Prune's

History:
The commercial dehydration of Prunes has evolved from the early days at turn of the twentieth century; when drying small amounts of fruit on wooden trays, laid out in the sun was common.  Today’s modern high tech & high volume systems are a far cry from those humble beginnings.   The first attempts at building a commercial Prune dryer (not requiring a sunny day), centered on the “Natural Draft” dryer.  This design required a small fire be built at the bottom of an inclined shaft, and trays loaded with fresh fruit were placed in the flue where the hot gases and smoke passed up through the trays.   The natural tendency of the warm air to rise provided the air velocity, and the heat from the fire raised the core temperature of the fruit, which helped to accelerate the drying process.  Even at this early date, the operators knew enough to move the trays ever closer to the fire as the product dried, to prevent case hardening.  From 1910 to 1920 this was the “state of the art” in dehydration technology.  

Prunes:
Early on, we referred to the fruit on the tree as Plums, and when it was dried, we called this wrinkled, oversized raisin a Prune.  Today there are many varieties; from baseball sized fruit to the tiny “Breakfast fruit”, all requiring dehydration to stabilize the fruit that will suffer degradation within hours of picking.  Dehydration allows for the ambient air storage of Prunes for months if not years.

Prunes are dried whole, normally with a quick water rinse, to remove dust and rudimentary debris removal helps keep the dried product at high standards.  The skin of the fruit is remains intact, and the pit remains inside the fruit.  While most processors dry the fruit without sizing (field run), there is a move to pre-size the fruit into two or more sizes, and dry each size separately, thus improving the quality and uniformity of the dried product, while cutting the time and expense in the operator.  This effort insures the small fruit are not over dried and lost, while the large fruit are dried to standard and without re-drying or equilibration (mixing the dried fruit so as to achieve desired moisture content), makes pre-sizing very desirable.

Tray drying is the norm for commercial and industrial level processors, but drying on continuous flow belt dryers will slowly replace the tray dryers in the future due to the high labor costs and the continuing costs of wood trays.  
When tray drying prunes, there are two procedures currently in use; Counter-Flow and Parallel-Flow (see definitions page) for large production applications.   The only known application of Parallel-Flow dehydration is in the California Prune Industry.   While Parallel-Flow is acknowledged to be faster, it engenders quality issues that have limited the viability of the fruit and customer satisfaction of the end product.  The Counter-Flow method takes more time in the dryer, but generally turns out a product that is less affected by case hardening, is dry uniformly from pit to skin, and is softer to the touch.    For smaller volume applications, Prunes can be dried in Batch, Batch to Dry, and Batch Rotation methods. 

In all but the Parallel-Flow method, the process air temperature should be 165 degrees (F), with a Relative Humidity (RH) measured at the hot end at about 17% to 19%.  Fruit on the trays should be 1 to 1-1/2 fruit deep on standard wooden trays.  The fruit needs to be placed on the tray uniformly.  Dwell times vary based on fruit size and sugar content, but will generally range from 20 to 24 hours.  The fruit is removed only when it is at the correct moisture content.   In the Parallel-Flow process air temperatures (PAT) run from 185 to 190 degrees (F), and dwell times are mandated by a time schedule required due to the fruit’s propensity to case harden.  Pulling the fruit to a schedule, not when it is at the correct moisture content, requires the re-dry and equilibration procedures.