Oxalate contents of kiwifruit (Actinidia deliciosa) juice extracted by pressing or enzyme extraction

Kiwifruit juice is a popular drink made from fruit that does not meet the standard shape and size required for direct consumption of the fruit. Extraction of the juice and pasteurisation may have an effect on the oxalate concentration of the processed juice. Total and soluble oxalate contents of extracted and processed juice were determined by HPLC chromatography. The levels were determined in the juice and pomace of pressed and enzyme-treated fruit and in the juice following pasteurisation. Oxalate contents of peeled green kiwifruit (Actinidia deliciosa var. Hayward) were 21.2 ± 0.33, 6.9 ± 0.03, 14.3 ± 0.05 (mg/100 g fresh weight) for total, soluble and insoluble oxalates, respectively. Treatment of the homogenised kiwifruit at 21°C for four hours with pectolase increased (p < 0.01) the yield of extracted juice. Higher levels of total and soluble oxalates (p < 0.01) were found in green kiwifruit juice obtained by enzymatic extraction compared with juice obtained by pressing. Most of the insoluble oxalate from the fresh kiwifruit was retained in the pomace during juice extraction. Soluble oxalate made up 59.3% of the total oxalate in the juice extracted by pressing while it constituted 66.8% of the juice extracted by enzyme extraction, compared with 32.7% in the fresh pealed tissue. Pasteurisation at 90°C significantly (p < 0.01) reduced the soluble oxalate content of both the pressed and enzyme treated kiwifruit juice but had no effect on the total oxalate content of the two juices