Another typical California scene, in which the prunes, spread in trays, are being subjected to the lye bath before being placed in the drying yard. The lye bath destroys any germs that are present and cracks the skins of the fruit to facilitate evaporation. It is the only artificial process to which the prunes are subjected. The dried prune is merely a plum preserved by its own sugar. All prunes are plums, but only plums having a high sugar content are ranked as prunes.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 2 Chapter 8