Determination of Protein Digestibility in Atlantic Cod (Gadus morhua) with Internal and External Indicators

Abstract

Chromium(III)-oxide added to feed was determined by flame atomic absorption spectrophotometry in the same Kjeldahl digest as protein (N × 6.25). The recovery was 98.0% compared with only 40.0% using nitric/perchloric acid digestion. Samples of apparent indigestible protein in Atlantic cod (Gadus morhua) could be collected from the middle and distal part of the intestine 24 h after feeding a diet based on fillets of saithe (Pollachius virens). The level of reference substance and the apparent digestibility of protein indicated that feces samples from the rectum representing the feed could not be obtained until 72 h after the first feeding. Water-insoluble protein from the intestine and rectum accounted for 35–47% and 27%, respectively, of the total dry matter protein. Postdefecation loss of water-soluble nitrogenous components would therefore cause overestimation of protein digestibility. Chromium(III)-oxide and titanium(IV)-oxide were equivalent as external indicators. With titanium(IV)-oxide as indicator substance, calcium, iron, and zinc present in the feed were examined as internal reference substances. The gastric ratios of protein over calcium or zinc were compared with the corresponding ratios in the distal part of the intestine. The apparent protein digestibility agreed with the corresponding estimates obtained by titanium(IV)-oxide. Estimates of protein digestibility from capelin (Mallotus villosus) predating cod caught at sea using the gastrointestinal levels of calcium and zinc as internal reference substances corresponded well with estimates from experiments with cod fed whole capelin with titanium(IV)-oxide as external reference substance. The estimates indicate that samples of feces should be taken from the distal part of the intestine excluding the rectum. Because of large differences between individuals, a sufficient number of fish should be included in digestibility studies.Key words: cod, protein digestibility, digestibility indicators, feces sampling