How Long Will They REALLY last? Part II: Perishables

perishable foodPart II: Perishable Foods


Perishable foods--the ones kept in the fridge--are the ones consumers are most afraid of.  They worry that expired perishables might make them sick.  It's an almost wasted worry. In general, refrigeration keeps bacteria from growing to sufficient numbers to cause illness. Moreover, the bacteria scientists call "spoilage bacteria"--the ones that ruin the taste, looks, texture, and/or smell of food--grow faster than those that cause illness, so food usually turns yucky and gets discarded before it becomes a menace. 


Food-borne illness is not generally caused by eating old food; most of the time, it's caused by cross-contamination or by foods that have been kept for at the wrong temperature--not hot enough or cold enough. Scientists refer to this as temperature abuse.  Perishable foods kept in the "danger zone" (40°F - 140°F) allow bacteria to grow rapidly to a level that can cause illness. (For an elaboration of these points, click here.)


"Can I Eat It?" General Guidelines


Refrigerated foods will, of course, spoil eventually.  How can you tell when?  Food scientist Dr. Susan Brewer says that, to some extent, you can rely upon your senses.  If a refrigerated food looks, smells, and feels okay (that is, it's doesn't feel slimy), it is probably safe to eat for 4-7 days after the "use-by" date, although it may not taste as good as it did prior  to its expiration. 


The guidelines for keeping leftovers are usually given at about 3-4 days, but some foods last a little longer.  For more information about how long to keep specific leftovers, click here.


You may have noticed that perishable foods typically have a "sell-by" or "best-by" date (as opposed to the "use by" or "best by" date on a shelf-stable product.)  You can assume that a "sell-by" date gives you at least a few more days of high quality.


Ever wonder what happens to food that makes it spoil?  Food scientist Dr. Karin Allen lists the damaging effects in three categories: physical changes, chemical reactions, and microbiological changes.  Let's see what happens in each of these categories.


Physical Changes:

Separation: Examples are an oil layer in salad dressings and solids settling to the bottom of liquids.


Losing water or becoming stale: These changes can affect baked goods, pudding, gravy, and vegetables (causing veggies to wilt).


Chemical Reactions:

Oxidation: This can cause rancidity in oils, a cardboard flavor in milk, and the browning of red meat.


Enzymatic reactions:  These can cause the browning of cut fruits, a softening of fruits, and nutrition loss.


Microbiological Changes:

Spoilage: In fruit, spoilage may be caused by yeast and molds.  In vegetables, it may be caused by bacteria and molds.


Microbial enzymes:  These cause fermentation.


Safety issues: These can result from improper storage of minimally processed foods.  There are also safety issues involving deli meats and cheeses, which are discussed later in this article.


Specific Food Groups


Fresh fruits and vegetables:


Many fresh fruits and vegetables are not given a "sell-by" or "use-by" date.  Visual condition is commonly used as a guideline.  For example, if bagged lettuce (which usually is dated) has been kept in the fridge and continues to look fresh, it will be safe to eat. 


Dairy products:

The general guideline for dairy products is that quality will remain high for 7 days after the "sell-by" date.  Changes that occur in these products are usually a matter of chemical reaction not bacteria. 


MILK:  Spoilage bacteria may cause souring, which is not unhealthy to consume. 


YOGURT AND SOUR CREAM:  These products are already fermented.  Whenever fermentation (with yeast, mold, or certain types of bacteria) is controlled so that one is allowed to grow, it will outcompete harmful bacteria, and the product will have a relatively long shelf life.  However, if yogurt or sour cream develop mold, discard the entire container.


CHEESE:   Cheeses are also already fermented.  They are most often spoiled by molding.  If a fresh or soft cheese looks moldy, discard all of it.  Dangerous spores may have spread throughout the cheese. Mold in a hard cheese can be cut away (cut off about an inch from the mold) and the rest of the piece safely consumed.


Soft cheeses, especially those unpasteurized and not aged, should be discarded by the "use-by" date.  These cheeses can become contaminated easily and cause illness. 


