The Science of Food

Ernest Rutherford, the famous physicist, supposedly claimed that “Science is either physics or stamp collecting”¹. At the cost of no longer being welcome in the biology department Rutherford was making the point that everything is subject to the laws of physics and ultimately these laws can describe any natural phenomenon. In real life we usually take the laws of physics as a given and describe many natural phenomena using higher level language but the fact always remains that what we are observing has been moulded by the unavoidable laws of physics. They aren’t the “Guidelines of Physics” and as Homer Simpson once said “In this house we obey the laws of thermodynamics!”.

Among the wonders of the modern world we sometimes forget that we are very much bound by physics, especially when it comes to our food systems. But when we are thinking about our food systems what we are really thinking about is the physics involved in the flow of energy from the sun to our bodies. The sun is where all our energy comes from and an unavoidable bottleneck in that flow is that plants are the only thing we have that are able to efficiently convert sunlight to biomass. We cannot photosynthesise nor can any of the animals we rely on for food. Only plants can photosynthesise and without the efforts of our botanical friends all that energy we receive from the sun would just be heating rocks.

All life relies on plants ability to capture the suns energy and turn it into biomass. To model this ecologists use the idea of an energy pyramid. Everyone has probably seen the energy pyramid; plants are at the bottom level, they are the primary producers making biomass from sunlight. On top of that level are the primary consumers that eat the primary producers converting plant biomass into herbivore biomass. Subsequent levels contain secondary consumers who eat the primary consumers and turn herbivore biomass into carnivore biomass, then tertiary consumers who eat secondary consumers and so on. Like a pyramid these layers go all the way up till you get to the apex predators like eagles or tigers who are no ones food (well until they die anyway).

The technical term for a layer in this pyramid is a trophic level and ecologists use trophic levels to model the flow of energy through an ecosystem. Plants that, apart from a few edge cases, don’t eat other organisms absorb energy from the sun and convert between 1 and 5% of that energy into plant biomass. From there on, though there can be much variation, about 10% of the energy in any trophic level makes it’s way up to the next level. Animals can eat plants or each other, but 90% of the energy in the biomass they consume gets used by the organisms in the normal progress of their day, gets lost as heat or is not digested and gets excreted (like fibre in human diets). Only about 10% of the lower levels biomass gets turned into biomass in the following trophic level.

There are a few consequences of this remorseless physics. The first is that each trophic level can support fewer organisms than those below it. If you can only convert 10% of the biomass of the trophic level you are feeding on into your biomass then this puts a hard limit on the amount of biomass you’re species is able to produce. This means that in any ecosystem there will be fewer eagle-like organisms and more mice-like organisms because eagles are apex predators and have much less potential energy to play with than mice who have plenty of plant energy to eat.

But, you may say, what about humans? We’re apex predators, aren’t we, why are there so many humans running around? The explanation for this is that we aren’t really apex predators (though there is some debate about this). Humans, like other organisms, are capable of occupying multiple trophic layers at once. The energy pyramid is not rigid thing and humans in hunter/gatherer mode, for example, are part primary consumer (the gathering) and part secondary, tertiary or apex consumer (the hunting). If, in prehistoric times, we had been only hunters then our population would have been limited by the energy available to the particular trophic level we inhabited. Whether that would have been apex predator I’m not sure given the existence of tigers, lions and other highly evolved predators (we may have also been pretty rubbish at hunting).

Whether we are apex predators or not, when we developed agriculture we kind of moved down the energy pyramid. We became, for the most part, a primary consumer (a herbivore). Throughout most of history, in agricultural societies, meat was generally a rarer commodity than grains and other vegetable matter. To feed growing populations we took advantage of the greater energy available in the primary consumer level by growing and eating crops. Our ability to not only consume plant food but use technology to increase the amount of plant food available to us meant we had much more energy available than in hunter/gatherer mode. We didn’t, however, leave our meat eating ways behind us, in agricultural mode instead of hunting we also farmed animals that we slaughtered and used for food. This basic form of agriculture is still the one we use to this day, farming crops and animals to access energy.

