Dextran modulates physical properties of rennet-induced milk gels

Physical properties of rennet-induced milk gels as core intermediates of cheese production are mainly affected by milk composition, type and amount of coagulation enzyme and starter culture activity. We investigated model systems of reconstituted milk and dextran, which triggers effects on milk gels similar to exopolysaccharides from lactic acid bacteria. Furthermore, clotting activity (0.02 or 0.04 IMCU mL⁻¹) and milk pH adjusted prior to renneting (6.5–5.7) were studied. A lower pH at renneting resulted in an earlier gelation onset, a higher gelation velocity and gel stiffness. The addition of dextran stabilised the gels especially at higher pH, and microstructural analysis revealed larger, more interconnected protein aggregates. However, at pH 5.7, a reverse effect was observed, indicating a destabilisation of the casein network. The current study indicates that altering milk pH and addition of polysaccharides gives the potential to change textural properties of cheese by affecting rennet-induced gelation.     

Influence of molecular weight and degree of substitution of carboxymethylcellulose on the stability of acidified milk drinks

The influence of molecular weight (M_w, 250,000, 700,000) and degree of substitution (DS, 0.7, 0.9 and 1.2) of carboxymethylcellulose (CMC) on the diameter and ζ-potential of casein micelles during acidification in diluted dispersions and on the stability of acidified milk drinks was investigated. The experimental results suggested that CMC with high M_w or low DS would result in thick adsorbed layer onto casein micelles. The ζ-potential of CMC-coated casein micelle increased with increasing the M_w of CMC with the same DS while at a fixed M_w the ζ-potential for CMC with high DS (1.2) increased in comparison with those for CMC with low DS (0.7 and 0.9). Both M_w and DS of CMC influenced the stability of acidified milk drinks. CMC with high M_w increased the viscosity of acidified milk drinks significantly and therefore contributed to the stability. CMC with high DS resulted in high ζ-potential of CMC-coated casein micelles, increasing the electrostatic repulsion between casein particles, which prevented the phase separation in acidified milk drinks. It was also found that the amount of CMC needed for efficient coverage of casein micelles increased with increasing the M_w of CMC. Above the efficient coverage concentration, the long-term stability of acidified milk drinks with high M_w CMC was better than that with low M_w CMC.     

Short communication: Physicochemical features and microbial community of milk kefir using a potential probiotic Saccharomyces cerevisiae KU200284

The aim of this study was to analyze the β-glucan contents, physicochemical features, and microbial communities in milk kefir prepared using Saccharomyces cerevisiae KU200284 isolated from cucumber jangajji, a fermented vegetable commonly eaten in Korean. Three types of milk kefir were manufactured, with (1) activated kefir grain, (2) activated kefir grain with commercial S. cerevisiae BOF, and (3) activated kefir grain with S. cerevisiae KU200284. β-Glucan contents of milk kefir using kefir grain and kefir grain with S. cerevisiae strains BOF and KU200284 were 8.29, 8.59, and 8.57%, respectively. The pH, titratable acidity, viscosity, Brix level, and alcohol contents of milk kefir using kefir grain with S. cerevisiae strains were acceptable compared with milk kefir using only kefir grain. In milk kefir produced using kefir grains and S. cerevisiae strains, 16S rRNA reads showed representative strains of Lactobacillus kefiranofaciens (>72% relative abundance) and Acetobacter fabarum (>16% relative abundance). In particular, milk kefir using kefir grain with S. cerevisiae KU200284 had the highest relative abundance of L. kefiranofaciens. In addition, the internal transcribed sequence (ITS) rRNA reads in tested milk kefir showed representative strains of Kluyveromyces marxianus (>52% relative abundance) and Saccharomyces cerevisiae (>16% relative abundance). In contrast, milk kefir using S. cerevisiae strains had higher relative abundance of S. cerevisiae (>37%). The β-glucan production, physicochemical properties, and microbial community profiling indicate that S. cerevisiae KU200284 could be used in functional dairy products as a starter culture.     

