Food groups were developed by the Food Surveys Research Group (FSRG) for use in reporting estimates of food intakes in past USDA food surveys. In this scheme, foods are classified into 71 food groups and subgroups. For each food group or subgroup, a short description of the kinds of food that are included and excluded is provided and the food codes included in each group/subgroup are identified. You can use these food groups in your analysis or you can devise your own grouping scheme.
As an example, here is the description for the dried fruit subgroup. The list of food codes for this group is very simple, with all codes beginning in 621-. Some other groups have much more complicated lists.