It was one of those "you could fry an egg on the sidewalk" days. Trucks were parked bumper to bumper in the parking lot of Kern County's Services Building on M Street. Official-looking people with sweat-drenched shirts scurried around with clipboards in their hands.

From the Mr. Big Stix ice cream truck to the Tita's Pupuseria rig, county inspectors were running their "regulatory gloves" over nearly every nook and cranny.

It was a roundup. Before hundreds of food trucks could head 'em up and move 'em out onto the streets of Bakersfield and other parts of the county, the operators had to prove to county inspectors that their vehicles were sanitary and the food they served was safe.

They were applying for Health Department permits for the fiscal year that runs from July 2012 to June 2013.

Supervising Environmental Health Specialist Diana Wilson estimated about 400 vending trucks -- selling tacos to ice cream, and just about everything in between -- would be inspected during two June roundups in Bakersfield and others in outlying areas of the county.

It's all part of efforts by the state and county to assure consumers that they can safely buy and eat food from Kern County's army of rolling restaurants. Donna Fenton, Kern County's chief environmental health specialist, advises that before buying food from a truck, people should look for a current health inspection sticker, which is orange in color and placed on the vehicle's upper left rear corner.

Fenton can tell horror stories about the reports her department has received on uninspected and unlicensed food trucks being operated in Kern County. In one case, an unlicensed vendor would go home every night and wash out pots and pans with a backyard hose. Fenton said rogue operators are often reported to health officials by their neighbors or by properly licensed competitors.

A safely operated and licensed food truck is required to have a commissary, which often is a restaurant, explained Wilson. The commissary, inspected by the Health Department, is used for the purchase or preparation of food products. Food trucks and cooking equipment are washed at the commissary, where waste is discarded and extra food items stored.

Wilson pointed out that food trucks are compact units with a limited ability to refrigerate food items. The roundup inspections focus on refrigeration systems, as well as the availability of hot water for sanitation. Fire Department inspectors test trucks' safety systems and fuel supplies.

Being mobile is key when it comes to food trucks, which are generally required to limit their operations to two hours in one spot unless the operator has made an arrangement with a nearby restaurant or business to give truck employees access to restroom facilities. Then trucks can stay in one location for a longer period of time.

If you think you're seeing an increasing number of colorfully decorated food trucks on Kern County's streets, you probably are correct. Wilson says the common lunch wagons that once pulled up outside a business and sold prepackaged sandwiches are being replaced by bigger trucks that serve hot meals and feature varied menus.

"We see very few of the old catering trucks anymore," Wilson said, explaining the increased need to thoroughly inspect and license food trucks, and the need to conduct occasional, unannounced field inspections to make sure rules are being followed and customers protected.

The food truck craze seems to be spreading everywhere. In Kern County, the operators cater mostly to workers' appetite. In urban areas, such as Southern California, gourmet food trucks have joined the ranks of their blue-collar brethren and are luring affluent, adventuresome customers.

But wherever they are and whatever they are serving, Alfonso Morales says they are continuing a centuries-old business tradition in the United States.

Speaking during a recent convention of the American Planning Association, Morales said the existence of street vendors can be traced back to the nation's beginnings.

A professor at the University of Wisconsin at Madison, Morales has written extensively on the topic and operates the website, which features articles about open-air markets and street vendors.

Street vendors were so common and accepted in early America that the U.S. Census once listed them as an "occupation category," Morales said, noting rivalries began to develop between brick and mortar businesses and street vendors in the 1930s and 1940s as a strong retail sector emerged.

But the advent of gourmands and foodies in the 1970s and 1980s brought an explosion of popularity to outdoor markets and launched consumers' search for the alternative menu choices that can be found in food trucks, said Morales.

Over the centuries, outdoor markets and food carts (or trucks) have served as "business incubators," leading transient operators to eventually open traditional, permanent stores, he said.

And that can help explain why some food trucks in Kern County are operated as an extension of traditional restaurants, while the operators of other popular food trucks have branched out to open their own restaurants.

Street vendors have been "historically important" to the nation's economic health, Morales said, noting they continue to be substantial contributors to every community in which they operator.