Quality Loss when saving Jpeg from Camera Raw

Question

I am try to do some color correction on 400 wedding photos. I open like photos in Camera Raw then synchronize, make a few additional adjustments and then save the photos as psd files. When I save the files, the quality drops dramatically. I can see pixels and jagged edges (looks like steps) on anything curved. The photographer will not give me the RAW files, so I have to work with these high quality jpegs and I would prefer to use camera raw as it is nice and easy to fix many of the photos quickly. How can I save these files so as to not lose quality? Oh, I am on a Mac.
--K

Answer 1

Ramón,
now I am quite baffled. For years ago I conducted respective tests and demonstrated, that repeated saving with the same "quality" does not introduce image deterrioration; others did the same tests and came to the same result.
Now, to be sure, I did it again, and the result contradicts my earlier findings. Although the deterrioration is after five consecutive savings practically irrecognizabe without hugely amplifying it, it is there, and that is an issue of principle.
While it is possible, that I and others made some error in those tests, the question is (for me) what causes the difference.
I verified, that the encoding parameters are unchanged. Thus the difference must come from the calculation required to transform between the original pixel values and the encoded values by
a. the conversion between RGB and YCbCr,
b. the discrete cosine transform (a variation of Furier transform).
However, any differences resulting from the calculation (iteration, rounding) should be eliminated by the subsequent lossy step, the "quantization" (which is an euphemism for "throwing away most of the data"). Even if there is a difference between the first and second savings, there should not be any between the fourth and fifth savings - but there is.
One explanation could be, that the encoding and decoding functions work with different precision, though this sounds very outlandish.
Anyway, darn.
Re the encoding parameters: they *have to be* stored in the JPEG file, otherwise the data could not be decoded, except by that program, which knows the parameters. However, JPEG is not designed for human consumption but for computers (this fact makes testing a program, which reads or writes JPEG data a serious PITA). The metadata can be analyzed only by a hex editor, and one has to know the structure of that data.
Re knowing, if the file has to be compressed or not: JPEG is *always* encoded. There are different variants of the encoding method, but there is no "unencoded" version.