When The Coca-Cola Co. and PepsiCo Inc. dropped saccharin from many of their diet drinks in favor of aspartame late last year, a group of small companies figured that their own fortunes would be sweetened.

G. D. Searle & Co., the pharmaceutical giant, is the sole U.S. producer of aspartame under a patent that expires in 1992. But Searle depends on contractors for much of its supply of L-phenylalanine, an amino acid that is the principal and most expensive ingredient in aspartame. The substance is made using biotechnology techniques.

A Japanese food company, Ajinomoto, had been the main supplier of L-phenylalanine in a market that could double this year, to $140 million. Scott R. King, an analyst with Montgomery Securities, predicts that L-phenylalanine sales could reach $400 million by 1990. "Phenylalanine could be the most spectacular specialty chemical over the next two years, and there will be some companies that will make a killing with it," says King. "It is a big enough market so that just about every biotechnology company has to take a hard look at it."

It might be a hard and careful look, though.Searle has invested $150 million in a new L-phenylalanine plant and won't say if it plans ultimately to shut out its suppliers. But that possibility isn't stopping entrepreneurs from entering the business.

Genex Corp., of Rockville, Md., has invested more than $10 million in a new plant, and Purification Engineering Inc., of Baltimore, plans to double its capacity in June. "We believe Searle will take most of the market," says Gary Calton, Purification's chairman. "We will, hopefully, take a substantial portion of [what is left]." BioTechnica International Inc., of Cambridge, Mass., has joined with H. J. Heinz Co. to commercialize its new process for making L-phenylalanine.

King says that the use of aspartame will lift the diet share of the soft-drink market from 22% in 1984 to 25% within a few years. If so, Coke and Pepsi could help carry a group of suppliers to fat times.