Title: Characterization of molecular structural changes in pectin during juice cloud destabilization in frozen concentrated orange juice Authors
Submitted to: Food Hydrocolloids Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 12, 2014
Publication Date: December 1, 2014
Citation: Galant, A.L., Widmer, W.W., Luzio, G.A., Cameron, R.G. 2014. Characterization of molecular structural changes in pectin during juice cloud destabilization in frozen concentrated orange juice. Food Hydrocolloids Journal. 41:10-18. Interpretive Summary: Juice cloud is a complex mixture of polysaccharides, proteins and lower molecular weight compounds that are responsible for the turbid appearance of citrus juices. The major polysaccharide present in cloud is pectin, which is comprised primarily of galacturonic acid. Galacturonic acid can be methylated, and many of the functional properties of pectin depend upon the extent and pattern of this methylation. When extensively demethylated, pectin molecules can crosslink in the presence of calcium ions; in juice, this aggregation can cause the cloud to precipitate, leaving behind a commercially undesirable serum. One means by which pectin can be demethylated is through the activity of a class of enzymes known as pectin methylesterases. Four of these enzymes are naturally present in juice, and may be active if the juice is under- or unpasteurized. We systematically treated reconstituted frozen concentrated orange juice with pectin methylesterases purified from citrus pulp cells, and monitored the resultant changes to the juice cloud. We also extracted a series of pectins from juice in various states of cloud loss, and assessed the resultant changes to their structure and function. We found a strong correlation between cloud loss and pectin demethylation, with greater demethylation leading to more pronounced cloud loss. We also determined that a relatively small reduction in pectin methylation (<10%) is sufficient to initiate pectin aggregation and cloud loss.
Technical Abstract: Pectin comprises one of the major components of cloud material in citrus juices. Juice cloud is a complex mixture of polysaccharides, proteins and lower molecular weight compounds that are responsible for the turbid appearance of citrus juices. The stability of juice cloud depends on a number of factors, including pectin degree of methylation (DM) and the availability of sufficiently-sized, charged demethylated blocks, but detailed information on the precise relationship between cloud state and pectin architecture is limited. To address this gap, we have systematically treated commercial frozen concentrated orange juice (FCOJ) with pectin methylesterases extracted from orange pulp cells to mimic the aggregation of pectin that naturally occurs in the presence of calcium in unpasteurized or under pasteurized juice. We then assessed the nano/macro-structural and functional properties, including juice optical density (OD) and pectin soluble sugar composition, DM, degree of blockiness (DB), and rheological capacity from juice in various states of cloud loss. A strong positive correlation ([r] = 0.916) was observed between juice optical density and DM, while a strong negative correlation ([r] = -0.914) was observed between OD and the degree of absolute blockiness (DBabs). Most critically, a reduction of average DM from 74.7% to 68.3% in the course of cloud loss resulted in the formation of rheologically active pectin, which has important implications for the industry practice of fortifying orange juice with calcium.