As the saying goes, one person’s trash – or compost – is another person's treasure. While many people throw away citrus fruit peels because they are usually too bitter to eat raw, this outer layer of the fruit’s skin is a rich source of many sought after beneficial compounds and is in demand for companies seeking to use those compounds for health and pharmaceutical products. 

Florida citrus farmers and producers have been battling to save their industry for over 20 years from Huanglongbing (HLB).  This disease, also known as citrus greening, was first found in 2005 in Florida and is now endemic. It poses a serious threat to the U.S. citrus industry and is now present in Arizona, Texas, and California. Citrus greening infects citrus trees and their fruits, resulting in reduced fruit production and increased bitterness in fruit, which equates to economic losses for farmers and producers. 

“One way that ARS is helping citrus farmers is developing citrus varieties and hybrids that show tolerance to citrus greening,” said Christina Dorado, Lead Scientist and Chemist at the U.S. Horticultural Research Laboratory’s Citrus and Other Subtropical Products Research Unit in Fort Pierce, FL. 

“It’s important to develop varieties and hybrids that are viable for commercial production. For citrus farmers, it is not just about growing fruits for food or juicing. The citrus peels are valuable too because they are good sources of antioxidants and anti-inflammatory compounds.” 

In a recent study, Dorado and the U.S. Horticultural Research Laboratory researchers assessed 22 different citrus varieties and new unique hybrids for the presence of these anti-oxidants and anti-inflammatory compounds, also known as their phenolic profile, in the fruit peel. Each citrus variety and hybrid has a phenolic profile that has the potential to be utilized for precision nutrition and health supplement applications. 

The key research findings were:

- The mandarin varieties had high levels of beneficial flavonoids called polymethoxylated flavones (PMFs), especially nobiletin and tangeretin.

- The mandarin x pummelo hybrids contained both PMFs and coumarins, which are largely absent in citrus and have shown potentially beneficial biological actions.

- The two trifoliate oranges, Flying Dragon and Gotha Road, showed big differences in their phenolic profiles. Gotha Road had more coumarins, while Flying Dragon had more psoralens, compounds that are structurally related to coumarins. 

- Hybrids of mandarins with trifoliate orange parents often combined unique compounds from both species, creating new chemical profiles not found in traditional citrus varieties.

While the fruits tested in the study are not available for commercial production yet, the research shows promise that these fruits can be valuable for citrus farmers. 

“We found that these hybrids and varieties have a lot of beneficial compounds that are not found in traditional citrus fruits like oranges, limes, lemons, and grapefruits, and so have not been studied as much or at all,” said Dorado. 

According to Dorado, more research is underway to study the health effects of the compound mixtures of these fruits. – by Jessica Ryan, ARS Office of Communications