Aged Butter part 1: background and basics

by Johnny Drain

This series is about oxidation, rancidity, and making aged butter. In this first part, I’ll give some background about butter, rancidity, and the cultural context for eating aged butter. In the second part, I’ll explore the science of oxidation in fats and the safety of eating them. I’ll then describe the results of my work on culturing butters with unusual sources of bacteria in Part 3 and on aging butters in Part 4.

Butter. A symbol of purity in India and of depravity in the hands of, amongst other things, Marlon Brando.

Butter is a vector for taste. It carries fat-soluble flavour and odour compounds and therefore facilitates the expression of non-water-soluble flavours and aromas, such as of spices and herbs, in our food. In this role it is a workhorse of many classical canons of cookery, most notably that of France.

It’s also delicious as a primary, characteristic flavour in its own right: cultured butter, made by culturing cream with lactic acid bacteria prior to churning, has a gently refreshing acidity and rich, well, butteriness.

The Project

My project at the lab focused on butter, but not normal butter: aged butter. I started thinking about this when I made a salted caramel with butter that had been kicking around my fridge for a little too long. The outer surface had yellowed and it smelled slightly sweet and a little goat-like, but I thought it probably wouldn’t affect the flavour of the caramel. I was wrong. The caramel was very tasty, but with some unusual, subtle, savoury, meaty notes (step aside bacon jam).

Most people, including myself, would find the idea of eating ‘rancid’ butter stomach turning. However, as I dug into what rancidity was, and whether it is even okay to eat old butter, I found a 2011 study by sensory scientists at UC Davis [1] that found that 44% of tasters preferred olive oils with some degree of rancidity, and, furthermore, that one of the compounds found most prominently in rancid butter—butyric acid—is also found in significant amounts in sheep and buffalo milk, and a wide range of cheeses, including Roquefort, Limburger and Gruyère [2].

Similarly, American peanut butter, which contains rancidity-prone polyunsaturated fats, can taste rancid to non-Americans; yet the US is said to spend a whopping $800 million a year on the stuff. “In some cases, we [Americans] have gotten used to rancid flavours,” says food science writer Harold McGee, “so we assume that they are normal.”[3]

So perhaps, I thought, many people’s attitudes to certain funky, mildly rancid flavours are unnecessarily negative? Maybe there is scope for making butters with some controlled degree of rancidity, to develop and accentuate certain desirable flavours?

Conscious that this might be a hard sell for even (or perhaps especially) the most ardent of butter lovers, I was heartened to find that precedents do exist—some cultures like aged flavours in fats and have developed culinary uses for them.

In Morocco, highly-prized ‘smen’—or as it is sneeringly called in French, beurre rance—is made by mixing normal butter with herb-infused water (often with oregano), leaving it overnight, straining, and then aging for anywhere between a couple of months and several decades. It has a texture similar to that of normal butter but tastes like a spicy blue-cheese or Parmesan rind. The oft-recounted story is that families would make a batch of smen on the day their daughter was born, and only open it to stir through the ceremonial couscous on the day of her wedding. It is also known as a signifier of wealth, with reports of royalty hoarding smen [4]. From my own trials, roasting chickens in it and adding some to stews or stocks (indeed, as is often done with parmesan rind) also works wonders: it adds a rich, unique complex flavour that would be difficult to attain with any other combination of ingredients.

Mounds of glorious smen in a Marrakech souk.

It is also of note that Yak’s butter, which forms a central part of the Tibetan diet in butter tea and tsampa dumplings, is often described by westerners as tasting ‘rancid’, and that slight rancidity is part of the flavour of traditional confit [5]. Perhaps the closest northern European analogue to smen is bog butter—butter packed into barrels and buried in peat bogs, a tradition known in Scotland, Ireland and Scandinavia, but which seems to have died out before the 19th century—despite the fact that the acidic, anoxic environment of the bog actually prevents, or at least dramatically slows, oxidation and degradation of the butter. 

