CHICAGO — Dwindling are the days of family dinners and three square meals, according to Information Resources, Inc. (I.R.I.), a Chicago-based market research firm that found in a recent report a growing number of consumers who graze on the go.

These “opportunists” represent 21% of Americans, and their unplanned eating style feeds a $90 billion market.

Click the infographic to learn more about opportunist eaters.

“The eating habits and attitudes of traditional three-square-meals-a-day eaters have been studied extensively for years,” said Susan Viamari, editor of Times & Trends, an I.R.I. publication. “While this segment is still important, you simply cannot deny the emergence of this new on-the-go eating segment. Since very little has been uncovered about these eaters, we are addressing how this group’s demographic, lifestyle and attitudinal characteristics impact their food and beverage shopping, buying and consumption behavior.”

Spanning diverse backgrounds, age groups and income brackets, opportunists are more likely to be female, of non-Hispanic origin and part of a single- or two-member household. Less than half are under the age of 45.

Driven by value and convenience, opportunists indulge moderately but are less likely than meal planners to select healthy foods. When it comes to food purchases, 31% of opportunists compared with 18% of planners are more likely to choose a sale item without regard to nutritional value, and 39% of opportunists tend to grab convenient foods without considering its role as a snack or meal. About half enjoy cooking, but two-thirds seek quick and easy preparation, and one-third prefer heat-and-eat foods over from-scratch options. In the past year, opportunists spent 60% more than planners on frozen appetizers and snack rolls, a category projected to continue growing among the group through 2015, according to I.R.I.