What is the significance of vanillin in understanding the age of the Shroud?
Vanillin is an aromatic compound that occurs naturally in plants, particularly vanilla beans (Vanilla Planifolia). Vanillin is found in flax fibers used in the production of linen. Specifically it is found in lignin, a complex chemical part of the secondary cell walls of the flax fibers. Over time, vanillin decomposes and it is possible to derive some estimates of the age of fibers, within limits, from the amount of vanillin that remains.
Under normal temperature conditions, vanillin should decompose completely in about 1300 years. Thus if a piece of linen cloth has no remaining vanillin, and it has not been subjected to excessive temperatures for a significant amount of time, we can be quite sure it is at least 1300 years old. For instance, linen wrappings for some of the Dead Sea Scrolls do not contain vanillin. Medieval linen, if the lignin has not been bleached away, does contain the chemical.
The main part of the Shroud of Turin does not test positive for vanillin. There is one exception: the place from which the carbon dating sample were cut in 1988.
Quantitative counts of lignin residues show large differences between the carbon dating area and the rest of the Shroud. Where there is lignin, in the sample area, it tests positive for vanillin. Other medieval cloths, where lignin is found, also test positive. Thus we can be quite certain that the carbon dating sample is medieval but that the rest of the cloth is not.