Spin polarized tunneling at finite bias.

A mesoscopic spin valve is used to determine the dynamic spin polarization of electrons tunneling out of and into ferromagnetic (FM) transition metals at finite voltages. The dynamic polarization of electrons tunneling out of the FM slowly decreases with increasing bias but drops faster and even inverts with voltage when electrons tunnel into it. A free-electron model shows that in the former case electrons originate near the Fermi level of the FM with large polarization whereas in the latter, electrons tunnel into hot electron states for which the polarization is significantly reduced. The change in sign is ascribed to the matching of the electron wave function inside and outside the tunnel barrier.

In vitro fermentation of swine ileal digesta containing oat bran dietary fiber by rat cecal inocula adapted to the test fiber increases propionate production but fermentation of wheat bran ileal digesta does not produce more butyrate.

This experiment evaluated three hypotheses: i) production of propionate is increased during fermentation of substrate containing oat bran (OB)(6); ii) production of butyrate is increased during fermentation of substrate containing wheat bran (WB) and iii) results of in vitro fermentations using physiological substrates and inocula agree with in vivo data. Ileal digesta collected from swine fed OB and WB were the substrates. Digesta was fermented for 0-96 h in an anaerobic in vitro system using inocula prepared from ceca of rats fed the same fiber sources. Carbohydrate and short-chain fatty acid (SCFA) contents in the fermentations were measured by gas chromatography. Fermentation of WB digesta did not produce more n-butyrate (P > 0.05) and was significantly slower (P < 0.05) than fermentation of OB digesta. OB digesta fermentation produced a significantly greater (P < 0.05) molar proportion of SCFA as propionate. Bacterial mass increased more and was maintained longer during fermentation of OB digesta than the WB digesta. Our results indicate that dilution of undigested WB fiber and not n-butyrate production is one mechanism by which WB may protect colonic mucosa; propionate production is increased during fermentation of beta-glucan in OB; and an in vitro system using physiological sources of inoculum and substrate containing WB and OB yields results that agree with in vivo findings in humans and rats.

Fermentation of carbohydrate in rat ileal excreta is enhanced with cecal inocula compared with fecal inocula.

The differential fermentative capacities of microflora from two regions of the large bowel and how fermentation was altered by prior exposure of the microflora to the substrate to be fermented were studied using an in vitro fermentation system. Bacterial inocula were prepared from cecal contents and feces from three groups of rats fed purified diets containing 100 g/kg dietary fiber from canned peas or psyllium seed husk, or a nonpurified diet containing 170 g/kg dietary fiber. The substrate for all fermentations was ileal excreta from colectomized rats fed a purified diet containing 100 g/kg dietary fiber from canned peas. Anaerobic fermentations were conducted for 0, 3, 6, 12, 24, 48 and 96 h. Sugars of the unfermented polysaccharides were measured by gas chromatography following acid hydrolysis; disappearance was the measure of fermentation. Independent of inoculum source, > 90% of the starch and arabinose and 75% of the uronic acids, but < 30% of non-starch glucose (the measure of cellulose), were fermented by 24 h. Cecal inocula fermented arabinose and uronic acids more quickly (P < 0.05) and fermented more (P < 0.05) non-starch glucose than fecal inocula. Inocula adapted to psyllium seed husk fermented starch faster (P < 0.05) and non-starch glucose, arabinose and uronic acids more slowly (P < 0.05) than inocula adapted to peas or nonpurified diet. Bacterial efficiency of carbohydrate utilization, the increase in muramic acid/mole carbohydrate fermented, was greatest (P < 0.05) with cecal inocula adapted to peas. We conclude that using cecal microflora as the inoculum source provides a more accurate index of fermentation during transit through the large bowel and that noncellulosic and storage polysaccharides of the plant cell wall are utilized before cellulose.

Rat cecal inocula produce different patterns of short-chain fatty acids than fecal inocula in in vitro fermentations.

The effects of bacterial collection site, bacterial adaption to substrate and duration of fermentation on short-chain fatty acid (SCFA) production were studied using an in vitro fermentation system. Cecal and fecal inocula from rats fed purified diets containing 100 g/kg dietary fiber from canned peas or psyllium seed husk, or a nonpurified diet containing 170 g/kg dietary fiber fermented ileal excreta from colectomized rats fed a purified diet containing 100 g/kg dietary fiber from canned peas. The SCFA concentration, measured by gas chromatography, in anaerobic fermentations of 0, 3, 6, 12, 24, 48, 72 and 96 h, increased (P < 0.05) through 72 h. Compared with fecal inocula, cecal inocula produced SCFA at greater (P < 0.05) initial rates, more (P < 0.05) total SCFA and a greater (P < 0.05) proportion as n-butyrate. Inocula from rats fed the pea or nonpurified diets produced SCFA at greater (P < 0.05) molar proportions as acetate and less (P < 0.05) as propionate than inocula from rats fed psyllium seed husk at most time points. We conclude that collection site, adaptation of bacteria to the substrate to be fermented and duration of fermentation significantly influence the results of in vitro fermentation studies.

Determination of fermentable carbohydrate from the upper gastrointestinal tract by using colectomized rats.

The primary aim of this study was to characterize the carbohydrate that would be supplied to the colon for fermentation under physiological conditions. Colectomized rats were fed fiber-free diets or diets containing 5% (wt/wt) gum arabic. Four (fucose, galactose, glucosamine, and galactosamine) of 11 analyzed sugars accounted for 77% of the total sugar in ileal excreta from colectomized rats fed fiber-free diets. The three sugars in gum arabic, rhamnose, arabinose, and galactose, accounted for 84% of the total sugars in gum arabic ileal excreta. Comparisons of the sugar compositions of the ileal excreta, the water-soluble fractions of the excreta, and three gel filtration fractions of the water-soluble material with those of the water-soluble fraction of rat mucosa, the acetone-soluble fraction of pancreas, and pancreatin suggested that the major source of endogenous carbohydrate is mucin. Gum arabic increased the daily excretion of the four mucin-derived sugars (fucose, galactose, glucosamine, and galactosamine) by the colectomized rats from 473 mumol per day to 634 mumol per day. We conclude that mucin is the major endogenous carbohydrate excreted from the upper gut and that gum arabic increases the amount of this endogenous carbohydrate.