Cheese in various forms has been around for thousands of years, and refrigeration for just over a century, so it's clear that not all cheeses must be refrigerated all the time. As a rule, the firmer and drier a cheese is, the longer it takes to spoil at room temperature. For example, hard, dry Parmesan doesn't need refrigeration at all. High-moisture soft cheeses, including familiar favorites such as cream cheese, should always be kept refrigerated.
Processed But Perishable
Many popular "cheese food" spreads and related products are designed to be shelf-stable at room temperature, through a combination of pasteurization and preservatives. Cream cheese, despite its common use as a spread, doesn't fall into the same category. It's a natural cheese, made -- like many other soft cheeses -- by encouraging lactic-acid bacteria to gently curdle whole milk. The various stabilizing agents used in cream cheese help preserve its thick but spreadable texture, and to prevent whey from separating. The cheese itself, like most other dairy products, is highly perishable and provides a nutritious breeding ground for bacteria.
The Danger Zone
Bacteria and molds flourish in moist environments with plenty of nutrients to fuel their growth, and cream cheese delivers on both counts. It can legally be up to 55 percent moisture, with sufficient protein and lactose -- milk's natural sugar -- to feed microorganisms. As with fish, chicken and other perishable foods, cream cheese and other soft cheeses should not be left out at room temperature for longer than two hours. Temperatures above 40 degrees Fahrenheit and below 140 F are ideal for bacterial growth, and cream cheese should not be left in this "danger zone."
The Prime Suspect
Once opened, cream cheese can easily be contaminated by dangerous or "pathogenic" bacteria from your fingers or a spreading knife that's previously been used for other foods. The most problematic of these is Listeria monocytogenes, the bacteria responsible for listeriosis. Usually listeriosis results in several days of nausea, low fever and diarrhea, but occasionally it's more severe. It can cause stillbirth in pregnant women, and septicemia and meningitis in the elderly or ill. It's especially troublesome because unlike most pathogens, L. monocytogenes can continue to grow and flourish in the refrigerator after you put the cheese away. In other words, even if it doesn't make you sick today, tomorrow could be different.
Foods Containing Cream Cheese
Bakers sometimes leave cream cheese to soften overnight on the counter, before making a cheesecake, reasoning that it will be baked afterwards and should therefore be safe. That's true to a point. But if bacteria are present, some will usually survive and begin to reproduce again after the cheesecake cools. As a rule, any food is as perishable as its most perishable ingredient, so cheesecakes, cream cheese spreads and similar dishes should also be kept refrigerated. Some commercial cream cheese icings are manufactured with enough preservatives to be safe without refrigeration, but their homemade counterparts should always be refrigerated.