New research suggests that the antioxidant activity of orange juice and juices from other citrus fruits is undervalued.

Researchers from the University of Granada developed a new technique for measuring this property and found that antioxidant values are ten times higher than those indicated by current analysis methods. The results suggest that tables on the antioxidant capacities of food products that dieticians and health authorities use must be revised.

Orange juice and juices from other citrus fruits are considered healthy due to their high content of antioxidants, which help to reduce harmful free radicals in our body, but a new investigation shows that their benefits are greater than previously thought.

In order to study these compounds in the laboratory, techniques that simulate the digestion of food in the digestive tract are used, which analyze only the antioxidant capacities of those substances that can potentially be absorbed in the small intestine: the liquid fraction of what people eat.

"The problem is that the antioxidant activity of the solid fraction (the fiber) isn't measured, as it's assumed that it isn't beneficial. However, this insoluble fraction arrives at the large intestine and the intestinal microbiota can also ferment it and extract even more antioxidant substances, which we can assess with our new methodology," José Ángel Rufián Henares, professor at the University of Granada, said in a statement.

His team has developed a technique called "global antioxidant response" (GAR), which includes an in vitro simulation of the gastrointestinal digestion that occurs in our body, whilst taking into account the "forgotten" antioxidant capacity of the solid fraction.

The method includes assessments of various physical and chemical parameters, such as color, fluorescence and the relationship between the concentrations analyzed and compounds indicators such as furfural.

Upon applying the technique to commercial and natural orange, mandarin, lemon and grapefruit juices, it has been proved that their values greatly increase. For example, in the case of orange juice, the value ranges from 2.3 mmol Trolox/L (units for the antioxidant capacity) registered with a traditional technique to 23 mmol Trolox/L with the new GAR method.

"The antioxidant activity is, on average, ten times higher than that which everyone thought up until now, and not just in juices, but also in any other kind of food analyzed with this methodology," Rufián said. "This technique and the results derived from it could allow dieticians and health authorities to better establish the values of the antioxidant capacity of foods."

The findings are detailed in the journal Food Chemistry.