H. J. Heinz Co. said yesterday it overstated last year's profits by an estimated $8.5 million because of "certain improper practices."

The Pittsburgh-based food processor also reported a 51.5 percent earnings gain for its first fiscal quarter, and announced a 10 percent dividend increase.

But because fiscal 1979 earnings are to be restated, Heinz said the results for the 1980 quarter ended Aug. 1 shouldn't be compared directly to last year's results.

Heinz said an investigation by a board of directors' audit committee has revealed "certain improper practices in subsidiaries and divisions . . . including those arising out of payments to vendors and treatment of sales" resulted in profits being "improperly transferred to fiscal year 1979 from prior years."

Heinz did not elaborate on its statements.

In the latest quarter, Heinz earned $32.06 million ($1.40 a share) on sales of $657.1 million, up from $21.16 million (91 cents) on sales of $555.6 million a year earlier.

Heinz boosted its common-stock dividend to 55 cents a share from 50 cents payable Oct. 10 to shareholders of record on Sept. 24..

Meanwhile, Campbell Soup Co. reported record sales and earnings for its frouth quarter and full year, citing productivity gains and increased volume.

The multinational soup and food concern said that its profits in the fourth quarter ended July 29 rose to $29.3 million (89 cents a share) from $27.7 million (82 cents) in the same 1978 period. Sales were up 19 percent to $539.3 million from $456 million.

For all of fiscal 1979, Campbell's profits rose to $132.7 million ($3.98) from $121.4 million ($3.61) in the previous year, while sales increased 13 percent to $2.248 billion from $1.983 billion.

The company also announced that it is beginning to market the food industry's first line of frozen breakfasts and dinner entrees packaged in paper trays suitable for use in either microwave or conventional ovens.