Release and recovery of pectic hydrocolloids and phenolics from culled citrus fruits

Abstract

The citrus industry worldwide is threatened by a bacterial disease (Huanglongbing, HLB) spread by a sap sucking hemipteran, the Asian citrus psyllid (Diaphorina citri). Before tree death there is a period of increased preharvest fruit drop and the production of smaller fruit containing off-flavored juice. The increasing frequency of HLB symptomatic fruit moving into the juice processing chain may become a challenge for maintaining flavor quality. An alternative use is sought for flavor degrading fruits, to recover value for fruit growers and juice processors. The purpose of this study was to investigate a steam explosion process for the release and recovery of highly functional pectic hydrocolloids and phenolic compounds from culled fruit. Symptomatic fruit from two varieties were culled and either entire fruits or juice-extracted fruit peels were submitted to steam explosion. Released pectic hydrocolloids, a useful hydrocolloid readily functionalized via alkaline demethylesterification, were co-isolated with a solvent-free fraction enriched in phenolic compounds. Recovery of pectic hydrocolloids ranged from 58% to 78%. Weight average molecular weight of this material ranged from 5.78 × 105 to 1.02 × 106 kDa and the degree of methylesterification ranged from 61% to 79%. Functionalization using alkaline, calcium sequestering compounds resulted in viscosities up to 250 mPa s−1. Additionally, recovery ranges of 5%–100% of the polymethoxylated flavones, 2%–71% of the flavanone glycosides, >50% of the limonoids were obtained. These results indicate this process would be useful for recovering value from culled fruit which would otherwise either degrade juice quality or be shunted for conversion to low value animal feed.