Chemistry in the News -- A Smelly Business: A feature by Festival Nifty Fifty Speaker Joe Schwarcz

Meat production stinks.  And I’m not referring to worries about bacterial contamination.  I mean it literally stinks.  Here’s the story.  Hold your nose.

We crave meat.  To satisfy our hunger Canadian feedlots ready some three and a half million cattle for slaughter every year.  And those cattle produce more than meat.  A single steer can crank out up to 30 kg of manure and urine every day, and some feedlots house over 40,000 animals!  That means more than a thousand tons of pee and poo have to be dealt with in some fashion every day!  Obviously, all that waste creates a massive odour problem caused by a huge array of smelly compounds that are released as a result of bacterial action on manure.  Most of these compounds have been identified, with indole, skatole and dimethyl sulphide, also present in human excreta, having been found to be particularly noxious.

Any effective odour control process has to destroy these compounds one way or another.  “Oxidation” as the term implies, requires reaction with oxygen, the simplest example of which is the process we know as combustion.  Just think of how natural gas, basically methane, combines with oxygen as it burns to yield carbon dioxide and water.  Other organic compounds, such as the variety of aldehydes, ketones, acids, esters and amines released from manure can also “burn,” but of course setting fire to feedlots is not a solution.  There are, however, “oxidizing agents” that have the ability to transfer oxygen to a variety of other compounds in what can be called a “cold” combustion process.  Potassium permanganate is one of these, and a solution can be easily sprayed on the ground to control the odour of manure.  Spraying with a 1% solution at the rate of about 8 kilograms of permanganate per acre three times a year is very effective at reducing the odours from cattle feedlot operations.  If animals are raised in an enclosed environment, ozone, a very strong oxidizing agent can be used to reduce odours.  In such feedlots air can also be collected and passed through beds of activated carbon that adsorb the smelly components, but this requires periodic replacement of the carbon which has a relatively short adsorbent life, making the process expensive

Methods of altering feed in order to reduce odours have also been investigated.  Humic acid is the black spongy matter present in soils that is a mix of compounds formed when dead plant matter undergoes microbiological degradation.  When incorporated into animal feed, it can lead to substantial reduction in the odour of manure.

Of course cattle feedlots are not the only place where odour problems are encountered.  Dairy, poultry, hog and sheep farms also battle the problem.  And it’s a big problem.  The total number of animals raised to meet human demands is stunning.  At a given time Canadian farms house some 12 million cattle, 12 million hogs, a million sheep and close to a hundred million chickens.  Just imagine the total amount of smelly compounds produced by all that manure.  But smell is not the only issue.  High odorant concentrations can kill animals and the effects of long term exposure of sub-lethal amounts are unknown.  Then there is the problem of workers being affected by gases and particulate matter released from manure.  Hydrogen sulphide, the classic odour of rotten eggs, is one of the components of manure smell and is highly toxic.  A number of farm workers have died while attempting to clean manure pits after being overcome by hydrogen sulphide.  There is also the problem of developing a lung disease known as “allergic alveolitis” after long term exposure to particulate matter containing a variety of antigens (compounds capable of causing allergies), particularly in chicken droppings.  Even aside from health issues, the revolting smells released by animal raising operations can be a nuisance to anyone living downwind.

Smells are also a major problem in animal processing and rendering operations.  Fish meal processing produces trimethylamine and putrescine, both with terrifying smells.  Rendering of beef offal yields many odourous compounds and processing of feathers generates the likes of acrolein, acetaldehyde, methyl mercaptan, diethylamine, n-propylamine, ammonia and hydrogen sulphide.  Many of these can be eliminated by circulating the air inside the facility through “chemical scrubber” solutions.  A solution of lime water can be used to remove ammonia, hydrogen sulphide can be removed by potassium permanganate, aldehyde smells can be removed using a sodium bisulphite solution, sodium carbonate can neutralize acids and calcium hypochlorite is a powerful oxidizing agent.

Environmental issues also crop up.  Ammonia released from animal excreta can be absorbed by nearby bodies of water where it can stimulate the growth of algae which in turn uses up the dissolved oxygen content of the water depriving fish of oxygen.  Basically, animal production facilities are a major source of air pollution as well as of water pollution from feedlot run-off.

And then there is the problem of methane production.  Ruminant animals, such as cattle, sheep, buffalo, and goats have a digestive system that can convert otherwise unusable plant materials into nutritious food and fiber.  But this same helpful digestive system also produces methane, a gas that has no smell but is a potent “greenhouse” gas that plays a role in global warming.  Livestock production systems can also emit other greenhouse gases such as nitrous oxide and carbon dioxide.  Globally, ruminant livestock produce about 80 million metric tons of methane annually, accounting for about 28% of global methane emissions from human-related activities.  An adult cow may be a very small source by itself, emitting only 80-110 kgs of methane a year, but with about 1.2 billion large ruminants in the world, they constitute one of the largest methane sources.

There is yet another issue.  The raising of animals requires huge amounts of water.  It’s not only the water they drink, it’s all the water used to grow the crops they eat.  So, there’s no question about it, raising animals is not an environmentally friendly process, and is not an efficient use of crops or water.  So why do we do it?  I think the simple answer is that most of us like the way they taste.  And when environmental issues raise a stink, we just hold our noses.

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I guess it applies to all the dairy products we love too.

By the way, those are dairy cattle in the attached picture at the top there.

"I think the simple answer is that most of us like the way they taste."

Are we really all so very shallow? Well maybe. I suspect a better answer is that these are habits formed and continually reinforced by a combination of cultural conditioning and ubiquitous sophisticated food marketing. We don't exercise real choice in these matters. As our strings are pulled so do we dance.

By Ed Harris (not verified) on 02 Nov 2012 #permalink