Yeast

Within Wall Sina, a special type of yeast was produced that could be used in storehouses and tents to significantly reduce the rate of decay of products such as soybeans, wheat, or fodder.

Titan's Regeneration
Many people speculate that the Titans use this special kind of yeast to operate. Specifically, to quickly regenerate or build themselves. Some say that a modified yeast could provide the necessary organic fiber that a titan needs, and that the heated gas of a titan is what causes the yeast to act so quickly.