Gregory Ziegler is exploring the commercial potential of a brilliant orange found in avocado pits as a natural food coloring.
For a few months, a jar of orange liquid has sat on the windowsill of the food science professor's office in the College of Agricultural Sciences, and the color is still just as vibrant as the day he made it, using the flesh of an avocado pit.
“I’m doing a very informal light stability test,” said Ziegler, who was working on avocados when he noticed an orange color that would not go away. “It’s been doing pretty well.”
Ziegler is exploring using that brilliant orange as a natural food coloring in the production of other foods.
He discovered the persistence of the bright orange color as he worked on a project to extract starch from avocado pits.
“It was difficult to extract the starch without there being an orange color. I’d wash it away and I kept extracting orange starch,” said Ziegler. "When pits are pulverized an enzymatic reaction produces a bright orange color."
Annoyed, yet intrigued about this color, Ziegler conducted a literature search and could not find any answers -- which only further puzzled him.
Like Ziegler, Joshua Lambert shared the same curiosity when learning about the avocado pit color. “It’s interesting that nobody has talked about it before. It’s not hard to make [the color reaction] happen. You can cut or smash an avocado seed and it turns orange,” said Lambert, associate professor of food science and co-director of the Center for Plant and Mushroom Foods for Health.
Lambert, with a background in toxicology, pharmacology, and natural products chemistry, joined the project when he arrived at Penn State in 2008.
“Ziegler thought it would be useful for us to collaborate and use some of the expertise I had in isolation and identification to figure out what was in this extract that was the orange compound,” explained Lambert.
So Ziegler and Lambert did more experiments concerning the stability of the compound and learned a few things about avocado color: It has a stable biological structure under many conditions, and may be applicable to use as a food color. It is non-toxic as far as they know and would require Food and Drug Administration approval to be used in food.
The color found in avocado pits can withstand heat, light, and oxidation, says Ziegler, all qualities that could make it useful for beverages and confections. A natural coloring could withstand the heat of baking or boiling, and be shelf-stable.
This color could be used to make drinks like grapefruit juice more appealing, to enhance the appeal for consumers. Many grapefruit juices use red #40, an artificial dye.
“The last thing that people want is to buy a fruit juice with an artificial color in it,” said Ziegler. "A small amount of avocado colorant, fewer than 10 mg to a milliliter, can be added for a dramatic effect."
Avocados pits produce this bright orange due to an unusual form of oxidation. When fruit or vegetable flesh is cut, it begins to brown once exposed to oxygen. The process is due to the enzyme polyphenol oxidase — known among food scientists as PPO.
This is the same reason the inside of an apple turns brown once it’s cut. Avocado flesh goes through this process rapidly, and the light green color soon turns to brown once exposed to oxygen.
The same process starts to happen once the inside of an avocado pit is exposed to air -- and then it stops.
“For some reason, it appears that the reaction in avocado pits stops at an intermediate stage of oxidation,” Ziegler explains." Instead of turning brown or black, the avocado pit stops at bright orange."
Developing a natural colorant to replace artificial dyes and finding a use for avocado pits instead of tossing them in the landfill or compost pile can help reduce waste. Others unsuccessfully have tried to convert that waste into bioethanol, but the pits apparently don’t decompose quickly enough to be suitable.
Many restaurants and factories that use avocados in their products have mountains of leftover avocado pits.
The next step for Ziegler and his team is to develop a technology with a consistent process of extracting the color from avocado pits. They are also working on additional toxicology studies.
They believe they are very close to understanding the chemical structure of this new colorant -- laying the groundwork to then explore its commercial potential.
“This is a cool story of being observant in science. The fact that Dr. Ziegler was doing an experiment and noticed this color and thought, ‘I’m going to do something with this,’ is important,” said Lambert.