The influence of fermentation on extruded food products
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
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
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.