Borden Inc.'s food division has given sudden notice that it will be giving up its sales force within the week and instead will be relying completely on a network of 56 food brokers.

The announcement last Friday caught many of Borden's salespeople by surprise. A company spokesmen said yesterday, however, that nearly all the employes affected have been assured of new jobs with the brokers Borden will be using.

Food companies traditionally have relied on a mix of distribution methods for delivering their goods to market, including both salespeople paid directly by the company and brokerage firms that work on commission.

A number of Borden food items -- including Cracker Jack, Wyler drink mixes, Sacremento brand tomato juice and Bama jams and jellies -- have been sold through brokers for some time. Under the new arrangement, brokers will take on the remainder of Borden's food line, including cheese, condensed milk, instant coffee, breakfast drink, instant mashed potatoes and Cremora nondairy creamer.

"There are two reasons for the change," explained Borden spokesman Maurice O'Reilly. "First, grocers can now deal with one broker for all our products. Second, our brokers will represent the full line of our products."

Borden's food division represents more than one-quarter of the company's sales but only one-fifth of its operating income. Its weak 4 percent sales growth last year lagged behind the 9.2 percent rate for the company as a whole.

Borden's other divisions include dairy and chemical products. In 1978, the company reported total sales of $3.8 billion and net income of $135.8 million.