Book Stalingrad by Antony Beevor covers also situation of prisoners. Few paragraphs:

The state of most [Axis] prisoners at the time of the surrender was so pitiful, that a considerable death rate was predictable in the weeks and months to follow. How far this was exacerbated by ill-treatment, casual brutality, and above all logistic deficiencies is impossible to calculate. Of the 91,000 taken prisoner at the end of the battle, almost half had died by the time spring arrived. The Red Army itself acknowledged in subsequent reports that orders for the care of prisoners had been ignored, and it is impossible to tell how many Germans were shot out of hand during or soon after their surrender, often as vengeance for the deaths of relatives or comrades. (pg 408)

Chances of survival proved brutally dependent on rank. Over 95 per cent of soldiers and NCOs died, 55 per cent of junior officers and just 5 per cent of senior officers. (pg 415)

As for Soviet POWs caught inside the Kessel:

The 3,500 Russian prisoners of war in the camps at Voroponovo and Gumrak were dying at a rapidly accelerating rate. Several German officers were deeply shocked to discover during January that these prisoners were reduced to cannibalism, and made verbal reports. When Russian troops reached the camps at the end of January, the Soviet authorities claimed that only twenty men remained alive out of the original 3,500. (pg 350)