Browsing by Subject "fermented foods"
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(2020)Sourdoughs are a natural habitat for microbial communities predominated by lactic acid bacteria (LAB) and yeasts. How microbial communities assemble and function is, to a large extent, determined by inter-species interactions. However, evidence for LAB-yeast interactions in rich environments, such as sourdough, is yet largely unavailable. In this study, a set of LAB and yeast species was isolated from rye and wheat sourdoughs. While mainly typical sourdough species were identified, Pediococcus parvulus was, to the best of our knowledge, for the first time isolated from sourdoughs. The isolates were characterized in rich chemically defined culture conditions and screened for interactions. Potential interactions were discovered where LAB growth was enabled by a yeast, or where stable communities were formed despite competition. These findings, the resource of naturally co-occurring species, and the designed chemically defined growth medium present the grounds for future research for uncovering the underlying mechanisms of LAB-yeast interactions in rich environments. LAB and yeasts commonly co-occur rich environments of fermented food processes and also in human gut and soil microbiomes. Therefore, the outcomes of this study support not only the optimization of food fermentations but provide also model systems for complex communities directly influencing human health.
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(2014)Idli is a popular cereal-legume fermented food of Indian origin. It is steam cooked from fermented (lactic acid-yeast) batter of rice (cereal) and black gram (legume). Idli preparation process includes three major steps – soaking of rice and black gram, grounding and fermentation. The idli preparation process is laborious, as the whole procedure takes about 20 hours. Further, the fermented batter has a shelf life of 4-5 days at 4 ºC. Literature studies reveal less efforts has been taken to improve shelf life and nutritional quality of idli. The overall aim of this thesis was to improve the quality of idli batter by mild heat treatment (Objective 1) and through microbial applications (Objective 2-4). First, the fermented idli batter was mild heat (MH) treated (57, 60, 63, 66 and 70 ºC ) to reduce the high (10.5 log cfu/g) lactic acid bacteria and yeast counts for enhancing the shelf stability at refrigerated storage. MH treatment (at 70 ºC) induced the highest reduction (3.6 log cfu/g) without affecting the pasting profile of idli batter. During storage study (upto 10 days at 4 ºC) the microbial counts further decreased without change in pH.The second objective was to monitor the changes in physicochemical properties and B-vitamin (riboflavin, folate and vitamin B12) levels in idli batter fermentation on addition of starters - Lactococcus lactis N8 (SAA1) and Saccharomyces boulardii (YEA1). Fermentation profiles were recorded individually and in combination of starters. SAA1 and YEA1 were able to enhance or retain riboflavin and folate levels, but no change in vitamin B12 levels were observed during fermentation. Further, YEA1 individually and in combination with SAA1 significantly improved the idli batter volume, implying high gas production. The third objective was to produce nisin in idli batter by addition of SAA1 (nisin producer). The results highlighted SAA1 was capable of producing nisin in idli. However, the produced nisin was degraded by the activity of indigenous LAB and yeast in idli batter. The final objective of this thesis was to determine the viability of probiotic Bacillus coagulans (BAC1) spores after cooking (steaming and microwaving) and during storage (at 4 ºC) of idli batter. Microwave cooking resulted in higher reduction of BAC1 than steam cooking. However, 5.4 log cfu/g of BAC1 spores were still viable in steamed idli from the initial added amount (8.2 log cfu/g). The BAC1 spores were not stable in idli batter suggesting spore outgrowth during storage. In summary, these results present different strategies and information for future process and product developments in idli.
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