What do congee, paella, risotto, and chimichangas have in common?
Nearly half of the world’s population eats rice on a daily basis, making it a staple food for roughly 3.5 billion people. As delicious and filling as rice is, it is also the main source of arsenic for humans and its cultivation is one of the greatest contributors of methane emissions in the atmosphere. Two papers published last week in the journals PLoS ONE and Nature highlight the most recent efforts by researchers to find solutions for rice’s arsenic and methane problems.
Over the years, human activities like mining and coal burning have released tonnes of arsenic from underground mineral deposits to seep into the soil and water. Rice takes up ten times more arsenic from its environment than other cereal grains largely due to the fact that it is the only grain grown in fields that are completely underwater. The lack of oxygen in flooded rice paddies converts the arsenic into a more mobile form that is easier for the rice plants to take up. Arsenic accumulates in all parts of the plant but it reaches the highest levels in the husk. That’s why brown rice has higher levels of arsenic than white rice.
Chronic arsenic exposure has been linked to several different types of cancer including cancers of the lung, skin and prostate. Those most at risk of arsenic poisoning are people who eat rice multiple times a day and infants whose first solid meals are commonly rice-based baby foods. Several initiatives underway to reduce arsenic levels in rice include creating a low arsenic variety of rice and using drier cultivation methods. These ideas will help provide long-term solutions but in the meantime, more immediate solutions are needed to address the problem of arsenic exposure in places like Bangladesh where an estimated 100 million people are suffering from arsenic poisoning.
The first paper in PLoS ONE looks at an innovative way of cooking rice that is more efficient at removing arsenic than the conventional method of rice preparation. Led by Dr. Andrew Meharg at Queen’s University Belfast in Northern Ireland, the researchers created two different set ups for cooking rice with percolating water. In the first set up, distilled water was heated and allowed to drip through the rice. The flow-through water was collected and heated up again to become steam, which was passed through a condensation tube to drip through the rice again. The water was cycled through this way three times before the rice was fully cooked. In the second set up, the researchers bought a coffee maker, which normally drips hot water through coffee grinds to make coffee. They put the rice in a paper filter, as you would your coffee grinds, and allowed the machine to do its job. Depending on the type of rice, it took the machine two to three runs to produce fully cooked rice. To determine how much arsenic was removed, they compared each batch of cooked rice with uncooked rice from the same manufacturer.
Using the lab set up, approximately 60% and 70% of the arsenic was removed from white and brown rice, respectively. With the coffee maker, roughly 50% of the arsenic was removed from the rice, regardless of the type of rice. What the researchers did not do was compare these methods with the standard way of cooking rice, which uses about one-and-half to two cups of water for each cup of rice. While there are studies showing that the conventional way of cooking rice does not remove significant amounts of arsenic, it would have been useful to include this method so that the different cooking methods could be directly compared.
However, the researchers did test cooking rice the standard way with higher ratios of water. At a water to rice ratio of 12:1, roughly 57% of the arsenic was removed on average. At lower ratios of 6:1 and 3:1, only 30% of the arsenic was removed from the cooked rice when compared to the uncooked rice.
The researchers also noted that while brown rice had higher levels of arsenic compared to white rice, the arsenic was also more efficiently removed from brown rice.
One thing that wasn’t clear from the experiments was whether or not the rice was washed beforehand. Previous studies have shown that rinsing rice thoroughly with water multiple times before cooking can remove as much as 30% of the arsenic. I was curious to know whether their effects would still be that dramatic if they washed their rice before cooking.
A potential pitfall of this new cooking method is nutrient loss. The percolating water carries away harmful arsenic but could it also carry away beneficial nutrients? The researchers found that in rice cooked with percolating water, there was a 53% decrease in potassium levels and a 7% decrease in phosphorous levels compared to uncooked ice. The levels of calcium, iron, manganese and other minerals stayed the same. However the researchers did not measure vitamin levels in the cooked and uncooked rice so we don’t know if this cooking method caused vitamins like niacin (vitamin B3) and thiamine (vitamin B1) to leach out.
Despite the holes in this paper, it still provides a convincing proof of principle for a low cost, easy to implement solution that can be used by both home cooks and commercial rice product manufacturers. Importantly, this method can be adapted for situations where there is a shortage of clean water or where the water itself is also contaminated with arsenic.
