In May 2020, I started researching how to make a crunchy bean snack similar to corn nuts. I wondered how important it was to soak and cook beans. I wanted to know if I could break the rules of bean preparation. This led me to investigate the deactivation of phytohaemagglutinin (PHA) which is a serious toxin that's found in many beans, especially kidney beans. This article is not about my crunch bean experiments, it's about what I learned about PHA. After my research, I came to this conclusion...

TLDR;

As a home cook, you should follow the standard bean-cooking-procedure recommend by the FDA. Having said that, the FDA recommendation is not the only method for deactivating PHA. There are lots of studies that have experimented with different methods. But it seems clear to me that there are a lot of variables to consider when working with a deactivation method. Things like time, temperature, heating medium, bean variety, and hydration of the bean all matter. There's also the question of reproducibility. How reliable is the method and can others duplicate the results.

Because PHA is not deactivated like a bacteria or virus might be, it seems risky for a home cook to venture beyond the safety recommendations. You can't just heat it to a certain temperature and call it good. Now let's look at the other methods of bean preparation that some others have explored.

Standard FDA Approved Method

The official recommendation by the FDA is to boil Kidney Beans for 30 minutes. That's 100C for 30 minutes. The full recommendation is as follows from the "Bad Bug Book 2012."

PHA is destroyed by adequate cooking. Some variation in toxin stability has been found at different temperatures. However, Bender and Readi found that boiling the beans (Kidney Beans) for 10 minutes (100°C) completely destroyed the toxin. Consumers should boil the beans for at least 30 minutes to ensure that the product reaches sufficient temperature, for a sufficient amount of time, to completely destroy the toxin. source

Denaturing PHA in Navy Beans

The book Nutritional Improvement of Food and Feed Proteins contains a section titled "Nutritional Studies, Comparison of Dry-Roasted Beans with Autoclaved Beans." In it the author compares two processes for Navy bean treatment - one being autoclaved (a sort of industrial-strength pressure cooker) and dry-roasting with heated salt (page 402). The author observed that both were effective in deactivating the phytohaemagglutinin (page 405).

The destruction of the PHA, which are also known to be a factor contributing to the poor nutritive value of raw legumes, was virtually complete in both the autocalved and roasted beans.”

In that experiment, the dry roasting process was 20-25 seconds at 204C (400F) with the beans coming in direct content with heated salt.

Denaturing PHA in Fresh Kidney Beans

In the study "Phytohemagglutinin content in fresh kidney bean in China", the authors compare braising vs stir-frying and conclude that it takes only 10 minutes of braising vs 18 minutes of stir frying to see hemagglutinin denature. In the section titled "Effect of cooking methods on PHA" they say...

“Protein denaturation need enough temperature and time… Stir-frying need more time to remove PHA than braising. The reason may be that fresh kidney bean can't be uniformly heated in stir-frying as good as that in braising. Our result indicated that stir-frying or braising for sufficient amounts of time can effectively remove PHA. Although the Zihuayoudou cultivar was found to contain high levels of PHA, it could be safe when cooked completely.”

Should we only worry about kidney beans?

In 2013, Kathleen Purvis a food editor at the Charlotte Observer, responded to the question Are there toxins in dried, uncooked beans?. She says...

"This is a case of a little bit of truth leading to a lot of worry. Dried beans do have lectin, a type of protein, called phytohemagglutinin, or PHA. And while PHA is toxic in large amounts, only red kidney beans are high enough in PHA to be an issue. Most dried beans, including garbanzos (also called chickpeas), have much smaller amounts."

I think this is probably true, but I don't think this should be taken as a sign that dried, uncooked beans are necessarily safe. As the phrase goes, the dose makes the poison, and I wouldn't want to risk it.

Autoclave, Dry Heat, Microwave, Infrared

From G.P. Savage, in Encyclopedia of Food Sciences and Nutrition (Second Edition), 2003 link

Dry heat treatment (roasting) to destroy hemagglutinins is remarkably ineffective. For example, winged beans (Psophocarpus tetragonolobus) heated at 100 °C for 2 h show less than 5% reduction in lectin activity. The generally low but quite variable levels of hemagglutinins in this seed could be easily destroyed by autoclaving at 120 °C for 5 min.

The treatment of whole cow peas (Vigna unguiculata) with microwaves (2400 MHz) for 6 min was almost ineffective in destroying hemagglutinins, while dry roasting (160 °C for 50 min) led to between 28 and 47% reduction in lectin content. Treatment of cow peas with mild alkali (0.5% sodium bicarbonate for 12 h) led to an 80% reduction in hemagglutinin content. (See COOKING | Domestic Use of Microwave Ovens.)

Infrared heating of winged bean (Psophocarpus tetragonolobus) for 60 s was remarkably effective in reducing the hemagglutinating activity of this seed, whereas oven and microwave heating had little effect.

Fermentation?

Fermentation is used in some parts of Africa to improve the nutritional value of beans by removing toxins (source). It's unclear whether or not this includes PHA and I have not researched that.