Orange juice: NMR health test

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  • Published: Feb 1, 2013
  • Author: David Bradley
  • Channels: NMR Knowledge Base
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Orange composition

The amino acid composition of juice from oranges grown on trees infected with HLB, Huanglongbing, a pathogen that causes citrus greening disease, can now be distinguished from that produced from healthy trees, while NMR spectroscopy points to a possible way to understand infection and block it.

The amino acid composition of juice from oranges grown on trees infected with HLB, Huanglongbing, a pathogen that causes citrus greening disease, can now be distinguished from that produced from healthy trees, while NMR spectroscopy points to a possible way to understand infection and block it.

Scientists at the US Department of Agriculture (USDA) based in Fort Pierce, Florida - a state renowned for its citrus fruit - are helping farmers and juice processors cope with an issue that costs them millions of dollars each year - citrus greening disease, which goes by the Chinese name Huanglongbing (HLB) meaning "Yellow Dragon Disease" and is also referred to as citrus dieback. The disease is caused by gram negative motile bacteria, Candidatus Liberibacter spp, which are carried by sap-sucking insects, psyllids and were first identified scientifically in the 1920s, had spread to China by the time of World War II, and Africa just after. The disease-carrying psyllid was seen in Florida in 1998 and has since spread to Louisiana, Georgia and South Carolina. It emerged in Southern California in 2008 in domestic citrus trees and has been the subject of quarantine and eradication programmes since.

Unfortunately, citrus dieback is not simply about green and misshapen oranges, the fruit usually fall before harvest and the disease kills the trees outright within a few years. USDA horticulturalist Elizabeth Baldwin has studied the effects of HLB on the taste of orange juice from diseased trees to assist the industry while others search for a permanent solution to citrus dieback.

Fruity disease

The researchers have used gas and liquid chromatography to analyse juice from the fruit of infected and healthy trees. Specifically, they compared Midsweet, Hamlin and Valencia strains of oranges, which are the primary US varieties harvested for processing, and used to analyze juice compounds. The results showed that fruit with HLB symptoms often had higher concentrations of the bitter-tasting compounds limonin and nomilin, although these remained at levels below testers could actually taste. In a parallel study, they investigated how HLB infection affected the quality of the juice in terms of cultivar, maturity and processing methods. Harvest date and variety led to greater variability than disease status with Hamlin oranges more deleteriously affected than diseased trees of the Valencia and Midsweet varieties. The researchers point out that using some fruit that has HLB symptoms would not be problematic in juice production provided that mixed varieties, locations, and seasons are used.

Last year, USDA chemist Andrew Breksa and Carolyn Slupsky of the University of California Davis and their colleagues investigated the NMR spectroscopic amino acid profile of orange juice in the hope of revealing just how HLB attacks citrus trees. Their hope is to find a safe, effective and environmentally friendly approach to undermining the pathogen. [J. Proteome Res., 2012, 11 (8), pp 4223-4230; DOI: 10.1021/pr300350x]. In that study, the team revealed three distinct profiles for concentrations of 11 amino acids in three types of oranges: fruit from healthy trees; symptom-free fruit from HLB-positive trees; and fruit, with HLB symptoms, from HLB-positive trees.

Blocking defence blockades

The same profiles could lead to an understanding of the mode of action of the infectious agent and perhaps, based on how amino acids are affected, lead to a treatment for the trees. For instance, a citrus tree converts the amino acid phenylalanine to cinnamic acid, which is itself a precursor for defence chemicals. Juice from infected orange trees had higher concentrations of phenylalanine, suggesting that the pathogen blocks formation of those defence chemicals.

Related Links

J Proteome Res, 2012, 11, 4223–4230: "Metabolomic Analysis of Citrus Infection by ‘Candidatus Liberibacter’ Reveals Insight into Pathogenicity"

J Agric Food Chem, 2010, 58, 1247–1262: "Effect of Liberibacter Infection (Huanglongbing Disease) of Citrus on Orange Fruit Physiology and Fruit/Fruit Juice Quality: Chemical and Physical Analyses"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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