Proteolysis of casein micelles by heat-stable protease secreted by Serratia liquefaciens leads to the destabilisation of UHT milk during its storage

Serratia liquefaciens is a psychrotrophic species, frequently found in raw milk, which secretes Ser2, a heat-resistant protease. Involvement of this species in UHT milk destabilisation was investigated in the present study. Microfiltered milk was inoculated independently with strains S. liquefaciens L53 or L64. Then, UHT treatment was performed and stability of the corresponding UHT milk was investigated during three months of storage. The residual proteolytic activity of strain L53 led to destabilisation of UHT milk, with sedimentation and formation of aggregates. Hydrolysis of casein micelles was confirmed by the increase in the content of non-casein nitrogen and the identification of numerous peptides coming from the four caseins using mass spectrometry. For strain L64, no visual and biochemical alteration were found. This study showed that Ser2 resists UHT treatment and could be a cause of UHT milk destabilisation; however, this destabilisation by S. liquefaciens was strain-dependent.     

Effect of storage time and temperature of milk protein concentrate (MPC85) on the renneting properties of skim milk fortified with MPC85

This study investigated the effect of storage temperature (20–50 °C) and time (0–60 days) on the renneting properties of milk protein concentrate with 85% protein (MPC85). Reconstituted skim milk was fortified with the MPC85 (2.5% w/w) and the renneting properties of the skim milk/MPC85 systems were investigated using rheology. It was found that the final complex modulus (final G∗) and the yield stress of the rennet-induced skim milk/MPC85 gels decreased exponentially with storage time of the MPC85 for storage temperatures greater than 20 °C, with a greater effect at the higher storage temperatures. Changes in the solubility of MPC85 with storage time were correlated with the rheological properties. The primary phase of renneting (cleavage of κ-casein) was not affected by the storage of the MPC85; hence the effect was related to the secondary stage of renneting (aggregation/coagulation of rennet-treated casein micelles). Using a temperature–time superposition method, a master curve was formed from the final G∗, yield stress and solubility results. This suggested that the same physical processes affected the solubility and rennet gelation properties of the milks. It is proposed that the MPC85 protein in rennet-treated skim milk/MPC85 solutions may transform from an interacting material, when solubility is high, to an inert or weakly interacting material, when solubility is low, and that this results in the reduced final G∗ and yield stress of the rennet gels when MPC85 is stored at elevated temperatures for long periods.     

Monitoring the chemical and textural changes during ripening of Iranian White cheese made with different concentrations of starter

The effect of the concentration of starter inoculated to milk on the composition, free tyrosine-tryptophan content, microstructure, opacity, and fracture stress of Iranian White cheese (IWC) was studied during 50 d of ripening in brine. Three treatments of cheese were made using 1-fold (IWC1S), 2-fold (IWC2S), and 4-fold (IWC4S) concentrations of a direct-to-vat mesophilic mixed culture containing Lactococcus lactis ssp. cremoris and Lactococcus lactis ssp. lactis as starter. As ripening progressed, moisture and protein contents of the treatments continuously decreased, whereas their total ash, salt, and salt in moisture contents increased. Fat content and pH of cheeses remained stable during ripening. The pH of cheese milk at the time of renneting, which decreased by increasing the concentration of starter (6.57, 6.49, and 6.29 for IWC1S, IWC2S, and IWC4S, respectively), significantly affected most of the chemical characteristics and opacity of cheese. Lower pH values at renneting decreased moisture and ash contents, whereas cheese protein content increased. The concentration of free tyrosine-tryptophan in curd increased at first 29 d but decreased between d 29 and 49 of aging. The changes observed in cheese whiteness followed the changes in moisture content of the treatments. As the concentration of starter inoculated to milk increased, the value of fracture stress at a given ripening time significantly decreased, leading to a less resistant body against applied stress. A similar trend was also observed for fracture strain during cheese ripening. The micrographs taken by scanning electron microscopy provided a meaningful explanation for decrease in the value of fracture stress. As the cheese ripening progressed or the concentration of starter increased, the surface area occupied by the protein fraction in cheese microstructure decreased, leading the way to lower the force-bearing component in cheese texture.     

