Putting the Squeeze on Pomace
Left to right, Keerthi Srinivas and Jerry King use a modified expeller to extract antioxidants from grape pomace; Grape pomace is made up of leftover skin, seeds and stems
The United States juice and wine industries produce 10 to 15 million tons of grape waste each year. This waste, known as pomace, is usually sent to landfills, fed to livestock or used to make a renewable biofuel. But a group of researchers at the Engineering Research Center and the department of food science think there may be a better use for the leftover grape skin, seeds and stems.
The researchers spent more than four years studying grape pomace and came up with an eco-friendly and health-enhancing way to use the typically discarded material.
Chemical engineering professor Jerry King and food science professor Luke Howard used funding from a multi-year U.S. Department of Agriculture grant to support the research project, and each recruited a doctoral student to assist them in attaining its objectives. King selected Keerthi Srinivas, a Doctoral Academy Fellow, who at the time was just beginning his journey toward a doctorate in chemical engineering.
King and Srinivas, along with their co-workers, developed an antioxidant extraction technique using hot water under pressure that allowed the removal of bioactive and health-benefitting compounds called flavonoids from the pomace. Nearly 50 percent of the total antioxidants found in grapes are located in its pomace.
Flavonoid antioxidants are thought to aid in the prevention of certain illnesses and help promote general wellness. They counteract the production of harmful and unstable molecules called free radicals that, when prevalent in a living being, can lead to heart disease and certain cancers. Extracting antioxidants from food waste allows them to be combined into a variety of foods, vitamins and skin products.
“All fruits and vegetables and many food sources have amounts of these flavonoids that are destroyed in conventional processing,” King said. “Our method allows them to be removed and permits the leftover pomace to be used for other purposes.
Srinivas examines the color of a water-based extract of antioxidants.
“By using water at a lower pressure and temperature you can preferentially extract from the pomace these nutraceutically active components – or antioxidants,” King said. “Then, the pomace material that remains can be treated at a higher temperature and pressure to make certain kinds of products – such as a renewable biofuel.”
Using water as an extraction medium is a safer alternative to the more commonly used flavonoid solvents that contain ethanol. The consumption and use of ethanol is highly regulated by the U.S. government when used in extraction processes. Water, on the other hand, quells these concerns and is abundant, cheap and environmentally benign.
“If we could replace the use of hydroethanolic solvents with water, it would ease the regulatory requirements and have financial benefits to the food and nutraceutical markets,” Srinivas said. “I think we have established a platform that has expanded the use of water as a solvent.”
King and Srinivas worked from 2006-2010 experimenting with different water temperature and pressure conditions. The research extended beyond Srinivas’ doctoral education and led him to apply for a post-doctoral fellowship under King so he could continue his work from 2010-2012. Much of this research involved finding an extraction technique that maximized antioxidant output.
The grape pomace research has resulted in more than 10 published articles in engineering and food science journals, as well as a large number of presentations and collaborative opportunities. For Srinivas, an up-and-coming researcher, the experience of writing peer-reviewed articles and making presentations was invaluable.
“Dr. King provided me with a considerable number of opportunities where I was required to take responsibility and complete tasks with deliberation, punctuality and a high degree of perfection,” Srinivas said. “The opportunities included writing book chapters, presenting at conferences, writing grants to industrial and government agencies, publishing papers and attending workshops.”
The fact that Srinivas was so highly published in peer-reviewed journals as a student is a rare and important accomplishment. Oftentimes, graduate students produce only their thesis and aren’t published until later in their professional lives.
“Unfortunately this is often the case in graduate school,” King said. “But, with my assistance, he produced 15 peer-reviewed publications from his combined graduate school and postdoctoral experience at the University of Arkansas, with even more to come.”
Since the conclusion of the grape pomace research, Srinivas has moved on to a post-doctoral fellowship at Washington State University where he is conducting similar water-related research in their Richland, Wash., laboratories.