Phytase is the enzyme that neutralizes phytic acid and liberates the phosphorus. This enzyme co-exists in plant foods that contain phytic acid.
Ruminant animals such as cows, sheep and goats have no trouble with phytic acid because phytase is produced by rumen microorganisms; monogastric animals also produce phytase, although far less. Mice produce thirty times more phytase than humans,26 so they can be quite happy eating a raw whole grain. Data from experiments on phytic acid using mice and other rodents cannot be applied to humans.
In general, humans do not produce enough phytase to safely consume large quantities of high-phytate foods on a regular basis. However, probiotic lactobacilli, and other species of the endogenous digestive microflora can produce phytase.27 Thus, humans who have good intestinal flora will have an easier time with foods containing phytic acid. Increased production of phytase by the gut microflora explains why some volunteers can adjust to a high-phytate diet. Sprouting activates phytase, thus reducing phytic acid.28 The use of sprouted grains will reduce the quantity of phytic acids in animal feed, with no significant reduction of nutritional value.29
Soaking grains and flour in an acid medium at very warm temperatures, as in the sourdough process, also activates phytase and reduces or even eliminates phytic acid.
Before the advent of industrial agriculture, farmers typically soaked crushed grain in hot water before feeding it to poultry and hogs. Today, feed manufacturers add phytase to grain mixes to get better growth in animals. Commercial phytases are typically produced using recombinant DNA technology. For example, a bacterial phytase gene has recently been inserted into yeast for commercial production.
Not all grains contain enough phytase to eliminate the phytate, even when properly prepared. For example, corn, millet, oats and brown rice do not contain sufficient phytase to eliminate all the phytic acid they contain. On the other hand, wheat and rye contain high levels of phytase—wheat contains fourteen times more phytase than rice and rye contains over twice as much phytase as wheat.30 Soaking or souring these grains, when freshly ground, in a warm environment will destroy all phytic acid. The high levels of phytase in rye explain why this grain is preferred as a starter for sourdough breads.
Phytase is destroyed by steam heat at about 176 degrees Fahrenheit in ten minutes or less. In a wet solution, phytase is destroyed at 131-149 degrees Fahrenheit.31 Thus heat processing, as in extrusion, will completely destroy phytase—think of extruded all-bran cereal, very high in phytic acid and all of its phytase destroyed by processing. Extruded cereals made of bran and whole grains are a recipe for digestive problems and mineral deficiencies!
Phytase is present in small amounts in oats, but heat treating to produce commercial oatmeal renders it inactive. Even grinding a grain too quickly or at too high a temperature will destroy phytase, as will freezing and long storage times. Fresh flour has a higher content of phytase than does flour that has been stored.32 Traditional cultures generally grind their grain fresh before preparation. Weston Price found that mice fed whole grain flours that were not freshly ground did not grow properly.33
Cooking is not enough to reduce phytic acid—acid soaking before cooking is needed to activate phytase and let it do its work. For example, the elimination of phytic acid in quinoa requires fermenting or germinating plus cooking (see Figure 3). In general, a combination of acidic soaking for considerable time and then cooking will reduce a significant portion of phytate in grains and legumes.
THE PHYTATE THRESHOLD
It appears that once the phytate level has been reduced, such that there is more available phosphorus than phytate in the grain, we have passed a critical point and the food becomes more beneficial than harmful. Retention of phosphorus decreases when phytate in the diet is 30-40 percent or more of the total phosphorus.35
For best health, phytates should be lowered as much as possible, ideally to 25 milligrams or less per 100 grams or to about .03 percent of the phytate-containing food eaten. At this level, micronutrient losses are minimized. (For phytate content of common foods as a percentage of dry weight, see Figures 4 and 5.)
White rice and white bread are low-phytate foods because their bran and germ have been removed; of course, they are also devitalized and empty of vitamins and minerals. But the low phytate content of refined carbohydrate foods may explain why someone whose family eats white flour or white rice food products may seem to be relatively healthy and immune to tooth cavities while those eating whole wheat bread and brown rice could suffer from cavities, bone loss and other health problems.
