The latest word on food prices -- as gleaned from the September Consumer Price Index -- is a stunning 19 percent annual rate of increase.

I know the pain, having just shelled out 85 cents for a slice of pizza.

Think that's bad? Listen to this forecast from one of the country's top trackers of food price trends, John Schnittker, former undersecretary of Agriculture under Lyndon Johnson. It's enough to take away your appetite.

He's telling clients of his Washington-based consultative service that the explosion in food prices will spill over into the first three months of 1981 and part of the second quarter as well. His gloomy numbers: A sustained 15 to 20 percent rate of increase during this period.

That's a much faster rate of increase than we have experienced in 1980, which Schnittker projects at 10 percent for the full year.

The summer crop failures in the United States, the People's Republic of China and the Soviet Union have already raised prices 25 to 30 percent on wheat, corn and soybeans, Schnittker says. And those failures -- plus the recently announced U.S.-China grain agreement -- tighten an already difficult world gain situation, he adds.

OK, what does it all mean?

For starters, a higher-than-expected rate of inflation.

Since food accounts for about 18 percent of the CPI, next year's gain in the index -- regardless of who's in the White House -- should run around 12 1/2 percent, according to Schnittker's calculations.

Needless to say, the weekly basket of food for the family is also headed considerably higher -- 12 to 13 percent higher in '81, in Schnittker's judgment. And that's on top of another 2 percent gain that's in store for us during the balance of '80.

At present, a weekly baseket of food for a family of four runs $53.70 for the thrift-conscious (usually recipients of food stamps); $69.20 for the low-income purchaser; $86.70 for the moderate- or middle-income bracket, and $103.90 for the more liberal spender.

The biggest increases, as Schnittker sees it, will be in beef and sugar.

Many of the cattle herds were badly depleted in the '75-'77 period when ranchers and farmers were discouraged from fattening their stocks because of low prices. But with prices a good deal higher, herds are being rebuilt -- though it won't be until '82 and '83 before there are abundant supplies, explains Schnittker.

"We won't have any severe beef shortages, just slow growth," he says.

Schnittker predicted that choice rib roast, recently about $3.05 a pound, should rise -- in just the next six months alone -- nearly 15 percent to $3.50, he says.

A gain of nearly 15 percent is also projected for sugar over the same period. A two-to five-pound pack of sugar is going for around 47 cents a pound -- heading, predicts Schnittker, to 54 cents by April.

In the face of all of this dismal commentary, Schnittker concluded our chat on what is certainly not the most optimistic note, as far as future food prices are concerned:

In the next 12 months, the world will consume 3 percent more food than will be produced this year.

Add the recognition that food at retail is about two-thirds labor, transportation and processing -- all of which are rising in price -- and maybe it's not such a crazy idea (as we hear from the doomsayers) to store away some foodstuff in the basement for what's likely to be some rougher times ahead. Those rougher times may be on us sooner than we think.

Beware of the postelectoral syndrome: That's the word from Yale Hirsch, author of the monthly investment newsletter called Smart Money.

Out of the last 17 national elections, the first year of the presidential term has produced 12bear markets -- four of them downright brutal.

And it apparently makes no difference who's in office. In 1977, following Jimmy Carter's election, the Dow Jones Industrials tumbled 173 points. And following the previous election -- in which Richard Nixon won a second term -- the Dow plummeted nearly 500 points. That year -- 1973 -- it should be pointed out also produced the Arab oil embargo and Watergate.

You probably have read that the market, initially, is likely to do better if Ronald Reagan wins than if Carter is reelected. Hirsch -- who's done an analysis of the last 20 national elections -- backs this up with statistical evidence. It shows that a Republican victory produces an average 0.9 percent gain the first day after the election. In the first week, the gain expands to 2.3 percent. And after the first month, 3.2 percent. On the other hand, it's all losses if the Democrats make it: 0.8 percent the first day, 0.6 percent the first week and 2.1 percent the first month.

Hirsch, who boasts a better-than-average record in calling market turns, takes a bearish view of '81. He figures that whichever party wins, the immediate task will be to get rid of some of messier economic problems (notably inflation) that have been shoved under the rug the past six months. And that means, he tells me, a further tightening of credit reins -- which should cause renewed weakness in both the economy and corporate profits. And that, as we all know, is not the stuff of which robust stock markets are made.