The first guideline now listed in the guidelines section, which wasn't there the day before, reads:
Use AMP HTML (Accelerated Mobile Pages) to build your recipe pages and mark up the content with structured data to provide rich cards and instant-loading host-specific lists for your recipes.
They also changed the recipe properties section to read:
Build your recipe pages with AMP HTML, and mark up your recipe content with the following properties of the schema.org Recipe type. In addition to the required fields, mark up as many of the recommended fields as possible to drive better engagement in Search.
Here are before and after screen shots of both sections.
Old recipes guidelines:
New recipes guidelines:
Old recipe properties:
New recipe properties:
Aaron explained "the obvious TL;DR here is "use AMP"! But Google's advice is verging on being prescriptive here, although AFAIK AMP is still not actually required to generate recipe rich cards."
Forum discussion at Google+.