Ten years ago, fragile leafy plants like arugula and baby spinach could only be harvested by hand. Today a single machine can collect 40,000 pounds a day, and bagged salad has become a $2.6 billion industry.
TRIM REAPER This harvester, built by Valley Fabrication and owned by Earthbound Farm in San Juan Bautista, California, can do the work of 35 skilled laborers. The reaper is 20 feet long and 12 feet wide and is propelled by an 80-horsepower Kabota diesel engine. It sells for more than $100,000.
SETTING UP To cut leaves at a uniform height, the harvester needs fields that lie perfectly flat and plants that grow straight up. The denser the crop, the more upright it grows, so lettuce farmers pneumatically inject as many as 3.5 million seeds per acre into the ground.
Salad greens are harvested in the morning or evening, when plants are most cool and crisp. Unfortunately, insects and rodents also favor those times. To keep pests from getting sliced up with the greens, engineers created “the tickler,” a comblike device at the front of the harvester that runs long, flexible tines through the leaves, shooing off any lurking beasts.
Once cut, greens drop onto a conveyor belt positioned a quarter inch behind the band saw. They then ride up through the machine to a loader. Engineers first tried installing a vacuum device onto the front of the harvester, but it bruised the leaves.
When a leaf of lettuce is torn or cut, it tries to heal the wound by respiring heavily—using oxygen to break down its own cellulose in a process we call wilting. Smaller pieces wilt faster because they’re the most cut up. To keep them out of the mix, the harvester sends leaves across a vibrating grid. Small pieces fall through holes in the grid.
Each conveyor belt is separated from the next by a six- to eight-inch space, known as an air jump. A small fan blows up through the gap, lofting leaves to the next level up. Heavier objects—such as rocks, dirt, insects, or rodents—fall through the space and are left behind.
KEEPING CRISP Harvested greens are washed, spun dry, and bagged in a factory. The bags are made of complex, multilayered polymers that allow controlled amounts of oxygen to enter and exit. Bags of chopped iceberg and romaine lettuce, for example, have no oxygen at all. It is flushed from the bags with nitrogen to avoid “pinking,” a reddish tinge at the cut edge. From the moment lettuce leaves the field until it’s bought in stores, it’s kept refrigerated between 34 and 39 degrees Fahrenheit. For every 10 degree drop from field temperatures down to just above freezing, chemical reactions in leaves (such as those that cause wilting and fermenting) are cut in half.