Effect of carbon dioxide under high pressure on the survival of cheese starter cultures

A new processing method that rapidly forms curds and whey from milk has the potential to improve cheesemaking procedures if cheese starter cultures can tolerate the processing conditions. The survival of Lactobacillus delbrueckii ssp. bulgaricus, Lactococcus lactis ssp. lactis, or Streptococcus thermophilus through this new process was evaluated. Inoculated milk containing 0, 1, or 3.25% fat or Lactobacillus MRS broth or tryptone yeast lactose broth (depending on microorganism used) was sparged with CO2 to a pressure of 5.52 MPa and held for 5 min at 38 degrees C. Broth contained 7.93 to 8.78 log CFU/ ml before processing and 7.84 to 8.66 log CFU/ml afterward. Before processing, milk inoculated with L bulgaricus, L. lactis, or S. thermophilus contained 6.81, 7.35, or 6.75 log CFU/ml, respectively. After processing, the curds contained 5.68, 7.32, or 6.50 log CFU/g, and the whey had 5.05, 6.43, or 6.14 log CFU/ml, respectively. After processing, the pHs of control samples were lower by 0.41 units in broth, 0.53 units in whey, and 0.89 units in curd. The pH of the processed inoculated samples decreased by 0.3 to 0.53 units in broth, 0.32 to 0.37 units in whey, and 0.93 to 0.98 units in the curd. Storing curds containing L. lactis at 30 degrees C or control curds and curds with L. bulgaricus or S. thermophilus at 37 degrees C for an additional 48 h resulted in pHs of 5.22, 5.41, 4.53, or 4.99, respectively. This study showed that milk inoculated with cheese starter cultures and treated with CO2 under high pressure to precipitate casein-produced curds that contained sufficient numbers of viable starter culture to produce lactic acid, thereby decreasing the pH.     

Antimicrobial effects of a bioactive glycolipid on spore-forming spoilage bacteria in milk

The growth of psychrotolerant aerobic spore-forming bacteria during refrigerated storage often results in the spoilage of fluid milk, leading to off-flavors and curdling. Because of their low toxicity, biodegradability, selectivity, and antimicrobial activity over a range of conditions, glycolipids are a novel and promising intervention to control undesirable microbes. The objective of this study was to determine the efficacy of a commercial glycolipid product to inhibit spore germination, spore outgrowth, and the growth of vegetative cells of Paenibacillus odorifer, Bacillus weihenstephanensis, and Viridibacillus arenosi, which are the predominant spore-forming spoilage bacteria in milk. For spore germination and outgrowth assays, varying concentrations (25–400 mg/L) of the glycolipid product were added to commercial UHT whole and skim milk inoculated with ～4 log₁₀ spores/mL of each bacteria and incubated at 30°C for 5 d. Inhibition of spore germination in inoculated UHT whole milk was only observed for V. arenosi, and only when glycolipid was added at 400 mg/L. However, concentrations of 400 and 200 mg/L markedly inhibited the outgrowth of vegetative cells from spores of P. odorifer and B. weihenstephanensis, respectively. No inhibition of spore germination or outgrowth was observed in inoculated UHT skim milk for any strain at the concentrations tested (25 and 50 mg/L). The effect of glycolipid addition on vegetative cell growth in UHT whole and skim milk when inoculated with ～4 log₁₀ cfu/mL of each bacteria was also determined over 21 d of storage at 7°C. Glycolipid addition at 50 mg/L was bactericidal against P. odorifer and B. weihenstephanensis in inoculated UHT skim milk through 21 d of storage, whereas 100 mg/L was needed for similar control of V. arenosi. Concentrations of 100 and 200 mg/L inhibited the growth of vegetative cells of B. weihenstephanensis and P. odorifer, respectively, in inoculated UHT whole milk, whereas 200 mg/L was also bactericidal to B. weihenstephanensis. Additional studies are necessary to identify effective concentrations for the inhibition of Viridibacillus spp. growth in whole milk beyond 7 d. Findings from this study demonstrate that natural glycolipids have the potential to inhibit the growth of dairy-spoilage bacteria and extend the shelf life of milk.     

