Adding milk to coffee changes its nutrient content significantly and can also counteract some of the negative effects coffee may have on human health, such as the possible increased risk of osteoporosis caused by drinking caffeinated beverages. Studies carried out on postmenopausal women showed that decreased bone mineral density was offset by consuming the equivalent of two cups of coffee with milk a day,9 although another study showed that milk may impair absorption of the polyphenols and antioxidants in coffee, affecting the bioavailability of chlorogenic acids and its metabolites.10 However, milk alone provides many health benefits, because it is a complete protein and a good dietary source of other nutrients, such as calcium and the B vitamins. There are three major constituents of milk that are important to understand in regard to its flavor and frothing capability, and how milk interacts with coffee.
FATS
Fats are an important part of the content of milk, giving it a rounded mouthfeel. Milks with a higher fat content generally result in a fuller flavor and a richer, silkier beverage. Milk can be purchased with varying levels of milk fat: from fat-free milk (also called skim or nonfat milk), with less than 0.2 percent; to low-fat or reduced fat milk; with about 1 percent and 2 percent milk fat respectively; to whole milk, with about 3.25 to 3.5 percent milk fat. Lower-fat milks generally create more foam, because there is less fat to compete with the proteins. However, once the fat content rises above whole milk level—for example in half-and-half, which contains 10 to 18 percent milk fat—foaming capability rises again. This explains why the higher the fat content in cream, the easier it is to whip.
PROTEINS
It is the proteins in milk that are largely responsible for foaming; as the milk heats to more than 140°F (60°C), these proteins denature, coating and stabilizing the air bubbles that are being introduced by the steam wand.
LACTOSE
Lactose, also known as milk sugar, is what gives milk its sweetness. Because lactose is less soluble than sucrose, it seems less sweet, but heating milk increases its solubility, breaking down the sugars and, therefore, increasing the sweetness of the milk. Nondairy milk can often react unpredictably in coffee. The extraction methods used in producing soy and nut milk can result in low lipid content, and lipids are necessary for holding the air bubbles in the liquid. There is also a risk of the acids in the coffee coagulating the proteins in nondairy milk if it is added cold to hot coffee. While many nondairy milk products now contain stabilizers to reduce this risk, it is advisable to warm homemade or natural store-bought nondairy milk before adding it to coffee or to let the coffee cool slightly before combining. Several methods can prevent this milk from curdling, such as making sure not to overheat them or adding the coffee to the milk as you stir instead of the other way around.
HOW TO FROTHING MILK
what is milk froth ?
1- Start with milk of your chosen fat content, taken straight from the refrigerator— it must be chilled. Make sure the milk is fresh—the older it is, the harder it will be to get proper foam. Pour the milk into a frothing pitcher until it reaches the bottom of the pouring spout; this will allow ample room in the pitcher for the milk to froth without spilling.
2 Insert the steam wand just below the surface of the milk on a slight angle, forcing the milk to move around in a whirlpool motion. This is called “stretching” the milk, creating macrofoam (tight air bubbles and silky foam) by letting air into the milk gently. It should make a hissing sound. Place your hand on the outside of the pitcher, and when the milk has almost reached body temperature, you want to stop introducing air, so change the position of the steam wand, moving it a fraction farther into the pitcher. This will texture the milk, increasing the temperature without introducing more air.
3 Continue spinning the milk in a whirlpool motion, but with the steam wand slightly more submerged—you should no longer hear any hissing. The key to frothing the milk correctly is to tilt the pitcher slightly, texturing the milk until the pitcher becomes just too hot to touch, about 140°F (60°C).
4 Once the milk has been heated and frothed, tap the pitcher on the counter to pop any large bubbles. Let the milk rest while you prepare the espresso shots, then when you are ready to pour, swirl the milk around the pitcher to make sure it is even in consistency, tapping it on the counter again if any large bubbles are remaining the milk should be smooth and shiny, resembling wet paint
FROTHING MILK
how do you froth milk ?
The hotter your milk, the sweeter it becomes—up to a point. Lactose (the sweet part of milk) is perceived as five times less sweet than regular sugar. Heating milk up increases lactose’s solubility and hence the sweetness you taste. Aim for milk around the 136 to 140°F (58 to 60°C) mark. Above that, you start to denature the milk proteins and create lower-quality foam.