Milk, yogurt, cheese and other dairy foods: composition, varieties, processing, storage and quality.
Fresh cheese is a broad category. Cottage cheese holds moist curds in a creamy dressing, while queso fresco is pressed into a firmer cheese; their water and fat contents create large nutritional differences even before brand-to-brand variation.
Removing milk fat mainly lowers calories, total fat and saturated fat. USDA references per 100 g: whole milk 61 calories and 3.25 g fat; 2% milk 50 calories and about 2 g fat; skim milk 34 calories and 0.08 g fat. Protein stays close to 3.2–3.4 g and calcium remains around 113–122 mg.
The biggest predictable change is fat. USDA plain yogurt references provide about 61 calories, 3.25 g fat and 3.47 g protein/100 g for whole-milk yogurt versus 50 calories, 0.1 g fat and 4.2 g protein for nonfat. Carbohydrate is product-dependent and can be higher in nonfat yogurt because milk solids and processing differ.
Opened mozzarella has no single storage time because fresh mozzarella packed in liquid and low-moisture pizza mozzarella are different products. For fresh mozzarella, follow the package's “use within X days of opening” direction and think of roughly 3–5 days as a common practical window; USDA's broader soft-cheese guidance is about a week. Low-moisture mozzarella can last longer, but the specific manufacturer instruction should take priority.
Opened shredded cheese should stay at 40 °F or below, and the package's after-opening direction is the most specific guide. FoodKeeper gives about 1 month as a broad refrigerated reference for shredded cheese, but some manufacturers recommend a much shorter period after opening. If mold appears in shredded cheese, discard the whole package.
Hard, aged cheeses are usually the lowest in lactose. Analytical reviews place Parmigiano Reggiano, Grana Padano and Pecorino Romano below 0.01 g/100 g, with Emmental, Gruyère and some blue cheeses below 0.1 g/100 g. Actual content varies by process and brand, and lactose intolerance is not the same as milk-protein allergy.
Many aged cheeses naturally develop white or crunchy crystals. Two common types are calcium lactate and the amino acid tyrosine; they can form on the surface or inside the cheese and are not, by themselves, signs of spoilage.
There is no single storage time for every cheese. USDA gives about 3–4 weeks after opening for hard cheeses such as cheddar, Swiss and Parmesan, while soft cheeses such as cottage cheese, ricotta or Brie are generally around 1 week. A shorter package instruction should take priority.
Fresh cheese should stay refrigerated and the package's use-by and after-opening directions should come first. As a broad reference, USDA gives about 1 week in the refrigerator for several soft cheeses at 40 °F or below, but individual fresh cheeses may carry a shorter after-opening instruction.
A hard edge on refrigerated cheese is usually a moisture problem, not the refrigerator somehow cooking it. Exposed cheese loses water to the surrounding air, concentrating the protein-and-fat matrix at the surface. Protecting the cut face slows that drying.
The shiny oil on melted cheese is milk fat that has separated from the protein matrix. Melting is not simply turning a solid into a liquid: casein has to soften while still holding water and liquid fat. When that balance breaks, free oil collects on the surface.
Fresh, reduced-fat cheeses dominate the low-fat end. USDA references put 1% cottage cheese at about 1 g fat/100 g and part-skim ricotta at 7.9 g, compared with 15.9 g for part-skim mozzarella, 21.3 g for feta and roughly 33 g for cheddar. Product labels still matter because recipes vary.