The Role of Phytase and Phytic Acid
Phytase is a type of enzyme that plants naturally produce to break down phytic acid, also known as phytate. Phytic acid is the primary storage form of phosphorus in many plant seeds, including whole grains like brown rice. It is considered an “anti-nutrient” because its chemical structure allows it to bind to minerals such as iron, zinc, calcium, and magnesium in the digestive tract, which reduces the body’s ability to absorb them.
Unlike ruminant animals, humans do not produce their own phytase, which means we must rely on the phytase present in our food or on external processing methods to degrade phytic acid. When brown rice is consumed without proper preparation, a portion of its minerals may pass through the digestive system unabsorbed. Therefore, understanding the phytase activity in brown rice is important for maximizing its nutritional benefits.
Phytase Activity in Raw and Processed Brown Rice
Brown rice contains endogenous phytase, but its activity is generally considered low compared to other whole grains. A study published in the Journal of Food Science and Technology investigated the changes in brown rice's phytase activity during processing.
- Raw brown rice: The study found that raw brown rice has a relatively low baseline level of phytase activity. For instance, one measurement indicated activity at 214 U/kg, placing it in the low category for cereals.
- During steeping: Surprisingly, soaking brown rice in water can initially decrease phytase activity, especially at certain temperatures. The decrease is thought to be due to the enzyme diffusing into the steeping water, where it is washed away.
- During sprouting: The most effective way to increase phytase activity is through sprouting, or germination. As the grain sprouts, its metabolic processes trigger a significant increase in phytase production to mobilize the stored nutrients. After just a few days of sprouting, phytase activity can increase dramatically.
- Effect of heat: Phytase is sensitive to high temperatures. While warmer soaking can degrade phytic acid faster, the enzyme itself can be denatured (destroyed) if the temperature is too high. This is why cooking brown rice in boiling water and rinsing it afterwards, as some methods suggest for reducing arsenic, can deplete the grain of its natural phytase.
Methods to Maximize Phytase and Reduce Phytic Acid
Fortunately, home cooks can use simple methods to reduce phytic acid and activate the enzyme in brown rice, thereby enhancing its nutritional value.
- Soaking: Soaking brown rice for at least 12-24 hours in warm, demineralized water is a simple and effective method. The water should be changed periodically to remove the phytic acid that leaches out. Soaking is the first step in the sprouting process, and it helps initiate enzyme activity.
- Sprouting (Germination): This process involves soaking and then leaving the rice to sprout until tiny tails appear. The germination process powerfully activates endogenous phytase, leading to a substantial breakdown of phytic acid. Studies have shown that phytase activity peaks after a few days of sprouting.
- Fermentation: Using a lactic acid culture to ferment brown rice is another powerful way to reduce phytic acid. The microorganisms introduced during fermentation often have high phytase activity, which works alongside the grain's natural enzymes to break down phytic acid. A study using Lactobacillus sakei with brown rice showed significant phytate reduction.
The Brown Rice vs. White Rice Comparison
The differences in nutritional and phytic acid content between brown and white rice are primarily due to the milling process.
| Feature | Brown Rice | White Rice |
|---|---|---|
| Processing | Only the inedible outer husk is removed. The nutrient-rich bran and germ layers remain intact. | The outer husk, bran, and germ are all removed, leaving only the starchy endosperm. |
| Phytic Acid Content | Higher levels, as phytic acid is concentrated in the bran and germ layers. | Significantly lower levels, since the bran layer has been removed. |
| Phytase Content | Endogenous phytase is present in the bran and germ, though at low levels in the raw state. | Practically non-existent, as the bran and germ containing the enzyme are removed during milling. |
| Mineral Bioavailability | Lower in its raw state due to high phytic acid levels, but can be significantly improved with proper preparation. | Higher in its raw state due to low phytic acid content, but is also less mineral-rich overall. |
| Other Nutrients | Higher in fiber, magnesium, phosphorus, and zinc. | Lower in most micronutrients, though often fortified with nutrients like iron and folate. |
Conclusion: Making Brown Rice More Bioavailable
In conclusion, brown rice does have phytase, but its natural activity is relatively low in the raw grain. The key to unlocking brown rice's full nutritional potential is to boost this enzyme activity and reduce its phytic acid content through careful preparation. Techniques such as soaking and, most effectively, sprouting, can dramatically increase phytase levels and significantly enhance the bioavailability of minerals like zinc and iron. While the phytic acid in brown rice may inhibit some mineral absorption, it can also act as an antioxidant and offer other health benefits. Ultimately, understanding and utilizing these preparation methods allows you to enjoy brown rice as a highly nutritious whole grain while mitigating the anti-nutrient effects of phytic acid.
References
- Phytase activity in brown rice during steeping and sprouting.
- Effects of Brown Rice Extract Treated with Lactobacillus sakei.
- Brown Rice vs. White Rice: Nutrient Comparison.
- Phytic Acid in Brown Rice Can Be Reduced by Increasing Soaking Temperature.
- Iron absorption from brown rice/brown rice-based meal and milled rice.
- Brown Rice as Useful Nutritional Source.
