrication of multiple emulsions where both the enclosed and the external water phases are structured using a combination of two non-gelling biopolymers. Emulsions (with gelled inner water droplets and gelled water continuous phase) were created using a simple ‘one-step’ process where the oil phase (triglyceride oil and polyglycerol polyricinoleate) and the water phase (containing a combination of locust bean gum and carrageenan) were emulsified at an elevated temperature (70 °C) followed by cooling to room temperature. The temperature triggered gelling of the synergistic biopolymer combination led to the formation of structured emulsions on cooling. Flowable to self-standing emulsion gels could be prepared by changing the total concentration of polymers (and the ratios of the individual polymers) as confirmed from low amplitude oscillatory shear rheology and creep recovery measurements. The cryo-scanning electron microscopy images of freeze-fractured emulsion samples revealed the presence of gelled inner water droplets. Further, when subjected to heating and cooling cycles, emulsions displayed reversible rheological changes which could be tuned by simply changing the total polymer concentration and the proportions of individual polymers. Such biopolymer-based structured emulsions with interesting microstructure and rheological properties could find potential applications in bio-related fields like food structuring.

Biochemical and rheological characterization of a protease from fruits of Withania coagulans with a milk-clotting activity

A protease from Withania coagulans fruits was evaluated for milk-clotting properties. The optimum temperature and pH for enzyme activity were 70°C and 4, respectively. The protease also exhibited an excellent thermal stability at 60°C for 30 min. Fractional precipitation using ammonium sulfate showed that the 40–50% fraction (F5) possessed the highest milk-clotting activity. The F5 fraction showed a Mw band of 66 kDa by SDS-PAGE. Gel formation was monitored using low amplitude oscillatory rheology at different temperatures. An Arrhenius plot was used to describe the temperature dependence of the gelation rate parameter. Time sweep testing showed that an increase in temperature was accompanied by a decrease in the gelation onset time, a higher gel formation rate, and higher values for final gel strength. W.coagulans fruits have potential for use as a rennet substitute.

Effect of camel chymosin on the texture, functionality, and sensory properties of low-moisture, part-skim Mozzarella cheese

The objective of this study was to compare the effect of coagulant (bovine calf chymosin, BCC, or camel chymosin, CC), on the functional and sensory properties and performance shelf-life of low-moisture, part-skim (LMPS) Mozzarella. Both chymosins were used at 2 levels [0.05 and 0.037 international milk clotting units (IMCU)/mL], and clotting temperature was varied to achieve similar gelation times for each treatment (as this also affects cheese properties). Functionality was assessed at various cheese ages using dynamic low-amplitude oscillatory rheology and performance of baked cheese on pizza. Cheese composition was not significantly different between treatments. The level of total calcium or insoluble (INSOL) calcium did not differ significantly among the cheeses initially or during ripening. Proteolysis in cheese made with BCC was higher than in cheeses made with CC. At 84 d of ripening, maximum loss tangent values were not significantly different in the cheeses, suggesting that these cheeses had similar melt characteristics. After 14 d of cheese ripening, the crossover temperature (loss tangent = 1 or melting temperature) was higher when CC was used as coagulant. This was due to lower proteolysis in the CC cheeses compared with those made with BCC because the pH and INSOL calcium levels were similar in all cheeses. Cheeses made with CC maintained higher hardness values over 84 d of ripening compared with BCC and maintained higher sensory firmness values and adhesiveness of mass scores during ripening. When melted on pizzas, cheese made with CC had lower blister quantity and the cheeses were firmer and chewier. Because the 2 types of cheeses had similar moisture contents, pH values, and INSOL Ca levels, differences in proteolysis were responsible for the firmer and chewier texture of CC cheeses. When cheese performance on baked pizza was analyzed, properties such as blister quantity, strand thickness, hardness, and chewiness were maintained for a longer ripening time than cheeses made with BCC, indicating that use of CC could help to extend the performance shelf-life of LMPS Mozzarella.

Effects of the Concentration of Insoluble Calcium Phosphate Associated with Casein Micelles on the Functionality of Directly Acidified Cheese