Looking for more contemporary examples, since starting the project I found that Zak Pelaccio from Restaurant Fish and Game in the Hudson Valley, NY, ages butter brushed with local whiskey and wrapped in horseradish or turmeric leaves, so it takes on a gaminess and sherry-like nuttiness, and thenwhips it with fresh yoghurt. Furthermore, Matt Lightner from Atera in New York ages butter made from cream cultured with cheese rind for a week at room temperature.

So, while examples of aged butters can be found throughout the world, they have limited notoriety, popularity and availability. In marked contrast, cheeses such as Gorgonzola, Roquefort and Stilton [6], in which some of the key flavour compounds—and the processes that create them (such as the transformation of fatty acids into methyl ketones)—are the same as in rancid butter, are gastronomically prized and appreciated across the globe.

My project therefore aimed to explore what we really mean by rancidity, from a chemical and cultural perspective, why old butter seems to have developed such a bad rep whereas cheeses with similar compositions are celebrated, and, ultimately, whether mild, controlled rancidity in butter can be a positive hedonic quality.

Butter Basics

First, a little overview of what butter is. Its composition is normally a minimum of 80% fat, 15–17% water, and 1–2% other stuff—non-fat milk solids—which includes milk proteins, phospholipids, free fatty acids, calcium, fat-soluble vitamins (A, D and E), and fat-soluble pigments such as carotenoids that give butter their colour. The more carotenoids, the more yellow the butter; goats milk contains few carotenoids and is therefore much paler than that of cows. Butter contains only small traces of lactose so, like many cheeses, it’s suitable for lactose intolerants, but people allergic to milk proteins should avoid both.

Cream, the precursor to butter, is an oil-in-water emulsion. Churning damages the protective layer of phospholipids and milk proteins that cover the fat globules, and which normally prevents them from coagulating. With these protective layers broken, the fat globules coalesce into a solid mass—butter, a water-in-oil emulsion.

Freshly churned butter. The milky droplets are buttermilk, the liquid that remains once the butterfat has coagulated.

The fat in butter (and milk) is composed primarily of triglycerides. Triglycerides are shaped a bit like the letter ‘E’ (Figure 1). They have a backbone of a glycerol unit, with one fatty acid chain bonded to each of its three oxygen atoms—this arrangement of bonds make it what chemists call an ester, or more specifically a triester. Triglycerides are the central component in vegetable oils and animal fats [7], and are also found in human skin oils [8].

In all triglycerides, the glycerol backbone is always the same, but the length and composition of the chains dictate the properties of the fat. The fatty acid chains are made of links of either single or double carbon-carbon bonds. When all these bonds are single carbon-carbon bonds we call the chain saturated, but if there are one or more carbon-carbon double bonds we say it is unsaturated.

Figure 1. Short- and medium-chain saturated fatty acids.

Fats with more unsaturated triglycerides, such as vegetable oils, tend to be softer, while those with more saturated triglycerides tend to be firmer. Butter is mostly saturated triglycerides and is therefore solid at room temperature. Importantly, unsaturated fats are more prone to rancidity than saturated ones, hence butter is more resistant to rancidity than, say, soy or sunflower oil. (‘Trans-fats’ refer to the stereochemistry of unsaturated fats, but we won’t be going into that here.)

If the bonds between the fatty acid chains and the glycerol backbone break, this liberates ‘free‘ fatty acids, which are key to the development flavours (‘good’ and ‘bad’) in fats. But I’m getting ahead of myself, more on that in Part 2.

A Short History of Butter Making

The word butter comes from the Latin butyrum, which in turn comes from the Greek bouturon (where bous means ‘grazing ox’ and turos means ‘cheese’) [9]. However, its manufacture, with sheep and goats’ milk rather than cows, can be traced back 4,000 years, meaning it predates olive oil.