BUTTER: Food process engineer Dr. Tim Bowser says he considers butter a shelf-stable product.  It does not need refrigeration, which is why some restaurants leave it out on the table for customer after customer.  However, at home, most people keep butter in the fridge because that prolongs its shelf life. 


ICE CREAM:  Sometimes ice cream doesn't even hold up as long as the manufacturer's expiration date promises.  Ice cream can get gummy or dry out.  I put ice cream in a freezer-strength plastic bag to give it a little more protection.  Before serving ice cream you've had for awhile, taste a little to see if it still tastes good.


EGGS: Uncooked, refrigerated eggs can be used 3-5 weeks after the "use-by" date.  Hard-boiled eggs, if refrigerated, will last a week.



Here are some tips from food scientist Dr. Karin Allen:



  • Catsup, hot sauces, and barbecue sauces contain preservatives that prevent mold or yeast growth.




  • The following should be refrigerated after opening and checked for mold after a couple of    weeks: salsas, tartar sauce, and creamy condiments.  If mold is found, discard the entire jar.




  • Mustard and horseradish are natural preservatives.  However, they can lose flavor as a result of oxidation. 



Raw meat, poultry, and fish:

These foods will taste best if you follow the date suggested on the packaged products or either use them or freeze them within a few days. If you're buying directly from a butcher and the product isn't dated, ask how long you can keep the raw item.  All of these foods should be wrapped tightly when refrigerated.  The packaging from the store is usually fine.  If the item is going uncooked into your freezer, ask or it to be freezer-wrapped.


Deli meats and hot dogs: 

Many consumers have the mistaken idea that deli meats can last almost forever because they have a lot of preservatives in them.  However, they are actually quite susceptible to contamination and spoilage. Therefore, they have a short shelf life unless frozen. Deli meats can be contaminated by listeria or by staph, either of which can cause serious illness.  Also, spoilage bacteria can grow on these and cause them to become "slimy."


Unopened deli meats can be consumed up to the "use-by" date.  Once the package is opened, they should be eaten or discarded within 3-5 days, as should luncheon meats purchased from a  deli counter.  Food scientist Dr. Catherine Cutter advises buying these products pre-packaged rather than from a deli counter.  Meats bought there can be contaminated by unsanitary handling by employees, by a slicer not adequately cleaned, or by cross-contamination from another product sliced on the same machine. A store slicer should be cleaned every two hours, but the consumer has no way of knowing if a store is actually following that safety procedure.


Furthermore, Cutter points out that pre-sliced deli-counter meats have a lot of surfaces exposed to air, and are, therefore, likely to spoil faster than the pre-packaged slices.  Buy these, she suggests, only if you're going to use them within a day or two.


Be sure your hot dogs are cooked well (to 165°F), and don't keep them forever in your fridge.  Pay attention to the use-by date.  Even an unopened package should be discarded once that "use-by" date comes. According to, if there is no product date on the package, unopened hot dogs can be safely stored in the fridge for two weeks.  Once opened, keep them only one week. If you open the package, use some, and don't plan to use the rest within a week, freeze the remainder, wrapped well in a freezer-wrap plastic bag.  They taste just fine when boiled from the frozen state.  For maximum quality, keep frozen hot dogs no longer than 1 or 2 months.


To reach links to additional Shelf Life Advice articles and Q/As about proper handling and health risks of deli meats and hot dogs, just type "deli-meats" or "hot dogs" into the search box on the home page. 


Refrigerator Care


To cool foods properly and avoid rapid bacterial growth, keep your fridge sufficiently cold, uncrowded, and clean.   Here are a few tips on refrigerator care:



  • Your fridge should be no warmer than 40°F, but you can keep it cooler than that. Cooler will extend the shelf life of many foods.  Try 35-38°F.  But check to be sure you aren't getting ice crystals in you milk or freezing the lettuce.  (Frozen lettuce is ruined.) 




  • Don't stuff your fridge to the gills.  For a refrigerator to cool properly, there must be space for the air to circulate.  Get rid of old leftovers to make room for newer items. 




