Ultimate pH and storage related changes to sheepmeat odour and flavour

Abstract

Gas chromatography/mass spectrometry (GC/MS), gas chromatography/olfactometry and a trained sensory panel were used to study the effects of meat ultimate pH on the odour and flavour of cooked sheepmeat. As part of methods development, supercritical carbon dioxide elution was evaluated as an alternative to thermal desorption of volatile compounds from Tenax-T A traps, but this technique did not offer any significant advantage. In a separate study, the sensory panel and an emerging technology, an electronic nose, were used to investigate the changes in cooked meat odour and flavour arising from long term storage of chilled lamb legs in a carbon dioxide atmosphere.

Pre-slaughter injection of adrenalin was used to produce sheepmeats of normal (5.66), moderate (6.26) and high (6.81) ultimate pH. Meat with a moderate or high ultimate pH had a significantly lower overall cooking odour and flavour intensity, as assessed by the sensory pane, than sheepmeat of a more acceptable pH (5.66). Panellists also found that desirable odour and flavour notes decreased and undesirable ones increased as ultimate pH increased. Purge-and-trap GC/MS of fat rendered during cooking identified 57 (out of a total of 325) volatile compounds that significantly decreased in concentration with increasing meat pH. The most common of these compounds were aldehydes. GC/olfactometry identified 54 odour-active compounds, 10 of which were also found to be responsive to changes in meat ultimate pH. Most of these compounds were again aldehydes.

High ultimate pH raw meat which was titrated with hydrochloric acid to pH levels close to that of low ultimate pH had odour and flavour intensities comparable to those of normal (low) ultimate pH meat. Most of the volatile compounds found by GC/MS analysis to decrease in concentration with increasing ultimate pH did not have a reduced concentration after this downward pH adjustment. These results and parallel analyses of soluble protein, free amino acids and free fatty acids led to the conclusion that pH-related changes in the odour and flavour of cooked sheepmeat are probably mediated by direct pH effects during the cooking process, rather than by a pH-related development of odour and flavour precursors in raw meat before cooking begins.

Meat pH values of lamb legs increased significantly between 4 and 14 weeks storage in the CO₂ aging. Sensory panellist found there were significant storage time effects in “overall”, “sheepmeat”, and “sweet” odours of cooked meat. Panellists also detected a significant decrease in “sweet” and “roasty” flavours in the CO₂-packed lamb legs stored at -1.5°C compared with vacuum packed controls stored frozen (-35°C). Of particular concern, after 14 weeks chilled storage in CO₂, the intensity of a “livery/offaly” flavour was significantly higher than for the frozen-stored control.

The electronic nose could detect differences in headspace volatiles of both raw and cooked minces from meat subjected to the different storage treatments. The results suggest that, under these experimental conditions, the electronic nose was more discerning and possibly gave more reproducible results than sensory panellists. What relationship the electronic nose’s measurements have to human perception of odour has yet to be determined.

The multidisciplinary approach used in this thesis has helped to explain some of the variations in meat flavour quality important to the New Zealand meat industry.