Nixtamalization: A Very Old Cooking Process

Around 3,000 years ago, ancient people from places that are now Mexico and Guatemala developed a cooking process for transforming hard-to-eat and unhealthy raw maize grains into soft, delicious, and nutritious cooked kernels, known as nixtamal (Figure 1A). They discovered that boiling maize grains with alkali substances (also known as “bases” in chemistry) such as wood ashes or minerals such as calcium hydroxide (lime) made the cooking liquid more basic. This process, called nixtamalization, allows the maize grains to soak up water and grow bigger and softer, by making the hard skin of the corn come off easily [1]. Then, they used these cooked grains to make a soft maize dough that could be turn into products that you may have tried at a Mexican restaurant or in a bag of Doritos® from the supermarket. Little did they know that their cooking process would last for hundreds of years and that tacos, tortilla chips, and other delicious dishes would be eaten all over the world (Figure 1B).

The Downsides of Nixtamalization

Cooking maize requires lots of water. This might not have been a problem for ancient civilizations in a world where resources like drinking water seemed infinite. However, nowadays we have less drinking water, and food industries that produce maize-based products are having trouble getting enough water because of dry weather—which is getting worse because the Earth is getting hotter (Figure 2A). However, one of the most worrying aspects of nixtamalization is that nejayote, the water left over after cooking and rinsing maize, is very dirty (Figure 2B). Nejayote contains particles of the removed maize peel, proteins, sugars, nutrients, and large amounts of leftover lime in the form of dissolved calcium [2]. If nejayote is poured into drains or into rivers or lakes, it can cause severe problems that could damage our planet.

Transforming Waste Into Treasure

The leftover calcium in nejayote is an especially big problem, since it makes nejayote difficult to treat. Filtering nejayote to remove it does not work well because it quickly clogs up the expensive filters [3]. Treating nejayote with hungry bacteria that eat pollutants also does not work because these bacteria get stressed by the large amounts of calcium and they stop eating [2].

As a way to avoid these problems, scientists have developed a method to first collect the leftover calcium [4]. To do so they use phosphate, a cheap and widely available substance, to transform calcium into hydroxyapatite. Hydroxyapatite is a white, soft powder quite similar to the material that our teeth and bones are made of. To create hydroxyapatite, phosphate salts are mixed into a small amount of water until they are completely dissolved. This phosphate solution is slowly mixed into nejayote. When phosphate gets close to calcium, they attract each other like magnets and bond tightly like puzzle pieces (Figure 3A). After this fusion, a new molecule called calcium phosphate settles out of the nejayote as tiny solids. After recovering calcium phosphate, washing it, and heating it at very high temperatures (such as 500°C or higher), hydroxyapatite nanoparticles are formed, and the remaining nejayote contains less calcium [5].

Hydroxyapatite, an Expensive Tooth and Bone Building Block

Our bones are very hard but still have some flexibility because they have both soft and hard components. The soft part (30%) is made of proteins, while the hard part (70%) is made of minerals including hydroxyapatite and calcium phosphate. Maintaining healthy bones is very important for a good quality of life. Many people need help fixing bone problems when they suffer sports injuries, accidents, or medical conditions. Hydroxyapatite is an amazing material that scientists use to fix bones. One great thing about hydroxyapatite is that our bodies accept it as a “friend” instead of attacking it as an “enemy”. It also helps new bone cells grow; thus, it can help bones connect to fake teeth and body parts called implants, allowing bone and tissue implants to work together. Also, dentists use hydroxyapatite to repair and strengthen teeth. It helps by filling tiny holes in teeth to make them hurt less (Figure 3B). Hydroxyapatite is also used to make fake teeth, which replace missing teeth. Thanks to hydroxyapatite, scientists can create better treatments to help people with bone and tooth problems heal faster.

Conclusion

We can consider hydroxyapatite a treasure, since it is very expensive and extremely useful. Obtaining this mineral from nejayote can help doctors and dentists fix bones and teeth more cheaply, so that more people can afford to treat their health problems. This work is very special since it was the first time that this mineral was recovered from waste such as leftover water from cooking maize. In addition to producing hydroxyapatite, recovering the calcium from nejayote helps to make the leftover water cleaner, so that it will not hurt nature once it is released into the environment. So the next time you eat a taco, remember: even the water used to cook corn might hide a scientific treasure!

Glossary

Alkali: ↑ Substances that, when dissolved in water, result in a high pH. Alkali substances are the opposite of acids.

Nixtamalization: ↑ A method for cooking corn with lime in water to make it soft and healthy for foods like tortillas.

Nejayote: ↑ The yellow, highly polluted wastewater from preparing nixtamalized corn for tortillas.

Dissolved Calcium: ↑ Calcium ions with a positive charge that remain free in a liquid and do not settle to the bottom.

Phosphate: ↑ A phosphorus compound with a negative charge formed by a phosphorus atom surrounded by four oxygen atoms; it can bind with dissolved calcium.

Hydroxyapatite: ↑ A hard calcium and phosphorous mineral that gives bones and teeth their strength. About two-thirds of your bones are made of this mineral.

Nanoparticles: ↑ Tiny particles with a size smaller than 100 nanometers, which is approximately 1,000 times smaller than a grain of sand.

Implants: ↑ Man-made teeth and bone pieces that doctors put in the body to replace a missing tooth or broken bone

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The financial support from Tecnológico de Monterrey through the Challenge-Based Research Funding Program (E051EICGI10-A-T8-E) is acknowledged. The funding from the Second National Challenge of Sustainability (2024): Preservation, utilization, and sanitation of water in Mexico from BBVA Mexico is also acknowledged.

AI Tool Statement

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