NEWS FROM THE FACULTY OF AGRICULTURE AND LIFE SCIENCES, LINCOLN UNIVERSITY
By Janette Busch
Rhubarb is easy to grow, is found in many home gardens throughout New Zealand and is enjoyed begin eaten by many people. This tasty food originates in China where it is valued for its medicinal properties.
Scientists from the Food Group in the Faculty of Agriculture and Life Sciences at Lincoln University, have a long-standing research interest in a number of compounds that occur naturally in foods, in particular, those containing oxalates such as rhubarb. Raw rhubarb contains high levels of oxalates.
Oxalates are part of the defence system of plants against disease and predators such as birds and insects but have no role in humans. When foods containing oxalates are eaten the oxalates pass out of the body as waste, either through the urine or in the faeces (depending on whether the oxalates are soluble or insoluble).
The absorption of excessive amounts of soluble oxalates from high oxalate foods can lead to kidney stone formation in susceptible people (those who have already had one attack), an unpleasant and painful disease.
As part of her large PhD project Ha Nguyen, analysed the levels of oxalates in both raw and cooked rhubarb with and with the addition of standard or trim milk.
“There have been previous studies undertaken on the levels of oxalates in rhubarb but none on cooked rhubarb, which is the way most people in New Zealand eat rhubarb,” said Ha.
Ha found that adding the milk to rhubarb significantly reduced the soluble oxalate levels in the rhubarb after cooking. The soluble oxalate from is implicated in stone formation.
Although boiling reduced the total and soluble contents by dilution, the levels of soluble oxalate and intestinal available oxalate were further reduced by the presence of added calcium when the rhubarb was cooked in standard or trim milk.
“The calcium in the milk binds to the soluble oxalates in the rhubarb making it insoluble. These insoluble oxalates then pass through the digestive tract without being absorbed and are excreted as waste,” said Ha.
“People who like eating rhubarb and who are concerned about kidney stones may want to consider eating rhubarb with a milk-containing product like custard or yoghurt,” said Ha.
Associate Professor Geoffrey Savage who supervised Ha’s project said that this research provided relatable information on how to minimise oxalate absorption from this widely used food.
“For vegetarian and vegan populations who consume large amounts of green leafy vegetables the prolonged consumption of high oxalates foods such as rhubarb this can lead to a mineral deficiency, said Dr Savage.”
“The most significant feature of this study is that a significant beneficial reduction in soluble and intestinal available oxalates occurs when rhubarb is cooked with trim milk.
While trim milk contains more calcium per 100 mL than standard milk both types of milk significantly reduced the soluble oxalate levels.”
“We were intrigued to learn recently of a quotation from an entry in The Penny Encyclopaedia of 1841, published by the Society for the Diffusion of Useful Knowledge in Great Britain, that warned people subject to “calculous” (stone formation) against eating rhubarb stalks. It seemed the potential for harm for some people from eating cooked rhubarb stems was already known in the early 19th century,” said Dr Savage.