How Data Was Used in Sourdough Sketches

The data used in RISE: Sonic Sketches of Sourdough Cultures is depicted in the graph you see below. This is the Optical Density growth profile over a 48 hour period for the 8 most prevalent strains of yeasts and lactic acid bacteria (LAB) found in The Sourdough Project’s 500 starter samples. Using these data defined shapes was suggested by their similarity to the motifs of Terry Riley’s In C, a piece that continually shapes and sharpens my appreciation of timbre and harmonics.

This data set turned out to be less important in the great scheme of the final Sourdough Project paper, however by assigning chromatic pitches to the OD levels from the lowest measured amount (.0867) to the highest amount (.8816) among all 8 taxa, a unique motif emerges for each one. The intervals between sampling points/tones reveal the growth rate and expansiveness of each taxa. The notes at each sampling point when strung together create a pentatonic pattern spread out over four octaves that will be the sonic profile of each strain of yeast and LAB. Here is an example of the motif for L Sanfrancisensis, a lactic acid bacteria common to sourdough starters.

There were 40 density amounts over 4 octaves, so 10 notes were needed in each octave and two notes had to go. Leaving out C and F in a scale with G as the fundamental pushed the scale toward more dissonance, which helps to create the “sour” part of the sound. The chromatic scale runs from G0 to G5 (the scale runs from G0 to F#4, and then jumps to G5. G5 is heard only in W. Anomalus). Here is the piece that introduces the yeast voices and pattern profiles –  String of Yeasts

The LAB voices are horn, synth, brass and a plucked resonant instrument. LAB do not reach levels higher than .5 on the OD scale thus are lower in pitch class range overall. Several of the yeasts soar into the 4th octave, but the LAB all stay in the 0-3 octaves as they grow slower and less abundantly.

And then there are the Acetic Acid Bacteria that have not received much attention in previous research. One of the findings of the Sourdough Project is that highly variable abundances of AAB are a key driver of functional diversity across the 500 starters in the study. The AAB also contribute heavily to starter aroma. In the soundscape AAB will take the form of sculpted noise- mixing various shades of noise with audio of watery bubbling sounds. And since AAB are drivers, percussion will be used as well. The primary AAB, Acetobactor Malorum, is represented by a polyrythmic frame drum statement.

The Yeast and LAB sonification profiles are what I call “data-driven” in that specific data points have been used to depict each Yeast and LAB voice. The AAB sonification is “data-derived” in that the use of percussion as a driver, of burbling, watery sounds as fermentation, and of post-soundscape frequency artifacts as VOCs were all suggested by descriptions of AAB in the published paper.

Three individual starters were sonified for the album. SD_522 was chosen because it may demonstrate the impact of Acetobacter Malorum on functional diversity in starter microbiomes. This starter had 6 of the 8 articulated taxa in measurable amounts and Acetobacter Malorum as the primary AAB. SD_131 contained Acetobacter Malorum and hit 4 of the 6 aromatic notes, so the last 30 seconds of the soundscape are the audio artifacts representing volatile organic compounds (VOC). SD_299 was chosen because it is mostly LAB and DOES NOT have any S Cerevisiae and very little AAB. This allowed me to play with a very different sonic pallette from SD_522 and _131.

The album is available March 30, 2021 on Bandcamp, and within the month on all other music platforms! Thank you for your support!

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