Plum Good Ingredients

Like cherries, nectarines, peaches and apricots, plums are called drupes, or stone fruit, because of their hard center pit. They also share the same genus, Prunus. However, plums come in a wide range of shapes, sizes and skin colors, with flavors ranging from sweet to tart.

Generally, they fall into one of three groups—European, Japanese and native North American. Japanese varieties are typically found in the fresh market, and North American plums are not grown commercially. European plums include the purplish-blue fruits found in markets and the dense, high-sugar varieties that, when dried, make dried plums, or prunes, the form most often used as ingredients.

California connection

In the United States, the vast majority—over 90%—of plums are grown in California. California dried plums are unique, both in the fruit itself and in their processing. The California dried plum (Prunus domestica L.) is a Petit d’Agen variety with origins in France. In 1850, while others were attempting to mine for gold in California, Louis Pellier, a French vineyardist, acquired a tract of rich topsoil near Mission San Jose, located in what is now Fremont, CA. Here, he began experimenting with the cultivation of dried plums, grafting choice cuttings of the d’Agen rootstock onto wild plum trees growing in the valley. As the seasons turned, Pellier’s patient work began to bear fruit, and the California dried plum was born. More than 150 years later, California has upwards of 72,000 acres planted in prune plum trees. With less than 1% of that comprising such varieties as Imperials, Robes de Sergeant, Sugars and more, the descendant of the original Petit d’Agen still rules the orchard. California provides 99% of the U.S. supply of dried plums, and 60% of the world’s supply.

Plum processing

In mid-August, the orchards are ready for harvest. Prune plums are tree-ripened, so growers determine harvest time by checking fruit firmness and sugar content. A mechanical shaker grabs a tree’s trunk and, in a matter of seconds, shakes the fruit onto a fabric catching frame spread underneath.

Processors transfer the freshly harvested fruit to dryers, where 3 lbs. of fresh fruit become 1 lb. of dried plums. After thorough washing, the fruit is placed on large wooden trays and dehydrated in a sequence of scientifically determined, fully automated and strictly sanitary operations that maximize efficiency while protecting product quality. Super-sensitive thermostats keep temperatures under tight control (85°C to 90°C) to yield uniform, high-quality fruit.

From the dehydrator, the newly dried plums go to state-of-the-art packing plants, where they are inspected, graded by size and stored ahead of final processing and packaging. Because dried plums store best at 21% moisture—a point at which they’re known as "natural condition dried plums"—post-harvest dehydration targets this level. There they remain, in cool (not refrigerated) storage facilities, until needed for further processing.

Sweet facts

More than 130 varieties of plums are grown in California, but only about a half dozen varieties have a high enough sugar content to be dried without fermenting while still containing the pits. In fact, prune-making plums contain twice as much total sugar at harvest than other varieties. The production processes of dried plums have a significant influence on their chemical composition. The main sugars found in fresh plums are glucose, fructose and sucrose. Sorbitol is also present.

When compared with fresh prune plums, the concentration of sugar increases in dehydrated California dried plums, but qualitative changes also occur in the proportion of individual sugars. The most striking change is the nearly total disappearance of sucrose, which is hydrolyzed to glucose and fructose. The high temperature of drying disrupts cell structure, releasing fruit acids and invertase, which catalyze the conversion during the first few hours of drying.

The sugars in dried plums do not elevate blood sugar levels rapidly, and their glycemic response, or glycemic index (GI), is in the low range, i.e., 29 +/-4. This low GI is due to the variation in sugar profiles and fiber content of dried plums.

Multifunctional ingredient

For food processors, California dried plum and fresh plum products offer a line of natural, multifunctional ingredients that improve and add value to a wide range of products. Thanks to a unique combination of fiber (7.5%), sorbitol (14.0%) and organic acids (primarily malic acid at 1.5% to 2.0%), the dried plum fruit system can function, for example, either as a fat substitute in baked goods and confections, or as a humectant to retain moisture in bakery and precooked meat products. Substituting dried plum carbohydrates for shortening, butter or oil in baked goods also reduces total calories. Dried plum and fresh plum ingredients can also function as natural sweeteners, caramel colorants, preservatives and flavor enhancers.

Pastes, purées, butters, powders and juice concentrates give processors a variety of forms to consider in wet and dry batch systems. The naturally occurring organic acids in dried plums, combined with high antioxidant power, have been linked to the suppression of microbial development in meats. Research also points to the effectiveness of dried and fresh plums in alleviating the “warmed-over flavor” that characterizes many cooked and reheated meat products.

A nutritional powerhouse

California dried plums provide potassium, soluble and insoluble fiber, phytochemicals that function as antioxidants, as well as some iron and vitamin A. Dried plums can also play an important role in digestive health.

Research at the Agricultural Research Service’s Jean Mayer Human Nutrition Research Center on Aging at Tufts University, Boston, has shown dried plums have the highest ORAC value (5770) of a group of 22 of the most commonly eaten fruits and vegetables tested. The specific source of this antioxidant activity is not yet clear. However, one clue can be found in their phenolic composition, i.e., naturally occurring chlorogenic and neochlorogenic acids that contribute significantly to the fruit’s antioxidant properties.

Dried plums can serve as a natural ingredient to prolong the shelf life and improve the safety, nutrition and eating quality of various processed foods. The growing body of dried-plum research continues to underscore the multifunctional benefits of adding dried plums to processed foods

James Degen, C.M.C., is president of J. M. Degen & Company, Inc., a marketing consulting firm headquartered in Templeton, CA. Contact the company at 805/434-2400, or degenconsulting.com. For further information about California dried plums, including technical information and applications, visitcaliforniadriedplums.org.