Gerard Brule

Mineral and protein equilibria between the colloidal and soluble phases of milk at low temperature

Mechanisms controlling exchange and partition of minerals and β-casein between micellar and soluble phases during cooling have been studied using suspensions of milk proteins of varying concentrations in the presence of aqueous phases having varying mineral compositions. In these studies the final equilibrium between Ca and phosphate was found to be regulated by formation of Ca phosphates of varying solubilities. The effect of the total casein level on mineral and protein equilibria was also studied. In milk, part of the β-casein content is bound to micelles by hydrophobic bonds; the proportion present in this form was increased by spontaneous micellar demineralization during cooling as well as by addition of a complexant to milk. At low temperatures, an equilibrium between the micellar and the monomeric states was reached which depends upon the total casein level and on the hydrophobic-bonded β-casein content. Not all of these hydrophobic bonds were broken when milk was cooled.

Mineral balance in skim-milk and milk retentate: effect of physicochemical characteristics of the aqueous phase

The effect of physicochemical characteristics (pH, temperature, composition) of the aqueous phase on the mineral balance in milk and milk retentate has been studied. The ratio of colloidal Ca to total protein decreased with pH, but at any given pH the higher the protein concentration, the higher was the ratio of colloidal Ca to total protein. The solubilization of Ca during cooling and the decrease in soluble Ca during heating were approximately the same in retentates and in milk. Among the components of the aqueous phase, soluble Ca and citrate ions were related to the amount of colloidal Ca.

Study of calcium binding to phosphoserine residues of β -casein and its phosphopeptide (1–25) by 31 P NMR

The effect of Ca 2+ . binding and ionic strength on the physicochemical characteristics of phosphoserine residues was studied on O -phospho-DL-serine, bovine β -casein and its phosphopeptide (1–25) using 31P NMR. pK in various experimental conditions were determined. The pK of the phosphoserine residues of β -casein and its phosphopeptide (1–25) respectively ranged from 6·46 to 7·21 and from 6·57 to 7·10. pK of O -phospho-DL-serine, β -casein and its phosphopeptide decreased when Ca 2+ was bound to the phosphoserine residue or when ionic strength was increased. The binding of Ca 2+ to the phosphopeptide (1–25) took place at first on phosphoserine residues 17, 18, 19 which had the highest pK, then, when these were saturated, on residue 15 whose pK was the lowest. When the four sites had bound Ca 2+ , peaks corresponding to a different complex form appeared in the spectrum.

Multinuclear NMR study of the pH dependent water state in skim milk and caseinate solutions

The effects of decreasing pH and micellar calcium concentrations of reconstituted skim milk and caseinate solution were studied by 1 H and 17 O NMR spectroscopy. The proton transverse relaxation rate 1/ T 2 of skim milk decreased as the pH decreased, reaching a minimum at pH 5·3. However, as the pH fell sodium caseinate solution showed a continuous increase in 1/ T 2 , with no minimum. Analysis of proton relaxation as a function of the interpulse time in the CPMG (Carr-Purcell-Meiboom-Gill) sequence demonstrated that both the proton exchange mechanism and ‘bound’ water contributed to proton relaxation in skim milk. The study of 17 O relaxation rate as a function of pH confirmed the change in protein hydration upon acidification. Increasing the amount of EDTA showed that the proton transverse relaxation rate of skim milk decreased until a plateau was reached when the micellar calcium was totally solubilized. With excess EDTA the relaxation rates of skim milk and caseinate solution were identical. A strong correlation was also found between the pH dependent relaxation rate and the solubilization of micellar phosphorus as detected by 31 P NMR. Together, these results suggested that aggregation of caseins by calcium and colloidal calcium phosphate is mainly responsible for the excess hydration in skim milk micelles.

Improvement of rheological properties of firm acid gels by skim milk heating is conserved after stirring

The enhancement of the strength of set acid gels by heating milk was related to rheological parameters (water retention capacity, storage modulus) of corresponding stirred gels. To obtain accurate rheological data from stirred gel it was necessary to maintain a constant granulometry of gel particles and to recognize time after stirring as a contributing factor. Two hours after stirring, the gel exhibited a higher storage modulus when milk was heated above 80 degrees C. A measurement of viscosity of just-stirred yoghurt was sufficient to predict correctly the quality of a stirred gel analysed by viscoelastic measurements. Increased resistance to syneresis of just-stirred gels was related to higher viscosity. The quantity of beta-lactoglobulin (beta-Ig) bound to casein micelles explains the improvement of these gel qualities. We have considered that the structure of the initial firm gel (mesostructure level) was conserved in fragments within the stirred gel. Consequently, the explanation given by various authors for the effect of heating milk on the properties of set gels can also be applied to stirred gels. The same mechanism, described in literature for structure formation of set gels from acidified milk is purposed to explain the role of heating milk on the recovery of gel structure after stirring. The beta-Ig association with casein micelles during heating favoured micelle connections during the acidification. It also favoured the association of gel fragments after stirring during the recovery in gel structure.

Handbook of Food Science and Technology 3: Food Biochemistry and Technology

This third volume in the Handbook of Food Science and Technology Set explains the processing of raw materials into traditional food (bread, wine, cheese, etc.). The agri-food industry has evolved in order to meet new market expectations of its products; with the use of separation and assembly technologies, food technologists and engineers now increasingly understand and control the preparation of a large diversity of ingredients using additional properties to move from the raw materials into new food products. Taking into account the fundamental basis and technological specificities of the main food sectors, throughout the three parts of this book, the authors investigate the biological and biochemical conversions and physicochemical treatment of food from animal sources, plant sources and food ingredients.

Handbook of Food Science and Technology 1: Food Alteration and Food Quality

This book serves as a general introduction to food science and technology, based on the academic courses presented by the authors as well as their personal research experiences. The authors’ main focus is on the biological and physical-chemical stabilization of food, and the quality assessment control methods and normative aspects of the subsequent processes. Presented across three parts, the authors offer a detailed account of the scientific basis and technological knowledge needed to understand agro-food transformation. From biological analyses and process engineering, through to the development of food products and biochemical and microbiological changes, the different parts cover all aspects of the control of food quality.