Food distributor Dean Foods Co. agreed to buy closely held Berkeley Farms for an undisclosed price to expand in Northern California as it vies with Suiza Foods Corp. for leadership among national dairy processors. Hayward-based Berkeley Farms is family-owned and -managed and had 1997 sales of about $160 million from its dairy businesses. Dean Foods had $3.29 billion in sales in its fiscal year ended May 31. This is the 10th purchase that Franklin Park, Ill.-based Dean Foods has announced or completed this year. The dairy industry, once dominated by family businesses, has been consolidating rapidly. Dean Foods shares fell 13 cents to close at $42.56 on the NYSE. Shares of Dallas-based Suiza fell $9.69 to $30.06, also on the NYSE, after the company said a surge in dairy costs will push its earnings below forecasts for the third quarter and possibly the fourth.