Nutrient enrichment of dairy curd by incorporation of whole and ruptured microalgal cells (Nannochloropsis salina)

This work investigated incorporation of Nannochloropsis salina into renneted dairy gels and curd. Whole and ruptured microalgal cells did not impair κ-casein macropeptide cleavage by the rennet enzyme. However, insoluble components of ruptured cells impeded gelation, presumably by hindering interactions between renneted casein micelles. Confocal imaging showed that whole cells were retained and homogenously distributed within the protein network of the gels and cooked curd, whereas ruptured algae formed large aggregates that altered the protein matrix. Eicosapentaenoic acid (EPA) in the whole microalgal cells was incorporated within the curds, with considerably less EPA retained for ruptured cells. Soluble algal debris did not impair gelation, however EPA wasn't retained in the curd. The study demonstrates that nutrient enrichment of renneted dairy products is possible by incorporating whole microalgal cells to displace milk fat with protein and the beneficial long-chain omega-3 fatty acid EPA. Future research into the optimisation of product organoleptic properties is required.     

Serum Protein Aggregates in the High-Heated Milk and Their Gelation Properties in Rennet-Induced Milk Gel

The influence of different heat treatments on the protein aggregates and changes in gelation properties of rennet-induced milk gels were investigated. In the heated milk, a visible difference in milk serum proteins was found resulting from the formation of protein aggregates. Meanwhile, the size of protein aggregates increased from 25 to 170 nm with increasing the intensity of heat treatment. Furthermore, the differences in textural variables of rennet gels were found among the heat treatments using the principal component analysis. The water holding capacity and cheese curd yield of rennet gels obtained from the heated milk were significantly greater than those of raw milk (p < 0.05). It was also found heat treatments above 80°C could endow rennet-induced milk gels with novel textural properties.     

Influence of variation in calcium content on casein micelle stability and techno-functional properties of buffalo milk

Changes in techno-functional properties of buffalo milk were evaluated due to variation in calcium content. Decalcification resulted in significant variation in ζ-potential, casein size, colour and η_app. However, calcium addition only influenced ζ-potential of milk. In case of acid gelation, the time and temperature required for coagulation decreased significantly for both calcium-depleted and -added milks. However, during chymosin gelation, only 20%–30% of calcium-depleted milk coagulated with an increased clotting time. Furthermore, calcium addition increased firmness, consistency and cohesiveness of both chymosin and acid-induced gelation.     

Effect of CSN1S1-CSN3 (α(S1)-κ-casein) composite genotype on milk production traits and milk coagulation properties in Mediterranean water buffalo

