Is it possible for prion-infected meat to be processed safely (such as in Soylent Green) and still provide some form of nutrition?

Question

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.

Answer 1

No.

There are two processes that we need to consider.

First off: incineration at 1000 C (1832 F) in an oxidizing atmosphere will result in almost total destruction of the material. Carbon based material, such as protein, will likely be mostly converted into CO₂ and water (ignoring the amine groups, sulphur groups). Calcium based substances (bones, teeth) will be significantly damaged and some residual material will be deposited as ash, similar to the remains you get from a cremation. There is no nutritional value in these remains other than some mineral content, certainly no energy value (i.e. food value).

Secondly. Pyrolysis. In a non-oxidising atmosphere (anaerobic, or in the absence of oxygen), you get something called pyrolysis, which results in permanent chemical changes in the substances treated that way. I had a look through the literature for pyrolysis of meat and it is commonly used to dispose of remains from meat processing plants/slaughterhouses, especially meat-and-bone meal (the stuff formerly fed to cattle, causing the spread of Bovine Spongiform Encephalopathy in the UK). A common theme is that there are several remains from pyrolysis carried out at 500 C (932 F) - a solid material, a liquid oil, an aqueous phase, and gas.

Typical output (from A. Chaala and C. Roy¹)

and in summary of outputs and uses (ibid):

The process generated oil, gas, a solid residue, and an aqueous phase. The oil, which exhibits a high heating value of 34.2 MJ/kg, can be used alone or mixed with petroleum products as a fuel in boilers or gas turbines. However, the presence of nitrogenated compounds may cause some environmental problems if the oil is used alone. The complex chemical composition of the pyrolysis oil allows it to be potentially used for various applications, for example, as a softener for polymer processing and as an additive for the fabrication of well drilling mud. The gas with its calorific value of 12.9 MJ/kg and the burnable aqueous phase can be cofired to provide heat to the vacuum pyrolysis reactor. The solid residue can be used as a fertilizer, a moisturizer, an adsorbent, a filler for road bitumen, or a solid fuel in cement kilns.

Long story short: no nutritional value left after heating to the sorts of temperatures used for destroying prions.

1: A. Chaala and C. Roy Environmental Science & Technology 2003. 37 (19), 4517-4522. DOI: 10.1021/es026346m. Reprinted with permission from American Chemical Society.

Answer 2

Removal of prions in organic material by thermal degradation completely destroys the organic material. However, prions can also be digested by some enzymes, particularly keratinase, under relatively mild denaturing conditions. The resulting short peptides would retain their nutritive value. From Enzymatic Formulation Capable of Degrading Scrapie Prion under Mild Digestion Conditions:

An enzymatic formulation combining N22 keratinase and biosurfactant derived from Pseudomonas aeruginosa degraded PrPSc at 65°C in 10 min to undetectable level

A study of protease digestion of infected brain homogenates under milder conditions (50°C and pH 9.6 for 2 h.) showed partial digestion of prion protein PrP reduced infectivity more than proportionally. From High CJD infectivity remains after prion protein is destroyed

In contrast to PK, NAP digestion left 0.8% residual PrP after 2hr, yet decreased titer by >2.5logs; few residual protein bands remained. FU-CJD infected cells with 10x the infectivity of brain by both animal and cell culture assays were also evaluated. NAP again significantly reduced cell infectivity (>3.5 logs).

Another study looked at the penetration of protease into actual meat and bone meal (MBM), which is the ground-up scraps of butchery refuse that was banned for use as livestock feed after the BSE epidemic in the UK. This study is the most relevant to preparing potentially prion-infected tissues for consumption, but they don't actually investigate changes in infectivity. From Diffusion of Protease into Meat & Bone Meal for Solubility Improvement and Potential Inactivation of the BSE Prion:

Results showed that surrogate distributed in bone particles was more susceptible to degradation than that in soft tissue particles. Three factors controllable by unit operations in an industrial-scale process were also tested. It was found that removing the lipid content and hydrating MBM prior to incubation both significantly increased the rate of surrogate degradation. In a test of particle size, the smallest collected diameter range demonstrated the largest degradation of the prion surrogate, suggesting milling would be beneficial.

So, by grinding up everything into tiny pieces, removing the fat content, then treating with enzymes, it may be possible to reduce transmissions to levels that are acceptable in your hypothetical cannibalistic dystopia.