The object is to "make the world's greatest wine." The participants are Baron Philippe Rothschild, owner of Chateau Mouton Rothschild, and Robert Mondavi of Napa Valley, Calif.Rumors of negotiations between the men, two of the strongest personalities in the wine world, were confirmed last fall. Now there will be an official announcement, at a California press luncheon scheduled for April 16, to launch what is being called "the joint venture."

Michael Mondavi, Robert's eldest son, was in Washington last week. He revealed details of the blueprint for "combining California technology and science with the art and philosophy of Bordeaux to make the world's greatest wine."

The plan is to produce about 5,000 cases a year of a "cabernet sauvignon type" wine (cabernet franc and "maybe some merlot" will be part of the blend) that will sell in a price class with such Bordeaux first growths as Lafite Rotchschild and Petrus. Eventually grapes for the wine would come from a parcel of Napa Vally land, 25 to 50 acres, that the joint venture hopes to purchase. In the short term, Mondavi will supply the grapes and the wine will be blended under the hoint supervision of winemaker Tim Mondavi and Lucian Sianneau, the winemaker ate Mouton.

The first wine has been made already. Sianneau spent four days in the Napa Valley last December and there will be about 2,000 cases of the wine that emerged from the tastings. It should be released in 1983. While marketing strategy has not yet been worked out, Michael Mondavi said he expects the majority will be sold in this country, but "some," probably 25 percent, will be earmarked for sale in France and other Western European countries.

Mondavi would not rule out the possibility of other wines from the joint venture, but said expansion was highly unlikely for at least the next five to seven years.

For the Mondavi family such an alliance will help further Robert Mondavi's unceasing efforts to win "world class" recognition for his wines (and for Napa and other California wines as well). Baron Philippe, whose late wife was an American and who knows California well, was among the first in French to recognize the achievements and potential of American winemakers and vineyards. The basis of his involvements is "both idealistic and practical," Michael Mondavi said. "The joint venture should help improve winemaking techniques for both firms."

The people at the Somerset Wine Company are blowing their horns for a flute. Aware of the rising price and declining sales of that long-time popular brand Blue Nun, they have introduced an Austrian wine called "Magic Flute." Aimed at a mass audience in homes and restaurants, it is on sale at wine shops in town and in the suburbs at prices ranging from $3.49 to $3.99.

This is not, the importer insists, a wine created to Madison Avenue specifications. It already existed and not even the packaging designed by the Austrian producer, Alois Morandell & Sohn, has been changed for domestic consumption. Tasting a sampling of the wine last week tended to support this contention. Magic Flute is neither flabby nor sweet. Low in alcohol, relatively dry and with a lively fruit-acid balance, it can be served as an aperitif or with light foods. The taste won't linger, but the importers don't mind. They hope you'll just sip some more.

Somerset is bringing in some other Morandell wines as well, notably a fragrant, fruity Mueller-Thergau and a Rhineriesling called Steiner Hund. These Austrian varietals are priced from $4.99 to $6.29.

The West germans haven't shown us everything yet, either. Wines from Lorch, a township at the southern end of the Rhinegau region, are being shipped here by the Friedrich Altenkirch Schwanenkellerei. Peter Breuer, the young man who manages his family's firm, explained that while his town is not so well known as some of the great Middle Rhine appellations, it does produce distinctive wines. The cause, he said, is geography and soil. Steep slopes with a high proportion of slate run down toward the Rhine, catching the sun early in the day and holding its heat into the evening. The wines of Lorch, therefore, tend to show some Moselle-like spirtz when young and age well.

A range of Friedrich Altenkirch wines, including spatlese and auslese class, will be on sale here soon. Breuer argues that German wines in the $5 to $8 range still represent very good value for money and offers two rays of hope to the American consumer: There was a large harvest of good to very good quality in Germany last fall, and the increased strength of the dollar already is being felt in wine export negotiations.