Inmates are paid the same as regular coffee pickers - $5 per 28-pound bucket of ripe beans - and get 10 days taken off their sentences for each month of work.
Puerto Rico was among the world's leading coffee exporters in the 19th century, then hurricanes and increased competition hurt the business. Today, nearly all Puerto Rican coffee stays on the island, where many people start their day with a "cafe con leche" - coffee with milk.
William Cintron, a former mayor of Yauco and one of 40 coffee farmers who volunteered to use the prisoners, said the labor shortage hurts the coffee industry's growth by limiting how much farmers can plant. He wants even more prisoners used.
"We can't plant more coffee without manual labor, and the manual labor is in the prisons," he said.
But Jorge Gonzalez, mayor of the coffee-growing town of Jayuya, said inmates are too slow at picking the beans, which over ripen if they are not harvested quickly.
He wants to bring in pickers from the Dominican Republic instead of using inmates. "Foreign workers are the salvation and the solution for the coffee industry," he said.
This U.S. territory in the Caribbean has just 10,000 coffee pickers - 5,000 less than needed. The number drops each year as workers quit to take less strenuous jobs in manufacturing and other sectors that pay more than triple the average wages for coffee pickers.
Few prisoners can pick more than 150 pounds of beans in a five-hour day, a fraction of what a skilled bean picker can pluck. Skilled pickers earn on average $50 a day - twice what most prisoners make - although really speedy ones make up to $100.
Fabre said there have been few complaints about the prisoners. None has tried to escape, and the ones picking the beans in Yauco said they enjoyed it.
"It takes away the boredom of being locked up. It's a form of therapy for us," said Andre Rivera, a 24-year-old convicted armed robber with a wide grin and tattoos snaking up his arms.
"It's been a tremendous experience," chimed in Juan Coyazo, 31, who is serving a seven-year sentence for assault.
"We're in the mountains of our Puerto Rico and we're proud to be picking the fruit of our soil."
Officials hope some prisoners will become coffee pickers after they are released. But Coyazo, for one, doubts it will be his career choice.
"If they put me in charge of a coffee plantation, then I'd do it," he said. "But not as a worker."