ISO 6655:1997

Overview

ISO 6655:1997 specifies an analytical method for determining the soluble nitrogen content of animal feeding stuffs after treatment with pepsin in dilute hydrochloric acid. The procedure measures nitrogen solubilised by in vitro pepsin digestion and reports results as soluble nitrogen (g/kg) or converted to soluble crude protein (factor 6.25). The standard clarifies that results do not distinguish protein nitrogen from non‑protein nitrogen and are not a direct measure of in vivo digestibility.

Key topics and technical requirements

• Principle: Incubate a prepared feed test portion with a pepsin solution in dilute HCl, filter, then determine nitrogen in the filtrate or in the residue by the Kjeldahl method (see ISO 5983).
• Reagents: Use analytical‑grade reagents and low‑nitrogen water. Key specifications include dilute HCl c(HCl) = 0.075 mol/L and pepsin with activity 2.0 units/mg; a pepsin working solution of about 400 units/L is prepared immediately before use.
• Procedure highlights:
  • Test portion ~2 g in 500 mL flask with 450 mL pepsin solution pre‑heated to 40 °C.
  • Maintain pH