Presentation Information
[O09-01]Bread making technique inspired by scoria★Invited Papers
*Atsushi Toramaru1 (1.Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University)
Keywords:
pumice,scoria,bread,bubble nucleation
Scoria is a type of volcanic ejecta, often described as dark-colored pumice. The crucial point is that both scoria and pumice are formed through explosive eruptions. These rocks contain numerous vesicles which are the direct evidence of the driving force for volcanic explosions. On the other hand, we are well aware that bread also contains many air bubbles; however, we know that bread is not created through an explosive process. Interestingly, a close observation of the vesicular textures in both pumice and bread reveals strikingly similar characteristics. Focusing on this similarity, this talk examines the physical processes underlying bread-making techniques on the basis of our understanding of the formation process of volcanic pumice. Vesicles in pumice are formed by decompression when magmas ascent in the volcanic conduit, whereas bubbles in bread are created by yeast fermentation. While the decompression of magma and the fermentation of yeast are entirely different phenomena, they follow the same fundamental physical processes such as nucleation, growth, expansion, coalescence, and deformation. Finding and analyzing similarities in such quite different phenomena is one of the true joys of scientific research, as it often leads to unexpected discoveries and insights. Moreover, this allows us to use more tractable substances (analog materials) to investigate the same underlying processes. For instance, creating pumice requires high temperature and pressure apparatus; however, the study of bread can be conducted easily in a kitchen. With the similarities between pumice and bread in mind, we conducted several experiments on bread, and here I will introduce our findings regarding bubble nucleation. In magma, it is known that the bubble number density increases with increasing decompression rate. Before the experiments, we initially thought that in bread, a larger amount of yeast would generate CO2 more rapidly, increasing the rate of CO2 accumulation and thus resulting in a higher number of bubbles. However, the experimental results were quite the opposite: we found that as the amount of yeast increased, the number of bubbles actually decreased. We interpret this result as more competition among the yeast cells (i.e., the struggle for sugar as nutrition), which reduces the overall activity during the fermentation process. It is intriguing to see such a phenomenon unique to the biological system within the same vesiculation process. The timing of nucleation in magma is governed by the time required to reach a specific level of supersaturation (amount of decompression), meaning the timing becomes shorter as the ascent rate, or decompression rate, increases. In order to know the nucleation time in bread, we conducted experiments by varying the fermentation time. The results revealed that the nucleation rate takes maximum values at approximately 6 minutes at 30oC and 3 minutes at 40oC. These are surprising results because the bread fermentation typically takes 30 minutes or more. Furthermore, in the two-step fermentation experiments, we found that the final bubble size distribution changed depending on the respective fermentation times. Specifically, a shorter fermentation time at 30oC resulted in bread with a higher number of bubbles. Applying this finding to magma suggests that fewer pre-existing bubbles in a magma chamber prior to decompression lead to a higher number of bubbles during the magma ascent, providing crucial insights for interpreting the vesicular textures of natural pumice as well as bread. I would like to express my gratitude to Ms. Hiroe Ogawa (then at Kanazawa University) and Ms. Kyoko Ikeda (then at Kyushu University) for their contributions to the bread experiments.