The effects of high hydrostatic pressure and protein concentration on the denaturation and gelation of whey protein were investigated. Industrial whey protein isolate (WPI) and whey protein concentrate (WPC) solutions (pH 6.8) at various concentrations were pressurized for 10 min at 30 degrees C under 200-1000 MPa. With the WPI solution, the concentration for affecting the turbidity was 1% and was 6% for the viscosity at 400 MPa, while for inducing gelation, it was 10% at 600 MPa. With the WPC solution, the viscosity changed at a concentration >12%, and gel formation began at >18% at 400 MPa. The hardness and breaking stress of pressure-induced WPI gels increased with increasing concentration of WPI (12-18%) and hydrostatic pressure, the ratings for the 20% WPC gels being one-third those of the 20% WPI gels. The solubility of proteins from the pressure-induced WPI gels decreased with increasing pressure, while that of WPC gel induced at >600 MPa remained constant at approximately 50%. The microstructure of the WPI gels had a porous network form, whereas the WPC gels were irregular particulates. beta-Lactoglobulin, alpha-lactalbumin, and serum albumin preferentially participated in pressure-induced aggregation and gelation through S-S bonding.

Milk fat globules (MFG) were reconstituted with milk fat globule membrane (MFGM) and milk fat (MF). Viscosity of the reconstituted MFG was highest at pH 5.0 and 4 min emulsifying, and rose with an increase of MFGM between 40-80 mg/g fat. Adsorbed protein/unit fat increased at acid pH with increase of MFGM. The composition of proteins adsorbed on the surface of MFG was not influenced by factors of reconstitution. The size and specific surface area of globules were influenced by emulsifying time, MFGM and MF concentrations, and pH. The size range of MFG prepared by standard method was 0.9-17 km in diameter. Median diameter was 5 p,m and specific surface area was 15,600 cm2/cm3 of emulsion.

Mucosal immunoglobulin (Ig)A dominance has been proposed to be associated with preferential class switch recombination (CSR) to the IgA heavy chain constant region, Cα. Here, we report that B cell activation in nasal-associated lymphoid tissue (NALT) upon stimulation with the hapten (4-hydroxy-3-nitrophenyl)acetyl (NP) coupled to chicken γ globulin caused an anti-NP memory response dominated by high affinity IgA antibodies. In the response, however, NP-specific IgG+ B cells expanded and sustained their number as a major population in germinal centers (GCs), supporting the view that CSR to IgG heavy chain constant region, Cγ, operated efficiently in NALT. Both IgG+ and IgA+ GC B cells accumulated somatic mutations, indicative of affinity maturation to a similar extent, suggesting that both types of cell were equally selected by antigen. Despite the selection in GCs, high affinity NP-specific B cells were barely detected in the IgG memory compartment, whereas such cells dominated the IgA memory compartment. Taken together with the analysis of the VH gene clonotype in GC and memory B cells, we propose that NALT is equipped with a unique machinery providing IgA-specific enrichment of high affinity cells into the memory compartment, facilitating immunity with high affinity and noninflammatory secretory antibodies.

A simple and rapid procedure for the preparation of milk fat globule membrane (MFGM)is proposed. The membranefragments released from bovine milk fat globules were recovered as MFGM by acidification at pH 4.8 and centrifugation (AC-MFGM). The yield, gross compositions, enzymatic activities, and electrophoretic properties of the resultant MFGM were compared with those of MFGMs recovered by ultracentrifugation (UC-MFGM) and by salting out (SA-MFGM). The different methods for recovering MFGMhad significant effects principally on the lipid content and protein composition of MFGM. Of the three MFGMs, AC-MFGM had a moderate lipid content, while UC-MFGM had the lowest and SA-MFGM the highest. High activities of marker enzymes for plasma membranein AC-and UC-MFGM were retained but not in SA-MFGM. Glycoproteins PAS-6 and-7 were preferentially released from UC-MFGM. The pH was a factor in causing the release of these glycoproteins. The release of PAS-6and-7 was also ascertained by the decrease in UC-MFGM of proteins extractable with 1 m KC1and 8murea. The yield of MFGM was influenced mostly by the smaller fat globules and releasable protein and, consequently, was low in UC-MFGM, moderate in AC-MFGM, and high in SA-MFGM. Acidification to the isoelectric point was the easiest method for recovering MFGM, and resultant AC-MFGM had advantages over both the UC-and SA-MFGMs. in the presence of sodium dodecyl sulfate (SDS-PAGE).3~6) From a comparison of the 2845 electrophoretic profiles of MFGM polypeptides in the literature5~12) (see also refs. 3 and 4 for reviews), it is evident that there are differences in the relative amounts of major polypeptides. In particular, glycoproteins PAS-6 and-7 (=CB-7 and-8),5) corresponding to components 15 and 16 of Mather and Keenan,7) were major components as well as PAS-5 (=CB-5) in any MFGMprepared by salting out,5'10'11} but were less than PAS-5 in MFGMpelleted by ultracentrifugation.7'9) The compositional heterogeneity of the total MFGMin part has been due to different preparative methods.

A procedure for the reconstitution of milk fat globules (MFG) stabilized with milk fat globule membrane (MFGM) was developed. MFG was reconstituted by homogenizing a mixture of 1% MFGM and 25% milk fat at 45°C and at oH 7.0 for 1 min. The emulsifying properties of MFGM were evaluited by emulsifying activity (~A~,'en&lsion stability (ES), whippability and foam stability. Of the variables affecting the reconstitution of MFG, prolonged homogenization decreased EA and ES. About 25% milk fat gave maximal EA and ES, increasing the MFGM concentration increased both EA and ES, which were also influenced by the pH level. Foam disappeared at ~30°C. INTRODUCTIONMILK FAT exists as an immiscible emulsion of liquid fat in the aqueous phase of milk plasma (Brunner, 1974). The fat particles take the form of finely divided spheres and are stabilized by a third phase oriented at the fat surface, which is referred to as the milk fat globule membrane (MFGM). MFGM is originally derived from the apical plasma membrane of the secretory cell of the lactating mammary gland. The apical plasma membrane enveloping milk fat droplets in the apical region of the cells is discharged into the lumen, and part of the membrane seems to be modified there. Milk fat globules enveloped with the membrane consist of MFGM and core lipid accounting for about 98% of the milk triacylglycerol. Studies of MFGM have been conducted by many investigators, as can be seen in the recent reviews (Patton and Keenan, 1975;Kanno, 1980;