The aim of this study was to estimate effects of CSN1S1-CSN3 (α_S1-κ-casein) composite genotypes on milk production traits and milk coagulation properties (MCP) in Mediterranean water buffalo. Genotypes at CSN1S1 and CSN3 and coagulation properties [rennet clotting time (RCT), curd firming time (K₂₀), and curd firmness (A₃₀)] were assessed by reversed-phase HPLC and computerized renneting meter analysis, respectively, using single test-day milk samples of 536 animals. Alternative protein variants of α_S1-CN and κ-CN were detected by HPLC, and identification of the corresponding genetic variants was carried out by DNA analysis. Two genetic variants were detected at CSN1S1 (A and B variants) and 2 at CSN3 (X1 and X2 variants). Statistical inference was based on a linear model including the CSN1S1-CSN3 composite genotype effect (7 genotypes), the effects of herd-test-day (8 levels), and a combined days in milk (DIM)-parity class. Composite genotype AB-X2X2 was associated with decreased test-day milk yield [?0.21 standard deviation (SD) units of the trait] relative to genotype BB-X2X2. Genotypes did not affect milk protein content, but genotype AB-X1X1 was associated with increased fat content compared with genotype BB-X2X2 (+0.28 SD units of the trait) and AB-X1X1 (+0.43 SD units of the trait). For RCT, the largest difference (+1.91 min; i.e., 0.61 SD units of the trait) was observed between genotype AA-X1X2 and AB-X1X1. Direction of genotype effects on K₂₀ was consistent with that for RCT. The maximum variation in K₂₀ due to genotype effects (between AA-X1X2 and AB-X1X1 genotypes) was almost 0.9 SD units of the trait. Magnitude of genotype effects was smaller for A₃₀ than for RCT and K₂₀, with a maximum difference of 0.5 SD units of the trait between genotype AA-X1X2 and AA-X1X1. The B allele at CSN1S1 was associated with increased RCT and K₂₀ and with weaker curds compared with allele A. Allele X2 at CSN3 exerted opposite effects on MCP relative to CSN1S1 B. Because of linkage disequilibrium, allele B at CSN1S1 and allele X2 at CSN3 tend to be associated and this likely makes their effects cancel each other. This study indicates a role for casein genes in variation of MCP of buffalo milk. Further studies are necessary to estimate the effects of casein genetic variants on variation of cheese yield.     

Effects of gelation temperature on Mozzarella-type curd made from buffalo and cows’ milk. 1: Rheology and microstructure

The rheology and microstructure of Mozzarella-type curds made from buffalo and cows’ milk were measured at gelation temperatures of 28, 34 and 39 °C after chymosin addition. The maximum curd strength (G′) was obtained at a gelation temperature of 34 °C in both types of bovine milk. The viscoelasticity (tan δ) of both curds was increased with increasing gelation temperature. The rennet coagulation time was reduced with increase of gelation temperature in both types of milk. Frequency sweep data (0.1–10Hz was recorded 90 min after chymosin addition, and both milk samples showed characteristics of weak viscoelastic gel systems. When both milk samples were subjected to shear stress to break the curd system at constant shear rate, 95 min after chymosin addition, the maximum yield stress was obtained at the gelation temperatures of 34 °C and 28 °C in buffalo and cows’ curd respectively. The cryo-SEM and CLSM techniques were used to observe the microstructure of Mozzarella-type curd. The porosity was measured using image J software. The cryo-SEM and CLSM micrographs showed that minimum porosity was observed at the gelation temperature of 34 °C in both types of milk. Buffalo curd showed minimum porosity at similar gelation temperature when compared to cows’ curd. This may be due to higher protein concentration in buffalo milk.     

Interactive Effects of Milk Fat Globule and Casein Micelle Size on the Renneting Properties of Milk

The size of the casein micelles (CM) and the milk fat globules (MFG) vary depending on farming factors, such as seasonal variation and stage of lactation, and cow genetics. The MFG and CM size of milk can influence the renneting behavior and texture of manufactured dairy products. In this work, we investigated the combined effects of MFG and CM size on the onset of gelation, the maximum rate of gelation, the value for G′_60 min (the final storage modulus) and G″_60 min (the final loss modulus), and tan δ upon renneting. Fractionation of MFG on the basis of size was carried out using laboratory-based centrifugation, whereas milk of predominantly large (184–218 nm) or small (147–159 nm) CM was selected naturally on-farm. Casein micelle size had the dominant effect on curd firmness and gelation rates of milk, where small CM milk formed rennet gels earlier and resulted in a firmer gel than milk with large CM. However, MFG size also influenced the renneting properties. The strongest rennet gels were obtained when large MFG (3.88–5.78 μm) was combined with small CM (153–159 nm). Selecting milk on the basis of the microstructure of key milk components could be achieved by natural selection of dairy cows or via fractionation technologies. Selection may provide a useful tool for efficient manufacturing of different dairy products based on the desirable characteristics specific to each.     

