In Mexico, this staple — made with heirloom corn — is under pressure from mass production and modernity, but small producers are pushing back.

Petra Cruz González wakes at 6 every morning to make some 400 tortillas by hand. Despite a few modern advancements, like an electric flour mill and a metal hand press, she still cooks them over a wood fire as she learned to do when she was 8 years old.

Ms. González, 49, sells tortillas on the street and from her home. As the president of the Union de Palmeadoras in Tlaxiaco, which started in 1990 to organize this Oaxacan city’s handmade tortilla producers, she believes this is important work. The union’s 89 members (all but one a woman) are fighting to keep this millenniums-old tradition alive in the face of cheaper competitors.

The city’s 25 tortillería shops have severely cut into their sales, and Ms. González often has to work until 10 p.m. to sell all her tortillas.