There's a lot of concern about updating the modified date on posts that's fear-driven and in our opinion, overblown. Dates do not matter - only content does.
We no longer recommend attempting to restrict the modified date from being updated (explanation below). If you still feel like this is important to you, we've heard the change last modified date plugin is the best one to use.
This doesn't apply to your top 5-10 posts which you generally shouldn't edit unless you're okay with the small chance you lose all traffic to it. It's simply not worth having your traffic/income devastated when there's little to no gain in editing the top performing posts.
However, for the vast majority of your posts this simply doesn't matter. We've spoken to far more bloggers who update their posts without limiting the modified date and see no difference or an improvement than we know people who are negatively impacted.
This is totally anecdotal.
If you're editing a post to improve user experience, just edit it and stop worrying. It doesn't matter how big or small it is. If you delete a single letter, just let the modified date update.
The two concerns people have about modified dates are that (1) your readers will notice and (2) Google will see the modified date and it will trigger some crazy update.
Myth: your readers will notice
For 99% of food blogs, 99% of visitors will never return to your recipe or notice a difference. They're simply not monitoring every change you make on your website and wondering if your recipe changed.
99% of your traffic is one-time visits to that recipe, who will bounce off the site, make the recipe, or print it off.
You are paid per impression, not per active returning user. Stop worrying about returning users. It's a total waste of time.
The only reason to edit a recipe is to improve it to make it more user friendly, and any returning user will benefit from re-reading the recipe and following the updated instruction.
So don't worry about informing readers about what was changed in a recipe
Myth: Google will notice
Google has explicitly said that they don't use modified dates as a ranking factor. This is because it's an easy-to-fake user input that if used, would be gamed/spammed to death immediately.
I can set the modified date to always display todays date, or never change, depending on my use-case and how I want to try manipulate Google.
But Google ignores this data and keeps its own caches/copies of your page every time it crawls it.
Let's make this explicitly clear: if you change a piece of content, or don't change a piece of content, Google knows no matter what date is there. It keeps multiple copies of your page that it compares against every time it visits your page.
Your published and modified dates has no impact whatsoever on whether Google notices content has changed.
The date does not matter to Google, period.
The only thing that matters is the quantity and quality of the content that changes, and how it affects the overall topic of the page, sub-sections and user experience.