The use of artificial flavourings and colours is common in the production of extruded snack food products. The problem with this process is two-fold, firstly the flavour is only present on the exterior of the product and secondly there is the risk of contaminating the product. The latter of these two problems is heightened by the fact that there is usually no further preservation treatment (heat, pressure, radiation) after flavouring. The aim of this study was to investigate the use of fermentation as a means of introducing both colour and flavour in extruded products. Model systems based upon a simplified traditional sourdough rye bread, was developed in order to study the influence of fermentation on the physical and chemical properties of extruded products. Using both the dry and liquid feeds, various fermented mixtures were extruded under a standard set of process conditions (feed rate 20 Kg/Hr, screw speed 200 rpm, feed moisture content 17% and die aperture 4mm). The fermentation process, although started with bakers yeast was greatly influenced by the establishment of a population of lactic acid bacteria from the natural microflora of the cereal flours. The total titratable acidity (measured as lactic acid) and pH of the slurries extruded via the liquid feed system were found to be in the ranges 0.73-1.28 % (dry basis) and 4.84-5.39 respectively. Levels of acidity and pH in samples extruded via the dry feed were determined to be in the ranges 0.77-1.67 % and 4.92-5.41 respectively. Increases in the fermentation time and hence levels of acidity, produced changes in the colour of the extrudates. Changesin the pressuret,o rque,s crews peed,a nd productt emperatured uring extrusionw ere also affected by changes in the fermentation process. This in turn was reflected in the physicalp roperties( expansiond, ensitya ndb reak strength)o f the extrudate. Analysis of the compounds found in the headspace of the fermented samples, using gas chromatography/mass spectrometry (GCMS), identified 97 compounds including alcohols, esters and carbonyl compounds. Concentrations of these compounds varied between 3 and 5012 ppb, with a number of compounds found to be present at levels well above their reported aroma thresholds. Retention of the identified compounds in the extrudate was found ix to be low however, samples produced from lower moisture dough systems showed retention of a number of compounds at levels known to be detectable by the human senses. Factors influencing the retention of the aroma compounds appear complex and could not be directly attributed to changes in the fermentation processes. Future studies of the effects of fermentation on the starch and protein fractions at a molecular level were suggested. It was concluded that, fermentation could present opportunities for the development of novel expanded snack products that avoid the pitfalls of current flavouring processes. X