The Antarctic cold front swept suddenly out of the south, plunging temperatures here to their lowest levels in 53 years. In the countryside, coffee growers tried, often in vain, to keep their vulnerable trees safe from the penetrating cold by using nebulizers and fire pots.

A wave of severe and unseasonably early frosts has struck three key coffee-growing states in Brazil, the world's largest coffee producer. Nervous growers, still recovering from the 1975 "black frost" that crippled Brazilian production and led to a quintupling of world coffee prices, are now wondering if history is about to repeat itself.

"In 1975, the frost did not come until mid-July," said one worried trader. "This year we have had two bad frosts even before winter officially begins. That's a bad omen."

The Brazilian government responded to the threat on June 4 by announcing the suspension of all coffee exports. Octavio Rainho da Silva Neves, head of the Brazilian Coffee Institute, said the measure was taken to "evaluate the real effects of the frost and prevent speculation."

But news of the frost and the Brazilian reaction to it immediately drove coffee prices on the New York and London markets to their highest levels in more than a year. In less than a month, coffee prices have jumped 37 percent - from $1.48 to $2.03 a pound.

The Brazilian suspension also prompted three other coffee growing countries - Mexico, Colombia and El Salvador - to follow suit. Within a week, both Brazil and Colombia resumed exports - but at a minimum price 15 to 20 percent above the pre-frost minimum export price.

Coffee Institute president Rainho said that an "initial and very superficial evaluation" indicated that "about 1 billion coffee trees, or one third of the total Brazilian coffee population" has been affected. Hardest hit was the state of Minas Gerais, where 50 percent of the trees were estimated to have been damaged.

Some coffee producers immediately began comparing this year's frost to the one that struck on the night of July 17, 1975 and wiped out over 90 percent of the Brazilian crop. But industry sources here are viewing such statements as an exaggeration intended to forced prices up even higher and more rapidly.

They point out that this year's frosts have thus far damaged only leaves on trees in low-lying areas and have not destroyed entire trunks and branches, as occurred in 1975. Experts also note that the state of Parana, Brazil's chief coffee-growing area and the main victim of the 1975 frost, has suffered comparatively little thus far.

Since 1975, Brazil has spent over $1 billion to replace destroyed trees. But it is precisely these trees that are now in jeopardy, just as they enter the period in which they are finally mature enough to produce beans.

According to Rainho, frost damage will have only a "slight impact" on this year's mostly harvested crop; its principal effects will be felt in the 1980 harvest. But market observers here say that political instability in El Salvador and a stevedore's strike in Colombia are likely to "amplify" the effect of the frost and create a short-term shortage.

Even so, prices are still a long way from reaching their 1977 high of $3.40 a pound. Growers here, buoyed by reports of a U.S. Department of Agriculture study admitting that prices could pass the $3 mark within the next few months, are holding on to their crops in anticipation of higher prices.

Should more severe frosts continue to drive prices up, Brazil does not stand to be the principal beneficiary. After the 1975 frost, stocks in the hands of the Brazilian Coffee Institute amounted to more than 20 million 132-pound sacks of coffee.

Those supplies were sold during the bonanza years of 1976 and 1977, leaving the Institute with current stocks estimated at about 8 million sacks. That is barely enough to meet Brazilian domestic demand, industry sources say.

Instead, it is Colombia, the second largest producer, that figures to gain the most. Colombian Finance Minister Jaime Garcia Parra reacted to this year's Brazilian frost by promising that "Brazil's misfortune and Colombia's good luck will be managed with discretion and moderation."