Milk. Our complex relationship with this white liquid starts at birth, and from an early age we are taught that it is an important part of a balanced diet. Milk is extremely versatile – it is the primary ingredient in cheese, butter, and ice cream and is added to countless recipes to enhance flavor and add a little something extra to the final dish. But before we can jump into the wonderful world of milk and cooking, we need to go over some basics when it comes to the chemistry of milk and modern methods of production.
The first thing to know about milk is that it is liquid at room temperature. The milks that most humans drink, cow being the most popular, are almost 90% water, so milk being liquid at room temperature (roughly 77 oF or 25 oC) shouldn’t come as much of a surprise. The other 10% of milk’s weight comes from fats (roughly 3.7% in cows), protein (3.4% in cows), lactose (4.8% in cows), and various minerals (0.7% in cows). These percentages are quite variable as cow breed changes, and even more variable between different species of milk-producing mammals. Different types of diets that the cows are fed are also a factor that influences the relative concentrations of the components in milk, as well as the overall flavor of the milk. Although differences exist between the types of milk available at the supermarket, this variation is largely not as common anymore due to modern methods of production.
Speaking of which, it is important to understand that milk production is highly modernized, consisting of large-scale dairy farms that collect milk from thousands of cows at once and follow strict FDA guidelines to ensure the safety of the product they produce.
Pasteurization, the heating of milk to a certain temperature for a period of time, preserves milk by killing pathogenic and spoilage microbes by inactivating milk enzymes that cause milk to go bad over time. Large-scale operations use the high-temperature, short-time (HTST) to pasteurize their milk. This process involves pumping milk continuously through a heat exchanger that is held at a minimum of 162 oF or 72 oC for 15 seconds.
The other process used by dairy farms is homogenization. Homogenization involves pumping hot milk at high pressure through small nozzles in order to tear apart the fat globules floating freely in the milk. This prevents the fat globules from congealing and creating an uneven consistency in the milk. Homogenization ensures that fat is evenly dispersed throughout milk.
Although these modern processes are marvelous for extending milk’s shelf life and ensuring an even consistency, they come at a price. Modern milk has less health benefits than less processed options and has far less flavor than “real milk” (milk that has been untouched by society’s innovations. But what plain old milk lacks in flavor, it surely makes up for in versatility. The goal of today’s class was to show the different ways that milk can be used in cooking, and to examine the chemistry behind the recipes.
The first thing we made was ice cream…eight different ways! Here are the recipes we used to make each ice cream mix:
As you may notice, each recipe is a variation of a simple formula: cream + milk + sugar + vanilla = ice cream! Philadelphia-style, or standard, ice creams are usually served hard and have a rich and creamy flavor. Philly-style ice cream is ideally served at 8-10 oF (-13 oC) so that it doesn’t numb the tongue but still maintains its hard consistency. This type of ice cream can also be served at a warmer temperature, usually around 22 oF or -6 oC, and is known as soft-serve. At this temperature, about half of the water in the ice cream is in the liquid form, contributing to its soft and smooth texture.
French ice cream, also known as custard, contains egg yolks. The proteins in the yolks help keep ice crystals small, which leads to a smooth, even, texture. The yolks also make the custard taste great, giving it a richer, eggier flavor. Gelato is a distinct style of custard and is high in butterfat. It has a very rich and dense profile, making it a favorite on summer afternoons!
The process of making ice cream is actually quite simple – once the mix is made and chilled (we used the cold weather to our advantage and put the mixes on the deck!), it’s as simple as putting the mix in a stand mixer and adding some liquid nitrogen!
Relax! It’s not nearly as dangerous as it sounds (but it is just as cool!) By adding the liquid nitrogen to the mixture while it is mixing, air pockets can form in the mix (which makes it smoother and more “fluffy”) and the cold is evenly distributed.
A quick side note about nitrogen – at room temperature it exists as a gas, and to keep it in the liquid phase it must be super cooled to at least -320.4 oF or -195.8 oC!! This temperature is the boiling point of liquid nitrogen, so when it comes in contact with the ice cream mix (and the air around it) that is warmer (relatively speaking) than it, the liquid nitrogen changes phase and with that phase change comes a drastic decrease in temperature of its surroundings. Using this to our advantage, we were able to almost instantaneously solidify the liquid ice cream mixes.
After the excitement of cooking with liquid nitrogen, it’s hard to believe that anything could be more interesting! The way we made butter, although not quite as cool (pun intended), was really interesting because we made it in a jar, only using slightly cultured cream and a smidge of sour cream! In an attempt to culture the cream, we left container of it open on the kitchen counter for a day or so, and adding a small dollop of sour cream about halfway through.
During class we all took turns churning the cream by vigorously shaking the jar. Agitating the cream damaged the fat globules in the cream, causing them to rupture and release the fat they stored. These globs of fat stuck together and soon connected with other globs, and before we knew it we had a large mass of solid butter in our jar!
Before it solidified, the cream aerated so much that it resembled a whipped spread that took up nearly the entire container. At this point the aerated cream took up almost 100% of the space in the jar, so mixing it more was quite difficult. By churning the cream more, the network of air pockets we had created collapsed, allowing the fat globs to coagulate and group together. This resulted in a more solidified texture to emerge.
As the butter became harder and less aerated, white milk, known as buttermilk, appeared. True buttermilk, like the one we made, has the consistency of skim milk with a sweeter flavor. Because all of the fat from the cream was in the butter glob, only water, amino acids, and trace other materials remained in the buttermilk.
After we removed the butter glob from the jar, we kneaded in some salt. Adding salt has a few benefits. First, it adds a salty flavor that complements the butter’s rich profile and, when the butter is used in conjunction with other ingredients, it brings out much of the natural flavors that would otherwise be hidden. Adding salt also helps prevent the growth of bacteria and other organisms that could cause the butter to spoil.
Mixing the butter by hand also helps create an even consistency, allowing for a more hands-on (pun intended) approach.
Milk has so many wonderful characteristics that make it an ideal ingredient in countless recipes. From the very beginning of our lives, we humans rely on milk to provide us with the sustenance we need to grow strong. Although our dependence on milk dwindles as we age (in fact nearly 90% of American adults are lactose intolerant!), our fascination with the white suspension of fats and proteins in water remains with us throughout our lives.
Thanks for reading about our adventures with milk, and be sure to check back next week for our exploits with emulsions, foams, and other mixtures! And remember…