Minimization of aflatoxin M1 content in Iranian white brine cheese

A chemometric approach was used to minimize the aflatoxin M1 (AFM1) content of Iranian white brine cheese. The effects of processing factors such as renneting temperature (30–40°C), cutting size (0.5–1 cm), stirring time (10–20 min), press time (1–2 h), curd size (64–256 cm³) and saturated brine pH (4.6–6) on the AFM1 content of the cheese curds were explored. Renneting temperature, press time and saturated brine pH were, respectively, the most significant factors. The aflatoxin content of the cheese samples decreased with increasing renneting temperature and press time. Lowering of the saturated brine pH reduced AFM1 in the cheese curds. Taking account of all of the factors studied, optimum processing conditions for minimization of AFM1 in the cheese curds were: renneting temperature = 39.91°C, cut size = 0.51 cm, stirring time = 17.71 min, press time = 19.48 min, curd size = 73.27 cm³ and saturated brine pH = 4.79.     

Improving the yield of Mozzarella cheese by phospholipase treatment of milk

Part-skim Mozzarella cheese was manufactured from milk hydrolyzed with fungal phospholipase A₁ prior to renneting. The phospholipase treatment reduced fat losses in whey and cooking water and increased cheese yield as a result of improved fat and moisture retention in the cheese curd. The amount of phospholipids in the whey was reduced because of improved retention of lysophospholipids in the cheese curd. Water binding in the fresh curds and young cheeses up to 3 wk of storage was investigated by a ¹H nuclear magnetic resonance spin-spin relaxation technique. In the fresh curds, 2 dominant water fractions were present, characterized by average spin-spin relaxation times (T₂) of 14 and 86 to 89 ms, respectively. These 2 fractions of low- and high-molecular-mobility water were similar in all cheeses and presumed to represent water associated with the casein matrix and water present in the pores. A few hours after manufacture, cheeses made with phospholipase showed decreased T₂ of the high-mobility fraction, indicating improved water-holding capacity. It is suggested that lysophospholipids released from the fat globule membranes act as surface-active agents in the cheese curd, helping emulsification of water and fat during processing and reducing syneresis. During 3 wk of storage after manufacture, the mobility of both water fractions increased in all cheeses, but was highest in the cheeses made with phospholipase. The increase in mobility during the first weeks of storage has earlier been ascribed to structural changes in the protein matrix, which in principle could be accelerated because of the higher moisture content. However, the microstructure of phospholipase-treated cheese was investigated by confocal laser scanning microscopy and found to be very similar to the control cheese during processing and up to 28 d of storage. In addition, flowability, stretchability, and browning were acceptable and similar in all the manufactured cheeses. Thus, phospholipase hydrolysis of cheese milk improved the cheese yield without changing the cheese microstructure, and resulted in cheese with functional properties that were identical to traditional Mozzarella cheese.     

Novel genetic variation associated to CSN3 strongly affects rennet-induced milk coagulation

The effect on rennet-induced milk coagulation of three novel genetic haplotypes in close proximity to CSN3 encoding κ-casein was evaluated. Milk samples were collected from 71 Danish Holstein cows homozygous for the three novel haplotypes named according to which genetic variants of CSN3 they were characterised by: AE, A and B, respectively. The results documented that haplotype AE had significantly longer rennet coagulation time and lower curd firming rate compared with haplotypes A and B. Haplotype AE milk was further characterised by larger casein micelles and lower relative content of κ-casein, whereas the total protein contents did not differ among haplotypes. These findings indicate that the genetic κ-casein A variant can be divided into two groups with poor and good milk coagulation properties. Furthermore, three milk samples were identified as non-coagulating. These were all associated with the haplotype AE.     

