Though perhaps impractical on a large scale, making butter is a fun process through which we can better understand the wonders of milk. In speaking of butter, it helps to place it context within a broad view of milk and its derivations. All mammals produce milk whose purpose is to feed infant mammals of that species. The same is true of humans, although we didn’t always seem to be hard-wired for consuming milk as adults; in childhood our bodies are supposed to stop producing the enzyme needed to break down lactose – the sugar found in milk. Indeed it is lactose that the dairy intolerant have problems with. Depending on whom you ask, it is believed that an ancient genetic mutation or biological imperative (perhaps vitamin deficiencies due to human migration further away from the equator) led to a growing toleration of lactose well into adulthood, leading to widespread fresh milk consumption in the neighborhood of about 7,500 years ago, if not earlier.
Long before, however, domestication of animals for meat as well as dairy had led to the development of fermented milk products of many kinds – from yogurt to hard cheeses – which were not only easily digestible (bacterial fermentation ‘eats up’ most of the lactose), but nutritious, portable, and stable for longer periods of time. Think about it: fresh milk is highly perishable and made up of 88% water; if you’re a nomadic herder constantly on the move, it makes sense to remove some of that heavy water and concentrate milk’s energy and nutrients, in turn increasing its shelf-life, and in many cases expanding its flavor profile.
Below, an approximate composition for cow’s milk and another that compares it against other mammalian species:
Not only is the composition important in determining the properties of milk, but looking at the physical structure is also helpful. Milk in its raw form is relatively unstable, in that the milk fat has a tendency to flocculate – to cluster together and rise to the surface. Because of milk fat’s natural tendency to form this ‘cream layer’, most commercial milk products are homogenized in order to separate the fat into small, stable droplets evenly dispersed throughout the product. In a process that has remained fundamentally unchanged for over a century, the milk is pumped through a screen of tiny openings at high pressure and velocity, effectively breaking the individual fat droplets into smaller ones. Homogenization is typically carried out immediately after pasteurization – the process of heat plus time used to destroy harmful microbes.
The structure of milk can be expressed in three different ways:
- An oil-in-water emulsion with the fat globules dispersed in the continuous water phase. An emulsion is an evenly dispersed mixture of two substances that do not readily mix – in this case the milk fat and water. Homogenization of the milk creates this emulsion.
- A colloidal suspension of protein particles; a suspension involves solid particles that are, in effect, ‘floating’ within in the milk.
- A solution of lactose, soluble proteins, minerals, vitamins other components; a solution refers to all of the substances dissolved within the milk.
Generally speaking, both the price and perceived level of quality of any dairy product is proportional to the percentage of milk fat it contains. The fat component in milk adds richness, aroma, and flavor to pastry preparations, and can contribute a smooth texture and creamy body, adding a sensation of moisture and lubrication to the palate as it is consumed. In baked goods, milk fat shortens, or tenderizes the finished product. The moisture in butter can create leavening as the tiny droplets turns to steam in the oven. Milk fat can be provided by whole milk, but also from concentrated forms, such as heavy cream and butter. With regard to whipped cream and ice cream, it is the fat that provides their primary solid structure, a ‘scaffolding’ of sorts surrounding air that has been whipped into it.
Below, a table comparing the composition of the most commonly used products derived from milk:
On our way to butter, we must also discuss cream – that portion of milk that naturally clusters and rises before homogenization. As mentioned, traditionally this is referred to as the ‘cream layer’ skimmed off of fresh milk. Today, a range of high fat products are produced, tailored for specific uses. Like milk, creams are categorized according to their fat percentage. Below, a table that compares common consumer and commercial products by composition:
Heavy cream or heavy whipping cream contains between 35% and 40% fat and is most commonly used in those two forms, providing flavor and texture to pastry preparations. The high fat content provides desirable boiling and whipping properties; the reason why boiled milk forms a ‘skin’ of protein and cream does not is because the extra fat in cream buffers the casein proteins, preventing coagulation when the cream is boiled (there’s also, of course, less protein cream to begin with). The fat also stabilizes whipped cream by forming a continuous, partially coalesced network around air that is whipped in. Because even a 5% difference in fat can adversely affect some delicate preparations, recognizing fat content and being able to adjust a formula is important.
An intermediate along the spectrum of cream and butter is clotted cream – also known as Devon or Cornish cream – a unique high fat product. Clotted cream is cream which has had a portion of its water cooked off, to produce a minimum fat content of 55%, though it can be higher.
As with ice cream, we can better understand butter by first looking at liquid cream’s ability to whip up into a solid foam – an everyday example of shear-thickening. Technically speaking, all foams can be described as colloidal, two-phase systems in which air forms the dispersed phase and water forms the surface phase. In the case of whipped cream, the surface phase contains casein proteins that complex with the partially coalesced fat -destabilized by the physical force of whipping – forming that ‘scaffolding’ surrounding the air cells that are incorporated during the whipping process.
