Abstract
Sourdough is simply fermented ground cereal and water, but in actuality it contains a complex ecosystem of microorganisms, mainly yeast and lactic acid bacteria (LAB). The metabolic activity of the microorganisms is used to not only leaven dough, but to produce a complex array of volatile and non-volatile flavour compounds that influence the sensory properties of the final bread product. The initial sourdough culture and the processing parameters applied in the production of sourdough bread products influences not only the diversity of microorganisms present but also their metabolic activity and flavour generation during fermentation. A wide range of volatile compounds have been identified in sourdough, including acids, alcohols, aldehydes, esters, ketones, lactones, hydrocarbons, pyrroles, pyrazines and sulfur compounds. Typically, the study of non-volatile compounds has focused on the key measures of acidity, pH and total titratable acidity (TTA) and the production of lactic and acetic acid, although a greater variation in organic acids have recently been identified. It is due to this complex flavour profile that consumer interest in sourdough bread has experienced a recent surge. However, a holistic understanding of sourdough flavour is lacking and the differences between sourdough cultures and the impact of different fermentation conditions on flavour perception are not well understood.
The overall objective of this thesis was to characterise the variation in the flavour compounds produced in bread made using different activated wholemeal sourdough starters and to relate these to the sensory perception of sourdough bread flavour. The first study characterised the volatile compounds and physicochemical parameters in the bread crumb produced using wholemeal sourdough starters. The second study related perceivable differences in wholemeal sourdough breads found using the free sorting sensory method to instrumental measures of flavour. The final study investigated the effect of fermentation temperature on the production of flavour related compounds of doughs and bread crumb produced using wholemeal sourdough starters and to relate these to the sensory properties obtained using flash profiling.
Volatile profiles of breads produced using 12 different sourdough starters were characterised using headspace solid phase microextraction (SPME) with gas chromatography mass spectrometry (GC-MS) and related to the mass ion identified from proton transfer reaction mass spectrometry (PTR-qMS). Key acidity measures were determined (pH, TTA and lactic acid), along with the size related measures (height, width, bake loss %) and colour measurements (L*, a* and b*), and related to the volatile profiles of the breads using multiple factor analysis (MFA). Three clusters of sourdoughs were identified related to their volatile profiles. One cluster was characterised by ethanol, 2-methyl-1-propanol, 3-methyl-1-butanol, phenylethyl alcohol, 2,3-butanedione and acetaldehyde, which have all been associated with yeast fermentation and homofermentative LAB fermentation. Another cluster was associated with acetic acid, ethyl acetate, hexyl acetate and 1-hexanol, that have all been related to heterofermentative LAB fermentation. The final cluster was characterised by those volatiles related to fermentation of lipids by yeasts, such as gamma-pentalactone, gamma-octalactone and gamma-nonalactone or lipid oxidation, such as 1-octen-3-ol, (E,E)-2,4-decadienal and (E,E)-2,4-nonadienal. These results indicated that the dominant microorganisms differ between the sourdough starters by the end of dough fermentation.
These same 12 sourdough cultures were then used to investigate the relationship between the flavour compounds identified instrumentally in the bread crumb, using SPME-GC-MS for the volatile fraction and high performance liquid chromatography (HPLC) for the non-volatile organic acids, with the perceived flavour of the sourdough breads identified using the free sorting sensory method. The free sorting method was successful in differentiating the sourdoughs based on their sorted groups, but identifying the key sensory variations was difficult due to the large number of descriptors applied to the samples by the sensory panellists (391 distinct terms). Textual processing reduced the number to 59 distinct terms, but semantic interpretation was difficult. When related to the instrumental flavour results, there were clear associations between a number of descriptors and instrumental measures, such as the volatiles 2,3-butanedione, acetoin, ethyl lactate and 2-methylbutanoic acid were related to the descriptors butter and sour cream; and the volatile isobutyl acetate related to the descriptor fruity. It was not possible to relate the most frequently used descriptors, including those relating to sourness intensity (low sour, medium sour and high sour), to the instrumental measures of flavour and they did not clearly discriminate between the samples. This indicated that key flavour characteristics were indiscriminate and that a more structured intensity scaling sensory method was required to understand sourdough flavour.
Four of the sourdough cultures were then used to investigate the influence of fermentation temperature on the production of flavour related compounds and relate them to the sensory perception of flavour using the flash profile method. In addition to SPME-GC-MS and HPLC, PTR-ToF-MS was used for real-time monitoring of volatile production during dough fermentation at 15°C, 25°C and 35°C. PTR-ToF-MS demonstrated that the production rate of volatiles varied during fermentation and highlighted that peak volatile production varied with the temperature applied, with the maximum rate of production for 25°C and 35°C samples located between 9 to 13 hours. Temperature had a greater significant impact on the production of flavour compounds than the sourdough type with a clear separation of the 15°C samples from the higher temperature samples. The flash profile method was successful in also distinguishing the breads fermented at different temperatures with the same temperature related clusters.
This research highlights that even though the sourdough starters impact on the flavour compounds produced during fermentation, the temperature applied during dough fermentation has a much greater influence. It also demonstrates that rapid sensory methods, free sorting and flash profiling, can be utilised with an experienced sensory panel to relate instrumental and sensory analysis of flavour, even in a complex food system such as sourdough bread. This research will increase the understanding of how sourdough parameters impact on volatile generation and flavour of sourdough bread and how this knowledge might be utilised to manipulate flavour and achieve consistency in sourdough products.