Enzymatic debittering of grapefruit peel juice

[摘要] Vast amounts of waste consisting of peels, segment membranes and seeds are generatedduring grapefruit juice processing. The peels can be used for juice extraction to obtaingrapefruit peel juice. Grapefruit peel juice can be a relatively cheap product and can beused as juice fillers. Extreme bitterness due to the compounds naringin and limonin limitsthe use of grapefruit peel juice in such applications. The aim of this study was todetermine the effects of the enzymes aromase and laccase on the bitter compoundsnaringin and limonin and other physico-chemical properties of grapefruit peel juice.Grapefruit peel juice was prepared by freezing milled peel residues, defrosting andpressing the juice through a screen. The peel juice was treated with aromase (0, 0.4 and0.8% w/v) and laccase (0, 1.5 and 3.0% w/v) in a 3 x 3 factorial experiment. ReversephaseHPLC was used to determine naringin, naringenin and limonin contents. Sugars(glucose, fructose, sucrose and rhamnose) were determined using liquid chromatography,anion-exchange chromatography with pulsed amperometric detection and gaschromatography-mass spectrometry. The colour and clarity were also determined. A 25-member consumer sensory panel was used to rate the juice samples for bitterness.Treating grapefruit peel juice with increasing concentrations of aromase decreasednaringin content by 80% and increased naringenin by 85 times. Increasing concentrations laccase only decreased naringin by up to 40% and increased naringenin by 4 times.Aromase-laccase combination treatment at their highest concentrations produced thegreatest decrease in naringin. Glucose content increased by 1.2 times on treating witharomase and by 0.95 times on treatment with laccase. The combination enzyme treatmentproduced the greatest increase in glucose by 2.0 times. There was no evidence of releaseof rhamnose upon aromase treatment. The rhamnose moiety (from the disaccharidemoiety of naringin) may be broken down into other compounds due to other activities ofaromase.Limonin was decreased by 8 times on treatment with aromase and by 1.2 times ontreatment with laccase. The combination enzyme treatment decreased limonin by up to 6times. The untreated grapefruit peel juice showed an increase in limonin content byalmost 30% after storage for 7 months while the aromase-treated sample showed adecrease in limonin by 35%, an indication that aromase can be used to prevent delayedbitterness in grapefruit peel juice.The grapefruit peel juice became darker on treatment with laccase and lighter ontreatment with aromase. The combination treatment made the grapefruit peel juice darkercompared to treatment with laccase on its own. Treatment with aromase increased clarityby 25% by making it less hazy.Although the decrease in naringin due to treatment with aromase on its own was less thanthe combination enzyme treatment, the aromase-treated sample was ranked by thesensory panel as least bitter followed by the combination enzyme-treated, laccase-treatedand the untreated samples. This may be due to the greater decrease in limonin in thearomase-treated sample compared to the other samples.In summary, this research shows that aromase can be used either on its own or incombination with laccase to debitter grapefruit peel juice, although it can also be used incombination with laccase. The use of these enzymes provides the citrus processing industry with alternative and possibly more cost-effective methods of debittering citrusproducts.

[发布机构] University of Pretoria