Washington — Food prices will go up 12.2 percent in 1981, with much of the inflationary pressure coming from price increases of meat, poultry, and eggs. The Agriculture Department sees meat and poultry prices increasing about 18 percent, and egg prices nearly 17 percent. Other pressure will come from an estimated 21 .5 percent increase in the cost of sugar and sweets. Higher meat prices will stem from farmers' losses this year that led to declines in pork production and smaller supplies. More demand for eggs, as consumers substitute them for meats and poultry, and a production decline will push up prices.