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Not all processing has an equally detrimental effect on the nutritional quality of rice, according to new research. [Photo: Bigstock]

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New evidence shows that processing rice, for instance polishing to remove the husk, but parboiling it to make it quicker for consumers to cook, significantly reduces its nutritional benefits.

As a cereal grain, rice is the most widely consumed staple food for a large part of the world’s human population, and it is mainly consumed as polished rice. This polishing process is performed on the rice so consumers can benefit from improved physical, sensory and shelf life properties. This process removes the bran and the germ, and is thought to be detrimental to the nutritional qualities of rice as the bran layers are rich in proteins, vitamins and fat, whilst the germ is mainly fat.

A common secondary processing technique is parboiling, in which the rice is soaked, steamed and dried. Parboiling ca increase the storage life of rice and make it quicker to cook. when cooked parboiled rice tends to be firmer and less sticky.

Different effects

A new in the journal Food Science looked at the processing of specific black and red pigmented strains of rice grains (Oryza sativa L.),

These varieties contain healthful phenolic compounds distributed in their bran layers. Phenolics compounds have been shown to promote several health benefits for consumers, including antioxidant, anticancer, antiallergic, anti-inflammatory, as well as being heart healthy and having anti-diabetic effects. Most recently they have also been shown to be important for gut health.

Anthocyanins, mainly cyanidin-3-O-glucoside, are the main phenolic constituent of black rice grains, while proanthocyanidins – also known as condensed tannins – have been reported as the main phenolic constituent of red rice grains.

In this study researchers found that processing – which is mainly done for aesthetic reasons, caused several significant losses in nutritional quality:

• Polishing caused a 16.2% decrease in protein of black rice and 6.2% in red rice.
• In addition, the soaking stage of parboiling decreased the protein content in both types of rice further.
• Polishing the rice, which removed the germ of the grain, also caused a drop in fat content of 86.2% and 77.4% in black and red rice respectively.
• Parboiling the grains also caused a further decrease in fat content of over 10%.
• Additionally, the polishing treatment removed 92.6% and 97.5% of antioxidant phenolic content of black and red rice respectively.

But say the researchers, there were some pluses from processing. Parboiling did not seem to have a negative effect on protein or phenolic compounds (and even allowed the partial preservation of some types of phenolics in black and red rice) and reduced cooking time for both black and red rice types.