Characterization of molecular structural changes in pectin during juice cloud destabilization in frozen concentrated orange juice

• Orange juice cloud was destabilized with pectin methylesterases from orange pulp.
• Degree of methylation of extracted pectin correlated with juice optical density.
• Soluble and insoluble pectic fractions were extracted from destabilized juice.
• Other correlations were observed between pectin nanostructure and rheology.
• A degree of methylation reduction of 6.4% was sufficient to induce gel formation.

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 pectin structural and functional properties, including juice optical density (OD) and soluble sugar composition, DM, degree of blockiness (DB), degree of absolute blockiness (DBabs) and rheological capacity from juice in various states of cloud loss. A strong positive correlation ([r] = 0.916) was observed between juice OD and DM, while a strong negative correlation ([r] = −0.914) was observed between OD and 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.

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