Refrigerated

All fluid milk except unopened, aseptic packs of ultrahigh-temperature pasteurized milk and certain canned milk products should be stored in the refrigerator. They need to removed only long enough to take what is to be used and then quickly returned to the refrigerator. Containers should be closed or covered to avoid rancidity, microbial contamination, and the absorption of odors from other foods in the refrigerator. Both oxidative and hydrolytic rancidity are a potential problem in milk and milk products because of the substantial amounts of short-chain fatty acids. One should never drink milk directly from the container, because bacteria in the mouth can wash back into the product. Pouring unused milk products back into the original container is also not recommended, because microbial contamination could have occurred from exposure to the air or other sources. Proper opaque containers will reduce exposure to light, which can trigger oxidation, resulting in off-flavors and the loss of riboflavin (B₂). The shelf life of certain dairy products such as cottage cheese can be doubled with the addition of carbon dioxide, which disrupts microbial functions.

The following guidelines should be followed when storing milk products in the refrigerator:

• Milk. No more than three weeks.
• Yogurt. Best consumed within the first ten days, but can last up to three to six weeks. If it separates, simply stir the liquid back into the curd before serving.
• Buttermilk. Best when used within three to four days after purchase, because it will continue to sour, but it can last up to three or four weeks.
• Sour cream. Unopened, up to one month, but best when used within a few days.

Dry Storage

Nonfat dry milk, ultrapasteurized milk, evaporated milk, and sweetened condensed milk are stored at or slightly below room temperature (72^oF/22^oC). Nonfat dry milk should not be exposed to moisture, because humidity will cause it to turn lumpy and become stale. Keeping containers tightly closed minimizes any contact with oxygen from the air. Nonfat dry milk stored in this manner will keep for about one year. Unopened cans of evaporated and sweetened condensed milks will keep up to a year in dry, ventilated areas, but double that if refrigerated. Both should be turned over every few weeks to prevent the solids from settling, thickening, and producing clots. Ultrapasteurized milk can be stored unopened at room temperature for up to three months. Once opened all these milks must be treated like fresh milks and refrigerated. The “sell by” or “expiry” date is the last day the item should be sold by the store, and this should always be checked before purchase.

Liquid milk products such as cream, evaporated milk, and reconstituted nonfat dried milk can be made into a foam by whipping air into the liquid. During whipping, the protein in these milk products is mechanically stretched into thin layers that trap air bubbles, fat particles, and liquid.

Whipped Cream

Cream expands two to three times its volume when whipped. The stability of milk foams, especially whipped cream, is dependent on several factors: the fat content, the temperature of the cream and the equipment used, the age of the cream, the sugar content, and the length of whipping time.

Fat Content. The higher the fat content, the more stable the whipped cream, because solid fat particles provide rigidity to the foam. Heavy whipping cream beats more easily than lower-fat whipping creams, but becomes lumpy and buttery with overbeating. An advantage of using heavy whipping cream (36 to 40 percent butterfat) in one particular baking application is that its lower moisture content will prevent pastry crust from becoming soggy when it is filled with cream fillings.

Most whipping creams are sold unhomogenized to allow for easier aggregation of the fat globules. When the cream is homogenized, much of its protein surrounds the now smaller and more numerous fat globules instead of being available to envelop the air bubbles that are essential for foam formation. Vegetable gums and gelatin are sometimes added to improve the foaming ability of the commercial creams.

Temperature. Cooling cream increases its viscosity or firmness and its tendency to clump. For best result, refrigerate the cream, bowl, and beaters at 45^oF (7^oC) or less for at least two hours before whipping. Cream allowed to warm to room temperature or even to about 50^oF (10^oC) has more widely dispersed fat globules, which reduces the cream’s ability to be whipped and creates a softer texture. Most cream is pasteurized, but the heating process denatures an enzyme that helps fat globules to cluster, so nonpasteurized creams whip more readily and have a smoother texture. Although ultrapasteurized whipping cream takes even longer to beat to a peak-holding consistency, other ingredients can be added to improve its whipping ability and dramatically extend its shelf life.

Age. The older the cream, the greater its viscosity and ability to foam. Whipping cream must be at least one day old in order to allow it to incorporate the air necessary for the optimum increase in volume.

