In prune-making the fruit is allowed to remain on the trees until ripe enough to fall to the ground, as the maximum proportion of solids is thus obtained. After picking, the plums are passed over graders to remove rubbish and to secure uniformity in size, this being essential to obtain evenness in curing, since the small fruits dry more rapidly than large ones. Usually before evaporation begins the fruit is dipped in boiling lye or pricked by needles in a pricking machine to make tender the tough skin and so allow the moisture to escape more readily. The dipping consists of immersing the fruit for a minute or less in a solution of lye in the proportion of 1 pound of concentrated lye to from 10 to 50 gallons of water maintained at the boiling point. The fruit is carried mechanically through the lye vat and a rinser by a modified endless chain, or it may be dipped in wire baskets. After rinsing the plums are ready for curing.
Exposure to the sun in curing varies from five to twelve days, depending upon the heat of the sun and the size and the variety of the plum. Curing in evaporators varies with the fruit and with the make of the machine. In general the temperature in the evaporator is from 120° to 140° at the start, increasing to from 160° to 180° and decreasing when the prunes are taken out. Too much heat at first causes the cells of the fruit to burst, thereby producing drip and discoloration. Important factors in evaporating in machines are the circulation of air, convenience, cost of fuel and power. The time required for curing ranges from twelve hours for a small plum to forty-eight hours for a large, juicy one. If not cured enough fermentation and molding result; if cured too much the weight is lessened, the quality is injured, the prune is harsh and coarse and has a dried up appearance.
When sufficiently dried the prunes are put in bins or piles to sweat, a process taking from one to three weeks, after which they are graded, processed and packed. In grading, the prunes are separated into sizes indicating the number of prunes required to make a pound, as 30’s to 40’s, 40’s to 50’s and so on to the smallest size, 120’s to 130’s. The processing is done by dipping the prunes in boiling water and glycerine or by steaming or by using some special preparation in the final dipping or by rattling in a revolving cylinder. Processing is reputable if it adds beauty to the color, or kills insects’ eggs or sterilizes the prunes. It is disreputable when the aim is to add to the weight. The best prunes are packed in boxes, in which process lining with paper, filling facing, pressing and labeling are important details. A well cured prune is soft and spongy, the pit is loose but does not rattle, the skin is bright, the product is free from drippings or exudation, the flesh is meaty, elastic, and of a bright, lively color.
The custom has been to bleach light colored prunes with sulphur fumes. This process injures the quality and possibly makes the product somewhat poisonous. Sulphuring is now regulated by the Federal Pure Food Law.
If poorly managed or if the plums are not of the best, several difficulties are encountered in curing prunes. Thus, a syrupy liquid sometimes oozes from the prunes, besmearing and making unattractive the final product. Again, the finished prunes may be covered with globules of sugar, rendering them sticky and destroying the lustre. Fruit grown on poor soils, on unhealthy trees or picked green may cure into small prunes of an abnormal shape called “Frogs” or they may ferment and swell up in large soft prunes called “Bloaters.”
The plum chiefly used in California in making prunes is the Agen, usually called Petite, a prune curing into a bright amber-colored product. This plum is easily cured, and the prune from it needs little sugar in cooking. In the states north of California the Italian Prune is the favorite, producing a dark red, almost black product, more tart but on the whole rather better flavored than the prune from the preceding variety. Other varieties more or less used are Golden Drop, the product from which is known as the Silver Prune; Reine Claude, which makes a fancy product often used as a confection; Yellow Egg, which sells as the Silver Prune when evaporated; the German Prune, making a product much like the Italian Prune; “Hungarian Prune,” from a very large plum and making a fancy product but very difficult to cure; the Tragedy Prune, an early plum of the Italian type; Golden Prune, much like the Silver and possibly better; and the Champion, Willamette, Pacific, Tennant, Steptoe and Dosch, all of the Italian type.
[156] Farlow, W. G. The Black Knot, Bulletin Bussey Institution 440-453. 1876. Halsted, B. D. Destroy the Black Knot, etc. N. J. Sta. Bul. 78:1-14. 1891.
[157] Smith, E. F. Peach Rot and Peach Blight Journ. Myc. 5:123-134. 1889. Quaintance, A. L. The Brown Rot, etc. Ga. Sta. Bul. 50:237-269, figs. 1-9. 1900.
[158] Atkinson, G. F. Leaf Curl and Plum Pockets Cornell Sta. Bul. 73:319-355, Pls. 1-20. 1894.
[159] Ibid.