The San Jose Mercury News reports that investigations into the latest in a 10-yr string of E. coli outbreaks is forcing food producers to re-examine their entire process, tracing a path from the seed in the ground to the salad on the table.

This time the tainted produce is spinach, with one death and 146 people sickened in 23 states after eating contaminated spinach traced back to the Salinas Valley. This valley has been implicated in eight of 19 previous outbreaks of potentially deadly E. coli O157:H7 since 1995, most involving lettuce.

This past week, investigators fanned out across the Salinas fields, suspecting the most likely source of contamination initially is on the farm somewhere.

The U.S. Department of Agriculture has credited the boom in spinach and lettuce sales to the advent in the early 1990s of pre-washed greens in sealed bags. California farmers sold $258 million of spinach last year alone, a huge jump from the $56 million sold in 1995.

Growers say they’re constantly fine-tuning the way they prepare the soil, irrigate, fertilize and harvest. The industry also follows its own guidelines. Despite such voluntary guidelines, contamination of a crop from a neighboring cow pasture seems a reasonable possibility when one takes a drive along Metz Road between Soledad and King City. Along this winding two-lane road, cattle graze just uphill from farms, and ditches take potentially manure-tainted rainwater through culverts directly into fields full of lettuce.

Scientists are also looking at the next stage: harvesting. They are looking into whether or not workers are failing to follow proper hygiene practices, or if occasional government inspections actually guarantee that iceberg lettuce isn’t tainted by a dirty knife the moment it’s removed from the ground.