pH-Induced cold gelation of whey proteins is a two-step process. After protein aggregates have been prepared by heat treatment, gelation is established at ambient temperature by gradually lowering the pH. To demonstrate the importance of electrostatic interactions between aggregates during this latter process, beta-lactoglobulin aggregates with a decreased iso-electric point were prepared via succinylation of primary amino groups. The kinetics of pH-induced gelation was affected significantly, with the pH gelation curves shifting to lower pH after succinylation. With increasing modification, the pH of gelation decreased to about 2.5. In contrast, unmodified aggregates gel around pH 5. Increasing the iso-electric point of beta-lactoglobulin via methylation of carboxylic acid groups resulted in gelation at more alkaline pH values. Comparable results were obtained with whey protein isolate. At low pH disulfide cross-links between modified aggregates were not formed after gelation and the gels displayed both syneresis and spontaneous gel fracture, in this way resembling the morphology of previously characterized thiol-blocked whey protein isolate gels (Alting, et al., J. Agric. Food Chem. 2000, 48, 5001-5007). Our results clearly demonstrate the importance of the net electric charge of the aggregates during pH-induced gelation. In addition, the absence of disulfide bond formation between aggregates during low-pH gelation was demonstrated with the modified aggregates.

No abstract

It is now well established that genetically obese rodents have an increased ability to convert dietary energy to body energy relative to their lean counterparts (1). Alterations in energy metabolism, as indicated by decreased rates of oxygen consumption, are observable in obese (ob/ob) mice by 3 weeks of age (24); however, few studies have focused on younger obese mice. Marked changes in oxygen consumption of obese and lean mice have been reported at the time of weaning. Oxygen consumption per g body weight decreased by 25-50% within the first several days after weaning (3, 4). These studies and others (5-9) indicate that alterations in energy metabolism may appear very early in the life of obese mice.The present study was designed to monitor oxygen consumption, an indirect indicator of energy expenditure, of lean and obese mice from birth to 112 days of age; major emphasis was focused on developmental changes prior to 21 days of age. Body fat content of lean and obese mice at 7 and 14 days of age was also determined.Materials and methods. Heterozygote breeding mice (C57BL/6J ob/+)2 were housed in solid-bottom cages with wood shavings for bedding. They were fed a stock diet3 ad libitum. Ambient temperature was 25 f 2". Pregnant dams were removed from the breeding cages and placed in separate similar cages. Litter size was routinely standardized to six pups per litter; however, in a few instances litters with five or seven pups were utilized. Pups were weaned at 21 days of age unless indicated otherwise.In the first experiment pups were individually identified at 1 day of age. From 1 to 19 days of age daily oxygen consumption measurements and body weights were obtained on individual pups. Litters were also killed at 7 and 14 days of age. Food residue was removed from the stomach and the carcasses were homogenized in an equal weight of water. Carcass fat was extracted with chloroform/methanol (2: 1) and quantitated gravimetrically .Oxygen consumption measurements and body weights of pups on days 18-20 and again on days 22-24 of age were obtained in the second experiment. Only lean mice were used. In one treatment pups remained with the dam; in the other treatments pups were weaned to a high-carbohydrate or a high-fat diet at 21 days of age. The high-fat diet was formulated to approximately simulate mouse milk on a protein-energy basis (10) and contained per 100 g diet: 43.5 g casein, 32.0 g tallow, 5.0 g corn oil, 9.6 g glucose, 5.0 g cellulose, 4.0 g mineral mix (1 I), 0.4 g vitamin mix (ll), 0.2 g choline chloride and 0.3 g methionine. The high-carbohydrate diet was formulated by replacing the tallow with glucose on an equal-energy basis.In the last experiment oxygen consumption measurements and body weights were recorded weekly from 3 weeks of age to 16 weeks of age. Lean and obese male and female mice were utilized.The apparatus described by Watts and Gourley (12) was utilized to obtain estimates of oxygen consumption in the mice. Food was available to the mice until the oxygen consumption measurements commence...

The ozone ultraviolet spectrum has been re-examined at low temperature to clarify features of photochemical interest. The origin of the system is a diffuse band at 28 450 cm−1, the long wavelength limit of band structure in the cold spectrum. The bands toward higher frequencies are identified as sequences of the upper state bending (333 cm−1) and symmetrical stretching (600 cm−1) modes. This band structure converges to a limit near 32 400 cm−1, consistent with the energy required to form O(1D) plus O2(1Δ). Both the shading of the bands and sequences which appear suggest that the Huggins bands are that part of the Hartley-Huggins system which lies below the dissociation limit, the system being a single transition 1B2← X 1A1. The component of the continuum due to excitation of the ν010″ bending mode is shown to be similar to that arising from the ν000″ ground state, except for a shift toward lower energy equal to the vibrational excitation energy. A rationale for predicting the effect of temperature on the shape of the continuum is presented.

Experimental evidence obtained in this study verifies reciprocal relations for two nonassociating ternary systems within the limits of experimental error. In addition, it demonstrates the applicability of hydrodynamic theory to multicomponent liquid diffusion. These conclusions are based on the fact that good agreement between the diffusion and phenomenological coefficients obtained by optical methods and those obtained from friction coefficients was found for the nonassociating systems studied.Hopefully experimental techniques will be developed in the future which will enable measurement of quaternary or higher diffusion coefficients, Dij, so that those calculated from friction coefficients, ui, can be checked. Since hydrodynamic theory requires fewer independent friction coefficients than independent diffusion coeficients to describe diffusion in systems of more than three components, such experimental techniques would be extremely valuable in checking the applicability of hydrodynamic theory to such systems.A study of the geometric and structural isomerization rates of chemically activated cis-1,2-dimethylcyclopropane formed by singlet methylene radical addition to the double bond of cis-2-butene is reported. The singlet methylene radicals were produced by diazomethane photolysis a t 4358 and 3660 d in the presence of added oxygen. The structural isomerization rate was determined by an internal comparison method that eliminates uncertainties due to pentene decomposition. The rates are in excellent agreement with RRKM theory calculations at an adjusted energy for each photolysis wavelength. The energies carried into the activated molecule by the methylene radical were 113.0 and 116.8 kcal/mol at 4358 and 3660 A, respectively. These values suggest that the methylene radical carries into the addition reaction a relatively small fraction (0.30) of the total excess energy available from its formation reaction. Energies determined in this work, applied to related systems, give results that differ by at most -3 kcal/moI from earlier determinations.