PHYTATES AND GERMINATION
Beer home brewers know that in order to make beer, they need malted (sprouted) grains. Soaking and germinating grains is a good idea, but it does not eliminate phytic acid completely. Significant amounts of phytic acid will remain in most sprouted grain products. For example, malting reduces wheat, barley or green gram phytic acid by 57 percent. However, malting reduces anti-nutrients more than roasting.36 In another experiment, malting millet also resulted in a decrease of 23.9 percent phytic acid after 72 hours and 45.3 percent after 96 hours.37
In legumes, sprouting is the most effective way to reduce phytic acid, but this process does not get rid of all of it. Germinating peanuts led to a 25 percent reduction in phytates. After five days of sprouting, chick peas maintained about 60 percent of their phytate content and lentils retained about 50 percent of their original phytic acid content. Sprouting and boiling pigeon pea and bambara groudnut reduced phytic acid by 56 percent.38 Germinating black eyed beans resulted in 75 percent removal of phytate after five days sprouting.
Germination is more effective at higher temperatures, probably because the heat encourages a fermentation-like condition. For pearled millet, sprouting at 92 degrees F for a minimum of 48 hours removed 92 percent of the phytate. At 82 degrees F, even after 60 hours, only 50 percent of phytic acid was removed. Higher temperatures above 86 degrees F seem less ideal for phytate removal, at least for millet.39
Sprouting releases vitamins and makes grains and beans and seeds more digestible. However it is a pre-fermentation step, not a complete process for neutralizing phytic acid. Consuming grains regularly that are only sprouted will lead to excess intake of phytic acid. Sprouted grains should also be soaked and cooked.
ROASTING AND PHYTIC ACID
Roasting wheat, barley or green gram reduces phytic acid by about 40 percent.40 If you subsequently soak roasted grains, you should do so with a culture that supplies additional phytase, as phytase will be destroyed by the roasting process.
ACIDIC SOAKING AND PHYTIC ACID
For grains and legumes that are low in phytase, soaking does not usually sufficiently eliminate phytic acid. Soaking of millet, soya bean, maize, sorghum, and mung bean at 92 degrees F for 24 hours decreased the contents of phytic acid by 4–51 percent.43 With these same grains and beans, soaking at room temperature for 24 hours reduced phytic acid levels by 16–21 percent.44 However, soaking of pounded maize for one hour at room temperature already led to a reduction of phytic acid by 51 percent.45
Sourdough fermentation of grains containing high levels of phytase—such as wheat and rye—is the process that works best for phytate reduction. Sourdough fermentation of whole wheat flour for just four hours at 92 degrees F led to a 60 percent reduction in phytic acid. Phytic acid content of the bran samples was reduced to 44.9 percent after eight hours at 92 degrees F.46 The addition of malted grains and bakers yeast increased this reduction to 92-98 percent. Another study showed almost complete elimination of phytic acid in whole wheat bread after eight hours of sourdough fermentation (See Figure 6).47
A study of phytates in recipes used typically by home bread bakers found that leavening with commercial yeast was much less effective at removing phytates. Yeasted whole wheat breads lost only 22-58 percent of their phytic acid content from the start of the bread making process to the complete loaf.48
PHYTIC ACID AND YOU
The purpose of this article is not to make you afraid of foods containing phytic acid, only to urge caution in including grains, nuts and legumes into your diet. It is not necessary to completely eliminate phytic acid from the diet, only to keep it to acceptable levels.
An excess of 800 mg phytic acid per day is probably not a good idea. The average phytate intake in the U.S. and the U.K. ranges between 631 and 746 mg per day; the average in Finland is 370 mg; in Italy it is 219 mg; and in Sweden a mere 180 mg per day.49
In the context of a diet rich in calcium, vitamin D, vitamin A, vitamin C, good fats and lacto-fermented foods, most people will do fine on an estimated 400-800 mg per day. For those suffering from tooth decay, bone loss or mineral deficiencies, total estimated phytate content of 150-400 mg would be advised. For children under age six, pregnant women or those with serious illnesses, it is best to consume a diet as low in phytic acid as possible.
In practical terms, this means properly preparing phytate-rich foods to reduce at least a portion of the phytate content, and restricting their consumption to two or three servings per day. Daily consumption of one or two slices of genuine sourdough bread, a handful of nuts, and one serving of properly prepared oatmeal, pancakes, brown rice or beans should not pose any problems in the context of a nutrient-dense diet. Problems arise when whole grains and beans become the major dietary sources of calories— when every meal contains more than one whole grain product or when over-reliance is placed on nuts or legumes. Unfermented soy products, extruded whole grain cereals, rice cakes, baked granola, raw muesli and other high-phytate foods should be strictly avoided.