LODI, Calif. — Roaring up and down the rows of ripe pinot grigio grapes, the mechanical picker is a model of efficiency on a bright harvest day.

Slim rods grip the vines and shake vigorously, sending grapes tumbling into the machine’s maw. Conveyor belts carry the grapes up to fans which blow out leaves and other debris before a metal arm shoots the grapes into a large bin trailing alongside.

The machine moves on, leaving denuded clusters quivering in its wake.

Once, all California grapes destined for wine were hand-picked. But during the last 30 years, more acres have gone to machines as the size of the crop has grown and the technology behind mechanical harvesters has improved.

“You can do a better job and you don’t have to use a lot of people,” says Rodrigo Sandoval, a vineyard manager for Bronco Wine Co. who supervised the phalanx of machines picking pinot grigio in California’s Central Valley as the 2009 harvest got into full swing.

How fast can a mechanical harvester pick? Very.

Bronco co-founder Fred Franzia estimates five people working with mechanical harvesters can do the work of about 100 people picking by hand.

It would take 5,000 people to bring in Bronco’s large harvest — they produce a number of brands, including Charles Shaw, known as Two Buck Chuck for its $1.99 price in California — something that’s just not practical.

“We couldn’t physically manage a work force that large in the amount of time needed to get the ripe grapes into the winery,” he said.

But for some premium wineries, hand-picking remains the method of choice, says Pat Garvey, who uses hand-picking crews on 650 acres of grapes.

Garvey estimates it runs about $250 to $400 an acre to use a mechanical harvester, and $750 for hand-picking.

But for more expensive wines, the price difference isn’t such a big thing — and high-end wineries generally prefer hand-picking because they believe the pickers can do a better and more selective job.

Beyond that, a big, rowdy machine doesn’t really fit the image of a hand-crafted, artisan product.

“When you have boutique wineries, you also have boutique prices. In many cases that boutique winery doesn’t want machine-picking,” says Garvey, a proprietor of Flora Springs Winery & Vineyards.

Another thing he likes about hand-picking — he can employ people nearly year-round (with pruning and other work), boosting morale.

Which method is superior depends on who you ask.

On hilly terrain, hand-picking may be easier. But in general, machine technology has improved over the years and some argue mechanical harvesting does less damage to grapes than cutting the clusters by hand and running them through a destemmer/crusher.

Others disagree.

“Hand-picking is definitely a more gentle process,” says Keith Hock, winemaker at Schramsberg Vineyards, a Napa Valley sparkling wine producer. “We hand-pick the fruit because we don’t want the berries or the clusters crushed before they get to the winery.”

Lumbering and loud, mechanical harvesters may be short on romance. But, for Franzia there is grace in the way skilled machine operators can coast up and down the vines, tractor-driven fruit bins chugging alongside in perfect synchronization.

“It’s like music,” he says.