Protein Digestibility in Rats Determined with TiO2and Cr2O3, and Comparisons with a Rapid Enzyme Test

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Abstract
TiO2 was reexamined as an indicator substance in protein digestibility studies with rats. This was necessary partly because of new analytical techniques and partly because it had been found unsuitable with fish as experimental animals. Comparisons with the conventional balance sheet method using fish meals or fish flours as protein sources showed good agreement, but the sensitivity of the Ti-determination was not satisfactory. Cr2O3 could conveniently be determined by atomic absorption spectrophotometry in the Kjeldahl digests, a comparison was made between TiO2 and Cr2O3. The 2 indicators gave comparable digestibilities. When analytical sensitivities and amounts of work involved were taken into account, Cr2O3 was a more convenient indicator substance than TiO2. In some of the experiments, relative digestibilities also were determined by a rapid in vitro enzyme test in which a decrease in pH (.DELTA.pH) was measured. There was a negative correlation between apparent recovery of ingested N in the feces (ARN) and .DELTA.pH. This method warrants further study.