Directly acidified cheeses with different insoluble Ca (INS Ca) contents were made to test the hypothesis that the removal of INS Ca from casein micelles (CM) would directly contribute to the softening and flow behavior of cheese at high temperature. Skim milk was directly acidified with dilute lactic acid to pH values of 6.0, 5.8, 5.6, or 5.4 to remove INS Ca (pH trial). Lowering milk pH also reduced protein charge repulsion, which could influence melt. In a second treatment, EDTA (0, 2, 4, or 6mM) was added to skim milk that was subsequently acidified to pH 6.0 (EDTA trial). Both types of milks were then made into directly acidified cheese. Cheese properties were determined at approximately 10h after pressing to reduce possible confounding effects of proteolysis. The INS Ca content was determined by the acid-base titration method. Dynamic low-amplitude oscillatory rheology was used to measure the viscoelastic properties of cheese during heating from 5 to 80°C. The composition of all cheeses was as similar as possible, with cheese-making procedures being modified to obtain similar moisture contents (∼55%). Insoluble Ca contents of cheeses significantly decreased with a reduction in pH or with the addition of EDTA to skim milk. The pH values of cheeses in the pH trial varied, but all cheeses in the EDTA trial had similar pH values (∼5.73). In the pH trial, the reduction in cheese pH and consequent decrease in INS Ca content resulted in a reduction in the G′ values of cheeses at 20°C. In contrast, the G′ values at 20°C in cheeses from the EDTA trial increased with EDTA addition up to 4mM EDTA. The G′ values at 70°C of cheeses from the pH trial decreased with a decrease in cheese pH, and a similar decrease was observed in the G′ values of cheese from the EDTA trial with an increase in EDTA concentration even though these cheeses had a similar pH value. In both trials, loss tangent (LT) values increased with temperatures >30°C and reached a maximum at approximately 70°C. In the pH trial, LT values at 70°C increased from 1.50 to 4.24 with a decrease in cheese pH from 5.78 to 5.21. The LT values increased from 1.43 to 3.23 with an increase in the concentration of added EDTA from 0 to 6mM. In the EDTA trial, the decrease in G′ and increase in LT values at 70°C were due to the reduction in INS Ca content, because the pH values of these cheeses were the same. It can be concluded that the loss of INS Ca increases the melting in cheeses that have the same pH and gross chemical composition, and removal of INS Ca can even make cheese at high pH (∼5.73) exhibit reasonable melt characteristics.

Use of Cold Microfiltration Retentates Produced with Polymeric Membranes for Standardization of Milks for Manufacture of Pizza Cheese

Pizza cheese was manufactured with milk (12.1% total solids, 3.1% casein, 3.1% fat) standardized with microfiltered (MF) and diafiltered retentates. Polymeric, spiral-wound MF membranes were used to process cold (<7°C) skim milk, and diafiltration of MF retentates resulted in at least 36% removal of serum protein on a true protein basis. Cheese milks were obtained by blending the MF retentate (16.4% total solids, 11.0% casein, 0.4% fat) with whole milk (12.1% total solids, 2.4% casein, 3.4% fat). Control cheese was made with part-skim milk (10.9% total solids, 2.4% casein, 2.4% fat). Initial trials with MF standardized milk resulted in cheese with approximately 2 to 3% lower moisture (45%) than control cheese (∼47 to 48%). Cheese-making procedures (cutting conditions) were then altered to obtain a similar moisture content in all cheeses by using a lower setting temperature, increasing the curd size, and lowering the wash water temperature during manufacture of the MF cheeses. Two types of MF standardized cheeses were produced, one with preacidification of milk to pH 6.4 (pH6.4MF) and another made from milk preacidified to pH 6.3 (pH6.3MF). Cheese functionality was assessed by dynamic low-amplitude oscillatory rheology, University of Wisconsin MeltProfiler, and performance on pizza. Nitrogen recoveries were significantly higher in MF standardized cheeses. Fat recoveries were higher in the pH6.3MF cheese than the control or pH6.4MF cheese. Moisture-adjusted cheese yield was significantly higher in the 2 MF-fortified cheeses compared with the control cheese. Maximum loss tangent (LTmax) values were not significantly different among the 3 cheeses, suggesting that these cheeses had similar meltability. The LTmax values increased during ripening. The temperature at which the LTmax was observed was highest in control cheese and was lower in the pH6.3MF cheese than in the pH6.4MF cheese. The temperature of the LTmax decreased with age for all 3 cheeses. Values of 12% trichloroacetic acid soluble nitrogen levels were similar in all cheeses. Performance on pizza was similar for all cheeses. The use of MF retentates derived with polymeric membranes was successful in increasing cheese yield, and cheese quality was similar in the control and MF standardized cheeses.