Sugar. Sugar increases the stability of whipped cream, but it should be added gradually, toward the end of the whipping period. Added earlier, it increases the whipping time and reduces overall volume and rigidity by delaying the clumping of fat. Sugar has the benefit, however, of lessening the likelihood of overbeating the cream. For the best stability, powdered sugar instead of granulated sugar should be used, because it dissolves more readily in the cold cream, and the cornstarch in the powdered sugar acts as stabilizer.

Whipping Time. Physical agitation of the cream is necessary, because it disrupts the phospholipid membranes surrounding the fat globules, preventing them from aggregating. Overbeating, for even a few seconds over the peak point turns whipped cream into butter and whey. To make whipped cream with an electric beater, it is best to beat on medium high and then slow to a lower speed as soon as the cream starts to thicken. To check for sufficient whipping, the beating is stopped and the beaters are lifted to see if the cream is falling into glossy, large globs with soft peaks. The formation of stiff, yet moist, peaks signals the completion of the whipping process. The cream should be underbeaten slightly if ingredients such as sugar are to be whipped into the cream.

Whipped Evaporated Milk

The high concentration of milk solids in evaporated milk makes it possible to whip it to three times it volume, but the flavor, texture, and stability are less acceptable than for whipped cream. The flavor of evaporated milk has a tendency to overpower other flavors; thus it is best used with highly flavored foods. The stability of evaporated milk foams is much more tenuous than that for whipped cream partly because of the former’s lower viscosity and its lower fat content. This can be compensated for to some degree by chilling the can of evaporated milk in the refrigerator for 12 hours or in the freezer until ice crystals form. Adding 1.5 tablespoons of sugar per chilled cup can further stabilize the protein and resulting foam. The main advantage of whipped evaporated milk over whipped cream is its lower cost.

Nonfat Dry Milk (NFDM)

Prepared nonfat dried milk powder can actually be whipped into a foam. This whipped milk product is very unstable, but it is much less expensive and lower in both calories (kcal) and fat than whipped cream. It is prepared by dissolving equal parts of nonfat dried milk and cold water, chilling, and beating until the mixture stands in soft peaks. Stability is increased by adding 1 tablespoon of lemon juice or 2 to 4 tablespoons of sugar during beating, which continues until the peaks bend over slightly on top.

Milk also coagulates and forms curds when it combined with certain enzymes originating from animal, plant, or microbial sources. Enzymes used to coagulate milk include pepsin from the stomach of swine, proteases from fungal sources, and certain enzymes from fruits. The enzyme most commonly used to coagulate milk is rennin, which is used in the production of cheese and ice cream.

One of the major differences between coagulation caused by enzymes and that initiated with acid is that rennin-coagulated clots are rich in calcium and have a tough, rubbery texture, unlike those created by acid, which are less elastic and more fragile in consistency. Cottage cheese, which is normally coagulated by acid, contains less calcium per ounce (19 mg) than cheddar cheese (204 mg), which is usually made with rennin.

For example, extra caution is required when combining milk or cream with lemon-flavored tea, tomato soup, and coffee (which is acidic). The key to preventing the milk from coagulating is to add the acid to the milk base instead of the other way around. Avoiding high temperatures after milk has been mixed with acid also helps to prevent coagulation.

When milk is heated to near the boiling point, the whey proteins lactalbumin and lactoglobulin become insoluble, mesh with the milk’s calcium phosphate, and precipitate, forming a film on the bottom and sides of the pan. This film can scorch easily. Scorching can be prevented by constant stirring, slow temperature increases, or use of a double boiler.

The skin that forms on the surface of the milk during heating is caused by the evaporation of water, which is accompanied by an increased concentration of casein, fat, and mineral salts. This thin skin also scorches easily; in addition, it can trap steam that is trying to escape and cause the milk to boil over.

Several steps can be taken to avoid this problem, including using a lid, continual stirring during heating, floating a small pat of butter on top of the milk, or, in the case of hot chocolate, adding whipped cream or marshmallows. Casein will not coagulate with heat unless it is boiled for long periods of time. Canned evaporated milk, however, which contains higher concentrations of casein, may coagulate during the high heats of sterilization. This is prevented by warming the milk prior to sterilization.