Oxalate and antioxidant concentrations of locally grown and imported fruit in New Zealand

Abstract

Locally grown and some imported fruits were analysed for their antioxidant and oxalate concentrations. Total phenolic and ascorbic acid concentrations and the antioxidant capacity using ABTS and ORAC methods showed that the fruits were a source of beneficial nutrients. In contrast, the fruits contained variable amounts of soluble and insoluble oxalates as anti-nutritive compounds. Fruit available in New Zealand contained a wide range of total phenolic compounds (27.4 – 2731.9 mg gallic acid equivalents (GAE)/100g fresh weight (FW)) and vitamin C (6.2-201.3 mg ascorbic acid/100 g FW). The ABTS capacity of these fruits ranged from 19.5 - 6045.9 μmol Trolox equivalents (TE)/100 g FW and the ORAC values ranged from 1121.2-13631.6 μmol TE/100 g FW. Extraction using 2 M HCL at 21°C was the optimum condition to extract total oxalate from fruit. This resulted in the highest mean value for total oxalate concentration(123.1 ± 2.5 mg/100 g DW) as compared to extraction using 0.2 M HCL at the same temperature (102.2 ± 2.1 mg/100 g DW) or extracting with 0.2 M HCL and 2 M HCL at 80°C (85.2 ± 3.0 and 114.5 ± 2.2 mg/100 g DW, respectively). Similarly, at 21°C, the mean soluble oxalate concentrations for the four analysed fruits were significantly higher than those extracted at 80°C (54.7 vs. 39.8 mg/100 g DM). The total oxalate concentration of locally grown fruit determined by extracting with 2M HCL at 21°C ranged from 2.9 – 7566.5 mg/100 g FW while the soluble oxalate extracted with deionised water ranged from 5 to 56% of the total oxalate concentration. Very high concentrations of total oxalate were measured in Indian gooseberry, rhubarb and carambola, 7566.5, 640.2 and 436.1 mg/100 g FW, respectively. Storage of kiwifruit for 15 days at 20 ± 1°C showed reductions in soluble oxalate concentration and increases in insoluble oxalate concentration in the three kiwifruit fractions measured (whole fruit, skin and pulp), but not the seeds. Green kiwifruit (Actinidia deliciosa) contained significantly higher concentrations of oxalates (p < 0.001) than golden kiwifruit(A. chinensis). Processing kiwifruit to produce juice considerably reduced the oxalate concentration because most of the insoluble oxalate was retained in the pomace. Concentrations of total and soluble oxalates found in green kiwifruit juice obtained by enzymatic extraction were significantly higher (p < 0.01) than the values obtained by pressing. In the juice, soluble oxalate concentration contributed to 60.3 - 70.3% of total oxalate concentration compared with 31.7% found in the fresh tissue. Pasteurization temperatures significantly affected the soluble oxalate concentration but did not impact on the concentrations of total oxalate in the juice. Boiling rhubarb petioles (Rheum rhabarbarum L.) in water or cooking with trim (low fat) or standard milk significantly (p < 0.05) reduced the total and soluble oxalate concentrations of the mixtures by dilution. Total oxalate in the raw rhubarb was 902.7 mg/100 g FW and in cooked rhubarb it was 454.3 mg/100 g FW. Cooking rhubarb petioles with standard and trim milk resulted in further reductions in soluble oxalate concentrations of 65.9% and 74.5%, respectively, when compared to the soluble oxalate concentration of the raw petioles. When using an in vitro method (to simulate the gastric and intestinal environments) to extract oxalates from the raw and cooked rhubarb, the amounts of soluble oxalates binding with fibre at pH 2.0 were significantly higher (p < 0.01) than at pH 7.0 (an average of 74.5 vs. 39.4 mg/100 g FW, respectively), while, the insoluble oxalate concentrations in the fibre increased when the extractions were carried out in pH conditions similar to the intestinal environment. This trapping of oxalate in the fibre fraction suggests that dietary fibre can partly reduce oxalate absorption.