I watched Soylent Green again today, and it's clear that our understanding of the health effects from cannibalism have improved since filming. Since then, we have now already experienced first-hand a localized epidemic as a result of the outbreak of Mad Cow Disease in the United Kingdom. Morality aside, any such product could cause the suicide of Humans as a species, due to the systemic spread of incurable and fatal disease from the ingestion of misfolded proteins.
It's already known that these proteins cannot be destroyed through the normal cooking process, and can survive temperatures of over 600 °C. I've read that it would require, by temperature alone, up to 1,000 °C to destroy prion infectivity.
Incineration of prion-contaminated material is considered the most effective method of disposal. Combustion at 1,000°C can destroy prion infectivity, however, low infectivity remains after treatment at 600°C. Despite its effectiveness, incineration may not be a practical solution, such as during a large outbreak of BSE, scrapie or CWD requiring a mass culling. Incineration of contaminated soil, vegetation and farm infrastructure (paddocks, fences) to eliminate CWD or scrapie environmental infectivity is also not practical.
I am not a biologist, and I don't know anything about food preparation or nutrition. But I am curious as to whether Soylent Green as a concept would even be possible. For example, if temperatures must reach the point of incineration, then any form of nutrition and other desirable proteins would effectively be destroyed as well. The remains would become carbonized, which wouldn't leave anything of nutritional value.
I thought about asking this as a science fiction question, as it is dystopian in origin, but I'm interested in the biology of the problem, as it exists in reality. I also think that it would be somewhat inappropriate to ask this on the nutrition stack exchange, for obvious reasons.