Much like how flooded rice paddies enable rice plants to become excellent scavengers of arsenic, those conditions also create the perfect environment for methane generation. Through photosynthesis, rice plants convert carbon dioxide to useable sugars, like starch, which it stores in its grains, shoots, and importantly, roots. From the roots, the sugars are released into the soil where they are gobbled up by communities of soil microbes. Flooded rice fields create soil conditions that are anaerobic, or completely lacking in oxygen. Under these conditions, special groups of bacteria called methanogens (for methane-generating) thrive, taking up the sugars released from the roots and converting it to methane. The methane is taken back up by the rice plant and emitted into the air.
Methane emissions from rice paddies account for 7-17% of the total amount of methane in the atmosphere, making it one of the largest human-driven sources of methane. In comparison, livestock farming is responsible for roughly a quarter of global methane emissions. Although methane is not as abundant in the atmosphere as carbon dioxide, it has a higher heat absorbing capacity, pound for pound, than carbon dioxide, making it the second most important greenhouse gas.
In 2002, researchers found that rice plants with more grains emitted less methane. This seemed to be because more of the plants’ sugars were going to the grains and less to the roots, where they could be used by methanogens to create methane. Suppose you could create a rice plant that shifted a greater proportion of its sugary starches from its roots to its grains. This high-starch, low-methane rice plant could kill two birds with one stone—increase the grain yield and nutritional value of rice while decreasing the amount of methane emissions from its cultivation.
In a paper published in Nature last week, a group of researchers led by Dr. Chuanxin Sun at the Swedish University of Agricultural Sciences describe, for the first time, a high-starch, low-methane variety of rice that significantly cut methane emissions in field tests.
The researchers generated this new variety of rice by borrowing a gene from barley called SUSIBA2. In barley SUSIBA2 controls the expression of genes that lead to increased sugar storage. A version of these genes also exist in rice so the researchers hypothesized that if they could use SUSIBA2 to turn on the rice genes only in seeds and stems, they could trick the plant into diverting sugars away from the plant’s root system and storing it in the grains. They did so by only expressing the barley SUSIBA2 gene in the seeds and stems of the rice plants.
During field trials in China, the SUSIBA2 plants were planted alongside a common rice variety called Nipponbare (or Nipp, for short). Before flowering, methane emission levels from the SUSIBA2 plants were roughly 10% of levels from the Nipp plants. At 28 days after flowering, methane levels from the SUSIBA2 plants were even lower—around 0.3% of the Nipp plants.
The SUSIBA2 plants also produced more grains with higher starch content than the Nipp plants. In contrast, the root system of the SUSIBA2 plants was smaller than that of the Nipp plants. By borrowing the SUSIBA2 gene from barley, the researchers had shifted the distribution of sugars in the plant from belowground to aboveground.
Looking more closely at the bacterial communities in the soil, the researchers found that there were fewer methanogens associated with the roots of the SUSIBA2 plants compared to the Nipp plants. Less sugary compounds being sent to the roots meant less food for methane-generating bacteria, which led to a lot less methane being produced and emitted into the atmosphere.
At this point the high-starch, low-methane SUSIBA2 rice plant is sounding pretty awesome. But consider this: with less carbon going to the roots, we’re returning less nutrients to the soil and giving less food to all the soil microbes. These microbes decompose organic materials and generate important nutrients that improve the land’s fertility. How will this new rice strain affect soil quality and the microbial community over time? Will we have to add more fertilizers to compensate for the shifted balance of nutrients? These questions can only be addressed through long term field studies but I think they are important ones to think about before we start planting the SUSIBA2 plants en masse.
That being said, I think this plant, and the innovative approach taken by these researchers, has huge potential to help reduce methane emissions from rice paddies. As the world population continues to rise and rice cultivation expands to meet the growing demands for food, we need more ideas and research like the ones presented in these two papers to help us produce rice that is both better for us and the world in which we live.
References: Carey M, Jiujin X, Gomes Farias J, & Meharg AA (2015). Rethinking Rice Preparation for Highly Efficient Removal of Inorganic Arsenic Using Percolating Cooking Water. PloS one, 10 (7) PMID: 26200355
Su J, Hu C, Yan X, Jin Y, Chen Z, Guan Q, Wang Y, Zhong D, Jansson C, Wang F, Schnürer A, & Sun C (2015). Expression of barley SUSIBA2 transcription factor yields high-starch low-methane rice. Nature PMID: 26200336