Bovine beta-Lactoglobulin (BLG) was cleaved by BNPS-skatole (2-(2'-nitrophenylsulfenyl)-3-methyl-3'-bromoindolenine), trypsin, or pepsin in 40% ethanol before emulsification with hexadecane in order to characterize the peptides active at the interfaces. The total digests and the different phases obtained after emulsification were analyzed by RP-HPLC to separate the peptides according to their gradual order on a hydrophilicity-to-hydrophobicity scale. In each case, hydrophobic peptides were recovered in the creamed phase and characterized by mass spectrometry and sequencing. After tryptic hydrolysis, short peptides were identified at the interfacial layer as fragments S21-L32, V41-L57, V41-K60, and W61-K70 linked to L149-I162 by a C66-C160 bond. It indicates that the hydrophilic/hydrophobic distribution of the amino acids in the sequence of the fragments is more relevant to adsorption than the length of the peptide. BNPS-skatole and peptic hydrolysis produced larger hydrophobic peptides which were also recovered in the creamed phase of the emulsion and characterized.
(Reçu le 26 juin 1989; accepté le 14 janvier 1991) Résumé -L'étude porte sur l'influence du polymorphisme génétique de la~-lactoglobuline (~-Lg) et de la x-caséine (K-Cn) sur la composition du lait et ses aptitudes à la coagulation et à la transformation fromagère. Quatre-vingt-trois fabrications de type Camembert au lait cru ont été réalisées au laboratoire à partir du lait de 230 vaches Pie Noire. Huit combinaisons génétiques de la~-Lg et de la K-Cn ont été étudiées; les mesures ont porté sur la composition physico-chimique des laits de fabrication, caillés et sérums, les paramètres d'aptitude à la coagulation mesurés à l'aide d'un Formagraph et les bilans et pourcentages de récupération des matières lors des fabrications. L'influence positive du génotype BB et de la~-Lg est mise en évidence essentiellement sur le taux de caséines du lait (+ 4,7% par rapport au génotype AA) et sur la rétention des matières azotées totales dans le caillé (+ 2,9%). Le génotype BB de la K-Cn exerce des effets bénéfiques plus marqués, notamment sur le temps de coagulation des laits et la fermeté maximale des gels (respectivement -24% et +37% par rapport à l'homozygote AA), expliqués notamment par une composition protéique plus favorable. Il en résulte une meilleure récupération des constituants du lait dans les caillés BB en K-Cn. Le génotype AB occupe une position intermédiaire. L'analyse factorielle discriminante souligne le caractère additif de l'interaction entre les variants gé-nétiques de la~-Lg et de la K-Cn. Un classement de qualité fromagère est proposé, le double homozygote BB en~-Lg/BB en K-Cn occupant la place la plus favorable. variant génétique / x-casèlne 1~-Iactoglobuline 1 aptitude fromagère Summary -Influence of genetic variants of~-Iactoglobulin and x-caseln on milk composition and cheese-making capacity.
Bovine beta-Lactoglobulin (BLG) was cleaved by BNPS-skatole (2-(2'-nitrophenylsulfenyl)-3-methyl-3'-bromoindolenine), trypsin, or pepsin in 40% ethanol before emulsification with hexadecane in order to characterize the peptides active at the interfaces. The total digests and the different phases obtained after emulsification were analyzed by RP-HPLC to separate the peptides according to their gradual order on a hydrophilicity-to-hydrophobicity scale. In each case, hydrophobic peptides were recovered in the creamed phase and characterized by mass spectrometry and sequencing. After tryptic hydrolysis, short peptides were identified at the interfacial layer as fragments S21-L32, V41-L57, V41-K60, and W61-K70 linked to L149-I162 by a C66-C160 bond. It indicates that the hydrophilic/hydrophobic distribution of the amino acids in the sequence of the fragments is more relevant to adsorption than the length of the peptide. BNPS-skatole and peptic hydrolysis produced larger hydrophobic peptides which were also recovered in the creamed phase of the emulsion and characterized.
A comparative study of various procedures for tryptophanyl peptide bond cleavage by BNPS-skatole [2-(2-nitrophenyl)-3-methyl-3-bromoindolenine] was carried out on native and on reduced and alkylated bovine beta-lactoglobulin (BLG). The reaction yield and the composition of the derived products were studied in acetic acid, trifluoroacetic acid (TFA), and ethanol/TFA. For BNPS-skatole removal, extraction by water or ethyl ether was compared with dialysis and gel filtration. The three expected peptides (1-19, 20-61, 62-162) and incomplete cleaved fragments (1-61, 20-162) were separated and characterized by electrophoresis, reverse-phase high-performance liquid chromatography, and mass spectrometry. The highest hydrolysis yield (67.4%) occurred with native BLG cleaved in 88% acetic acid at 47 degrees C for 60 min. Subsequent water extraction and gel filtration led to total recovery of the material, but reagent elimination was only quantitative after gel filtration. Cleavage specificity was ensured by mass spectrometry and the amino acid composition of peptides 1-19 and 62-162. The chemical side reactions identified are discussed.
The foaming properties of bovine beta-lactoglobulin (BLG), BNPS-skatole (2-(2'-nitrophenylsulfenyl)-3-methyl-3'-bromoindolenine) (BNPS), trypsin (T) and pepsin in 25, 30 and 40% ethanol (P25, P30, P40) hydrolysates were investigated in the 0.2 to 1 mg/ml range. Foaming capacity and foam stability were assessed in terms of drained liquid volume and foam volume. Foam texture was analyzed from video images obtained during foam decay. The foaming capacity of BNPS, P30 and P40 was similar to that of BLG and greater than that of T or P25. All hydrolysates except BNPS were less stable than BLG at all concentrations tested. This result was insured by texture analysis. Principal component analysis confirmed the distribution of the samples into three groups based on their increasing stability: (i): P25, (ii): P30 and P40, and (iii): BLG and BNPS. Tryptic hydrolysate had the poorest foaming properties. The results are considered in relation to the molecular characteristics of the peptides, particularly their size and hydrophobicity.
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