Effect of Insoluble Calcium Concentration on Rennet Coagulation Properties of Milk

Rennet-induced gels were made from milk acidified to various pH values or milk at pH 6.0 that had added EDTA. The objective was to examine the effect of removing insoluble Ca (INS Ca) from casein micelles (CM) on rennet gelation properties. For the pH trial, diluted lactic acid was added to reconstituted skim milk to decrease the pH to 6.4, 6.0, 5.8, 5.6, and 5.4. For the EDTA trial, EDTA was slowly added (0, 2, 4, and 6mM) to reconstituted skim milk, and the final pH values were subsequently adjusted to pH 6.0. Dynamic low amplitude oscillatory rheology was used to monitor gel development. The Ca content of CM and rennet wheys made from these milks was measured using inductively coupled plasma spectroscopy. The INS Ca content of milk was altered by the acidification pH values or level of EDTA added. In all samples, the storage modulus (G′) exhibited a maximum (GM), with a decrease in G′ during longer aging times. Gels made at pH 6.4 had higher GM compared with gels made at pH 6.7 probably due to the reduction in electrostatic repulsion, whereas the INS Ca content only slightly decreased. The highest GM value of gels was observed at pH 6.4 and the GM value decreased with decreasing pH from 6.4 to 5.4. This was due to an excessive loss of INS Ca from CM. There was a decrease in GM with the increase in the concentration of added EDTA, which was probably due to the loss of colloidal calcium phosphate, which weakens the integrity of CM. Loss tangent (LT) values at GM increased with a reduction in milk pH and the addition of EDTA to milk. Rennet gels at the point of the GM were subjected to constant low shearing to fracture the gels. With a reduction in INS Ca content, the yield stress decreased, whereas LT values increased indicating a weaker, more flexible casein network. Microstructure of rennet-induced gels near the GM point and 2 to 10h after this point was studied using fluorescence microscopy. At GM, gels made from milk acidified to pH 6.4 exhibited more branched, interconnected networks, whereas strands and clusters became larger with a reduction in milk pH to 5.4. Gels made from milk with EDTA added had more finely dispersed protein clusters compared with gels made from milk with no EDTA added. These microscopic observations supported the effect of loss of INS Ca on GM and LT. There was a decrease in apparent interconnectivity between strands in gel microstructure during aging, which agreed with the decrease in G′ after GM. It can be concluded that low levels of solubilization of INS Ca and the decrease in milk pH resulted in an increase in GM. With greater losses of INS Ca there was excessive reduction in cross-linking within CM, which resulted in weaker, more flexible rennet gels. This complex behavior cannot be explained by adhesive hard sphere models for CM or rennet gels made from these CM.

Effect of Type of Concentrated Sweet Cream Buttermilk on the Manufacture, Yield, and Functionality of Pizza Cheese

Sweet cream buttermilk (SCB) is a rich source of phospholipids (PL). Most SCB is sold in a concentrated form. This study was conducted to determine if different concentration processes could affect the behavior of SCB as an ingredient in cheese. Sweet cream buttermilk was concentrated by 3 methods: cold (<7°C) UF, cold reverse osmosis (RO), and evaporation (EVAP). A washed, stirred-curd pizza cheese was manufactured using the 3 different types of concentrated SCB as an ingredient in standardized milk. Cheesemilks of casein:fat ratio of 1.0 and final casein content ∼2.7% were obtained by blending ultrafiltered (UF)-SCB retentate (19.9% solids), RO-SCB retentate (21.9% solids), or EVAP-SCB retentate (36.6% solids) with partially skimmed milk (11.2% solids) and cream (34.6% fat). Control milk (11.0% solids) was standardized by blending partially skimmed milk with cream. Cheese functionality was assessed using dynamic low-amplitude oscillatory rheology, UW Meltprofiler (degree of flow after heating to 60°C), and performance of cheese on pizza. Initial trials with SCB-fortified cheeses resulted in ∼4 to 5% higher moisture (51 to 52%) than control cheese (∼47%). In subsequent trials, procedures were altered to obtain similar moisture content in all cheeses. Fat recoveries were significantly lower in RO- and EVAP-SCB cheeses than in control or UF-SCB cheeses. Nitrogen recoveries were not significantly different but tended to be slightly lower in control cheeses than the various SCB cheeses. Total PL recovered in SCB cheeses (∼32 to 36%) were lower than control (∼41%), even though SCB is high in PL. From the rheology test, the loss tangent curves at temperatures>40°C increased as cheese aged up to a month and were significantly lower in SCB cheeses than the control, indicating lower meltability. Degree of flow in all the cheeses was similar regardless of the treatment used, and as cheese ripened, it increased for all cheeses. Trichloroacetic acid-soluble N levels were similar in the control and SCB-fortified cheese. On baked pizza, cheese made from milk fortified with UF-SCB tended to have the lowest amount of free oil, but flavor attributes of all cheeses were similar. Addition of concentrated SCB to standardize cheesemilk for pizza cheese did not adversely affect functional properties of cheese but increased cheese moisture without changes in manufacturing procedure.

