Enzymatic debittering of grapefruit peel juice

Abstract

Vast amounts of waste consisting of peels, segment membranes and seeds are generated during grapefruit juice processing. The peels can be used for juice extraction to obtain grapefruit peel juice. Grapefruit peel juice can be a relatively cheap product and can be used as juice fillers. Extreme bitterness due to the compounds naringin and limonin limits the use of grapefruit peel juice in such applications. The aim of this study was to determine the effects of the enzymes aromase and laccase on the bitter compounds naringin and limonin and other physico-chemical properties of grapefruit peel juice. Grapefruit peel juice was prepared by freezing milled peel residues, defrosting and pressing the juice through a screen. The peel juice was treated with aromase (0, 0.4 and 0.8% w/v) and laccase (0, 1.5 and 3.0% w/v) in a 3 x 3 factorial experiment. Reversephase HPLC 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 gas chromatography-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 decreased naringin 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 the greatest decrease in naringin. Glucose content increased by 1.2 times on treating with aromase and by 0.95 times on treatment with laccase. The combination enzyme treatment produced the greatest increase in glucose by 2.0 times. There was no evidence of release of rhamnose upon aromase treatment. The rhamnose moiety (from the disaccharide moiety of naringin) may be broken down into other compounds due to other activities of aromase. Limonin was decreased by 8 times on treatment with aromase and by 1.2 times on treatment with laccase. The combination enzyme treatment decreased limonin by up to 6 times. The untreated grapefruit peel juice showed an increase in limonin content by almost 30% after storage for 7 months while the aromase-treated sample showed a decrease in limonin by 35%, an indication that aromase can be used to prevent delayed bitterness in grapefruit peel juice. The grapefruit peel juice became darker on treatment with laccase and lighter on treatment with aromase. The combination treatment made the grapefruit peel juice darker compared to treatment with laccase on its own. Treatment with aromase increased clarity by 25% by making it less hazy. Although the decrease in naringin due to treatment with aromase on its own was less than the combination enzyme treatment, the aromase-treated sample was ranked by the sensory panel as least bitter followed by the combination enzyme-treated, laccase-treated and the untreated samples. This may be due to the greater decrease in limonin in the aromase-treated sample compared to the other samples. In summary, this research shows that aromase can be used either on its own or in combination with laccase to debitter grapefruit peel juice, although it can also be used in combination with laccase. The use of these enzymes provides the citrus processing industry with alternative and possibly more cost-effective methods of debittering citrus products.