Effect of condensed tannin and fresh forage diets on the formation of indole and skatole in the rumen and on the pastoral odour and flavour of sheep meat : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand
Flavour is a factor that has a large influence on meat quality. Pastoral flavour that results from the grazing of pasture is an undesirable characteristic of meat flavour for consumers more accustomed to meat produced by grain and concentrate feeding systems. In New Zealand there is a reliance on grazing systems for sheep meat production, however the resulting meat flavour is one factor that impedes the increase of sheep meat exports to discerning markets. Correlation of chemical analyses to sensory evaluations of sheep meat has identified that a high concentration of indole and skatole in the fat is associated with pastoral flavours. Indole and skatole are formed in the rumen from the microbial fermentation of tryptophan. New Zealand pasture is high in protein, which is both highly soluble and rapidly degraded in the rumen. As such, pasture diets provide a rich and ready source of tryptophan for the formation of indole and skatole in the rumen. Condensed tannins are known to slow the degradation of protein in the rumen. Therefore, one of the objectives of this study was to establish if dietary condensed tannin can reduce the ruminal biogenesis of indole and skatole and consequently, ameliorate pastoral flavour in sheep meat. White clover usually comprises up to 30% of the botanical composition of pastures in New Zealand, is highly degradable in the rumen and likely to result in a high availability of tryptophan in the rumen for conversion to indole and skatole. Therefore, another objective of this study was to determine if feeding white clover gave a significant increase in the formation of indole and skatole in the rumen compared to perennial ryegrass and if this has an effect on pastoral flavour in meat. These hypotheses were tested using a series of in vitro rumen fermentations that incorporated the use of fresh forages (Chapter 3 and 6). In vivo experiments were utilised to assess rumen formation of indole and skatole with different forages (Chapter 4) and to assess effects of dietary condensed tannin (CT; Chapter 5 and 7). Meat and fat samples from lambs used in Chapters 5 and 7 underwent sensory evaluation to determine if forage or CT treatments were having an effect on the fat odour or meat flavour. From the in vitro and in vivo experiments of (Chapter 3, 4, 6 and 7) it was calculated that the formation of indole and skatole with perennial ryegrass is generally only 6-41% of that formed with white clover. A higher concentration of indole and skatole was also observed in the blood plasma of lambs that were fed white clover compared to those that were fed perennial ryegrass (Chapter 7) and white clover gave an overall more intense flavour in the meat. Comparison of forages fermented in vitro (Chapter 3) showed that with forage legumes of a higher CT concentration, such as Lotus pedunculatus (98 gCT kg-1 DM) and Dorycnium rectum (122 gCT kg-1 DM), the indole and skatole formed was only 7-21% of that formed with white clover. With forages of an intermediate CT concentration such as sulla (Hedysarum coronarium) and Lotus corniculatus the indole and skatole concentration formed was 53-68% of that of white clover. From in vitro rumen fermentation of mixtures of white clover and Lotus pedunculatus it was concluded that the CT in Lotus pedunculatus was not reacting with the protein in white clover. Fermentation of fresh white clover in the presence of an increasing concentration of added CT extract showed that at a higher CT, indole and skatole formation were reduced to low levels. It was inferred that this was due to optimal protein binding and the availability of free condensed tannin to bind other sources of protein, including rumen microbes. However, in vivo dosing with a CT extract resulted in only a small reduction in rumen indole and skatole concentration. This indicated that when CT was dosed into the in vivo rumen of lambs fed fresh forages the CT probably passed from the rumen before adequate protein release from the forage had taken place. Thus, in the grazing situation it will be optimal to provide CT in planta to maximise protein binding and this, in combination with the high CT concentration needed (approximately 80 gCT kg-1 DM), makes Lotus pedunculatus or Dorycnium rectum the prime candidates for further grazing trials into pastoral flavour amelioration using CT forages. Grazing Lotus corniculatus (40 gCT kg-1 DM) in a field experiment resulted in a lower rumen and blood plasma and fat concentration of indole and skatole in comparison to the grazing of perennial ryegrass/white clover pasture. However, a change in the pastoral odour of the fat was not perceived by the sensory panel when comparing fat samples from lambs that had grazed Lotus corniculatus and perennial ryegrass/white clover pasture. The concentration of skatole in the body fat was less variable in the lambs that had grazed Lotus corniculatus and resulted in no lambs with a high outlying concentration (>100 ng g-1) of indole and skatole. This finding holds some potential for reducing pastoral flavour for consumers sensitive to high indole and skatole concentration in the fat. When condensed tannin was dosed to lambs that were fed white clover or perennial ryegrass in the form of a grape seed extract the intermittent supply of CT slightly reduced indole and skatole formation in the rumen and reduced the plasma concentration of indole and skatole. Flavour assessment of meat from the lambs fed white clover or perennial ryegrass with or without CT suggested that CT reduced the intensity of pastoral flavours. However, there were minimal effects on indole and skatole concentration in the body fat. It was possible that other pastoral flavour related compounds derived from the degradation of amino acids, in addition to indole and skatole that were measured, were having an effect on the meat flavour. It was concluded that dietary condensed tannin is able to reduce the formation of indole and skatole in the rumen and can alter the sensory attributes of sheep meat including reducing pastoral flavours. A higher CT concentration present within the forage plant (approximately 80 g kg-1 DM) will be best to minimise indole and skatole formation in the rumen and reduce pastoral flavours in the meat. Further research is required to confirm this in the grazing situation. Feeding white clover results in a greater rumen biogenesis of indole and skatole compared to perennial ryegrass and therefore, may be the primary contributor to pastoral flavours when ruminants graze conventional pastures. Further research is required to evaluate the flavour attributes that result from feeding white clover to meat producing ruminants in the New Zealand grazing situation.