This is an exciting moment for olive oil, akin to the toddler years of the wine industry in California. Since Franciscan missionaries planted the first olive trees in 1769, California has become a prolific olive growing region with 33,000 acres in the San Joaquin and northern Sacramento valleys. More than 90 percent of the crop is processed as black ripe olives, and the remainder used for olive products, including olive oil. In the past decade a few dozen excellent producers have sprung up in Northern California, some of them too small to have their own processing equipment. Which is where the Olive Press comes in.

From October through early March, the big Pieralisi press, visible through the glass wall at the back of the tasting room, crushes olives harvested from the orchards of the partners. Among them are Lila Jaeger, founder of the California Olive Oil Council, whose silken, grassy oil comes from her century-old nevadillo and cornezuelo olive trees in Rutherford; Phil Woodward of Chalone Vineyards, whose smooth and buttery Pinnacles oil is a mix of olives like French provincial aglandaus and picholines grown in Monterey; and B.R. Cohn who, with Greg Reisinger, produces richly flavored oils from a blend of four olive varieties. Under the Olive Press label, Stolman offers four oils: early harvest sevillano, master blend, California mission, and the lush lunigiana, made from Tuscan olives grown on his properties nearby.

You can buy or sample the oils pressed here at the tasting bar, where little plastic cups and cubes of bread await. Someone is on hand to guide you through the fundamentals, from warming the cup of oil between your palms and breathing in its aroma to appreciating nuances from grassy and flowery to spicy and peppery.

Olive oil tastings are becoming popular in specialty shops around the West. At Katz & Company in Napa, owners Albert and Kim Katz stock a half dozen of the finest California extra virgin olive oils for sampling. These include Storm Olive Ranch from Pope Valley, Poplar Hill from Napa Valley, and the rich, fruity DaVero, made from trees imported from Tuscany, the first and only Ameri-can extra virgin to win a blind tasting in Italy. Two labels from Frantoio are available, including the proprietors select, made from sevillano olives, a robust oil with hints of artichoke and pepper.

Frantoio, which is Italian for olive press, is the brainchild of Roberto Zecca, president of the California Olive Oil Council and an olive oil producer in Tuscany. At his Mill Valley restaurant, Frantoio, diners enjoy entr&#EACUTE;es like tapenade pizza and porcini-encrusted albacore tuna while watching olive oil flow from the Pieralisi equipment in its own glass room. Like the Olive Press, Frantoio presses olives from small producers, which once included the McEvoy Ranch, Marin County's first commercial olive orchard.

Nan Tucker McEvoy, former chair of the Chronicle Publishing Co. who is now in her very vigorous 80s, imported Italian olive trees for her 550-acre estate about a decade ago. Now the hills glitter with thousands of silver-leafed trees while lucky swans make their homes in the irrigating ponds. By arrangement, visitors can view, in this one breathtaking setting, the process of estate-produced oil, from planting seedlings and hand harvesting to olive processing and the hand-tying of bottles.

T O U R S   A N D    T A S T I N G S The Olive Press, 14301 Arnold Drive, Glen Ellen. (707) 939-8900, www.theolivepress.com. Katz & Company, 101 South Coombs-Y3, Napa. (707) 254-1866, www.katzandco.com. McEvoy Ranch, P.O. Box 341, Petaluma, CA 94953. (707) 778-2307, www.mcevoyranch.com. Nick Sciabica & Sons, 2150 Yosemite Boulevard, Modesto. (209) 577-5067, www.sciabica.com. Frantoio, 152 Shoreline Highway, Mill Valley. (415) 289-5770, www.frantoio.com. The California Olive Oil Council, (888) 718-9830, www.cooc.com.

"Most people have no idea how labor-intensive this process is," says Michael Coon, McEvoy's sales and marketing consultant, who explains that olive trees must mature for several years before bearing fruit. During picking, netting is draped below the branches to prevent bruising. Harvested olives must be quickly sorted, cleaned, and rushed to the press. Delay can cause deterioration, tainting the oil. After crushing, water is removed from the paste and the oil is graded. To be classified extra virgin, the oil can contain no more than 1 percent oleic acid, which causes oxidation and spoilage. Labels with the words "first cold press" indicate that no heat or chemical processes were used to extract the oil. Some producers also list acid content, vintage, the varieties of olives, and notes about the harvest.

All these producers specialize in extra virgin oil, but lesser grades of olive oil can be made. Subsequent pressings, filters, chemicals, and additives extract more oil, lower the acid content, remove off-flavors and colors, and otherwise refine the resulting liquid. "Virgin" olive oil may have an acidity level up to 2 percent. "Pure" olive oil has an acidity of less than 1.5 percent and is a mixture of refined oils plus a dollop of extra virgin oil for taste. So-called "lite" oil contains the same number of calories as all other olive oils but is rectified oil containing little or no extra virgin olive oil. Pomace is the pulpy residue treated with solvents and blended with extra virgin olive oil; it cannot be sold as olive oil. These lesser oils are fine for general purpose cooking, but are no match for the voluptuous aroma and rich taste of extra virgin olive oil. Admittedly expensive—a half liter can cost upwards of $8—extra virgin olive oil might be compared with a bottle of wine and lasts much longer.

Whereas foreign oils must meet the specifications of the International Olive Oil Council to qualify for the designation extra virgin, there is no obligatory standard in this country. Thus even the most elegantly packed oil can be just a bulk oil with a designer label. To remedy this, the California Olive Oil Council, founded in 1992, has established a certification program that replicates the standards of the IOOC, requiring that producers submit their oil for sensory and chemical analysis to determine acidity. Only those oils with less than 1 percent acidity can display the seal "certified extra virgin" on the label.

Olive oil should be stored in a cool place away from light and heat. Tinted bottles provide extra protection. If refrigerated, the oil can become cloudy, but this will disappear quickly at room temperature. Shelf life is about two years.

Olive oils range in color from deep green to sunny yellow. Color may indicate the ripeness of the olives but is no determination of quality. Early harvest, younger olives produce spicy, lusty oils in the green spectrum. Late harvests tend to be smoother and more mellow, with buttery hues.

According to the North American Olive Oil Association, almost one-third of U.S. households now use olive oil, sales of which increased about 10 percent last year. This popularity is partly due to olive oil's health benefits. Cholesterol- and sodium-free, olive oil is rich in antioxidants and cholesterol-lowering monounsaturates. But its popularity also stems from its versatility, the subtleties of its earthy taste, and the magical way it can transform a plain dish into a simple feast—enlivening garden greens or finishing a bean soup or a plate of pasta.

None of this is news to Modesto's Sciabica family, which has been making olive oil since Nicola Sciabica started the company in 1936. Visitors can wander the tasting room at the plant, wheremore than a dozen types of extra virgin varietal oils are produced from the same types of olive trees first planted in the California missions. At farmers' markets in Modesto and at San Francisco's Ferry Plaza, Dan Sciabica and his 85-year-old father, Joe, sell their oils, some just pressed the night before. They can also answer any—and that is any—question about California olive oil.