The Role of B-Lactoglobulin in the Development of the Core-and-Lining Structure of Casein Particles in Acid-Heat-Induced Milk Gels

Acid-heat-induced gels were obtained by coagulating casein micelle dispersions at 90 C using glucono-and-lactone. The casein micelles used were isolated from raw skim milk by centrifugation, washed free of whey proteins and soluble salts, and dispersed in water or a milk dialyzate. The pH values of the gels varied from 4.7 to 6.3. A core-and-lining ultrastructure developed in casein particles coagulated at pH 5.2 to 5.5 from casein micelle dispersions in the milk dialyzate provided that B-lactoglobulin or whey proteins (10 mg/ml) were added to them prior to coagulation. Addition of B-lactoglobulin to aqueous casein micelle dispersions led to the development of a considerably less distinct core-and-lining ultrastructure of the resulting gels. Coagulated casein particles obtained from casein micelle dispersions in water or in the milk dialyzate to which neither B-lacto-globulin nor whey proteins were added, did not show the core-and-lining ultrastructure but constained void spaces inside and were covered with loosely aggregated protein on the surface. It was concluded that both B-lactoglobulin or whey proteins and the milk salt system are essential for the formation of the core-and-lining ultrastructure in the casein micelle dispersions gelled by heating at 90 C at pH 5.2 to 5.5

Composition and Some Properties of Spray-Dried Retentates Obtained by the Ultrafiltration of Milk

Retentates containing 20, 27, and 34% total solids, obtained on commercial scale by the ultrafiltration of milk were spray-dried on laboratory scale using centrifugal atomization and single stage drying with the inlet air temperature of 220 C and the outlet air temperature of 90 C. The protein content in the powders was 31% to 35% compared to 24.8% protein in the control whole-milk powder. Lactose contents were markedly lower in the retentate powders (-10.6%) than in the milk powder (40.4%). Storage of the powders at 37 C resulted in a marked increase in the 5-hydroxmethylfural contents with doubling of this content in the retentate powders and tripling in the milk powder. When viewed by scanning electron microscopy, the spray-dried retentate powder particles had smooth surfaces free from wrinkles usually seen in spray-dried milk powders. When the same products were exposed to atmospheres having 75%, 85%, or 100% relative humidity, the retentate powders exhibited less lactose recrystallization than the milk powder. The melting temperature (Tm) (as determined by differential scanning calorimetry) of lactose present in the retentate powders was not affected by the reduced lactose content in the powders but the fusion enthalpy of lactose was reduced in the retentate powders compared to the control milk powder

Impact of in vitro gastrointestinal digestion on peptide profile and bioactivity of cooked and non-cooked oat protein concentrates

Oat (Avena sativa) is one of the most cultivated and consumed cereals worldwide. Recognized among cereals for its high protein content (12% to 24%), it makes it an excellent source of bioactive peptides, which could be modified during processes such as heating and gastrointestinal digestion (GID). This work aims to evaluate the impact of heat treatment on the proteolysis of oat proteins and on the evolution of antioxidant peptide release during in vitro static GID, in terms of comparative analysis between cooked oat protein concentrate (COPC) and non-heated oat protein concentrate (OPC) samples. The protein extraction method and cooking procedure used showed no detrimental effects on protein quality. After GID, the proportion of free amino acids/dipeptides (40% for both samples (OPC and COPC), thus producing peptides with low molecular weight and enhanced bioactivity. Furthermore, during GID, the amino acid profile showed an increase in essential, positively-charged, hydrophobic and aromatic amino acids. At the end of GID, the reducing power of OPC and COPC increased >0.3 and 8-fold, respectively, in comparison to the non-digested samples; while ABTS•+ and DPPH• showed a >20-fold increase. Fe2+ chelating capacity of OPC and COPC was enhanced >4 times; similarly, Cu2+ chelation showed a >19-fold enhancement for OPC and >10 for COPC. β-carotene bleaching activity was improved 0.8 times in OPC and >9 times in COPC; the oxygen radical antioxidant capacity assay increased 2 times in OPC and >4.7 times in COPC, respectively. This study suggests that OPC after cooking and GID positively influenced the nutritional and bioactive properties of oat peptides. Thus, COPC could be used as a functional food ingredient with health-promoting effects, as hydrothermal treatment is frequently used for this type of cereals

Relationship Between Microstructure and Susceptibility to Syneresis in Yoghurt Made from Reconstituted Nonfat Dry Milk

Increase in the density of protein matrices i n yoghurt samples conta ining 10 to 30% total milk sol ids was studied by scanning electron micros copy and was corr e lated with a decrease in syneresis. Casein particles which formed chains and clusters in the protein matrix were largest in the 10% total solids yoghurt and their dimensions were decreased as the total solids contents were increased. This observation was confirmed by transmission electron microscopy of thin sections. An attempt has been made to explain the discrepancy between results on syneresis obtained by a drainage and a centrifugation method applied to yoghur t samples made at pi! 3.85 and 4.5. The explanation is based on a difference in rigidity of t he yoghurts under study and in t he forces affecting t he protein matrix during tests for syneresis

Effect of Heating to 200 C on Casein Micelles in Milk: A Metal Shadowing and Negative Staining Electron Microscope Study

Milk was heated to 200 C for 3 min in sealed inverted-Y-shaped glass vials and reacted with a glutaraldehyde solution at that temperature. Electron microscopy of the metal-shadowed and negatively stained samples revealed that casein micelles in the milk did not disintegrate extensively at the high-temperature used but, rather, became enlarged. Some of them were found to be either clustered or distorted