A few months ago I had a brief exploratory foray into modifying kombucha biofilm with the aim of modifying it’s physical properties. The goal was to increase the tensile strength and suppleness that tends to be lost as bacterial leather dries and becomes more brittle.
The inspiration was drawn from the multitude of research papers on making films and hydrogels formed via crosslinking of cellulose. (Take your pick from google scholar at leisure) From there I made the assumption that I could probably change the extracellular cellulose via crosslinking, despite most of the above papers dealing with powdered cellulose.
Kombucha scoby was grown in jam jars - there were two thickness, those of the original scoby halves used to start the cultures, a few centimeters thick, and week long cultures which were about a centimeter thick when wet.The treatments mainly consisted of soaking the freshly harvested scoby in acidic solutions for 18 hours at room temperature after harvesting, then allowing to air dry. Solutions of 10% tannic acid, 2% citric acid and 10% citric acid were used. In addition a few untreated pieces were rubbed down with glycerol after the drying stage.
Untreated thick: Hard, shrunken, like beef jerky. Invulnerable.
Untreated thin: Translucent, like dry rice paper. Tears easily
2% Citric acid thick: Slight fruity odour, Flexible, Stretchy
2% Citric acid thin: dry but flexible, Slight fruity odour, tear resistant.
10% Citric acid thick: tacky, strong fruit oder, flexible, stretchy
10% Citric acid thin: same as above. hard to tear.
10% Tannic acid Thick: Dry, flexible, darkened. feels similar to dry vegetable tanned leather that needs oiling. can be torn easier than the citric treatments as it has less stretch.
10% Tannic acid thin: similar to above but is more stretchy. Got a bit folded when drying which is why it looks a bit munted in the picture.
Glycerol: feels soft and very flexible, tears easier than citric treatments, but harder than tannic acid treatment.
Because I didn’t do many replicates of each treatment I don’t know how reproducable my results will be. If anyone wants to repeat this, then heating the acidic solution should give a quicker result, saving quite a few hours, though more care should be taken around hot acids. Also from my understanding of it, glycerol mainly seems to crosslink cellulose when water has been removed. - I also tried to add some glycerol into a living kombucha culture to see if it would be integrated into the biofilm formation, but instead it inhibited the formation of any biofilm at all. Perhaps I added too much? Who knows.
Overall I think the thick piece of 2% citric acid leather is the most durable, although the pieces treated with tannic acid feel the most like actual leather. Hopefully this information, while it’s hardly conclusive, can help other makers out there.