Response surface methodology modeling of protein concentration,coagulum cut size,and set temperature on curd moisture loss kinetics during curd stirring

The effects of the independent variables protein concentration (4–6%), coagulum cut size (6–18 mm³), and coagulation temperature (28–36°C) on curd moisture loss during in-vat stirring were investigated using response surface methodology. Milk (14 kg) in a cheese vat was rennet coagulated, cut, and stirred as per semihard cheesemaking conditions. During stirring, the moisture content of curd samples was determined every 10 min between 5 and 115 min after cutting. The moisture loss kinetics of curds cut to 6 mm³ followed a logarithmic trend, but the moisture loss of curds from larger cut sizes, 12 or 18 mm³, showed a linear trend. Response surface modeling showed that curd moisture level was positively correlated with cut size and negatively correlated with milk protein level. However, coagulation temperature had a significant negative effect on curd moisture up to 45 min of stirring but not after 55 min (i.e., after cooking). It was shown that curds set at the lower temperature had a slower syneresis rate during the initial stirring compared with curds set at a higher temperature, which could be accelerated by reducing the cut size. This study shows that keeping a fixed cut size at increasing protein concentration decreased the level of curd moisture at a given time during stirring. Therefore, to obtain a uniform curd moisture content at a given stirring time at increasing protein levels, an increased coagulum cut size is required. It was also clear that breakage of the larger curd particles during initial stirring can also significantly influence the curd moisture loss kinetics. Both transmission and scanning electron micrographs of cooked curds (i.e., after 45 min of stirring) showed that the casein micelles were fused at a higher degree in curds coagulated at 36°C compared with 28°C, which confirmed that coagulation temperature causes a marked change in curd microstructure during the earlier stages of stirring. The present study showed the dynamics of curd moisture content during stirring when using protein-concentrated milk at various set temperatures and cut sizes. This provides the basis for achieving a desired curd moisture loss during cheese manufacture using protein-concentrated milk as a means of reducing the effect of seasonal variation in milk for cheesemaking.     

Structural changes in milk from different species during gastric digestion in piglets

This study investigated the structural and physicochemical changes that occur in milk, a naturally designed complex structured emulsion, during gastric digestion using the bottle-fed piglet as an animal model. The gastric digestions of cow, goat, and sheep milk were compared in male piglets euthanized at different postfeeding times to collect the stomach chyme. The cow and noncow milks separated into curd (aggregated caseins) and liquid (mostly soluble whey) phases in the piglet's stomach. For milk from all the species, the curd remained longer in the stomach because of its slow disintegration, whereas the liquid phase emptied readily. The majority of the fat globules were found to be entrapped within the protein network of the curd. The rate of release of fat globules was strongly dependent on the breakdown of the surrounding protein network of the curd. The consistency of the gastric curds changed as digestion progressed, with goat and sheep milk curds having relatively softer curd consistency and less fused protein networks, especially toward the end of digestion. This might have led to the lower protein and fat retention in the goat and sheep milk curds and relatively faster gastric emptying of these nutrients from goat and sheep milk in comparison to cow milk. This in vivo study provided new and enhanced understanding of the mechanisms of the gastric digestion of milk from different species. It may have implications for developing bioinspired structures for the controlled digestion and delivery of nutrients.     

Staphylococcus aureus and Listeria monocytogenes in Norwegian raw milk cheese production

The aim of this study was to survey the presence of Staphylococcus aureus and Listeria monocytogenes during the cheese making process in small-scale raw milk cheese production in Norway.The prevalence of S. aureus in bovine and caprine raw milk samples was 47.3% and 98.8%, respectively. An increase in contamination during the first 2-3 h resulted in a 73.6% prevalence of contamination in the bovine curd, and 23 out of 38 S. aureus-negative bovine milk samples gave rise to S. aureus-positive curds. The highest contamination levels of S. aureus were reached in both caprine and bovine cheese after 5-6 h (after the first pressing). There was no contamination of L. monocytogenes in caprine cheeses and only one (1.4%) contaminated bovine cheese.This work has increased our knowledge about S. aureus and L. monocytogenes contamination during the process of raw milk cheese production and gives an account of the hygiene status during the manufacture of Norwegian raw milk cheeses.