The occurrence of furan in some food products has already been known for a few decades, and it has been reconfirmed in more recent investigations that furan is present in a variety of foodstuffs. This list of products includes roasted coffee, which has been shown to generate furan as a result of the heat treatment at roasting which is applied to achieve the desired aroma and flavour profile of a roasted coffee. The objective of this study is to provide data to allow a better understanding of the available data of furan in coffee, the kinetics of furan generated during roasting, and to estimate the reduction of furan levels afterwards due to subsequent processing steps and consumer handling. Finally, the study is meant as a contribution to establish exposure data on the basis of scientific data at the stage of coffee consumption. This paper shows that the formation of furan during roasting is dependent on roasting conditions and is, therefore, directly linked to achieving targeted flavour profiles. Furthermore, it is demonstrated that modifications in process conditions potentially to reduce furan levels may have the opposite effect on other undesired reaction products of the roasting chemistry such as, for example, acrylamide. Due to the high volatility of furan, any subsequent processing step or consumer handling has an impact on the level of furan. As a guidance from this study and in consideration of the identified losses of each process and handling step on the basis of the trial conditions, it is estimated that only approximately 10% of the initially generated furan during roasting gets into the cup of coffee for consumption.
The xylose metabolism of Bacteroides xylanolyticus X5-1 was studied by determining specific enzyme activities in cell free extracts, by following 13C-label distribution patterns in growing cultures and by mass balance calculations. Enzyme activities of the pentose phosphate pathway and the Embden-Meyerhof-Parnas pathway were sufficiently high to account for in vivo xylose fermentation to pyruvate via a combination of these two pathways. Pyruvate was mainly oxidized to acetyl-CoA, CO2 and a reduced cofactor (ferredoxin). Part of the pyruvate was converted to acetyl-CoA and formate by means of a pyruvate-formate lyase. Acetyl-CoA was either converted to acetate by a combined action of phosphotransacetylase and acetate kinase or reduced to ethanol by an acetaldehyde dehydrogenase and an ethanol dehydrogenase. The latter two enzymes displayed both a NADH- and a NADPH-linked activity. Cofactor regeneration proceeded via a reduction of intermediates of the metabolism (i.e. acetyl-CoA and acetaldehyde) and via proton reduction. According to the deduced pathway about 2.5 mol ATP are generated per mol of xylose degraded.
During cultivation on a mixture of xylose and glucose, Bacteroides xylanolyticus X5-1 showed neither diauxic growth nor a substrate preference. Xylose-limited continuous-culture cells were able to consume xylose and glucose both as single substrates and as mixed substrates without any lag phase. When glucose was the growth-limiting substrate, the microorganism was unable to consume xylose. However, in the presence of a small amount of glucose or pyruvate, xylose was utilized after a short lag phase. In glucose-limited cells, xylose isomerase was present at low activity but xylulose kinase activity could not be detected. On addition of a mixture of xylose and glucose, xylose isomerase was induced immediately and xylulose kinase was induced after about 30 min. The induction of the two enzymes was sensitive to chloramphenicol, showing de novo synthesis. Xylose uptake in glucose-grown cells was very low, but the uptake rate could be increased when incubated with a xylose-glucose mixture. The increase in the uptake rate was not affected by chloramphenicol, indicating that a constitutive uptake system had to be activated. The inability of B. xylanolyticus X5-1 cells undergoing glucose-limited continuous culture to induce the xylose catabolic pathway after the addition of only xylose probably was caused by energy limitation.
Bacteroides xylanolyticus X5-1 was grown in pure culture and in mixed culture with Methanospirillum hungatei JF-1 under xylose limitation in the chemostat. In the pure culture, ethanol, acetate, C02, and hydrogen were the products. In the mixed culture, acetate, C02, and presumably hydrogen were the only products formed by B. xylanolyticus X5-1. The biomass yield of B. xylanolyticus X5-1 increased because of cocultivation. In cell extracts of the pure culture, both NADand NADP-dependent acetaldehyde dehydrogenase and ethanol dehydrogenase activities were found. In cell extracts of the mixed culture, activities of these enzymes were not detected. Inhibition of methanogenesis in the mixed culture by the addition of bromoethanosulfonic acid (BES) resulted in an accumulation of H2, ethanol, and formate. Immediately after the addition of BES, NADdependent acetaldehyde dehydrogenase and ethanol dehydrogenase activities were detected. After a short lag phase, a NADP-dependent ethanol dehydrogenase was also detectable. The induction of acetaldehyde dehydrogenase and ethanol dehydrogenase was inhibited by chloramphenicol, suggesting de novo synthesis of these enzymes. These results are consistent with a model in which the shift in product formation caused by interspecies electron transfer is regulated at the level of enzyme synthesis.
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