For every gallon of olive oil that's pressed from the ripe fruit, about 38 pounds of olive skins, pulp and pits are left behind. Known as pomace, these leftovers typically have low-value uses. But U.S. Department of Agriculture (USDA) agricultural engineer Rebecca R. Milczarek and her colleagues are working with olive growers and olive-oil processors in California—where most of the nation's commercial olives are grown—to find new, environmentally friendly, and profitable uses for pomace.

According to Milczarek, pomace from California mills is usually a wet, heavy goulash that ranges in color from green to brown to black to purple, and has an aroma somewhat like that of olive tapenade, a flavorful spread made of finely chopped or puréed olives, anchovies, capers, garlic and olive oil.

Milczarek notes that one key to creating higher-value uses for pomace is to develop techniques that millers can use to quickly and affordably dry it on-site. That would make the pomace lighter, and easier and less expensive to ship to, for example, a centralized processing plant. There, specialized equipment could be used to extract additional oil or perhaps compounds for use in new foods, pharmaceuticals, cosmetics or other products.

In her research, Milczarek is investigating the dynamics of drying pomace. The goal of these studies is to determine precisely how long it would take for water to diffuse from the pomace under specific conditions.

Olives of various shapes, sizes, and colors. ARS scientists are helping to discover the secrets of a crop that is a lot more than just delicious. Once the oil is extracted, even the byproducts, or leftovers, have significant potential value to consumers. Photo credit: University of California-Davis Olive Center

In preliminary experiments, documented in a 2011 peer-reviewed article in the Journal of Food Engineering, Milczarek's team dried small batches of fresh pomace, using a combination of microwave and convection (hot forced air) heating. The drying rates for the four internal temperatures studied—104 degrees, 122 degrees, 140 degrees, and 158 degrees Fahrenheit—averaged about 28 percent lower than those reported in some studies conducted by other scientists.

The bottom line? Lower drying rates mean more drying time is needed in order for the pomace to dry sufficiently.

What can olive mills do about that? For commercial drying, pomace would be carried on a conveyor belt through a "drying tunnel." With the drying rates in mind, the tunnel could be lengthened, or the conveyor belt could be slowed, to ensure that pomace emerging from the tunnel isn't damp and prone to mold.

Of course, drying adds to mills' energy costs. However, the combination of microwave and convection drying that Milczarek tested is inherently more energy-efficient than drying options that are based solely on convection, she points out.

Two features of Milczarek's study—keeping the pomace's internal temperature steady when testing each temperature regimen, and taking pomace shrinkage into account—likely made the research unique among olive-pomace-drying experiments and contributed to the accuracy of her results.

Milczarek is with the USDA Agricultural Research Service (ARS) Western Regional Research Center in Albany, Calif. ARS is the USDA's chief intramural scientific research agency.

Read more about this and other olive oil studies in the May/June 2013 issue of Agricultural Research magazine.