The two primary factors responsible for cream’s ability to whip are fat content and temperature.A minimum 25% is necessary to create a solid structure. In order for the fat to properly adhere or coalesce, the milk fat within the cream must also be partially solid, or crystalline. Success also depends on a minimum of 5˚C/40˚F. Half and half, for example will never whip up to a solid because it lacks sufficient fat. Likewise, cream that is room temperature or warmer will fail to form a stable structure because a large portion of its milk fat will have begun to liquefy, simply forming larger fat globules rather than a network of small globules that retain some of their own identity.
Counter-intuitively perhaps, whipping cream to ‘stiff’ or firm’ peaks does not necessarily provide for maximum stability and volume of whipped cream. A firm textured whipped cream is a result of too much coalesced milk fat, which will eventually ‘squeeze out’ some of the air and water phases; an unstable, over-whipped cream will likely have a slightly grainy texture, will ‘weep’ water over time, and will be difficult to incorporate into another mixture, such as when using cream to lighten a mousse. Fat coalescence in whipped cream is irreversible, meaning that once adhered, the milk fat cannot be separated without heating and chilling the cream to liquefy and recrystallize the fat. Thus, over-whipped cream can rarely be ‘rescued’; it is often better to use the over-whipped cream for another use and start over, whipping the cream to a ‘soft’ peak for maximum volume and stability.
As noted above, cream can be described as an oil-in-water emulsion with milk fat globules dispersed in a continuous water phase. In whipped cream, that milk fat has partially coalesced to form a semi-solid structure. When the cream is whipped or beaten further, those milk fat globules continue to coalesce – they irreversibly increase in size, expelling much of the air and water phase to eventually form a water-in-oil emulsion where the remaining water is dispersed in a continuous fat phase. This simple inversion of the original cream’s original structure is how butter is produced.
Below, various stages of whipped cream stable soft peak to finished butter.
As more and more of the fat becomes tightly packed together, it is further separated from much the cream’s original liquid, referred to as buttermilk. This buttermilk (a little less than half the original weight of the cream) contains only a trace of the cream’s original fat, but a majority of its nonfat solids. The buttermilk we produce as a byproduct of making butter ourselves is a bit different than the commercial buttermilk available. Although its name survives from the traditional practice of allowing ‘true’ buttermilk to spontaneously ferment, modern commercial buttermilk is produced by inoculating low-fat or skim milk with a bacterial culture (Lactobacillus bulgaricus or Lactococcus lactis), sometimes supplemented with additional nonfat milk solids to create a thicker texture (as in all cultured dairy products, the acids produced by the bacteria denature the proteins, which gives the product body). Even though it differs from the commercial version, the possible uses of fresh buttermilk are many; I recently made a fairly decent ricotta from it, as there remain a substantial amount of curd-forming proteins.
To salt, or not to salt: up to 1.5% salt can be added to butter (at the final kneading stages, otherwise it will dissolve and drain away with the buttermilk) in part for flavor, but traditionally relied upon as a preservative. It is generally preferable to use an unsalted butter, which allows the chef to better control the salt content of a preparation as well as to ensure a fresher product. If I intend to use salt with butter for bread service, I actually prefer to serve it with a sprinkle of coarse Maldon salt at the table.
Federal law requires that butter contain a minimum 80% milk fat. Most commercial butters are composed of roughly 82% fat, 16% water, and 2% nonfat milk solids. High fat butters, sometimes referred to as European-style, may have a fat content of 83% to 84%, and are desirable for their plasticity, especially in the preparation of laminated dough products. A simple means of determining the fat content is to cut a thin slice from the cold butter and try to bend or fold it – a low fat butter will break and feel brittle, whereas a high fat butter will maintain a greater degree of flexibility, even when cold.
A simple method for making cultured-style butter by hand from heavy cream and crème fraiche is given below. The butter-making process is relatively simple; the cream and crème fraiche are whipped until all of the fat has coalesced. The resulting liquid, or butter milk is drained off and further kneaded from the fat. This method can produce a butter of higher fat content more complex flavor than most commercially available butter:
A few in-depth resources on milk:
On Food and Cooking, by Harold McGee
Milk and Milk Products: Technology, chemistry and microbiology, by A. Varnam and Jane Sutherland
Dairy Science and Technology, by Pieter Walstra, Jan Wouters, and Tom Geurts
University of Guelph (Ontario), Dairy Education Series
Also of interest, from my dairy and ice cream guru, Cesar Vega:
The Kitchen as Laboratory: Reflections on the Science of Food and Cooking, by César Vega, Job Ubbink and Erik van der Linden
And did you know that Kitchen Arts and Letters in NYC now sells online? Awesome!