Like many of the foods we eat, butter likely started off as a way to preserve nutrients—in this case, by extending the shelf life of milk in a rich, delicious, energy-dense way. Northern Europe, with its combination of pastures for livestock and cool climates, was perfect for butter, but in the hotter climes of southern Europe, butter spoils more quickly than cheese. Perhaps for this reason, the Greeks and Romans were a little snooty about the stuff and considered it to be food for northern barbarians. Greek comic poet Anaxandrides referred to Thracians as boutyrophagoi or “butter-eaters” (admittedly, other scholars said far meaner things about them), while Pliny the Elder, in his Natural History, calls butter “the most delicate of food among barbarous nations”, and then proceeds to describe its medicinal properties. Interestingly, Galen described butter purely as a medicinal agent rather than as a food—a notoriously tricky distinction best left for another time, but suffice it here to say the awareness that butter is good for you is millennia-old.

Introduction to rancidity and oxidation:

One of the central questions of the project was: what do we mean by ‘rancid’? As I explored this, I realised that there are in essence two relevant meanings—a more scientific one and a more cultural one. The two are not well correlated, and there lacks, among the general populace, including myself and most of us at the lab, a coherent lexicon to describe the latter.

Something rancid is typically defined as smelling or tasting unpleasant as a result of being old and stale, which is of course incredibly subjective: when we describe something as rancid what we often mean is “this tastes too old/weird for me” or “I am not willing to eat this”.

Context is a critical factor in determining what we think is palatable. I’ve discussed some examples of rancid butters above but, more widely, encouraging rancidity is actually not such a crazy idea: Cognac and Armagnac are both aged to develop ‘rancio’ character, which derives from oxidative processes [10]; in the UK, butter for toffee was often stored to develop some rancidity, to produce a more desirable stronger dairy flavour; and in the USA, chocolate manufacturers encourage their milk fat to undergo some rancidification by fat-digesting enzymes to develop modest cheesy and animal notes which enhance the complexity of the chocolate’s flavour [11]. In analogous processes, the enjoyment of many nuts is augmented by roasting-induced lipid oxidation, which cause the development of new flavours [12], and highly prized Sherry, Marsala, Vin Jaune, Maury, Banyul and Madeira wines rely on flavour development via oxidative processes [13]. In particular, because Madeira wines are so oxidised there is little scope for further oxidation and they are very stable, meaning that they can be stored for many years before being enjoyed.

In all these cases, particular audiences of people enjoy the results of ‘rancidity’—the flavours and value it adds to a product—without thinking of it as such. Furthermore, despite the words ‘rancid’ and ‘oxidised’ being more typically used in general parlance to imply negative hedonic qualities, there are also cases (eg. ‘rancio’ and ‘oxidised’ in the context of some wines and spirits) where the same words are used to describe positive attributes. Nonetheless, my experience of explaining to people the concept of this project showed me that phrases like ‘rancid butter’ and ‘oxidised fats’ do typically elicit a pained expression, and a sudden unwillingness to let me cook for them. So, if we want to develop a delicious, mildly oxidised, hedonically-positive butter—which has a flavour profile similar to cheeses that we know people love—then perhaps the first step should be simply to not describe it as ‘rancid’, or even the somewhat more technical-sounding but not entirely neutral ‘oxidised’ (more on the intricate chemical and linguistic relationships between ‘rancidity’ and ‘oxidation’ in Part 2). We could instead, for example, use the term ‘aged’ to draw parallels with the worlds of cheese and wine, where instances of similar physico-chemical processes elicit positive hedonic response. 

