The fermentation of milk to produce cheese and yogurt are traditional processes for preserving milk's nutrients. Hard cheeses, such as Cheddar, which have a low moisture content and contain salt as a preservative, will last for many months if stored appropriately. Ideally cheese should be eaten fresh, but if properly stored it will retain its flavor for long periods. It should be wrapped in clear film or foil to prevent drying and then stored in a cool larder or refrigerator. Soft, unripened cheeses, such as cottage cheese, are highly perishable, need to be stored in the refrigerator, and have a relatively short shelf life. At low temperatures, microbiological growth will be reduced, as will enzyme action and biochemical changes that might change the flavor, color or texture of the product. Ripened soft cheeses such as Brie should also be kept refrigerated, wrapped in airtight film or aluminum foil.
Nearly all yogurt sold in the UK contains live bacteria (derived from the starter culture used to produce the yogurt). It is necessary to refrigerate the product to restrict the activity of these bacteria, to prevent development of excess acidity and impairment of flavor. At temperatures of about 5 °C, yogurt has a shelf life of about 14 days, after which time acidity levels may rise above acceptable levels. Spoiled yogurts are often referred to as 'blown.' This is because pressure has built up in the pot via the fermentation of the sugar in the yogurt by the growth of yeasts.
A small proportion of yogurts are heat treated to prolong their shelf life. As a result, they no longer contain live bacteria and do not need to be refrigerated.
See also: Carbohydrates: Requirements and Dietary Importance. Food Intolerance. Lactation: Dietary Requirements. Microbiota of the Intestine: Prebiotics; Probiotics. Protein: Quality and Sources.
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