Creating high-value products from leftovers

For around every 4 L of olive oil that’s pressed from the ripe fruit, about 17 kg of olive skins, pulp and pits are left behind. Known as pomace, these leftovers typically have low-value uses. But US 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 US’s commercial olives are grown - to find new, environmentally friendly and profitable uses for pomace.

Photo by University of California-Davis Olive Center.

According to Milczarek, pomace from California mills is usually a wet, heavy goulash that ranges in colour from green to brown to black to purple, and has an aroma somewhat like that of olive tapenade, a flavourful 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 centralised processing plant. There, specialised 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.

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 - 40, 50, 60 and 70°C - averaged about 28% lower than those reported in some studies conducted by other scientists.

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

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 mould.

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.