Additionally, cultural priming is important, and people from different cultures have very different levels of sensitivity to rancidity. We have seen how Moroccans prize smen, yet for their French colonisers beurre rance was barbaric, offensive stuff. Roland Barthes, no less, writes on the subject:

“One day I was invited to eat a couscous with rancid butter; the rancid butter was customary; in certain regions it is an integral part of the couscous code. However, be it prejudice, or unfamiliarity, or digestive intolerance, I don’t like rancidity. What to do? Eat it, of course, so as not to offend my host, but gingerly, in order not to offend the conscience of my disgust…”[14]

Barthes raises the notion of disgust which is key to understanding what rancidity is, and isn’t. As Rozin and Fallon note, “It is the subject’s conception of the object, rather than the sensory properties of the object, that primarily determines the hedonic value.”[15] Thus, disgusting foods need not even have negative sensory properties: many people find the idea of eating some animal species (rats! dogs! horses! snakes!) revolting, and yet, from a nutritional and sensory perspective, they are pretty similar to those that they are perfectly happy to eat (piglets, sheep, cows, geese). Put another way, even stuff that tastes good can be perceived as disgusting. Rozin expands:

“The sense of disgust that overwhelms someone who bites into a wormy apple—the feeling of revulsion, the spewing of chewed apple pieces, the approach to the brink of vomiting—is not brought on because the worm tastes bad. The half worm is ‘found’ by seeing it writhing in the bitten apple; it is not tasted at all.”

Elicitors of disgust vary enormously from culture to culture, and to a lesser extent from individual to individual within cultures (with some possible exceptions, such as vomit and faeces, which may be universal disgust objects—though surprisingly there is limited evidence to support this notion) [16].

Some conjecture on cheese and butter

Pondering the relationship between cheese and butter, I became deeply interested in why rancid butter and blue cheese, for example, are regarded so differently. Why is there widespread cultural acceptance for one but not the other? Here are some ideas—granted, they are speculative.

DeLaval cream separator.

Firstly, butter requires more raw ingredients. It takes 20 litres of milk to make 1 kg of butter, but only 10 litres to make 1 kg of hard cheese, and even less for softer cheeses: 1 kg of Brie might require 7 litres [17].

Secondly, the milk from which cheeses are made has long been readily available in significant amounts and people have had time to perfect strong-tasting, challenging varieties of cheese, and their appreciation has been firmly embedded in the cultures that produce or import [get_bloginfo]url[/get_bloginfo]/files/blog/2016/1/21/them. in contrast, cream was historically very much a luxury and only available in smallish quantities until the late 19th century when mechanical cream separators were invented. prior to this, using traditional shallow setting pans, which could only handle a modest amount of milk per batch, it would take 3 days to generate a small amount of cream. the method was time-consuming and unreliable; the cream could easily sour in the process. However, once De Laval invented his continuous cream separator in 1878, the cream could be separated from 150 kg milk in just one hour.

Thus, until relatively recently, making butter was more expensive and impractical than making cheese, meaning that the resulting product was more precious and valuable, and so people might have been less willing to let it age or develop unusual flavours and aromas. Without widespread cultural precedent for enjoying it—we have highlighted a few pockets of the world where the tradition did develop—aged butter seems to have been largely overlooked as a source of deliciousness, and rather become one of Rozin’s “objects of disgust”.

Compounding this issue could be the central role that butter plays in the classic French cooking that has exerted such strong influence over global gastronomy, and via a trickle-down effect to ordinary households in many regions, for such a long time. With France’s wealth of delicious, funky-tasting cheeses—which provide plenty of sources of goaty, meaty, methyl ketoney flavours—perhaps there was little motivation to explore the aging of butters to create new, alternative additional sources of these flavours, especially when butter was already in high demand, used in baking, basting, sauce-making and pastry, and being eaten as a condiment in its own right. Contrast this with Morocco, where butter plays a far less critical role in the cuisine and there is a notable lack of aged cheeses; the complex flavour profile that smen can add to a dish sits quite uniquely amongst other traditional foods of the region.

Stay tuned for Part 2 of this series, in which I will explore the science behind rancidity, what it means chemically and linguistically, and how safe it is to eat aged fats.

A sampling of various aged butter trials



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[16] Bitton, D., Current theories of sensory and interpersonal disgust.