It used to be that restaurant menus stuck to a script, with a few variations each night recited by servers as "specials" or "menu additions."
Take Dish Osteria. In 2008 when former Post-Gazette restaurant critic China Millman reviewed the South Side restaurant, she wrote, "... eating at Dish is such a timeless experience that much of the menu remains the same as when the restaurant opened. This constancy may help explain why so many people speak so fondly of the restaurant, although they haven't eaten there in quite some time." She went on, to "encourage the kitchen to let loose a bit with the menu."
Fast-forward to 2013 and Dish changes much of the menu weekly, if not daily. Case in point: Chef Michele Savoia offered a memorable linguini with morels, fiddleheads and ramps as well as shad roe with greens in early spring, then soft shell crab later in the season.
Such is the case with many Pittsburgh restaurants, where menus change frequently, seemingly driven by seasonal items.
Yet that's not all that drives menu changes.
They are expected by many Pittsburgh diners, especially as younger people with disposable income -- or not so disposable income -- dine out more frequently than ever before.
While the farm-to-table movement may have inspired a changing menu, it's not necessarily what maintains it.
Cooks' desire to test new techniques, to sharpen line cooks' skills and to attract more regulars also drive the trend.
Swapping out dishes on a nightly basis is also a tactic among the most creative chefs in the world. As a result, eating at restaurants can be like pursuing a crush, as diners are seduced by the flavors in a dish one night, only to find it has disappeared from the menu the next.
Consider the example of chef Paul LieBrandt of Corton in Manhattan. A profile in The New York Times last month addressed how his exacting work and lofty presentations have secured his spot as one of the country's most creative chefs in fine dining.
Changing dishes -- to the delight and dismay of his customers -- has helped secure the chef's reputation.
"I think Paul's food changes, and it changes rather rapidly," said business partner and restaurateur Drew Nieporent. "And sometimes you run the risk if you compliment him about something, that then he'll change that, too."
One restaurant that's demonstrative with menu changes is Salt of the Earth in Garfield. A giant chalk board on a back wall notes the day's starters and main dishes, cocktails, beer and wine selections. And yet there's more to be had.
Beginning in January, the restaurant started a separate late-night menu of smaller plates that are less expensive than the dinner menu.
This week's late night menu, which runs from 10 p.m. to 1 a.m. daily, includes the burger with homemade American cheese -- the mainstay. The restaurant has also reintroduced Korean fried chicken, with skin as crisp as glass and quite delicious.
Other dishes rotate among a Monday red sauce item, Tuesday night oysters for a buck a piece and Wednesday night's Bo Ssam, which arrives as a slab of pork for (at least) two with a glistening savory-sweet skin. For $35, it comes with condiments and herbs and requires a hearty appetite to finish.
Thursday nights offer seasonal pies, which often sell out within the first hour.
This menu serves several purposes beyond feeding a late-night crowd. For one, it keeps line cooks engaged and lets them master techniques with limited ingredients and specific parameters.
It also "keeps things fresh," said Salt's chef de cuisine Chad Townsend, not just for cooks, but for diners looking for a restaurant's evolution, not a repeat episode.
" 'What are they doing right now?' is the question diners ask before they go out to a restaurant," said Cavan Patterson, owner of Wild Purveyors in Lawrenceville that supplies to more than 40 restaurants. The question is reinforced by the consumption of social media as much as food itself, he said.
Mr. Patterson said he constantly has to accommodate restaurants' requirements for new ingredients and does so by foraging for increasingly unusual items and finding new creameries and meat purveyors.
Menu changes affect farmers, too. "It creates an environment agriculturally that changes how and what farmers grow," said Mr. Patterson. "Either by growing something for an individual restaurant, or by becoming specialists in a few items, farmers are tailoring their businesses to people's desire for rarity."
Changing menus often display dishes that are the equivalent of a new band's cover of a Beatles tune, or a nod to a restaurant in another city. Take Salt's dollar oyster night, a tribute to Foreign & Domestic in Austin.
"Kevin [Sousa, executive chef] has a tangential relationship with them," said Mr. Townsend. "We knew people here were craving oysters. When we saw how that restaurant does it, we thought it was cool."
Some restaurants build a name around changing menus. In Bethel Park, the whole concept of Slate Bistro on Donati Road was developed to start with a "clean slate" or menu every month.
Its June menu, for example, included 16 items -- including spinach and berry salad, grilled Chilean sea bass, chicken fried steak, and hot banana pepper and asiago ravioli -- but it urges "no substitutions please."
Each dish is also a work of sorts, as it adheres to how chef Dave Sgro conceived the dish for the month it's on the menu.
Over at Casbah in Shadyside, sous chef Dustin Gardner said he and executive chef Eli Wahl change menus according to what's in season, but the changes are gradual and "less avant garde" than the menu at Salt of the Earth.
Casbah has to please bigger crowds at the 250-seat restaurant that offers rustic fare and tailored technique.
"We keep a dish on the menu for about two weeks, based on where we source things," said Mr. Gardner. Many of the more challenging items are introduced as menu additions.
Take the rabbit pappardelle. Inspired by a food trip to Amish country for Big Burrito chefs, the dish includes Cerignola olives, apricots, an aged cheese called Grayson from Meadow Creek Farm, rabbit confit, sausage and rosemary.
"We sell at least 14 plates a night," said Mr. Gardner, who is surprised by the brisk sales of a rabbit dish.
Chef Justin Severino has changed a menu to lighten things, as charcuterie and complex meat dishes at Cure in Lawrenceville often demand "a ton of time" and concentration from sous chef Nathan Hobart.
Enter crab tacos. "Here's a dish I brought on to entertain myself," Mr. Severino joked. The casual starter was served with tortillas and salsa on a wooden board, paired with a Negra Modelo "corked" with a marinated shrimp.
At nearly every restaurant, some menu items are constants, such as a dish at Casbah introduced by Big Burrito corporate chef Bill Fuller, comprising orecchiette, grilled chicken, cream, goat cheese, cranberries and sage.
"That dish will never go away," Mr. Gardner said.
Mr. Townsend said that he also keeps evergreens on the menu.
"While 80 or 90 percent of our diners want new options, a few really like that hanger steak that's been on the menu," he said.
Although restaurants have become hyper-aware of diners' appetites for all things new, they take measures to avoid alienating them, too, said Mr. Townsend.
"Pittsburgh still has a crowd that likes what they're familiar with, so we have to accommodate the comfort factor."
Melissa McCart: 412-263-1198 or on Twitter @melissamccart.