First, click on the Turn Off Citizen Automation button for a City, if it isn't already off.
Then, click on the Turn On Emphasize Food button, which is located a little bit below the whipping button in a City screen.
If you leave Citizen Automation turned on, then you may find that your existing citizen assignments will get changed, probably in a way that you don't want (such as working a G For square in place of a GH Mine square).
But, with Citizen Automation turned off, your existing citizen assignments should remain how they are now, while with Emphasize Food turned on, newly-born citizens (the new citizens that appear when your Cities grow in City Size) should get assigned to a role that provides the most Food, which will almost certainly not mean being turned into a Specialist, such as not being turned into a Scientist Specialist, at least until your Cities are large enough that they have run out of any Food-earning squares to work.
There is a bit of a tradeoff to having Citizen Automation turned off, though, as when you switch to building a Settler or a Worker, the squares which citizens are working will not be automatically adjusted in your favour, and the same is particularly true when you whip a Settler or a Worker with Citizen Automation turned off (for example, a GH Mine might get whipped away while the G Farm will continue to get worked... which means that you'll only earn 3 Hammers instead of 4 Hammers for the last turn of completing that Settler or Worker after having just whipped it).
For this reason, when I switch to building a Worker or a Settler, I will manually adjust the squares that are being worked, and even more so when I whip the Worker or Settler, and then I may manually adjust the squares back once I am finished producing a Worker or a Settler (I will probably work a G Farm when not building the Worker or a Settler but I would favour working a GH Mine while building the Worker or Settler, but then again, when done with a Worker or Settler that I didn't whip, I might continue to choose to work the GH Mine if the City is already near its Happiness cap). The alternative approach will probably be something like:
1. The City has:
Citizen Automation = off
Emphasize Food = on
2. I start to build a Settler or a Worker, so I set:
Citizen Automation = on
(Emphasize Food being on or off doesn't seem to matter)
3. After the Settler or Worker has been completed, I will:
i. Pick a non-Settler or non-Worker build item
ii. Set Emphasize Food = off, if I didn't do it previously
iii. Set Citizen Automation = off
iv. Set Emphasize Food = on
That way, the citizen assignments will be "the best as chosen by the City Governor in terms of its regular way of assigning citizens," instead of being overly focused on Food.
Meanwhile, newly-born citizens will continue to work the high-Food squares and thus are unlikely to become Specialists unless I've run out of Food-earning squares for new citizens to work.