Pastoral flavour in meat products from ruminants fed fresh forages and its amelioration by forage condensed tannins

摘要

Pastoral flavour which occurs when animals graze pasture can have a negative impact on the consumer acceptance of sheep meat. Several options are available for amending pastoral flavour in sheep meat although the utilisation of condensed tannin (CT) forages holds the most potential. The formation of pastoral flavour compounds in the rumen depends on the forage consumed by the ruminant and is linked to differences in forage protein solubilisation and rumen degradability. White clover (Trifolium repens), with its rapidly solubilised and degraded protein, consistently results in a greater formation of the pastoral flavour compounds, indole and skatole, in the rumen. As such, white clover may be the primary contributor to pastoral flavour when animals graze New Zealand (NZ) pastures (which generally contain 5-30% white clover). Although CT-containing legumes contain a similar crude protein content to white clover, the formation of indole and skatole in the rumen is considerably less with these forages. The CT slowed the rumen degradation of forage protein, limited the availability of amino acids in the rumen for conversion to indole and skatole and other flavour compounds and consistently reduced the concentration of indole and skatole in the blood plasma. A higher CT concentration (70-90g/kg DM) and the provision of CT within a forage diet (rather than exogenously) were most effective for reducing rumen indole and skatole formation. Grazing the CT forage Lotus corniculatus L. lowered indole and skatole concentration in body fat compared to grazing perennial ryegrass (Lolium perenne)/white clover pasture, and supplementing lambs with an exogenous CT in the form of a grape seed extract reduced pastoral meat flavour but did not reduce fat indole and skatole concentration. Further studies are needed to investigate the factors that influence the deposition of blood indole and skatole into body fat and to define the length of time that a high CT legume needs to be fed before slaughter to reduce fat indole and skatole to ameliorate pastoral meat flavour.