Even when we began farming animals instead of hunting we moved down the energy pyramid. We almost exclusively farm herbivores, the primary consumers, and this is because of the 10% biomass rule. Say, for some reason, we decide to farm coyotes, a secondary consumer, for food. To do this we’d have to ‘grow’ a whole bunch of herbivores to feed the coyotes and then we’d only get 10% of the energy we invested in the herbivores back from the coyote meat. It’s easier and more efficient to just eat herbivores that eat plant matter that we may not want to eat anyway. The higher up the trophic pyramid the less efficient farming animals becomes. That’s not to say we can’t eat carnivores but we tend to hunt those animals, we don’t domesticate and farm them.

In real life ecosystems are much more complicated, but, as we’ve seen above, the idea of trophic levels does provide some insight into why we do things the way we do. Take milk as another example. You can think about milk in the context of trophic levels. Milk is a handy way for us to extract some energy from plants, in the form of animal fats, without actually eating the herbivore. This makes it a kind of renewable resource; the fruit of the primary consumer trophic level. We can milk the cow everyday and get some energy from plants in the form of animal fats. The 10% rule still holds as the milk contains only 10% of the energy that a cow used to produce it but we don’t have to consume all the cow, we can get some more milk the next day. This is why in places with a strong dairy culture even the poorest households often kept a dairy cow (you can read about this in Bridget Ann Henisch’s brilliant book ‘The Medieval Cook’).

The energy pyramid has consequences for our food systems as well. In the 18th century Thomas Malthus predicted widespread famine and death as the human population exceeded its capacity to feed itself. Famously, this did not happen, we avoided it by improving our technology and increasing our agricultural output. But what we mostly did, in terms of energy, was increase the amount of plant biomass not improve the photosynthetic efficiency of our crops.

Of course we cannot continue to do this for ever, if our population continues to increase, there will come a time when we have optimised our food production as much as we can and we run up against the fact that we can still only get about 10% of the energy from our primary producers. As I discussed in the banana post we may also have made food systems more fragile. Many of the agricultural improvements we have made have also diminished the genetic diversity of our primary producers making us dangerously exposed if conditions change. Like, for example, if our climate starts changing for some reason.

So, just like every other organism on the planet, we are still bound by the flow of energy through our ecosystems. And the amount of that energy in that system is defined by the plants that we grow or harvest in our food chains. In a way we are still as vulnerable to crop failures as the typical medieval peasant, just on a global scale. To really break free of the physics behind the energy pyramid we could devise a more efficient way of converting sunlight to edible biomass. Current solar technology, able to capture between 10-20% of incoming solar energy, is more efficient than photosynthesis but it produces electricity not biomass. As it stands we have no technology that can convert that electricity to biomass with the same efficiency as plants, though people are working on it and it would make a good topic for a future post (see here and here if you want to skip ahead)².

I’ve written before how a modern supermarket would have seemed a marvel to your average medieval peasant. Tomatoes in winter, milk by the quart, fresh food all the time and every time. But the wonder of the modern supermarket also acts in reverse for us modern humans, distancing us from the fundamental laws underlying the energy flow in our ecosystems. Entering a supermarket now, one could begin to think that we have conquered physics and have thrown off the shackles of the energy pyramid. But, like Plato’s Allegory of the Cave, the occasional egg shortage or increase in the price of rice are shadows on the wall reminding us that we are just as much prisoners of physics as any of our ancestors.

Footnotes

1. It’s commonly attributed to Rutherford but no one can find a source, the closest is an indirect attribution in a book by John Desmond Bernal (see here) ↩︎
2. The other approach could be to live within our means. We could cut down on the shockingly high amount of food waste that occurs in the modern world. We could cut down on some less efficient food systems (as much as I hate to say it beef is a pretty low efficiency way of obtaining energy). We could also slow our population increase, a process that seems to be underway already. ↩︎