Diffusion NMR as a New Method for the Determination of the Gel Point of Gelatin

The gelation of gelatin has been investigated using pulsed field gradient (PFG) NMR. For the first time, diffusion results have been used to determine the gelation point, which is indicated by a minimum in the self-diffusion coefficient of the free polymer fraction vs temperature. Biexponential analysis of the diffusion decay data allowing the diffusion of free and network-bound gelatin to be determined separately has been applied to provide an extended insight into the gelation process. Low-amplitude oscillatory shear rheology and time-resolved dynamic light scattering (DLS) as classical polymer characterization methods were applied as control experiments. All three methods yielded a gelation temperature of 24-25 degrees C for the cooling regime. Hysteresis effects could also be observed.

Structure and Physical Properties of Yogurt Gels: Effect of Inoculation Rate and Incubation Temperature

The effects of inoculation rates and incubation temperatures on the physical properties and microstructure of yogurt gels were investigated. A 2-factor experimental design with 3 replicates was used for data analysis. Yogurt gels were made with 0.5, 1, 2, 3, or 4% (wt/wt) inoculation rates and incubated at 40 or ∼46°C. Dynamic low amplitude oscillatory rheology was performed to monitor the formation of yogurt gels. Gel permeability and the amount of surface whey were determined. Gel structure was examined by confocal scanning laser microscopy. Higher storage modulus values were observed in yogurt gels made at higher inoculation rates and lower incubation temperatures. Gels made at higher inoculation rates and incubation temperatures exhibited higher yield stress and maximum loss tangent values, respectively. Permeability, pore size, and whey separation of yogurt gels increased with decreased inoculation rate and increased incubation temperature. An increase in the inoculation rate resulted in a decrease in the pH where the maximum in the loss tangent occurs, presumably reflecting less efficient solubilization of colloidal calcium phosphate (which is a slow process) and the need to attain a lower pH to complete the solubilization. Significant positive correlations were observed between whey separation and the value of maximum in loss tangent (r = 0.94) and permeability (r = 0.89). Whey separation was negatively correlated with storage modulus (r = −0.48). It was concluded that rearrangements of casein particles in the gel network and the rate at which the solubilization of colloidal calcium phosphate occurred were important driving forces for whey separation and weak gel.

The application of microscopy and rheology to study the effect of milk salt concentration on the structure of acidified micellar casein systems

The microstructural and rheological properties of glucono-delta-lactone induced acid casein gels made from micellar casein dispersed in water and in lactose-free simulated milk ultrafiltrate (SMUF) containing 40, 70, 100 and 200% of the salts normally present in milk were studied. Confocal scanning laser microscopy (CSLM) and low amplitude oscillatory rheology (LAOR) were used to monitor structural development during acidification. Image analysis of CSLM micrographs was used for measurement of mean apparent equivalent circular diameter of pores, mean volume-weighted size (star volume) of pores and for measurement of homogeneity by grey-level pixel variance. Transmission electron microscopy (TEM) was used to determine mean particle diameter and star volume of protein aggregates within the gel networks. At low salt concentrations, gel networks formed more rapidly with consequently larger pores compared to more slowly forming, homogeneous gels being formed on acidification at higher salt concentrations. TEM analysis showed increased fusion of casein particles as salt concentration decreased, indicating significant destabilisation of micellar caseins. Dynamic LAOR indicated differences in storage modulus and phase angle depending on ionic strength. Results indicated an inverse relationship between pore or particle size and SMUF concentration. Some length-scale dependence was noted for pixel variance and apparent equivalent circular diameter.

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.

Effect of gelation temperature on the properties of skim milk gels made from plant coagulants and chymosin

Reconstituted skim milk was gelled at 25–40°C with the plant-origin coagulants from Cynara cardunculus L. or Cynara humilis L. or with fermentation-produced chymosin. Gel formation and ageing were monitored by low amplitude oscillatory rheology and confocal scanning laser microscopy. Arrhenius plots for the rate of milk gelation were also determined. Plant coagulants had shorter gelation time (tg) at 25°C, 35°C and 40°C, and higher initial rate of increase in G′ values at all temperatures tested. The firmest gels at long ageing times were produced by chymosin at 30°C and 32°C. At a gelation temperature of 25°C, the differences in rheological and microstructural characteristics between plant coagulants and chymosin were considerable; plant coagulants had shorter tg and higher G′ values. For the lowest gelation temperatures, plant coagulants had smaller activation energy values for gelation. Most of the gelation results were similar between plant coagulants, but some differences were found in the values of tg, the rate of increase in G′ and loss tangent parameter. The characteristics of gels produced with plant coagulants were influenced less by the changes in temperature compared with chymosin-produced gels, which may be an important consideration in using plant-origin coagulants in the production of cheeses with a wider range of gelation temperatures.