Oxalate content of green juices and strategies for reduction of soluble oxalate content : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Green juices are an example of a modern food innovation that has developed partly by itself in response to general nutritional advice in the media imploring people to consume more fruits and vegetables. Unfortunately, the risks of developing acute oxalate nephropathy is high following the consumption of some green juices and fruit juices. This study investigates the occurrence and levels of oxalates in green juices made from common fruit and vegetables available in New Zealand. The influence of the type of juicer used to prepare the juices, and the use of treatments and/or additions to reduce the oxalate content of these mixed juices, was also investigated. A Design of Experiments multi-factor approach was used to investigate the extraction of oxalates from a green juice. Fresh spinach samples were juiced in a high speed blender and the oxalate contents were extracted using seven levels of pH, eight levels temperature and six extraction times. A statistically significant quadratic model with an overall level of significance of P < 0.0002 was achieved by using 20 experimental runs performed over one day. The optimum conditions for extracting all the oxalate from the model green spinach juice was at pH 0.93, with a temperature of 65°C and at any time greater than 15 minutes. The least amount of oxalic acid was extracted at pH 4.59 with a temperature of 25°C. To investigate the levels of oxalates found in homemade green juices, five green juice recipes containing between three and nine ingredients were made. Fresh spinach was the common component, and ranged from 20.1% to 37.9% w/w. The oxalate content ranged from 90.34 to 152 and 13.0 to 82.85 mg/100 g fresh weight (FW) for total and soluble oxalates, respectively. The percentage of soluble oxalate varied from 11.9 to 67.8%. The juices were made using a masticating juicer (MJ), which separates out a waste pulp fraction that is normally discarded. This fraction contained large amounts of oxalic acid, 134.6 to 348.09 mg/100 g FW in total. The waste pulp also had a higher concentration of minerals of all 11 minerals present, apart from Al, compared to the green juice. It is proposed there is an interaction between the oxalate, calcium and the pulp fraction of the green juice. A 200 g glass of green juice containing a mix of apple, celery and spinach, contained 165.7 mg of soluble oxalate; this is a considerable amount of soluble oxalate to ingest. In the home, juicing commonly uses two basic types of juicer, a high speed blender (HSB) or a masticating juicer (MJ); the MJ separates out a pulp fraction while the HSB does not. High and low spinach containing juices were made using both juicers. The MJ had a concentrating effect during processing, resulting in 528.41 mg total oxalate/100 g FW compared to the HSB, at 369.47mg total oxalic acid/100 g FW. They both produced green juices that had similar ratios of soluble oxalate, for both the high and low spinach juices. A significant amount of the oxalate and minerals were separated out into the waste pulp fraction from the juice made using the MJ. The waste pulp contained 238.10 to 55.67 mg total oxalate/100 g FW and 61.7 to 74.7% of the calcium content of the original spinach leaves. Two strategies to lower the soluble oxalate content of green juices were investigated; direct addtion of a food grade calcium salt, and soaking the raw materials used to make the fresh green juice. Four calcium salts tested. Calcium chloride was the most effective, reducing the soluble oxalate by 98.3% after the addtion of 500 mg/100 g to the fresh juice. Fresh spinach leaves were soaked in either tap water, a 1% w/v/ NaCl or a 1%w/v CaCl solution, prior to processing into a juice. The tap water and 1% w/v NaCl showed no loss of soluble oxalate but the 1% w/v CaCl treatment showed a 50% reduction in soluble oxalate and a 28.% increase in insoluble oxalate. This treatment had the added benefit of increasing the bioavailabilty of calium in the soaked spinach leaves. This study has shown that there are high levels of oxalate in green juices and, if consumed, this could pose a risk for diet induced nephropathy.