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Abstract:

One of the oldest and most known leavening methods in the production of bakery products is fermentation with Saccharomyces cerevisiae yeast. During yeast fermentation, CO2 and ethanol are produced, which cause the dough to rise. Next to CO2 and ethanol, yeast metabolism also results in the release of other metabolites, such as glycerol, organic acids and flavour compounds, that can have a large impact on dough rheology and end product texture, flavour, and colour. Next to yeast, carbohydrates are also crucial in the production of bakery products, as they act as yeast substrates but also impact dough and product properties, like dough rheology and end product texture. Although in recent years, much research has been done on carbohydrates and yeast fermentation in bread, there is still a lack of knowledge on carbohydrates and yeast fermentation in other bakery products, such as fermented pastry. Fermented pastry products consist of alternating dough and fat layers. During fermentation, the produced CO2 gets stacked in between the different layers and pushes the layers upwards. After baking, this results in the typical flaky texture of fermented pastry products. More insight into the fermentation dynamics and the role of carbohydrates during fermented pastry making could allow to develop yeast-based strategies in order to obtain technological, nutritional, or sensorial benefits. Some strategies, including changing dough formulations or modifying fermentation dynamics, have already been investigated in bread. For example, optimising the CO2 production rate can result in a better product volume, while steering the degradation and consumption of carbohydrates during fermentation can result in high dietary fibre-containing products. This is also of great interest in fermented pastry products, as consumers are increasingly looking for a wider variety of healthy and tasty food products. To broaden the variety of tasty food products, using non-conventional yeast strains has a large potential for possible application in the production of fermented bakery products. However, in fermented pastry products, these strategies have not been explored yet. Against this background, this doctoral dissertation aimed to contribute to a better understanding of the fermentation and sugar dynamics, as well as the interplay between yeast and sugar in fermented pastry making. The knowledge gained will be used to explore yeast-mediated sugar reduction strategies. Finally, the possibility of producing fermented pastry with non-conventional yeast strains will be investigated in order to obtain different aroma profiles that can be used to improve end product flavour in future research. In the first part, the impact of yeast fermentation and different sugar concentrations on pastry dough properties and product quality characteristics was investigated. The comparison of yeasted and unyeasted pastry samples with 14% added sucrose revealed that sucrose was almost entirely degraded into glucose and fructose by invertase in yeasted samples after mixing. At least 23.6 ± 2.6% of the released glucose was consumed during fermentation. The production of yeast metabolites significantly impacted dough and end product, like increasing product height and reducing dough strength and extensibility. However, yeast fermentation did not significantly affect the sweetness factor of the pastry product with 14% added sucrose. When sucrose was omitted from the recipe, yeast was less exposed to osmotic stress, resulting in a higher initial CO2 production. However, the productive fermentation time was shortened due to sugar depletion. Therefore, the dough height of samples with 14% sucrose or without sucrose was not significantly different after fermentation. After baking the samples without sucrose, a light-coloured end product was obtained due to the reduced Maillard and caramelisation reactions. When 7% sucrose was added, a higher total CO2 production was obtained because sugar was not depleted. However, this increased CO2 production did not result in an increased dough height after fermentation, as the maximum gas holding capacity was probably already reached. Increasing the sugar content to 14 or 21% resulted again in a lower CO2 production due to higher osmotic stress to the yeast. Moreover, an increased sugar concentration led to a darker end product colour and a higher sweetness factor. Finally, dough rheology was affected to a limited extent by changes in sucrose addition, although no sucrose addition or a very high sucrose level (21%) reduced the maximum dough strength. Sugar reduction in fermented pastry seems possible, but significant changes in end product quality (e.g. colour, volume, sweetness) were observed. Further research to overcome negative consequences in end product quality is needed to make sugar reduction successful for this kind of product. In the second part, the impact of yeast and sugar on dough rheology and product structure setting was explored more in detail by investigating the impact of yeast and different added sucrose concentrations (0, 7, 14 and 21% on dm flour base) on gluten polymerisation and starch gelatinisation and on the heating rate during baking. The addition of yeast greatly increased the heating rate, probably due to the change in porosity. Yeast addition also facilitated the incorporation of gliadins in the gluten network, and the starch gelatinisation, which could be explained by changes in sugar concentration due to invertase activity and sugar consumption, and the production of yeast metabolites, such as GSH. The addition of sucrose slightly decreased the heating rate, which might be explained by the change in composition or by the reduced porosity as a consequence of lower CO2 production due to osmotic stress. During baking, sucrose addition inhibited gluten network formation by decreasing the water availability or increasing the proteins' thermal stability. In the next part, the possibility of using fructose instead of sucrose in fermented pastry to decrease the end product total sugar content was investigated. Therefore, the fermentation dynamics, product height and sweetness factor of samples with 10% or 14% added fructose (dm flour base) were compared with samples with 14% added sucrose. The replacement of 14% sucrose with 14% fructose had a small impact on fermentation dynamics and no significant impact on end product height. Fermented pastry samples made with 10% fructose did not have significantly more fructose in the end product than those made with 14% sucrose, which is remarkable and of interest, as some studies revealed that fructose could increase the risk of non-communicable diseases. Moreover, samples made with 10% fructose had the same sweetness factor as samples made with 14% sucrose, while the end product contained 33% less sugar. In the third part, the fermentation dynamics of 23 commercially available yeast strains were investigated in sweet dough with 14% added sucrose (% w/w dm flour). All yeast strains were able to ferment, although significant differences in fermentation dynamics were found. A positive correlation (R2 = 0.74) was measured between sugar consumption and metabolite production. Several non-conventional yeast strains produced more aroma compounds that are considered positive in bakery products and/or less aroma compounds that are considered off-flavours compared to the reference baker's yeast. These results form a basis for further research on non-conventional yeast strains in bakery products. In the last part, four non-conventional yeast strains were selected to further test their impact on end product quality of fermented pastry samples. The selection was based on their fermentation dynamics and aroma production in sweet dough. After some adaptations in the production process, like reducing the glutathione content and adapting the fermentation time, a dough texture and end product height comparable to the reference product were obtained for all samples. Moreover, the yeast strains significantly influenced the aroma profile of the end product. It can be concluded that non-conventional yeast strains that produce different aroma compounds can be used in fermented pastry production, although sensory analyses in the future are needed to explore if the use of these yeast strains results in an improved end product flavour. This doctoral dissertation has led to new insights into fermentation dynamics in fermented pastry production, the impact of sugar on these fermentation dynamics, and the impact of fermentation and sucrose addition on dough and end product properties. The research performed has led to possible strategies to steer the fermentation process and, hence, reduce the final sugar content. Furthermore, the results obtained provide a basis for further research on fructose-based sugar reduction strategies or improvement of the end product quality using non-conventional yeast strains.