Seattle's minimum wage ordinance did not affect supermarket food prices by food processing category.

Objective: To examine the impacts of Seattle's minimum wage ordinance on food prices by food processing category.

Design: Supermarket food prices were collected for 106 items using a University of Washington Center for Public Health Nutrition market basket at affected and unaffected supermarket chain stores at three times: March 2015 (1-month pre-policy enactment), May 2015 (1-month post-policy enactment) and May 2016 (1-year post-policy enactment). Food items were categorized into four food processing groups, from minimally to ultra-processed. Data were analysed across time using a multilevel, linear difference-in-differences model at the store and price level stratified by level of food processing.

Setting: Six large supermarket chain stores located in Seattle ('intervention') affected by the policy and six same-chain but unaffected stores in King County ('control'), Washington, USA.

Subjects: One hundred and six food and beverage items.

Results: The largest change in average price by food item was +$US 0·53 for 'processed foods' in King County between 1-month post-policy and 1-year post-policy enactment (P < 0·01). The smallest change was $US 0·00 for 'unprocessed or minimally processed foods' in Seattle between 1-month post-policy and 1-year post-policy enactment (P = 0·94). No significant changes in averaged chain prices were observed across food processing level strata in Seattle v. King County stores at 1-month or 1-year post-policy enactment.

Conclusions: Supermarket food prices do not appear to be differentially impacted by Seattle's minimum wage ordinance by level of the food's processing. These results suggest that the early implementation of a city-level minimum wage policy does not alter supermarket food prices by level of food processing.