Updated: Dec 14, 2018
There isn't any fix or best aperture to use. It all depend on what type of mood/shot that you want to create.
For more editorial type of shots, using a longer focal lens (100mm macro), you can use large aperture like f/2.8 or f/4 to create a more shallower depth of field so that you can direct the viewer's attendtion to the subject that is in the foreground. With large aperture, you can also make the background look softer.
Take a look at the 3 images attached below using natural lighting. The ISO is kept constant at ISO 400 in order to show you the difference. The only changes is the Aperture and Shutter Speed.
Starting from f/2.8 to f/5.6. The flower at the background becomes more soft focus and blurred when you use larger aperture like f/2.8.
Did you also notice that the table top become brighter when a smaller aperture f/5.6 is being used? The reason is because of the slower shutter speed is being used. Slower shutter speed like 1/30s will let more light entering into the sensor and will also captured the ambient light. So remember to off the ceiling lights and just use the natural lighting.
How to control the depth of field?
1) Type of F-Stop used. A larger f-stop such as f2, f2.8, f4 or f5.6 gives you a smaller depth of field with less area in focus. That will bring viewers attention straight to the area you want them to see. The image also appears to be more soft focus feel.
2) Type of Lens used. Compare a macro lens with a wide-angle lens using the same f-stop will have narrower depth of field because a wide-angle lens has a wider angle of view.
3) Distance used When a camera is focused on a subject that is farther to it, it will give you larger depth of field.
To learn more about basic food photography, go to https://www.bene-tan.com/basic-food-photography