Comparison of Food Processing Classification Systems

Two PhD-level registered dieticians (authors 3 and 5) independently coded the top 100 foods by processing category using each system. Coders were instructed to follow guidelines from the original published documents outlining system classification criteria for IFIC [17 ] and UNC [13 (link)]. In the case of Nova, multiple versions of the system have been published [14 (link),29 (link),30 (link),31 (link),32 (link)]. For this analysis, we used the criteria described in a 2014 review of classification systems by the authors of Nova [29 (link)], which has been referenced in subsequent publications by the authors. Published studies employing the systems were used to clarify application of the processing system [15 (link),19 (link),33 (link),34 (link)]. NHANES food descriptors (Appendix A) associated with unique food codes were used in classifying foods. For mixed dished (e.g., pizza) foods were assumed to be homemade unless the food descriptor included place of production/production method (e.g., fast-food restaurant). In cases of ambiguity, coders were instructed to choose the more conservative processing category (i.e., less processed). For the IFIC and Nova systems, foods are classified into five and four categories, respectively, as presented in Table 1 [17 ,29 (link)]. The UNC system utilizes the same scheme as Nova, but further subdivides foods into seven processing categories (unprocessed/minimally, basic—preservation, basic—ingredient, moderately—grain product, moderately—flavor, highly—ingredient and highly) [13 (link)]. To examine inter-rater reliability, original processing category assignment was compared between coders (category 1–5 for IFIC; category 1–4 for Nova and category 1–7 for UNC).
A third coder (author 1) evaluated coding discrepancies and determined a final coding decision by consultation with authors 3 and 5 for use in analyses examining the relationship between nutrient concentration and processing category. In order to compare systems on a common scale, processing classifications were collapsed to four categories: for IFIC, categories four (ready-to-eat processed) and five (foods/meals) were combined into category four. For UNC, categories two (basic—preservation) and three (basic—ingredient) were combined into category two; categories four (moderately—grain product) and five (moderately—flavor) were combined into category three, and categories six (highly—ingredient) and seven (highly) were combined into category four. Due to insufficient numbers of category two foods, categories one and two were combined for all systems. Other studies have found small proportions of foods classified as basic/processed for preservation compared to other categories, supporting the decision to combine this category with category one [15 (link),34 (link),35 (link)]. The resulting categories were category one (unprocessed/minimally), category two (moderately processed), and category three (highly processed) (Figure 1).