  • Don't put a large container or quantity of hot food in the fridge in one space.  It may take several hours to cool, and, by that time, bacteria have had a feast on it, and your chili, casserole, or whatever may be ruined or even contaminated.  Divide hot food into smaller amounts, or put containers of hot leftovers into an ice bath and cool them down before refrigeration.




  • Keep your fridge clean!  Bacteria love spills, so wipe these up promptly.  Periodically, take everything out of the fridge and clean and sanitize it.  How often?  We recommend once a month, more often if it smells bad or looks unclean.  For more tips on proper refrigerator care, click here




  • Proper wrapping of perishables will lengthen shelf life.  Shelf Life Advice has a few articles on this topic. Here are the links to these:


If you put "wrapping" into the search box on the home page, you'll find links to Q/As and articles about specific foods and specific products for storing food (plastic containers, aluminum wrap, fish, etc.)


Frozen Foods


Foods that were not contaminated when they went into your freezer will remain safe forever.  BUT that doesn't mean you can keep them forever.  Quality deteriorates over a long period of time, especially if they are not wrapped well. Furthermore, says food scientist Dr. Joe Regenstein, if oxidation gets bad enough, some scientists believe the food can become unhealthy. For tips on wrapping for the freezer, click here:


Food scientist Dr. Karin Allen says that freezer burn is the main reason frozen food deteriorates. But freezer burn does not necessarily mean the food must be discarded.  Here are Allen's suggestions if your frozen food develops freezer burn: 


1) Discard thin cuts or deli slices.

2) Use freezer-burned ground meats or sausage in dishes that are heavily spiced, such as chili. 

3) Roasts can be thawed in the fridge.  Then, freezer-burned sections can be trimmed away. 


Here's what Dr. Regenstein says about freezer burn and techniques for avoiding it:  "Everything is subject to freezer burn.  The more 'water' in the product, the more likely there will be freezer burn.  The key to prevention is TIGHT wrapping. If 'oxidation' is a problem, then putting certain foods in water will help prevent that. Food processors can do this with a process called 'glazing.'   The product is usually frozen and then dipped into a glazing solution-- often it's plain water but other times processors may add a few other compounds, such as gums, to make the solution more viscous.  This then freezes on the surface. Sometimes, water is added prior to freezing, for example, with shrimp and some fruits." 


Give a little thought to what you store in the freezer door, especially if your household includes folks who frequently open the freezer looking for ice cream or just browsing through the frozen meal choices.  Dr. Bowser keeps these items in or near his freezer door: ice cubes, freezer packs, nuts, flour, cereal, and other low-moisture foods. What does Dr. Regenstein keep in his freezer door?  Batteries, coffee, and other items that don't have freezer-burn considerations. 


Semi-perishable Foods


Between perishable and non-perishable foods, there is a third category called "semi-perishable."  Here's how the USDA defines this group: "The category of semi-perishable foods can encompass a wide variety of foodstuffs that have varying shelf lives. Generally, semi-perishables require no refrigeration and are shelf stable, except under extreme temperatures, due to the presence of preservatives, specialized processing or packaging. Examples include dried fruit, ultra high temperature treated milk and pasteurized fruit juices in aseptic packaging."  To this list, Dr. Allen adds most hard cheeses, such as cheddar and Parmesan.  "They're better if refrigerated," she says, "but still safe if they haven't been."


To read about the shelf life of non-perishables, click here:




Karin E. Allen, Ph.D., Utah State University, Dept. of Nutrition, Dietetics, and Food Sciences


Timothy J. Bowser, Ph.D., Oklahoma State University, Dept. of Biosystems and Agricultural Engineering


Catherine N. Cutter, Ph.D., Pennsylvania State University, Dept. of Food Science


Joe Regenstein, Ph.D., Cornell University, Dept. of Food Science


Susan Brewer, Ph.D., R.D. University of Illinois, Department of Food Safety and Human Nutrition "Tips for Tailgating: Hot Dog Safety Basics" "Report to Congress on Use of Perishable Commodities and Live Animals in Food Aid Programs, III. A. "Perishable and Semi-perishable Commodities"



Links to additional sources within this website are provided in the article.


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